Radiation detector and method for manufacturing same

Abstract

The present invention enables easy acquisition of an excellent electrical contact while using electrodes made of a metal other than thallium.　A radiation detector 1 is provided with: a crystal 2 containing thallium bromide as the main component; and electrodes 3 and 4 which are in contact with the crystal 2 so as to sandwich the crystal 2 therebetween in an X direction and which are made of a metal other than thallium. Portions of the crystal 2 in contact with the electrodes 3 and 4 are doped with at least one of strontium, lead, thallium, and bromine.

Description

Radiation detector and method of manufacturing the same

[0001] The present invention relates to a radiation detector and a method for manufacturing the same.

[0002] In radiation detectors used in environmental measurement, medical care, etc., research has been conducted into the use of crystals containing thallium bromide as a main material from the viewpoints of detection efficiency, cost reduction, etc. Patent Document 1 is an example of a radiation detector using such a crystal, and employs a structure in which a thallium bromide crystal is sandwiched between a pair of thallium electrodes.

[0003] Patent No. 5083964

[0004] Thallium is easily corroded by oxidation. Therefore, the detector's performance is likely to deteriorate when thallium is used for the electrode. However, when metals other than thallium are used for the electrode, the radiation detection performance depends on whether good electrical contact can be obtained between the electrode and the crystal.

[0005] An object of the present invention is to provide a radiation detector that uses electrodes made of a metal other than thallium and that can easily ensure good electrical contact, and a method for manufacturing the same.

[0006] The radiation detector according to the present invention comprises a crystal whose main component is thallium bromide, and first and second electrodes made of a metal other than thallium, which are in contact with the crystal so as to sandwich the crystal between them in one direction, and the portion of the crystal in contact with at least one of the first and second electrodes is doped with at least one of strontium, lead, thallium, and bromine.

[0007] According to the radiation detector of the present invention, a metal other than thallium is used for the electrodes. Therefore, deterioration of detection performance due to corrosion of thallium does not occur. Furthermore, the portion of the crystal that contacts the electrodes is doped with at least one of strontium, lead, thallium, and bromine. This makes it easy to ensure appropriate ohmic contact. As a result, a radiation detector that uses a metal other than thallium for the electrodes and that easily ensures good electrical contact is realized.

[0008] In the present invention, both the portion of the crystal in contact with the first electrode and the portion in contact with the second electrode may be doped with at least one of strontium, lead, thallium, and bromine, which makes it easier to ensure appropriate ohmic contact with both the first electrode and the second electrode.

[0009] In the present invention, it is also preferable that at least one of the first and second electrodes is made of gold, which makes it easier to ensure electrical conductivity between the electrode and the crystal.

[0010] In the present invention, the other of the first and second electrodes is preferably made of any one of gold, platinum, aluminum, indium, and chromium, which makes it easier to ensure ohmic contact between the electrode and the crystal.

[0011] Another aspect of the present invention relates to a method for manufacturing a radiation detector, which includes a crystal containing thallium bromide as a main component, and first and second electrodes in contact with the crystal so as to sandwich the crystal between them in one direction, wherein the crystal is partially doped with an element that forms ohmic contact between the crystal and at least one of the first and second electrodes, and at least one of the first and second electrodes made of a metal other than thallium is placed in contact with the portion of the crystal doped with the element.

[0012] According to the method for manufacturing a radiation detector of the present invention, a metal other than thallium is used for the electrodes. Therefore, deterioration of detection performance due to corrosion of thallium does not occur. Furthermore, the portion of the crystal that contacts the electrode is doped with an element that establishes ohmic contact between the crystal and the electrode. As a result, a radiation detector that uses a metal other than thallium for the electrodes and that easily ensures good electrical contact is realized.

[0013] Fig. 1 is a conceptual diagram showing the configuration of a radiation detector according to an embodiment of the present invention. Fig. 2 is a plan view of a sample used in an experiment to confirm the characteristics of electrical contact between the crystal and the electrodes in the radiation detector of Fig. 1. Fig. 3 is a graph showing experimental results of current-voltage characteristics when strontium is used as a dopant and various metals are used as electrodes in the sample of Fig. 2. Fig. 4 is a graph showing experimental results of current-voltage characteristics when lead is used as a dopant and various metals are used as electrodes in the sample of Fig. 2. Fig. 5 is a graph showing experimental results of current-voltage characteristics of a substrate surface when various substances are used as dopants in the substrate used in the sample of Fig. 2.

[0014] A radiation detector 1 according to one embodiment of the present invention will be described below with reference to FIG. 1 . The radiation detector 1 includes a crystal 2 and electrodes 3 and 4. The crystal 2 is mainly composed of bromine and thallium. The term "main components" refers to the presence of bromine and thallium in the crystal at a composition ratio of 0.3 or greater. In other words, when the total number of bromine and thallium atoms is X and the total number of atoms is Y, X / Y is 0.3 or greater. This allows the bromine and thallium in the crystal 2 to function as the main components for radiation detection ("Realization of Cherenkov PET with a Photon-Electron Hybrid Detector Using Transparent Semiconductors" (Keitarou Hitomi et al., 2019-2020, https: / / kaken.nii.ac.jp / ja / grant / KAKENHI-PROJECT-19K22942 / )).

[0015] Crystal 2 may contain elements other than the main component throughout. Elements other than the main component include those inevitably contained in the crystal during the synthesis of thallium bromide crystals, as well as elements other than bromine and thallium that are present in the crystal as a result of doping the thallium bromide crystal for various purposes, such as compensating for defects in the crystal. Inevitable impurities remaining in the crystal form donor or acceptor levels within the band gap, resulting in increased leakage current, i.e., noise sources. Doping for defect compensation refers to doping with a dopant that acts as an acceptor or donor to compensate for this level. A specific example of other doping is the chlorine doping described above in "Realizing Cherenkov PET with a photon-electron hybrid detector using a transparent semiconductor." This is done to make the crystal transparent by increasing the band gap of the crystal, thereby allowing Cherenkov light emitted when gamma rays interact with crystal 2 to pass through crystal 2 and be measured by an external photodetector.

[0016] Electrodes 3 and 4 are formed on the surface of crystal 2 and are in contact with crystal 2. Electrodes 3 and 4 sandwich crystal 2 between them in the X direction. One of electrodes 3 and 4 is used as a cathode, and the other as an anode. One of electrodes 3 and 4 is made of gold, and the other is made of gold, platinum, aluminum, indium, or chromium. The electrodes may be made of an alloy of these metals. Electrodes 3 and 4 are formed on crystal 2 using vapor deposition or sputtering. The specific vapor deposition or sputtering method is selected from RF sputtering, DC sputtering, resistance heating vapor deposition, and electron beam vapor deposition, depending on the type of metal. Electrodes 3 and 4 can take various forms. For example, each may consist of a single electrode, or multiple electrodes. Each electrode may be rectangular, circular, or elliptical. When multiple electrodes are used, the electrodes may be arranged in a grid pattern on the surface of crystal 2.

[0017] The portions of the crystal 2 including the surfaces on which the electrodes 3 and 4 are formed are doping layers 5 and 6, respectively, which have been doped by thermal diffusion. The elements doped into the doping layers 5 and 6 are at least one of strontium, lead, thallium, and bromine. Magnesium, calcium, zinc, germanium, indium, gallium, cobalt, tellurium, and selenium are also candidates for doping elements.

[0018] The radiation detector 1 is used as follows: When a voltage is applied between the electrodes 3 and 4, electron-hole pairs generated by radiation incident on the crystal 2 are moved by the applied voltage. The current generated between the two electrodes in this way is detected by an appropriate detection device, thereby detecting the radiation incident on the crystal 2.

[0019] [Experimental Examples] (Experimental Example 1) As shown in FIG. 2, sample A in this experimental example simulates the contact structure between the crystal 2 and the electrode 3 or 4 in the radiation detector 1, and includes a substrate α, one electrode β, and five electrodes γ. The substrate α is a disk-shaped single crystal made of thallium bromide. The substrate α is doped with strontium or lead from one circular surface α1. The doped region does not reach the surface opposite to surface α1, but extends to a predetermined depth from surface α1 in the thickness direction perpendicular to surface α1. Electrode β and electrode γ are both formed on surface α1. Electrode β is made of gold and is located approximately in the center of surface α1. The five electrodes γ are made of gold, platinum, aluminum, indium, and chromium, respectively. These electrodes γ are arranged around electrode β so as to surround electrode β.

[0020] In this experimental example, Sample A was prepared as follows. Using 4N purity thallium bromide powder, a 12 mm diameter thallium bromide crystal was obtained by purification and single crystal synthesis using the vertical Bridgman method and zone melting. This was then cut into a disk-shaped crystal approximately 1 mm thick. A disk-shaped pellet of the dopant bromide was placed on the thallium bromide crystal, and the resulting crystal was heated in an atmosphere primarily consisting of argon and hydrogen. Specifically, the temperature was changed from room temperature to a maximum temperature exceeding the melting point of the bromide. The pellet-doped crystal was then maintained at the maximum temperature for a predetermined time and then slowly cooled over 10 hours or more. This process was repeated for both strontium bromide and lead bromide as the bromide. This resulted in two types of substrates α, each doped with strontium and lead.

[0021] Next, electrodes β and γ were formed on the surfaces α1 of the two substrates α by sputtering or vapor deposition of gold, platinum, aluminum, indium, and chromium, respectively. Resistance heating vapor deposition was used for gold, aluminum, and indium, DC sputtering was used for platinum, and RF sputtering was used for chromium. Thus, two types of samples A, each doped with strontium and lead, were fabricated.

[0022] Next, for the two types of sample A, the current-voltage characteristics between electrode β and each electrode γ were evaluated using a measuring device (B1505A Power Device Analyzer, Keysight Corporation). Furthermore, as a control, the current-voltage characteristics between two points on the crystal surface of thallium bromide that was not doped with any element were also evaluated. The results are shown in Figures 3 and 4. Figure 3 relates to sample A doped with strontium, and Figure 4 relates to sample A doped with lead. In both Figures 3 and 4, "Au-X" (X: any of Au, Pt, Al, In, and Cr) corresponds to the current-voltage characteristics between electrode β and electrode γ consisting of X. "Undoped" corresponds to the current-voltage characteristics of the control.

[0023] As shown in Figures 3 and 4, the voltage between electrode β and each electrode γ was approximately proportional to the current, and this characteristic was obtained for each of the two types of sample A. In other words, it was shown that ohmic contact was ensured between electrode β or γ and the crystal. It was also shown that ohmic contact was properly ensured when gold was used for one of the two electrodes and one of gold, platinum, aluminum, indium, and chromium was selected for the other. Furthermore, it was shown that using gold as the electrode is preferable from the viewpoint of improving conductivity.

[0024] (Experimental Example 2) Six types of substrates α, each doped with strontium, lead, calcium, cadmium, mercury, and magnesium, were prepared by the same method as in Experimental Example 1. Next, the measurement probe of the measurement device of Experimental Example 1 was directly applied to two points on the doped surface of each substrate α, and the current-voltage characteristics between these two points were evaluated. As a control, the current-voltage characteristics between two points on the crystal surface of thallium bromide, which was not doped with any element, were also evaluated. The results are shown in Figure 5. As shown in Figure 5, the current-voltage characteristics of substrates α doped with calcium, cadmium, mercury, or magnesium were not different from those of the control. In contrast, the current-voltage characteristics of substrates α doped with strontium or lead exhibited significant changes compared to those of the control. In addition, a similar evaluation was performed on substrates α doped with sulfur, and the current-voltage characteristics were not different from those of the control.

[0025] (Experimental Example 3) For the substrate α produced in Experimental Example 1, the distribution of each atom in the substrate α was measured by secondary ion mass spectrometry (SIMS) using a quadrupole mass analyzer (PHI ADEPT-1010, ULVAC-PHI, Inc.). As a result, for the strontium-doped substrate α, the result was obtained that strontium was distributed in a range from the surface to a depth of about 10 μm. Furthermore, for the lead-doped substrate α, the result was obtained that lead was distributed in a range from the surface to a depth of about 3 to 5 μm.

[0026] [Effects of the Present Embodiment] The radiation detector 1 according to the present embodiment described above uses a metal other than thallium for the electrodes 3 and 4. Therefore, degradation of detection performance due to thallium corrosion does not occur. Furthermore, the portions of the crystal 2 that contact the electrodes 3 and 4 are doped with at least one of strontium, lead, thallium, and bromine. This facilitates proper ohmic contact in each of the electrodes 3 and 4, as shown in the above-described experimental examples involving doping with strontium and lead. Furthermore, proper ohmic contact is also facilitated with doping with at least one of thallium and bromine. This is because doping with at least one of thallium and bromine changes the composition ratio of thallium to bromine in the crystal 2 from 1:1, thereby placing the doped layer 5 or 6 in an electron-excess state or a hole-excess state. Therefore, a detector that facilitates good electrical contact while using a metal other than thallium for the electrodes is realized. The same applies to crystals 2 doped with magnesium, calcium, zinc, germanium, indium, gallium, cobalt, tellurium, and selenium.

[0027] <Modifications> The above is a description of a preferred embodiment of the present invention, but the present invention is not limited to the above-described embodiment, and various modifications are possible within the scope described in the means for solving the problems.

[0028] For example, in the above-described embodiment, doping layers 5 and 6 doped with strontium or the like are formed on both surfaces of crystal 2 on which electrodes 3 and 4 are formed, respectively. Alternatively, a doping layer of strontium or the like may be formed on only the surface on which either one of electrodes 3 and 4 is formed.

[0029] In the above-described embodiment, ohmic contact is ensured between both the electrodes 3 and 4 and the crystal 2. In this regard, it is sufficient that ohmic contact is ensured between at least one of the electrodes 3 and 4 and the crystal 2. In other words, one of the electrodes 3 and 4 is an ohmic contact, but the other may be either an ohmic contact or a Schottky contact.

[0030] In the above-described embodiment, the electrodes 3 and 4 are each made of any one of gold, platinum, aluminum, indium, and chromium. Alternatively, at least one of the electrodes 3 and 4 may be made of a metal oxide semiconductor such as indium tin oxide or tin dioxide. These materials are oxides and therefore less susceptible to oxidation by bromine, and the presence of the doping layer 5 or 6 of strontium or the like near the interface between the electrodes 3 and 4 and the crystal 2 tends to make them stable electrodes.

[0031] 1 Radiation detector 2 Crystal 3, 4 Electrode 5, 6 Doping layer

Claims

1. A radiation detector comprising: a crystal whose main component is thallium bromide; first and second electrodes made of a metal other than thallium that are in contact with the crystal so as to sandwich the crystal between them in one direction; and the portion of the crystal that is in contact with at least one of the first and second electrodes is doped with at least one of strontium, lead, thallium and bromine.

2. A radiation detector as described in claim 1, characterized in that both the portion of the crystal in contact with the first electrode and the portion in contact with the second electrode are doped with at least one of strontium, lead, thallium and bromine.

3. A radiation detector as claimed in claim 1 or 2, characterized in that at least one of the first and second electrodes is made of gold.

4. A radiation detector according to claim 3, wherein the other of said first and second electrodes is made of any one of gold, platinum, aluminum, indium and chromium.

5. A method for manufacturing a radiation detector comprising a crystal whose main component is thallium bromide, and first and second electrodes in contact with the crystal so as to sandwich the crystal between them in one direction, characterized in that the crystal is partially doped with an element that provides ohmic contact between the crystal and at least one of the first and second electrodes, and at least one of the first and second electrodes made of a metal other than thallium is placed in contact with the portion of the crystal doped with the element.