JP7347805B2 - Nuclear material distribution measuring device - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act applies Nuclear Instruments and Methods in Physics Research Section A Copy of published paper

The present invention relates to a nuclear material distribution measuring device for measuring the spatial distribution of nuclear material that emits radiation.

In evaluating radioactive materials in nuclear reactors, etc., it is necessary to detect plutonium (Pu: ²³⁹ Pu, ²³⁸ Pu, etc.), which is an artificial radionuclide, and it is especially necessary to measure the distribution of Pu in the facility. . For this purpose, for example, a method of measuring the spatial distribution of α rays emitted by ²³⁹ Pu with a peak energy of 5.1 MeV is used. However, the abundance of ²³⁹ Pu is generally small, and because ²²² Rn (radon), which is a natural radionuclide and emits alpha rays with energy close to the above, exists, it is difficult to measure the distribution of ²³⁹ Pu with high precision. is difficult. That is, when detecting the presence of ²³⁹ Pu by detecting α rays, the presence of α rays originating from ²²² Rn as a background component is an obstacle to the detection of ²³⁹ Pu.

Here, the α rays caused by ²²² Rn are α rays emitted when ²²² Rn decays into progeny nuclides in its decay series. Figure 10 shows the decay series of ^222Rn . Here, the half-life of each process, the emitted radiation (type of α-ray or β-ray), and its peak energy are also shown. In this way, ²²² Rn decays in the decay sequence ²²² Rn→ ²¹⁸ Po→ ²¹⁴ Pb→ ²¹⁴ Bi→ ²¹⁴ Po→ ²¹⁰ Pb, and α and β rays are emitted in each process. At this time, the half-life of each process and the energy of the emitted alpha and beta rays vary.

For this reason, a technique has been proposed for reducing the influence of α rays caused by ²²² Rn when detecting α rays emitted from Pu. For example, Patent Document 1 states that ^222Rn is a natural radionuclide and is generally uniformly distributed over a wide range, whereas Pu does not exist in nature and is generally extremely distributed. Taking advantage of the fact that the α-rays are locally distributed and that the peak energy of the α-rays emitted by Pu and the peak energy of the α-rays caused by ^222Rn are different, the two-dimensional mapping results of the detected α-rays are used to calculate the α-rays. Techniques are described for estimating the background level and attributing this background level to ²²² Rn. With this method, by removing such background components from the detected radiation, it is possible to appropriately recognize the distribution of the nuclear material (Pu, etc.) to be detected.

Furthermore, in Patent Document 2 and Non-Patent Document 1, the background component caused by ²²² Rn was investigated by focusing on the reaction from ²¹⁴ Bi to ²¹⁰ Pb in the decay series shown in FIG. Techniques to reduce this are described.

Here, ²¹⁴ Bi emits β rays of about 3.26 MeV and decays to ²¹⁴ Po with a half-life of about 20 minutes. Thereafter, ²¹⁴ Po emits α rays of 7.68 MeV with a half-life of 164 μs and decays to ²¹⁰ Pb. At this time, the half-life of ²¹⁴ Po is the shortest in the process of the above-mentioned decay series, and it can be considered that the β rays emitted by the decay of ²¹⁴ Bi and the α rays emitted by the decay of ²¹⁴ Po are emitted almost simultaneously.

For this reason, in the technology described in Patent Document 2, when detecting α rays emitted from Pu, two detectors are used for α rays and β rays, and the detection time of α rays and the detection time of β rays are By excluding from detection α rays corresponding to those that are close to (recognized as β-α correlated events), the background component caused by ²²² Rn is reduced. Thereby, the distribution of the nuclear material (Pu, etc.) to be detected can be recognized with higher accuracy.

Japanese Patent Application Publication No. 2016-151454 JP2007-163280A

Tetsuo Fukumoto, Hirotaka Ishiyama, Shigeki Ito, "Continuous observation of natural radionuclides in standby dust using β-α correlated event measurement - Towards online detection of artificial radionuclides from nuclear fuel handling facilities, etc.", RADIOISOTOPES, Volume 57, page 670 (2008)

In the method described in Patent Document 2 and the like, detectors for α rays and β rays are required. Furthermore, in order to recognize nuclear materials, it is necessary to measure the energy spectrum of the detected alpha rays, but this requires a dedicated signal processing system, making the measurement equipment complicated. . Furthermore, in the method described in Patent Document 2, among the α rays detected for the detection of nuclear material (Pu), although the α rays emitted by ²¹⁴ Po are excluded, other background components, In 10, for example, the influence of 5.49 MeV α rays emitted when ²²² Rn decays to ²¹⁸ Po could not be removed.

In the technique described in Patent Document 1, the background component is estimated only from the spatial distribution of detected radiation, so for example, when ^the nuclear material to be detected is spatially spread out, When the presence of Rn is not uniform, it may not be possible to appropriately estimate the background component.

For this reason, there has been a desire for a technology that can appropriately recognize the distribution of nuclear material by appropriately recognizing the background components of radiation.

The present invention has been made in view of these problems, and it is an object of the present invention to provide an invention that solves the above problems.

In order to solve the above problems, the present invention has the following configuration.
The nuclear material distribution measuring device of the present invention includes:
A nuclear material distribution measurement device that measures the spatial distribution of nuclear material that emits radiation by detecting the spatial distribution of the intensity of the radiation, and absorbs incident α-rays and β-rays to generate a pulse-shaped output signal. and a position detection type radiation detector that recognizes the incident position and time of incidence of the incident α rays and the incident β rays, and identifies whether the output signal is due to the α rays or the β rays. Based on the incident time of the incident α ray and the incident β ray, a combination of the incident α ray and the β ray with a time difference within a certain range is recognized as an α-β correlation event, and the combination is recognized as an α-β correlation event. From the incident position corresponding to the α-β correlated event, the spatial distribution of the α-β correlated event within the measurement time is calculated, and from the spatial distribution, the detection result of the radiation by the position-detecting radiation detector is calculated. an analysis unit that estimates a background component in the area and corrects the spatial distribution of the intensity of the radiation measured by the position-detecting radiation detector using the estimated background component. shall be.
The nuclear material distribution measuring device of the present invention is characterized in that the α rays and the β rays corresponding to the α-β correlation event are α rays and β rays emitted in the decay series of ²²² Rn, respectively.
In the nuclear material distribution measuring device of the present invention, the analysis section calculates the energy of the α-ray or β-ray from the output signal.
In the nuclear material distribution measuring device of the present invention, the position detection type radiation detector absorbs radiation and emits pulsed fluorescence, and determines the decay time constant of the fluorescence when absorbing α rays and the decay time constant of the fluorescence when absorbing β rays. a plate-shaped scintillator made of a material that has a different decay time constant of the fluorescence when a detection type photodetector, the analysis unit is configured to calculate an integral value I1 of the output signal within a first period including at least a period in which the output signal attenuates after a peak in the output signal. and a second period in which the output signal attenuates after a peak in the output signal, whose start period is later than the first period , and whose end period is the same as the first period. The method is characterized in that it calculates I2, which is an integral value of the output signal, and identifies whether the output signal is due to alpha rays or beta rays based on the value of I2/I1.
In the nuclear material distribution measuring device of the present invention, the material is an organic compound.
In the nuclear material distribution measuring device of the present invention, the material is stilbene.
In the nuclear material distribution measurement device of the present invention, the analysis unit recognizes that the output signal is due to α rays when I2/I1 is large in the magnitude relationship between I2/I1 and a preset threshold value. , I2/I1 is small, the output signal is recognized as being due to β rays.
In the nuclear material distribution measuring device of the present invention, the position detection type photodetector is a position detection type silicon photomultiplier tube.

Since the present invention is configured as described above, the distribution of nuclear material can be appropriately recognized by appropriately recognizing the background component of radiation.

1 is a diagram showing the configuration of a nuclear material distribution measuring device according to an embodiment of the present invention. 3 is a flowchart showing the operation of the nuclear material distribution measuring device according to the embodiment of the present invention. It is a figure which shows typically the example (1) of correction|amendment of the background component in the nuclear material distribution measurement apparatus based on embodiment of this invention. It is a figure which shows typically the example (2) of correction|amendment of the background component in the nuclear material distribution measurement apparatus based on embodiment of this invention. 1 is a diagram showing the configuration of a position detection type radiation detector used in a nuclear material distribution measuring device according to an embodiment of the present invention. FIG. 3 is a diagram showing pulse waveforms of output signals due to α-rays and β-rays when using a scintillator material used in a position-detecting radiation detector used in a nuclear material distribution measuring device according to an embodiment of the present invention. FIG. 3 is a diagram illustrating a method of discriminating between α rays and β rays using an output signal. Energy spectra of α-rays (a) and β-rays (b) obtained in the presence of α-rays and β-rays by a position-detecting radiation detector used in the nuclear material distribution measurement device according to the embodiment of the present invention It is. The pulse waveform of the output signal due to alpha rays and beta rays is shown when another example of the scintillator material used in the position detection type radiation detector used in the nuclear material distribution measuring device according to the embodiment of the present invention is used. It is a diagram. ²²² is a diagram showing the decay series of Rn.

A nuclear material distribution measuring device according to an embodiment of the present invention will be described below. This nuclear material distribution measuring device uses a radiation detector that detects α-rays and β-rays. This radiation detector uses a two-dimensional position detection type, and can also recognize positional information within the detector of detected α-rays and β-rays.

FIG. 1 is a diagram showing a simplified configuration of a nuclear material distribution measuring device 1 that operates according to the above principle. The plate-shaped position detection type radiation detector 10 absorbs alpha rays and beta rays, and emits a pulse-like output signal by each absorption. At this time, the incident position of the absorbed α rays and β rays into the position detection type radiation detector 10 can also be recognized. For this reason, for example, by bringing the position-detecting radiation detector 10 close to a plate-shaped sample in which nuclear material to be detected is distributed, the position-detecting radiation detector 10 detects α-rays and β-rays emitted by the nuclear material. By detection, the two-dimensional distribution of this nuclear material in the sample can be determined. The position-detecting radiation detector 10 operates with a voltage supplied by a power source 40. Further, this position detection type radiation detector 10 can detect both alpha rays and beta rays, but at this time, it is possible to recognize whether the detected radiation is alpha rays or beta rays. Details of the position detection type radiation detector 10 will be described later.

Since the pulsed output signal emitted from the position detection type radiation detector 10 is an analog signal, it is digitized by the digitizer 20 and then input to the computer 30. As the digitizer 20, a digital oscilloscope can actually be used.

The analysis unit 31 in the computer 30 can recognize this output signal together with its position information as a detection result, and can store this in the storage unit 32. At this time, the detection results can be stored separately for alpha rays and beta rays. The computer 30 is also provided with a display unit 33 that can display two-dimensional distributions of various types of information (measurement results). Here, since the nuclear substances to be detected are ²³⁸ Pu and ²³⁹ Pu, it is assumed that α rays of 5.499 MeV emitted by ²³⁸ Pu and 5.156 MeV emitted by ²³⁹ Pu are mainly detected. Therefore, the two-dimensional intensity distribution of α rays after the background component is removed reflects the distribution of ²³⁸ Pu and ²³⁹ Pu.

FIG. 2 is a flowchart showing the operation of this nuclear material distribution measuring device 1. Here, first, the position detection type radiation detector 10 detects alpha rays or beta rays, thereby emitting a pulsed output signal (S1). This output signal is digitized by a digitizer 20 and input to a computer 30. The analysis unit 31 recognizes the waveform data of this output signal together with its position information and detection time.

Thereafter, the analysis unit 31 recognizes from this waveform data whether this output signal is due to α rays or β rays, as will be described later (S2). The analysis unit 31 also recognizes the energy of the α-ray or β-ray in this case from the peak value of this output signal. Thereafter, the energy, position information, and detection time thus recognized can be stored in the storage unit 32 by classifying them into α-rays and β-rays (S3). The above operation is repeated every time the position detection type radiation detector 10 detects α-rays or β-rays until the measurement time ends (S4: Yes). As a result, the energy, incident position information, and detection time of the α-rays and β-rays detected within the measurement time are stored in the storage unit 32 in a manner that is distinguished for each α-ray and β-ray.

After the above information is stored by all detections within the measurement time, the analysis unit 31 extracts combinations (α-β correlation events) in which the β-ray detection time and the α-ray detection time are close (S5). . This combination is estimated to correspond to the decay of ²¹⁴ Bi, which emits β rays, and the decay of ²¹⁴ Po, which emits α rays, in FIG. 10. The perceived time difference for this combination is set corresponding to the half-life of ²¹⁴ Po (164 μs) in FIG. 10 and can be, for example, 1 ms. This point is similar to the technology described in Patent Document 2. In this case, we exclude from this combination the β rays whose energy is significantly different from the energy of the β rays emitted by ²¹⁴ Bi, and the α rays whose energy is significantly different from the energy of the α rays emitted by ²¹⁴ Po, and we use energy close to these energies. The above α-β correlation event may be extracted only from the β rays and α rays. Alternatively, α-β correlation events may be extracted for each pixel in the position-detecting radiation detector 10.

Similar to the technique described in Patent Document 2, it is estimated that the α-β correlation event recognized in this way is caused by the natural radionuclide ^222Rn , rather than the nuclear material to be measured ( ^239Pu ). I can do it. Here, the analysis unit 31 can two-dimensionally map the individual incident positions of α-rays and β-rays in the combination of α-β correlation events, and display the map on the display unit 33 (S6). This distribution is estimated to reflect the distribution of ^222Rn in the sample.

The analysis unit 31 can calculate the background component caused by ²²² Rn from this result (S7). For example, if there is no particular tendency in the two-dimensional distribution of α-β correlated events and this two-dimensional distribution is a random distribution, this background component is the average value of this two-dimensional distribution ( It can be estimated that the constant value) corresponds to the background component. However, since it is actually difficult to detect all the background components caused by ²²² Rn as the above α-β correlation event, in reality, the estimated background component is the α-β correlation event. This constant can be set as appropriate based on experimental results, etc. In this case, the value obtained by subtracting this background component from the detected α-ray detection intensity (c/s) can be set as the corrected α-ray detection intensity (S8).

FIG. 3 is a diagram schematically showing this operation. Here, for convenience, it is assumed that the intensity distribution of α-rays is a one-dimensional distribution, and the horizontal axis corresponds to the one-dimensional position, and the vertical axis is the α-ray intensity distribution. This is the detection strength. FIG. 3(a) shows the intensity distribution before correction, FIG. 3(b) shows the estimated background component (constant value), and FIG. 3(c) shows the intensity distribution after correction. Here, it is assumed that ²³⁸ Pu and ²³⁹ Pu actually exist at position X, and the actual background component before correction (FIG. 3(a)) is B1. In reality, there are background components other than those caused by ^222Rn , whereas the background component B2 estimated as above is caused by ^222Rn , so it is shown in Figure 3(b). The estimated background component B2 shown in is smaller than B1. Therefore, although it is difficult to reduce the background component to zero even after correction, it is possible to reduce the background component B3 (=B1-B2) (FIG. 3(c)) after correction, and ²³⁸ Pu The contrast of the intensity of ²³⁸ Pu and 239 Pu with respect to the background component (B3) can be increased, and the detection accuracy of ²³⁸ Pu and ²³⁹ Pu can be improved.

Alternatively, if there is a clear tendency in the two-dimensional distribution of α-β correlated events, for example, take this tendency into account and set the background component using an approximation formula that approximates this distribution, and You can perform operations. FIG. 4 is a diagram showing an example of such a case in correspondence with FIG. 3. In FIG. 4A, which is the state before correction, it is assumed that the background component B4 increases linearly toward the right side in the figure. Even in such a case, if at least a part of this background component B4 is recognized as the above α-β correlation event, the background level B5 can be plotted as a function of position (in the case of FIG. 4, a linear function). 4(b). Therefore, as shown in FIG. 4(c) after correction, the background component B6 (=B4-B5) after correction can be made small. The same applies to cases where the background component has other distributions.

Although the intensity distribution is assumed to be a one-dimensional distribution in FIGS. 3 and 4 for simplicity, in reality, the analysis unit 31 can display the corrected two-dimensional intensity distribution of α-rays on the display unit 33. I can do it (S9). This intensity distribution reflects the distribution of ²³⁸ Pu and ²³⁹ Pu with higher precision than before correction.

Note that the above two-dimensional intensity distribution may be an intensity distribution in an energy band near the peak energy of α rays emitted by ²³⁸ Pu and ²³⁹ Pu. In this case, the background component subtracted in the above-described correction is calculated from the number of α-β correlated events as being in this energy band.

The above method is similar to the technique described in Patent Document 1 in that the background component is estimated and corrected from the actually measured two-dimensional intensity distribution of α-rays. However, since the α-β correlation event is used to estimate the background component here, the accuracy is higher than that of the technique described in Patent Document 1. Therefore, even if the background component itself has a distribution as in the example of FIG. 4, this correction can be performed more appropriately.

Furthermore, in the technique described in Patent Document 2, the influence of ²²² Rn is reduced by excluding the α ray corresponding to the α-β correlation event described above from the detection results. In this case, only the α rays emitted by ²¹⁴ Po in FIG. 10 are excluded; for example, the α rays emitted by ²²² Rn and ²¹⁸ Po are not excluded. On the other hand, the background component estimated by the technique of the present invention corresponds to the abundance of ^222Rn recognized from the intensity of the α-β correlation event, and as a result, it is not emitted in all the processes in Fig. 10. Correction is performed after including the influence of alpha rays. Therefore, according to the above method, ²³⁸ Pu and ²³⁹ Pu can be detected with higher accuracy than the technique described in Patent Document 2.

Note that the operations S1 to S4 in FIG. 2 can be performed every time a pulse is output from the position-detecting radiation 10 (one time of detection of α rays or β rays); After storing all the outputs from the storage unit 32 in the storage unit 32, the operations S1 to S4 in FIG. 2 may be performed for each output. In this case, when a digital oscilloscope is used as the digitizer 20, the output may be stored on the digital oscilloscope side.

Next, the position detection type radiation detector 10 will be explained. FIG. 5 is a diagram showing the configuration of this position detection type radiation detector 10. Here, a plate-shaped scintillator 11 that emits fluorescence by absorbing α-rays and β-rays, and a position detection type light that detects the light emission (fluorescence) from the plate-shaped scintillator 11 by recognizing its incident position. A detector 12 is used.

The position detection type photodetector 12 is configured to be able to recognize the intensity of weak visible light incident on the surface and its incident position, and a position detection type silicon photomultiplier is particularly preferable. used. In FIG. 5, the position detection type photodetector 12 and the scintillator 11 are shown separated for convenience, but they can actually be provided close to each other. The position detection silicon photomultiplier tube is a photomultiplier tube constructed using silicon, and can have, for example, 8×8 pixels. By analyzing the output of each pixel for a single output, it is also possible to calculate the incident position of light (light emission from the scintillator 10) with finer precision than the above-mentioned pixels, for example, equivalent to 512×512 pixels.

The material constituting the scintillator 11 used here is an organic compound, such as stilbene. FIG. 6 shows the pulse waveform of the output of fluorescence detected by a photomultiplier tube when alpha and beta rays are incident on stilbene, and corresponds to the waveform of the output signal from the position detection type photodetector 12. Here, the peak values are standardized and unified. In this way, by using stilbene as a scintillator material, both α-rays and β-rays can be detected. However, the attenuation time constant of the pulse is significantly different between the two, and the attenuation time constant of the α ray is larger than that of the β ray. Therefore, it is possible to discriminate between the two using the difference in the attenuation time constant. Such attenuation characteristics are determined by the physical properties of the material constituting the scintillator.

FIG. 7 is a diagram illustrating a method for this purpose. FIG. 7 schematically shows the pulse shape shown in FIG. Here, first, an integral value I1 during a period (first period) T1 in this pulse output is calculated. Here, T1 is set as a period spanning almost the entire attenuation period from the rise of the pulse (before the peak) until the intensity sufficiently attenuates after the peak, and for example, in the example of FIG. 6, it is a period of 0 to 1200 ns.

On the other hand, another period (second period) T2 within the period T1 is also set here. T2 is set so that the end period is equal to T1 and the beginning period is before T1 and in the middle of the attenuation period after the peak of the pulse. The integral value I2 during the period T2 of this pulse output is also calculated, and I2<I1. When I2 and I1 are set in this way, in FIG. 6, I2/I1 for the α-ray, which has a large decay time constant, becomes larger than I2/I1 for the β-ray. That is, the output of FIG. 6 can be identified by the value of I2/I1.

Further, in FIG. 6, the output peaks due to α rays and β rays are shown as unified, but in reality, these peak values vary depending on the energies of α rays and β rays. Here, if the scintillator is made thick enough to absorb α-rays and β-rays in the target energy range, almost all of the energy of these α-rays and β-rays will be absorbed by this scintillator. All absorbed energy contributes to the fluorescence mentioned above. Therefore, the energy of the detected α-rays and β-rays can be appropriately recognized based on the peak value of the pulse output. Moreover, the energy may be calculated from I1, which is the integral value of the entire pulse waveform, instead of the above-mentioned peak value.

At this time, since the above I1 and I2 are approximately proportional to this peak value for α rays of different energies, the value of I2/I1, which is the ratio between them, does not depend on the energy of the α rays. The same applies to β-rays in this regard. Therefore, I2/I1 can be used as an energy-independent index for discrimination between α rays and β rays. Therefore, by using the above-mentioned scintillator and this discrimination method, it is possible to obtain the energy spectra of each of α-rays and β-rays.

I2, I1, or even the peak value of the pulse can be easily calculated by digitizing and storing the output signal from the position-sensitive photodetector 12. At this time, the start and end times of T1 and T2 in FIG. 6 can be set, for example, by detecting α rays and β rays generated from the standard sample, so that discrimination based on I2/I1 can be easily performed. .

Further, as described above, the computer 30 can also recognize the energy of the α-ray or β-ray from the peak value of the output signal or I1. FIG. 8 shows the energy spectra of α rays (a) and β rays (b) recognized in this way. Here, the horizontal axis is the channel of the peak value (corresponding to energy), and the peak of the energy spectrum of the α-ray (FIG. 8(a)) corresponds to 7.68 MeV. Similarly, the energy spectrum of β-rays is also obtained (FIG. 8(b)), but the correspondence between channels and actual energy values differs between α-rays and β-rays.

In this way, the position detection type radiation detector 10 can distinguish and detect α-rays and β-rays. Therefore, in the nuclear material distribution measuring device 1 described above, this position detection type radiation detector 10 can be particularly preferably used.

In the example of FIG. 6, stilbene was used as the scintillator material, but other organic compounds with similar properties can also be used. FIG. 9 shows a pulse waveform corresponding to FIG. 6 in a plastic scintillator (trade name: EJR-299-33, manufactured by Eljen Technology) as such an example. Even in this case, it can be confirmed that the pulse waveform due to α rays has a longer decay time constant than the pulse waveform due to β rays. Therefore, such materials can also be used as the scintillator 11 described above.

However, other position-detecting radiation detectors can also be used as long as they can similarly detect α-rays and β-rays together with their incident position information. In this case, for example, similarly to the technique described in Patent Document 2, a detector for α rays and a detector for β rays may be used simultaneously. Alternatively, a scintillator for alpha rays and a scintillator for beta rays may be used in combination, and a common photodetector (position detection type photodetector) may be used.

In addition, in the above example, it is assumed that ²³⁹ Pu is the target to be detected, but even other nuclear substances can be detected by α rays, and there is a backlash during detection in the decay sequence of ^222Rn . Anything that emits alpha rays, which serves as a ground, can be detected in the same way. Furthermore, even if it is a nuclide other than ^222Rn . The same applies if the background is caused by a nuclide whose decay sequence has something that can be recognized as an α-β correlation event similar to the above.

1 Nuclear material distribution measuring device 10 Position detection type radiation detector 11 Scintillator 12 Position detection type photodetector 20 Digitizer 30 Computer 31 Analysis section 32 Storage section 33 Display section 40 Power supply

Claims (8)

A nuclear material distribution measurement device that measures the spatial distribution of nuclear material that emits radiation by detecting the spatial distribution of the intensity of the radiation, comprising:
It absorbs the incident α rays and β rays and emits a pulse-like output signal, and the incident position and time of incidence of the incident α rays and the incident β rays are recognized, and the output signal is transmitted to the α rays, a position detection type radiation detector that can identify which of the β rays is generated;
Based on the incident time of the incident α rays and the incident β rays, a combination of the incident α rays and the β rays with a time difference within a certain range is recognized as an α-β correlation event,
Calculating the spatial distribution of the α-β correlated event within the measurement time from the incident position corresponding to the recognized α-β correlated event ,
Estimating a background component in the detection result of the radiation by the position detection type radiation detector from the spatial distribution,
an analysis unit that corrects the spatial distribution of the radiation intensity measured by the position-detecting radiation detector using the estimated background component;
A nuclear material distribution measuring device comprising: The nuclear material distribution measuring device according to claim 1, wherein the α-ray and the β-ray corresponding to the α-β correlation event are α-ray and β-ray, respectively, emitted in the decay series of ²²² Rn. . The nuclear material distribution measuring device according to claim 1 or 2, wherein the analysis section calculates the energy of the alpha ray or beta ray from the output signal. The position detection type radiation detector includes:
A plate-shaped plate made of a material that absorbs radiation and emits pulsed fluorescence, and has a different decay time constant of the fluorescence when absorbing α-rays and a different time constant of fluorescence when absorbing β-rays. scintillator and
a position detection type photodetector that receives the fluorescence and outputs the output signal corresponding to the fluorescence, and recognizes the incident position of the fluorescence;
Equipped with
The analysis section includes:
I1, which is an integral value of the output signal within a first period that includes at least a period in which the output signal attenuates after the peak in the output signal;
The output signal within a second period, which is a period in which the output signal attenuates after a peak in the output signal, whose start period is later than the first period, and whose end period is the same as the first period. I2, which is the integral value of
Calculate,
The nuclear material distribution according to any one of claims 1 to 3, wherein it is determined whether the output signal is due to α rays or β rays based on the value of I2/I1. measuring device. The nuclear material distribution measuring device according to claim 4, wherein the material is an organic compound. 6. The nuclear material distribution measuring device according to claim 5, wherein the material is stilbene. In the magnitude relationship between I2/I1 and a preset threshold, the analysis unit recognizes that the output signal is due to alpha rays when I2/I1 is large, and recognizes that the output signal is caused by α rays when I2/I1 is small. The nuclear material distribution measuring device according to claim 5 or 6, characterized in that the signal is recognized as being due to β rays. The nuclear material distribution measuring device according to any one of claims 4 to 7, wherein the position detection type photodetector is a position detection type silicon photomultiplier tube.

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