Radiation detection device, radiation detection method, radiation analysis method, and nuclide analysis device

Abstract

The present invention obtains a radiation detection device that, by means of a simple configuration, can accurately recognize the energy and the radiation intensity of radiation.　A scintillator 10 and a photomultiplier tube 20 are used in combination. The photomultiplier tube 20 is provided with a voltage change suppressing means. A power source 30 is connected to the photomultiplier tube 20, and a current sensor 31 that measures a supply current ID is provided. Also provided is a pulse signal processing unit 40 that reads output pulses outputted from an anode of the photomultiplier tube 20, and counts the output pulses within a fixed period. A data processing unit 50 recognizes a radiation intensity Y1 as the counted number of the output pulses using the pulse signal processing unit 40, recognizes the supply current ID recognized by the current sensor 31, and recognizes an actual radiation intensity Y0 from the supply current ID.

Description

Radiation detection device, radiation detection method, radiation analysis method, nuclide analysis device

[0001] The present invention relates to a radiation detection device for measuring the intensity of radiation (γ-rays, X-rays, neutrons, etc.), a radiation analysis method using the same, and a nuclide analysis device.

[0002] In order to detect the presence of a radioactive nuclide, a technique of detecting radiation (such as γ-rays) emitted when this nuclide decays is effective. For this purpose, a radiation detector capable of measuring the energy spectrum of radiation is used. In this case, generally γ-rays are used as the radiation. Since the energies of γ-rays emitted from different nuclides are generally different, the nuclides present in this sample can be identified from the energy spectrum (spectrum) of γ-rays detected from the sample (radiation environment). Such techniques are described in, for example, Patent Document 1 and the like.

[0003] As a radiation detector capable of measuring the spectrum of γ-rays in this way, a detector using a scintillator is known. In this case, the scintillator absorbs γ-rays and emits photons in the vicinity of the visible region, and the pulsed light (scintillation light) composed of these photons is detected by a photomultiplier tube. For a detector combining a scintillator and a photomultiplier tube, in principle, every time a γ-ray is detected, an electrical output pulse with a short duration corresponding to this is obtained from the photomultiplier tube, and the pulse height of this output pulse (more precisely, the waveform integral value or charge amount of the pulse) corresponds to the energy of the detected γ-ray. Also, the count value (count number) per unit time of this output pulse is recognized as the intensity of this γ-ray (or the dose of the radiation source). Thereby, an energy spectrum is acquired by this radiation detector, and by performing this analysis, the nuclides present in the measurement object (environment) can be recognized.

[0004] However, in the presence of high-dose radiation, in fact, this analysis is not easy. For example, in the measurement of a sample (measurement environment) related to a nuclear reactor, as radioactive nuclides, １３７ Cs and ６０The abundance of certain known radionuclides, such as Co (hereinafter referred to as background radionuclides), is high. As a result, a particularly large amount of gamma rays emitted by these background radionuclides are detected, and in the energy spectrum mentioned above, the gamma ray peaks emitted by radioactive radionuclides other than the background radionuclides are greatly affected by the gamma rays emitted by the background radionuclides, reducing the accuracy of detection.

[0005] Patent Document 2 describes a situation in which, especially high doses １３７ This document describes a technique that can accurately recognize the gamma ray peaks emitted by radionuclides other than the background nuclide in the presence of Cs (background nuclide), thereby recognizing the presence of radionuclides other than the background nuclide. In this technique, by switching between multiple shielding materials of different thicknesses and multiple scintillators of different sizes, the component attributable to the background nuclide in the acquired energy spectrum is recognized with high precision, thereby enabling the accurate recognition of the gamma ray peaks emitted by radionuclides other than the background nuclide, and thus the accurate recognition of radionuclides other than the background nuclide.

[0006] In this case, it is necessary to accurately recognize the energy spectrum as described above. To do this, it is necessary to accurately recognize the energy of the detected gamma rays and the number of incident particles (radiation intensity) per unit time at the detector. Here, as mentioned above, the pulse height and waveform integral value of the output pulse output from the photomultiplier tube correspond to this energy, and in order to recognize the energy of the gamma rays with high accuracy, it is required that the pulse height and waveform integral value of this output pulse are proportional to the intensity of the received pulsed light.

[0007] In this photomultiplier tube, multiple dynodes are provided between the cathode and anode. A constant voltage applied between the cathode and anode is distributed and applied to each dynode, and photoelectrons generated by the incident pulsed light are sequentially multiplied between the dynodes and output from the final anode. In this case, as described in Patent Document 2, when the amount of incident light is large, the distribution of the voltage applied between the dynodes changes, which increases the gain compared to when the amount of incident light is small, causing the output of the photomultiplier tube to be higher than in the ideal case and disrupting the linearity of the output characteristics.

[0008] Therefore, as described in Patent Document 2, a photomultiplier tube equipped with a voltage change suppression means that suppresses changes in the voltage distribution between dynodes even when the intensity of pulsed light is high is particularly effective in the above measurement. In other words, this makes it possible to accurately recognize the energy of the incident gamma rays.

[0009] Japanese Patent Publication No. 2016-513256, International Publication No. 2022 / 075455

[0010] As described above, in order to recognize the energy spectrum with high accuracy, it is necessary to accurately recognize the energy of the detected gamma rays and the number of gamma rays incident on the detector per unit time (radiation intensity). The energy of the gamma rays was obtained with high accuracy by using a photomultiplier tube equipped with the voltage change suppression means described above.

[0011] On the other hand, radiation intensity is recognized as the number of counts per unit time in the radiation detector (hereinafter referred to as detection intensity), specifically as the number of counts per unit time of the output pulse from the photomultiplier tube. However, when the radiation intensity is high, even if radiation enters the radiation detector, it may not be counted as output, and the radiation intensity and detection intensity may not match.

[0012] Figure 15 schematically illustrates this situation, showing the output pulses from the photomultiplier tube (horizontal axis: time, horizontal axis: output voltage) when the radiation intensity is low (a) and high (b). Within the range shown, the number of output pulses is 2 in Figure 15(a) and 5 in Figure 15(b). The time interval between each output pulse is random, and the number of each output pulse within a certain time (count) represents the detected intensity of this radiation. Here, since each output pulse has a finite time width, the first two output pulses in Figure 15(b) overlap (pile up) due to the short time interval, and they cannot be separated and recognized. In reality, in the case of Figure 15(b), the number of output pulses is recognized as 4. In other words, when the radiation intensity is high, the detected intensity is counted lower than the actual detected intensity.

[0013] Therefore, there was a need for a radiation detection device that could accurately recognize not only the energy of the detected radiation but also its radiation intensity under high-dose conditions. In particular, for radiation detection devices used in applications such as those described in Patent Document 2, it was necessary to transport the device into the environment to be measured and perform the measurement, so a simple configuration was also required.

[0014] Therefore, there was a need for a radiation detection device with a simple configuration that could accurately recognize the energy and intensity of radiation, or to use such a device to perform highly accurate analysis of the radionuclides that emitted the radiation.

[0015] This invention has been made in view of the above-mentioned problems, and aims to provide an invention that solves the above-mentioned problems.

[0016] To solve the above problems, the present invention has the following configuration. The radiation detection device of the present invention comprises a scintillator that absorbs radiation and emits scintillation light, and a photomultiplier tube that sequentially multiplies the photoelectrons generated by the scintillation light between a cathode, a plurality of dynodes, and an anode, and outputs an electrical output pulse from the anode side, wherein the photomultiplier tube comprises a divider circuit that distributes a DC voltage from a power supply to the cathode, each of the dynodes, and the anode, and when a current due to the photoelectrons flows between the cathode and the anode, a voltage change suppression means is provided to suppress the change in the voltage applied to each of the dynodes due to the current from when no current flows, the photomultiplier tube comprises a current sensor that recognizes the supply current supplied from the power supply when the photomultiplier tube is operating, and a data processing unit that calculates the intensity of the radiation based on the supply current. The data processing unit may calculate the intensity of the radiation by correcting the count value of the output pulses within a certain time period based on the supply current within the same time period. The data processing unit may calculate a histogram of the intensity of each emission of scintillation light calculated from the count value corrected based on the supply current and the intensity of each emission of scintillation light calculated from each of the output pulses, as a spectrum. The radiation is gamma rays emitted from the object being measured, and the data processing unit may calculate the energy spectrum of the gamma rays, which is the spectrum in which the intensity of each emission is the energy of the gamma rays. The data processing unit may recognize radioactive nuclides included in the object being measured from the energy spectrum. The voltage change suppression means may suppress fluctuations in the voltage applied between the final stage dynode and the dynode adjacent to the final stage dynode due to an increase in the current flowing between the final stage dynode and the dynode adjacent to the final stage dynode.The present invention provides a radiation detection method that uses a photomultiplier tube comprising a scintillator that absorbs radiation and emits scintillation light, and a divider circuit that sequentially multiplies the photoelectrons generated by the scintillation light between a cathode, a plurality of dynodes, and an anode to output an electrical output pulse from the anode side, distributes a DC voltage from a power supply to the cathode, each of the dynodes, and the anode, and is provided with a voltage change suppression means to suppress the change in the voltage applied to each of the dynodes due to the current that flows between the cathode and the anode when the current due to the photoelectrons flows, from the voltage when the current does not flow, and is characterized in that the radiation intensity is calculated by recognizing the supply current supplied from the power supply when the photomultiplier tube is operating and correcting the count value of the output pulses within a certain period of time based on the supply current within the certain period of time. The present invention provides a radiation detection method that includes a photomultiplier tube comprising a scintillator that absorbs radiation and emits scintillation light, and a divider circuit that sequentially multiplies the photoelectrons generated by the scintillation light between the cathode, a plurality of dynodes, and an anode to output an electrical output pulse from the anode side, distributes a DC voltage from a power supply to the cathode, each of the dynodes, and the anode, and is provided with a voltage change suppression means to suppress the change in the voltage applied to each of the dynodes due to the current that flows between the cathode and the anode when the current due to the photoelectrons flows, from the voltage when the current does not flow, and is characterized in that the current supplied from the power supply is recognized when the photomultiplier tube is operating, and the intensity of the radiation is calculated from the supply current.The present invention provides a radiation analysis method comprising: a sample measurement step, in which multiple conditions are defined as setting conditions such that the detection intensity for the same measurement target differs in the radiation detection device, one of the scintillators of different sizes is switched and used, the selection of the size of the scintillator is included in the setting conditions, and an actual measurement spectrum is obtained, which is the spectrum of the sample measured by the radiation detection device; a background nuclide-derived component estimation step, in which a background nuclide-derived component is a component in the actual measurement spectrum that is caused by the background nuclide, which is the nuclide that has the largest contribution to the radiation among the nuclides, is estimated from the results obtained by measuring with the radiation detection device for each of the setting conditions; and a corrected spectrum calculation step, in which a corrected spectrum is calculated by subtracting the background nuclide-derived component from the actual measurement spectrum, and the nuclide in the sample is analyzed using the corrected spectrum. The system includes a reference source measurement step in which radiation emitted from a reference source composed of the aforementioned background nuclides is measured by the radiation detection device for each of the set conditions to obtain a reference spectrum, and in the background nuclide-induced component estimation step, the background nuclide-induced component may be estimated by comparing the reference spectrum with the measured spectrum. The radiation detection device is equipped with a plurality of shielding bodies that can be switched between to absorb and limit the radiation incident on the radiation detection device to different degrees, and the selection of the shielding bodies may be included in the set conditions. The setting of the distance between the radiation detection device and the reference source in the reference source measurement step may also be included in the set conditions.The nuclide analyzer of the present invention is characterized by comprising a radiation detection device, wherein the radiation detection device has multiple conditions set as setting conditions such that the detection intensity for the same measurement target differs, one of the scintillators of different sizes is switched and used, the selection of the size of the scintillator is included in the setting conditions, and an analysis unit is provided which obtains an actual spectrum which is the spectrum of the sample measured by the radiation detection device, estimates a background nuclide-derived component which is the component caused by the background nuclide that has the largest contribution to the radiation in the actual spectrum, from the results obtained by measuring with the radiation detection device for each of the setting conditions, calculates a corrected spectrum by subtracting the background nuclide-derived component from the actual spectrum, and performs nuclide analysis in the sample using the corrected spectrum. The analysis unit may also obtain a reference spectrum which is the spectrum obtained by measuring the radiation emitted from a reference source composed of the background nuclide with the radiation detection device for each of the setting conditions, and estimate the background nuclide-derived component by comparing the reference spectrum with the actual spectrum. The radiation detection device is equipped with a plurality of shielding bodies that can be switched between to absorb and limit the radiation incident on the radiation detection device to different degrees, and the selection of such shielding bodies may be included in the setting conditions.

[0017] As the present invention is configured as described above, it is possible to obtain a radiation detection device with a simple configuration that can accurately recognize the energy and intensity of radiation, or to use it to perform high-precision analysis of the nuclide that emitted the radiation.

[0018] This diagram schematically shows the configuration of a photomultiplier tube without a voltage change suppression means. This diagram schematically shows the current flow in a photomultiplier tube without a voltage change suppression means when there are no photoelectrons. This diagram schematically shows the current flow in a photomultiplier tube without a voltage change suppression means when there are photoelectrons. This is an example of the configuration of a photomultiplier tube using a divider circuit equipped with a voltage change suppression means. This diagram schematically shows the relationship between the light intensity received by the photomultiplier tube and the supplied current (a: without a voltage change suppression means, b: with a voltage change suppression means). This is a diagram showing the configuration of a radiation detection device according to an embodiment of the present invention. This is a diagram showing the relationship between the radiation intensity recognized as the count value of the output pulse and the actual radiation intensity. This is the result of actual measurement of the relationship between the count value of the output pulse and the dose (radiation intensity) of the radiation source. This is the result of actual measurement of the relationship between the increase in the supplied current and the dose (radiation intensity) of the radiation source. This is an example of the time course of the output pulse (upper panel) and supplied current (lower panel) from the photomultiplier tube when acquiring an energy spectrum. This figure shows the configuration of the photomultiplier tube and its surroundings when a divider circuit without an auxiliary power supply is used in a radiation detection device according to an embodiment of the present invention. This figure shows the configuration of the photomultiplier tube and its surroundings when a divider circuit with an auxiliary power supply is used in a radiation detection device according to an embodiment of the present invention. This figure shows the configuration of an apparatus for performing a radiation analysis method (nuclide analysis method) according to an embodiment of the present invention. This is a flowchart of the radiation analysis method according to an embodiment of the present invention. This shows the output pulses output from the photomultiplier tube when the radiation intensity is low (a) and high (b).

[0019] The following describes a radiation detection device (radiation detection method) according to an embodiment of the present invention. This radiation detection device uses a scintillator that emits light (scintillation light) with a wavelength near the visible range when incident on gamma rays (radiation), and a photodetector that detects this light emission. In particular, a photomultiplier tube equipped with a voltage change suppression means is used as the photodetector. Therefore, the energy of the detected gamma rays can be recognized with high precision from the output pulse output from the anode of the photomultiplier tube.

[0020] In this process, in addition to the output pulses emitted from the anode side of the photomultiplier tube, the current supplied to the photomultiplier tube from the power supply (supply current) is monitored. Even if a situation like that shown in Figure 15(b) occurs, this supply current reflects the original radiation intensity, so the counting rate of the output pulses can be corrected according to this supply current value, and the original detection intensity can be calculated. In other words, this allows for accurate recognition of both the energy of the gamma rays and the number of incident rays per unit time (radiation intensity), and enables accurate recognition of the energy spectrum of the incident radiation.

[0021] First, we will explain the configuration and characteristics of a photomultiplier tube equipped with such voltage change suppression means. The following explanation is based on Chapter 5, "Divider Circuits and Accessories for Photomultiplier Tubes" (hereinafter referred to as "reference") of "Photomultiplier Tubes: Their Fundamentals and Applications, 4th Edition, Hamamatsu Photonics K.K."

[0022] Figure 1 is Figure 5-6 of the above-mentioned reference, showing the basic configuration of a photomultiplier tube without a voltage change suppression means. Here, when the pulse light (scintillation light) to be detected is incident on the cathode K on the left, photoelectrons proportional to the number of photons are generated. These photoelectrons (electrons) are accelerated and incident on the first-stage dynode Dy1, which is set to a potential positive to the cathode K, and are multiplied there. The multiplied electrons are accelerated by the potential of the next-stage dynode Dy2 and incident on dynode Dy2, where they are multiplied again. Similarly, the electrons multiplied on dynode Dy3 are finally incident on the anode P and output as a current Ip. In reality, the voltage when this current Ip flows through the load resistor (not shown in Figure 1) becomes the output pulse.

[0023] In reality, a constant high voltage VD is applied between the cathode K and anode P by a power supply (DC power supply), and this voltage VD is used in a divider circuit (resistor R １ ~R ４ The electrons are distributed according to the voltage (V) and applied to the dynodes Dy1 to Dy3. At this time, the electron multiplication factor in each dynode is determined by the potential difference (V) between each dynode and the preceding dynode (or cathode K). １ ~V ４Since it is determined by ( ), the multiplication factor of each stage is determined by the distribution of this potential. In FIG. 1, the current between the cathode K and the dynode Dy1 is I １ , the current between the dynodes Dy1 and Dy2 is I ２ , the current between the dynodes Dy2 and Dy3 is I ３ , the current between the dynodes Dy3 and the anode P is I ４ . According to the above principle, when electron multiplication is occurring, I １ < I ２ < I ３ < I ４ , and I ４ becomes Ip which is the output current.

[0024] FIG. 2(a) (FIG. 5-7 of the above-cited reference) schematically shows the current situation when pulsed light is not incident (no photoelectrons are generated) in this case by black arrows. Here, assuming there is no dark current, I １ = I ２ = I ３ = I ４ = Ip = 0, and all the current flows through the resistors R １ to R ４ . The voltage across each of the resistors R １ to R ４ becomes V １ to V ４ , and V １ + V ２ + V ３ + V ４ = VD. That is, in this case, the potential difference between each dynode and the dynode in the previous stage or the cathode K in the subsequent stage is determined to be V １ to V ４ as shown in a divider circuit having the resistors R １ to R ４ . At this time, the current I Ｒ１ to I Ｒ４ flowing through each resistor is I Ｒ１ = I Ｒ２ = I Ｒ３ = I Ｒ４ , which is equal to the current ID supplied from the power source of the voltage VD.

[0025] Figure 2(b) (Figure 5-8 in the above-mentioned reference) schematically shows the current situation when pulsed light is incident and photoelectrons are generated, causing a current to flow between the dynodes, etc., with black arrows. In this case, as the output Ip is generated, a portion of the current flows to the divider circuit side as shown in the figure, and the thickness of the black arrow corresponds to the magnitude of the current that flows in this case. １ ~I ４ Ip is I １ '~I ４ If we denote it as ', Ip', then electron multiplication will result in I １ '(≠0) < I ２ '<I ３ '<I ４ '=Ip'. In this case, current flows from each dynode to the divider circuit, therefore the I in this case is Ｒ１ ~I Ｒ４ I Ｒ１ '~I Ｒ４ 'If so, as shown in the diagram, I Ｒ１ '>I Ｒ２ '>I Ｒ３ '>I Ｒ４ 'It will be.'

[0026] Generally, Ip' << ID, and when the light intensity is low and Ip' is small, V １ ~V ４ This is not significantly different from the situation in Figure 2(a) where Ip = 0, and the electron multiplication factor in each stage can be considered constant. In this case, it can be assumed that an Ip proportional to the received light intensity is output, corresponding to this electron multiplication factor.

[0027] On the other hand, when Ip' is large (light intensity is large), １ '~I ４ As ', Ip' increases, the change from the situation in Figure 2(a) to the situation in Figure 2(b) becomes significant. In Figure 2(b), V in Figure 2(a) １ ~V ４ The voltage corresponding to V １ '~V ４ If so, V will be the same as in the case of Figure 2(a). １ '+V ２ '+V ３ '+V４ '=VD, but I Ｒ１ '~I Ｒ４ Since ' becomes as described above, V on the preceding side １ '>V １ For example, in the later stages, V ４ '<V ４ And so on. In other words, the distribution of the voltage applied to the dynode (the voltage used for electron multiplication) changes from the case shown in Figure 2(a). Such a change in voltage is caused by the current flowing from the dynode to the divider circuit, and this current is particularly large on the downstream side, as shown in the figure.

[0028] Figure 3 (Figure 5-5 in the above-mentioned reference) shows an example of the actual Ip / ID light intensity dependence. In this case, region A is the region with ideal characteristics where Ip is proportional to the light intensity, as described above. Region B, which deviates from these characteristics, arises for the reasons mentioned above. In contrast, as also described in the above-mentioned reference, V is caused by Ip as described above. １ ~V ４ (Especially the V on the later side) ４ It is known that a voltage change suppression means is provided in the divider circuit to suppress fluctuations such as (etc.).

[0029] Figure 4 shows three examples of such divider circuits described in the above-mentioned reference. Figure 4(a) (Figure 5-11 in the above-mentioned reference) shows a configuration using transistors, Figure 4(b) (Figure 5-15 in the above-mentioned reference) shows a configuration using a Cockcraft-Walton circuit, and Figure 4(c) (Figure 5-18 in the above-mentioned reference) shows a configuration using separate power supplies for the downstream dynodes. In Figures 4(a) and (c), as mentioned above, a transistor circuit and separate power supplies are provided only in the downstream side where the voltage change is particularly large. In the example in Figure 4, the anode P side is grounded via the load resistor RL, and a negative high voltage is applied to the cathode K side. The output pulse shown in Figure 15 is output as the voltage across the load resistor RL.

[0030] In a divider circuit equipped with such voltage change suppression means, the resistor R on the downstream side in the situation shown in Figure 2(b) ４ Ya R ３It can be considered that this function increases the current flowing through it according to Ip', and in this case, this increase in current is reflected in the current (supply current) ID supplied from the power supply with voltage VD. That is, in a divider circuit provided with such a voltage change suppression means, ID is the voltage VD as described above. ４ It increases by the amount that the above increases. Alternatively, in this case, ID includes a component to compensate for the voltage change as described above, and this component is proportional to the light intensity (number of incident photons). This light intensity is proportional to Ip, which is the output (that should be obtained) when there is no voltage change. That is, the current ID supplied to the divider circuit equipped with a voltage change suppression means increases depending on the light intensity. Generally, Ip is recognized in the pulse signal processing unit described later, and since the output via the preamplifier etc. is recognized here, a direct comparison between Ip and ID is generally not possible, but the increase in such supplied current ID is proportional to the above V, similar to the case of Ip. ４ These changes are handled separately from Ip (pulse output) and can be detected by an ammeter.

[0031] In this case, for example, the average value of the increase in supply current ID over a certain period corresponds to the number of photons received during that period. Since this number of photons corresponds to the integral value of the pulse waveform in Figures 15(a) and 15(b), in the case of Figure 15(a), it is the value of two pulses, and in the case of Figure 15(b), it is the value of five pulses. In other words, the supply current ID reflects the light intensity received by the photomultiplier tube, regardless of whether or not pile-up occurs when the radiation intensity is high.

[0032] On the other hand, if no voltage change suppression means is provided as shown in Figure 1, the supply current ID is determined by each resistor and therefore does not change with light intensity. Considering this point, the relationship between light intensity and supply current ID in a photomultiplier tube is schematically shown in Figure 5 (a: without voltage change suppression means, b: with voltage change suppression means). That is, if there is no voltage change suppression means, the supply current ID is constant regardless of light intensity, whereas when a divider circuit with a voltage change suppression means is used, there is a certain relationship (proportional relationship) between the increase in supply current ID from the case where the light intensity is zero and the value of the received light intensity. This situation holds true regardless of whether or not pile-up occurs when the radiation intensity is high. In Figure 5, the light intensity on the horizontal axis corresponds to the actual radiation intensity Y0 to be detected. Therefore, even when the radiation intensity is high, the radiation intensity Y0 to be detected can be recognized from the supply current ID using the characteristics in Figure 5(b). In Figure 5(b), the increase in supply current ID is assumed to be proportional to the radiation intensity Y0, but in reality, these may not be strictly proportional. In this case as well, according to the principle described above, the supply current ID increases with increasing radiation intensity Y0.

[0033] On the other hand, as shown in Patent Document 1, it is also important to recognize the energy of the detected gamma rays. To do this, it is necessary to recognize the individual output pulses shown in Figure 15 and calculate their pulse height and waveform integral value. For this reason, the radiation detection device according to the embodiment of the present invention is provided with a configuration that recognizes individual output pulses from the photomultiplier tube and determines their energy, or further counts them, as in the conventional method, and a separate configuration that measures the supply current ID supplied from the DC power supply of the photomultiplier tube. The radiation intensity Y0 recognized from the supply current ID is a quantity determined by all the radiation (gamma rays) detected within a certain period, whereas, for example, when obtaining an energy spectrum, the intensity for each gamma ray energy is recognized. For this reason, in practice, the energy spectrum can be measured with high accuracy by correcting this intensity using the supply current ID.

[0034] Figure 6 is a block diagram showing the configuration of the radiation detection device 1. This radiation detection device 1 is used to measure the energy spectrum of gamma rays emitted from a sample (or radiation environment). Here, as described above, a scintillator 10 that emits scintillation light by absorbing radiation (especially gamma rays) and a photomultiplier tube 20 that receives this scintillation light and emits an electrical output pulse are used in combination. Here, the photomultiplier tube 20 is equipped with the voltage change suppression means described above.

[0035] A power supply 30 is connected to the photomultiplier tube 20. The DC voltage from this power supply 30 is distributed to each electrode (cathode, each dynode, and anode) of the photomultiplier tube 20 via a divider circuit equipped with a voltage change suppression means. At this time, a current sensor 31 is provided to measure the supply current ID.

[0036] On the other hand, a pulse signal processing unit 40 is provided that reads the output pulses output from the anode of the photomultiplier tube 20, recognizes the pulse height and waveform integral value of each output pulse as quantities corresponding to the energy of the gamma rays, and counts these output pulses within a certain period of time. The operation of this pulse signal processing unit 40 is no different from that used in conventional photomultiplier tubes that do not have a voltage change suppression means.

[0037] The data processing unit 50 recognizes the energy of the gamma rays corresponding to each output pulse output by the pulse signal processing unit 40 from the photomultiplier tube 20, as described above, and recognizes the count value of these output pulses within a certain period (measurement period) as the gamma ray intensity. This makes it possible to recognize an energy spectrum for each measurement period, with the horizontal axis representing the gamma ray energy and the vertical axis representing the count value for each energy. Here, as described above, the gamma ray energy is determined with high precision because a voltage change suppression means is provided.

[0038] In this case, if the gamma ray intensity (count) is high, a lower light intensity may be recognized than the actual light intensity under the conditions shown in Figure 15(b). Figure 7 shows the light intensity recognized by the pulse signal processing unit 40 (recognized count: radiation intensity Y1) on the vertical axis and the actual radiation intensity Y0 on the vertical axis, with solid lines. Ideally, these should be the same, but as shown by the dashed lines, on the high-intensity side, the recognized radiation intensity Y1 deviates from the actual radiation intensity Y0 and becomes lower. In this case, the radiation intensity is the count value (count) of all gamma rays incident on (or detected by) the radiation detection device 1 within a certain period, and is different from the count value for each energy in the energy spectrum mentioned above.

[0039] Figure 8 shows the results of actually changing the dose rate of the radiation source using this radiation detection device 1 and measuring the count rate (counts per second) recognized by the pulse processing unit 40 as the radiation intensity Y1. From these results, it can be seen that when the dose rate is about 2 Gy / h or less (low dose rate), a proportional relationship exists between the dose rate and the count rate, whereas at higher dose rates, the count rate decreases below this proportional relationship. Specifically, this decrease is about 13% at 4.5 Gy / h.

[0040] In contrast, Figure 9 shows the results of actual measurements of the characteristics of the radiation detection device 1 shown in Figure 5(b). The divider circuit configuration used here is the same as that shown in Figure 4(a). In Figure 4(a), only the last three stages are controlled by the voltage change suppression means, but in this case, all stages are controlled. Here, the vertical axis represents the increase in the supply current ID (proportional component) in Figure 5(b), and the horizontal axis represents the dose rate, similar to Figure 8. This dose rate can actually be converted to radiation intensity Y0. As shown in Figure 5(b), it can be confirmed that a highly accurate proportional relationship exists between these values.

[0041] Here, a storage unit 60, such as a hard disk, is used to pre-store the characteristics of Figure 7 (actual values ​​are in Figure 8) and Figure 5(b) (actual values ​​are in Figure 9). The data processing unit 50 uses the pulse signal processing unit 40 to recognize the radiation intensity Y1 as the count of the output pulses, and also recognizes the supply current ID recognized by the current sensor 31. From this supply current ID, the actual radiation intensity Y0 can be recognized from the characteristics in Figure 9. As described above, the supply current ID does not reflect information for each gamma ray energy; rather, the detection results of all detected gamma rays are integrated and reflected, and the supply current ID can be easily recognized by the current sensor 31.

[0042] When calculating radiation intensity Y0 by counting pulses as shown in Figure 15, the pulse signal processing unit 40 shown in Figure 6 is used. In contrast, when calculating radiation intensity Y0 from the characteristics shown in Figure 9 (Figure 5(b)), only the current sensor 31 that measures the supply current ID is used, and the pulse signal processing unit 40 is unnecessary. The pulse signal processing unit 40 is necessary because it is necessary to recognize the pulse height in order to obtain the energy spectrum, but if the measurement of the energy spectrum is not necessary for the radiation detection device, the pulse signal processing unit 40 shown in Figure 6 becomes unnecessary, and the configuration can be greatly simplified. As shown in Figure 5, the fact that radiation intensity Y0 can be calculated with high accuracy from only the supply current ID is unique to the case when a photomultiplier tube 20 with voltage change suppression means is used. That is, in order to calculate radiation intensity Y0 from only the supply current ID as described above, it is sufficient to use a photomultiplier tube 20 with voltage change suppression means and a current sensor 31 directly connected to the power supply 30 as shown in Figure 6.

[0043] When acquiring an energy spectrum, it is necessary to obtain a large number of detected events (the total number of detected gamma rays), which increases the measurement time required. The upper part of Figure 10 shows an example of the time course of the output from the photomultiplier tube 20 (output pulse train) in this case. In Figure 15, each output pulse is represented here as a line segment with a time width of approximately zero. Since radiation is basically emitted at random time intervals, as shown here, the output pulses may be dense in the T1 section and sparse in the T2 section.

[0044] In this case, as shown in the lower part of Figure 10, if the supply current ID in section T1 is ID1 and the supply current ID in section T2 is ID2, then ID1 > ID2. In particular, if ID2 is in the range where Y1 = Y0 in Figure 7, and ID1 is in the range where Y1 < Y0 in Figure 7, then if the count in section T1 in this case is C1, then if Y0 corresponding to ID1 in Figure 5(b) (Figure 9) is Y01, and Y1 when Y0 = Y01 in Figure 7 (Figure 8) is Y11, then by replacing C1 with C1 × K (K = Y01 / Y11: correction coefficient), and setting the contribution of the output pulse in section T1 to the correction coefficient K (>1), the effect of pile-up as shown in Figure 15(b) can be removed, and a more accurate energy spectrum can be obtained. In the interval T2, this correction is not necessary, but since Y1 = Y0 in the interval T2, it is also possible to perform the same processing by assuming that the correction coefficient K is 1 (corresponding to Y1 = Y0).

[0045] In the example above, the target section was simplified by limiting it to sections T1 and T2. However, in reality, a correction coefficient K can be calculated similarly for more sections based on the characteristics in Figures 5(b) and 7 according to the supply current ID. A more accurate energy spectrum can then be calculated using the result obtained by multiplying the count for each section by the correction coefficient K for each section. Furthermore, when the dose rate is high, the interval between output pulses in Figure 9 can be made shorter (the count value is larger), and the section for recognizing the supply current ID can be set to be shorter. In this case, the supply current ID may be measured continuously on the time axis. In the configuration of Figure 6, the count is measured by the pulse signal processing unit 40, and the current ID for determining the correction coefficient K is measured by a current sensor 31 in a separate system.

[0046] Figure 11 is a diagram showing in more detail the relationship between the photomultiplier tube 20, power supply 30, current sensor 31, etc., in Figure 6. Here, the photomultiplier tube 20 is assumed to use a divider circuit 21 that employs a voltage change suppression means that does not use any power supply other than the power supply 30, as configured in Figure 4(a) or Figure 4(b). As described above, a load resistor RL is provided on the anode P side, and this voltage is taken out as the output. The current sensor 31 is connected to the power supply 30 as described above, and the supplied current ID is measured by this.

[0047] Figure 12 shows a similar configuration when the divider circuit 22 using the voltage change suppression means configured in Figure 4(c) is used. In this case, in addition to the power supply 30, auxiliary power supplies 221 and 222 are also provided in the divider circuit 22. Therefore, current sensors 223 and 224 are connected to these auxiliary power supplies 221 and 222, respectively, and the sum of the currents detected by current sensors 31, 223 and 224 is used as the supply current ID, thereby performing the same operation.

[0048] In either case, the radiation detection device 1 described above can be obtained simply by adding a current sensor to a photomultiplier tube that uses a divider circuit equipped with voltage change suppression means, and performing analysis using the supply current ID recognized by the current sensor. In other words, this radiation detection device 1 can be obtained at low cost.

[0049] In the radiation analysis method described in Patent Document 2 (a technique for recognizing nuclides that emit gamma rays under high background conditions), the difference in energy spectra measured during the analysis is calculated, and therefore, it is required that the energy spectra be obtained with particularly high accuracy. For this reason, the above-described radiation detection device 1 is particularly suitable for use in such a radiation analysis method.

[0050] Figure 13 shows the configuration of a nuclide analyzer 2 that implements this radiation analysis method using the radiation detector described above and performs analysis of radioactive nuclides contained in a sample. In this configuration, similar to Figure 6, a scintillator 10, a photomultiplier tube 20 equipped with a voltage change suppression means, a power supply 30, a current sensor 31, a pulse signal processing unit 40, a data processing unit 50, and a storage unit 60 are provided. In addition to the configuration of Figure 6, a shielding body 15 that shields the gamma rays to be detected to a certain extent is provided on the source side of the scintillator 10. In this configuration, the object to be measured is a sample 100 or a reference source 200, which is the nuclide that emits gamma rays and is the target for recognition. The sample 100 is generally not a specific object, but a specific environment (the environment under measurement). The reference source 200 is a nuclide (background nuclide) that exists at a high concentration (high dose) in the sample 100 (the environment under measurement), separate from the nuclide to be recognized, for example, １３７ Cs and ６０ It is Co.

[0051] In Figure 13, as described above, the data processing unit 50 recognizes the gamma rays emitted from the sample 100 or the reference source 200 and detects their energy spectrum. As described above, by using the above configuration, this energy spectrum can be obtained with particularly high accuracy. The computer (PC: analysis unit) 70 controls the photomultiplier tube 20, pulse signal processing unit 40, data processing unit 50, etc. to acquire this energy spectrum and, by performing the following analysis using this energy spectrum, recognizes the radionuclides contained in the sample 100.

[0052] Here, multiple types of scintillators 10 made of the same material but with different sizes (thickness along the direction of incidence, width perpendicular to the direction of incidence) are used. Also, multiple types of shielding bodies 15 made of the same material but with different thicknesses along the direction of incidence of gamma rays are switched between and used. The shielding body 11 is made of, for example, a heavy metal (W, etc.), and its thickness range is set to a range in which the measured spectrum (detection intensity of gamma rays) changes significantly, and is appropriately set according to the intensity of radiation (gamma rays) emitted by the sample 100.

[0053] Figure 14 is a flowchart illustrating this radiation analysis method (nuclide analysis method) as described in Patent Document 2, and is identical to Figure 4 in Patent Document 2. The computer 70 controls the photomultiplier tube 20, pulse signal processing unit 40, data processing unit 50, etc., to perform this operation.

[0054] Here, first, the energy spectrum (measured spectrum) of the gamma rays emitted from the sample 100 is measured (sample measurement step: S1). Here, as described in Patent Document 2, measured spectra are obtained for each size of the scintillator 10 and thickness of the shielding body 15 (set conditions).

[0055] Next, the same measurements as above are performed on the reference source 200 (reference source measurement step: S2), and energy spectra (reference spectra) are obtained for each setting condition (scintillator size, shielding thickness). Subsequently, from the reference spectra obtained in the reference source measurement step (S2), the background nuclides in the measured spectrum are identified. １３７ A background nuclide-derived component, which is a component caused by Cs, is estimated (background nuclide-derived component estimation step: S3). This background nuclide-derived component includes monochromatic gamma rays emitted by the background nuclide (for example) １３７ This includes not only the peak corresponding to the 662 keV monochromatic gamma rays emitted by Cs, but also other interfering components resulting from this peak.

[0056] As described in Patent Document 2, in the measured spectrum and the reference spectrum, the low-energy component (low-energy interfering component) is based on the energy of this peak and includes components generated by the Compton effect of gamma rays emitted from the background nuclide, while the high-energy component (high-energy interfering component) includes components generated by the Sum effect and pile-up of gamma rays emitted from the background nuclide. Of these, the high-energy interfering component largely depends on the size of the scintillator, and the low-energy interfering component largely depends on the thickness of the shielding. Therefore, performing measurements using the size of the scintillator and the thickness of the shielding as parameters, as described above, is effective in recognizing appropriate background nuclide-induced components across the entire energy range.

[0057] In this case, as described in Patent Document 2, the difference spectrum (difference spectrum) between the measured spectrum and the reference spectrum is calculated under conditions where the size of the scintillator and the thickness of the shielding material (setting conditions) are the same. As described in Patent Document 2, when calculating this difference, for example, normalization is performed so that the measured spectrum and the reference spectrum can correspond. As a normalization method, for example, peaks due to background nuclides (for example) １３７ Normalization can be performed such that the intensity of the 662 keV peak corresponding to Cs is equal in both spectra (i.e., the difference at this peak energy is zero). In this difference spectrum, the one with the smallest energy component of most interest, or the one with the highest contrast peak corresponding to the gamma-ray energy of the candidate nuclide to be recognized, is selected. Using this reference spectrum and various fittings, the background nuclide-derived component in the measured spectrum is calculated.

[0058] Next, the corrected spectrum is calculated as the difference between the measured spectrum under the set conditions and the background nuclide-induced component in this difference spectrum (corrected spectrum calculation step: S4). By analyzing this corrected spectrum, １３７The compositional analysis of nuclides other than Cs can be performed (corrected spectral analysis step: S5). Furthermore, as described in Patent Document 2, if the reference spectrum is known in advance, or if the background nuclide-derived components can be calculated solely from the measured spectrum, the reference source measurement step (S2) is unnecessary.

[0059] In this case, since the difference between the measured spectrum and the reference spectrum is used, it is required that these be measured precisely. In this case, by using the configuration shown in Figure 13 (or the radiation detection device 1 described above), these spectra can be accurately recognized, thereby enabling the acquisition of a particularly accurate corrected spectrum. This allows for the following: １３７ This allows for more accurate compositional analysis of nuclides other than Cs. The computer 70 in Figure 13 can perform various calculations shown in Figure 14, and the memory unit 60 can be used as appropriate during this process.

[0060] In the above example, multiple sizes of the scintillator 10 and the thickness of the shielding body 15 were set, and these values ​​(types) were switched and used as setting conditions. Of these, the thickness of the shielding body is particularly dependent on the low-energy side interference component, and is therefore especially effective in accurately determining the low-energy side component of the background nuclide-induced component. However, a component that is similarly highly dependent on the low-energy side component can be used, and parameters related to this component can be used as appropriate in place of the thickness of the shielding body 15. For example, a collimator may be used instead of the shielding body 15. When a collimator is used, the incident direction of gamma rays is restricted by the aperture of the collimator, and the degree of this restriction changes depending on the energy of the gamma rays. For this reason, the aperture diameter, etc., can also be used as a setting condition in place of the shielding body. In other words, such a collimator is also included in the shielding body 15 described above. Furthermore, as described in Patent Document 2, multiple distances between the radiation detection device (scintillator) and the reference source 200 may be set and included in the setting conditions.

[0061] Even when measuring radiation intensity other than the radiation analysis method described in Patent Document 2, it is clear that the above radiation detection device is effective. However, as described in the above cited document, since the above voltage change suppression means (divider circuit) is particularly effective in accurately recognizing the energy of γ-rays, the above configuration that can accurately recognize the intensity for each energy of γ-rays is particularly effective when measuring an energy spectrum. Also, since the measurement principle is the same, the above configuration is also effective in detecting X-rays. In this case, this radiation detection device can be used for elemental analysis and the like.

[0062] Also, neutrons can be detected by a radiation detector combining a scintillator and a photomultiplier tube. As described above, for γ-rays (X-rays), energy is recognized from the pulse height of each output pulse (intensity for each emission of scintillation light), and the histogram of this energy becomes an energy spectrum. On the other hand, when neutrons are detected, for example, in the case of thermal neutrons, high-energy charged particles generated by the reaction when neutrons are absorbed by a neutron absorber (e.g., ６ Li or １０ B, etc.) in the scintillator, and in the case of fast neutrons, high-energy recoil protons emit scintillation light in the same manner as the above γ-rays. Therefore, unlike the case of γ-rays, the intensity of the scintillation light (number of generated photons) in this case does not directly correspond to the energy of the detected neutrons. Therefore, although neutrons can be detected using a scintillator, unlike the case of γ-rays or X-rays, the energy of the incident neutrons and the pulse height of the output pulse do not necessarily correspond, and generally, no proportional relationship holds between them. Therefore, the histogram obtained in the same way by detecting neutrons is generally different from the energy spectrum of the detected neutrons. Furthermore, the energy of neutrons emitted from a nuclide generally has a continuous spectrum rather than a line spectrum with specific energy as in the example of the above γ-rays.

[0063] Therefore, the spectrum obtained when neutrons are detected by the above-mentioned radiation detector differs from the neutron energy spectrum. However, in some cases, a certain correspondence exists between this spectrum and the neutron energy for neutron detection. In this case, it is possible to use the spectrum obtained when neutrons are detected to analyze the nuclide that emitted the neutron.

[0064] During this analysis, along with neutron-emitting nuclides １３７ When background radionuclides such as Cs are present, the same problems as described above occur, and similar problems such as pile-up also occur. Therefore, it is clear that the above configuration is also effective when detecting neutrons and analyzing (recognizing) the radionuclide that emitted these neutrons.

[0065] As for the scintillator material (fluorescent material) mentioned above, any material capable of detecting the target radiation (gamma rays, X-rays, neutrons) can be used as appropriate. As for the shielding material, lead, tungsten, etc. can be used for gamma rays and X-rays, and for neutrons, materials with added moderators containing hydrogen or thermal neutron trapping materials such as boron can be used. Furthermore, when a collimator is used instead of a shielding material as described above, the materials constituting the collimator other than the opening can be the same as those for these shielding materials, and the degree of radiation transmission can be adjusted by providing not only an opening but also cavities as appropriate.

[0066] 1 Radiation detection device 2 Nuclide analyzer 10 Scintillator 15 Shielding 20 Photomultiplier tube 21, 22 Divider circuit 30 Power supply 31, 223, 224 Current sensor 40 Pulse signal processing unit 50 Data processing unit 60 Memory unit 70 Computer (PC: Analysis unit) 221, 222 Auxiliary power supply

Claims

1. A radiation detection device comprising: a scintillator that absorbs radiation and emits scintillation light; a photomultiplier tube that sequentially multiplies photoelectrons generated by the scintillation light between a cathode, a plurality of dynodes, and an anode, and outputs an electrical output pulse from the anode side, wherein the photomultiplier tube comprises a divider circuit that distributes a DC voltage from a power supply to the cathode, each of the dynodes, and the anode, and provides a voltage change suppression means to suppress the change in the voltage applied to each of the dynodes due to the current when the current due to the photoelectrons flows between the cathode and the anode, from the voltage when the current does not flow; a current sensor that recognizes the supply current supplied from the power supply when the photomultiplier tube is operating; and a data processing unit that calculates the intensity of the radiation based on the supply current.

2. The radiation detection device according to claim 1, characterized in that the data processing unit calculates the intensity of the radiation by correcting the count value of the output pulses within a certain period of time based on the supply current within the certain period of time.

3. The radiation detection device according to claim 2, characterized in that the data processing unit calculates a histogram of the intensity of each emission of scintillation light calculated from the count value corrected based on the supply current and the intensity of each emission of scintillation light calculated from each of the output pulses, thereby calculating a spectrum.

4. The radiation detection device according to claim 3, wherein the radiation is a gamma ray emitted from the object to be measured, and the data processing unit calculates the energy spectrum of the gamma ray, wherein the intensity of each emission is the energy of the gamma ray.

5. The radiation detection device according to claim 4, characterized in that the data processing unit recognizes radionuclides included in the measurement target from the energy spectrum.

6. The radiation detection device according to claim 1 or 2, characterized in that the voltage change suppression means suppresses fluctuations in the voltage applied between the final stage dynode and the dynode adjacent to the final stage dynode due to an increase in the current flowing between the final stage dynode and the dynode adjacent to the final stage dynode.

7. A radiation detection method comprising: a scintillator that absorbs radiation and emits scintillation light; a photomultiplier tube that sequentially multiplies photoelectrons generated by the scintillation light between a cathode, a plurality of dynodes, and an anode to output an electrical output pulse from the anode side, distributes a DC voltage from a power supply to the cathode, each of the dynodes, and the anode, and is provided with a voltage change suppression means that suppresses the change in the voltage applied to each of the dynodes due to the current when the current due to the photoelectrons flows between the cathode and the anode from the voltage when the current does not flow; and a method for detecting radiation characterized by recognizing the supply current supplied from the power supply when the photomultiplier tube is operating, and correcting the count value of the output pulses within a certain period of time based on the supply current within the certain period of time to calculate the intensity of the radiation.

8. A radiation detection method comprising: a scintillator that absorbs radiation and emits scintillation light; a photomultiplier tube that sequentially multiplies photoelectrons generated by the scintillation light between a cathode, a plurality of dynodes, and an anode to output an electrical output pulse from the anode side, distributes a DC voltage from a power supply to the cathode, each of the dynodes, and the anode, and is provided with a voltage change suppression means that suppresses the change in the voltage applied to each of the dynodes due to the current when the current flows between the cathode and the anode; and the method comprising: recognizing the supply current supplied from the power supply during the operation of the photomultiplier tube; and calculating the intensity of the radiation from the supply current.

9. A radiation analysis method comprising: detecting radiation emitted from a sample containing radioactive nuclides using a radiation detection device according to claim 3 or 4 and measuring the spectrum; in the radiation detection device, a plurality of conditions with different detection intensities for the same measurement target are defined as setting conditions, one of the scintillators of different sizes is switched and used, and the selection of the size of the scintillator is included in the setting conditions; a sample measurement step to obtain an actual spectrum which is the spectrum measured by the radiation detection device of the sample; a background nuclide-derived component estimation step to estimate a background nuclide-derived component, which is a component in the actual spectrum that is caused by the background nuclide, which is the nuclide that has the largest contribution to the radiation among the nuclides, from the results obtained by measuring with the radiation detection device for each setting condition; and a corrected spectrum calculation step to calculate a corrected spectrum by subtracting the background nuclide-derived component from the actual spectrum; and performing analysis of the nuclides in the sample using the corrected spectrum.

10. The radiation analysis method according to claim 9, comprising a reference source measurement step of obtaining a reference spectrum which is the spectrum obtained by measuring the radiation emitted from a reference source composed of the background radionuclides with the radiation detection device for each of the set conditions, and characterized in that in the background radionuclide-induced component estimation step, the background radionuclide-induced component is estimated by comparing the reference spectrum with the measured spectrum.

11. The radiation analysis method according to 9 or 10, characterized in that the radiation detection device is provided with a plurality of shielding bodies that can be switched between to absorb and limit the radiation incident on the radiation detection device to different degrees, and the selection of the shielding bodies is included in the setting conditions.

12. The radiation analysis method according to claim 10, characterized in that the setting of the distance between the radiation detection device and the reference radiation source in the reference radiation source measurement step is included in the setting conditions.

13. A radionuclide analyzer comprising a radiation detection device according to claim 3 or 4, wherein a plurality of conditions are defined as setting conditions such that the detection intensity for the same object to be measured differs in the radiation detection device, one of the scintillators of different sizes is switched and used, the selection of the size of the scintillator is included in the setting conditions, and an analysis unit is provided which obtains an actual spectrum which is the spectrum of a sample measured by the radiation detection device, estimates a background nuclide-derived component which is the component caused by the background nuclide that has the greatest contribution to the radiation in the actual spectrum from the results obtained by measuring with the radiation detection device for each setting condition, calculates a corrected spectrum by subtracting the background nuclide-derived component from the actual spectrum, and performs analysis of the nuclides in the sample using the corrected spectrum.

14. The radionuclide analyzer according to claim 13, characterized in that the analysis unit obtains a reference spectrum, which is the spectrum obtained by measuring the radiation emitted from a reference source composed of the background radionuclides with the radiation detection device for each of the set conditions, and estimates the background radionuclide-induced component by comparing the reference spectrum with the measured spectrum.

15. The radionuclide analyzer according to claim 13 or 14, characterized in that the radiation detection device is provided with a plurality of shielding bodies that can be switched between to absorb and limit the radiation incident on the radiation detection device to different degrees, and the selection of the shielding bodies is included in the setting conditions.

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