POLYMER NUCLEAR RADIATION DETECTOR

Field of the Invention

The present invention relates to the detection of nuclear radiation. More specifically, the present invention relates to a nuclear radiation detector, dosimetry apparatus and method of detecting nuclear radiation. The invention also extends to a polymer-nanoparticle composite or polymer nanocomposite (PNC) material for use in the detection of nuclear radiation and related methods of manufacture.

Background to the Invention

Known nuclear radiation detectors comprise semiconductor devices, typically of silicon or germanium. These solid-state devices detect nuclear radiation by measuring the number of charge carriers, e.g. electrons and holes, generated in the detector volume in response to incident nuclear radiation. Nuclear radiation may comprise energetic particles. When these particles collide with the semi-conductor, they tend to cause ionisation, and hence generate charge carriers, in the semiconductor. Photons originating from nuclear events can also be energetic enough to generate charge carriers in the semi-conductor. This can occur by processes known as the photoelectric effect, Compton scattering or pair production.

When a bias voltage is applied across the semiconductor, the charge carriers are accelerated under the influence of the electric field generated by the applied bias voltage. This leads to a current being generated across the semi- conductor and this current can be readily detected. By monitoring the current, or charge pulse, the existence of radiation incident on the detector can therefore be identified.

It is known that PNC materials can be suitable for use in photovoltaic applications, in particular for solar cells that generate electricity from solar radiation, as they can be made to behave in a similar way to silicon or germanium based semiconductors in the presence of certain wavelengths of light. However, the organic polymers of most PNC materials tend to have relatively low charge carrier mobility of the organic semiconductor, e.g. between around 10 ^"6 cm ² V ^"1 s ^"1 to 10 ^'1 cm ² V ^"1 s ^"1 . This means that, in order to extract a useful current, the PNC material is fabricated to be only a few micrometres thick. Such arrangements are not suitable for detecting nuclear radiation, which is too energetic to reliably generate charge carriers over such a small thickness of material. Rather, most nuclear radiation would pass straight through such a thin volume of PNC material without interaction.

Summary of the Invention

According to a first aspect of the present invention, there is provided a nuclear radiation detector comprising: a layer of polymer-nanoparticle composite material comprising a plurality of electrically conductive nanoparticles dispersed in an organic polymeric substance; wherein, in use, when the polymer-nanoparticle composite material is irradiated by nuclear radiation, the presence of the radiation is indicated by the generation of a current in the polymer-nanoparticle composite material. In this aspect, the present invention advantageously solves the abovementioned problems by demonstrating how to create a radiation detector using nanoparticles in an organic polymeric substance.

In the context of the invention, the term "nuclear radiation" is considered to extend to alpha particles, beta particles, X-rays, gamma rays and so on. It should also be noted that the term "polymer-nanoparticle composite" is considered to extend to any material comprising nanoparticles dispersed within a polymeric

substance, the nanoparticles having a diameter of between about 0.3 nm and 100 nm. Typically, the nanoparticles have an aspect ratio of between about 20 and 1000000. They might therefore be referred to as nanofibres or nanowires. In preferred embodiments of the invention, the nanoparticles are either single- or multi- walled' carbon nanotubes. Alternatively, the nanoparticles may be C-60 (Buckminsterfullerene). Preferably, the nanoparticles are semiconductive.

The organic polymeric substance may comprise a polymer or copolymer. Usually, the organic polymeric substance comprises any one of a long chain molecule, carbon atoms being covalently bonded along the backbone of the molecule; a conjugated polymer; or an acrylic copolymer composition. Preferably, the organic polymeric substance is formed from a dispersion of colloidal particles of these materials in either solvent or water. Typically, the particles have an average size of approximately 200 nm.

According to a second aspect of the present invention, there is provided a composite material comprising electrically conductive nanoparticles dispersed in an organic polymeric substance, wherein the weight fraction of nanoparticles to organic polymeric substance is from 0 %w to about 20%w. In preferred embodiments of the invention, the nanoparticles are either single- or multi-walled carbon nanotubes. Alternatively, the nanoparticles may be C-60 (Buckminsterfullerene). Preferably, the nanoparticles are semiconductive. Preferably, the weight fraction of nanoparticles to organic polymeric substance is from about 0.05 %w to about 2 %w. In this aspect, the present invention advantageously demonstrates how to provide a suitable polymer-nanoparticle composite material for use in detecting radiation.

A further advantage of the present invention is that the polymer-nanoparticle composite material is carbon based as opposed to known radiation detectors which are based on semiconductor materials such as silicon and germanium. This means that the polymer-nanoparticle composite material is better suited to act as a 'tissue

equivalent' model for human radiation absorption characteristics as compared to previous detectors.

A yet further advantage is that a thick film of the polymer-nanoparticle composite material can be fabricated simply using low-cost water based technologies such as screen printing, spin coating or painting which can take place at room temperature. Doctor blading and ink jet printing could also be used. There is no restriction on the size into which the film can be formed and so the polymer- nanoparticle composite material is therefore suitable for use in the production of large-area, cheap, radiation sensors. Furthermore, the film is both strong and flexible and can therefore be folded or bent into unusual shapes and other geometries. This allows the detector to be used in the production of 'active' textile radiation dosimeters, for example as "detector clothing" for workers who risk exposure to radiation. The polymer-nanoparticle composite material could be formed as a fibre and woven into the fabric of clothing to be worn by a person at risk of exposure to radiation. Likewise, the film could be used as the basis for a model of a curved section of the human body to which radiation is to be applied, or be a component of a bandage or such like over a body part being irradiated, with the purpose of quantifying the amount of radiation to which a patient is exposed during an irradiation procedure. A yet further advantage of the present invention is that the polymer- nanoparticle composite material is able to detect radiation directly. By detecting radiation directly, the effective lifetime of the detector is increased as compared to other detector systems which use secondary indicators to detect the presence of radiation, such as chemical changes or discolouration of the polymer-nanoparticle composite material itself. Given that the present invention operates without any chemical reaction or degradation occurring to the polymer-nanoparticle composite

material, the total amount of polymer-nanoparticle composite material available to detect radiation remains constant.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 shows a schematic diagram of a radiation detector according to the invention; Figure 2 shows a graphical representation of the response of the radiation detector against radiation dose at different reverse bias voltages, a PNC material of the detector having a percentage weight fraction of CNT of 2%w;

Figure 3 shows a graphical representation of current generated in the radiation detector against reverse bias voltage after different radiation doses, a PNC material of the detector having a percentage weight fraction of CNT of 2%w; and

Figure 4 shows a graphical representation of current generated in the radiation detector against time as the radiation dose rate applied to the detector is varied, a PNC material of the detector having a percentage weight fraction of CNT of 2%w.

Detailed Description of the Preferred Embodiments

Referring to figure 1 , in a first preferred embodiment of the invention, a radiation detector 1 comprises a layer 2 of polymer-nanoparticle composite (PNC) material on a substrate 3. On the surface of the layer 2 of PNC material facing away from the substrate 3 there is a top electrode 4 and between the layer 2 of PNC material and the substrate 3 there is a bottom electrode 5. The top and bottom

electrodes 4, 5 are connected to wires 6 that allow a voltage to be applied across the electrodes 4, 5 from a power source (not shown) and for a current between the electrodes to be detected by a detection circuit (not shown).

In this embodiment, the PNC material is a composite of an organic polymeric substance and carbon nanotubes (CNTs). The CNTs are either single- or multi- walled carbon nanotubes, having diameters between about 0.1 nm and 2 nm and lengths many hundreds of times that of their diameters. The organic polymeric substance comprises a dispersion of colloidal particles of an acrylic copolymer composition in water. It might therefore generally be referred to as 'latex'. One particularly suitable copolymer composition comprises ethyl hexyl acrylate, butyl methacrylate, and methyl methacrylate. However, other combinations of materials which, when combined, exhibit suitable characteristics for use in the PNC may also be used. In particular, it is envisaged that other suitable materials may be found within the families of the compounds mentioned above. The average size of the colloidal particles is of the order of 200nm and the glass transition temperature is of the order of 2O ⁰ C.

PNC materials having low electrical conductivity are most suitable for use in the nuclear radiation detector 1. As carbon nanotubes are electrically conductive, keeping the weight fraction of carbon nanotubes low helps to keep the conductivity of the PNC material low, although the electrical conductivity of a PNC material is in part also determined by the conductivity of the polymer. Bearing this in mind, in this embodiment, the percentage by weight of CNTs in the latex should typically be between 0.05%w and 2%w.

Manufacture of the radiation detector 1 can include manufacturing the PNC material. In order or achieve this, the CNTs can be produced by any suitable method, including chemical vapour deposition, arc discharge, laser ablation and so on. A solution of the organic polymeric substance in water is also prepared. The

desired percentage weight of CNTs is then added to the solution, which should be thoroughly mixed, using an automated device such as a sonicator or such like, to ensure the CNTs are well dispersed in the mixture. The mixture is then applied to the substrate 3 over the bottom electrode 5 by screen printing, spin coating, painting, doctor blading, ink jet printing or such like. Over time, water in the mixture evaporates to leave the layer 2 of PNC material. The evaporation is controlled such that the layer 2 of PNC material is between about 0.5 μm and 1000 μm thick and preferably between about 0.5 μm and 50 μm thick. The layer 2 of PNC material has a surface area (substantially parallel to the plane of the substrate) of about 10 mm ² to 25 mm ² .

In this embodiment, the substrate 3 is preferably a ceramic and the top and bottom electrodes 4, 5 are gold. The electrodes 4, 5 are about 10 nm to 50 nm thick. The top and bottom electrodes 4, 5 are formed by evaporation or such like, with the bottom electrode 5 being formed on the substrate 3 before the mixture forming the layer 2 of PNC material is applied and the top electrode being formed once the layer 2 of PNC material has been formed. The wires 6 are then attached . to the electrodes 4, 5.

In use, a voltage between about 1 V and 10 V is applied between the top and bottom electrodes 4, 5 via the wires 6 by the power source. The applied voltage creates an electric field in the layer 2 of PNC material, with a gradient extending between the electrodes 4, 5. When the radiation detector 1 is exposed to nuclear radiation, the radiation causes ionisation in the layer 2 of PNC material. Electrons generated by the ionisation tend toward the carbon nanotubes, which are highly conductive. Provided the electrons can be transferred from one carbon nanptube to another and permeate through the layer 2 of PNC material to reach one of the electrodes 4, 5, a current is generated in the detector 1. The current is proportional to the rate of ionisation in the layer 2 of PNC material.

In this embodiment, the radiation detector 1 is arranged as a dosimeter. In other words, the detector is arranged to monitor the total radiation dose to which it is exposed during a given period of time. This is achieved by the detection circuit monitoring the current generated across the electrodes 4, 5 during the given time period. Provided the detector 1 has been well calibrated, e.g. using controlled radiation doses in a laboratory, the monitored current can give a good indication of the radiation dose to which the detector 1 is exposed. For example, an indication of the dose can be provided by computer processing means by measuring a signal output by the circuit representing the current across the electrodes 4, 5 for the time period and comparing the result with a look up table of calibration values.

Alternatively, in other embodiments, real-time inspection of the current across the electrodes can be used to give an indication of the rate at which radiation is being applied to the detector 1.

Experimental results have been obtained for a radiation detector 1 , in which the PNC material is a composite of the above acrylic copolymer composition and 2%w CNT, exposed to X-ray radiation. Referring to figure 2, the current (in Cs ^"1 ) generated across the electrodes 4, 5 is seen to be linearly proportional to the dose rate (in mGy.s ^"1 ) of X-ray radiation to which the detector 1 is exposed. Furthermore, it can be seen that the sensitivity of the detector 1 , e.g. the ratio (in CmGy ^'1 ) of the current generated across the electrodes 4, 5 to the radiation dose rate increases with increased bias voltage across the electrodes 4, 5. More specifically, with a bias voltage of OV, the sensitivity of the detector 1 is 4.47x10 ^"12 CmGy ^"1 ; with a bias voltage of 2.5V, the sensitivity of the detector 1 is 2.53x10 ^"11 CmGy ^"1 ; with a bias voltage of 5V, the sensitivity of the detector 1 is 4.14x10 ^"11 CmGy ^"1 ; with a bias voltage of 7.5V, the sensitivity of the detector 1 is 5.43x10 ^"11 CmGy ^"1 ; and with a bias voltage of 10V, the sensitivity of the detector 1 is 6.60x10 ^"11 CmGy ^"1 .

Referring to figure 3, the dark current generated between the electrodes 4, 5 when the bias voltage was applied across the electrodes 4, 5 in the absence of the X-ray radiation was measured after the detector 1 had been exposed to different amounts of X-ray radiation. It can be seen that only a small dark current is present in the device, rising to a maximum of about 1.8x10 ^"11 A at a 10 V bias voltage.

Furthermore, the dark current does not vary significantly after the detector has been exposed to X-ray radiation, up to a 19700 mGy dose of X-ray radiation. This indicates that the composite of the above acrylic copolymer composition and 2%w CNT PNC material has good radiation hardness. In other words, the properties of the material do not appear to deteriorate significantly with exposure to X-ray radiation.

Referring to figure 4, the current generated across the electrodes 4, 5 was monitored as the X-ray radiation applied to the detector 1 was varied. Initially, the dose rate was set to 2.47 mGy.s ^"1 for 950 s. The X-ray radiation was then turned on and off repeatedly and finally reduced stepwise to 1.53 mGy.s ^"1 , 0.58 mGy.s ^'1 and 0.01 mGy.s ^"1 in turn. It can be seen that the current changed rapidly at each change in radiation dose rate and stabilised quickly. More specifically, the switching time of the current was faster than the switching time of the X-ray beam, estimated at ~1 s. This indicates that the detector 1 is suitable for detecting short term changes in radiation exposure or bursts of radiation exposure.

The radiation detector 1 is described above in relation to only some very specific choices of materials. However, the invention is not limited to these choices. For example, in another embodiment, the substrate 3 may be a glass sheet and the bottom electrode 5 may be a layer of Indium Tin Oxide (ITO). Naturally, in these embodiments, the detector 1 is rigid and stiff. However, this need not be the case in other embodiments. Rather, through judicious choice of materials, the radiation detector 1 can be flexible or deformable. This allows it to be moulded or shaped into

any of a number of desirable shapes, in one such embodiment, the detector 1 can be deformed to model a structure to which radiation is to be applied. In particular, the detector 1 can be shaped to take the form of another carbon based structure, such as a human or animal limb etc. In this way, the detector can be used to model how much radiation an object of equivalent shape would absorb during a particular irradiation regime. This is of particular use, for example, to a radiologist in devising an irradiation regime for a particular patient; following shaping of the sheet to model the patient's own shape (or a portion thereof), radiation can be applied and the detector 1 used to quantify the amount of radiation absorbed in the PNC material. The advantage of this arrangement is that because the PNC material is carbon based (i.e. 'tissue equivalent'), the amount of radiation absorbed by the PNC material could be used to accurately represent the amount of radiation that would be absorbed by a portion of a patient (human or animal etc.) having the same shape. In another embodiment, the detector 1 can be used to form of part of an item of clothing on a person (for example, the radiologist of the previous example). The clothing could then be used to detect the amount of radiation to which a person wearing the clothing was exposed. Furthermore, the clothing could take the form of a bandage or the like applied to a patient and could subsequently be used to detect the amount of radiation applied to the patient across that bandage during a radiation regime. In an alternative arrangement, the radiation detector could be formed as fibres by, for example, spinning, which could be woven together to form the clothes, bandages or the like indicated above.

It should be noted that in any of the above embodiments, the atomic mass of the PNC material may increased to improve the efficiency of gamma-ray detection by the addition of nano-particle metallic or semi-conducting materials.

It should be noted that it is envisaged that this technology could be used for the detection of radiation in a variety of other applications, including

satellite/spacecraft, and synchrotron biomolecular imaging, as well as in the nuclear power industry.

The described embodiments of the invention are only examples of how the invention may be implemented. Modifications, variations and changes to the described embodiments will occur to those having appropriate skills and knowledge. For example, it should be noted that, in any of the above embodiments, the atomic mass of the PNC material may increased to improve the efficiency of gamma-ray detection by the addition of nano-particle metallic materials or such like. These modifications, variations and changes may be made without departure from the scope of the invention defined in the claims and its equivalents.