Radiation analysis method and radiation analysis device

Abstract

The present invention carries out highly accurate assay of a sample, even under high-dose radiation, by analyzing an energy spectrum obtained by a radiation detector.　In this radiation analysis method, first, the spectrum (actual spectrum) of a sample is measured by a radiation detector (sample measurement step: S1). In this step, two or more types of scintillators having different sizes are used and two or more types of shielding substances having different thicknesses are used to obtain an actual spectrum for each condition (setting condition). Next, measurement similar to that described above is carried out with respect to a reference radiation source (reference radiation source measurement step: S2). Next, a background nuclide-derived component, which is a component derived from a background nuclide (137Cs) in the actual spectrum, is inferred from the reference spectrum obtained in the reference radiation source measurement step (S2) (background nuclide-derived component inference step: S3). Next, a corrected spectrum, which is the difference between the actual spectrum and the background nuclide-derived component, is calculated (corrected spectrum calculation step: S4).

Description

[Technical field]

[0001] The present invention relates to a radiation analysis method and apparatus for analyzing a radiation source contained in a sample by measuring the spectrum of radiation (gamma rays, X-rays, etc.), and a radiation detector used therein. [Background technology]

[0002] In order to detect the presence of radioactive nuclides, it is effective to use a method for detecting radiation (such as gamma rays) emitted when the nuclides decay, and for this purpose, a radiation detector capable of measuring the energy spectrum of radiation is used. In this case, gamma rays are generally used as the radiation, and since the energies of gamma rays emitted by different nuclides are generally different, the nuclides present in the sample (radiation environment) can be identified from the energy spectrum (spectrum) of gamma rays detected from the sample. Such a technique is described, for example, in Patent Document 1, etc.

[0003] Known radiation detectors capable of measuring the gamma-ray spectrum include detectors using semiconductor elements such as Ge and CdTe, and scintillators. In the latter, the scintillator absorbs gamma-ray photons and emits visible light photons, which are detected by a photomultiplier tube. In principle, a detector combining a scintillator and a photomultiplier tube produces a corresponding short-duration pulse output each time a gamma-ray photon is detected, and the charge (or waveform integral) of this pulse corresponds to the energy of the detected gamma ray. Therefore, the histogram of the charge (or waveform integral) of the detected pulse corresponds to the detected energy spectrum. Even if a sample contains a mixture of multiple types of nuclides, the presence of each nuclide can be recognized if the peaks corresponding to the energy of the gamma rays emitted by each of them can be separated and recognized in this spectrum.

[0004] However, the resolution (energy resolution) of the spectrum obtained in this way is determined by the type of radiation detector, and in reality, when two or more types of nuclides that emit gamma rays with close energies are mixed, it is difficult to recognize these nuclides individually. For this reason, when using a scintillator, for example, a scintillator material is selected that can obtain an energy resolution according to the purpose. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2016-513256 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, in the measurement of samples (measurement environment) related to a nuclear reactor, there are many radioactive nuclides, among which, 137 Because of the high Cs content 137 The gamma rays emitted by Cs are particularly detected in large amounts, resulting in high doses. 137 To recognize the presence of radionuclides other than Cs, 137 In the gamma ray spectrum, the influence of gamma rays emitted by Cs is strongly evident. 137 It is necessary to recognize the gamma ray peaks emitted by radioactive nuclides other than Cs.

[0007] Furthermore, 137 Cs emits monochromatic gamma rays with an energy of 662 keV, but when a radiation detector actually detects this monochromatic gamma ray, various components that are generated during the process of absorbing this monochromatic gamma ray in the radiation detector are also detected in addition to the monochromatic gamma ray with an energy of 662 keV. 137When the abundance ratio of Cs is high, this component also increases, hindering the recognition of gamma rays emitted by other nuclides. For example, in a scintillator, Compton scattering of monochromatic gamma rays with an energy of 662 keV produces gamma rays (X-rays) with a continuous spectrum on the lower energy side, and these components are also detected by a radiation detector at the same time.

[0008] As mentioned above, the charge (or waveform integral) of the pulse output by the radiation detector corresponding to one detected gamma-ray photon corresponds to the energy of this gamma-ray photon. However, as a prerequisite for such recognition, individual gamma-ray photons may be detected separately by the radiation detector. In contrast, two or more gamma-ray photons may be detected as a single pulse as a result of superposition. For example, if a gamma-ray photon is detected during the duration of a pulse corresponding to one gamma-ray photon due to a high dose of gamma rays, the pulses corresponding to these two gamma-ray photons may be superposed, resulting in the recognition of a single pulse with a charge (or waveform integral) different from the energy of either gamma-ray photon (pile-up). A similar phenomenon also occurs when a gamma-ray-emitting nuclide undergoes decay in two stages with a short time constant and gamma rays are emitted in each stage (sum effect). In such cases, false peaks or background components corresponding to these pulses occur in the gamma-ray spectrum.

[0009] In this way, certain radionuclides ( 137 When the abundance ratio of Cs is particularly high, the components detected at energies other than the monochromatic gamma ray energy due to the monochromatic gamma ray emitted by this nuclide as described above adversely affect the detection of gamma rays emitted by other radionuclides. This adverse effect can be seen, for example, in 137 When the abundance ratio of Cs is high, this also extends to the recognition of radioactive nuclides that emit gamma rays with energies far from 662 keV. This situation is similar not only when the abundance ratio of only one type of radioactive nuclide is high, but also when the abundance ratio of multiple types of radioactive nuclide is high, so it occurs in general when analyzing nuclides by analyzing gamma ray spectra under high doses.

[0010] For this reason, it has been desirable to be able to perform highly accurate analysis of samples by analyzing the energy spectrum obtained by a radiation detector even under high radiation doses.

[0011] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide an invention that solves the above problems. [Means for solving the problem]

[0012] In order to solve the above problems, the present invention has the following configuration. The radiation analysis method of the present invention is a radiation analysis method for measuring an energy spectrum of radiation emitted from a sample containing a mixture of a plurality of radioactive nuclides and recognizing the nuclides contained in the sample, the method comprising: a sample measurement step in which a plurality of conditions under which detection intensities for the same measurement target are different are set as set conditions in a radiation detector that detects the radiation, and an actual measured spectrum is obtained, which is the energy spectrum measured by the radiation detector; a background nuclide-attributed component estimation step in which a background nuclide-attributed component in the actual measured spectrum is a component attributable to a background nuclide that is the nuclide that contributes the largest amount of radiation among the nuclides, is estimated from results obtained by measuring with the radiation detector for each of the set conditions; and a corrected spectrum calculation step in which a corrected spectrum is calculated by subtracting the background nuclide-attributed component from the actual measured spectrum, and the sample is analyzed using the corrected spectrum. The radiation analysis method of the present invention comprises a reference source measurement step of obtaining a reference spectrum, which is an energy spectrum obtained by measuring radiation emitted from a reference source constituted by the background nuclides for each of the set conditions with the radiation detector, and is characterized in that in the background nuclide-attributable component estimation step, the background nuclide-attributable component is estimated by comparing the reference spectrum with the actual measured spectrum. The radiation analysis method of the present invention is characterized in that the setting of the distance between the radiation detector and the reference radiation source in the reference radiation source measurement step is included in the setting conditions. In the radiation analysis method of the present invention, the radiation detector is provided with a plurality of switchable shields that absorb and limit the radiation incident on the radiation detector to different degrees, and the selection of the shield is included in the setting conditions. In the radiation analysis method of the present invention, the radiation detector comprises a scintillator that absorbs the radiation and emits light, and a photodetector that detects the light, and the scintillators of different sizes are switched for use, and the selection of the size of the scintillator is included in the setting conditions. In the radiation analysis method of the present invention, the photodetector is a photomultiplier tube. In the radiation analysis method of the present invention, the photomultiplier tube has a cathode, an anode, and a plurality of dynodes between the cathode and the anode, and the photomultiplier tube is provided with a voltage change suppression means for suppressing fluctuations in the voltage applied between the anode and the final dynode, which is the dynode adjacent to the anode, when the light is detected. In the radiation analysis method of the present invention, the voltage change suppression means suppresses fluctuations in voltage applied between the final dynode and the dynode adjacent to the final dynode, which are caused by an increase in current flowing between the final dynode and the dynode adjacent to the final dynode. In the radiation analysis method of the present invention, the background nuclide is 137 Cs or 60 It is characterized in that it is Co. A radiation analysis apparatus according to the present invention is a radiation analysis apparatus for executing the radiation analysis method, and is characterized in that it includes a calculation unit for executing the background nuclide-attributable component estimation step to calculate the background nuclide-attributable component and for executing the corrected spectrum calculation step to calculate the corrected spectrum. 。 Effect of the Invention

[0013] Since the present invention is configured as described above, it is possible to perform highly accurate analysis of a sample by analyzing the energy spectrum obtained by the radiation detector even under high radiation dose conditions. [Brief description of the drawings]

[0014] [Figure 1] This is an example of a measured spectrum obtained from a high-dose sample. [Diagram 2] This is an example of a reference spectrum obtained from a reference source composed of 137Cs. [Diagram 3] FIG. 11 is a diagram showing a configuration of a reflection type spectroscopic analysis device according to a third embodiment of the present invention. [Figure 4] 1 is a diagram showing an apparatus configuration for carrying out a radiation analysis method according to an embodiment of the present invention; [Diagram 5] 2 is a flowchart showing a radiation analysis method according to an embodiment of the present invention. [Figure 6] 4 is a flowchart showing an example of a background nuclide-induced component estimation step in the radiation analysis method according to the embodiment of the present invention. [Figure 7] 4 is a measured spectrum obtained in a radiation analysis method according to an embodiment. [Figure 8] 2 is a reference spectrum obtained in a radiation analysis method according to an embodiment. [Figure 9a] 1 shows a shape of the vicinity of the 60Co peak in an actual measured spectrum obtained by a radiation analysis method according to an embodiment. [Figure 9b] 1 is a corrected spectrum corresponding to the shape in the vicinity of the 60Co peak in an actual measured spectrum obtained by a radiation analysis method according to an embodiment. [Figure 10] FIG. 1 is a diagram showing a simplified configuration of a photomultiplier tube. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, a radiation analysis method according to an embodiment of the present invention will be described. In this radiation analysis method, the nuclide that emitted the radiation (gamma rays) is recognized from the spectrum of the radiation obtained by the radiation detector. Even when the radiation dose is high and the radiation emitted by the nuclide to be detected is difficult to recognize in the spectrum, the recognition can be performed with high accuracy.

[0016] Here, the sample to be measured is part of the environment of a nuclear facility or a radiation facility, so the radiation level is high, and the detected radiation (gamma rays) is 137 The influence of gamma rays emitted by Cs is assumed to be the strongest. The radiation detector used to detect gamma rays here is a combination of a scintillator and a photomultiplier tube, and when one gamma ray photon is incident on the scintillator, pulsed visible light (pulsed light) is emitted from the scintillator due to its absorption. This pulsed light is detected by the photomultiplier tube, and the voltage output becomes a pulse corresponding to this pulsed light as described above, and in the ideal case, the pulse charge (or waveform integral value) corresponds to the energy of the gamma ray photon absorbed by the scintillator. The scintillator and photomultiplier tube used are set so that gamma rays in the target energy range are measured with high efficiency, and details will be described later.

[0017] However, particularly under high radiation doses, as mentioned above, in reality, many components of this pulse charge (or waveform integral value) do not correspond to the energy of the gamma-ray photon absorbed by the scintillator. Figure 1 shows the simulation results of measurements in a high-dose environment related to a nuclear facility or radiation facility. The energy of each peak recognized here and the nuclides that emit gamma rays of this energy are shown in the figure, and here 137 Cs(662keV), 134 Cs(604keV, 796keV), 40A peak corresponding to the gamma rays emitted by K (1408 keV) is visible. Although the gamma rays emitted by these nuclides are monochromatic, in this spectrum they are recognized as a distribution whose peak corresponds to this energy and whose spread is determined by the energy resolution of the detector. Among these, 137 The Cs (662 keV) peak is the most prominent. However, in addition to these peaks, a continuum component with high intensity, especially on the low energy side, is observed. 137 The component resulting from the Compton scattering of the gamma rays of Cs (662 keV) in the scintillator is included. Also, the peak at 1334 keV (twice the 662 keV) on the high energy side is due to the above-mentioned 137 This peak is due to the sum effect of Cs (662 keV).

[0018] On the other hand, Figure 2 shows a reference sample. 137 This is the spectrum of the actual measurement when a Cs source is measured with a similar detector. Here, the horizontal axis is the detector channel, which corresponds to the gamma ray energy, and the vertical axis is the count number (the number of gamma rays detected). In this case, too, 137 A peak (corresponding to 662 keV) is seen in which the 662 keV monochromatic gamma ray emitted by Cs is recognized with a spread determined by the energy resolution of the radiation detector, and a continuous component on the low energy side including the effect of Compton scattering and a peak due to the sum effect are also seen. From Figure 1, the peak due to the sum effect is 40 Because it is close to the peak due to K, 40 This has a large effect on the detection accuracy of K. The components on the lower energy side of the 662 keV peak in Figure 2 (low-energy interference components) include components generated by the Compton effect, while the components on the higher energy side of the 662 keV peak (high-energy interference components) include components generated by the sum effect, pile-up, etc. Most of the low-energy interference components and high-energy interference components are 137 This is particularly noticeable when the amount of Cs is large, and in Figure 1 137 This has a significant effect on the recognition of peaks of nuclides other than Cs.

[0019] Therefore, the spectrum in Fig. 1 is as shown in Fig. 2. 137 If we subtract the component caused by Cs, 137 It is expected that a spectrum will be obtained in which the gamma ray peaks of nuclides other than Cs are prominent. If only the 662 keV peak is visible in the spectrum of Figure 2, this operation will be easy, and the spectrum of Figure 1 will be 137 It is possible to remove the components caused by Cs. However, in addition to this peak, the spectrum in Figure 2 also shows the above-mentioned interfering components (low-energy interfering components, high-energy interfering components), and these interfering components vary depending on the measurement conditions and environment. For this reason, the actual interfering components in the results measured under the actual measurement environment (corresponding to Figure 1) generally do not match the interfering components in the spectrum in Figure 2 measured in a different environment for a reference sample.

[0020] Of these, the high-energy interference components are as follows: 137 This includes the part that is generated by the pile-up and sum effect of gamma rays emitted by Cs. Specifically, this interference component is 137 This becomes noticeable when the number of counts per unit time of gamma rays emitted by Cs (total number of detections) is large. As mentioned above, a scintillator and a photomultiplier tube are used as the radiation detector used here, and what is detected here is the pulsed light emitted when the gamma rays are absorbed by the scintillator. This number depends on the size of the scintillator, and the larger the scintillator, the greater the number. Therefore, when the scintillator is large, the intensity of the peak of the nuclide to be measured in the spectrum becomes high, but the interfering components on the high energy side also become large. Conversely, when the scintillator is small, the intensity of the peak of the nuclide to be measured in the spectrum becomes low, but the interfering components on the high energy side become small and can be made negligible. In other words, by changing the size of the scintillator, the influence of the interfering components on the high energy side in particular can be adjusted.

[0021] In addition, the low-energy interference components in Figure 2 are 137 It contains gamma rays (X-rays) of lower energy that are generated by the Compton effect due to the gamma rays emitted by Cs. 137 Compared to the gamma rays emitted by Cs and the above-mentioned high-energy interfering components, it is more easily absorbed (its absorption coefficient in materials is large), so it can be reduced by using shielding. In other words, by changing the degree of absorption by changing the thickness of the shielding, it is possible to adjust the influence of low-energy interfering components in particular.

[0022] In the above, we simplified the high-energy interference component in Figure 2 as depending on the size of the scintillator, and the low-energy interference component on the thickness of the shielding, but in reality, both the high-energy interference component and the low-energy interference component are affected by both. For this reason, the influence in the actually measured spectrum (corresponding to Figure 1) is that of the dominant nuclide (background nuclide). 137 A reference source consisting of Cs ( 137 From the spectra measured by changing the size of the scintillator and the thickness of the shielding, the components to be subtracted from the spectrum in Figure 1 (components due to background nuclides) can be reproduced. These components due to background nuclides include: 137 This includes not only the 662 keV gamma rays emitted by Cs itself, but also components due to pile-up and summing effects that occur during the measurement, as mentioned above, and background components.

[0023] As described above, both the high-energy interference components and the low-energy interference components in the spectrum (actually measured spectrum) obtained for the sample 137 When it is caused by Cs, the detection strength of the high-energy interference component and the low-energy interference component, 137 It is clear that there is a correlation between the detection intensity (peak intensity) of the monochromatic gamma ray of 662 keV emitted by Cs. 137When a 1000 keV (Cs source) is measured under various conditions with different detection intensities using the same radiation detector as the sample, and measurement results (reference spectra) are obtained for each condition, every reference spectrum contains a 662 keV peak, high-energy interference components, and low-energy interference components. 137 The 662 keV peak, which most strongly reflects Cs, has the shape closest to the 662 keV peak in the sample spectrum (measured spectrum). 137 It can be estimated that the reference spectrum best reflects the component caused by Cs. In other words, this reference spectrum can be used as the component caused by background nuclides. The parameters that can result in different detection intensities when obtaining such a reference spectrum include the size of the scintillator and the thickness of the shielding, as described above, but in the simplest case, the distance between the radiation detector and the reference source can also be used as this parameter.

[0024] FIG. 3 is a schematic diagram of the entire measurement system for implementing the above-mentioned radiation analysis method. In this configuration, a radiation detector 10 for detecting gamma rays and a computer (calculation unit) 20 for recognizing the measurement result (energy spectrum of gamma rays: spectrum) of the radiation detector 10 and performing various arithmetic processing using the result to calculate the above-mentioned mass nuclide-attributable components and a spectrum obtained by subtracting the mass nuclide-attributable components from the actual measured spectrum are used. The radiation detector 10 is a combination of a shield 11, a scintillator 12, and a photomultiplier tube (photodetector) 13, and a plurality of shields 11 are used in a switchable manner, and the scintillator 12 is made of the same material but has different sizes (thickness in the optical axis direction, width perpendicular to the optical axis direction). The shield 11 is made of, for example, a heavy metal (W, etc.), and the range of its thickness 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 the radiation (gamma rays) emitted by the sample 100. Details of the scintillator 12 and the photomultiplier tube 13 will be described later. As described above, a pulsed voltage output is emitted from the photomultiplier tube 13, and for this processing, this signal is AD converted at short time intervals (for example, 2 ns or less) in the signal processing unit 30 and recognized as a digital signal in the computer 20.

[0025] Moreover, the radiation detector 10 obtains the spectrum of gamma rays emitted by the sample 100 and the reference radiation source 200 for each of the scintillators 12 of different sizes and the shields 11 of different thicknesses, and these are stored in the hard disk or non-volatile memory of the computer 20 and used for calculations. Here, the reference radiation source 200 is a radiation source composed of the background nuclide described above, and in the above case, 137 Cs, but is actually set according to the sample 100. This background nuclide is the nuclide whose most prominent peak appears in the spectrum of the sample measured by the radiation detector 10 as shown in FIG. 1. For example, when the sample 100 is in a nuclear reactor environment, 137 In addition to Cs 60 It is sometimes referred to as Co.

[0026] Fig. 4 is a flow chart showing a radiation analysis method executed using the above-mentioned configuration. In this radiation analysis method, first, the spectrum (measured spectrum) of the sample 100 as shown in Fig. 1 is measured by the radiation detector 10 (sample measurement step: S1). As described above, here, a plurality of different sizes of the scintillator 12 and a plurality of different thicknesses of the shielding body 11 are used, and a measured spectrum is obtained for each condition (set condition) and stored in the computer 20. Fig. 1 corresponds to one example of such measured spectrum.

[0027] Next, the reference radiation source 200 is subjected to the same measurement as described above (reference radiation source measurement step: S2), and the spectrum (reference spectrum) obtained for each set condition is stored in the computer 20. The nuclide (background nuclide) selected as the reference radiation source 200 can be selected as the most dominant nuclide from the measured spectrum obtained in the sample measurement step. If the background nuclide is known in advance, the reference radiation source measurement step may be performed before the sample measurement step. In this case, a common reference spectrum can be used even if the sample 100 is different, and this step does not need to be performed every time a different sample 100 is used. FIG. 2 shows one example of such a reference spectrum.

[0028] Next, from the reference spectrum obtained in the reference source measurement step (S2), the background nuclides ( 137 The background nuclide component, which is a component caused by 1Cs, is estimated (background nuclide component estimation step: S3). 137 The peaks include not only the peaks corresponding to the monochromatic gamma rays of 662 keV emitted by Cs, but also the above-mentioned interfering components. Various methods can be selected as the estimation method for this purpose.

[0029] Fig. 5 is a flow chart showing an example of this method. The calculation process for this is carried out by the computer 20. Here, the size of the scintillator 12 and the thickness of the shielding body 11 are determined as the setting conditions of the radiation detector 10 as described above, and an analysis is carried out for each setting condition. For this purpose, first, a certain setting condition (a combination of the size of the scintillator 12 and the thickness of the shielding body 11) is set (S31). From the actual measurement spectrum and the reference spectrum, one that satisfies this condition is selected, and normalization is carried out so that these can be compatible. Various normalization methods are used for this purpose, for example 137 Normalization is performed (S32) so that the peak intensity at 662 keV corresponding to Cs becomes equal for both (i.e., the difference at this peak energy becomes zero). Then, the spectrum of the normalized difference (difference spectrum) is calculated (S33) and stored in computer 20. This operation is performed until difference spectra are obtained for all conditions (S34: Yes).

[0030] Next, it is recognized which of the setting conditions (size of the scintillator 12, thickness of the shielding body 11) corresponds to the most appropriate difference spectrum (S35). The criteria for this judgment can also be set appropriately. For example, when the influence of the sum effect in the spectrum is large, the spectrum in which the component corresponding to this sum effect is the smallest may be selected, or the spectrum in which the peak corresponding to the gamma ray energy of the candidate nuclide to be recognized is obtained with the highest contrast may be selected.

[0031] This determines the setting conditions (size of the scintillator 12, thickness of the shield 11) corresponding to the background nuclide-induced component, and the corresponding reference spectrum is recognized. However, since this reference spectrum is the result of actual measurement like the actual measurement spectrum, it generally has a large statistical error. For this reason, it is desirable to select a background nuclide-induced component estimated to be plausible from this reference spectrum or to construct an interpolated spectrum. For this purpose, it is possible to compare the results of fitting the actual measurement result and the reference spectrum using modeling near the peak as described in chapter 10 (Radiation Spectroscopy with Scintillators) by Willy, "Radiation Detection and Measurement", for example, and to determine the plausible reference spectrum or interpolated spectrum as the background nuclide-induced component. In this case, for example, the 662 keV peak can be approximated by a Gaussian distribution, and a spectrum obtained by adding a linear component to this for energy can be fitted to this reference spectrum by the least squares method. This modeling can be appropriately set according to the shape of the reference spectrum. The results obtained by fitting in this way may be used at the stage of calculating the difference spectrum (S33).

[0032] In the background nuclide-induced component estimation step (S3), the background nuclide-induced component can be estimated using only the measured spectrum obtained for each set condition in the sample measurement step (S1) without using the reference spectrum obtained for each set condition in the reference radiation source measurement step (S2). This is particularly effective when there are many set conditions (the number of types of scintillators 12 and shields 11 used). In this case, the background nuclide-induced component for a certain size of scintillator 12 can be estimated by calculating the difference between the measured spectrum for each set condition, for example, the difference between the measured spectrum corresponding to two different sizes of scintillators 12. This estimation is particularly effective when the reference spectrum is approximated with high accuracy by the model described above. This is equivalent to estimating the reference spectrum by obtaining many differences between the measured spectrum under different set conditions instead of obtaining the reference spectrum by actual measurement. In this case, the reference radiation source measurement step (S2) is unnecessary, so that the time required for measurement can be shortened.

[0033] Next, as shown in Fig. 4, a corrected spectrum is calculated, which is the difference between the measured spectrum and the background nuclide-induced component B (corrected spectrum calculation step: S4). Fig. 6 shows such background nuclide-induced component B and the corrected spectrum SP when the result of Fig. 1 is the measured spectrum. In this corrected spectrum, 137 Since the components due to Cs are reduced, other nuclides ( 134 Cs and 40 Therefore, by analyzing the corrected spectrum, it is possible to perform composition analysis of these other nuclides (corrected spectrum analysis step: S5).

[0034] In the above example, the size of the scintillator 12 and the thickness of the shielding body 11 are specified as the setting conditions for the radiation detector 10. Here, the shielding body 11 functions as a filter for cutting the low energy side, so its thickness affects the shape of the spectrum. Furthermore, the material of the shielding body 11 affects the generation of characteristic X-rays. It is also possible to construct a multi-layered shielding body 11 with different shielding thicknesses and shielding materials. However, when low energy photons are not to be measured, the structure of the shielding body 11 may be kept constant (or the shielding body 11 may not be used), and only the size of the scintillator 12 may be changed to perform the above measurement. In addition, in the above example, multiple thicknesses of the shielding body 11 are set to change the degree of absorption of radiation by the shielding body 11, but multiple materials of the shielding body 11 may be set to change the degree of absorption. In addition, the above radiation detector 10 recognizes the energy of each detected gamma-ray photon. In such applications, the combination of the scintillator 12 and the photomultiplier tube 13 as described above is particularly preferable. However, a photodetector capable of recognizing the energy of each detected gamma ray photon in the same manner as the photomultiplier tube 13 may be used instead.

[0035] In the corrected spectrum analysis step (S5), other analyses may be performed instead of the composition analysis of nuclides. In this case, the above-mentioned corrected spectrum can be used to determine the abundance of nuclides. 137 In the above example, the effect of background nuclides can be reduced. 137 Cs, but other nuclides may be used depending on the circumstances, e.g. 60 The same applies when Co is used as the background nuclide. In the above example, gamma rays were detected and their energy spectrum was obtained, but if the sample is analyzed using other radiation, such as the energy spectrum of neutrons, and the influence of a particular nuclide is large, the same method can be applied.

[0036] In the flowchart of FIG. 5, the size of the scintillator 12 and the thickness of the shielding body 11 are determined as the set conditions. Meanwhile, as described above, in the reference radiation source measurement step (S2),137 Measurement results obtained under multiple conditions with different measured intensities of Cs can be used as the reference spectrum. The actual procedure for this case will be explained according to the flowchart in Figure 4.

[0037] In this case, in the sample measurement step (S1), the settings on the radiation detector 10 side (the size of the scintillator 12, the thickness of the shielding body 11) are kept constant, and the result of the measurement can be used as the actual measured spectrum.

[0038] Next, in the reference radiation source measurement step (S2), the reference radiation source 200 (background nuclide 137 Cs) is measured under a plurality of conditions (set conditions) that differ in measurement intensity in the radiation detector 10 to obtain a reference spectrum. As described above, the distance between the reference radiation source 200 and the radiation detector 10 can be set as the set condition in this case. That is, in the reference radiation source measurement step (S2), a plurality of measurements are performed while changing the distance between the reference radiation source 200 and the radiation detector 10, thereby obtaining a plurality of corresponding reference spectra.

[0039] Next, in the background nuclide-related component estimation step (S3), the shape around the 662 keV peak in the measured spectrum is compared with the shape around the 662 keV peak in each reference spectrum, and the reference spectrum whose shape is closest to the measured spectrum (has the smallest difference) can be estimated to be the background nuclide-related component. Thereafter, as in the above example, a corrected spectrum calculation step (S4) and a corrected spectrum analysis step (S5) are performed to obtain a corrected spectrum from the difference between the measured spectrum and this background nuclide-related component.

[0040] The results actually obtained in this case will be described. In the above example, 137 In the presence of Cs 40 The case of measuring K was shown here. 137 In the presence of Cs (background nuclide) 60The figure shows the case where Co was measured. Figure 7 shows the measured spectrum (Figure 4: S1) obtained in this case. The horizontal axis is the channel (corresponding to the gamma ray energy), and the energy of the recognized peak is written in the figure. 137 Since it is Cs, the 662 keV peak is the same as in the previous case. 60 Co has a peak at 1333 keV, but this peak is 137 It essentially overlaps with the Cs sum peak (1334 keV).

[0041] Next, change the distance as described above to measure the background nuclide ( 137 The reference spectrum (S2 in FIG. 4) obtained by measuring the distance (setting condition) is shown in FIG. 8. Here, the distance (setting condition) is set in seven ways, and as mentioned above, 137 Components due to Cs (662 keV peak, associated sum peak, other low-energy interference components, and high-energy interference components) are visible. When comparing the shapes around the 662 keV peak in each of these reference spectra with the measured spectrum (Fig. 7), the difference between the second reference spectrum from the top in Fig. 8 and the measured spectrum was the smallest. For this reason, this reference spectrum was estimated to be a component due to background nuclides (Fig. 4: S3).

[0042] FIG. 9a is an enlarged view of the shape around 1333 keV in FIG. 7, and FIG. 9b is a corrected spectrum (FIG. 4: S4) calculated using the above-mentioned background nuclide-attributable components in the same range. In this corrected spectrum, 60 The Co peaks (1333keV, 1173keV) are clearly visible. 60 The Co peak is clearly observed.

[0043] Next, a description will be given of the configuration of the radiation detector 10 that is particularly preferably used in the above-mentioned radiation analysis method. As described above, the radiation detector 10 uses a combination of the scintillator 12 and the photomultiplier tube 13.

[0044] First, the material constituting the scintillator 12 will be described. It is preferable that this material has a high density to absorb gamma rays, a short emission decay time to prevent pile-ups and the like, a small self-radioactivity, and a high energy resolution. A material that satisfies these requirements is CeBr3 (emission wavelength 370 nm, decay time 19 ns, density 5.1 g / cm3). 3 ) and Ce-doped LaBr3 (emission wavelength 380 nm, decay time 16 ns, density 5.3 g / cm 3 As described above, the scintillator 12 is available in a variety of sizes, and the size can be, for example, a cube with one side ranging from 4 mm to 15 mm.

[0045] On the other hand, in the background nuclide-induced component estimation step (S3) in FIG. 4, the spectrum is compared (difference is calculated) whether the reference spectrum is used or only the measured spectrum is used. In this case, it is required that the energy scale on the horizontal axis of the spectrum in FIG. 1 and 2 is consistent between the spectra to be compared. On the other hand, since the corresponding setting conditions of these spectra are different, the absolute value of the gamma ray intensity (total number of detections per unit time) is different. This energy corresponds to the current intensity multiplied and output in the photomultiplier tube 13 in response to the pulsed light incident on the photomultiplier tube 13. Generally, the intensity of the pulsed light emitted from the scintillator 12 (the charge amount of the pulse or the waveform integral value) is proportional to the energy of the absorbed gamma ray, whereas, as described below, the intensity of the pulsed light in the photomultiplier tube 13 and the current intensity output are not necessarily in a good proportional relationship. In this case, the energy scale in the spectrum varies as described above, and the above analysis cannot be practically performed.

[0046] FIG. 10 is a simplified diagram showing the structure of the photomultiplier tube 13 and its driving circuit. In FIG. 7, the configuration is shown in a simplified manner, so the shapes and positional relationships of the components are different from the actual ones. In this example, only three dynodes are used, but in reality, more dynodes are provided. In the photomultiplier tube 13, when pulsed light (visible light) is incident from the scintillator 12 as shown by arrow A1, electrons (photoelectrons) proportional to the intensity of the light are generated by photoelectric conversion of a photocathode (not shown) on the cathode 131 side in a vacuum. These electrons are focused by a focus grid 132 as shown by arrow A2 and incident on a dynode 133, generating more electrons, which flow to the subsequent dynode 134. Electrons are similarly multiplied in the dynodes 134 and 135, and the successively multiplied electrons finally flow to the anode 136 as shown by arrows A3, A4, and A5. Here, the multiplication factor between the dynodes depends on the potential difference between the dynodes.

[0047] In reality, however, in the photomultiplier tube 13, a DC voltage is applied between the cathode 131 and the anode 136, and part of this DC voltage is divided and applied between each dynode. Meanwhile, the flow of electrons (current) indicated by arrows A2 to A5 in Fig. 7 becomes larger toward the rear stage due to the above-mentioned multiplication. For example, Japanese Patent Application Laid-Open Nos. 7-142024, 10-208688, and 2010-54364 disclose that in such a case, as the current flowing between the dynodes (or between the dynode and the anode) becomes larger toward the rear stage, the voltage (absolute value) applied between the dynodes (or between the dynode and the anode) decreases (fluctuations).

[0048] As described above, this voltage is generated by dividing the voltage of the DC power supply, so when this voltage decreases on the rear stage side, the potential difference (absolute value) between the dynodes on the front stage side increases. This increases the multiplication factor on the front stage side, and since the contribution of the multiplication factor on the front stage side to the final electron multiplication factor in photomultiplier tube 13 is large, this increases the final electron multiplication factor compared to the case where there is no such voltage fluctuation. In this case, the intensity of the pulsed light in photomultiplier tube 13 and the output current intensity are not proportional to each other, and the output current intensity is greater than if they were proportional.

[0049] Japanese Patent Application Publication Nos. 7-142024, 10-208688, and 2010-54364 each provide a means (voltage change suppression means) for suppressing a drop in voltage applied between the anode 136 and the adjacent dynode 135 (final dynode) that accompanies an increase in current flowing between them when light is detected in this way. The technology described in Japanese Patent Application Publication No. 7-142024 suppresses the drop in voltage that accompanies an increase in current by connecting an auxiliary DC power supply to the subsequent dynode, the technology described in Japanese Patent Application Publication No. 10-208688 uses a Cockcroft-Walton circuit, and the technology described in Japanese Patent Application Publication No. 2010-54364 uses a circuit in which a transistor is connected to each dynode.

[0050] That is, it is particularly preferable to use a photomultiplier tube 13 provided with such a voltage change suppression means. However, even if such a voltage change suppression means is not used, it is sufficient as long as the linearity in the output as described above is maintained. [Explanation of symbols]

[0051] 10 Radiation detector 11 Shield 12 Scintillator 13 Photomultiplier tube 20 Computer (calculation section) 30 Signal Processing Section 100 samples 131 Cathode 132 Focus Grid 133~135 Dynode 136 Anode 200 Reference source

Claims

1. A radiation analysis method for measuring an energy spectrum of radiation emitted from a sample containing a mixture of multiple radioactive nuclides and recognizing the nuclides contained in the sample, comprising: In the radiation detector for detecting radiation, a plurality of conditions under which the detection intensity for the same measurement target is different are set as setting conditions, a sample measurement step of obtaining an actual measured spectrum, which is an energy spectrum measured by the radiation detector; a background nuclide-attributable component estimation step of estimating a background nuclide-attributable component, which is a component attributable to a background nuclide that is the nuclide that has the largest contribution to the radiation among the nuclides in the measured spectrum, from a result obtained by measurement with the radiation detector for each of the set conditions; a corrected spectrum calculation step of calculating a corrected spectrum by subtracting the background nuclide-attributable components from the measured spectrum; Equipped with A radiation analysis method, comprising: analyzing the sample using the corrected spectrum.

2. a reference radiation source measurement step of obtaining a reference spectrum, which is an energy spectrum obtained by measuring radiation emitted from a reference radiation source composed of the background nuclides with the radiation detector for each of the set conditions; 2. The radiation analysis method according to claim 1, wherein in the background nuclide-induced component estimation step, the background nuclide-induced component is estimated by comparing the reference spectrum with the measured spectrum.

3. 3. The radiation analysis method according to claim 2, wherein the setting conditions include setting a distance between the radiation detector and the reference radiation source in the reference radiation source measurement step.

4. 4. The radiation analysis method according to claim 1, wherein the radiation detector is provided with a plurality of switchable shields that absorb and limit the radiation incident on the radiation detector to different degrees from each other, and a selection of the shield is included in the setting conditions.

5. The radiation detector includes a scintillator that absorbs the radiation and emits light, and a photodetector that detects the light, Any of the scintillators having different sizes is used in a switchable manner, 5. The radiation analysis method according to claim 1, wherein the setting conditions include a selection of a size of the scintillator.

6. 6. The radiation analysis method according to claim 5, wherein the photodetector is a photomultiplier tube.

7. the photomultiplier tube has a cathode, an anode, and a plurality of dynodes between the cathode and the anode; In the photomultiplier tube, 7. The radiation analysis method according to claim 6, further comprising a voltage change suppression means for suppressing fluctuations in the voltage applied between the anode and a final dynode which is the dynode adjacent to the anode, which occurs when the light is detected.

8. The voltage change suppressing means is 8. The radiation analysis method according to claim 7, further comprising suppressing a fluctuation in voltage applied between the final dynode and the dynode adjacent to the final dynode, which is caused by an increase in current flowing between the final dynode and the dynode adjacent to the final dynode.

9. The background nuclide is １３７ Cs or ６０ 9. The radiation analysis method according to claim 1, wherein the metal is Co.

10. A radiation analysis apparatus for carrying out the radiation analysis method according to any one of claims 1 to 9, a calculation unit which executes the background nuclide-attributable component estimation step to calculate the background nuclide-attributable component, and executes the corrected spectrum calculation step to calculate the corrected spectrum.

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