Radiation-shielding materials and items and methods of their manufacture

Abstract

The invention provides a material with radiation-shielding properties, comprising a polymer matrix (3) through which particulate radiation-attenuation material (1,2) is distributed, the particulate ra

Description

The present invention is concerned with radiation shielding. There are many situations in which people or objects need to be protected from the potentially damaging effects of radiation, and particularly of ionising radiation. Some arbitrarily chosen example scenarios are:- imaging by use of ionising radiation, typically in the form of X-rays, which may be medical or non-medical. Operatives (such as radiographers) need to be protected from the potentially injurious effects of repeated exposure. therapeutic use of ionising radiation, as for example in radiotherapy, where the subject is necessarily exposed to the radiation but others in the vicinity need to be shielded from it. - space exploration. Without the shielding effect of the atmosphere, personnel and equipment in space are potentially exposed to elevated levels of radiation, with the potential for injury and damage if suitable protection is not provided, both by spacecraft and by suits used for extra-vehicular activity. storage of materials that emit ionising radiation through nuclear decay, which may be on a range of different scales from storage of radioactive material in a laboratory or medical facility to storage of nuclear waste from power plants. The store typically needs shielding to reduce radiation emission. nuclear reactors, which need to incorporate shielding to limit their emission of potentially dangerous radiation. Exposure to radiation can be fatal, due both to short-term effects of high-intensity exposure and to longer-term effects, often the result of cumulative exposure over a period of time, such as increased risk of cancer and other diseases, making shielding of vital importance in a range of scenarios. It is also sometimes necessary to shield equipment from radiation to prevent it from being damaged, or to prevent the radiation from adversely affecting performance, e.g. by affecting measurements taken using sensors. Examples of ionising radiation include, without limitation, X-rays, alpha, beta and neutron radiation, radiation from nuclear processes, cosmic rays and electromagnetic radiation of sufficient photon energy to have an ionising effect, including gamma rays. It is known to provide radiation shielding in the form of a physical barrier intended to block at least part of the incident radiation. Typically, such barriers are not wholly opaque to the relevant radiation, but intensity diminishes exponentially with the depth of penetration of the radiation into the shield, so that thick shields can have a greater blocking effect than thinner ones. Elements of high atomic number are found to be typically more effective in blocking some kinds of ionising radiation than elements of low atomic number. Lead is widely used for radiation shielding, but its disadvantages include its high density and toxicity, as well as its material properties, which do not lend themselves to certain advantageous manufacturing techniques nor - due to its softness - to its use in certain applications requiring structural strength. Another material widely used for radiation shielding is concrete, which has intrinsic radiationattenuation characteristics which may be improved by admixture of aggregates of magnetite or barite. Concrete is well suited to some radiation-shielding applications, especially in large, fixed structures such as walls, floors and ceilings in rooms used for medical imagine and radiation therapy, reactor containment structures, spent-nuclear-fuel storage and nuclear-waste disposal facilities, but concrete's material properties do not suit it to all radiationshielding applications. The manufacture of radiation-shielding apparatus and structures with both lead and concrete is energy intensive, which represents a burden on the environment. It is desirable to reduce the energy consumption involved in manufacture of radiation shields. A need exists for an improved radiation-shielding material. It is desirable that this material should provide one or more of the following properties: high radiation attenuation low toxicity low density, in comparison e.g. to metals adaptability to different manufacturing techniques and to the manufacture of products in a range of different shapes capability of providing structural strength low energy burden - tolerable financial cost, in terms of materials and manufacture recyclability. A paper by Elsafi, M., El-Na ha I, M.A., Sayyed, M.L et al. Novel 3-D printed radiation shielding materials embedded with bulk and nanoparticles of bismuth. Sci Rep 12, 12467 (2022). http$: / / doi.Qrg / lQ.103g / s41S98-022-16317--w concerns a study of radiation-shielding material developed using a 3D printing technique. Bismuth metal particles are said to have been added manually during printing of a cylinder from polylactic acid thermoplastic polymer ("PLA"). The paper mentions use of bismuth particle powders and of bismuth nanoparticles, and refers to them being incorporated as a filler between inner layers of 3-D printed PLA plastic. It also suggests an alternative where ABS plastic and bismuth, which have similar melting temperatures, are fused in an extruder machine during manufacture of a filament, the implication seemingly being that the bismuth would be melted during the manufacturing process. Bismuth is an expensive material with a relatively low melting point, both of which may impair its utility in this context. According to a first aspect of the present invention, there is a material comprising a polymer matrix through which particulate radiation-attenuation material is distributed. According to a second aspect of the present invention, there is a method of manufacture of a radiation-shielding material, the method comprising providing radiation-attenuation material in particulate form, providing polymer material in liquid form, blending the radiation-attenuation material with the polymer material to form a mixture and - setting of the polymer material. According to a third aspect of the present invention, there is a method of manufacture of a radiation-shielding item comprising providing radiation-attenuation material in particulate form, providing thermoplastic polymer matrix material, heating the matrix material to liquify it, blending the radiation-attenuation material with the liquified polymer material to form a mixture, - setting of the polymer material to form a solid radiation-shielding material, and forming the radiation-shielding material into a chosen shape (a) by liquifying the radiation-shielding material by heating and then moulding the radiation-shielding material or (b) by additive manufacturing. Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a graph of penetration of radiation through layers of material embodying the present invention, measured using indicator stickers; Figure 2 is a graph of penetration of radiation through layers of material embodying the present invention, measured using a Geiger counter; and Figure 3 is an electron micrograph of material embodying the present invention. The invention provides a composite radiation-shield material which offers on the one hand a high coefficient of radiation attenuation (which is to say that the material provides high radiation attenuation for a given thickness of shield) and on the other hand the adaptability and utility of a material that can for example be straightforwardly moulded or used in layer manufacturing, without need of high temperatures nor expensive manufacturing equipment. The material can also have modest raw-material cost. It can be of low density. It can incorporate reinforcement giving it high structural strength and even anti-baIlistic capabilities. The radiation-shield material comprises a matrix of polymer material, which in the present embodiment is a plastic material. The invention may be implemented using thermosetting plastics, but thermoplastic materials are also suitable and provide the advantage of recyclability. Suitable polymer matrix materials include (1) polycarbonate, (2) PEEK (polyether ether ketone) and (3) PTFE (polytetrafluoroethylene). This is not an exhaustive list. The polymer matrix material may be chosen for a particular shielding application. For example, PTFE is well suited to neutron shielding applications, PEEK is well suited to X-ray shielding applications and polycarbonate is well suited to shielding against beta and gamma rays. For many applications, the polymer matrix material in itself provides sufficient structural strength and rigidity. However, the polymer matrix material may be provided with structural reinforcement. Such reinforcement may for example comprise fibres and / or powder. The fibres may be of short-strand type compatible with the use of the radiation-shield material in 3D manufacturing techniques. Suitable materials for a reinforcing additive include carbon fibre, para-aramids including poly-paraphenylene terephthalamide (known and commercially available under the registered trade mark Kevlar), boron carbide, silicon carbide, or a combination of any of these materials. Such reinforcement may improve the radiation-shield material's structural properties including impact resistance, anti-ballistics, tensile strength and rigidity. Incorporated in and distributed through the polymer matrix material are particles of radiationattenuation material. In the present embodiment, the radiation-attenuation material comprises at least one of (a) titanium, which may be in pure form or may be present in any suitable mixture, compound or alloy and (b) tungsten, which again may be in pure form or in any suitable mixture, compound or alloy. In the present embodiment, the tungsten is in the form of tungsten carbide, and the radiation-shield material comprises both titanium and tungsten carbide. One or more components of the radiation-attenuation material may be in powder form and may more specifically be nanoparticulate. The radiation-attenuation material may be homogeneously, or at least substantially homogeneously, distributed through the polymer matrix. Attenuation of neutron radiation may be improved by incorporating a material with a high neutron capture cross-section, which may comprise boron. The boron may be in the form of boron carbide or borated tungsten carbide. The ratio of the radiation-attenuation material to the polymer matrix material may be varied to provide desired characteristics including coefficient of radiation attenuation, but byway of example and not limitation the radiation-shield material may comprise by volume approximately 20% polymer matrix material and 80% particulate radiation-attenuation material. In such an example, the radiation-attenuation material may comprise approximately equal parts of titanium and tungsten carbide. In other embodiments the bulk, pre-mixed volume ofthe particulate radiation-attenuation material may for example be from 30% to 80% of the volume ofthe radiation-shield material. The particles of the radiation-attenuation material may comprise a powder and / or may comprise and / or consist predominantly of nanoparticles, with a characteristic dimension between 1 nanometre and 100 nanometres. Small particle size maximises surface area in proportion to mass and volume, increasing photon absorption. A process of manufacture of a radiation-shield material implemented using a thermoplastic polymer matrix material may comprise the following steps:- 1. providing the polymer matrix material in suitable form. Typically, it is pelletised. Depending on the condition in which it is received, it may need to be dried. This can be achieved by heating (a temperature of 70 - 80 degrees Celsius is suitable) for a chosen period, which may be between two and six hours. 2. providing the radiation-attenuating material in suitable particulate form, typically powdered and / or nanoparticulate. The radiation-attenuating material may be dried prior to its use although this is not always necessary as it does not undergo and thermal reconfiguration. 3. mixing ofthe polymer matrix material with the radiation-attenuation material in the desired ratio to provide a homogeneous mixture. 4. heating ofthe mixture to liquidise the polymer matrix material. This may be done in an extrusion device in which, once the mixture has been heated to the necessary temperature, the liquified mixture is directed through an extrusion head to form a filament; and 5. setting of the mixture to form the solid radiation-shield material. The resultant filament may be chopped to create pellets of the radiation-shield material suitable e.g. for subsequent moulding, or in other examples the extruded filament may be in suitable form for use in a 3D printing (additive manufacturing) machine, the diameter of the filament being suitably selected and regulated during the extrusion process. The resultant radiation-shield material may be used in any suitable manufacturing process to form a radiation shield. Examples of suitable manufacturing processes include (a) 3D printing (additive manufacturing) and (b) moulding, particularly injection moulding. The suitability of the radiation-shield material for additive manufacturing and injection moulding means that radiation shields made with it can be designed with great freedom in terms of shape and size, reflecting the adaptability of these manufacturing processes. Additive manufacturing provides for economical manufacture of modest quantities of products, while injection moulding makes possible economical high-volume manufacture of radiation shields. The radiation-shield material can be suited to well-known and easily available additive manufacturing techniques including fused deposition modelling, without need of the relatively high temperatures and the expensive equipment needed e.g. to carry out additive manufacture using metals. Radiation shields according to the present invention may be adapted to attenuate certain nonionising electromagnetic radiation. Specifically, an electrically conductive layer may be incorporated or applied to the exterior of the radiation shield to provide shielding at radio EM frequencies. The conductive layer may comprise carbon nanotubes. Recycling of radiation shields comprising the radiation-attenuation material is straightforward, since the material can simply be cleaned if necessary, and then reworked. Reworking may comprise reducing the radiation shield to particulate form e.g. by chopping or shredding, then reusing the material thus obtained in manufacture of a new product, e.g. by injection moulding or by extruding filament for layer manufacturing. Examples of radiation shields which can be manufactured with the radiation-attenuation material according to the present invention include, without limitation, the following:- personal protective equipment for use by workers exposed to elevated levels of ionising radiation, e.g. workers in the nuclear industries, radiographers, radiologists and others involved in medical imaging or treatment using ionising radiation, and persons operating security scanners. body armour, including anti-ballistic armour (e.g. "bullet-proof vests"). Thanks to its low density and potential to incorporate anti-ballistic reinforcement such as para-aramids (e.g. the material sold under the registered trade mark Kevlar), the radiation-shield material is well suited to use in wearable body armour capable of providing protection both from radiation and from ballistic weapons and / or physical injury or impact. spacesuit components. - spacecraft components, including for example manned capsules, where radiation screening is needed to protect occupants, and unmanned spacecraft including satellites, where for example electronics may require screening from ambient radiation. components of devices for imaging by use of ionising radiation, including X-ray machines, or screens for use in that setting. The imaging devices in question may for example be for medical purposes or for security purposes, or for some other purpose such as weld testing. components of devices for medical treatment using ionising radiation, e.g. radiotherapy machines, and screens for use in that setting. - structures for containment of radioactive material, including nuclear waste, which may be small (e.g. for storage of small radioisotope samples in a laboratory or teaching establishment) or large, as in the case of industrial-scale waste storage facilities; components of nuclear reactors. The performance of a radiation-shield material embodying the present invention has been demonstrated by testing carried out confidentially on behalf of the applicant by the Science and Technologies Facilities Council in the UK. Samples of the radiation-shield material were prepared, each measuring 50mm x 50mm x 5mm. The polymer matrix material of the samples was PEEK, and they contained 80% by volume of titanium nanoparticles and tungsten carbide nanoparticles, in equal parts. The samples were stacked to create a multi-layer shield and exposed to high-energy emissions (>50KGr) for a period of 8 hours in a linear accelerator ("LINAC"). Penetration of the radiation was measured by means of indicator stickers applied to each of the samples in the stack, and also with a Geiger counter. It was found that the radiation penetrated detectably, by means of the indicator stickers, only through two of the nine layers (Figure 1). Figure 2 is a graph of Geiger counter readings against the position of the 5 layer in the stack and shows a rapid reduction in the readings from one layer to the next. Figure 3 provides an electron micrograph image of a sample of the radiation-shield material in which particles of tungsten carbide are see at 1, titanium particles at 2, and the polymer matrix material, which in this example comprises polycarbonate, is seen at 3. The radiation-attenuation material according to the present invention may have a density 10 lower than that of aluminium. It may have a density of the order of 2.4 grams per cubic centimetre. The energy consumption involved in manufacture of items from the radiationattenuation material may be low - of the order of 0.1 - 0.3 kilowatt hours (360 to 1080 kilojoules) per kilogram, compared with a figure of the order of 2.5 kilowatt hours (9000 kilojoules) per kilogram for manufacture of concrete and of lead shielding.

Claims

1. A material comprising a polymer matrix through which particulate radiation-attenuation material is distributed.

2. A material as claimed in any claim 1 in which the radiation-attenuation material comprises at least one metal.

3. A material as claimed in claim 1 in which the radiation-attenuation material comprises nanoparticles.

4. A material as claimed in claim 1 in which the radiation-attenuation material comprises particles having a length between 1 and 100 nanometres.

5. A material as claimed in any preceding claim in which the radiation-attenuation material comprises titanium and / or tungsten.

6. A material as claimed in any preceding claim in which the radiation-attenuation material comprises titanium and tungsten.

7. A material as claimed in claim 5 or claim 6 in which the tungsten is in the form of tungsten carbide.

8. A material as claimed in any preceding claim in which the polymer matrix is thermoplastic.

9. A material as claimed in any preceding claim in which the polymer matrix comprises any of polycarbonate, polyether ether ketone) and polytetrafluoroethylene.

10. A material as claimed in any preceding claim further comprising fibrous reinforcement.

11. A material as claimed in any preceding claim further comprising any one or more of the following, incorporated in the polymer matrix:carbon fibre,para-aramid fibre,poly-paraphenylene terephthalamide fibre,boron carbide,silicon carbide.

12. A radiation-shielding item or component comprising a material as claimed in any of claims Ito 11.

13. A radiation-shielding item or component as claimed in claim 12, further comprising an electrically conductive outer layer.

14. A radiation-shielding item or component as claimed in claim 13 in which the electrically conductive outer layer comprises carbon nanotubes.

15. An item of body armour comprising material as claimed in any of claims 1 to 11.

16. A method of manufacture of a radiation-shielding material, the method comprisingproviding radiation-attenuation material in particulate form, providing polymer material in liquid form,blending the radiation-attenuation material with the polymer material to form a mixture and- setting of the polymer material.

17. A method as claimed in claim 16, further comprising extruding the mixture prior to setting of the polymer material.

18. A method as claimed in claim 17 in which the extrusion of the mixture forms a filament suitable for use in an additive manufacturing machine.

19. A method as claimed in any of claims 16 to 18 in which the radiation-attenuation material comprises titanium and tungsten.

20. A method as claimed in claim 19 in which the tungsten is in the form of tungsten carbide.

21. A method as claimed in any of claims 16 to 20 in which the polymer material is thermoplastic and the provision of the polymer material in liquid form involves heating the polymer material.

22. A method as claimed in any of claims 16 to 21 in which the polymer material comprises any of polycarbonate, polyether ether ketone) and polytetrafluoroethylene.

23. A method of manufacture of a radiation-shielding item comprisingproviding radiation-attenuation material in particulate form,providing thermoplastic polymer matrix material,heating the matrix material to liquify it,blending the radiation-attenuation material with the liquified polymer material to form a mixture,5 - setting of the polymer material to form a solid radiation-shielding material, andforming the radiation-shielding material into a chosen shape (a) by liquifying the radiation-shielding material by heating and then moulding the radiation-shielding material or (b) by additive manufacturing.

24. A method as claimed in claim 23 in which, prior to the setting of the polymer material to 10 form the solid radiation-shielding material, the mixture is extruded to form a filament, and in which forming the radiation-shielding material into the chosen shape comprises fused deposition modelling using the filament.

25. A radiation-shielding item produced by the method of claim 23 or claim 24.

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