RADIATION SENSORS

Abstract: A radiation sensor comprising a field effect transistor (1) is disclosed. The transistor (1) comprises a substrate (2) and a doped gate layer (3) on the substrate (2). An insulating layer (4) is provided on the gate layer (3), and interdigitated source (5) and drain (6) regions are provided on the insulating layer (4). An active layer (7) including at least one organic semiconductor material is arranged between the source and drain regions. Ionising radiation incident on the detector causes changes in the electrical properties of the detector, and the electrical properties provide an indication of the integrated radiation dose incident upon the detector.

Description: RADIATION SENSORS The present invention relates to radiation sensors, and relates particularly, but not exclusively, to sensors for detecting high energy ionising radiation. Semi-conductor radiation detectors are known in which excitation by radiation of electron-hole pairs n the depletion region of a semi-conductor device results in an electric current flow. However, such detectors give a transient response (i.e. representative of the instantaneous rate of irradiation) rather than a cumulative response (i.e. representative of the total amount of radiation over a particular interval) and further electronic apparatus is required to enable a cumulative reading to be obtained. Photographic film is commonly used as a detector of cumulative radiation over an interval However, because it is necessary to develop the film before taking a reading, each piece of film can only be used once and there is a delay before any reading is obtained. Readings are also affected by batch- to-batch variations in film processing It is also known to detect radiation by using thermo luminescent materials which trap electrons and holes generated in the material by radiation. Subsequent heating of the material releases the trapped charges, whicn can then re- combine radiatively, to emit light which can oe detected. Although such devices can be repeatedly used, the reading process erases the radiation dose information existing m the material, with the result that this information can only be read out of the device once. In addition, materials in which the accumulated reading does not decay at room temperature give weak luminescent signals, which necessitates an expensive measuring system. European patent application no. 0261286 relates to radiation detection by means of conjugated polymer films together with radiation sensitive materials in which optical absorption or conductivity changes associated with irradiation are measured. These techniques, however, are insensitive and are therefore unsuitable for many applications. Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art . According to an aspect of the present invention, there is provided a method of determining the total amount of ionising radiation irradiating an object over a predetermined interval, the method comprising: providing, at at least one location in the vicinity of the object for the duration of said interval, a respective field-effect transistor having an active layer including at least one organic semiconductor material, wherein the electrical characteristics of the or each field effect transistor are dependent upon the total amount of ionising radiation irradiating the organic semiconductor material; measuring predetermined electrical characteristics of the or each field effect transistor; and determining from said measured electrical characteristics the total amount of ionising radiation irradiating the object at each said location during the interval. By providing a field effect transistor having an active layer, a much enhanced sensitivity can be obtained. In a preferred embodiment, said measuring step is performed subsequently to removal of the or each field effect transistor from the vicinity of the object. The active layer is preferably arranged between source and drain regions of the or each field effect transistor. The measuring step may comprise measuring the source- drain current of the or each transistor while the gate voltage thereof is maintained at a predetermined level. This has the advantage of enabling a bias voltage chosen to give maximum sensitivity to be applied to the gate of the transistor. In a preferred embodiment, said measuring step comprises applying an ac signal having a predetermined frequency to the gate of the or each transistor and measuring the amplitude of the component of the source-drain current of said transistor at said predetermined frequency. The amplitude of the component of the source-drain current of the or each transistor may be measured by means of a lock-in amplifier controlled by the ac signal applied to the gate of said transistor. According to another aspect of the invention, there is provided a method of determining the total amount of ionising radiation irradiating an object over a predetermined interval, the method comprising: providing, at at least one location in the vicinity of the object for the duration of said interval, a respective radiation detector including at least one conjugated polymer, wherein the photoluminescence characteristics of the or each detector are dependent upon the total amount of ionising radiation irradiating the or each conjugated polymer; measuring photoluminescence characteristics of the or each detector; and determining from said measured photoluminescence characteristics the total amount of ionising radiation irradiating the object at each said location during the interval . The measuring step is preferably performed subsequently to removal of the or each detector from the vicinity of the object . In a preferred embodiment, the method further comprises the step of shielding a portion of the or each detector from the radiation and the measuring step includes comparing the measured luminescence characteristics of the shielded and unshielded portions of the or each detector. This provides the advantage of enhancing the sensitivity of the measurement . The method may further comprise the step of photo- oxidising the conjugated polymer. This has the advantage of enhancing the effect on the organic semiconductor material of irradiation. It is believed that irradiation of the organic semiconductor material results in scission of bonds in the material, many of which soon reform. Photo-oxidation of the material before the bonds can reform is believed to make the radiation induced changes more permanent and thereby enhance the sensitivity of the measurement . According to a further aspect of the invention, there is provided a field effect transistor for use in determining the total amount of ionising radiation irradiating an object over a predetermined interval, the transistor comprising: a substrate; a doped gate region on the substrate; an insulating layer on the gate region; separated source and drain regions on the insulating layer; an active layer including at least one organic semiconductor material between the source and drain regions; and a protective layer above the active layer; wherein the electrical characteristics of the transistor are dependent upon the total amount of radiation irradiating the organic semiconductor material . By providing a protective layer, this prevents ingress of air and other impurities which may tend to degrade the organic semiconductor material . The protective layer may be substantially transparent to the radiation to be detected. The protective layer may be a silicon dioxide film. The protective layer may be removable. This has the advantage of enabling subsequent photo- oxidation of the organic semiconductor material to be carried out . The transistor may further comprise a sensitising layer for generating secondary electrons in the organic semiconductor material . This provides the advantage of increasing the measurable response of the transistor to ionising radiation. The source and drain regions may comprise gold and / or chromium electrodes . In a preferred embodiment, the source and drain regions are in an interdigitated arrangement . This has the advantage of maximising the channel width of the transistor. According to a further aspect of the invention, there is provided a radiation detector for use in determining the total amount of ionising radiation irradiating an object over a predetermined interval, the detector comprising :- a substrate; an active layer comprising at least one conjugated polymer and arranged on the substrate; and a protective layer over the active layer, wherein the photoluminescence characteristics of the detector are dependent upon the total amount of ionising radiation irradiating the or each conjugated polymer. The distinctive advantage of conjugated polymers is that the excitonic state responsible for photoluminescence is mobile. This means that it normally samples a considerable volume before decaying (either radiatively or non-radiatively) . If it encounters a quenching defect (created by the ionising radiation) anywhere within the diffusion volume the photoluminescence from that exciton will be quenched. The ratio between the diffusion volume and the effective volume of the exciton represents a large amplification factor for the sensitivity of the photoluminescence to quenching defects. The protective layer is preferably a metal film. Alternatively, the protective layer may be a silicon dioxide film. The protective layer is preferably removable. The detector may further comprise a sensitising layer for generating secondary electrons in the conjugated polymer. In a preferred embodiment, the substrate is substantially transparent to photoluminescence emitted by the active layer. This has the advantage of enabling measurement of the photoluminescence characteristics of the detector to be carried out without necessitating removal of any protective layer on the detector. The detector may further comprise shielding means for shielding a portion of the detector from the radiation to be detected. In order that the invention may be better understood, preferred embodiments thereof will now be described n detail, by way of example only and not m any limitative sense, with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional elevation view of a field effect transistor embodying the present invention; Figure 2 is a plan view of the transistor of Figure 1 ; Figure 3 is a first embodiment of a measuring circuit including the transistor of Figure 1, Figure 4a is a graph of the current-voltage characteristics of the transistor of Figure 1 at various gate bias voltages before irradiation; Figure 4b is a graph of the current voltage characteristics of the transistor of Figure 1, corresponding to Figure 4a, after irradiation; Figure 4c is a graph, corresponding to Figure 4b, of the current voltage characteristics of the transistor of Figure 1, after a radiation dose of 69 kGy; Figure 5a is a second embodiment of a measuring circuit including the transistor of Figure 1, Figure 5b is a modification of the measuring circuit of Figure 5a; Figure 6 is a schematic view of a first embodiment of an apparatus for measuring the photoluminescence characteristics of a radiation detector embodying the present invention; Figure 7 is a graph of the relative photoluminescence efficiency of the radiation detector of Figure 6 exposed to X-rays of energy ranging from 8 keV to 40 keV. Figure 8a is a graph of the relative photoluminescence efficiency of the radiation detector of Figure 6 exposed to X- rays ranging m energy from 8 keV to 40 keV; Figure 8b is a graph, corresponding to Figure 8a, of the relative photoluminescence efficiency of the radiation detector of Figure 6 as a function of radiation dosage; Figure 9 is a second embodiment of a measuring apparatus for measuring the photoluminescence characteristics of a radiation detector embodying the present invention; and Figure 10 is a third embodiment of a measuring apparatus for detecting the photoluminescence characteristics of radiation detector embodying the present invention. Referring in detail to Figure 1, a field effect transistor 1 is constructed by doping an N-type silicon wafer 2 to provide a heavily N-doped layer 3 forming the gate layer of the transistor 1 and having a dopant density greater than 10 19 cm "3 for a depth greater than 500 nm. The surface of the gate layer 3 is then thermally oxidised to provide an insulating Sι02 layer 4 of thickness 150 nm. Source 5 and drain 6 electrodes are created in an mterdigitated arrangement (as shown m greater detail in Figure 2) by means of contact lithography, chromium being first deposited to a thickness of 30 nm, and then gold being deposited to a thickness of 50 nm. The mterdigitated electrodes 5, 6 have an active area of 3mm x 3mm and an electrode separation of 10 microns, an electrode width of 10 microns and a channel width of about 45cm. A contact pad is attached to each electrode 5, 6. A layer of conjugated polymer is constructed as described below. By "conjugated polymer" is meant a polymer which possesses a delocalised -jr - electron system along at least part of the polymer back-bone; - the delocalised 1 - - electron system confers semi -conducting properties to the polymer and gives it the ability to support positive and / or negative charge carriers with high mobilities along the polymer chain. A film of the polymer poly (p-xylene-o;- tetrahydrothiophene) chloride is formed over the insulating layer 4 and the separated source 5 and drain 6 electrodes by placing a few drops of a solution of the polymer in methanol onto the insulating layer 4 and spinning the substrate at high speed to create a thin solid film. The film is then chemically converted to poly (p-Phenylene vinylene) (PPV) by heating in a vacuum at 250 °C for 10 hours to produce a film 7 of thickness 110 nm. A non-conducting film 8 of thickness 50 nm is then formed over the polymer film 7 to encapsulate the device to prevent ingress of air, which may tend to degrade the conjugated polymer material. As will be appreciated by persons skilled in the art, the thickness of the encapsulation layer 8 needs to be sufficient to prevent degradation of the polymer film 7, but thin enough to be substantially transparent to the radiation to be detected. However, it is believed that by suitable choice of materials, the encapsulation layer 8 may alternatively enhance the response of the transistor 1 by increasing the energy deposition in the polymer film 7 due to secondary radiation from the encapsulation layer 8. It is believed that the source and drain electrodes similarly enhance the response of the transistor by acting as sources of secondary radiation. The operation of the device shown in Figures 1 and 2 will now be described with reference to Figure 3. In order to determine the total radiation dose irradiating the transistor 1 within a predetermined interval, the transistor 1 is connected in a measuring circuit 9 such that the source-drain current may be measured by any suitable measuring device 10 as will be known to persons skilled in the ar . The transistor 1 may be incorporated in a lapel badge type detector worn by personnel, and may be connected into the measuring circuit 9 at a convenient measuring station subsequent to irradiation. The source-drain voltage of the transistor 1 is held constant, and the gate voltage is held constant at a value providing the greatest sensitivity of measurement. The source-drain current is then measured and represents the total radiation absorbed by the device. It is believed that applying a bias voltage to the gate 3 of the transistor during read-out allows the appropriate setting of the minimum dopant density in the polymer layers 7 for efficient charge injection from the electrodes 5, 6, and that the gate bias voltage creates an accumulation layer allowing the mobility of the charge carriers to be measured even if the number density of carriers in the bulk of the material, and hence the conductivity, is very low. Referring to Figure 4a, which shows the current voltage characteristics of the device of Figure 1 at various gate bias voltages before irradiation, it can be seen that no appreciable field effect exists. After exposure to X-rays of energy 8.4 keV for 6 hours, as shown in Figure 4b, there is an appreciable field effect and the electrical characteristics of the device are indicative of the total radiation absorbed by the transistor, as shown also in Figure 4c. An alternative measuring circuit is shown in Figure 5a, in which the source-drain voltage of the transistor 1 is held constant and an AC signal voltage at a frequency f and constant amplitude, and a constant DC bias voltage, are applied to the gate of the transistor 1. The component of the source-drain current at frequency f is detected and the amplitude of this component is a measure of the cumulative radiation dose absorbed by the transistor 1. An alternative implementation of the measuring circuit of Figure 5a is shown in Figure 5b. The source-drain voltage of the transistor 1 is again held constant, and an AC signal voltage at a frequency f and constant amplitude, and a constant DC bias voltage, are applied to the gate of the transistor 1. The AC signal voltage applied to the gate of the transistor is also used to provide a reference signal to a lock-in amplifier connected to the source-drain circuit. This enables the component of the source-drain current at frequency f to be detected and its amplitude measured. Referring to Figure 6, a photolu inescent radiation detector 11 is formed by forming a film of the polymer poly (p- xylene-α-tetrahydrothiophene) chloride on a transparent substrate 13 comprising a spectrosil-B glass disk by spin coating from a solution of the polymer in methanol, and then chemically converting the film 12 to poly (p-phenylene vinylene) (PPV) by heating in a vacuum at 250°C for ten hours, in a similar manner to the formation of the active layer of the transistor 1 of Figure 1. An encapsulation layer 14 comprising an aluminium film is then formed on top of the photoluminescent layer 12. In a manner similar to the embodiment of Figure 1, the encapsulation layer 14 enhances the performance of the device by increasing the energy deposition in the photoluminescent layer as a result of secondary radiation. In order to measure the photoluminescence characteristics of the detector 11, the photoluminescent layer 12 is illuminated through the transparent substrate 13 by means of a light source 15 producing photons having energy above the optical absorption edge of the activated conjugated material used in the sensor 11, for example a blue light emitting diode (LED) . An optical filter 16 which is a coloured glass filter passing only light of wave-length 470 nm or less, is placed between the light source 15 and the detector 11. This prevents passage of light in the wavelength range of the photoluminescence of the device, so that the level of background light is reduced when the photoluminescence is subsequently measured. An optical filter 17, for example a spectrometer, is placed between the detector 11 and a CCD detector 18 and blocks light from the light source 15 which has been scattered from the detector 11 towards the detector 18. This also reduces the background when the photoluminescence is measured, and the photoluminescence signal is acquired by summing the output from those pixels of the CCD array of the detector 18 that correspond with the wavelength range 485 nm to 700 nm. The operation of the detector of Figure 6 is illustrated in Figure 7, which shows a plot of the relative photoluminescent quantum efficiency against radiation dosage when the detector is exposed to X-rays of energy 8.4 keV. Similarly, Figures 8a and 8b show the relative photoluminescence efficiency against radiation dosage when the detector 11 is exposed to X-rays ranging in energy from 8 keV to 40 keV. Referring to Figure 9, in which parts common to the embodiment of Figure 6 have like reference numerals but increased by 100, the radiation detector 111 differs from the detector 11 of Figure 6 in that a shield layer 120 is provided over one region of the detector 111 using material generally opaque to the radiation to be detected, and a sensitising layer 121 using additional layers or blends sensitises the region of the detector 111 thereunder to radiation. The apparatus is set up so as to measure the photoluminescence efficiency at a spot -lion the detector ill, and the detector is spun at a frequency F so as to bring the shielded and sensitised regions alternately under that spot. The component of the photoluminescence signal measured at frequency F is then a measure of the total amount of radiation absorbed by the detector. Referring to Figure 10, in which parts common to the embodiment of Figure 6 have like reference numerals but increased by 200, the detector 211 is provided with a shielding layer 220 and a sensitising layer 221 in the form of alternating stripes of a particular width. The detector 218 captures an image of the detector 211 using the photoluminescent light emitted, and numerical techniques which will be known to person skilled in the art, such as spatial filtering, extract the component of the photoluminescent signal corresponding to the pattern of the sensitising layers. Alternatively, the optical apparatus can be arranged so as to measure the photoluminescence efficiency at a spot on the detector 211, and the spot is scanned across the device. By moving the spot and / or the detector 211 the spot is scanned across the device and the time varying signal, together with the scan speed and direction can be used to calculate the size of the component of the photoluminescence signal corresponding to the sensitised layers. All of the embodiments described with reference to Figures 1 to 10 can be modified by making the encapsulating layer 8, 14, 114 or 214 removable to allow oxidation of the conjugated polymer layer after irradiation by the radiation to be detected. This has the effect of amplifying or making more permanent the effect of defects created in the polymer layer by irradiation, and thus improves the sensitivity of the subsequent measurements. For example, the operation of the embodiment described with reference to Figures 6 to 10 is enhanced by removing the encapsulation layer subsequently to irradiation, and then exposing the device to a reactive fluid, such as oxygen and / or water and simultaneously to light of wave length shorter than the optical absorption edge for the active conjugated material. As will be appreciated by persons skilled in the art, similar treatment can enhance the electrical measurements obtained from the embodiment of Figures 1 to 5. It is also believed that such modification and / or irradiation may improve the performance of gas sensors having conductive polymer layers. The optimum film thicknesses for all of the embodiments described above may be determined for the particular use of the device by means of a suitable modelling technique such as the Monte-Carlo technique. An example of such a technique is described in "Breiermeister J F, Ed, 1993 MCNP - A General Monte Carlo N-Particle Transport Code, Version 4A Los Alamos Report: LA-12625". In addition, the performance of the embodiments of Figures 1 to 5 may be enhanced by the use of one or more sensitising layers of lead or other suitable material, provided in the substrate for generating further secondary radiation which increases the energy deposition in the active layer. In addition, the performance of the embodiments of Figures 6, 9 and 10 may be enhanced by the use of a sensitising layer of lead in the encapsulation layer, or a transparent sensitising layer provided in the substrate 13, 113, 213. It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications may be made without departure from the scope of the invention as defined by the appended claims .

Claims: CLAIMS : 1. A method of determining the total amount of ionising radiation irradiating an object over a predetermined interval, the method comprising: providing, at at least one location in the vicinity of the object for the duration of said interval, a respective field-effect transistor having an active layer including at least one organic semiconductor material, wherein the electrical characteristics of the or each field effect transistor are dependent upon the total amount of ionising radiation irradiating the organic semiconductor material; measuring predetermined electrical characteristics of the or each field effect transistor; and determining from said measured electrical characteristics the total amount of ionising radiation irradiating the object at each said location during the interval . 2. A method according to claim 1, wherein said measuring step is performed subsequently to removal of the or each field effect transistor from the vicinity of the object. 3. A method according to claim 1 or 2 , wherein the active layer is arranged between source and drain regions of the or each field effect transistor. 4. A method according to any one of the preceding claims, wherein said measuring step comprising measuring the source-drain current of the or each transistor while the gate voltage thereof is maintained at a predetermined level. 5. A method according to any one of claims 1 to 3 , wherein said measuring step comprises applying an ac signal having a predetermined frequency to the gate of the or each transistor and measuring the amplitude of the component of the source-drain current of said transistor at said predetermined frequency. 6. A method according to claim 5, wherein the amplitude of the component of the source-drain current of the or each transistor is measured by means of a lock-in amplifier controlled by the ac signal applied to the gate of said transistor. 7. A method of determining the total amount of ionising radiation irradiating an object over a predetermined interval, the method comprising: providing, at at least one location in the vicinity of the object for the duration of said interval, a respective radiation detector including at least one conjugated polymer, wherein the photoluminescence characteristics of the or each detector are dependent upon the total amount of ionising radiation irradiating the or each conjugated polymer,- measuring photoluminescence characteristics of the or each detector; and determining from said measured photoluminescence characteristics the total amount of ionising radiation irradiating the object at each said location during the interval . 8. A method according to claim 7, wherein said measuring step is performed subsequently to removal of the or each detector from the vicinity of the object. 9. A method according to claim 7 or 8, further comprising the step of shielding a portion of the or each detector from the radiation and the measuring step includes comparing the measured luminescence characteristics of the shielded and unshielded portions of the or each detector. 10. A method according to any one of claim 7 to 9, further comprising the step of photo-oxidising the conjugated polymer. 11. A field effect transistor for use in determining the total amount of ionising radiation irradiating an object over a predetermined interval, the transistor comprising: a substrate; a doped gate region on the substrate; an insulating layer on the gate region; separated source and drain regions on the insulating layer; an active layer including at least one organic semiconductor material between the source and drain regions; and a protective layer above the active layer; wherein the electrical characteristics of the transistor are dependent upon the total amount of radiation irradiating the organic semiconductor material . 12. A transistor according to claim 11, wherein the protective layer is substantially transparent to the radiation to be detected. 13. A transistor according to claim 11 or 12, wherein the protective layer comprises a silicon dioxide film. 14. A transistor according to any one of claims 11 to 13, wherein the protective layer is removable. 15. A transistor according to any one of claims 11 to 14, further comprising a sensitising layer for generating secondary electrons in the organic semiconductor material. 16. A transistor according to any one of claims 11 to 15, wherein the source and drain regions comprise gold and / or chromium electrodes. 17. A transistor according to any one of claims 11 to 16, wherein the source and drain regions are in an interdigitated arrangement . 18. A transistor according to any one of claims 11 to 17, wherein the organic semiconductor material comprises at least one conjugated polymer. 19. A radiation detector for use in determining the total amount of ionising radiation irradiating an object over a predetermined interval, the detector comprising :- a substrate; an active layer comprising at least one conjugated polymer and arranged on the substrate ; and a protective layer over the active layer, wherein the photoluminescence characteristics of the detector are dependent upon the total amount of ionising radiation irradiating the or each conjugated polymer. 20. A detector according to claim 19, wherein the protective layer comprises a metal film. 21. A detector according to claim 19, wherein the protective layer comprises a silicon dioxide film. 22. A detector according to any one of claims 19 to 21, wherein the protective layer is removable. 23. A detector according to any one of claims 19 to 22, further comprising a sensitising layer for generating secondary electrons in the conjugated polymer. 24. A detector according to any one of claims 19 to 23, wherein the substrate is substantially transparent to photoluminescence emitted by the active layer. 25. A detector according to any one of claims 19 to 24, further comprising shielding means for shielding a portion of the detector from the radiation to be detected. 26. A method of determining the total amount of ionising radiation irradiating an object over a predetermined interval, the method substantially as hereinbefore described with reference to the accompanying drawings. 27. A field effect transistor substantially as hereinbefore described with reference to the accompanying drawings . 28. A radiation detector substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: ALDERSON ANDREW, FRIEND RICHARD HENRY, GRAHAM STEPHEN CHARLES, ALDERSON, ANDREW, FRIEND, RICHARD, HENRY, GRAHAM, STEPHEN, CHARLES
Applicants: ALDERSON ANDREW, BRITISH NUCLEAR FUELS PLC, FRIEND RICHARD HENRY, GRAHAM STEPHEN CHARLES, ALDERSON, ANDREW, FRIEND, RICHARD, HENRY, GRAHAM, STEPHEN, CHARLES
Application year: 1997
Main classification: H01L27
IPC codes: G01J1, G01T1, H01L27, H01L29, H01L51