detector

Abstract

There is provided a high-purity germanium, HPGe, detector element (100) comprising: an elongate body (102) having a longitudinal direction and comprising a HPGe crystal (101), the HPGe crystal (101) comprising one or more lateral faces defining an external surface; a first electrode apparatus (110), the first electrode apparatus (110) being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal (101); and a second electrode apparatus (112), the second electrode apparatus (112) being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal (101).

Description

[0001] DETECTOR

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to high-purity germanium (HPGe) detectors and methods for manufacturing the same.

[0004] BACKGROUND

[0005] High-purity germanium (HPGe) detectors are semiconductor devices that are used to detect ionising radiation, particularly x-rays and gamma-rays, or charged particles, such as alpha-rays and beta-rays. The interaction of ionising radiation / charged particles with the germanium crystal of the detector generates electrical charges (i.e., electron -hole pairs). When electrodes of the HPGe detector are used to reverse biasing the semiconductor junction and deplete the crystal, the detector is operated as a diode. The electrons and holes drift in opposite directions along electric field lines within the crystal, generating a detectable electrical signal at the electrodes. By analysing this electrical signal, characteristics of the ionising radiation / charged particles may be determined. For example, the high sensitivity of HPGe detectors enables radioactive contaminants to be identified, including reconstruction of isotopic composition, based on the determined energy of detected gamma-rays. This technique is known as gammaray spectroscopy.

[0006] As such, HPGe detectors are fundamental to a number of scientific and industrial applications. Their sensitivity to ionisation means that HPGe detectors are the primary radiopurity assay tool used during the construction of a number of flagship particle and astroparticle physics experiments / facilities (e.g., in selecting materials and components for use in ‘rare-event’ searches, such as in neutrino or dark matter research). HPGe detectors may also be used to detect radioactive isotopes for the purpose of nuclear non proliferation, nuclear forensics or nuclear waste monitoring and characterisation, for example. Another application for HPGe detectors is the localization of radioactive waste in nuclear plants, a technique known as gamma-ray imaging.

[0007] However, current HPGe detector designs have a number of fundamental limitations. Existing electrode configurations (discussed further below) inherently restrict the size of the HPGe crystal. Increasing the size beyond a certain limit results in electrodes located on the surface of the crystal being situated too far apart, such that the electric field inside the crystal becomes too weak to deplete the detector, thus preventing its operation as a diode. This size limitation restricts the detection efficiency of the detector (larger detectors providing for greater chances of interaction). Additionally, the size limitation means that multiple detectors are typically manufactured from the same raw HPGe crystal / ingot, thus increasing manufacturing complexity and material wastage / cost due to cutting and fabrication.

[0008] Given the widespread interest in large HPGe detectors, it is desirable to address at least some of these problems.

[0009] SUMMARY

[0010] According to a first aspect of the invention, there is provided a high -purity germanium (HPGe) detector element. The HPGe detector element comprises: an elongate body having a longitudinal direction and comprising a HPGe crystal, the HPGe crystal comprising one or more lateral faces defining an external surface; a first electrode apparatus, the first electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal; and a second electrode apparatus, the second electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal.

[0011] Providing both the first electrode apparatus and the second electrode apparatus longitudinally along at least a part of the length of the external surface of the HPGe crystal (i.e., on a lateral surface of the elongate body of the HPGe crystal) may allow for larger HPGe detector elements to be produced. The longitudinal first and second electrode apparatuses may be used to generate a substantially radial electric field - i.e., having electric field lines lying primarily in the cross-sectional plane of the HPGe crystal elongate body, with little or no electric field component in the longitudinal direction along the length of the HPGe crystal. The electric field gradient component in the longitudinal direction may be less than 5%, for example. The exact field gradient in the longitudinal direction may be dependent on the uniformity of impurities along the length of the HPGe crystal. Thus, the length of the HPGe crystal may be increased indefinitely without substantially changing the radial electric field gradient. Therefore, the HPGe detector element of the present invention may be made with longer HPGe crystals than detectors which require one or more electrodes disposed on an end surface of the HPGe crystal. For embodiments of the present invention which do comprise electrodes on an end surface of the HPGe crystal, the electric field may not be substantially radial near to the ends. Nonetheless, the electric field may be substantially radial along the majority of the length of the detector (e.g., along 80% or 90% of the length of the detector) and the length of the HPGe crystal may still be increased indefinitely.

[0012] By using longer HPGe crystals, detector elements of the present invention may have greater detection efficiencies than existing detectors. The increased length may provide for a greater HPGe crystal volume, which may increase the probability of charged particles / ionising radiation interacting with the germanium crystal. The increased crystal length may also reduce the cost and / or complexity of the HPGe detector element. The entire length of the raw HPGe crystal may be used to manufacture a single detector element, thereby reducing material wastage associated with slicing the raw crystal to produce multiple, shorter detectors. The HPGe detector element of the present invention may enable less electronics or fewer read-out channels to be utilised, compared to multiple smaller detectors of equivalent volume.

[0013] What is meant by an “elongate body”, a “longitudinal direction” and “lateral faces defining an external surface” can be better understood with reference to the enclosed figures. However, the elongate body may generally be regarded as comprising a prismatic portion, with lateral (or side) faces that are longer than a maximum cross - sectional dimension of the prismatic portion. The lateral faces may be parallel with the longitudinal direction. The cross-sectional dimension may be normal to the longitudinal direction. The longitudinal direction may be in the direction that the raw crys tal of the body was pulled in during production.

[0014] One example of said prismatic portion would be a cylinder. The HPGe crystal may be generally cylindrical, in that the elongate body comprises a cylindrical portion. In such an example, lateral faces are the curved side face of the cylinder, and the cross -section of the cylindrical portion is a circle. The length of the cylinder may be greater than the diameter of the circular cross -section. The skilled person will understand that a cylinder is merely just one simple example of the elongate body. The prismatic portion may be ‘generally cylindrical’, e.g., being a cylinder but with a notch in the curved side face. Alternatively, the prismatic portion may have a different cross-sectional geometry other than circular, e.g., a semicircle or another polygonal shape. Likewise, the elongate body may comprise additional surfaces / regions other than the prismatic portion but may still be regarded as ‘generally cylindrical’ . For example, the end faces of the elongate body may comprise conical crystal portions, rather than flat end faces as in a cylinder. The prismatic portion may comprise at least 90% of the volume of the detector element - e.g., where the HPGe crystal comprises portions on the end faces of a generally cylindrical prismatic portion, the prismatic portion of the elongate body may still comprise at least 90% of the volume of the HPGe crystal.

[0015] In a preferred embodiment the detector element is HPGe, but the features disclosed herein may apply equally to detectors comprising other semiconductor materials, e.g., silicon or cadmium telluride. The detector element may, for example, comprise high purity silicon, or cadmium telluride, instead of HPGe.

[0016] The first electrode apparatus may comprise a longitudinal strip. The first electrode apparatus and the second electrode apparatus may be separated by a longitudinal separation region. The second electrode apparatus may be disposed around the whole circumference of the elongate body except for portions of the circumference occupied by the first electrode apparatus and the separation region.

[0017] Generally, the first electrode apparatus may be provided as a thin, longitudinal strip. The first electrode apparatus may be a read-out electrode - i.e., an electrode where electrons / holes generated are collected and detected. The first electrode apparatus may be disposed across up to 10% of the circumference of the elongate body, for example. The first electrode apparatus may have a width of less than 10 mm, or less than 5 mm, or less than 2 mm. By providing the first electrode apparatus as a thin, longitudinal strip, the area of the first electrode apparatus may be minimised. Reducing the size of the read-out electrode may reduce the capacitance of the HPGe detector element, which may reduce electronic noise. A first electrode apparatus width of less than 2 mm may provide for a preferably small capacitance. Generally, the second electrode apparatus may ‘wrap around’ the elongate body of the HPGe detector. Providing the second electrode apparatus across the remainder of the circumference of the elongate body may contribute to providing a sufficient electric field gradient across the entire cross-section of the HPGe crystal, such that charges generated within the entire crystal volume are collected at the electrodes. The second electrode apparatus may be disposed across at least 80% of the circumference of the elongate body, for example.

[0018] The separation region may comprise: at least one groove in the external surface of the HPGe crystal, the at least one groove being located between, and along the length of, the first electrode apparatus and the second electrode apparatus; and / or a region wherein the elongate body comprises a passivated surface.

[0019] The separation region may ensure that there is sufficient isolation between the first electrode apparatus and the second electrode apparatus, so as to prevent current leaking between the two. The separation region may comprise one or more grooves (which may also be referred to as ‘ditches’) cut into the external surface of the HPGe crystal between the first and second electrode apparatus. The at least one groove may comprise a pair of grooves located on either side of the first electrode apparatus or a single groove that surrounds the first electrode apparatus, for example. Additionally, or alternatively, the separation region may comprise a passivated surface. For example, the separation region may comprise a modified portion of the HPGe crystal, e.g., an amorphous germanium or germanium oxide portion. The separation region may comprise additional material, e.g., an insulating material that is applied to the external surface of the HPGe crystal.

[0020] The external surface of the elongate body may be a cylindrical surface. The first electrode apparatus may be disposed across a first portion of a circumference of the cylindrical surface The second electrode apparatus may be disposed across a second portion the circumference of the cylindrical surface, the second portion being greater in circumference than the first portion.

[0021] The use of a generally cylindrical elongate body (i.e., a generally cylindrical HPGe crystal) may reduce the number of manufacturing steps required to produce the HPGe detector element. The raw HPGe crystal may be grown naturally in a generally cylindrical shape, thus reducing the need to machine the raw crystal. As discussed above, longer crystals may be used to produce the detector element. Thus, HPGe detector elements of the same or greater volume may be produced without the need for additional machining required for existing detector configuration (e.g., without providing a coaxial borehole). Material wastage may therefore be reduced.

[0022] The generally cylindrical elongate body may still be restricted in diameter. If the cross - sectional area of the HPGe is too large, the electric field across the entirety of the crystal area may not be sufficiently achieved by commercially available power supplies to reach operational conditions (i.e., to deplete the HPGe crystal and effectively collect the generated electron-hole pairs). The diameter of the elongate body may be increased using the configurations discussed below, thereby further increasing the volume of the HPGe detector element.

[0023] The external surface of the elongate body may comprise a cylindrical surface (i.e., a surface that comprises most of a cylindrical shape) and a radial notch. The first electrode apparatus may be disposed across a first portion of the circumference of the cylindrical surface, the first portion being located on the cylindrical surface opposite to the radial notch. The second electrode apparatus may be disposed across a second portion of the circumference of the cylindrical surface and an external surface of the radial notch, the second portion being greater in circumference than the first portion.

[0024] The external surface of the elongate body may comprise a curved surface and a planar surface. The external surface may, for example, be semicylindrical (or approximately semicylindrical). The first electrode apparatus may be disposed across a first portion of the planar surface. The second electrode apparatus may be disposed across the curved surface and a second portion the planar surface.

[0025] The external surface of the elongate body may comprise a semicylindrical surface comprising a curved surface and a planar surface. The first electrode apparatus may be disposed across a first portion of the curved surface. The second electrode apparatus may be disposed across the planar surface and a second portion of the curved surface, the second portion being greater in circumference than the first portion. The notch may be easily cut into the HPGe crystal external surface using conventional semiconductor slicing tools. Likewise, a semicylindrical HPGe crystal may be produced by slicing a cylindrical crystal along its length through the diameter of its cross -section.

[0026] As discussed above, although the various configurations are described as being cylindrical or semicylindrical in nature, the same general configuration of the first electrode apparatus and second electrode apparatus may be applied to HPGe crystals that are generally cylindrical (e.g., having conical end portions) or having other prismatic geometry.

[0027] The HPGe crystal may comprise p-type germanium having an impurity concentration of less than 5 x 107atoms / mm3. Alternatively, the HPGe crystal may comprise n-type germanium having an impurity concentration of less than 5 x 107atoms / mm3.

[0028] The HPGe crystal may have an impurity concentration of less than 5 x 107atoms / mm3, 2 x 107atoms / mm3or 1 x 107atoms / mm3. The HPGe crystal may have an impurity concentration of 0.1 - 2.0 107atoms / mm3, 0.4 - 1.0 x 107atoms / mm3, 0.5 - 1.5 x 107atoms / mm3, or 1.0 - 2.0 x 107atoms / mm3for example. The HPGe crystal may have both p-type and n-type impurities, but nonetheless be p-type or n-type overall. An impurity concentration of less than 5.0 x 107atoms / mm3may be regarded as ‘high-purity’ . Reducing the impurity concentration may increase the energy resolution and detection efficiency of the HPGe detector element; the probability of electrons / holes becoming trapped by impurities while drifting to the electrodes may be reduced.

[0029] The first electrode apparatus may comprise a P+ doped region of the external surface of the HPGe crystal and the second electrode apparatus may comprise an n-i- doped region of the external surface of the HPGe crystal.

[0030] Alternatively, the first electrode apparatus may comprise a n-i- doped region and the second electrode apparatus may comprise an P+ doped region. Using a doped portion of the HPGe crystal as the first and second electrode apparatus may reduce the materials used to produce the detector element. Alternatively, the first and second electrode apparatus may comprise additional material / components coupled to the external surface of the HPGe crystal. For example, a layer of conductive material (e.g., metal contacts) may be deposited on the external surface. The first and / or second electrode apparatus may each comprise a single longitudinal electrode.

[0031] Providing a single longitudinal electrode (i.e., a continuous electrode strip along the length of the first / second electrode apparatus) may provide a substantially two - dimensional, radial electric field across the length of the detector element.

[0032] The first and / or second electrode apparatus may each comprise a plurality of discrete electrode elements disposed along the length of the respective electrode apparatus.

[0033] Providing a plurality of discrete electrode elements may reduce the total area of the first / second electrode area. This may reduce the capacitance of the HPGe detector element, which may reduce electronic noise.

[0034] Providing a plurality of discrete electrode elements may also improve the sensitivity of the detector to the characteristics of the ionising radiation, for example the location where gamma-rays interact within the crystal. This may enhance the applicability of the detector to gamma-ray spectroscopy applications, by increasing the discrimination efficiency between gamma-rays fully absorbed within the detector and those only partially absorbed; a technique known as Compton scattering suppression. This may also enhance the applicability of the detector to gamma -ray imaging applications by increasing the sensitivity to the direction of incoming gamma-rays.

[0035] The elongate body may comprise an end surface. The end surface may comprise a passivated surface. A portion of the first and / or second electrode apparatus may be disposed on the end surface.

[0036] The end surface of the HPGe crystal may be machined, e.g., the top and / or bottom of the crystal may be sliced such that the end surface is a flat surface orthogonal to the longitudinal axis of the crystal. The HPGe crystal may comprise end portions (e.g., conical portions) disposed on one or both ends of the elongate body. The elongate body may have a length of at least 70 mm. The elongate body may have a length of at least 100 mm, or a length of at least 300 mm, or a length of at least 300 mm.

[0037] The elongate body may have a length to diameter aspect ratio of at least 2. The elongate body may have a length to diameter aspect ratio of at least 1.5 or at least 2.5. The elongate body may have a maximum diameter of 40 - 100 mm.

[0038] The HPGe crystal may have a mass of at least 1 kg, at least 2 kg, or at least 3 kg. The HPGe crystal may have a mass of 3 to 5 kg. The HPGe crystal may have a mass exceeding 5 kg.

[0039] The detector element of certain embodiments may have a greater volume HPGe crystal than existing detectors which have an electrode disposed on an end surface of the HPGe crystal. Existing detectors may have a diameter of 65 to 80 mm and / or a length of 25 to 50 mm. Existing detectors may be limited to a maximum HPGe crystal mass of 2 to 2.5 kg-

[0040] The first electrode apparatus and / or the second electrode apparatus may be disposed along at least 90% of the entire length of the external surface of the HPGe crystal.

[0041] Providing the electrode apparatus along the length of the HPGe crystal may mean that a substantially two-dimensional radial electric field is created across most or all of the volume of the crystal. The first and / or second electrode may be provided across the entire length of the elongate body, e.g., extending from a top end surface to a bottom end surface of the elongate body, relative to the longitudinal axis of the HPGe crystal.

[0042] The elongate body may comprise a single HPGe crystal.

[0043] According to a second aspect of the invention, there is provided a detector system comprising: a high-purity germanium (HPGe) detector element, the HPGe detector element comprising: an elongate body having a longitudinal direction and comprising a HPGe crystal, the HPGe crystal comprising one or more lateral faces defining an external surface; a first electrode apparatus, the first electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal; and a second electrode apparatus, the second electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal; a power supply, the power supply being electrically coupled to the first electrode apparatus and / or the second electrode apparatus and configured to generate a potential difference therebetween, thereby resulting in a substantially radial electric field being created between the first electrode apparatus and second electrode apparatus; and a processing device configured to detect an electrical signal received at the first electrode apparatus and / or the second electrode apparatus, the electrical signal corresponding to an electrical charge generated within the detector by an interaction between the HPGe crystal and a charged particle or ionizing radiation incident upon the HPGe crystal, the electrical charge moving along the substantially radial electric field to one of the first electrode apparatus or second electrode apparatus.

[0044] The detector system may comprise a plurality of HPGe detector elements, power supplies and processing devices.

[0045] The detector system of the second aspect may comprise a HPGe detector element according to the first aspect. The power supply and / or the processing device may comprise any suitable device that is used to operate and analyse a conventional HPGe detector.

[0046] The power supply may be a high voltage power supply, for example capable of generating potential differences across the electrodes of up to 5 kV. For example, the first electrode apparatus may be a read-out electrode at 0 kV while the second electrode apparatus may be provided with a voltage of 3 to 5 kV. The power supply may be coupled to one of the electrode apparatuses while the other electrode apparatus is grounded. The processing device may comprise a digitiser and / or a spectroscopy amplifier, configured to analyse the currents produced at one or both of the first electrode apparatus and the second electrode apparatus, for example.

[0047] The detector system may comprise additional components that are configured to improve the performance of the system (e.g., to increase the sensitivity of the HPGe detector element). For example, the system may comprise amplifiers and / or filters that are coupled to the read-out electrode. The detector system may comprise a low voltage power supply for operating said amplifiers.

[0048] The first and / or second electrode apparatus may each comprise a plurality of discrete electrode elements disposed along the length of the respective electrode apparatus. The processing device may be configured to detect an electrical signal received at each of the plurality of discrete electrode elements. The plurality of discrete electrode elements may be analysed separately. For example, where the first electrode apparatus comprises a plurality of electrode segments / point-like electrodes, each segment or electrode may be provided with separate read-out circuitry, such that the current signal at each segment / electrode is processed separately. The plurality of discrete electrode element s may be read-out and processed in parallel. This may allow different longitudinal portions of the HPGe detector element to be analysed simultaneously. A discrete electrode element near the top of the HPGe crystal may be read-out at the same time as another discrete electrode element near the bottom of the HPGe crystal, for example. As discussed above, the use of discrete electrode elements may allow the HPGe detector element of the present invention to be used for additional or more specialised applications. The discrete electrode elements may be used to determine the location of interactions within the crystal, determine whether gamma -rays have been fully or partially absorbed (for Compton scattering suppression) and / or determine the direction of incoming gamma-rays, for example.

[0049] Each of the discrete electrode elements may be provided with suitable circuitry such that the read-out electrode (e.g., the first electrode apparatus) has a multi-channel output. For example, each electrode element may be provided with a separate amplifier. The processing device may be a multi-channel processing device, configured such that all of the discrete electrode elements may be analysed simultaneously. The detector system may further comprise: a cryostat housing configured to surround the HPGe detector element; and / or a cryogenic control device configured to cool an interior of the cryostat housing so as to maintain the HPGe detector element at a cryogenic temperature.

[0050] The cryostat housing and control device may provide sufficient cooling to the HPGe crystal so as to ensure optimal operation (e.g., to mitigate resistive heating). The cryostat housing may be a vacuum cryostat and may comprise, for example, a holder for the HPGe detector element and thermal shielding. In certain embodiments, the increased length of the HPGe detector element may mean the cryostat is custom designed to hold the detector element.

[0051] The cryostat control device may comprise suitable temperature sensing devices and processing devices, for example. The cryostat control device may be operated as a closed loop control system, configured to maintain the temperature inside the cryostat housing and / or the temperature of the HPGe detector element within a desirable temperature range.

[0052] According to a third aspect of the invention, there is provided a method of manufacturing a high-purity germanium (HPGe) detector element, the method comprising: growing a HPGe crystal by crystal pulling in a longitudinal direction, the HPGe crystal comprising one or more lateral faces defining an external surface and the HPGe crystal forming an elongate body of the HPGe detector element; forming a first electrode apparatus on the external surface of the HPGe crystal, the first electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal; and forming a second electrode apparatus on the external surface of the HPGe crystal, the second electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal.

[0053] The method of the third aspect may be used to make a HPGe detector element according to the first aspect. The raw crystal may be grown using any suitable crystal pulling method known in the art. For example, the HPGe crystal may be grown using the Czochralski method. The crystal may be pulled in a longitudinal direction, the longitudinal direction of the cryst al pulling corresponding to the longitudinal, lengthwise dimension of the HPGe crystal. Thus, the length of the HPGe crystal may be controlled by the amount of crystal pulling / growth. Existing HPGe detectors may be manufactured by slicing the crystal into approximately 2 to 8 segments, so as to limit the length of the detector. In certain embodiments, by using the entire length of a HPGe crystal to manufacture a detector element material wastage and manufacturing complexity may be reduced.

[0054] The method may further comprise, prior to forming the first electrode apparatus and prior to forming the second electrode apparatus, machining at least part of the external surface of the raw HPGe crystal.

[0055] The HPGe detector elements of certain embodiments may regarded as using the ‘entire length’ of the raw HPGe crystal, in that the raw HPGe crystal may not be sliced into a plurality of shorter segments to produce multiple detectors, as done for existing methods / devices. However, the skilled person will understand that some machining of the raw HPGe crystal may still be performed. Typically, the output of crystal growing methods is a crystal that is elongate but having a very non-uniform surface. Thus, the external surface of the raw HPGe crystal may be machined (e.g., via cutting or grinding) so as to produce the elongate body (e.g., a generally cylindrical body) for the HPGe detector element. This machining may result in some reduction in the length of the crystal. For example, end surfaces / portions of the elongate body of the raw HPGe crystal may be machined so as to remove irregularities (e.g., to produce a HPGe detector element with cylindrical symmetry). Some HPGe detector element embodiments, e.g., the semicylindrical geometries discussed herein, do comprise slicing the HPGe crystal to produce two crystal segments. Such slicing may be along a lengthwise axis rather than cross-sectionally, such that the result crystal segments retain the total crystal length (minus any general machining, as discussed).

[0056] The HPGe crystal may be grown with a substantially cylindrical cross -section. The method may comprise: forming a radial notch in the external surface of the HPGe crystal; and / or forming, prior to forming the first electrode apparatus and prior to forming the second electrode apparatus, a longitudinal planar surface on the external surface of the HPGe crystal.

[0057] As discussed above, the raw HPGe crystal that is produced via crystal growing may be regarded as generally cylindrical, in that it a prismatic, elongate object. Additional machining may be performed to remove any irregularities / non -uniformity, such that the HPGe crystal comprises a cylindrical elongate body. The HPGe may comprise end portions disposed on end surfaces of the elongate body, e.g., conical portions.

[0058] The generally cylindrical HPGe crystal may be used to form the HPGe detector. However, additional machining may be performed to produce different crystal geometries. The radial notch may have a width of approximately 5 to 10 mm and may extend from the external lateral surface of the HPGe crystal to the center of the cross - sectional area. The notch may be provided along the entire length of elongate body, or at least along the length of the first and second electrode apparatus. The generally cylindrical elongate body of the HPGe crystal may be cut along a longitudinal axis, thereby producing a semicylindrical crystal geometry comprising a longitudinal planar surface (i.e., the surface through the crystal along which cutting occurred) and a curved, semicylindrical surface (i.e., the remainder of the external cylindrical surface). The addition of the longitudinal notch and / or the semicylindrical geometry may enable detector elements with a HPGe crystal of greater diameter.

[0059] The method may further comprise forming a longitudinal separation region along at least part of the length of the external surface of the HPGe crystal, the longitudinal separation region separating the first electrode apparatus and the second electrode apparatus. Forming the longitudinal separation region may comprise: forming at least one groove in the external surface of the HPGe crystal, the at least one groove being located between, and along the length of, the first electrode apparatus and the second electrode apparatus; and / or forming a region of the external surface wherein the elongate body comprises a passivated surface.

[0060] Forming the first electrode apparatus and / or the second electrode apparatus may comprise doping respective portions of the external surface of the HPGe crystal elongate body. Forming the first electrode apparatus or the second electrode apparatus may comprise doping a first portion of the external surface of the HPGe crystal with a p-type dopant. Forming the other of the first electrode apparatus and the second electrode apparatus may comprise doping a second portion of the external surface of the HPGe crystal with an n-type dopant.

[0061] For example, where the first electrode apparatus is a p+ read-out electrode, the first electrode apparatus may comprise a portion of the HPGe external surface that has been doped with a p-type dopant (e.g., via boron implantation, with a depletion length of approximately 0.3 microns). Where the second electrode apparatus is an n-i- electrode, the second electrode apparatus may comprise a portion of the HPGe external surface that has been doped with an n-type dopant (e.g., via lithium infusion, with a depletion length of approximately 0.4 to 0.8 mm). The doping used to form the two electrode apparatuses may be switched, i.e., the first electrode apparatus may be formed with an n-type dopant and the second electrode apparatus may be formed with a p-type dopant.

[0062] Forming the first and / or second electrode apparatus may each comprise forming a plurality of discrete electrode elements disposed along the length of the respective electrode apparatus.

[0063] Where the first and second electrode apparatus are a P+ electrode and an n-i- electrode produced via doping of the HPGe crystal, the plurality of discrete elements may be formed by doping (e.g., via implantation or infusion of the external surface) of specific, discrete areas of the HPGe crystal external surface, so as to produce segmented or point - like electrode elements. Areas of the first and / or second electrode apparatus between the plurality of discrete electrode elements may comprise a passivated surface and / or grooves, so as to isolate the electrode elements. The passivated surface may comprise amorphous germanium or germanium oxide, for example.

[0064] DETAILED DESCRIPTION

[0065] Embodiments of the invention will be described, purely by way of example, with reference to the accompanying drawings, in which:

[0066] Figure 1 is an illustration of a known example of a HPGe detector formed from a HPGe crystal; Figures 2a to 2c are schematic diagrams of known electrode configurations for a HPGe detector;

[0067] Figures 3a to 3c show electric field models and electron -hole collection paths for the electrode configurations of Figures 2a to 2c respectively;

[0068] Figure 4 is an illustration of a HPGe detector formed from a HPGe crystal according to the present invention;

[0069] Figures 5a to 5d are schematic diagrams of various electrode configurations for a HPGe detector according to the present invention;

[0070] Figures 6a to 6d show electric field and A / E estimator models for the electrode configurations of Figures 5a to 5c respectively;

[0071] Figures 7a to 7d are schematic diagrams of various electrode apparatus configurations according to the present invention;

[0072] Figure 8 is a schematic diagram of a detector system according to the present invention; and

[0073] Figure 9 is a flow chart of a method of manufacturing a HPGe detector element according to the present invention.

[0074] Similar features in the Figures are provided with like reference numerals.

[0075] Referring to Figures 1 to 3, examples of existing HPGe detector configurations are shown. As shown in Figure 1 , HPGe detectors are made from a crystal 1 of high -purity germanium. The crystal 1 can generally be said to have a longitudinal body 2 (i.e., substantially cylindrical body) and end surfaces 4a, 4b disposed on opposing ends of the longitudinal body 2.

[0076] The right-hand image of Figure 1 shows a cross-section through the crystal 1 in the x-z plane (i.e., along the length of the crystal 1). Due to restrictions on the size of the HPGe detectors - discussed further below - the crystal 1 is typically cut into slices, thereby producing a plurality of shorter HPGe detectors la to 1c. The HPGe detectors la to 1c shown in Figure 1 are similar to those shown in Figure 2c - i.e., having a coaxial borehole 13. The expected electric field Fx>y>zfor HPGe detectors of this configuration is illustrated in example detector la. More particularly, Fx>y>zin this instance is the “weighting field magnitude”. The weighting field is the field generated by only the electrodes; the contribution to the field due to negative charge distribution within the crystal created by the depleted region is neglected. The weighting field is typically shown when modelling detectors as it is proportional to the current signal induced by a charge at any specific location within the detector. The ‘field’ lines show the trajectories followed by electron-hole pairs created at points p. The skilled person will understand that the weighting field is similar to the total electric field (differing slightly due to the neglected contribution of the negative charge distribution), and that the electron -hole trajectories are similar to the maximum field gradient (differing slightly due to the crystal structure / defects resulting in different charge mobility in different directions).

[0077] Figures 2a to 2c show cross-sections of three different electrode configurations for existing HPGe detectors, the cross sections being taken in the x-z plane (i.e., along the length of the crystal 1). All three configurations comprise a first point -like / small area electrode 10 located on an end of the detector (i.e., on the top / bottom surface of the generally cylindrically shaped detector). A second electrode 12 is disposed across much of the remainder of the crystal 1 surface, including on the longitudinal cylindrical surface and the opposing end surface to the first electrode 10.

[0078] Figures 3a to 3c show models1for the corresponding electric fields (electric weighting field shown via heat map and lines showing trajectories for electron -hole pairs created at specific points) within the crystal 1 of the detector configurations shown in Figures 2a to 2c. The first electrode 10 is provided with a potential of 0 kV and the s econd electrode 12 is provided with a potential of 4 kV. In these embodiments, the electric field is axisymmetric, resulting from the cylindrical geometry of the detector. A substantial component of the electric field is in the z direction (parallel to the longitudinal axis) - i.e., the electric field lines run along the length of the crystal 1 to the first electrode 10 that is located on an end surface.

[0079] 1Agostini, M., et al. "Charge-carrier collective motion in germanium detectors for P0-decay searches." The European Physical Journal C 81 (2021) 1, 76. The electric field resulting from these configurations restricts the size of the crystal 1 that can be used for the detector, particularly in the z direction. A sufficient electric field gradient is required throughout the whole volume of the detector, such that the germanium crystal 1 is sufficiently depleted and generated hole -electron pairs are effectively collected. Increasing the length of the detector in the z direction - such that the ‘top’ regions of the crystal 1 are located further from the first electrode - means that a greater potential difference is needed between the electrodes to generate sufficient field gradient across the detector volume. However, commercially available power supplies are generally limited to ~ 5 kV, thus limiting the maximum dimensions of the detector. Additionally, given the current technologies used to electrically isolate the electrodes, potential difference exceeding ~ 4 kV often result in increased leakage current between the electrodes. This may result in unstable detector behaviour and / or poor detector performance.

[0080] Figures 2a and 2b show detector configurations where the length of the crystal 1 in the z direction is limited by the restrictions discussed above. The maximum mass of the germanium crystal 1 in detectors according to these examples is approximately 1 kg.

[0081] Figure 2c shows an alternative configuration, wherein a borehole 13 is provided on the end surface of the detector opposing the first electrode 10, the borehole 13 running coaxially through the centre of the crystal 1 along the z axis. The second electrode 12 is disposed over the surface of the borehole 13 also. The borehole 13 may reduce pinch- off effect, therefore enabling greater detector lengths than the configurations shown in Figures 2a and 2b (as illustrated in Figure 3c). However, detectors like that shown in Figure 2c and 3c are still limited to a maximum germanium crystal mass of ~ 2 - 2.5 kg. Furthermore, the addition of the borehole 13 increases manufacturing complexity and material wastage. Additionally, the overall drift time for electron and holes is increased up to a few microseconds, resulting in charge trapping and, in turn, lower energy resolution.

[0082] Other configurations, not shown in Figures 2 and 3, pose similar limitations. In a ‘fully coaxial’ configuration, a borehole may be created along the length of the crystal, or a substantial portion thereof, similar to that shown in Figures 2c and 3c. One electrode may be provided on the inner surface of the borehole and another electrode may be provided on the external surface of the crystal. Although this may result in a more radial electric field than shown in Figure 3, the length of the HPGe detector is still significantly limited. Increasing the length of the detector reduces the accessibility to the inner surface of the borehole, thereby making it increasingly difficult to provide an electrode on the inner surface. Therefore, such coaxial designs are impractical beyond a certain length - either very specialised manufacturing techniques are required, or the diameter of the borehole has to be increased in order to improve accessibility (thereby reducing the crystal volume and resulting in further wastage). In the present invention, an ‘external surface’ of the crystal may be regarded as a surface that is located on an outer face of the crystal, facing generally away from an axis of the crystal along its length. The inner surface of a borehole, as discussed above, may not be considered to be an ‘external surface’ . The surface of the borehole will instead generally face the centre of the crystal (in the case where the crystal is a cylinder, the axis through the centre of the cylinder along its length). Unlike the external surfaces discussed herein, the surface of the borehole cannot be interacted with when moving in a direction perpendicular to the longitudinal axis of the crystal from a location outside the body of the crystal.

[0083] Likewise, ‘planar’ configurations, whereby electrodes are disposed on opposing surfaces of a planar crystal cannot be indefinitely increased in size. Increasing the dimension of the crystal between the two electrode surfaces is limited due to the electric field restrictions discussed above, while increasing the planar surface area of the crystal results in a thin, slab-like HPGe detector that is impractical for most applications.

[0084] Referring to Figure 4, an example of a HPGe detector element 100 according to an embodiment is shown. The HPGe detector element 100 is manufactured from a high- purity germanium crystal 101. The crystal 101 comprises a generally elongate / longitudinal body 102 (i.e., substantially cylindrical body) and end surfaces / portions 104a, 104b disposed on the ends of the longitudinal body 102.

[0085] The crystal 101 is used to form an elongate body of the detector element 100. As discussed further below, the raw HPGe crystal may be machined and the outer surface ground, so as the form the elongate body of the detector element 100. The elongate body is thus generally prismatic in shape. The elongate body may be substantially cylindrical or a prism shape. The prism shape may have a polygonal cross section, or a cross section comprising one or more curved portions and one or more straight portions). The elongate body of the detector element has a longitudinal direction along the length of the detector element 100, the longitudinal direction corresponding to the direction in which the crystal is pulled as it is grown (i.e., the z axis direction of Figure 4). Lateral faces of the HPGe crystal define an external surface of the detector, the lateral faces being in (e.g. parallel to) the z direction (e.g., the curved surfaces of a cylindrical detector element 100). The elongate body further comprises a top end surface 104a and a bottom end surface 104b, located on opposing ends of the elongate body. The end surfaces may be machined to any suitable shape, e.g., substantially flat and orthogonal to the longitudinal direction, or substantially conical in shape (like those shown in Figure 4).

[0086] The detector element 100 comprises a first electrode apparatus 110 and a second electrode apparatus 112. The first electrode apparatus 110 is longitudinally disposed along part of the external surface of the HPGe crystal 101 - i.e., the first electrode apparatus 110 may comprises one or more electrode elements arranged in a strip disposed along the side of the HPGe crystal 101. The second electrode apparatus 112 is similarly longitudinally disposed along anther part of the external surface of the HPGe crystal 101. The first electrode apparatus 110 and the second electrode apparatus 112 are thus both disposed on the external surface of the elongate body of crystal 101. The first electrode apparatus 110 and / or second electrode apparatus 112 may be provided along the entire length of the elongate body (i.e., from a first end of a lateral face to an opposing second end of the lateral face) or a substantial portion thereof (i.e., there may be a relatively small portion of the lateral face near the first and / or second end which the electrode apparatus is not disposed on). Part of the first electrode apparatus 110 and / or second electrode apparatus 112 may extend onto either or both of the end surface 104a, 104b.

[0087] The first electrode apparatus 110 and the second electrode apparatus 112 may be separated by a separation region 114. The separated region 114 may be provided along the longitudinal length of the first electrode apparatus 110 and / or the second electrode apparatus 112. The separation region 114 is a longitudinal region of the HPGe crystal 101 elongate body’s external surface upon which no electrode is disposed. The separation region 114 may be a pair of grooves or a passivated surface region disposed on either side of the first electrode apparatus 110. The right-hand image of Figure 4 shows a cross -section through the HPGe detector element 100 in the x-y plane (i.e., across the generally cylindrical crystal 101). The expected electric weighting field Fx,yfor the HPGe detector element 100 is illustrated. As both the first electrode apparatus 110 and the second electrode apparatus 112 are longitudinally disposed on the external surface of the crystal 101 (i.e., on the curved cylindrical surface), the electric field is substantially two-dimensional in the x-y plane. Very little electric field direction gradient exists in the z direction. Thus, the length of the detector element 100 may be increased indefinitely without incurring performance degradation. Substantially the entire length of the longitudinal body 102 of the HPGe crystal 101 (minus any surface finishing) may be used to produce a single detector element 100.

[0088] Referring to Figures 5 and 6, different configurations of the first electrode apparatus 110 and the second electrode apparatus 112 are shown. The different electrode configurations shown in Figures 5a to 5d may be used to produce HPGe detector elements according to the present invention. Figures 6a to 6d show models for the electric fields (top graphs, electric field strength shown via heat map and electric field lines illustrated with lines) and the normalised A / E estimator2for events generated through the detector volume (bottom graphs) for the electrode configurations shown in Figures 5a to 5d.

[0089] The electrode configuration shown in Figure 5a corresponds to a detector element wherein the external surface of the elongate body is a substantially cylindrical surface (i.e., the elongate body of the HPGe detector element is substantially cylindrical). The first electrode apparatus 110a is disposed across a first portion of the circumference of the cylindrical external surface, e.g., as a relatively narrow strip. The second electrode apparatus 112a is disposed across the majority of the remainder of the circumference of the cylindrical external surface, except for separation regions 114 located b etween the first electrode apparatus 110a and the second electrode apparatus 112a.

[0090] Although the longitudinal (z axis) length of the HPGe detector element using an electrode configuration as shown in Figure 5a may be increased indefinitely (due to the

[0091] 2Agostini, M., et al. "Pulse shape analysis in Gerda Phase II." The European Physical Journal C 82 (2022) 4, 284. substantially two-dimensional electric field), the cross-sectional area of the HPGe detector element in the x-y axis may be limited. Increasing the diameter of the cylindrical elongate body will increase the maximum distance between the first electrode apparatus 110 and the furthest portions of the second electrode apparatus 112, thereby reducing the electric field gradient. The alternative electrode configurations shown in Figures 5b to 5d may enable HPGe detectors with a greater diameter than Figure 5a.

[0092] The electrode configuration shown in Figure 5b corresponds to a detector element wherein the external surface of the elongate body is a substantially cylindrical surface (i.e., the elongate body of the HPGe detector element is substantially cylindrical) but with a notch 113 formed on a side of the cylindrical surface. The first electrode apparatus 110a is disposed across a first portion of the circumference of the cylindrical external surface. The second electrode apparatus 112a, 112b is disposed across the majority of the remainder of the circumference of the cylindrical external surface (except for separation regions 114) and across the surface of the notch 113. The portion of the second electrode apparatus 112b that is disposed on the surface of the notch may still be regarded as being disposed on an ‘external surface’ of the HPGe crystal as the notch is directly adjacent to the lateral, cylindrical external surface (as opposed to being on a borehole surface located coaxially within the crystal).

[0093] The electrode configuration shown in Figure 5c corresponds to a detector element wherein the external surface of the elongate body is a substantially semicylindrical surface comprising a curved surface and a planar surface (i.e., the elongate body of the HPGe detector element is substantially semicylindrical). The first electrode apparatus 110b is disposed across a first portion of the planar surface. The second electrode apparatus 112a, 112c is disposed across the majority of the remainder of the planar surface (except for separation regions 114) and across the curved surface.

[0094] The electrode configuration shown in Figure 5d is similar to the semicylindrical configuration shown in Figure 5c. However, the first electrode apparatus 110a is instead disposed across a first portion of the curved surface. The second electrode apparatus 112a, 112c is disposed across the majority of the remainder of the curved surface (except for separation regions 114) and across the planar surface. A substantially semicylindrical HPGe detector element, like those discussed above with reference to Figures 5c and 5d, may be produced by cutting a substantially cylindrical HPGe crystal along the longitudinal axis so as to produce two substantially semicylindrical HPGe crystals (i.e., cutting perpendicular to the x-y plane of Figure 4).

[0095] For the electrode configurations shown in Figures 5b to 5d, the first electrode apparatus 110 is located either opposite to the notch or in the middle of the relevant surface. This may generate a symmetry in the electric field. Such a symmetry can be broken by positioning the first electrode apparatus 110 not opposite to the notch or in the middle of the relevant surface (i.e., by moving the first electrode apparatus circumferentially). This may result in an asymmetry in the electric field, which may enhance the sensitivity of the detector to the characteristic and direction of the gamma -rays interacting in the detector (or at least in some directions).

[0096] Table 1 below provides indicative geometrical and operational parameters for the four HPGe detector element configurations shown in Figures 5a to 5d. Detector diameter, impurity concentration, and bias voltage ranges correspond to fully depleted detectors with sufficient maximum hole-collection time and read-out electrode capacitance. The capacitance and mass may scale linearly with the detector length. The quoted values are for an illustrative 20 cm long HPGe detector element. The maximum detector length and diameter may depend on the properties of the HPGe crystal, which may be vary during manufacture.

[0097] Table 1 : Example geometrical and operational parameters that may be utilised for a suitable HPGe detector using the configurations shown in Figures 5a to 5d. Referring to Figure 7, alternative configurations of a first electrode apparatus 110 are shown. Each of the three alternative configurations may be regarded as an electrode apparatus that is longitudinally disposed (i.e., along the z axis) along at least part of the external surface of a HPGe detector element.

[0098] The configuration of Figure 7a comprises a first electrode apparatus comprising a single longitudinal strip electrode 110a, a second electrode apparatus 112 (e.g., an electrode disposed around the external surface of the HPGe detector element as shown in Figure 5) and a separation region 114 separating the first electrode apparatus 110 and the second electrode apparatus 112. The single strip electrode 110a is located continuously along the entire length of the first electrode apparatus, which may be along the entire length of the detector element or a significant portion thereof.

[0099] Alternatively, the first electrode apparatus 110 may comprise a plurality of discrete electrode elements disposed along the length of the electrode apparatus. The configuration of Figure 7b comprises a first electrode apparatus comprising a segmented strip electrode 110b. The configuration of Figure 7c comprises a plurality of small area / point-like electrodes 110c disposed along the length of the first electrode apparatus. Despite comprising discrete electrode elements with interstitial gaps, the first electrode apparatus of Figures 7b and 7c may still be disposed across the entire length of the detector element, or a significant portion thereof (i.e., the topmost and bottom most electrode elements may span the entire length, or a significant portion, of the HPGe detector element). For example, the field resulting from these electrodes may be sufficiently uniform along the z direction for the detector to function in much the same way as if the electrode were continuous. A bounding box drawn around the discrete electrode elements may encompass more than 80%, or 90% or 95% of the length of the detector element. The discrete electrodes 110b, 110c may be regarded as forming a longitudinal electrode strip.

[0100] The first electrode apparatus 110 may comprise one or more passivated surfaces 116 located between each of the discrete electrode elements 110b, 110c. The passivated surface may be formed in the same way as described for the separation region 114 located between the first and second electrode apparatus. Figures 7a to 7c show configurations where the separation region 114 is disposed on either side of the first electrode apparatus 110. The separation region 114 may be a pair of grooves or a pair of passivated surface regions, for example. Alternatively, as shown in Figure 7d, the separation region 114b may entirely or substantially surround the first electrode apparatus 110. The separation region 114b may comprise a single groove or passivated region provided around all sides of the first electrode apparatus 110. The surrounding separation region 114b may be used with a strip -strip like first electrode apparatus 110 as shown in Figure 7d, or with segmented electrode configurations like those of Figures 7b and 7c.

[0101] The first electrode apparatus 110 may comprise a plurality of segments arranged radially instead of or in addition to longitudinally, i.e., the discrete electrode elements may not be arranged in a single line. Although shown in Figure 7 for the first electrode apparatus 110, the second electrode apparatus 112 may similarly comprise discrete electrode elements rather than a single continuous electrode. The second electrode apparatus 112 may comprise a plurality of segments / point-like electrodes arranged longitudinally along the length of the detector element and / or radially around the circumference of the detector element. The second electrode apparatus 112 may comprise a plurality of pointlike electrodes, a plurality of longitudinal strips and / or a plurality of rings, for example. For example, the second electrode apparatus 112a shown in Figure 5a may comprise a plurality of vertical strips arranged around the circumference, or a plurality of rings arranged along the length of the detector element.

[0102] Part of the first electrode apparatus 110 and / or the second electrode apparatus 112 may extend onto an end surface of the HPGe crystal. Where the HPGe crystal is cylindrical, the first and second electrode apparatus may be provided along the entire length of the cylinder and also extend onto the flat end faces. The first and / or second electrode apparatuses may be strip-like or segmented, and one or more separation regions 114 may be provided. I.e., the various configurations shown in Figure 7 may extend onto the end surfaces.

[0103] Referring to Figure 8, an example of a detector system 200 is shown. The detector system 200 comprises a HPGe detector element 100 according to the present invention (i.e., comprising a first electrode apparatus 110 and a second electrode apparatus 112 disposed longitudinal along the external surface of a HPGe crystal). The first electrode apparatus 110 and second electrode apparatus 112 are separated by a separation region 114. The HPGe crystal comprises conical end portions 104 located on either end of an elongate body 102.

[0104] The detector system 200 further comprises power and control apparatus 210 coupled to both the first electrode apparatus 110 and the second electrode apparatus 112. The skilled person will understand that any suitable power and control devices used for existing HPGe detectors may be used in the detector system 200 of the present invention.

[0105] The power / control apparatus 210 comprises a power supply configured to bias one of the electrode apparatuses 110, 112 so as to generate an electric field across the HPGe detector element 100. The power supply may be a high-voltage power supply configured to bias the second electrode apparatus 112 with a voltage of up to 5 kV, for example . The first electrode apparatus 110 may be grounded.

[0106] The power / control apparatus 210 further comprises a processing device (or readout device) configured to analyse electrical charges (holes or electrons) produced within the detector element 100 when charged particles or ionising radiation collide with the HPGe crystal. The holes-electron pairs will drift under the electric field generated by the applied bias, thereby being collected at the first or second electrode apparatus respectively. The first electrode apparatus 110 may be configured as read-out electrode; the processing device may be coupled to the first electrode apparatus 110 so as to analyse signals produced by the captured electrical charges. The processing device may comprise a digitizer or spectroscopy amplifier, for example.

[0107] The power / control apparatus 210 may further comprise additional components configured to improve the detection characteristics of the detector system 200. For example, amplifiers and / or filters may be provided so as to increase the signal -to-noise ratio of the read-out electrode. Where the first electrode apparatus 110 comprises a plurality of discrete electrode elements (e.g., a segmented electrode strip or point -like electrodes), the power / control apparatus 210 may be configured to power and analyse each electrode element individually. For example, the processing device may be a multichannel device capable of analysing the output of each electrode element in parallel. A single processing / readout device may also be used to readout multiple detectors, which may be arranged in an array. Similarly, a power / control apparatus may be common to more than one detector. In large scale physics experiments there may be a need for a large number of such detectors.

[0108] The detector system 200 may further comprise a cryostat housing 220 and a cryostat control device 222. The cryostat housing 220 is configured to enclose the HPGe detector element 100 such that the crystal and the associated electronics may be kept at cryogenic temperatures. The cryostat housing 220 may comprise a vacuum capsule. The cryostat control device 222 may comprise pumps, tanks, conduits, etc. fluidly and / or thermally coupled to the cryostat housing 220 and configured to cool the housing via a cryogenic fluid (e.g., liquid nitrogen). The cryostat control device may comprise appropriate electronics - such as temperature sensors, processing devices, devices for operating valves, etc. - so as to control the temperature of the cryostat housing 220 / the HPGe detector element 100. In embodiments with multiple detectors, a cryostat may house more than one detector.

[0109] Cryogenic fluid within the cryostat control device 222 may not be fluidly coupled to the cryostat housing 220 itself. The cryostat control device 222 may comprise a bath of cryogenic fluid, into which a thermally conductive probe or ‘finger’ is placed. The thermally conductive probe may then be thermally coupled (e.g., using a thermally conductive conduit) to the cryostat housing 222 and / or the detector system 220, thereby providing cooling. However, it will be understood that cooling can be achieved using alternative methods / apparatuses. The cryostat control device 222 may comprise a plurality of fluid conduits that surround the cryostat housing 220, with cryogenic fluid being pumped around said conduits to provide cooling, for example.

[0110] Referring to Figure 9, an example method 300 of manufacturing a HPGe detector element according to the present invention is illustrated.

[0111] The method 300 comprises growing 302 a HPGe crystal by crystal pulling in a longitudinal direction, the HPGe crystal comprising one or more lateral faces defining an external surface and the HPGe crystal forming an elongate body of the HPGe detector element. The HPGe crystal may be grown using the Czochralski method, for example.

[0112] The method 300 may further comprise additional machining processes 304, prior to the formation of the electrode apparatus. For example, the raw HPGe crystal may be machined so as to remove non-uniformities in the crystal surface. The HPGe crystal may be cut into a desired detector element geometry (e.g., cylindrical, cylindrical with a longitudinal notch, or semicylindrical). The method 300 further comprises forming 306 a first electrode apparatus on the external surface of the HPGe crystal, the first electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal. The method further comprises forming 308 a second electrode apparatus on the external surface of the HPGe crystal, the second electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal.

[0113] Either of the first or second electrode apparatus may be formed first. Forming 306, 308 the first electrode apparatus and the second electrode apparatus may comprise doping portions of the HPGe external surface with p-type and n-type dopants respectively (e.g., via implantation / infusion of the HPGe with boron and lithium).

[0114] Although specific examples have been described, the skilled person will appreciate that variations are possible, within the scope of the invention, which should be determined with reference to the accompanying claims.

Claims

CLAIMS1. A high-purity germanium, HPGe, detector element comprising: an elongate body having a longitudinal direction and comprising a HPGe crystal, the HPGe crystal comprising one or more lateral faces defining an external surface; a first electrode apparatus, the first electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal; and a second electrode apparatus, the second electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal.

2. The HPGe detector element of claim 1, wherein: the first electrode apparatus comprises a longitudinal strip; the first electrode apparatus and the second electrode apparatus are separated by a longitudinal separation region; and the second electrode apparatus is disposed around the whole circumference of the elongate body except for portions of the circumference occupied by the first electrode apparatus and the separation region.

3. The HPGe detector element of claim 1 or claim 2, wherein the separation region comprises: at least one groove in the external surface of the HPGe crystal, the at least one groove being located between, and along the length of, the first electrode apparatus and the second electrode apparatus; and / or a region wherein the elongate body comprises a passivated surface.

4. The HPGe detector element of any preceding claim, wherein: the external surface of the elongate body is a cylindrical surface; the first electrode apparatus is disposed across a first portion of a circumference of the cylindrical surface; and the second electrode apparatus is disposed across a second portion of the circumference of the cylindrical surface, the second portion being greater in circumference than the first portion.

5. The HPGe detector element of any of claims 1 to 3, wherein:the external surface of the elongate body comprises a cylindrical surface and a radial notch; the first electrode apparatus is disposed across a first portion of the circumference of the cylindrical surface, the first portion being located on the cylindrical surface opposite to the radial notch; and the second electrode apparatus is disposed across a second portion the circumference of the cylindrical surface and an external surface of the radial notch, the second portion being greater in circumference than the first portion.

6. The HPGe detector element of any of claims 1 to 3, wherein: the external surface of the elongate body comprises a curved surface and a planar surface; the first electrode apparatus is disposed across a first portion of the planar surface; and the second electrode apparatus is disposed across the curved surface and a second portion the planar surface.

7. The HPGe detector element of any of claims 1 to 3, wherein: the external surface of the elongate body comprises a curved surface and a planar surface; the first electrode apparatus is disposed across a first portion of the curved surface; and the second electrode apparatus is disposed across the planar surface and a second portion of the curved surface, the second portion being greater in circumference than the first portion.

8. The HPGe detector element of any preceding claim, wherein the HPGe crystal comprises p-type germanium or n-type germanium, the germanium having an impurity concentration of less than 5 x 107atoms / mm3.

9. The HPGe detector element of any preceding claim, wherein the first electrode apparatus comprises a p-type doped region of the external surface of the HPGe crystal and wherein the second electrode apparatus comprises an n-type doped region of the external surface of the HPGe crystal.

10. The HPGe detector element of any preceding claim wherein the first and / or second electrode apparatus each comprise a single longitudinal electrode.

11. The HPGe detector element of any of claims 1 to 9, wherein the first and / or second electrode apparatus each comprise a plurality of discrete electrode elements disposed along the length of the respective electrode apparatus.

12. The HPGe detector of any preceding claim, wherein the elongate body comprises an end surface, and wherein: the end surface comprises a passivated surface; and / or a portion of the first and / or second electrode apparatus is disposed on the end surface.

13. The HPGe detector element of any preceding claim, wherein: the elongate body has a length of at least 70 mm; and / or the elongate body has a length to diameter aspect ratio of at least 2.

14. The HPGe detector element of any preceding claim, wherein the HPGe crystal has a mass of at least 3 kg.

15. The HPGe detector element of any preceding claims, wherein the first electrode apparatus and / or the second electrode apparatus are disposed along at least 90% of the entire length of the external surface of the HPGe crystal.

16. The HPGe detector element of any preceding claim, wherein the elongate body comprises a single HPGe crystal.

17. A detector system comprising: a HPGe detector element according to any of claims 1 to 16; a power supply, the power supply being electrically coupled to the first electrode apparatus and / or the second electrode apparatus and configured to generate a potential difference therebetween, thereby resulting in a substantially radial electric field being created between the first electrode apparatus and second electrode apparatus; and a processing device configured to detect an electrical signal received at the first electrode apparatus and / or the second electrode apparatus, the electrical signalcorresponding to an electrical charge generated within the detector by an interaction between the HPGe crystal and a charged particle or ionizing radiation incident upon the HPGe crystal, the electrical charge moving along the substantially radial electric field to one of the first electrode apparatus or second electrode apparatus.

18. The detector of claim 17, wherein the first and / or second electrode apparatus each comprise a plurality of discrete electrode elements disposed along the length of the respective electrode apparatus, and wherein the processing device is configured to detect an electrical signal received at each of the plurality of discrete electrode elements.

19. The detector system of claim 17 or 18, further comprising: a cryostat housing configured to surround the HPGe detector element; and / or a cryogenic control device configured to cool an interior of the cryostat housing so as to maintain the HPGe detector element at a cryogenic temperature.

20. A method of manufacturing a high-purity germanium, HPGe, detector element, the method comprising: growing a HPGe crystal by crystal pulling in a longitudinal direction, the HPGe crystal comprising one or more lateral faces defining an external surface and the HPGe crystal forming an elongate body of the HPGe detector element; forming a first electrode apparatus on the external surface of the HPGe crystal, the first electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal; and forming a second electrode apparatus on the external surface of the HPGe crystal, the second electrode apparatus being longitudinally disposed along at least part of the length of the external surface of the HPGe crystal.

21. The method of claim 20, further comprising, prior to forming the first electrode apparatus and prior to forming the second electrode apparatus, machining at least part of the external surface of the HPGe crystal.

22. The method of claim 20 or claim 21 , wherein the HPGe crystal is grown with a substantially cylindrical cross -section, and optionally or preferably the method further comprising:forming a radial notch in the external surface of the HPGe crystal; and / or forming, prior to forming the first electrode apparatus and prior to forming the second electrode apparatus, a longitudinal planar surface on the external surface of the HPGe crystal.

23. The method of any of claims 20 to 22, further comprising forming a longitudinal separation region along at least part of the length of the external surface of the HPGe crystal, the longitudinal separation region separating the first electrode apparatus and the second electrode apparatus, wherein forming the longitudinal separation region comprises: forming at least one groove in the external surface of the HPGe crystal, the at least one groove being located between, and along the length of, the first electrode apparatus and the second electrode apparatus; and / or forming region of the external surface wherein the elongate body comprises a passivated surface.

24. The method of any of claims 20 to 23, wherein: forming the first electrode apparatus or the second electrode apparatus comprises doping a first portion of the external surface of the HPGe crystal with a p-type dopant; and forming the other of the first electrode apparatus and the second electrode apparatus comprises doping a second portion of the external surface of the HPGe crystal with an n-type dopant.

25. The method of any of claims 20 to 24, wherein forming the first and / or second electrode apparatus each comprise forming a plurality of discrete electrode elements disposed along the length of the respective electrode apparatus .

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