Improvements in and relating to isotope production

Abstract

Methods for production of isotopes (such as Actinium-225, Lead-212, Terbium-161, Gold-199, Scandium-47, Copper-64, Copper-67, Bromine-77, and / or Yttrium-90 comprise irradiating a target isotope with

Description

Field of the Invention The present invention concerns methods and apparatus for production of isotopes, such as isotopes for use in nuclear medicine. More particularly, but not exclusively, this invention concerns methods and apparatus for production of product isotopes, such as Actinium-225, Lead-212, Terbium-161, Gold-199, Scandium-47, Copper-64, Copper-67, Bromine-77, and / or Yttrium-90 from various target isotopes. This invention also concerns methods and apparatus in which a target composition comprising a target isotope is irradiated with neutron radiation, wherein the neutron radiation comprises at least 50% neutrons generated by deuterium-tritium fusion, and optionally no more than 40% neutrons generated by deuterium-deuterium fusion, such as deuterium-tritium fusion generated by lattice confinement fusion in a nuclear fusion reactor. Background of the Invention Various radionuclides are used in nuclear medicine, for example for imaging and therapeutic applications. Isotopes such as Ac-225, Pb-212, Tb-161, Au-199, Sc-47, Cu-64, Cu-67, Br-77, and / or Y-90 are sought after, but currently only available via complex and / or expensive routes, and / or from expensive and / or limited availability target isotopes. Thus, there is a need for a cost-effective, high volume and high purity method of producing various product isotopes, such as Ac-225, Pb-212, Tb-161, Au-199, Sc-47, Cu-64, Cu-67, Br-77, and / or Y-90. The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide improved methods and apparatus for production of such product isotopes. Summary of the Invention According to a first aspect, the present invention provides a method for producing a product isotope, wherein the method comprises subjecting a target composition comprising a target isotope to neutron irradiation to produce the product isotope via a nuclear reaction, such as a nuclear transmutation reaction. Optionally, the target composition is irradiated by neutron radiation comprising (or optionally substantially consisting of, such as comprising at least 95%) neutrons having an energy in the range of about 1 to about 20 MeV. Optionally, the target composition is irradiated by neutron radiation comprising: at least 50% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV), and / or no more than 40% neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV. According to a second aspect of the invention, there is provided an isotope product, for example an isotope product produced according to the method of the first aspect of the invention. According to a third aspect of the invention, there is provided apparatus for production of isotopes, for example apparatus configured and arranged for performance of the method of the first aspect of the invention. Target isotopes Optionally, the target isotope is an isotope susceptible to a nuclear transmutation reaction by irradiation with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV. Additionally or alternatively, the target isotope is an isotope susceptible to a nuclear reaction (e.g. a nuclear transmutation reaction) by irradiation with neutrons having an energy in the range of about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV. Optionally, the method of the first aspect of the invention comprises irradiating the target composition with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV (e.g. about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV) to form the product isotope by one or more nuclear transmutation reactions. Optionally, the target isotope is susceptible to one or more charged particle producing reactions and / or multiple particle producing reactions when subjected to irradiation with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV (e.g. about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV). It will be understood that charged particle producing reactions include, for example, (n, p), (n, d) and (n, a) reactions, and that multiple particle producing reactions include, for example, (n, n'p), (n, n'a), (n, 2n) and (n, 3n) reactions. In contrast, (n, n), (n, n'y) and (n, y) reactions will be understood to be elastic, inelastic and absorption reactions, respectively, and thus not charged particle producing reactions or multiple particle producing reactions. Optionally, the target isotope has a mass no heavier than the heaviest naturally occurring isotope (i.e. no heavier than Uranium-238). Optionally, the target isotope is a naturally occurring isotope. Additionally or alternatively, the target isotope has a half life (1½) of at least 1 hour, such as at least 1 day, for example at least 14 days. Optionally, the target isotope forms a product isotope having a half life (t’A) of up to about 18 days (for example from about 1 hour to about 18 days) via a nuclear reaction (such as a nuclear transmutation reaction) when irradiated with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV (e.g. about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV). It has been found that such half lives provide a convenient balance between the half life being long enough (and thus radioactivity being low enough) and the half life being short enough (and thus reactivity high enough) for the product to be useful in, e.g., imaging and / or therapeutic applications. In particular, it is believed that product isotopes having shorter half lives may be less well suited because of the consequential time pressure for their use following production, while product isotopes having longer half lives may be more costly and / or challenging to produce in sufficient quantities for typical applications, particularly using relatively low flux nuclear fusion reactor systems. Furthermore, long half-life isotopes may be of lower clinical utility, for example if higher patient dose is required and / or more careful patient management is necessitated. It will be understood that the target isotope may optionally form the product isotope via one or more intermediate isotopes, for example via a sequence of nuclear reactions including spontaneous decay of an intermediate (parent) isotope to the (daughter) product isotope. In other words, the target isotope may form one or more intermediate isotopes via a nuclear reaction (optionally a nuclear transmutation reaction) when irradiated with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV (e.g. about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV), optionally wherein one or more of the intermediate isotopes spontaneously decay to form the product isotope by one or more nuclear reactions (optionally one or more spontaneous nuclear transmutation reactions). Alternatively, the target isotope may optionally form the product isotope directly, i.e. without forming an intermediate isotope. Optionally, the target isotope is Radium-226, Dysprosium-161, Mercury-202, Mercury-199, Titanium-47, Calcium-48, Zinc-64, Zinc-67, Krypton-78, Zirconium-90 or Niobium-93. Optionally, the product isotope is Actinium-225, Lead-212, Terbium-161, Gold-199, Scandium-47, Copper-64, Copper-67, Bromine-77, and / or Yttrium-90. It is believed that the target isotopes described hereinabove are particularly well suited to use in the isotope production method and apparatus of the present invention. Neutron radiation Optionally, the target composition is irradiated by neutron radiation comprising at least 50% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV), and / or no more than 40% neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV. Optionally, at least 75% of the neutrons of the neutron radiation have an energy of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV) and below about 4 MeV, such as about 0.5 to about 4 MeV. In other words, optionally no more than 25% of neutrons have energies outside those ranges. Additionally or alternatively, at least 90% of neutrons have an energy of no more than about 16 MeV. Optionally, the neutron radiation comprises at least 60% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV), such as at least 70% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV). Additionally or alternatively, the neutron radiation comprises at least 50%, for example at least 60%, such as at least 70% neutrons having an energy in the range of about 13 MeV to 15 MeV. Optionally, the neutron radiation comprises no more than 35% neutrons having an energy of below about 4 MeV (such as 0.5 to 4 MeV), such as no more than 18% neutrons having an energy of below 4 MeV (such as in the range of about 0.5 MeV to about 4 MeV). Optionally, the neutron radiation comprises at least 80% neutrons, for example at least 85% neutrons, having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV) and below about 4 MeV (such as in the range of about 0.5 to about 4 MeV). It has been found that such a radiation spectrum is especially effective in efficient and reliable production of target isotopes, and in some cases in production of high purity target isotopes. Specifically, a relatively low proportion of neutrons having energies outside the range 12-16 MeV (such as 4 MeV and / or above 16 MeV) has been found to provide an efficient device that can avoid alternative nuclear transmutation reactions that may product unwanted isotopes. It will be appreciated that isotope separation can be complicated and costly, if possible at all, and so it is advantageous to provide a process that avoids production of unwanted isotopes. It will understood that % values for neutrons having certain energies is readily determined from neutron spectra. Nuclear fusion reactor Optionally, the method of the first aspect of the invention comprises operating a nuclear fusion reactor to generate the neutron radiation. It will be appreciated that such a nuclear fusion reactor avoids the use of fissile material (such as that present in a nuclear fission reactor), and thus provides well defined radiation and the ability to entirely shut off emission of radiation when the reactor is turned off. Optionally, the method comprises operating the nuclear fusion reactor to initiate deuterium-tritium fusion. It will be understood that deuterium-tritium fusion produces neutrons having energies in the range of about 12 MeV to 16 MeV. Optionally, the method further comprises operating the nuclear fusion reactor to initiate deuterium-deuterium fusion and / or tritium-tritium fusion. It will be understood that deuterium-deuterium fusion provides neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV, and tritium-tritium fusion provides neutrons having an energy in the range of about 0.5 MeV to about 10 MeV. Optionally, the method comprises operating the nuclear fusion reactor to minimise deuterium-deuterium fusion reactions and / or tritium-tritium fusion reactions, especially deuteriumdeuterium fusion reactions. Additionally or alternatively, the method optionally comprises inducing deuterium-tritium lattice confinement fusion in a nuclear fusion reactor to produce neutron radiation by generating an electric field between electrodes of the nuclear fusion reactor. Optionally, at least part of a surface of one or more said electrodes of the nuclear fusion reactor is enriched with deuterium and / or tritium, for example enriched with at least 100 ppm deuterium and / or tritium by atomic percentage. Optionally, the method comprises producing neutron radiation comprising at least at least 50% (such as at least 60%, for example at least 80%) neutrons generated by deuterium-tritium fusion and no more than 50% (such as no more than 40%, for example no more than 20%) neutrons generated by deuterium-deuterium fusion. Optionally, the nuclear fusion reactor is configured and / or arranged for multistate fusion. It will be understood that multi-state fusion may comprise, for example, a combination of plasma-based fusion and solid-state fusion (such as lattice confinement fusion). It has been found that multi-state fusion reactors provide cost, energy and space efficient systems for production of suitable neutron radiation. Optionally, the nuclear fusion reactor comprises at least one electrode having an enriched surface, the enriched surface comprises a lattice substrate enriched with deuterium and / or tritium (preferably deuterium), for example enriched with at least 100 ppm deuterium and / or tritium by atomic percentage. Optionally the nuclear fusion reactor is arranged and operated to induce lattice confinement fusion (particularly deuterium-tritium fusion) in the enriched surface. Optionally, the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor. Optionally, the enriched surface is deliberately enriched. It will be understood that deliberate enrichment distinguishes from, for example, passive enrichment (through which a surface may become enriched to a minor extent during operation of a nuclear fusion reactor). It has been found that deliberate enrichment provides a significantly greater degree of enrichment as compared to passive enrichment. It will be understood that the enriched surface may be enriched by any suitable means, such as by one or more of electrolysis and high temperature, high pressure (HPHT) gas loading. Optionally, the enriched surface is enriched by electrolysis, for example electrolysis with a solution of deuterated and / or tritiated heavy water and conductive salts for electrolysis. It has been found that deliberate enrichment of an electrode surface, such as by electrolysis, provides a high level of deuterium and / or tritium enrichment, promoting effective lattice confinement fusion. It has further been found that enrichment by electrolysis results in adsorption of elements of the conductive salt into the surface, detectable for example by Energy-dispersive X-ray spectroscopy (EDX). It follows that the presence of such elements in an enriched surface max provide a characteristic fingerprint of enrichment by electrolysis. Suitable conductive salts include alkali metal-containing salts. Following enrichment by electrolysis, a sample of the enriched surface may comprise, for example, at least 0.05 %, such as at least 0.1 % of one or more elements of the conductive salt, such as one or more alkali metal elements, for example as determined by EDX. Optionally, the surface is subjected to electrolysis at a current density of least 0.1 mA / cm2 of surface for at least 10 minutes. A suitable nuclear fusion reactor having one or more enriched surface electrodes is described in WO 2022 / 263827 Al (Astral Neutronics Ltd), the contents of which are incorporated herein by reference. Optionally, the method comprises forming a plasma from gas, for example forming a plasma within the nuclear fusion reactor. Optionally, the method comprises initiating nuclear fusion in the presence of the plasma to generate the neutron radiation. Optionally, the gas comprises at least 50 mol% tritium and no more than 50 mol% deuterium. Additionally or alternatively, the gas optionally comprises at least 80 mol% tritium (such as at least 90 mol%, for example at least 95 mol%), and optionally no more than 20 mol% deuterium (such as no more than 10 mol%, for example no more than 5 mol%). Optionally, the gas is substantially entirely tritium gas, for example about 98 mol% (such as about 99 mol%, optionally about 100 mol%) tritium gas. Optionally, the method comprises maintaining the gas composition at a constant concentration of deuterium and / or tritium during operation of the nuclear fusion reactor. Optionally, when the gas is predominantly or entirely tritium, the enriched surface is enriched with deuterium, for example wherein the enriched surface is enriched with fusible isotope species and where the fusible isotope species enrichment is at least 95 mol% (such as at least 98 mol%, for example at least 99 mol%) deuterium, based on the fusible isotope species content of the enriched surface. Optionally, the gas is at least 95 mol% (such as at least about 99 mol%, optionally about 100 mol%) either deuterium or tritium, and the enriched surface comprises at least 95 mol% (such as at least about 98 mol%, optionally at least about 99 mol%) either deuterium and tritium, based on the fusible isotope species content of the enriched surface, wherein the gas is predominantly deuterium and the fusible isotope species of the enriched surface is predominantly tritium, or wherein the gas is predominantly tritium and the fusible isotope species of the enriched surface is predominantly deuterium. It has been found that having a gas comprising predominantly tritium and a surface enriched predominantly with deuterium, and vice versa, helps to favour deuterium-tritium fusion in preference to deuterium-deuterium and tritium-tritium fusion. Without wishing to be bound by theory, it is believed that in a multi-state fusion nuclear reactor, neutron radiation is produced predominantly by lattice confinement fusion reactions occurring between a plasma fusible isotopes species and an enriched surface fusible isotope species. Furthermore, it has been found that surface enrichment with deuterium may be more convenient than surface enrichment with tritium. Optionally, the apparatus of the third aspect of the invention comprises a nuclear fusion reactor configured and arranged to produce neutron radiation comprising at least 50% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV), and optionally no more than 40% neutrons having an energy of below about 4 MeV, such as about 0.5 to about 4 MeV. Optionally, the nuclear fusion reactor is arranged to produce neutron radiation comprising at least 75% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV) and below about 4 MeV, such as in the range of about 0.5 to about 4 MeV. Optionally, the apparatus is configured to produce neutron radiation as described in relation to the method of the first aspect of the invention. Optionally, the nuclear fusion reactor is configured and arranged to generate the neutron radiation by deuterium-tritium fusion, and optionally by deuteriumdeuterium fusion and / or by tritium-tritium fusion. Additionally or alternatively, deuterium-tritium fusion provides the neutrons having an energy in the range of about 12 MeV to about 16 MeV, deuterium-deuterium fusion provides neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV, and / or tritium-tritium fusion provides neutrons having an energy in the range of about 0.5 MeV to about 10 MeV. Optionally, the nuclear fusion reactor is a multi-state fusion reactor, for example wherein multi-state fusion comprises a combination of plasma fusion and lattice confinement fusion. Additionally or alternatively, the nuclear fusion reactor comprises at least one electrode having an enriched surface, the enriched surface comprising a lattice substrate enriched with deuterium and / or tritium (preferably deuterium), optionally wherein the nuclear fusion reactor is configured and arranged for lattice confinement fusion in the lattice substrate. Optionally, the nuclear fusion reactor comprises an anode structure and a cathode structure, wherein the anode and cathode structures are substantially concentric along at least a part of their lengths and are configured such that, in operation, an electric field is provided between the anode and cathode structures, and wherein at least one of the anode structure and the cathode structure comprises at least one said electrode comprising a surface enriched with deuterium and optionally tritium. Optionally, the enriched surface is deliberately enriched, for example as described in relation to the first aspect of the invention. Optionally, the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor. Optionally, the apparatus comprises a nuclear fusion reactor configured and arranged for inducement of deuterium-tritium lattice confinement fusion by generation of an electric field between electrodes of the nuclear fusion reactor. Optionally, at least part of a surface of one or more said electrodes of nuclear fusion reactor is enriched with deuterium and / or tritium. Optionally, the enriched surface is deliberately enriched, for example as described in relation to the first aspect of the invention. Optionally, the nuclear fusion reactor contains a gas and is operable to form a plasma from the gas. Optionally, the gas comprises at least 50 mol% tritium and no more than 50 mol% deuterium. Additionally or alternatively, the gas optionally comprises at least 80 mol% tritium, and optionally no more than 20 mol% deuterium. Optionally, the gas is as described in relation to the first aspect of the invention. Liquid target composition Optionally, the method of the first aspect of the invention utilises a liquid target composition, for example when the target isotope is Radium-226. Optionally, the target composition is an aqueous solution of the target isotope, for example the target composition comprises or consists of water and one or more salts comprising the target isotope. It will be understood that any water-soluble salt may optionally be used, particularly salts having a counterion having low or no activity or reactivity under the neutron irradiation conditions of the method. Optionally, the salt is present in the water in an amount of no more than 100 g / L (such as 0.1 g / L to 100 g / L), for example no more than 50 g / L (such as 1 g / L to 50 g / L). Additionally or alternatively, the concentration of target isotope ions in the aqueous solution is no more than about 0.2M (such as 0.00IM to 0.2M), for example as no more than about 0. IM (such as 0.01M to 0.1M). Optionally, the target isotope salt is present in the water in an amount of at least 50 g / L, for example at least 100 g / L. Additionally or alternatively, the concentration of target isotope ions in the aqueous solution is at least about 0. IM, such as at least about 0.2M. Optionally, the aqueous solution is a saturated solution of the one or more salts of the target isotope. Optionally, the salt is a halide, hydroxide, iodate or nitrate salt of the target isotope. Optionally, the volume of the target solution is from about 1 pl to 100 ml, such as about 10 pl to 50 ml. It will be understood that the target volume is the volume of solution irradiated by the neutron radiation. It has been found that a liquid target allows for particularly convenient extraction and handling of isotopes produced by the method. Optionally, the method is a continuous production method, for example wherein the method comprises removing a portion of the target composition, and optionally replenishing the target composition with additional target isotope, for example without interrupting irradiation of the target composition and / or without removing the entire target composition. Additionally or alternatively, the method comprises selectively removing one or more isotopes of elements other than the target isotope from the target composition, and / or adding additional target isotope to the target composition. Optionally, the step of selectively removing isotopes other than the target isotope from the target composition comprises removing a portion of the target composition solution from the target holder, extracting one or more isotopes of elements other than the target isotope from said portion of the target composition solution, and returning remaining target composition solution to the target holder. Optionally, the target composition has a thickness of about 0.5 mm to about 15 mm, such as about 1 mm to about 10 mm, for example when the target composition comprises Radium-226. It will be understood that the target composition thickness is the thickness of target isotope (e.g. Ra-226)-containing target composition through which neutron radiation travels when the target composition is irradiated. It has been found that having a relatively thin target may help to improve product purity, particularly when the target isotope is Ra-226. For example, interaction of 12-16 MeV energy neutrons with Ra-226, thereby producing Ra-225 by the (n, 2n) reaction may result in lower energy neutrons able to interact with further Ra-226 isotopes and product unwanted Ra-227 by the (n, y) reaction. In particular, it has been found that, for example, reducing target composition thickness from 50mm to 10 mm may result in a 20x reduction of Ac-227 production when the target isotope is Ra-226, but only a 1.6x reduction in Ac-225 production, and it is understood that further reduction of the target thickness will result in a close to isotopically pure Ac-225. Optionally, the method comprises irradiating a surface area of the target composition, for example wherein the surface area is at least about 1 cm2, such as at least about 10 cm2, for example at least about 50 cm2, optionally wherein the surface area is about 1 cm2 to 1.4 m2, such as about 10 to about Im2, for example about 50 cm2 to about 0.6 m2. It has been found that irradiating a large surface area of the target solution allows for efficient isotope production, for example while also allowing for use of a thin target composition. Furthermore, having a larger surface area target is particularly advantageous when the target composition is provided as a liquid solution, for example because it allows circulation of the solution to avoid development of ‘hotspots’ in locations with higher neutron flux, thereby helping to avoid accumulations of unwanted activation products (e.g. Ac-227 when the target composition comprises Radium-226) over time. In other words, circulation of liquid target composition across a large surface area allows for homogenisation of the neutron fluence over the entire target composition volume, thereby avoiding unwanted buildup of byproducts (e.g. Ac-227 when the target composition comprises Radium-226). Optionally, the neutron radiation is substantially isotropic, for example wherein the method comprises operating the nuclear fusion reactor to produce substantially isotropic neutron radiation. Optionally, the target composition is arranged in a target holder, such as a target holder as described in reference to the apparatus of the third aspect of the invention. Optionally, the apparatus of the third aspect of the invention comprises a target holder configured to hold a target composition (e.g. a target composition comprising Ra-226), for example wherein the target holder is arranged to cause exposure of the target composition to the neutron radiation. It will be understood that the target holder may be formed integrally with, or separate to, the nuclear fusion reactor. Optionally, the target composition is an aqueous solution of Ra-226, for example the target composition comprises or consists of water and one or more salts comprising Ra-226. Optionally, the target composition is a solution as described in relation to the first aspect of the invention. Optionally, the apparatus comprises the target composition, for example wherein the target composition is disposed in the target holder, for example wherein the target holder is in the form of a hollow vessel. Optionally, the apparatus is configured so that the target composition has a thickness of 0.5 mm to 15 mm, such as 1 mm to 10 mm. Additionally or alternatively, the target holder is configured to provide and / or hold a target composition having a thickness of about 0.5 mm to about 15 mm, such as about 1 mm to about 10 mm. It will be understood that the target composition thickness does not include the target holder (for example, the thickness of the walls of any vessel holding the target composition do not form part of the thickness of the composition. Additionally or alternatively, the target holder comprises a void for holding a target composition, the void being bounded by at least a first side facing the neutron source of the nuclear fusion reactor and a second side opposing the first side, optionally wherein the first and second sides are spaced apart by about 0.5 mm to about 15 mm, such as about 1 mm to about 10 mm. Optionally, the apparatus is configured so that, during operation of the nuclear fusion reactor, a surface area of the target composition irradiated by the neutron radiation is at least about 100 cm2, such as at least about 1000 cm2, optionally wherein the surface area is about 100 to 4000 cm2, such as about 500 to 3000 cm2. Additionally or alternatively, the target holder is configured to provide and / or hold a target composition having a surface area of at least about 100 cm2, such as at least about 1000 cm2, facing the neutron source of the nuclear fusion reactor, optionally a surface area of about 100 to 4000 cm2, such as about 500 to 3000 cm2. Additionally or alternatively, the target holder is configured to present a target composition surface area of at least about 100 cm2, such as at least about 1000 cm2, for neutron irradiation, optionally a target composition surface area of about 100 to 4000 cm2, such as about 500 to 3000 cm2. Optionally, the target holder is arranged to position the target composition at least partially around a neutron source (for example a nuclear fusion reactor, more particularly around vessel of the nuclear fusion reactor), such as arranged to extend circumferentially around the neutron source and / or to extend in an arc around the neutron source wherein the arc subtends an angle of at least 90°, such as at least 180°, for example at least 270°. Optionally, the target holder is a tube that extends around at least part of a neutron source, for example wherein the tube is in the form of a coil wrapped around the neutron source, optionally wherein the coil has a plurality of, such as at least three, for example at least five, turns that each extend circumferentially around the neutron source (for example wrapped around a vessel of a nuclear fusion reactor, thereby at least partially surrounding the neutron source). It has been found that such an arrangement may provide a particularly convenient and effective arrangement for exposure of a large surface area of the target composition to neutron radiation. Additionally or alternatively, the target holder is a in the form of a shell that extends around at least part of a neutron source and / or along a length of a neutron source. Optionally, the neutron source is a nuclear fusion reactor having an elongate reactor vessel and elongate electrodes, wherein the elongate vessel and electrodes each independently have a length at least 1.5 times, for example at least 2 times, their width. Optionally the target holder is configured to position the garget composition along at least 50%, such as at least 80%, for example at least 95% of the length of the vessel and / or the electrodes. Optionally, the nuclear fusion reactor comprises a reactor vessel having a length of at least about 20 cm, such as about 20 cm to about 200 cm, e.g. about 20 cm to about 80 cm, and / or a width of at least about 10 cm, such as about 10 cm to about 80 cm, e.g. about 10 cm to about 40 cm. Optionally, the reactor vessel is cylindrical for at least a portion of its length, such as for at least 15%, optionally at least 50%, for example at least 75% (or at least 85%), of its length. Optionally, the target holder is configured to be positioned at least partially across an end of the reactor vessel. Optionally, the reactor vessel has an elongated shape with a longitudinal peripheral surface and at least two opposed end surfaces, for example wherein the end surfaces are curved or planar, and optionally the target holder is configured to position the target composition adjacent the longitudinal peripheral surface and at least one of the two end surfaces, and / or wherein such a reactor vessel is substantially cylindrical for at least a portion of its length. It will be understood that a cylindrical reactor vessel having a length of 30 cm and a diameter of 15 cm provides a surface area across which target composition may be arranged of more than 1400 cm2 excluding the ends of the cylinder, or more than 1700 cm2 including the ends of the cylinder. Solid target composition Optionally, the method of the first aspect of the invention utilises a solid target composition, for example when the target isotope is not Radium-226, and / or when the target isotope is Dysprosium-161, Mercury-202, Mercury-199, Titanium-47, Calcium-48, Zinc-64, Zinc-67, Krypton-78, Zirconium-90 and / or Niobium-93. Optionally, the target composition comprises the target in elemental form and / or as one or more oxides or salts (e.g. metal salts). It will be appreciated that any suitable solid formulation of the target isotope may be utilised, for example using carrier materials known in the art. Optionally, the target composition comprises an isotopically enriched material comprising the element of the target isotope (i.e. enriched with the target isotope). Optionally, the apparatus of the third aspect of the invention comprises such a solid target composition, for example held in the target holder, such as enclosed and / or sealed in the target holder. Radium-226 / Actinium-225 / Pb-212 An example of a sought-after isotope is Ac-225, which has been found to have particularly useful radiochemistry. Ac-225 has a half-life of 10 days, and decays by alpha emission, making it well suited for use in medicines for targeting and killing cancer cells. In particular, alpha particle radiation is able to break DNA strands, and the 10-day half life has been found to provide enough time for medicines to be prepared and administered while also avoiding long-term presence in the body after treatment. While various processes have been developed for production of Ac-225, development and deployment of nuclear medicines based on Ac-225 has been limited by problems of cost and insufficient supply. Ac-225 is not a naturally occurring isotope in appreciable quantities, and so synthetic production is required. One approach to producing Ac-225 is via transmutation of Ra-226, which may be initiated by irradiation of a Ra-226-containing target material with high energy neutrons, protons or photons. Following such an approach, suitable irradiation of Ra-226 causes a (n, 2n) reaction to generate Ra-225, followed by decay to produce Ac-225. A challenge with such methodologies is avoiding formation of Ra-227 by lower energy irradiation of Ra-226 leading to a (n, y) reaction. Ra-227 decays to Ac-227, which due to its approximately 22-year half-life is considered to be a harmful contaminant. US2022 / 0108812A1 discloses a method of manufacturing Ra-225 from Ra-226 in a moderated nuclear reactor. The Ra-226 target is shielded with a thermal neutron absorption shield to improve Ac-225 purity. The method requires use of a highly complex, centralised nuclear reactor facility, limiting the availability of isotope products. It will be appreciated that long-distance transport of Ac-225 from a nuclear production facility to medicine production facility is likely to result in a prohibitively large quantity of Ac-225 given the isotope’s 10-day half-life. Furthermore, such nuclear reactors are complex, expensive to run, require fissile materials, produce nuclear waste, take a decade or more to build, and can only be constructed on very specific sites. Yet further, many of the existing reactors are scheduled for decommissioning over the next decade. WO2020 / 210147A1 discloses a method of producing Ac-225 from Ra-226 by irradiation of a Ra-226 with neutrons generated using a cyclotron. WO2004079751A2 discloses another cyclotron-based process, in which Ra-226 is bombarded with deuterons. WO9963550A1 discloses a further cyclotron-based process, in which Ra-226 is bombarded with protons. Only a small number of cyclotrons worldwide have the capability to produce Ac-225 because of the high energy requirements for Ra-226 transmutation to Ac-225. Thus, cyclotron-based production methods suffer from similar drawbacks to methods relying on use of nuclear fission reactors. Yet further, cyclotron-based production methods (usually involving bombardment of Th-232) typically result in a high level of Ac-227 contamination. US2024 / 0032182A1 discloses a system that uses a D-Li-7 neutron generator to irradiate Ra-226 and generate Ac-225 with ~1% Ac-227 contamination. High flux 10 and 13 MeV neutrons are generated by the D-Li-7 neutron generator. Neutron production is indirect (achieved by bombardment of a Li-7 source with a deuteron beam), resulting in relatively poor efficiency due to losses in the system. The system also generates gamma photons as well as neutrons. Furthermore, over time the Li-7 target will require replacement, interrupting production. A study into Ra-226 reactions is disclosed in CROSS-SECTIONS FOR THE REACTIONS Ra-226(n,2n)Ra-225 AND Ra-226(n,3n)Ra-224 WITH 14.5 MeV NEUTRONS, L. P. O'Connor and J. L. Perkin, J. Inorg. Nucl. Chern., 1960, Vol. 13. pp. 5 to 12. Further information on Ac-225 synthesis is found in the following journal articles: Yuto Sasaki, Aaru Sano, Shinji Sasaki, Nobuyuki Iwamoto, Kazuki Ouchi, Yoshihiro Kitatsuji, Naoyuki Takaki &Shigetaka Maeda (2024), Evaluation of the production amount of Ac-225 and its uncertainty through the Ra-226(n,2n) reaction in the experimental fast reactor Joyo, Journal of Nuclear Science and Technology, 61:4, 509-520; Iwahashi,D.; Kawamoto, K.; Sasaki, Y.; Takaki,N., Neutronic Study on Ac-225 Production for Cancer Therapy by (n,2n) Reaction of Ra-226 or Th-230 Using Fast Reactor Joyo. Processes 2022, 10, 1239; Yuto Sasaki, Nobuyuki Iwamoto, Naoyuki Takaki &Shigetaka Maeda (2024), Status of the Ra-226 nuclear data library and its impact on the production amount of Ac-225 via the Ra-226 (n,2n) reaction, Journal of Nuclear Science and Technology, 61:2, 251-260; Robertson et al., Development of Ac-225 Radiopharmaceuticals, Current Radiopharmaceuticals, 2018, Vol. 11, No. 3, 156-172; Higashi, T.;Nagatsu, K.; Tsuji,A.B.; Zhang,M.-R., Research and Development for Cyclotron Production of Ac-225 from Ra-226 — The Challenges in a Country Lacking Natural Resources for Medical Applications. Processes 2022, 10, 1215; G. Melville, P. Melville, A theoretical model for the production of Ac-225 for cancer therapy by neutron capture transmutation of Ra-226, Applied Radiation and Isotopes 72 (2013) 152-157. Alongside Ac-225, another sought-after medical isotope is Pb-212. It has been found that Pb-212 can be produced alongside Ac-225 from Ra-226. US2022 / 0415533A1 discloses a method of producing both Pb-212 and Ac-225 by high energy proton irradiation of Ra-226. Pb-212 is interesting as a product isotope in its own right. Pb-212 has a half life (tU) of 10.64 days, offers 100% P' decay, and mean P' energy of 101.3 keV. 203Pb / 212Pb has been suggested as a candidate for a “true” theranostic imaging / alpha-emitting therapy pair, with recent investigation into 203Pb / 212pb-labelled PSMA targeting ligands for prostate cancer and similar consideration for neuroendocrine tumour imaging / therapy. There is an interest in Pb-212 as an alpha-emitter, particularly since (in contrast to Ac-225) it does not have a particularly complex decay chain, thus lowering the risk of freely circulating radioactive nuclides. Lead-212 undergoes beta-decay with a convenient 10.6 h halflife to produce 212Bi and 2O8T1, which then rapidly decays to 208Pb by a combination of alpha and beta emission. See, for example Targeted Alpha-emitter Therapy of Neuroendocrine Tumors using 212Pb-octreotate (AlphaMedix TM), Tworowska, Izabela et al., Journal of Medical Imaging and Radiation Sciences, Volume 50, Issue 1, S34; dos Santos, J.C., Schafer, M., Bauder-Wust, U. et al. Development and dosimetry of 203Pb / 212Pb-labelled PSMA ligands: bringing “the lead” into PSMA-targeted alpha therapy?. Eur J Nucl Med Mol Imaging 46, 1081-1091 (2019); Izabela Tworowska et al.. Abstract LB-259: Pb,03-AR-RMX conjugates for image-guided TAT of neuroendocrine tumors (NETs). Cancer Res 1 July 2017, 77 (13_Supplement): LB-259. Optionally, the target isotope is Radium-226, and the product isotope is Actinium-225. Additionally or alternatively, the method of the first aspect of the invention comprises subjecting a target composition comprising Radium-226 to neutron irradiation to produce Radium-225 isotopes by a (n, 2n) reaction. It has been found that the radiation spectrum described hereinabove is especially effective at producing Ra-225 (which decays to the sought-after isotope Ac-225) at high purity, notably with low production of Ra-227 (which delays to the unwanted contaminant isotope Ac-227). Specifically, the relatively low proportion of neutrons having energies of 4 MeV or lower reduces production of Ra-227. Furthermore, a low proportion of neutrons having energies outside the ranges 12-16 MeV and below 4 MeV helps to avoid other unwanted reactions of Ra-226. It will be appreciated that isotope separation can be complicated and costly, if possible at all, and so it is advantageous to provide a process that avoids production of unwanted isotopes. Optionally, the target isotope is Radium-226 and the product isotope is Actinium-225, and the method of the first aspect of the invention comprises subjecting a target composition comprising Radium-226 to neutron irradiation to produce Actinium-225 isotopes by a 226Ra(n, 2n)225Ra->225Ac reaction. It will be understood that, in such a method, Ra-225 is an intermediate isotope, with a half life of 14.9 days. Optionally, the target composition is an aqueous solution of Ra-226, for example the target composition comprises or consists of water and one or more salts comprising Ra-226. It will be understood that any water-soluble radium salt may optionally be used, particularly radium salts having a counterion having low or no activity or reactivity under the neutron irradiation conditions of the method. Optionally, the radium salt is present in the water in an amount of no more than 100 g / L (such as 0.1 g / L to 100 g / L), for example no more than 50 g / L (such as 1 g / L to 50 g / L). Additionally or alternatively, the concentration of Ra-226 Ra2+ ions in the aqueous solution is no more than about 0.2M (such as 0.00IM to 0.2M), for example as no more than about 0. IM (such as 0.01M to 0. IM). Optionally, the radium salt is present in the water in an amount of at least 50 g / L, for example at least 100 g / L. Additionally or alternatively, the concentration of Ra-226 Ra2+ ions in the aqueous solution is at least about 0. IM, such as at least about 0.2M. Optionally, the aqueous solution is a saturated solution of the one or more salts of Ra-226. Optionally, the salt is a Ra2+ salt comprising Ra-226, such as a halide, hydroxide, iodate or nitrate salt of Ra-226. Optionally, the salt is one or more of Rah, RaCh, RaBr2, Ra(OH)2, Ra(IOs)2 or Ra(NOs)2, optionally Rah, RaCh, RaBr2 or Ra(OH)2. Optionally, the volume of the target solution is from about 1 pl to 100 ml, such as about 10 pl to 50 ml. It will be understood that the target volume is the volume of solution irradiated by the neutron radiation. It has been found that a liquid target allows for particularly convenient extraction and handling of isotopes produced by the method. Optionally, the method is a continuous production method, for example wherein the method comprises removing a portion of the target composition, and optionally replenishing the target composition with additional Ra-226, for example without interrupting irradiation of the target composition and / or without removing the entire target composition. Additionally or alternatively, the method comprises selectively removing one or more isotopes of elements other than radium from the target composition, and / or adding additional Ra-226 to the target composition. Optionally, the step of selectively removing isotopes other than radium from the target composition comprises removing a portion of the target composition solution from the target holder, extracting one or more isotopes of elements other than radium from said portion of the target composition solution, and returning remaining target composition solution to the target holder. Optionally, said one or more isotopes of elements other than radium include one of more of actinium isotopes and lead isotopes, such as one or more of Pb-212 and Ac-225 (such as a mixture of Ac-225 and Ac-227). Optionally, the method comprises allowing at least some Ra-225 isotopes to decay to Ac-225 isotopes, and optionally extracting Ac-225 isotopes. Optionally, the method comprises producing Ra-224 isotopes by a (n, 3n) reaction, and optionally allowing at least some of the Ra-224 isotopes to decay to Pb-212 isotopes via Rn-220 and Po-216. Optionally, the method comprises separating Ac-225 isotopes from isotopes of one or more other elements (such as isotopes of radium and / or lead). Additionally or alternatively, the method optionally comprises extracting Pb-212 isotopes and separating Pb-212 isotopes from isotopes of one or more other elements (such as isotopes of radium and / or actinium). It will be appreciated that any suitable method of extraction (and optionally purification) of isotopes may be used. A method of separation of radium and actinium using metal oxide sorbents is disclosed in M. Alex Brown, “Metal Oxide Sorbents for the Separation of Radium and Actinium”, Ind. Eng. Chern. Res. 2020, 59, 46, 20472-20477, the contents of which are incorporated herein by reference. Optionally, the target isotope is Radium-226 and the product isotope is Lead-212, and the method of the first aspect of the invention comprises subjecting a target composition comprising Radium-226 to neutron irradiation to produce Lead-212 isotopes by a 226Ra(n, 3n)224Ra->212Pb reaction. It will be understood that, in such a method, Ra-224, Rn-220 and Po-216 are intermediate isotopes, having half lives of 3.63 days, 56 seconds and 0.15 seconds, respectively. Additionally or alternatively, optionally the target isotope is Radium-225, the product isotope is a first product isotope, wherein the first product isotope is Actinium 225, and the method of the first aspect of the invention additionally comprises subjecting the target composition comprising the target isotope to said neutron irradiation to produce a second product isotope by a nuclear transmutation reaction, wherein the second product isotope is Lead 212. Thus, optionally, the target isotope is Radium-226, the first product isotope is Actinium-225 and the second product isotope is Lead-212, and the method of the first aspect of the invention comprises subjecting a target composition comprising Radium-226 to neutron irradiation to produce Actinium-225 isotopes by a 226Ra(n, 2n)225Ra->225Ac reaction and Lead-212 isotopes by a 226Ra(n, 3n)224Ra->212Pb reaction. Optionally, the isotope product of the second aspect of the invention comprises Ac-225 and / or Pb-212, for example when the isotope target comprises Radium-226. It will be appreciated that the unique method of the first aspect of the invention may provide an isotope product having a unique composition, for example an isotope product comprising a unique combination of Ac-225 and Ac-227 when the isotope target comprises Radium-226. In particular, the present invention may provide an isotope product comprising a mixture of Ac-225 and Ac-227, wherein the mixture comprises a particularly low proportion of Ac-227 as compared to Ac-225. Optionally, the isotope composition comprises Ac-225 and Ac-227, wherein the Ac-227 is present in an amount of no more than about 0.08%, such as no more than about 0.05%, for example no more than about 0.01%, based on the amount of Ac-225, wherein % is atomic percent. It will be understood that the isotope product may be in any form, for example a solution, and that actinium may be present for example as a salt. Optionally, the apparatus of the third aspect of the invention comprises such a liquid target composition held in the target holder. Dysprosium-161 / Terbium-161 Optionally, the target isotope is Dysprosium-161, and / or the product isotope is Terbium-161. Additionally or alternatively, the method comprises subjecting a target composition comprising Dysprosium-161 to neutron irradiation to produce Terbium-161 isotopes by a (n, p) reaction. Preferably, no intermediate isotope is formed in said method. It has been found that use of the neutron radiation spectrum described hereinabove provides a particularly efficient and effective route to production of Terbium-161. In particular, while current methods for production of Terbium-161 utilise Gadolinium-161 as the starting isotope, the method of the present invention allows use of cheaper and more widely available Dysprosium-161 as the starting isotope. Optionally, when the target isotope is Dysprosium-161, the target composition is provided as a solid, as described herein. Optionally, the target composition comprises isotopically enriched elemental dysprosium and / or one or more isotopically enriched dysprosium oxides or salts (such as 161Dy2O3). It will be understood that, in this example, isotopic enrichment means enriched with Dysprosium-161. It is believed that Terbium-161 offers promise as a therapeutic agent, with a half life (1½) of 6.91 days, 100% P- decay, and mean P- energy of 154 keV, giving it similar decay characteristics to Lutetium-177. Terbium predominantly exists in the trivalent state and the tetravalent state under certain conditions. Similar coordination chemistry exists for Lutetium-177, and therefore Terbium-161 is likely compatible with the same carrier molecules, such as DOTA, which can be used with a combination of different small molecules, peptides and antibodies currently employed with Lutetium-177. In addition, it co-emits a substantially larger number of conversion and Auger electrons at a favourable energy range (~12 e",~36 keV per decay for Terbium-161, and ~le , ~1.0 keV per decay for Lutetium-177, respectively). Terbium-161 therefore offers a useful alternative to Lutetium-177. See, for example, Gracheva, N., Muller, C., Talip, Z. et al. Production and characterization of no-carrier-added 161Tb as an alternative to the clinically-applied 177Lu for radionuclide therapy. EJNMMI radiopharm. chem. 4, 12 (2019). Mercury-202 or Mercury-199 / Gold-199 It is believed that Gold-199 offers promise as a therapeutic agent, with a half life (t’A) of 3.14 days, 100% P- decay, and mean P- energy of 82 keV. Optionally, the target isotope is Mercury-202 or Mercury-199, and / or the product isotope is Gold-199. Additionally or alternatively, the method comprises subjecting a target composition comprising Mercury-202 to neutron irradiation to produce Gold-199 isotopes by a (n, alpha) reaction. Additionally or alternatively, the method comprises subjecting a target composition comprising Mercury-199 to neutron irradiation to produce Gold-199 isotopes by a (n, p) reaction. Preferably, no intermediate isotope is formed in said method(s). It has been found that use of the neutron radiation spectrum described hereinabove provides a particularly efficient and effective route to production of Gold-199. In particular, while current methods for production of Gold-199 utilise more expensive and less readily available starting isotopes then Mercury-202 or Mercury-199. Optionally, when the target isotope is Mercury-202 or Mercury-199, the target composition is provided as a solid, as described herein. Optionally, the target composition comprises isotopically enriched elemental mercury and / or one or more isotopically enriched mercury oxides or salts. It will be understood that, in this example, isotopic enrichment means enriched with Mercury-202 or Mercury-199. Titanium-47 or Calcium-48 / Scandium-47 It is believed that Scandium-47 offers promise as an imaging and / or therapeutic agent, with a half life (1½) of 3.35 days, 100% P- decay with mean P-energy of 162.0 keV, and y decay of 68.3%. Optionally, the target isotope is Titanium-47 or Calcium-48, and / or the product isotope is Scandium-47. Additionally or alternatively, the method comprises subjecting a target composition comprising Titanium-47 to neutron irradiation to produce Scandium-47 isotopes by a (n, p) reaction. Preferably, no intermediate isotope is formed in said method. Additionally or alternatively, the method comprises subjecting a target composition comprising Calcium-48 to neutron irradiation to produce Scandium-47 isotopes by a 48Ca(n, 2n)47Ca->47Sc reaction. It will be understood that such a method forms Calcium-47 as an intermediate isotope, which spontaneously decays to Scandium-47. Calcium-47 has a half life of 4.54 days. It has been found that the methods described hereinabove provides a particularly efficient and effective route to production of Scandium-47. In particular, while current methods for production of Scandium-47 rely on the use of expensive and limited availability high energy cyclotrons, the method of the present invention is able to utilise small, efficient, lower cost nuclear fusion reactors as described herein. Optionally, when the target isotope is Titanium-47 or Calcium-48, the target composition is provided as a solid, as described herein. Optionally, the target composition comprises isotopically enriched elemental titanium or calcium and / or one or more isotopically enriched titanium oxides or salts or calcium oxides or salts. It will be understood that, in this example, isotopic enrichment means enriched with Titanium-47 or Calcium-48. Scandium has an ionic radius of 74.5 pm and is chemically similar to other heavy lanthanides, and so it is understood that ligands developed for these cations should also be suitable for chelating scandium. Scandium is a harder metal and therefore smaller, and it prefers a coordination number of 6-8. Macrocyclic ligands, such as DOTA and open-chain ligands, such as DPTA and EGTA are believed to form stable complexes with scandium. The emission properties of 47Sc are comparable to 177Lu, 47Sc also emits characteristic gamma-rays with an ideal energy for SPECT imaging. See, for example, Mikolajczak, R., Huclier-Markai, S., Alliot, C. etal. Production of scandium radionuclides for theranostic applications: towards standardization of quality requirements. EJNMMI radiopharm. chem. 6, 19 (2021); Chernysheva M, Loveless SC, Brossard T, Becker K, Cingoranelli S, Aluicio-Sarduy E, Song J, Ellison P, Nolen J, Rotsch DA, Lapi SE, Engle JW. Accelerator Production of Scandium Radioisotopes: Sc-43, Sc-44, and Sc-47. Curr Radi opharm. 2021; 14(4):359-373. Zinc-64 / Copper-64 It is believed that Copper-64 offers promise as an imaging and / or therapeutic agent, with a half life 0:½) of 12.70 hours, 38.5% P' decay with mean P' energy of 191 keV, 17.5% P+ decay with mean P+ energy of 278 keV, and y decay of 0.475%. Optionally, the target isotope is Zinc-64, and / or the product isotope is Copper-64. Additionally or alternatively, the method comprises subjecting a target composition comprising Zinc-64 to neutron irradiation to produce Copper-64 isotopes by a (n, p) reaction. Preferably, no intermediate isotope is formed in said method. It has been found that use of the neutron radiation spectrum described hereinabove provides a particularly efficient and effective route to production of Copper-64. In particular, while current methods for production of Copper-64 utilise Nickel-64 as the starting isotope, the method of the present invention allows use of cheaper and more widely available Zinc-64 as the starting isotope. Optionally, when the target isotope is Zinc-64, the target composition is provided as a solid, as described herein. Optionally, the target composition comprises isotopically enriched elemental zinc and / or one or more isotopically enriched zinc oxides or salts. It will be understood that, in this example, isotopic enrichment means enriched with Zinc-64. It will be appreciated that the biochemistry of copper is well known, it has no long-term residence in the body and neither it nor its daughter (stable 67Zn) is acutely toxic. The coordination chemistry of copper has been extensively studied, allowing for convenient identification of suitable ligands for radiopharmaceuticals. 64Cu is a particularly versatile radioisotope, owing to its unique decay scheme, which combines electron capture (41%), P' (40%) and P+ (19%) and results in Meitner-Auger electron emission. Its half-life of 12.7 hours is long enough for radiolabelling of therapeutic targets and is compatible with the in vivo kinetics of a variety of molecular carriers. Thus, 64Cu is believed to offer promise for PET imaging applications and for in vivo targeted radiation therapy applications. See, for example, Giulia Anna Follacchio, Maria Silvia De Feo, Giuseppe De Vincentis, Francesco Monteleone, Mauro Liberatore, Radiopharmaceuticals Labelled with Copper Radionuclides: Clinical Results in Human Beings, Current Radiopharmaceuticals; Volume 11, Issue 1, Year 2018. Zinc-67 / Copper-67 It is believed that Copper-67 offers promise as an imaging and / or therapeutic agent, with a half life (1½) of 61.83 hours, 100% P' decay with mean P' energy of 141 keV, and y decay of 72.93%. However, 67Cu is currently not widely used in nuclear medicine as it is difficult to produce clinical quantities by current methods. In particular, if natural Zn is used as the target, it is believed that an unacceptable level of 64Cu is produced due to the 64Zn(n, p)64Cu reaction, which has a cross-section of 31 mb. Coupled with the 10 fold increase in abundance of 64Zn over 67Zn, this leads to a large activity to decay away before final separation and distribution. Optionally, the target isotope is Zinc-67, and / or the product isotope is Copper-67. Additionally or alternatively, the method comprises subjecting a target composition comprising Zinc-67 to neutron irradiation to produce Copper-67 isotopes by a (n, p) reaction. Preferably, no intermediate isotope is formed in said method. It has been found that use of the neutron radiation spectrum described hereinabove provides a particularly efficient and effective route to production of Copper-67. In particular, while current methods for production of Copper-67 are inefficient and thus severely limit availability of Copper-67, the method of the present invention offers a low-cost and convenient route from relatively cheap and widely available Zinc-67 as the starting isotope. Optionally, when the target isotope is Zinc-67, the target composition is provided as a solid, as described herein. Optionally, the target composition comprises isotopically enriched elemental zinc and / or one or more isotopically enriched zinc oxides or salts. It will be understood that, in this example, isotopic enrichment means enriched with Zinc-67. As noted above, the biochemistry of copper is well known, it has no long-term residence in the body and neither it nor its daughter (stable 67Zn) is acutely toxic. The coordination chemistry of copper has been extensively studied, allowing for convenient identification of suitable ligands for radiopharmaceuticals. 67Cu is the longest-lived radioisotope of copper, a beta emitter and a strong gamma emitter, with a 2.6-day half-life, making it useful for both therapeutic and diagnostic applications. The range of the 67Cu beta particle in tissue is comparable to the diameter of a cell and its half-life is long enough for a modest dose to show high targeted uptake without appreciable decay losses. See, for example, Nicholas A. Smith, Delbert L. Bowers, David A. Ehst, The production, separation, and use of 67Cu for radioimmunotherapy: A review, Applied Radiation and Isotopes, Volume 70, Issue 10, 2012, Pages 23772383. Krypton-78 / Bromine-77 It is believed that Bromine-77 offers promise as an imaging and / or therapeutic agent, with a half life 0:½) of 2.4 days, 0.73% P+ decay with mean P+ energy of 151 keV, and y decay of 76.59%. Optionally, the target isotope is Krypton-78, and / or the product isotope is Bromine-77. Additionally or alternatively, the method comprises subjecting a target composition comprising Krypton-78 to neutron irradiation to produce Bromine-77 isotopes by a 78Kr(n, 2n)77Kr -> 77Br reaction. It will be understood that such a method forms Krypton-77 as an intermediate isotope, which spontaneously decays to Bromine-77. Krypton-77 has a half life of 73 minutes. It has been found that the method described hereinabove provides a particularly efficient and effective route to production of Bromine-77. In particular, while current methods for production of Bromine-77 rely on the use of inefficient cyclotron-based methods thereby limiting availability of Bromine-77, the method of the present invention is able to utilise small, efficient, lower cost nuclear fusion reactors as described herein. Optionally, when the target isotope is Krypton-78, the target composition is provided as a solid, as described herein. Optionally, the target composition comprises isotopically enriched elemental krypton and / or one or more isotopically enriched krypton oxides or salts. It will be understood that, in this example, isotopic enrichment means enriched with Krypton-78. Various nuclear medicine applications have been proposed for 77Br, including SPECT imaging and Auger electron therapy, and [76Br]MBBG, i.e. the bromine analogue of MIBG, has been evaluated as a radiobromine compound. See, for example Breunig, Katharina, Spahn, Ingo, Spellerberg, Stefan and Coenen, Heinz H.. "Production of no-carrier-added radiobromine: new nickel selenide target and optimized separation by dry distillation" Radiochimica Acta, vol. 103, no. 5, 2015, pp. 397-402. See also (2019), 23rd International Symposium on Radiopharmaceutical Sciences. J Label Compd Radiopharm, 62: S70 (0-47: Radiosynthesis of a novel 77Br-labeled PARP-1 inhibitor through Cu-mediated aryl boronic ester bromination, Ellison et al.; 0-48: Large-scale production and isolation of theranostic radionuclides 76Br and 77Br, Ellison et al.). Zirconium-90 or Niobium-93 / Yttrium-90 It is believed that Yttrium-90 offers promise as a therapeutic agent, with a half life (1½) of 2.7 days, 100% P- decay, and mean P- energy of 933.6 keV. Optionally, the target isotope is Zirconium-90 or Niobium-93, and / or the product isotope is Yttrium-90. Additionally or alternatively, the method comprises subjecting a target composition comprising Zirconium-90 to neutron irradiation to produce Yttrium-90 isotopes by a (n, p) reaction. Preferably, no intermediate isotope is formed in said method. Additionally or alternatively, the method comprises subjecting a target composition comprising Niobium-93 to neutron irradiation to produce Yttrium-90 isotopes by a (n, alpha) reaction. It has been found that the method described hereinabove provides a particularly efficient and effective route to production of Yttrium-90. In particular, while current methods for production of Yttrium-90 utilise Yttrium-89 thereby providing a carried added product with poor specific activity, the method of the present invention avoids Yttrium-89 production. Optionally, when the target isotope is Zirconium-90 or Niobium-93, the target composition is provided as a solid, as described herein. Optionally, the target composition comprises isotopically enriched elemental zirconium or niobium and / or one or more isotopically enriched zirconium oxides or salts or niobium oxides or salts. It will be understood that, in this example, isotopic enrichment means enriched with Zirconium-90 or Niobium-93. 90Y is used for selective internal radiation therapy (SIRT) and 90Y-labelled microspheres are used for radioembolisation therapy for liver cancer. 90Y-based materials are used clinically to treat a range of diseases including Cushing’s disease, acromegaly, haemophilia and a wide range of cancers. See for example WO2017004088A1; Tickner, B. J., Stasiuk, G. J., Duckett, S. B., &Angelovski, G. (2020). The use of yttrium in medical imaging and therapy: Historical background and future perspectives. In Chemical Society Reviews (Vol. 49, Issue 17, pp. 6169-6185). Royal Society of Chemistry; Vinjamuri, S., Gilbert, T. M., Banks, M., McKane, G., Maltby, P., Poston, G., Weissman, H., Palmer, D. H., Vora, J., Pritchard, D. M., &Cuthbertson, D. J. (2013); Peptide receptor radionuclide therapy with 90 Y-DOTATATE / 90 Y-DOTATOC in patients with progressive metastatic neuroendocrine tumours: Assessment of response, survival and toxicity. British Journal of Cancer, 108(1), 1440-1448; K.V. Vimalnath, Sudipta Chakraborty, A. Rajeswari, H.D. Sarma, Jitendra Nuwad, Usha Pandey, K. Kamaleshwaran, Ajit Shinto, Ashutosh Dash, Radiochemistry, pre-clinical studies and first clinical investigation of 90Y-labeled hydroxyapatite (HA) particles prepared utilizing 90Y produced by (n, y) route, Nuclear Medicine and Biology, Volume 42, Issue 5, 2015, Pages 455-464. It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a side cut-through schematic view of a nuclear fusion reactor in accordance with an embodiment of the invention; Figure 2 shows an end cut-through of the nuclear fusion reactor of Figure 1; Figure 3 shows a side cut-through schematic view of a nuclear fusion reactor in accordance with another embodiment of the invention; Figure 4 shows an end cut-through of the nuclear fusion reactor of Figure 3; Figure 5 shows a side cut-through schematic view of a nuclear fusion reactor in accordance with another embodiment of the invention; Figure 6 shows an end cut-through of the nuclear fusion reactor of Figure 5; Figure 7 shows neutron energy spectra of neutron radiation generated from various sources; Figure 8 shows a model plot of Ac-227 / Ac-225 isotope distribution for neutron radiation according to the spectra of Figure 7 against target composition thickness; and, Figure 9 shows an enlarged section of the model plot of Figure 8, showing results for two of the spectra of Figure 7. Detailed Description In man-made fusion, forces are utilised to impart adequate kinetic energy to fusible isotope species (FIS). As used herein, FIS are the isotopes of hydrogen (hydrogen, deuterium and tritium). Forces take the form of magnetic plasma confinement, particle beam accelerators, pulsed laser heating and any combination of electromagnetic confinement fields used to focus ions into a collision zone. The first ever fusion-based technology successfully commercialised was an Inertial-Electrostatic Confinement (TEC) neutron generator. In a typical IEC, the reactor vessel contains a meshed cathode in the centre and its inner wall acts as an anode. The chamber is filled with FIS gas and a high voltage is applied to the cathode, creating a strong electric field within the chamber which accelerates the ions, sparking a plasma at the centre of the cathode where fusion occurs. IEC neutron generators have demonstrated reliable capability to produce neutron fluxes up to 5 x 109 neutrons per second for deuterium-tritium fusion for tens of thousands of hours continuously with little to no maintenance. Various multi-particle interactions can be used to generate nucleons. Fusion reactions of deuterium and tritium for generation of neutrons (and the generated particle kinetic energies) include: 1. Df + Df He^(0-82 MeV) + nj(2.45 MeV) 2. Df + Df 7^(1.01 MeV) + p}(3.02 MeV) 3. Df+ T? He^(3.5 MeV) + nJ (14.1 MeV) wherein is deuterium, Tf is tritium, He is helium and nJ is a neutron. The reactions in equations 1 and 3 above demonstrate that deuteriumdeuterium fusion generates relatively low energy neutrons, which when used for example in Ra-226 irradiation tend to result in production of unwanted Ac-227, whereas tritium-tritium fusion generates higher energy neutrons suitable for forming sought-after Ac-225 from Ra-226. Where fusion can be produced in a controlled environment, such as a particle generator or fusion reactor, reactions of higher energy and lower mass can be collimated and attenuated to be made use of in many different ways. In particular, the neutrons of higher energy in equation 3 are unique in that they are otherwise difficult or impossible to produce by means other than fusion. A technology that has been used for neutron generation is an advanced tube neutron generator, or compact linear accelerator. Typically, this takes the form of a deuterium ion source which is linearly accelerated into a concentrated beam by an electric field into a solid or gaseous tritium target to induce fusion reactions and neutron production. Such systems have been commercialised, but their use is limited due to large upfront and running costs. This is mainly due to the infrastructure needed to run the accelerator, unintentional activation of materials from stray energetic deuterium fluence as well as erosion or burnup of the tritium target requiring regular replacement. This also implies the system is not continuously operable for long periods. Yet further, it has been found that use of such accelerators to generate neutrons for irradiation of, e.g., Ra-226 is not only costly, but also leads to significant problems in unwanted Ac-227 production. The theorisation and measurement of Lattice Confinement Fusion (LCF) has demonstrated fusion reactions taking place in a solid metal. It has been found that LCF may be promoted by electron screening in the solid metal, according to which the negatively charged electron cloud present in a conductive metal neutralises the positive charge of fusible ions located in the metal lattice. Thus, through a combination of metal lattice enrichment by FIS and metal electron screening, LCF may be promoted. More particularly, incident accelerated ions are not repulsed by the electrostatic force usually seen between two positively charged particles, allowing for fusion reactions to take place with less required energy and at enhanced rates. "Strong Screening by Lattice Confinement and Resultant Fusion Reaction Rates" by Prados-Estevez, F., Subashiev, A and Nee, H. discussed how the top 10 valence electrons in a given metal can nullify the coulomb potential between fusible isotope species thereby increasing the fusion reaction cross section and allowing for fusion at high rates in solid metals. A saturation effect is also anticipated and seen to occur. LCF effects change little between FIS or their associated reactions but vary for different host metals due to their innate screening potential. Optionally, the enriched surface comprises a metal having an electron screen potential of at least 200 eV, for example the metal of the surface subjected to enrichment has an electron screen potential of at least 200 eV. Optionally, the neutron source used in accordance with embodiments of the present disclosure is a particle producing apparatus of the inertial electrostatic confinement type, comprising an anode structure and a cathode structure provided in a vessel, wherein at least part of a surface of the anode structure and / or the cathode structure is an enriched surface. It will be understood that an enriched surface is a material (such as a conductive material) enriched with fusible isotope species (i.e. deuterium and / or tritium. It has been found that an enriched surface may prompt lattice confinement fusion. Optionally, the apparatus is configured to contain an ion and neutral gas mixture and, in operation, to cause the ion and neutral gas mixture to form a plasma. Optionally, the vessel comprises a central axis, and / or the anode structure and the cathode structure are optionally positioned to be substantially coaxial with the vessel. Optionally, the anode structure has a mean distance from the central axis that is larger than the mean distance from the central axis of the cathode structure. Optionally, the anode and cathode structures are substantially concentric along at least a part of their lengths and configured such that, in operation, an electric field is provided between the anode and cathode structures. Optionally, the enriched surface comprises a material having an electron screening potential for example an electron screening potential sufficient to encourage lattice confinement fusion. Optionally, the vessel has a substantially constant cross-section coaxially along the length of the cathode structure. It will be understood that the anode structure and / or the cathode structure may each independently be formed of a plurality of units. It has been found that an axial cylindrical IELC system may provide enhanced fusion rates. Optionally, the enriched surface comprises a basic or transition metal, such as an element with an atomic number greater than 40. Suitable metals include titanium, zirconium, palladium and / or erbium. Additionally or alternatively, the enriched surface optionally comprises a semiconductor material, such as a CVD diamond. The enriched surface may be provided in any suitable form, for example as a coating on or integrally with the anode structure and / or cathode structure. In some embodiments, a first enriched surface may form at least a part of the surface of the anode structure, and a second enriched surface may form at least a part of the surface of the cathode structure. Electrolysis represents quick and efficient way to load deuterium and / or tritium into a metal surface lattice at atomic percentage levels, avoiding the need for use of an ion beam or complex metallurgy. Electrolysis represents an available manufacture route for a FIS enriched cathode or FIS enriched anode suitable for use in the apparatus of the present disclosure. In embodiments of the present disclosure, the potential is positive at the anode and negative at the cathode so once neutral gases are ionized, they become positive ions which are repulsed from the anode surface and attracted to the cathode surface, and are therefore accelerated towards the plasma. Through a combination of thermionic and photo-electric effects, electron emission may occur at the cathode surface. The electrons emitted from the cathode are repulsed from the cathode and accelerated towards the anode due to their negative charge. These high energy electrons cause secondary electrons to ionize the neutral gas at the anode wall, which then accelerates towards the cathode due to its positive charge and contribute to the plasma. Ions born at the anode surface may contribute more to fusion due to being accelerated a larger distance by the electric field into the potential well inside the cathode. Through the use of materials enriched with FIS for the anode or cathode, not only may there be increase in lattice confinement fusion in the materials compared to conventional lEC's but at adequate temperature the FIS may diffuse into the reactor chamber, become ionised and contribute significantly to the neutron production rate of the system. It will be understood that control of the temperature of the anode through controlled cooling may allow for a controlled release of FIS into the chamber to maintain an increased particle production rate. This can be managed by an automated system which manages the power, particle production rate and temperature to provide a stable output. As is well established, all systems may saturate at a stable state where particle production is constant. In the present disclosure, saturation occurs but for an additional reason where at a given temperature there is an equilibrium between the steady release of ions due to degradation of the hydride surface layer on the enriched surface and assimilation of hydrogen isotopes into the lattice bulk. This process can be beneficial or detrimental to the particle production rate dependent on the material used for the anode and cathode. Embodiments of the present disclosure may include apparatus for producing nuclear fusion reactions by utilization of characteristics of an inertial electrostaticlattice confinement (IELC) device with ions initially produced by glow-discharge breakdown of a reactant gas plus ion-impact and electron-impact processes in a plasma gas mix and also a favourable production of secondary electrons of low energy which are well suited for further ion production after impact of high energy electrons and ions on structures located at or near an anode wall as well as production of FIS from the anode wall. Two complementary phenomena act to increase the neutron production rate: 1. Generation of secondary electrons from the cathode surface, increased by choice of material. These electrons gain significant energy from the electric field and are accelerated into the anode wall. The cathode material can also be enriched with fusible species to encourage lattice confinement fusion events and increase plasma density. 2. Enrichment of the inner surface of the anode with fusible species within an appropriate metal such as titanium, zirconium, palladium, erbium or semiconductor material e.g., CVD diamond, inducing lattice confined fusion events and producing secondary electrons which allow for anode-born ions to ionise and contribute to the particle production rate within the central cathode region. Figure 1 shows a side cut-through schematic view of a nuclear fusion reactor apparatus 100 comprising a cylindrical outer vessel wall 101, high voltage stand-off component 102, cathode assembly 103, and localised fusible isotope species enriched inner anode surface 104. Specifically, Figure 1 illustrates an IELC cut-through showing the flanged cylindrical cathode 103, encompassing anode surface 104 and appended ceramic insulators 105 at either end including voltage feedthroughs 106. The anode 104 is formed on the inner surface of the reactor vessel wall 101, and is enriched with deuterium (optionally and / or tritium), thus incorporating atomic % level deuterium (optionally and / or tritium) in the anode lattice substrate. Additionally or alternatively, the surface of the cathode 103 may be enriched with deuterium and / or tritium. Cathode and anode materials may be chosen for secondary electron emission properties as well as the ability to retain high levels of fusible ion species to high temperatures whilst remaining stable. Optionally, at least one of the anode and the cathode is enriched with deuterium, and the interior 107 of the reactor vessel 101 is filled with tritium. Figure 1 represents a simplified neutron generator configuration where the deuterium or tritium gas species are released and stored in a getter material within the sealed vessel (not shown in Figure 1). The nuclear fusion reactor apparatus 100 additionally comprises a target holder 110 in the form of a tube wrapped around the cylindrical outer vessel wall 101. The target holder tube 101 is configured to hold an aqueous solution of a target isotope, such as a Ra-226 salt, and has an internal diameter of about 10 mm. The tube is arranged as a continuous spiral coil having multiple turns around the outer vessel wall 101, thus presenting a large surface area of target material to radiation emitted from the reactor when filled with the target composition. Figure 2 shows an end cut-through of the nuclear fusion reactor 100 of Figure 1, along line A-A. Features shown in Figure 2 are labelled with the same reference numerals as used in Figure 1. Figure 3 shows a side cut-through schematic view of another nuclear fusion reactor apparatus 200. Features of the nuclear fusion reactor apparatus 200 that are the same as the reactor apparatus 100 of Figure 1 are labelled with the same reference numerals. The nuclear fusion reactor apparatus 200 comprises a target holder 210 in the form of a shell around the cylindrical outer vessel wall 101. The target holder shell is configured to hold an aqueous solution of a target isotope, such as a Ra-226 salt, and has an internal diameter of about 10 mm. The shell extends around the outside of the vessel wall 101 along a length of the reactor corresponding to the length of the cathode 103, thus presenting a large surface area of target material to radiation emitted from the reactor when filled with the target composition. Figure 4 shows an end cut-through of the nuclear fusion reactor 200 of Figure 3, along line B-B. Features shown in Figure 4 are labelled with the same reference numerals as used in Figure 3. Figure 5 shows a side cut-through schematic view of another nuclear fusion reactor apparatus 300. Features of the nuclear fusion reactor apparatus 300 that are the same as the reactor apparatus 100 of Figure 1 are labelled with the same reference numerals. The nuclear fusion reactor apparatus 300 comprises a target holder 310 adjacent the outer vessel wall 101. The target holder is configured to hold a solid target material comprising a target isotope. The target holder 310 extends adjacent the outside of the vessel wall 101 along a length of the reactor corresponding to the length of the cathode 103, thus presenting a large surface area of target material to radiation emitted from the reactor when holding the target composition. Figure 6 shows an end cut-through of the nuclear fusion reactor 300 of Figure 5, along line B-B. Features shown in Figure 6 are labelled with the same reference numerals as used in Figure 5. Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. For example, while reactor vessels have been shown having a cylindrical geometry, other geometries may be envisaged, such as elongate polygonal shapes (e.g. an elongate vessel having an octagonally shaped cross-section). Additionally or alternatively, anodes may be provided on a structure spaced apart from the vessel wall. Additionally or alternatively, a getter pump assembly may be located external to the reaction vessel, for example provided in a manifold assembly. Suitable reactor arrangements and structures, and reactor manufacturing techniques, are disclosed in WO 2022 / 263827 Al (Astral Neutronics Ltd), the contents of which are incorporated herein by reference. Embodiments of the present disclosure may provide apparatus for producing nuclear fusion reactions by utilization of characteristics of a so-called star mode of operation. Star mode of operation refers to a mode of operation in which a plasma is produced inside the apparatus. As a result, embodiments are envisaged wherein the apparatus is configured to contain an ion and neutral gas mixture and, in operation, to cause the ion and neutral gas mixture to form a plasma. Additionally, embodiments of the present disclosure may provide apparatus for producing nuclear fusion reactions by utilization of an ion capture phenomenon called "zone of acceptance", whereby a spatial region centred on each star beam and having a curved funnel-like shape with its broadest end at an anode wall defines a region where ions born with relatively low kinetic energy may be drawn into a local star beam from the gas plasma, anode and / or cathode surfaces. A cathode grid shape may be adapted so that formation and shape characteristics of star mode beams and a zone of acceptance are controlled to maximize or at least improve the above-described production and utilization of ions for fusion collisions. The grid may be constructed out of panels and may be shaped such that the panels run lengthways to form a hollow cylinder or cylindrical skeletal frame. The cathode grid may comprise flanges, and the flanges may be made from said panels. These flanges may distribute the electric field between the cathode grid and the anode such that the field is concentrated near the flanges to produce beams or channels. For example, these channels may form in between neighbouring, adjacent or proximate flanges. These beams or channels may result in improved acceleration of generated ions towards the plasma, which may be located in the centre of the electric field and / or where the beams or channels intersect. These electric field channels may be considered to be the zone of acceptance. Thus, embodiments of the present disclosure are provided wherein produced nucleons can escape from a sealed apparatus in all directions from a zone of origin that is elongated and able to replace point sources like point source neutron generator apparatus. Thus, embodiments of the present disclosure may provide apparatus for producing nuclear fusion reactions in a volume or zone as defined above which extends from a centreline to an anode and includes an internal cathode and a space external to it for a radial distance of approximately one half of a radius of the cathode, as well as to the inner surface wall of the anode where it has been suitably enriched with fusible isotope species. In addition, embodiments of the present disclosure may provide apparatus arranged to produce nuclear fusion reactions in an elongated zone or multiple zone segments in the case of a curvilinear geometry within a reactor vessel. In other words, embodiments of the present disclosure may provide apparatus for producing nuclear fusion reactions in a volume centred on a centreline axis or line of cylindrical symmetry of a reactor vessel. Unlike in a typical linear accelerator, neutrons produced by embodiments of the present disclosure may be multi-directional. Embodiments of the present disclosure seek to replace a multi-millimetre diameter intense mono directional beam of accelerated energetic neutrons made by a particle accelerator apparatus which impinge on a target causing relatively rapid damage and limit the surface area of the target that can be irradiated. In comparison to a linear accelerator, the present disclosure may spread out neutron production over larger target composition surface area. Hence, embodiments of the present disclosure may provide apparatus capable of maintaining nuclear fusion reactions for a duration of thousands of hours to several years with little or no maintenance to a reactor chamber containing a FIS enriched anode and / or cathode. Similarly, embodiments of the present disclosure may include apparatus for producing nuclear fusion reactions with little or no maintenance to a central electrode or an associated high voltage power input structure, and / or little or no maintenance to an internally mounted reactor chamber gas storage and pressure regulation device. The materials chosen for the cathode and anode may be selected to encourage secondary electron emission, resistance to thermal damage and lattice confinement fusion. Furthermore, as described above the apparatus may be adapted such that ions bom or generated within a zone of acceptance between the anode wall and the perimeter of the cathode grid may be drawn into the star mode beam around which the zone of acceptance may be substantially centred and which has cathode hole window side segment curvatures which may be suited to a shape of planes of equipotential in the electrostatic field to increase a size of the zone of acceptance and thereby capture most or substantially all ions produced by interactions of neutrals with secondary electrons near the anode wall. Optionally, the nuclear fusion reactor is arranged for and or operated in pulsed power mode. It has been found that there may be a neutron production rate benefit of operating in a pulsed power mode. This is likely due to a combination of lattice confinement fusion, the accompanying electron screening effect and secondary electron emission. The periodic relaxing of the voltage may allow for a higher concentration of valence electrons to briefly return to the surface which increases secondary electron yield and a stronger electron screening effect to increase lattice confinement fusion. Therefore, to maximise the gain from operation in pulsed mode, the frequency of pulses should match the relaxation time taken for electrons to repopulate the enriched surfaces after extensive ionisation. Hence, embodiments of the present disclosure may provide apparatus that may utilise pulsed power input whereby the electrical current is in the order of several to tens of amperes during the pulse thereby exploiting an observed fusion rate enhancement characteristic of super linear proportionality with the applied current. In embodiments employing a getter pump, such as a non-evaporative getter pump, the pump may be supported by a power supply, heating element and a temperature measurement circuit. The heater may be configured to raise the getter material to a temperature in the range of 400°C to 600°C. The heater may be controlled so that the getter material remains at a steady temperature. In use, the reactor vessel may be sealed and evacuated after it has been correctly baked out to eliminate residual volatile substances such as water. A conditioned getter of the appropriate material may release hydrogen isotopes (e.g. tritium) so that a partial pressure may rise to the level of 5xl0'3 mbar to 5xl0-1 mbar when it is in the abovementioned temperature range. At a particular steady temperature, the partial pressure may also be steady. The getter pump at constant temperature may serve as a pressure source and a pressure regulator of high precision. Very minor pressure fluctuations can cause significant departures of the star mode glow discharge voltage. The regulation of pressure can be fine enough with open bleed valve and turbo molecular vacuum pump configurations, but the getter pump may provide a superior means of pressurization of the sealed configuration IELC device. The capacity of the getter pump to store the reactant gas (e.g. tritium) may be a factor in determining the maximum number of operation hours of a sealed reactor chamber. A practical configuration may allow ten years of continuous consumption of deuterium at the rate of up to IxlO14 fusions per second. During such a period, the output of the sealed reactor can be expected to change very slowly as the mixture ratio of reactants changes. The fusion rate in a reactor according to the disclosure is likely to be IxlO10 to IxlO14 per second, depending on the fusible species enrichment level. It is feasible to perform maintenance on a sealed chamber by opening the fill and vent port (not shown), extracting the gas by heating the getter pump and baking the chamber to induce outgassing of the embedded volatile species in the inner wall surfaces of the reactor. The handling of Tritium may be subject to safety regulations. Preferably, the apparatus may be adapted to generate neutrons in a "macro" linear or curvilinear geometry, where the expression "macro" is used to distinguish between a relatively small "micro" sized neutron source geometry such as a single pellet of radioactive isotope and a "mega" sized neutron source such as a fission reactor core or a star. In other words, "macro" implies a size or scale that is useful for industrial applications. This may range from approximately 10 to 150 cm line source length. Examples of possible embodiment characteristic dimensions: I. Inside diameter of the anode and vessel wall 8 cm II. Diameter of the cathode grid electrode 3 cm III. Length of the cathode grid electrode 80 cm IV. Length of the proton line source 80 cm V. Overall length of the reactor chamber and power supply assembly 180cm It has been found that the relatively small footprint of the nuclear fusion reactor apparatus of the present disclosure allows facilities to be constructed quickly at locations adjacent to the site of isotope use, thereby minimising transport time and thus loss of valuable isotopes through radioactive decay. In order to control purity of isotope production, it may be advantageous to precisely define neutron flux. The quality of the neutron flux from accelerator sources or sealed tube neutron generators is deemed to not to be ideal. Accelerator spallation neutron sources may generate a range of neutron energies, making them more challenging to moderate and thermalise than for mono-energetic sources. Sealed tube devices do provide mono energy neutrons but suffer from poor reliability of the neutron output as do the accelerator spallation neutron sources. The solid targets that these devices use suffer from altered characteristics due to the damage they incur through use. The combined electrostatic-lattice confinement fusion of fusible ions from a neutral gas and ion mix plasma does not suffer from target degradation as damage is distributed across the anode and cathode surfaces, where diffusive and infusive processes reach an equilibrium during operation. The combined Inertial Electrostatic-Lattice Confinement (IELC) fusion device represents a very versatile particle generator, where the chosen isotopic fusible species produces mono-energetic particle spectra of energies seen through equations 1 and 3 above. In tandem with the capability of continuous neutron output, an advantage is the ability to switch on and off repeatedly to create a pulsed mode of operation. The pulse mode duty cycle may range from minutes or seconds of ON time and similar intervals of OFF time to milli-, micro- and even nano-seconds. Additionally, from a safety perspective the concept of a particle generator with a built- in kill switch is very attractive. Sealed tube neutron generator technology is inherently age-limited by the unavoidable erosion of the solid target. This component is a metal such as titanium that has been impregnated with tritium or deuterium gas. The incident high energy deuterons have the effect of causing sputter erosion of the target. The sputter product condenses as a metallic film on the inside surfaces of the sealed tube device. The use of voltages near 100 kilovolts results in a short circuit condition as the metallic film builds up. Even before this ultimate failure mode, the highly localized beam causes a hot spot and associated gas depletion within the target. Various neutron yield degradation mitigation schemes have been employed but the fact remains that the best guaranteed lifetime of a sealed tube neutron generator is only 4000 hours. The reactor apparatus of the present disclosure spreads the thermal and radiation damage during operation over a wider surface area of the anode and cathode resulting in an inherent advantage in durability compared to accelerator-based systems. A common issue regarding the longevity of both spherical and cylindrical IELC devices based on observations of experimental units where stainless-steel wire electrodes would suffer structural failure after perhaps 10-20 hours of operation at voltages ranging from 20 to 60 kilovolts and applied current of approximately 5 to 30 milliamperes. The mode of failure was metal vaporization or erosion and deposition on the surface of insulator components which would inevitably lead to short circuit conditions. The lifetime of systems in accordance with the present disclosure may exceed the claimed lifetime of systems based on commercial sealed tube beam-solid target neutron generators by 1050% and are expected to be able to be run indefinitely. A mean time between failures of 20,000 hours or more may be expected for some embodiments of the present disclosure. For commercial success, embodiments of the present disclosure are simple enough to enable manufacturing, operation and maintenance costs to be less than the life cycle costs associated with competing accelerator-based particle generators as a result of a reduction of the piece part count in the assembly, low piece part manufacturing costs, quick assembly and inexpensive quality assurance checks. Individual components or subassemblies have high durability in their intended function within embodiments of the present disclosure. Figure 7 shows neutron energy spectra of neutron radiation generated from various sources, thereby providing a comparison between the distributions of neutron energies obtained from such sources. The spectra show the relative proportions of neutrons of energies ranging from 1 to 16 MeV. The neutron spectrum labelled ‘LA’ (illustrated by solid grey triangles in Figure 7) is a typical linear accelerator spectrum, showing a high proportion of lower energy neutrons. The spectrum is based on spallation neutrons from high energy (p, n) reactions. The neutron spectrum labelled ‘BR2’ (illustrated by a dashed black line in Figure 7) is a typical neutron energy spectrum from a fission reactor used to make medical radionucleotides (more particularly the BR2 core spectrum of the SCK reactor in Belgium), and again sows a high proportion of lower energy neutrons in the 1-16 MeV range. The neutron spectra labelled DDDT and DT in Figure 7 (illustrated by grey crosses and black circles, respectively, in Figure 7) are produced by an IELC reactor of the type described herein. For the DDDT spectrum, the gas contained in the reactor comprises a 50 / 50 mol% mixture of deuterium and tritium, whereas the DT spectrum is generated when the reactor comprises only tritium as the gas. For both spectra, at least one of the cathode and the anode is enriched with deuterium. As shown in Figure 7, both the DDDT and DT spectra show generation of large quantities of higher energy neutrons (12-16 MeV), while the DT spectrum shows a lower proportion of lower energy neutrons (<4 MeV). It is understood that the 12-16 MeV neutrons of the DDDT and DT spectra result from deuterium-tritium fusion reactions, taking place in the lattice of the enriched surface between embedded deuterium and tritium originating from the plasma. Both DDDT and DT spectra are understood to comprise neutrons generated by tritium-tritium fusion taking place in the plasma. The Dt spectra is further understood to comprise neutrons generated by deuterium-deuterium fusion taking place both in the plasma and in the lattice of the enriched surface. Integration of the DDDT and DDT spectra provides the neutron energy distribution set out in Table 1 below, clearly indicating the high proportion of neutrons in the 12-16 MeV energy range. Table 1 Neutron energy (MeV) DDDT (% neutrons) DT (% neutrons) 0-1 0.30% 30.3% 0.40% 15.3% 1-2 2.80% 3.40% 2-3 20.00% 5.20% 3-4 7.20% 6.30% 4-5 2.10% 2.60% 5-6 1.70% 2.00% 6-7 1.20% 1.50% 7-8 1.00% 1.20% 8-9 1.00% 1.20% 9-10 0.70% 0.80% 10-11 0.70% 0.80% 11-12 0.70% 0.90% 12-13 0.60% 60.7% 0.70% 73.7% 13-14 12.90% 15.70% 14-15 36.10% 43.80% 15-16 11.10% 13.50% Total 100 100 Figure 8 shows a plot of Ac-227 production as a fraction of Ac-225 against target composition thickness when Ra-226 is irradiated with neutron radiation according to the spectra of Figure 7. The plots are produced from MCNP6.2 simulations, using neutron energy spectra are representative of the neutrons available for irradiation, rather than mono-energetic 2.45 or 14.1 MeV energies - it will be understood that the spectra shown in Figure 7, and used in the model to produce Figure 8, include for example scattered neutrons from the irradiation mechanism infrastructure. For the purposes of the model, the target composition is assumed to be pure Ra-226 metal, having a density of 5.5 g / cc. ‘Ac-227 / 225 Percentage’ (shown on the y axis of the graph of Figure 8) is the amount of Ac-227 as a fractional percentage of the amount of Ac-225 generated. Figure 8 shows that the amount of undesired Ac-227 produced increases with sample thickness. Furthermore, Figure 8 illustrates the striking difference in proportion of Ac-227 produced by irradiation of Ra-226 with neutron radiation from the various sources. When using BR2 neutron radiation (illustrated by the grey squares in Figure 8), Ac-227 production exceeds that of Ac-225, thus showing that such a radiation source is unsuitable for production of Ac-225 isotopes for medical use. The LA neutron radiation shows some improvement, but still produces around 0.5 % Ac-227. It is expected that such a contamination level of Ac-227 (which will increase during the time between isotope extraction and use, given the significantly shorter half-life of Ac-225) will not be accepted for medical use by health authorities in future. Irradiation of Ra-226 with either DDDT or DT neutron radiation provides a significant reduction in Ac-227 production as compared with the other radiation sources, with the DT neutron radiation providing the lowest Ac-227 content (below 0.01% at a target thickness of up to 10 mm (as low as 0.0047% with a thinner target). Figure 9 shows a similar plot with only the model of DDDT (grey crosses in Figure 9) and DT (black circles in Figure 9) shown, better illustrating the amounts of Ac-227 produced. Modelling further showed that both DDDT and DT neutron radiation also produced useful quantities of Pb-212 when used to irradiate Ra-226, which is also a very useful radiotherapeutic nuclide and readily extractable from the isotope product mixture. The models thus demonstrate that the apparatus of the present disclosure provides both a higher purity isotope product, and also a route to dual production of two clinically useful isotopes. The present invention may also be described according to the following numbered clauses: 1. A method for producing isotopes, wherein the method comprises subjecting a target composition comprising Ra-226 to neutron irradiation to produce Ra-225 isotopes by a (n, 2n) reaction, wherein the target composition is irradiated by neutron radiation comprising: at least 50% neutrons having an energy in the range of about 12 MeV to about 16 MeV; no more than 40% neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV. 2. A method for producing isotopes, wherein the method comprises subjecting a target composition comprising Ra-226 to neutron irradiation to produce Ra-225 isotopes by a (n, 2n) reaction, and wherein the method comprises inducing deuterium-tritium lattice confinement fusion in a nuclear fusion reactor to produce neutron radiation by generating an electric field between electrodes of the nuclear fusion reactor, wherein at least part of a surface of one or more said electrodes of the nuclear fusion reactor is enriched with at least 100 ppm deuterium and / or tritium; optionally wherein the method is a method according to clause 1. 3. A method according to clause 1 or clause 2, wherein the target composition is an aqueous solution of a salt comprising Ra-226, such as a Ra2+ salt comprising Ra-226. 4. A method according to clause 3, wherein the salt is a halide, hydroxide, iodate or nitrate salt, such as Ra(NOs)2, Ra(IO3)2, Rah, RaCh, RaBr2, Ra(OH)2. 5. A method according to any preceding clause, comprising allowing at least some of the Ra-225 isotopes to decay to Ac-225 isotopes, and extracting Ac-225 isotopes. 6. A method according to any preceding clause, wherein the method additionally comprises producing Ra-224 isotopes by a (n, 3n) reaction, allowing at least some of the Ra-224 isotopes to decay to Pb-212 isotopes via Rn-220 and Po-216, and extracting Pb-212 isotopes. 7. A method according to clause 5, wherein extracting Ac-225 isotopes comprises separating Ac-225 isotopes from isotopes of one or more other elements, and / or a method according to clause 6, wherein extracting Pb-212 isotopes comprises separating Pb-212 isotopes from isotopes of one or more other elements. 8. A method for producing isotopes, wherein the method comprises subjecting a target composition comprising a target isotope to neutron irradiation to produce a product isotope by a nuclear reaction (e.g. a nuclear transmutation reaction), optionally wherein the target composition is irradiated by neutron radiation comprising: at least 50% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV); and / or no more than 40% neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV. 9. A method according to clause 8, wherein the target isotope is an isotope susceptible to a nuclear transmutation reaction by irradiation with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV (optionally in the range of about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV), and / or wherein the method comprises irradiating the target composition with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV (optionally in the range of about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV) to form the product isotope by one or more nuclear transmutation reactions. 10. A method according to clause 8 or clause 9, wherein the target isotope is susceptible to one or more charged particle producing reactions and / or multiple particle producing reactions when subjected to irradiation with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV (e.g. about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV). 11. A method according to any one of clauses 8 to 10, wherein: the target isotope has a mass no heavier than the heaviest naturally occurring isotope, for example wherein the target isotope is a naturally occurring isotope; and / or the target isotope has a half life (1½) of at least 1 hour, such as at least 1 day, for example at least 14 days. 12. A method according to any one of clauses 8 to 11, wherein the target isotope forms a product isotope having a half life (t’A) of up to about 18 days (for example from about 1 hour to about 18 days) via a nuclear reaction (such as a nuclear transmutation reaction) when irradiated with neutrons having an energy of at least about 1 MeV, such as at least about 10 MeV (e.g. about 1 to about 20 MeV, such as about 10 MeV to about 20 MeV). 13. A method according to any one of clauses 8 to 12, wherein the target composition is a solution comprising the target isotope (for example a salt comprising the target isotope) or wherein the target composition is a solid comprising elemental target isotope and / or one or more oxides or salts of the target isotope. 14. A method according to any one of clauses 8 to 13, wherein the target isotope forms the product isotope via one or more intermediate isotopes, and wherein the method comprises forming the product isotope from the target isotope via one of more intermediate isotopes by a series of nuclear reactions including one or more nuclear transmutation reactions. 15. A method according to any one of clauses 8 to 14, wherein the target isotope is Radium-226 and the (first) product isotope is Actinium-225, and wherein the method comprises subjecting a target composition comprising Radium-226 to neutron irradiation to produce Actinium-225 isotopes by a 226Ra(n, 2n)225Ra->225Ac reaction, optionally wherein the method is a method according to any one of clauses 1 to 7; and / or, the (second) product isotope is Lead-212, and wherein the method comprises subjecting a target composition comprising Radium-226 to neutron irradiation to produce Lead-212 isotopes by a 226Ra(n, 3n)224Ra->212Pb reaction, optionally wherein the method is a method according to any one of clauses 1 to 7. 16. A method according to any one of clauses 8 to 13, wherein the target isotope is Dysprosium-161 and the product isotope is Terbium-161, and wherein the method comprises subjecting a target composition comprising Dysprosium-161 to neutron irradiation to produce Terbium-161 isotopes by a (n, p) reaction. 17. A method according to any one of clauses 8 to 13, wherein the target isotope is Mercury-202 and the product isotope is Gold-199, and wherein the method comprises subjecting a target composition comprising Mercury-202 to neutron irradiation to produce Gold-199 isotopes by a (n, alpha) reaction. 18. A method according to any one of clauses 8 to 13, wherein the target isotope is Mercury-199 and the product isotope is Gold-199, and wherein the method comprises subjecting a target composition comprising Mercury-199 to neutron irradiation to produce Gold-199 isotopes by a (n, p) reaction. 19. A method according to any one of clauses 8 to 13, wherein the target isotope is Titanium-47 and the product isotope is Scandium-47, and wherein the method comprises subjecting a target composition comprising Titanium-47 to neutron irradiation to produce Scandium-47 isotopes by a (n, p) reaction. 20. A method according to any one of clauses 8 to 14, wherein the target isotope is Calcium-48 and the product isotope is Scandium-47, and wherein the method comprises subjecting a target composition comprising Calcium-48 to neutron irradiation to produce Scandium-47 isotopes by a 48Ca(n, 2n)47Ca->47Sc reaction. 21. A method according to any one of clauses 8 to 13, wherein the target isotope is Zinc-64 and the product isotope is Copper-64, and wherein the method comprises subjecting a target composition comprising Zinc-64 to neutron irradiation to produce Copper-64 isotopes by a (n, p) reaction. 22. A method according to any one of clauses 8 to 13, wherein the target isotope is Zinc-67 and the product isotope is Copper-67, and wherein the method comprises subjecting a target composition comprising Zinc-67 to neutron irradiation to produce Copper-67 isotopes by a (n, p) reaction. 23. A method according to any one of clauses 8 to 14, wherein the target isotope is Krypton-78 and the product isotope is Bromine-77, and wherein the method comprises subjecting a target composition comprising Krypton-78 to neutron irradiation to produce Bromine-77 isotopes by a 78Kr(n, 2n)77Kr -> 77Br reaction. 24. A method according to any one of clauses 8 to 13, wherein the target isotope is Zirconium-90 and the product isotope is Yttrium-90, and wherein the method comprises subjecting a target composition comprising Zirconium-90 to neutron irradiation to produce Yttrium-90 isotopes by a (n, p) reaction. 25. A method according to any one of clauses 8 to 13, wherein the target isotope is Niobium-93 and the product isotope is Yttrium-90, and wherein the method comprises subjecting a target composition comprising Niobium-93 to neutron irradiation to produce Yttrium-90 isotopes by a (n, alpha) reaction. 26. A method according to any preceding clause, wherein the neutron radiation comprises: at least 60% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV), such as at least 70% neutrons having an energy in the range of about 12 MeV to about 16 MeV; no more than 35% neutrons having an energy of below 4 MeV (such as 0.5 to 4 MeV), such as no more than 18% neutrons having an energy of below 4 MeV (such as in the range of about 0.5 MeV to about 4 MeV); at least 75% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV) and below about 4 MeV, such as in the range of about 0.5 to about 4 MeV; and / or at least 90% of neutrons have an energy of no more than about 16 MeV. 27. A method according to any preceding clause, wherein the method comprises operating a nuclear fusion reactor to generate the neutron radiation by deuterium-tritium fusion, and optionally by deuterium-deuterium fusion and / or by tritium-tritium fusion, wherein deuterium-tritium fusion provides neutrons having an energy of about 12 MeV to about 16 MeV, deuterium-deuterium fusion provides neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV, and tritium-tritium fusion provides neutrons having an energy in the range of about 0.5 MeV to about 10 MeV. 28. A method according to clause 27, wherein the nuclear fusion reactor is configured and arranged for multi-state fusion, wherein multi-state fusion comprises a combination of plasma fusion and lattice confinement fusion; and / or wherein the nuclear fusion reactor comprises at least one electrode having an enriched surface, the enriched surface comprising a lattice substrate enriched with at least 100 ppm deuterium and / or tritium, and wherein the nuclear fusion reactor is configured and arranged for lattice confinement fusion in the lattice substrate; optionally wherein the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor. 29. A method according to any preceding clause, wherein the method comprises forming a plasma from a gas and initiating nuclear fusion in the presence of the plasma to generate the neutron radiation, wherein the gas comprises at least 50 mol% tritium, and optionally no more than 50 mol% deuterium, for example wherein the ion and neutral gas mixture comprises at least 80 mol% tritium, and optionally no more than 20 mol% deuterium. 30. A method according to any preceding clause, wherein the target composition has a thickness of about 0.5 mm to about 15 mm, such as about 1 mm to about 10 mm. 31. A method according to any preceding clause, wherein the method comprises irradiating a surface area of the target composition, and wherein the surface area is at least about 100 cm2, such as at least about 1000 cm2, optionally wherein the surface area is about 100 to 4000 cm2, such as about 500 to 3000 2 cm . 32. Apparatus for production of isotopes, wherein the apparatus comprises: a nuclear fusion reactor configured and arranged to produce neutron radiation, the neutron radiation optionally comprising: at least 50% neutrons having an energy in the range of about 12 MeV to about 16 MeV; no more than 40% neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV; and a target holder configured to hold a target composition comprising a target isotope, wherein the target holder is arranged to cause exposure of the target composition to the neutron radiation, for example to produce a product isotope by a nuclear transmutation reaction when the target isotope is irradiated with said neutron radiation; optionally wherein the target composition is a target composition according to any one of clauses 1, 3 or 4; optionally wherein the target isotope and / or product isotope is a target isotope according to any one of clauses 8 to 12; and / or optionally wherein the target composition is a solution comprising the target isotope (for example a salt comprising the target isotope) or wherein the target composition is a solid comprising elemental target isotope and / or one or more oxides or salts of the target isotope. 33. Apparatus according to clause 32, comprising said target composition held in the target holder. 34. Apparatus according to any one of clauses 32 to 33, wherein the nuclear fusion reactor is configured and arranged to produce neutron radiation comprising: at least 60% neutrons having an energy in the range of about 12 MeV to about 16 MeV, such as at least 70% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV); no more than 35% neutrons having an energy of below 4 MeV (such as 0.5 to 4 MeV), such as no more than 18% neutrons having an energy of below 4 MeV (such as in the range of about 0.5 MeV to about 4 MeV); at least 75% neutrons having an energy in the range of about 12 MeV to about 20 Mev (such as in the range of about 12 MeV to about 16 MeV) and below about 4 MeV, such as in the range of about 0.5 to about 4 MeV; and / or at least 90% of neutrons have an energy of no more than about 16 MeV. 35. Apparatus according to any one of clauses 32 to 34, wherein the nuclear fusion reactor is configured and arranged to generate the neutron radiation by deuterium-tritium fusion, and optionally by deuterium-deuterium fusion and / or by tritium-tritium fusion, wherein deuterium-tritium fusion provides neutrons having an energy in the range of about 12 MeV to about 16 MeV, deuteriumdeuterium fusion provides neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV, and tritium-tritium fusion provides neutrons having an energy in the range of about 0.5 MeV to about 10 MeV. 36. Apparatus according to clause 35, wherein the nuclear fusion reactor is a multi-state fusion reactor, wherein multi-state fusion comprises a combination of plasma fusion and lattice confinement fusion; and / or wherein the nuclear fusion reactor comprises at least one electrode having an enriched surface, the enriched surface comprising a lattice substrate enriched with at least 100 ppm deuterium and / or tritium, and wherein the nuclear fusion reactor is configured and arranged for lattice confinement fusion in the lattice substrate, optionally wherein: the nuclear fusion reactor comprises an anode structure and a cathode structure, wherein the anode and cathode structures are substantially concentric along at least a part of their lengths and are configured such that, in operation, an electric field is provided between the anode and cathode structures, and wherein at least one of the anode structure and the cathode structure comprises at least one said electrode comprising a surface enriched with at least 100 ppm deuterium and optionally tritium; optionally wherein the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor. 37. Apparatus for production of isotopes, wherein the apparatus comprises: a nuclear fusion reactor configured and arranged for inducement of deuterium-tritium lattice confinement fusion by generation of an electric field between electrodes of the nuclear fusion reactor, wherein at least part of a surface of one or more said electrodes of nuclear fusion reactor is enriched with at least 100 ppm deuterium and / or tritium; and a target holder configured to hold a target composition comprising a target isotope, wherein the target holder is arranged to cause exposure of the target composition to the neutron radiation; optionally wherein the target composition is a solution comprising the target isotope (for example a salt comprising the target isotope) or wherein the target composition is a solid comprising elemental target isotope and / or one or more oxides or salts of the target isotope. 38. The apparatus of claim 37, wherein the apparatus is apparatus according to any one of clauses 42 to 36. 39. The apparatus according to clause 37 or clause 38, wherein the target composition is a target composition according to any one of clauses 1, 3 or 4, and / or wherein the target isotope is a target isotope according to any one of clauses 8 to 12. 40. The apparatus according any one of 37 to 39, wherein the apparatus comprises said target composition held in the target holder. 41. Apparatus according to any one of clauses 32 to 40, wherein the nuclear fusion reactor contains a gas mixture and is operable to form a plasma from the gas mixture, and wherein the gas mixture comprises at least 50 % tritium, and optionally no more than 50 % deuterium, for example wherein the ion and neutral gas mixture comprises at least 80 mol% tritium, and optionally no more than 20 mol% deuterium. 42. Apparatus according to any one of clauses 32 to 41, wherein the target isotope is Radium-226, and the target composition is an aqueous solution of a salt comprising Ra-226, such as a Ra2+ salt, and wherein the target composition is disposed in the target holder. 43. Apparatus according to clause 42, wherein the salt is a halide, hydroxide, iodate and / or nitrate radium salt, such as one or more of Ra(NOs)2, Ra( I ()3)2, Rah, RaCh, RaBr2, and Ra(OH)2, wherein Ra is Ra-226. 44. Apparatus according to any one of clauses 32 to 43, configured so that the target composition has a thickness of 0.5 mm to 15 mm, such as 1 mm to 10 mm. 45. Apparatus according to any one of clauses 32 to 44, configured so that during operation a surface area of the target composition irradiated by the neutron radiation is at least about 100 cm2, such as at least about 1000 cm2, optionally wherein the surface area is about 100 to 4000 cm2, such as about 500 to 3000 2 cm . 46. Apparatus according to ay one of clauses 32 to 45, wherein the target holder is in the form of a shell or tube extending around at least a portion of the nuclear fusion reactor. 47. An isotope product comprising Ac-225 and Ac-227, wherein Ac-227 is present in an amount of no more than 0.08% based on the amount of Ac-225; optionally wherein the isotope product is produced according to the method of any one of clauses 1 to 15, or according to the method of any one of clauses 26 to 31 wherein the target isotope is Radium-226. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments. References in the present disclosure to "one embodiment", "an embodiment" and so on, indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. It should be understood that, although the terms "first", "second" and so on may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the disclosure. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed terms. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the 5 terms "comprises", "comprising", "has", "having", "includes" and / or "including", when used herein, specify the presence of stated features, elements, and / or components, but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof. 10

Claims

1. A method for producing isotopes, wherein the method comprises subjecting a target composition comprising a target isotope to neutron irradiation to produce a product isotope by a nuclear transmutation reaction, optionally wherein the target composition is irradiated by neutron radiation comprising: at least 50% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV); and / orno more than 40% neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV;wherein the target isotope is susceptible to one or more charged particle producing reactions and / or multiple particle producing reactions when subjected to irradiation with neutrons having an energy in the range of from about 1 to about 20 MeV.

2. A method according to claim 1, wherein:the target isotope has a mass no heavier than the heaviest naturally occurring isotope, for example wherein the target isotope is a naturally occurring isotope; and / orthe target isotope has a half life (1½) of at least 1 hour, such as at least 1 day, for example at least 14 days.

3. A method according to claim 1 or claim 2, wherein the target isotope forms a product isotope having a half life (1½) of from about 1 hour to about 18 days via a nuclear transmutation reaction when irradiated with neutrons having an energy in the range of from about 1 to about 20 MeV.

4. A method according to any preceding claim, wherein the target isotope is Dysprosium-161 and the product isotope is Terbium-161, and wherein the method comprises subjecting a target composition comprising Dysprosium-161 to neutron irradiation to produce Terbium-161 isotopes by a (n, p) reaction.

5. A method according to any one of claims 1 to 3, wherein the target isotope is Mercury-202 and the product isotope is Gold-199, and wherein the method comprises subjecting a target composition comprising Mercury-202 to neutron irradiation to produce Gold-199 isotopes by a (n, alpha) reaction.

6. A method according to any one of claims 1 to 3, wherein the target isotope is Mercury-199 and the product isotope is Gold-199, and wherein the method comprises subjecting a target composition comprising Mercury-199 to neutron irradiation to produce Gold-199 isotopes by a (n, p) reaction.

7. A method according to any one of claims 1 to 3, wherein the target isotope is Titanium-47 and the product isotope is Scandium-47, and wherein the method comprises subjecting a target composition comprising Titanium-47 to neutron irradiation to produce Scandium-47 isotopes by a (n, p) reaction.

8. A method according to any one of claims 1 to 3, wherein the target isotope is Calcium-48 and the product isotope is Scandium-47, and wherein the method comprises subjecting a target composition comprising Calcium-48 to neutron irradiation to produce Scandium-47 isotopes by a 48Ca(n, 2n)47Ca->47Sc reaction.

9. A method according to any one of claims 1 to 3, wherein the target isotope is Zinc-64 and the product isotope is Copper-64, and wherein the method comprises subjecting a target composition comprising Zinc-64 to neutron irradiation to produce Copper-64 isotopes by a (n, p) reaction.

10. A method according to any one of claims 1 to 3, wherein the target isotope is Zinc-67 and the product isotope is Copper-67, and wherein the method comprises subjecting a target composition comprising Zinc-67 to neutron irradiation to produce Copper-67 isotopes by a (n, p) reaction.

11. A method according to any one of claims 1 to 3, wherein the target isotope is Krypton-78 and the product isotope is Bromine-77, and wherein the method comprises subjecting a target composition comprising Krypton-78 to neutron irradiation to produce Bromine-77 isotopes by a 78Kr(n, 2n)77Kr -> 77Br reaction.

12. A method according to any one of claims 1 to 3, wherein the target isotope is Zirconium-90 and the product isotope is Yttrium-90, and wherein the method comprises subjecting a target composition comprising Zirconium-90 to neutron irradiation to produce Yttrium-90 isotopes by a (n, p) reaction.

13. A method according to any one of claims 1 to 3, wherein the target isotope is Niobium-93 and the product isotope is Yttrium-90, and wherein the method comprises subjecting a target composition comprising Niobium-93 to neutron irradiation to produce Yttrium-90 isotopes by a (n, alpha) reaction.

14. A method according to any preceding claim, wherein the neutron radiation comprises:at least 60% neutrons having an energy in the range of about 12 MeV to about 16 MeV, such as at least 70% neutrons having an energy in the range of about 12 MeV to about 16 MeV;no more than 35% neutrons having an energy of below 4 MeV (such as 0.5 to 4 MeV), such as no more than 18% neutrons having an energy of below 4 MeV (such as in the range of about 0.5 MeV to about 4 MeV); at least 75% neutrons having an energy in the range of about 12 MeV to about 16 Mev and below about 4 MeV, such as in the range of about 0.5 to about 4 MeV;and / orat least 90% of neutrons have an energy of no more than about 16 MeV.

15. A method according to any preceding claim, wherein the method comprises operating a nuclear fusion reactor to generate the neutron radiation by deuterium-tritium fusion, and optionally by deuterium-deuterium fusion and / or by tritium-tritium fusion, wherein deuterium-tritium fusion provides the neutrons having an energy of about 12 MeV to about 16 MeV, deuteriumdeuterium fusion provides neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV, and tritium-tritium fusion provides neutrons having an energy in the range of about 0.5 MeV to about 10 MeV.

16. A method according to claim 15, wherein the nuclear fusion reactor is configured and arranged for multi-state fusion, wherein multi-state fusion comprises a combination of plasma fusion and lattice confinement fusion; and / or wherein the nuclear fusion reactor comprises at least one electrode having an enriched surface, the enriched surface comprising a lattice substrate enriched with at least 100 ppm deuterium and / or tritium, and wherein the nuclear fusion reactor is configured and arranged for lattice confinement fusion in the lattice substrate;optionally wherein the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor.

17. A method according to any preceding claim, wherein the method comprises forming a plasma from a gas and initiating nuclear fusion in the presence of the plasma to generate the neutron radiation, wherein the gas comprises at least 50 mol% tritium, and optionally no more than 50 mol% deuterium, for example wherein the ion and neutral gas mixture comprises at least 80 mol% tritium, and optionally no more than 20 mol% deuterium.

18. A method according to any preceding claim, wherein the method comprises irradiating a surface area of the target composition, and wherein the surface area is at least about 100 cm2, such as at least about 1000 cm2, optionally wherein the surface area is about 100 to 4000 cm2, such as about 500 to 3000 2 cm .

19. Apparatus for production of isotopes, wherein the apparatus comprises: a nuclear fusion reactor configured and arranged to produce neutron radiation comprising:at least 50% neutrons having an energy in the range of about 12 MeV to about 20 MeV (such as in the range of about 12 MeV to about 16 MeV);no more than 40% neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV; anda target holder configured to hold a target composition comprising a target isotope, wherein the target holder is arranged to cause exposure of the target composition to the neutron radiation;wherein the target isotope is susceptible to one or more charged particle producing reactions and / or multiple particle producing reactions when subjected to irradiation with neutrons having an energy in the range of from about 1 to about 20 MeV.

20. Apparatus according to claim 19, wherein:the target isotope has a mass no heavier than the heaviest naturally occurring isotope, for example wherein the target isotope is a naturally occurring isotope; and / orthe target isotope has a half life (1½) of at least 1 hour, such as at least 1 day, for example at least 14 days.

21. Apparatus according to claim 19 or claim 20, wherein the target isotope forms a product isotope having a half life (1½) of from about 1 hour to about 18 days via a nuclear transmutation reaction when irradiated with neutrons having an energy in the range of from about 1 to about 20 MeV.

22. Apparatus according to any one of claims 19 to 21, wherein the nuclear fusion reactor is configured and arranged to generate the neutron radiation by deuterium-tritium fusion, and optionally by deuterium-deuterium fusion and / or by tritium-tritium fusion, wherein deuterium-tritium fusion provides the neutrons having an energy in the range of about 12 MeV to about 16 MeV, deuterium-deuterium fusion provides neutrons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV, and tritiumtritium fusion provides neutrons having an energy in the range of about 0.5 MeV to about 10 MeV.

23. Apparatus according to claim 22, wherein the nuclear fusion reactor is a multistate fusion reactor, wherein multi-state fusion comprises a combination of plasma fusion and lattice confinement fusion;and / or wherein the nuclear fusion reactor comprises at least one electrode having an enriched surface, the enriched surface comprising a lattice substrate enriched with at least 100 ppm deuterium and / or tritium, and wherein the nuclear fusion reactor is configured and arranged for lattice confinement fusion in the lattice substrate, optionally wherein:the nuclear fusion reactor comprises an anode structure and a cathode structure, wherein the anode and cathode structures are substantially concentric along at least a part of their lengths and are configured such that, in operation, an electric field is provided between the anode and cathode structures, and wherein at least one of the anode structure and the cathode structure comprises at least one said electrode comprising a surface enriched with at least 100 ppm deuterium and optionally tritium;optionally wherein the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor.

24. Apparatus for production of isotopes, wherein the apparatus comprises: a nuclear fusion reactor configured and arranged for inducement of deuterium-tritium lattice confinement fusion by generation of an electric field between electrodes of the nuclear fusion reactor, wherein at least part of a surface of one or more said electrodes of nuclear fusion reactor is enriched with at least 100 ppm deuterium and / or tritium; anda target holder configured to hold a target composition comprising a target isotope, wherein the target holder is arranged to cause exposure of the target composition to the neutron radiation;wherein the target isotope is susceptible to one or more charged particle producing reactions and / or multiple particle producing reactions when subjected to irradiation with neutrons having an energy in the range of from about 1 to about 20 MeV;optionally wherein the apparatus is apparatus according to claim 19.

25. Apparatus according to claim 24, wherein:the target isotope has a mass no heavier than the heaviest naturally occurring isotope, for example wherein the target isotope is a naturally occurring isotope; and / orthe target isotope has a half life (1½) of at least 1 hour, such as at least 1 day, for example at least 14 days.

26. Apparatus according to claim 24 or claim 25, wherein the target isotope forms a product isotope having a half life (t’A) of from about 1 hour to about 18 days via a nuclear transmutation reaction when irradiated with neutrons having an energy in the range of from about 1 to about 20 MeV27. The apparatus of any one claims 19 to 26, wherein the target isotope is one or more of: Dysprosium-161, Mercury-202, Titanium-47, Calcium-48, Zinc-64, Zinc-67, Krypton-78, Zirconium-90 and Niobium-93.

28. Apparatus according to any one of claims 19 to 27, wherein the nuclear fusion reactor is configured and arranged to produce neutron radiation comprising:at least 60% neutrons having an energy in the range of about 12 MeV to about 16 MeV, such as at least 70% neutrons having an energy in the range of about 12 MeV to about 16 MeV;no more than 35% neutrons having an energy of below 4 MeV (such as 0.5 to 4 MeV), such as no more than 18% neutrons having an energy of below 4 MeV (such as in the range of about 0.5 MeV to about 4 MeV); at least 75% neutrons having an energy in the range of about 12 MeV to about 16 Mev and below about 4 MeV, such as in the range of about 0.5 to about 4 MeV;and / orat least 90% of neutrons have an energy of no more than about 16 MeV.

29. Apparatus according to any one of claims 19 to 29, comprising said target composition held in the target holder, optionally wherein the target composition is a solid comprising elemental target isotope and / or one or more oxides or salts of the target isotope.

30. Apparatus according to any one of clauses 19 to 29, wherein the nuclear fusion reactor contains a gas mixture and is operable to form a plasma from the gas mixture, and wherein the gas mixture comprises at least 50 % tritium, and 5 optionally no more than 50 % deuterium, for example wherein the ion andneutral gas mixture comprises at least 80 mol% tritium, and optionally no more than 20 mol% deuterium.

31. Apparatus according to any one of claims 19 to 30, configured so that during 10 operation a surface area of the target composition irradiated by the neutronradiation is at least about 100 cm2, such as at least about 1000 cm2, optionally wherein the surface area is about 100 to 4000 cm2, such as about 500 to 3000 2 cm .15 32. Apparatus according to ay one of claims 19 to 31, wherein the target holder isin the form of a shell or tube extending around at least a portion of the nuclear fusion reactor.

Images