Diamond radiation detector

Abstract

In a radiation detector using a diamond crystal produced by a conventional heteroepitaxial method, the quality of the used crystal is not sufficient. A diamond crystal used in a radiation detector according to the present invention is heteroepitaxially grown by a chemical vapor deposition method on a substrate of a material other than diamond having a plane orientation inclined by a predetermined offset angle from a front orientation. And has crystallinity such that the half-width of the diffraction peak of the (004) plane of X-ray diffraction shows a value of 200 seconds or less.

Description

Technical Field

[0001] The present invention relates to a radiation detector using a diamond crystal for detecting radiation. Background Art

[0002] Radiation detectors that detect radiation such as Alpha (α) rays, Beta (β) rays, Gamma (γ) rays, and neutron rays are used in various occasions such as medical use and environmental measurement. Radiation detectors include scintillation type, GM tube type, ionization chamber type, semiconductor type, etc., which are used according to the purpose. In recent years, research on semiconductor detectors using Ge and Si that have excellent time responsiveness and energy resolution and can detect positions with high precision has become increasingly popular. However, these semiconductor detectors need to be cooled to low temperatures by liquid nitrogen before use, so there is a disadvantage that the detector is large and expensive as a whole.

[0003] In contrast, radiation detectors using diamond, a bandgap semiconductor, have attracted much attention in recent years due to their excellent characteristics, such as excellent radiation and heat resistance, fast time response due to high carrier mobility, an effective atomic number close to that of a living organism, and the ability to operate at room temperature.

[0004] As diamonds used in conventional radiation detectors, high-quality diamonds selected from natural single-crystal diamonds, high-quality single-crystal diamonds produced by HPHT (High Pressure and High Temperature) method, or high-quality single-crystal diamonds grown by CVD (Chemical Vapor Deposition) method using high-quality single-crystal diamonds as seeds can be used. It has been confirmed that radiation detectors using these high-quality single-crystal diamonds have excellent characteristics (Non-Patent Document 1).

[0005] In order to use diamond crystal as a radiation detector, single crystal diamond was initially used, but radiation cannot use lenses, and single crystal diamond is expensive and small in area, so there is a disadvantage of being difficult to improve sensitivity. Although small single crystal diamond crystals can also be arranged in a tiled shape to increase the area, the extraction of signals and the electrode structure become complicated and difficult to integrate. Polycrystalline diamond is easier to increase the area than single crystal diamond, so there is also an example of using polycrystalline diamond doped with boron to develop neutron detectors (patent document 1). However, in polycrystalline, due to the influence of grain boundaries and defects, the original excellent properties of diamond as above cannot be brought into play, and compared with single crystal diamond, the collection efficiency and resolution can only obtain lower values.

[0006] In order to make a highly sensitive radiation detector, the size of the diamond crystal is necessary. Therefore, the research on making a thin film of diamond on the surface of a crystal diamond or other base material by chemical vapor deposition (CVD) using gas as a raw material and applying it to the detection substrate of a radiation detector has become very popular. Among them, the heteroepitaxial growth method, which grows diamond on a substrate other than diamond instead of homoepitaxially growing diamond on a diamond single crystal, can achieve the size of the diamond substrate. For example, a plurality of columnar diamonds composed of diamonds are formed on an MgO substrate via an iridium (Ir) buffer layer using a microwave plasma CVD method, diamond crystals are grown from the front end of each columnar diamond, and each diamond crystal is integrated to form a diamond substrate layer, thereby obtaining a crystal with a diameter of 2 inches (Patent Document 2).

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 5-150051

[0010] Patent Document 2: Japanese Patent Publication No. 6142145

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 2011-155189

[0012] Non-patent literature

[0013] Non-patent document 1: Teruya Tanaka et.al, Diamond radiation detector made of anultrahigh-purity type IIa diamond crystal grown by high-pressure and high-temperature synthesis, Review of Scientific Instruments Vol.72,1406(2001)

[0014] Non-patent literature 2: S. Kim, R. Takaya, S. Hirano, and M. Kasu, Two-inch high-quality (001) diamond heteroepitaxial growth on sapphire (11-20) misoriented substrate by step-flow mode, Appl. Phys. Express, Vol. 14, 11551 (2021) Summary of the invention

[0015] Patent Document 3 discloses an example of a radiation detector using a diamond crystal produced by heteroepitaxial growth. The diamond layer grown by CVD on the Ir layer contains a high concentration of defects at the interface with Ir, so after peeling off from Ir, the defective layer on the Ir interface side of the diamond layer is removed by mechanical grinding and dry etching by 20 μm, and then a detector is produced to detect α rays.

[0016] However, even if the defective layer of the diamond layer on the interface side with the growth substrate is removed by 20 μm, defects will remain further inside (Patent Documents 3, Figure 2 ), in order to completely remove defects, it is necessary to remove a considerable portion of the thickness of the grown diamond layer by mechanical grinding or plasma etching, and the utilization efficiency of the crystal becomes very poor. Therefore, in order to obtain a high-quality diamond layer without the cumbersome process of removing the defects of the diamond layer and with a low defect density, the crystal needs to be of sufficiently high quality at the stage where epitaxial growth ends.

[0017] The inventors of the present application have found that if a sapphire (α-Al 2 O 3 ) is formed on a substrate whose (11-20) plane is tilted at a small angle, and a diamond layer is grown thereon, so that crystal growth is performed in a step-flow mode, and as a result, a high-quality crystalline diamond layer with little strain can be obtained (Non-Patent Document 2). Self-supporting flat-plate diamond crystals were made by this method and applied to radiation detectors. It was found that although the performance was inferior to that of radioactive detectors using small high-quality single crystal diamonds, it showed sufficient response characteristics for gamma rays and other radiation in practical use.

[0018] The radiation detector using the diamond of the present invention as a detector is configured as follows.

[0019] The radiation detector is composed of a diamond detector and a signal processing unit. The diamond detector is a flat diamond with its side surfaces insulated and electrodes provided on both surfaces. When radiation is incident, electric charges are generated. The signal processing unit performs digital processing using the electric charges as input signals.

[0020] The diamond is a diamond crystal of a heteroepitaxial growth layer and has such crystallinity that the half maximum width of a diffraction peak of the (004) plane in X-ray diffraction shows a value of 200 seconds or less.

[0021] The diamond crystal is a heteroepitaxial growth layer having a plane orientation tilted at a small angle on a substrate of a material other than diamond, and is peeled off and cut from the substrate.

[0022] In addition, the diamond crystals are tilted at a small angle from the plane orientation (001) to the

[110] direction.

[0023] Furthermore, the small-angle inclination angle (deviation angle) of the plane orientation of the diamond crystal is preferably 7° to 10°.

[0024] Furthermore, in order to improve the sensitivity to neutrons, a diamond layer doped with boron (B) may be further provided on the surface of the diamond crystal on which the radiation is incident.

[0025] The signal processing unit preferably includes a charge sensitive preamplifier for amplifying the charge, a digital converter for acquiring the output voltage signal, and a computer (PC) for obtaining the energy spectrum of the radiation.

[0026] The present invention also includes a method for manufacturing the diamond radiation detector.

[0027] The diamond crystal used in the above-mentioned diamond detector is tilted at a small angle from the plane orientation (001) to the

[110] direction and produced by the heteroepitaxial growth method.

[0028] The diamond crystal is peeled off from the substrate to cut out a flat plate-shaped self-supporting diamond crystal,

[0029] The side surfaces of the self-supporting diamond crystal are insulated.

[0030] The diamond detector is manufactured by arranging electrodes on the upper and lower surfaces of the self-supporting diamond crystal.

[0031] The diamond detector is connected to a signal processing unit configured to digitally process the input signal using the electric charge generated in the diamond crystal when radiation is incident thereon as an input signal.

[0032] The diamond detector of the present invention is easy to manufacture using a self-supporting heteroepitaxially grown diamond crystal of superior quality to conventional heteroepitaxially grown diamond crystals, and can provide a highly sensitive radiation detector at a lower price than conventional radiation detectors using high-quality single crystal diamond. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a diagram schematically showing the structure of the diamond detector of the present invention.

[0034] Figure 2 It is a diagram schematically showing the structure of the diamond detector for position detection according to the present invention.

[0035] Figure 3It is a block diagram showing the overall structure of the diamond radiation detector of the present invention.

[0036] Figure 4 The diamond radiation detector of the present invention detects 60 Co. 137 Cs, 133 Gamma-ray spectrum of Ba nuclide.

[0037] Figure 5 The results are shown in Figure 2 when the applied voltage of the diamond radiation detector of the present invention is increased from 100V to 500V by 100V. 137 A graph showing the change in the gamma-ray spectrum of Cs.

[0038] Figure 6 The results are shown in the case where the thickness of the diamond crystal of the present invention is changed to 500 μm, 300 μm, and 100 μm, the applied voltage is 500 V, 300 V, and 100 V respectively, and the internal electric field is kept constant. 137 A graph showing the changes in the gamma-ray spectrum of Cs.

[0039] Figure 7 The diamond crystal thickness of the present invention is 100 μm and the applied voltage is 100 V. 60 Co. 137 Cs, 133 Plot of the gamma-ray spectrum of Ba and the background (BG) spectrum.

[0040] Figure 8 This is a diagram comparing the gamma-ray spectrum of the diamond radiation detector of the present invention with the gamma-ray spectrum of a diamond radiation detector produced using the highest quality commercially available diamond crystal.

[0041] Fig. 9 The graphs show gamma-ray spectra of a radiation detector using the diamond crystal of the present invention grown on a substrate having a plane orientation at different off-angles.

[0042] Fig.10 This is a graph that plots the X-ray diffraction peaks of the (004) plane and the (311) plane of a diamond crystal at off-angles varying from 0° to 10°. DETAILED DESCRIPTION

[0043] The following is a detailed description of the embodiments of the present invention using examples. The diamond crystal used in the present invention is a substrate having a surface cut from the (11-20) plane of sapphire and tilted at a small angle toward the [1-100]m direction or the

[0001] c direction, on which an iridium (Ir) buffer layer is formed, and then the surface is heated by methane (CH 4) as the raw material gas, and the growth is carried out at a substrate temperature of 1000°C (Non-Patent Document 2). By using a substrate with an offset angle, a step of atomic size is formed on the growth surface, and the carbon atoms reaching the surface diffuse in the lateral direction and are taken to the step to grow in a step flow mode, resulting in a high-quality diamond crystal with a diameter of 2 inches with few defects. The diamond crystal grown in this way has a crystal orientation that is tilted at a small angle from the plane orientation (001) to the

[110] direction.

[0044] In order to evaluate the crystallinity of the grown diamond crystal, the diffraction peak of the (004) plane was measured by X-ray diffraction. The diffraction peak half-width (FWHM) was less than 200 seconds, and the best crystal obtained a value as small as 98 seconds (Non-patent Document 2). Although this value is slightly worse than the FWHM of the highest quality homoepitaxially grown diamond of less than 50 seconds, it is significantly improved compared with the value of the previous heteroepitaxially grown diamond crystal (less than 300 seconds) (Patent Document 2). In addition, even in the spectrum measurement of Raman spectroscopy, which is another method for evaluating crystallinity, the 1332cm －1 The FWHM of the peak is also 2.6 to 2.7 cm －1 This is smaller than the value in Patent Document 3. Figure 2 The small value (>4) shown reflects good crystallinity. In addition, the nitrogen concentration required for the radiation detector is 3 ppb or less.

[0045] In addition, with respect to the area of ​​the crystal, the size of the commercially available diamond crystal obtained by homoepitaxial growth is 4.5 mm square, while the diamond detector of the present invention can easily obtain diamond crystals of any size by cutting out wafers of about 2 to 4 inches in diameter, and can make a diamond detector using the diamond. In the following embodiment, a 4 mm square is cut out by laser to make a detector. At this time, the cut surface is graphitized by the heat of the laser and becomes conductive, so the side surface is insulated by grinding the side surface. In addition, since the diamond crystal used in the present invention generates tensile stress inside during the growth process, it is easy to peel off from the substrate after growth. Therefore, a self-supporting flat diamond crystal can be obtained by a simple process.

[0046] Figure 1 This is a schematic diagram showing the structure of the first diamond detector 110 used in the radiation detector of the present invention. (A) shows its cross-sectional view, and electrodes 2 and 3 of gold (Au) or titanium / gold (Ti / Au) are set on the upper and lower surfaces of a self-supporting diamond crystal 1 of 4 mm square and 100 to 500 μm thick. When a high DC voltage is applied to the upper and lower electrodes, electron-hole pairs are generated due to the radiation absorbed in the diamond crystal, and these charges reach the electrodes respectively through the electric field, and their charge amounts can be measured.

[0047] Figure 2 This is a schematic diagram showing the structure of the second diamond detector 120 used in the radiation detector for position detection of the present invention. (A) shows its cross-sectional view. Similar to the first diamond detector 110, Au or Ti / Au electrodes 4 and 5 are provided on the upper and lower surfaces of a self-supporting diamond crystal 1 of 4 mm square and 100 to 500 μm thick. When a DC high voltage is applied to the upper and lower electrodes, electron-hole pairs are generated due to the radiation absorbed in the diamond crystal, and these charges reach the electrodes respectively through the electric field, and their charge amounts can be measured. As shown in (B) and (C), the upper surface electrode 4 and the lower surface electrode 5 of the diamond crystal respectively form a plurality of parallel lines, and are arranged in a matrix of N rows and M columns with the directions of the upper and lower linear electrodes being at right angles to each other. Electrons and holes are generated due to the absorbed radiation, and are sent as charge signals to the signal processing unit 30 (from the electrodes at the positions (N, M) respectively reached by the electric field. Figure 3 ). Thus, the incident position of the radiation can be detected. The number of N and M can be set in a way that the spatial resolution is optimal.

[0048] The diamond detectors 110 and 120 of the present invention can also detect neutron rays. In this case, a boron-containing ( 10 B) Lithium ( 6 Preferably, if a diamond layer doped with boron is formed into a film, the conversion layer can be formed uniformly on the incident surface, so there is preferably no loss of charge collection.

[0049] The diamond radiation detector 200 of the present invention is composed of a diamond detector 110 or 120, a high voltage power supply 20, and a signal processing unit 30 for digitally processing the detection signal. Figure 3 . A high DC voltage is applied between the upper and lower electrodes 2, 3 or between the electrodes 4, 5 of the diamond detector 10 by a high voltage power supply 20, and the induced charge generated by the absorption of radiation is extracted from the electrodes. It is used as an input signal, amplified by a charge-sensitive preamplifier 32 and output as a voltage signal. Then, after the signal waveform is acquired by a digital converter 34, the signal waveform is digitally filtered in a computer (PC) 36 to perform waveform shaping and wave height analysis. Then, the wave height value of each channel is obtained and the energy spectrum is displayed. The structure for converting the input charge signal into an energy spectrum is not limited to the above-mentioned embodiment, and any structure can be used.

[0050] As an example of the performance evaluation of the diamond radiation detector, gamma ray detection was performed. The results are shown in Figure 4The thickness of the diamond crystal is 500 μm and the applied voltage is 500 V. The vertical axis is the logarithm of the count value of each channel, corresponding to the signal intensity, and the horizontal axis is the channel number, which is proportional to the energy of the gamma ray. It is known that a typical gamma ray source that can clearly distinguish different energies is 60 Co (1.17, 1.33 MeV), 137 Cs(662keV), 133 The nuclide of Ba(356keV).

[0051] Figure 5 It means that Figure 4 When the applied voltage is gradually increased from 100V to 500V in increments of 100V for the same detector with a thickness of 500μm, the 137 Changes in the γ-ray spectrum of Cs show that the charge collection efficiency can be improved as the applied voltage increases.

[0052] Figure 6 This indicates the case where the thickness of the diamond crystal is changed. 137 Changes in the gamma-ray spectrum of Cs. When the thickness is 500μm, 300μm, and 100μm, the applied voltage is 500V, 300V, and 100V respectively, so that the internal electric field is constant at 1000V / mm. It can be seen that even if the electric field strength is constant, as the thickness of the diamond crystal becomes thinner, the wave height value increases, and the charge collection efficiency also increases. In addition, as an advantage of the thinning of the diamond crystal, the short growth time of the crystal also has a great advantage in production cost.

[0053] Figure 7 The diamond crystal thickness is 100 μm and the applied voltage is 100 V. 60 Co. 137 Cs, 33 The gamma ray spectrum of Ba and the background (BG) spectrum. If the diamond layer thickness decreases, the absorption of gamma rays in the diamond layer decreases, so the detection count value decreases regardless of the gamma ray of any nuclide. When measuring alpha rays, charged particles, and neutrons, gamma rays may become noise, so their output needs to be suppressed. Figure 6 As shown, by reducing the layer thickness to 100 μm or less, the charge collection efficiency is improved, thereby achieving a high S / N detector for measuring α rays, charged particles, and neutrons. The diamond layer thickness can be set according to the type of radiation to be measured.

[0054] The gamma-ray spectrum of the diamond detector used in the present invention was compared with the gamma-ray spectrum of a diamond detector of the same structure produced using the highest quality commercially available single crystal diamond (produced by Element Six) produced by homoepitaxial growth. The results are shown in Figure 8 The thickness of the diamond single crystal was 500 μm and the applied voltage was 500 V. Although the wave height value was inferior to that of a detector using homoepitaxially grown diamond, it was found that it had sufficient characteristics as the performance of a general radiation detector.

[0055] Fig. 9 Figure 2 shows the detector using diamond crystals grown on substrates at different offset angles. 137 The gamma-ray spectrum of Cs. The applied voltage is 500V. If a diamond detector with the same thickness (500μm) is used for comparison, the charge collection is better when the deviation angle of the diamond crystal is 7° than when it is 5°. It should be noted that at the same deviation angle of 7°, if the thickness is increased to 800μm, the charge movement distance becomes longer and the wave height value decreases. As shown in Non-Patent Document 2 Figure 3 As shown in (a), it can be considered that the deviation angle dependence is consistent with the result that the FWHM value of the diffraction peaks of the (004) plane and the (311) plane of X-ray diffraction is smaller at a deviation angle of 5° than at 7°, indicating good crystallinity. Fig.10 This is a graph showing the results of measuring the FWHM of the X-ray diffraction peaks of the (004) plane and the (311) plane for diamond crystals produced with the deviation angle changed from 0° to 10°. As the deviation angle increases from 0°, the FWHM shows a decreasing trend, and within this measurement range, the peaks of the (004) plane and the (311) plane are saturated to a smaller value at 7 to 10°, and the crystallinity is improved. Therefore, the deviation angle of the diamond crystal used in the radiation detector of the present invention is preferably 7 to 10°.

[0056] It should be noted that although the present invention has been described based on the above-mentioned embodiments, the present invention is not limited to these embodiments, and various changes and modifications can be made within the spirit of the present invention and the scope of the appended claims.

[0057] Explanation of symbols

[0058] 1 Diamond Crystal

[0059] 2.4 Upper surface electrode

[0060] 3.5 Lower surface electrode

[0061] 20 High voltage power supply

[0062] 30 Signal Processing Unit

[0063] 32 Charge sensitive preamplifier

[0064] 34 Digital Converter

[0065] 36 Computer (PC)

[0066] 110 Diamond Detector

[0067] 120 Diamond detector for position detection

[0068] 200 Diamond Radiation Detector

Claims

1. A radiation detector using diamond in the detector. The radiation detector is composed of a diamond detector and a signal processing unit. The diamond detector is a diamond detector in which the side surfaces of the flat diamond are insulated, electrodes are provided on the upper and lower surfaces, and electric charges are generated when radiation is incident. The signal processing unit performs digital processing on the charge as an input signal. The diamond is a diamond crystal of a heteroepitaxial growth layer and has crystallinity such that the half maximum width of a diffraction peak of a (004) plane in X-ray diffraction shows a value of 200 seconds or less.

2. The radiation detector according to claim 1, wherein: The diamond crystal is a heteroepitaxial growth layer on a substrate of a material other than diamond having an offset angle, and is a self-supporting diamond crystal cut and peeled off from the substrate in a flat plate shape.

3. The radiation detector according to claim 2, wherein: The diamond crystal is tilted at a small angle from the plane orientation (001) to the [110] direction.

4. The radiation detector according to claim 3, wherein: The deviation angle of the small-angle inclination of the plane orientation of the diamond crystal is 7° to 10°.

5. The radiation detector according to claim 4, wherein: The diamond crystal further includes a diamond layer doped with boron (B) on the surface on the radiation incident side.

6. The radiation detector according to claim 1 or 5, wherein: The signal processing unit includes a charge sensitive preamplifier for amplifying the charge, a digital converter for acquiring the output voltage signal, and a computer for obtaining the energy spectrum of the radiation.

7. A method for manufacturing a radiation detector, comprising: providing a diamond detector using diamond as a detector; The diamond crystal used in the diamond detector is tilted at a small angle from the plane orientation (001) to the [110] direction and produced by a heteroepitaxial growth method. The diamond crystal is peeled off from the substrate to cut out a flat plate-shaped self-supporting diamond crystal, insulating the sides of the self-supporting diamond crystal, The diamond detector is manufactured by arranging electrodes on the upper and lower surfaces of the self-supporting diamond crystal. The diamond detector is connected to a signal processing unit, and the signal processing unit is configured to use electric charges generated in the diamond crystal when radiation is incident as an input signal and to digitally process the input signal.

8. The method for manufacturing a radiation detector according to claim 7, wherein: The deviation angle of the small-angle inclination of the plane orientation of the diamond crystal is 7° to 10°.

9. The method for manufacturing a radiation detector according to claim 8, wherein: The diamond crystal has such crystallinity that the half maximum width of a diffraction peak of a (004) plane in X-ray diffraction shows a value of 200 seconds or less.

10. The method for manufacturing a radiation detector according to claim 9, wherein: The heteroepitaxial growth method is a plasma chemical vapor deposition method using methane as a raw material.

11. The method for manufacturing a radiation detector according to any one of claims 7 to 10, wherein: A diamond layer doped with boron (B) is further provided on the surface of the diamond crystal on the radiation incident side.

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