JP6583855B2 - Radiation detector - Google Patents

Description

  The present invention relates to a radiation detection apparatus that detects radiation.

  Conventionally, radiation detectors having an energy discrimination function have been developed. Radiation detectors with energy discrimination functions are expected to be used in the medical and industrial fields, and are essential for developing from conventional black and white X-ray imaging technologies to new imaging technologies that can identify materials. It is a device.

  For example, as an example of a conventional radiation detector, a radiation image sensor is known that includes a scintillator that emits light when irradiated with radiation and a photosensor array in which light receiving portions that receive the emitted light are two-dimensionally arranged (the following patents). Reference 1). This radiation image sensor has a scintillator in which a large number of columnar portions are formed inside CsI and NaCl, and this scintillator is arranged so that its light propagation direction is perpendicular to the light receiving surface of the optical sensor array. Have In such a radiation image sensor, radiation having different energies can be distinguished from the signal intensity profile in the row direction of the optical sensor array.

JP 2013-24731 A

  However, since the conventional radiation image sensor described above employs a configuration in which light propagating inside the scintillator is received and detected by the photosensor array, the light spreads on the light receiving surface of the photosensor array, and the radiation energy The resolution of discrimination tends to decrease. Furthermore, since the intensity of the light is detected by the optical sensor array after converting the radiation into light, the influence of noise due to Compton scattering specific to the radiation in the detection signal cannot be ignored.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a radiation detection apparatus capable of realizing radiation energy discrimination with high resolution and high S / N.

  In order to solve the above problems, a radiation detection apparatus according to an aspect of the present invention includes a side surface on which radiation is incident, a first main surface connected to the side surface, and a second main surface connected to the side surface and facing the first main surface. A flat plate-shaped conversion member having a main surface and containing CdTe crystals for converting radiation into electric charge; a first electrode member two-dimensionally separated on the first main surface; and a second And a second electrode member disposed on the main surface of the first electrode member and connected to the plurality of first electrode members, and is generated between each of the plurality of first electrode members and the second electrode member inside the conversion member. A readout circuit that independently reads out the electric charge as a readout signal, and the readout circuit is provided independently for each of the plurality of first electrode members, and when the electric charge exceeds a preset threshold, A plurality of threshold setting circuits for outputting corresponding electrical signals; The electrical signal output from the accumulated for each of a plurality of first electrode members, and a plurality of signal output circuit for outputting a read signal the accumulated electric signal at a predetermined timing.

  According to the radiation detection apparatus of the above aspect, when radiation is incident on the side surface of the conversion member, the radiation is converted into charges while passing through the conversion member by a distance corresponding to the energy, and the charge is converted into the conversion member. A readout signal having a two-dimensional distribution is output by being independently read out by the readout circuit through the plurality of first electrode members that are two-dimensionally arranged on the main surface. The energy of the radiation can be detected by the distribution of the readout signal output in this way in the direction perpendicular to the side surface of the conversion member. At this time, the energy discrimination resolution can be improved by directly taking out the charge converted from the radiation by the conversion member as a readout signal. Further, by providing a threshold value setting circuit in the reading circuit, when the electric charge generated in the conversion member exceeds a specified threshold value, the electric signal is accumulated, and the electric signal is output as a reading signal. As a result, noise in the readout signal caused by Compton scattering of radiation can be reduced. As a result, it is possible to realize radiation energy discrimination with high resolution and high S / N.

  Here, the plurality of threshold setting circuits are set such that thresholds corresponding to the plurality of first electrode members arranged in a direction perpendicular to the side surface gradually increase along the vertical direction. It is good as well. In this case, noise due to the influence of scattered radiation by the radiation detection device is easily removed, and the accuracy of energy discrimination is further improved.

  The plurality of signal output circuits include a first capacitor that accumulates an electric signal, a second capacitor that transfers the electric signal accumulated by the first capacitor, and an electric signal that is transferred to the second capacitor. It is good also as having having the amplifier which outputs. In this case, the charge generated in the conversion member can be accumulated and output as a read signal at an appropriate timing.

  Further, a control circuit that variably sets threshold values of the plurality of threshold setting circuits may be further provided. In this case, the accuracy of energy discrimination can be further improved by dynamically changing the threshold value.

  According to the present invention, it is possible to realize radiation energy discrimination with high resolution and high S / N.

1 is a side view of a radiation detection apparatus 1 according to an embodiment. 3 is a plan view of a conversion member included in the radiation detection apparatus 1. FIG. 2 is a circuit diagram of a main part of a readout circuit included in the radiation detection apparatus 1. FIG. It is a graph which shows the energy response of the radiation which injects perpendicularly | vertically on the board | substrate comprised by CdTe used for the conversion member 3 of FIG. It is a figure which shows the example of the image signal output from the radiation detection apparatus 1 when X-rays of various energy were made to enter.

  DESCRIPTION OF EMBODIMENTS Hereinafter, a preferred embodiment of a radiation detection apparatus according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant description is omitted.

  First, the structure of the radiation detection apparatus 1 which concerns on embodiment is demonstrated using FIGS. 1-3. 1 is a side view of the radiation detection apparatus 1, FIG. 2 is a plan view of a conversion member included in the radiation detection apparatus 1, and FIG. 3 is a circuit diagram of a main part of a readout circuit included in the radiation detection apparatus 1. . The radiation detection apparatus 1 is an apparatus that detects the energy distribution of radiation incident from the outside. Examples of radiation to be detected include X-rays and γ-rays, as well as α-rays and β-rays. X-rays and γ-rays are preferable because they have a high absorption rate when the radiation is converted into electrical signals. Detected.

  As shown in FIGS. 1 and 2, the radiation detection apparatus 1 includes a conversion member 3 that converts incident radiation into an electrical signal (charge), electrodes 5 and 7 disposed on both surfaces of the conversion member 3, and a conversion member. 3 and a readout circuit 9 that reads out an electrical signal from the electrical circuit 3.

  The conversion member 3 is a rectangular flat plate member made of a CdTe crystal that is a direct conversion type semiconductor material that directly converts radiation into an electrical signal (charge), and a side surface 3a on which the radiation to be detected is incident; It has a main surface 3b connected to the side surface 3a and a main surface 3c connected to the side surface 3a and facing the main surface 3b. The conversion member 3 using such a CdTe crystal does not cause a decrease in radiation detection sensitivity and energy resolution due to light diffusion, scattering, attenuation, etc., compared to a scintillator that converts radiation into light. Has characteristics. Further, the conversion member 3 has a large absorption rate of X-rays, γ-rays, etc., has high detection sensitivity to such radiation, and can be operated at room temperature. The size of the conversion member 3 is not limited to a specific size. For example, the main surfaces 3b and 3c have a size of 20 mm × 20 mm, and the thickness in the direction perpendicular to the main surfaces 3b and 3c along the side surface 3a is 0.5. It is set to about ~ 1.0mm. In the following description, in the conversion member 3, the direction along the principal surfaces 3b and 3c perpendicular to the side surface 3a is the X axis direction, the thickness direction of the conversion member 3 along the side surface 3a is the Z axis direction, and the X axis. A direction perpendicular to the direction and the Z-axis direction is taken as a Y-axis direction.

  A plurality of electrodes (electrode members) 7 are two-dimensionally separated and arranged on the main surface 3 b of the conversion member 3. Specifically, each of the plurality of electrodes 7 is a substantially rectangular flat plate electrode, and is arranged two-dimensionally at a predetermined pitch along the X-axis direction and the Y-axis direction on the main surface 3b. Is formed. For example, when the size of the main surface 3b is 20 mm × 20 mm, 200 electrodes 7 are arranged at a pitch of 0.1 mm in each of the X-axis direction and the Y-axis direction. That is, a total of 200 × 200 electrodes 7 are formed. Furthermore, an electrode (electrode member) 5 that is a flat plate electrode is formed on the main surface 3c of the conversion member so as to cover almost the entire surface of the main surface 3c.

  The material of the electrodes 5 and 7 is not limited to a specific material. For example, a combination of Pt and In, a combination of Pt and Al, or the like is used as the material of the electrodes 5 and 7, and the electrodes 5 and 7 are main surfaces. On the 3c and 3b, a Schottky electrode is formed in the case of a combination of Pt and In, and an ohmic electrode is formed in the case of a combination of Pt and Al. In this case, by applying a negative voltage to the Pt electrode and applying a positive voltage to the In (or Al) electrode, it is possible to apply a high voltage because the leakage current is small. Electric charges can be taken out from the electrode 7 efficiently. The electrodes 5 and 7 may be formed as ohmic electrodes using both materials as Pt.

  In the conversion member 3 having the plurality of electrodes 7 arranged two-dimensionally in this way, the charges generated inside the conversion member 3 are two-dimensionally distributed along the X-axis direction and the Y-axis direction of the charges. Each of the divided positions can be taken out as an independent electrical signal via the electrode 7 in the vicinity of the position.

  A readout circuit 9 is provided on the main surface 3b of the conversion member 3 configured as described above. That is, the readout circuit 9 includes a number of cell circuits 9 a corresponding to the plurality of electrodes 7, and each cell circuit 9 a is electrically connected to the corresponding electrode 7 via the electrode pad 11 that is a metal connection member. It is connected to the. The readout circuit 9 is an integrated circuit such as an LSI for reading out the electric charges generated between each of the plurality of electrodes 7 and the electrode 5 in the conversion member 3 as an independent readout signal. As shown in FIG. 3, the cell circuit 9 a is a circuit unit provided independently for each of the plurality of electrodes 7, and includes a threshold setting circuit 13 and a signal output circuit 15.

  The threshold setting circuit 13 generates a voltage signal (electric signal) corresponding to the amount of charge when the charge generated in the region sandwiched between the corresponding electrode 7 and electrode 5 exceeds a preset threshold. Output. The threshold setting circuit 13 includes a differential amplifier 17, one input terminal of the differential amplifier 17 is connected to the corresponding electrode 7, and the other input terminal of the differential amplifier 17 is a resistor for adjusting a threshold value. The element 19 is connected to the ground. Such a differential amplifier 17 receives a voltage signal corresponding to the amount of charge generated in a region sandwiched between the electrode 7 and the electrode 5 corresponding to one input terminal, and the other input terminal. A voltage signal corresponding to the resistance value of the resistance element 19 is input to the amplifier, and the difference between the voltage signals is amplified. With such an operation, the differential amplifier 17 responds to the amount of charge when the amount of charge generated in the region of the corresponding electrode 7 exceeds a threshold value set in advance by the resistance value of the resistance element 19. When the voltage signal is output and the amount of charge generated in the corresponding electrode 7 region is equal to or less than the threshold value preset by the resistance value of the resistance element 19, the voltage value is 0 (has a significant value). No) Output voltage signal.

  The signal output circuit 15 is provided in the subsequent stage of the threshold setting circuit 13, accumulates the voltage signal output from the differential amplifier 17 of the threshold setting circuit 13 for each corresponding electrode 7, and stores the accumulated voltage signal at a predetermined timing. Output as a read signal. The signal output circuit 15 includes a signal storage capacitor 21, a signal readout capacitor 23, an amplifier 25, and switches 27a, 27b, 27c, and 27d. A predetermined voltage is applied to one end of the capacitor 21, and the output of the differential amplifier 17 is connected to the other end, and is connected to the ground via the switch 27a. A predetermined voltage is applied to one end of the capacitor 23, and the other end of the capacitor 21 is connected to the other end via a switch 27c and to the ground via a switch 27b. Further, the other end of the capacitor 23 is connected to the output of the cell circuit 9a through the switch 27d and the amplifier 25.

  The signal output circuit 15 having such a configuration operates as follows when the on / off of the switches 27a, 27b, 27c, and 27d is controlled by an external control circuit (not shown). That is, the switches 27a and 27b are turned on to reset the charges accumulated in the capacitors 21 and 23, and then the switches 27a, 27b, 27c, and 27d are turned off, so that the voltage signal output from the differential amplifier 17 is changed. The accumulated charge is accumulated in the capacitor 21. Next, by turning on the switch 27 c, charges corresponding to the voltage signal accumulated in the capacitor 21 are transferred to the capacitor 23. Thereafter, the switch 27c is turned off and the switch 27d is turned on to amplify and output a voltage signal corresponding to the charge transferred to the capacitor 23 via the amplifier 25. As a result, the signal output circuit 15 can output a voltage signal corresponding to the amount of charge generated in the region of the corresponding electrode 7 as a read signal at a predetermined repetition timing.

  The readout circuit 9 configured as described above sequentially outputs readout signals output from the plurality of cell circuits 9a, thereby outputting an image signal of one image composed of a plurality of pixel signals. As the output control method of the image signal, a conventional method used by a CMOS image sensor, a CCD image sensor, or the like is used.

Here, it is preferable that the threshold value set in the threshold value setting circuit 13 of each cell circuit 9a built in the readout circuit 9 is set to a different value among the plurality of cell circuits 9a. Specifically, it is preferable that the threshold value is set to gradually increase as the distance in the X-axis direction from the side surface 3a of the conversion member 3 of the corresponding electrode 7 increases. For example, when the electrodes 7 are arranged at a pitch of 0.1 mm in the X-axis direction, the threshold value of the cell circuit 9a corresponding to the electrode 7 in each column having the same distance from the side surface 3a is converted into the energy conversion of absorbed radiation. Is set to a voltage value corresponding to the following value.
Threshold corresponding to the 10th row (distance 0.1 cm from side surface 3a): 100 keV
Threshold corresponding to the 50th row (distance 0.5 cm from side surface 3a): 200 keV
Threshold value corresponding to the 100th row (distance 1.0 cm from side surface 3a): 300 keV
Threshold value corresponding to the 200th row (distance 2.0 cm from side surface 3a): 600 keV

  The reason why the threshold value of the cell circuit 9a is set in this way is as follows. FIG. 4 is a graph showing the energy response of radiation that is perpendicularly incident on a substrate made of CdTe used for the conversion member 3. In this graph, the sensitivity (absorption rate) to the energy of incident radiation is shown for substrates set to various thicknesses. As can be seen from this characteristic, the threshold value described above corresponds to such energy response, and corresponds to energy at which the absorptance at the position where each electrode 7 of the radiation incident from the side surface 3a is arranged is 60%. It can be seen that it is set as follows. For example, the distance from the side surface 3a of the electrode 7 in the 10th row is 0.1 cm, and the energy of radiation at which the absorption rate at such a distance is 60% is about 100 keV, and the side surface corresponding to the 50th row. The distance from 3a is 0.5 cm, and the energy of radiation at which the absorption rate at such a distance is 60% is about 200 keV. With this setting, for example, in the electrode 7 in the 50th column whose distance from the side surface 3a is 0.5 cm, the cell circuit 9a uses the energy of the radiation of about 200 keV having an absorption rate of 60% or more as a readout signal. It can be prevented from taking in.

  According to the radiation detection apparatus 1 described above, when radiation is incident on the side surface 3a of the conversion member 3 in the X-axis direction, the radiation is transmitted through the conversion member 3 in the X-axis direction by a distance corresponding to the energy. The charges are converted into electric charges, and the electric charges are independently read out by the reading circuit 9 via the plurality of electrodes 7 arranged two-dimensionally on the main surface 3b of the conversion member 3, thereby obtaining a two-dimensional distribution. Is read out. The energy of the radiation can be detected by the distribution of the readout signal output in this way in the X-axis direction perpendicular to the side surface 3a of the conversion member 3. Specifically, the position where the charge is converted inside the conversion member 3 changes in the X-axis direction according to the energy of the radiation, and as a result, the low energy radiation is photoelectrically converted in a region close to the side surface 3a, and the high energy This radiation is photoelectrically converted not only in the region near the side surface 3a but also in the region proceeding in the X-axis direction. Therefore, the energy of the incident radiation can be estimated by quantitatively evaluating the distribution in the X-axis direction of the two-dimensional image signal obtained from the readout signal. At this time, it is possible to improve the energy discrimination resolution by directly taking out the electric charge converted from the radiation by the direct conversion type conversion member 3, and measure the radiation such as X-rays irradiated to the object to be measured. This makes it possible to identify the material of the object to be measured. Furthermore, since the electrodes 7 are also arranged in the Y-axis direction and correspondingly provided with the cell circuit 9a, the distribution of the energy of the incident radiation along the Y-axis direction can also be output, and as a line sensor Can function.

  The radiation detection apparatus 1 is suitable for detecting X-rays having a long absorption length (attenuation length) and high energy. For example, if the conversion member 3 made of CdTe having a size of 20 mm × 20 mm is provided, energy discrimination is possible even for high-energy X-rays having an effective energy of about 400 to 500 keV (tube voltage 1000 kV). .

  Further, a threshold value setting circuit 13 is built in each cell circuit 9 a of the reading circuit 9. As a result, noise in the readout signal caused by Compton scattering of radiation can be reduced. In other words, radiation with relatively low energy is generated as noise due to Compton scattering, but such noise can be prevented from being mixed in the readout signal. For example, the distance from the side surface 3a of the electrode 7 in the 10th row is about 100 keV at an absorption rate of 60% at 0.1 cm, and the distance from the side surface 3a corresponding to the 50th row is 0.5 cm. The energy of radiation with an absorption rate of 60% is about 200 keV. Here, the radiation having the energy of about 100 keV that has reached the position corresponding to the electrode in the tenth row generates radiation having an energy lower than about 100 keV due to Compton scattering. Even if this low energy radiation is radiated in the X direction and reaches the position corresponding to the electrode 7 in the 50th row whose distance from the side surface 3a is 0.5 cm, for example, Since the cell circuit 9a does not take in the radiation energy such as about 200 eV or less with an absorption rate of 60% or more as a readout signal, readout of radiation due to Compton scattering as noise can be avoided. As a result, it is possible to realize radiation energy discrimination with high resolution and high S / N.

  Here, the threshold value setting circuit 13 of the present embodiment is set so that threshold values corresponding to the plurality of electrodes 7 arranged in the X-axis direction gradually increase along the X-axis direction. Thereby, the noise by the influence of the scattered radiation by the radiation detection apparatus 1 becomes easy to be removed according to the position in the X-axis direction, and the accuracy of energy discrimination is further improved.

  Each cell circuit 9 a of the readout circuit 9 includes a signal output circuit 15. Thereby, while accumulating the charge generated in the conversion member 3, the charge can be output as a read signal at an appropriate timing.

  FIG. 5 shows an example of an image signal output from the radiation detection apparatus 1 when X-rays of various energies are incident at one point. FIG. 5A shows an output example when X-rays are made incident at an acceleration voltage of 50 kV for the X-ray tube, and FIG. 5B shows an output when X-rays are made incident at an acceleration voltage of 100 kV for the X-ray tube. For example, FIG. 5C is an output example when X-rays are incident at an acceleration voltage of 150 kV of the X-ray tube. As shown in these output examples, it can be seen that as the energy of the incident X-rays increases, the high-luminance region subjected to photoelectric conversion extends in the X-axis direction.

  The present invention is not limited to the embodiment described above. In the said embodiment, you may change into the following structure.

  For example, in the radiation detection apparatus 1, the readout circuit 9 may operate as a control circuit that variably sets the threshold in the threshold setting circuit 13 in the plurality of cell circuits 9a. That is, in the above-described embodiment, the threshold value in the threshold setting circuit 13 is set so as to correspond to the energy at which the absorption rate at the position where each electrode 7 of the radiation incident from the side surface 3a is disposed is 60%. . The readout circuit 9 changes the threshold value in each threshold setting circuit 13 to energy that becomes the absorption rate in which the radiation absorption rate is set at the position where each electrode 7 is arranged, according to the parameter of the absorption rate set from the outside. You may operate | move so that it may set so that it may respond | correspond. More specifically, when the absorptance is set to 80%, 60%, 40%, and 20% from the outside, the readout circuit 9 uses the energy response data as shown in FIG. The energy value of the radiation corresponding to the absorption rate is specified, and the threshold value of the cell circuit 9a corresponding to the electrode 7 of each column is set based on the energy value.

  According to such a modification, in the detection of the same radiation, it is possible to acquire a tendency of how the scattered radiation is generated by dynamically changing the threshold value. As a result, the accuracy of energy discrimination can be further improved.

  DESCRIPTION OF SYMBOLS 1 ... Radiation detection apparatus, 3 ... Conversion member, 3a ... Side surface, 3b, 3c ... Main surface, 5, 7 ... Electrode (electrode member), 9 ... Read-out circuit, 13 ... Threshold setting circuit, 15 ... Signal output circuit, 21 , 23: Capacitor, 25: Amplifier.

Claims (4)

A CdTe crystal having a side surface on which radiation is incident, a first main surface connected to the side surface, and a second main surface connected to the side surface and opposed to the first main surface, which converts the radiation into an electric charge. A flat conversion member;
A plurality of first electrode members arranged two-dimensionally on the first main surface;
A second electrode member disposed on the second main surface;
Charges connected to the plurality of first electrode members and generated between each of the plurality of first electrode members and the second electrode member inside the conversion member are independently read out as readout signals. A readout circuit,
The readout circuit is
A plurality of threshold setting circuits that are provided independently for each of the plurality of first electrode members and that output an electrical signal corresponding to the charges when the charges exceed a preset threshold;
A plurality of signal output circuits for storing the electrical signals output from the plurality of threshold setting circuits for each of the plurality of first electrode members and outputting the stored electrical signals as the readout signals at a predetermined timing; Have
Radiation detection device. The plurality of threshold setting circuits are set such that the threshold corresponding to the plurality of first electrode members arranged in a direction perpendicular to the side surface gradually increases along the vertical direction. Yes,
The radiation detection apparatus according to claim 1. The plurality of signal output circuits are transferred to a first capacitor for storing the electric signal, a second capacitor to which the electric signal stored by the first capacitor is transferred, and the second capacitor. And an amplifier for outputting the electric signal,
The radiation detection apparatus according to claim 1 or 2. A control circuit that variably sets the threshold values of the plurality of threshold setting circuits;
The radiation detection apparatus of any one of Claims 1-3.