JP2021162421A - Radiation detector - Google Patents

Abstract

To provide a radiation detector which can sensitively detect an α-ray and a β-ray separately.SOLUTION: A scintillator 10 made of stilbene is used. A light detector 20 emits a pulse output for each entrance of an α-ray and a β-ray to the scintillator 10. An analysis unit 41 of a computer 40 calculates integrated values I1 and I2 in the pulse signal on a pulse-by-pulse basis and calculate I2/I1. The output is thus determined to be due to the α-ray when the value of I2/I1 is larger than a threshold value K, and the output is thus determined to be due to the β-ray when the value of I2/I1 is not larger than the threshold value K.SELECTED DRAWING: Figure 3

Description

The present invention relates to a radiation detector that detects a plurality of types of radiation.

As radiation, for example, a detector using a ZnS (Ag) scintillator is used as a detector capable of detecting α rays with high efficiency in order to detect α rays, and high β rays are used to detect β rays. For example, a GM tube is used as a detector that can be detected efficiently. Therefore, in order to detect α-rays and β-rays in a certain environment, it is often the case that measurements are performed individually, and it often takes time to measure both of them.

On the other hand, for example, when detecting nuclides that emit α-rays and β-rays in the process of decay, it is desirable to be able to detect α-rays and β-rays at the same time. It is preferable that it can be detected. Patent Document 1 describes a detector capable of distinguishing between α-rays and β-rays in this way. This detector is composed of a scintillator that absorbs radiation and emits visible light fluorescence, and a photomultiplier tube that detects the fluorescence. Here, as a scintillator, a scintillator composed of ZnS (Ag) for detecting α rays and a scintillator composed of plastic for detecting β rays are laminated in the incident direction of α rays and β rays. Is used. Since the flight distance of α rays in ZnS (Ag) is short and ZnS (Ag) has low sensitivity to β rays (does not emit fluorescence due to β rays), ZnS (Ag) scintillators and plastic scintillators are sequentially laminated from above. By providing a photoelectron multiplier tube on the lower side thereof, the fluorescence emitted by the ZnS (Ag) scintillator due to α rays and the fluorescence emitted by the plastic scintillator due to β rays can be detected by the photoelectron multiplier tube, respectively. ..

Here, both the fluorescence emitted by the ZnS (Ag) scintillator due to α rays and the fluorescence emitted by the plastic scintillator due to β rays are pulse-shaped, whereby a pulse-shaped output can be obtained from the photomultiplier tube. However, these pulse shapes (rise time, fall time, etc.) are significantly different. Therefore, the two can be discriminated by the pulse waveform of the output signal of the photomultiplier tube, and α-rays and β-rays can be detected (counted) individually. That is, by using the scintillator having a two-layer structure in this way, it is possible to distinguish between α rays and β rays by using a single detector.

Japanese Unexamined Patent Publication No. 5-341407

However, the ability of the detector described in Patent Document 1 to detect α-rays and β-rays was not sufficient.

For example, in order to identify the nuclei that emitted radiation (α-rays, β-rays), it is required to recognize not only the count number of radiation but also the energy of the detected radiation. It is preferable that the energy spectrum of the emitted radiation can also be recognized. Since the fluorescence in the scintillator is emitted by the scintillator material absorbing the energy of radiation, this energy is proportional to the fluorescence intensity (peak value of the pulsed output signal), and the energy can be recognized by this peak value. .. However, in this case, it is necessary that almost all of the radiation energy is absorbed by the scintillator and contributes to fluorescence.

On the other hand, when a scintillator having a two-layer structure is used as in the technique described in Patent Document 1, the entire scintillator having a two-layer structure is tentatively thickened, thereby almost all of the energy of α rays and β rays. It is difficult for the α-ray scintillator (ZnS (Ag)) to absorb the total energy of the α-rays and the β-ray scintillator (plastic material) β-rays. be. In this case, for example, of the β-ray energy, the component absorbed by the α-ray scintillator does not contribute to light emission. Therefore, even if the pulse output corresponding to α rays and the pulse output corresponding to β rays are discriminated as described above, these peak values do not necessarily reflect the energies of α rays and β rays. Therefore, in the detector described in Patent Document 1, it is difficult to obtain an energy spectrum with high resolution.

Further, in such a scintillator having a two-layer structure, not only the energy is not properly recognized in this way, but also when the energy is small, the scintillator may absorb the energy but not fluoresce. In this situation, for example, β-rays having a small energy are not detected. Therefore, the detector described in Patent Document 1 cannot obtain sufficient detection performance.

Therefore, a radiation detector capable of distinguishing between α-rays and β-rays and detecting them with high sensitivity has been desired.

The present invention has been made in view of such problems, and an object of the present invention is to provide an invention for solving the above problems.

The present invention has the following configurations in order to solve the above problems.
The radiation detector of the present invention is a radiation detector having a scintillator that absorbs radiation and emits pulsed fluorescence, and a light detector that receives the fluorescence and emits a pulsed output signal corresponding to the fluorescence. The scintillator is made of a material in which the decay time constant of the fluorescence when absorbing α-rays and the decay time constant of the fluorescence when absorbing β-rays are different from each other, and is after the peak in the output signal. I1 which is the integrated value of the output signal within the first period including at least the period during which the output signal is attenuated, and the period during which the output signal is attenuated after the peak in the output signal. I2, which is the integrated value of the output signal in the second period whose start is later than the period, is calculated, and the output signal is based on either α-ray or β-ray based on the value of I2 / I1. It is characterized by having an analysis unit for identifying a thing.
In the radiation detector of the present invention, the analysis unit calculates the energy of α-rays or β-rays corresponding to the output signal based on the peak value or I1 of the output signal.
In the radiation detector of the present invention, the material is an organic compound.
In the radiation detector of the present invention, the material is stilbene.
In the radiation detector 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, and I2. When / I1 is small, the output signal is recognized as being due to β rays.
In the radiation detector of the present invention, the photodetector is a silicon photomultiplier tube.

Since the present invention is configured as described above, it is possible to obtain a radiation detector capable of distinguishing between α-rays and β-rays and detecting them with high sensitivity.

It is a figure which shows the pulse waveform of the output signal by α ray and β ray when the material of the scintillator used in the radiation detector which concerns on embodiment of this invention is used. It is a figure explaining the method of discriminating α ray and β ray by an output signal. It is a figure which shows the structure of the radiation detector which concerns on embodiment of this invention. It is a flowchart which shows the operation in the radiation detector which concerns on embodiment of this invention. It is an energy spectrum of α ray (a) and β ray (b) measured by the 1st Example of this invention. It is a figure which shows the pulse waveform of the output signal by α ray and β ray when the material of the scintillator used in the 2nd Example of this invention is used. It is an energy spectrum of α ray (a) and β ray (b) measured by the 2nd Example of this invention.

Hereinafter, the radioactivity detector according to the embodiment of the present invention will be described. In this radiation detector, a scintillator that absorbs α-rays and β-rays and emits visible light fluorescence is used. This fluorescence is detected by a photomultiplier tube like a conventional radiation detector.

First, the principle of this radiation detector will be described. The material constituting the scintillator used here is an organic compound, for example, stilbene. FIG. 1 is a pulse waveform of an output signal obtained by detecting fluorescence when α rays and β rays are incident on stilbene with a photomultiplier tube. Here, the peak value is standardized and unified. In this way, by using stilbene as a material for the scintillator, both α-rays and β-rays can be detected. However, the decay time constants of the pulses are significantly different between the two, and the α-ray has a larger decay time constant than the β-ray. Therefore, it is possible to discriminate between the two by using the difference in the decay time constant. Such damping characteristics are determined by the physical properties of the materials that make up the scintillator.

FIG. 2 is a diagram illustrating a method for this purpose. FIG. 2 schematically shows the pulse shape shown in FIG. Here, first, the integrated value I1 during the period (first period) T1 in this pulse output is calculated. Here, T1 is set as a period over almost the entire attenuation period from the rise of the pulse (before the peak) to the sufficient attenuation of the intensity after the peak. For example, in the example of FIG. 1, it is a period of 0 to 1200 ns.

On the other hand, here, another period (second period) T2 within the period of T1 is also set. T2 is set so that the end is equal to T1 and the start is before T1 and in the middle of the decay period after the peak of the pulse. The integral value I2 during the period T2 in this pulse output is also calculated, and I2 <I1. When I2 and I1 are set in this way, in FIG. 1, I2 / I1 in the α ray having a large decay time constant is larger than I2 / I1 in the β ray. That is, the output of FIG. 1 can be identified by the value of I2 / I1.

Further, in FIG. 1, the peaks of the output by α-rays and β-rays are shown in a unified manner, but in reality, these peak values change according to the energy of α-rays and β-rays and are various. Here, if the scintillator is made thick enough that the α-rays and β-rays in the target energy range are sufficiently absorbed by this scintillator, almost all of the energy of these α-rays and β-rays is absorbed by this scintillator. All the absorbed energy contributes to the above fluorescence. This point is significantly different from the detector described in Patent Document 1 in which a scintillator having a two-layer structure is used. Therefore, the detected α-ray and β-ray energies can be appropriately recognized from the peak value of the pulse output. Alternatively, even when the energies of α-rays and β-rays are small, these can be detected appropriately. Further, instead of the above peak value, the energy may be calculated from I1 which is the integrated value of the entire pulse waveform.

At this time, since the above-mentioned I1 and I2 are substantially proportional to the peak value with respect to α rays having different energies, the value of I2 / I1 which is a ratio thereof 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 index for discriminating between α-rays and β-rays, which does not depend on energy. Therefore, the energy spectra of α-rays and β-rays can be obtained by using the above scintillator and this discrimination method.

The peak value of I2, I1, or even the pulse can be easily calculated by digitizing and storing the pulse output from the photomultiplier tube. At this time, the start of T1 in FIG. 2 can be set by detecting, for example, α-rays and β-rays generated from a standard sample so that such discrimination by I2 / I1 can be easily performed.

FIG. 3 is a diagram showing a configuration of a radiation detector 1 that operates according to the above principle. Here, the scintillator 10 composed of stilbene as described above is used. The thickness of the scintillator 10 is set so as to sufficiently absorb α-rays and β-rays of the energy to be measured, and is, for example, about 6 mm when α-rays and β-rays of 2 MeV or less are targeted.

As the photodetector 20, a photodetector 20 capable of recognizing the intensity of weak visible light incident on the surface is used, and a silicon photomultiplier tube (Si Photomultiplier) is particularly preferably used. In FIG. 3, the photodetector 20 and the scintillator 10 are shown apart for convenience, but they can actually be provided close to each other. The silicon photomultiplier tube is a photomultiplier tube constructed of silicon. The photodetector 20 emits a pulse output as shown in FIG. 1 for each incident of α-rays and β-rays on the scintillator 10. Although not shown, the photodetector 20 is appropriately provided with a preamplifier or the like. Further, a light guide unit may be provided between the scintillator 10 and the photodetector 20. Further, in order to measure the two-dimensional distribution of α-rays and β-rays, the scintillator 10 is made into a plate shape, and the photodetector 20 is a position detection type corresponding to the surface of the scintillator 10 (for example, a position detection type silicon photomultiplier tube). Tube: Position Sensitive Si Photomultiplier) may be used. The photodetector 20 is driven by the voltage supplied by the power source 50.

Since this pulse output is an analog signal, it is digitized by the digitizer 30 and then input to the computer 40. As the digitizer 30, a digital oscilloscope can actually be used.

The analysis unit 41 in the computer 40 performs the above-mentioned analysis on the output signal (pulse signal) digitized in this way. Therefore, the data of such a pulse signal is stored in the storage unit 42 of the computer 40, and then, as described above, the integrated values I1 and I2 in the pulse signal are calculated for each pulse, and I2 / I1 is calculated. It is calculated. At this time, the peak value of the pulse output is also recognized at the same time. Further, when the photodetector 20 is a position detection type as described above, the analysis unit 41 can also recognize the incident position (incident position information) of the light corresponding to this pulse signal in the photodetector 20. can.

FIG. 4 is a flowchart showing the operation of the analysis unit 41 at this time. First, the analysis unit 41 recognizes the peak values of I1, I2, and the pulse in this way (S1). As a result, when the value of I2 / I1 is larger than the threshold value K (S2: Yes), this output is recognized as being due to α rays, this output is counted as that of α rays, and the energy is higher than the peak value. Is calculated (S3). On the other hand, when the value of I2 / I1 is equal to or less than the threshold value K (S2: No), this output is counted as due to β rays, and the energy is calculated from the peak value or I1 (S4). In either case, these results (energy for each α-ray and β-ray) are stored in the storage unit 42 (S5).

At this time, as described above, the threshold value K is set in advance so that this determination (S2) is properly performed. Further, the specific relationship between the peak value and the energy (proportional constant) is set individually for α-rays and β-rays, and is similarly set in advance.

The operation of FIG. 4 is performed for each pulse output from the photodetector 20 (one detection of α-ray or β-ray), but the storage unit 42 outputs all the outputs from the digitizer 30. After storing in the storage unit 42, the operation shown in FIG. 4 may be performed for each output. In this case, when a digital oscilloscope is used as the digitizer 30, the output may be stored on the digital oscilloscope side.

After the measurement is completed and all the above data obtained at the time of measurement is stored in the storage unit 42, the analysis unit 41 can display the result on the display unit 43. Here, for example, in the case of the optical detector 20 g position detection type, if the detection result for each position information is mapped for each of α ray and β ray, this result is a two-dimensional intensity distribution of α ray and β ray. This result corresponds to the two-dimensional distribution of these radiation-emitting nuclides.

On the other hand, the energy histograms recognized for each α-ray and β-ray correspond to the energy spectra of α-rays and β-rays, respectively. When the light detector 20 is a position detection type, this energy spectrum can also be created for each position region (pixel). The analysis unit 41 can also display these results on the display unit 43.

The result of actually measuring the spectrum with the above radiation detector 1 will be described. Here, as the scintillator 10, a scintillator 10 having a size of 6 mm × 6 mm × 6 mm made of stilbene was used. A single-element silicon photomultiplier tube was used as the photodetector, and the energy spectra of α-rays and β-rays were measured here. Here, as a measurement target, a radiation source in which 241 Am that emits an α ray having a peak energy of 5.5 MeV and 90Sr that also emits a β ray of 0.55 MeV / 90Y that emits a β ray of 2.28 MeV is mixed is used. It was used.

FIG. 5 is an energy spectrum of α-rays (a) and β-rays (b) obtained as a result. Here, the horizontal axis is the energy channel (corresponding to the peak value), and the value is proportional to the energy. The results corresponding to the characteristics of the radiation source are appropriately obtained for each, and the energy resolution for the α-ray measurement is 23.6% in FWHM.

In the example of FIG. 5, stilbene was used as the material for the scintillator, but other organic compounds having similar properties can also be used. FIG. 6 is a pulse waveform corresponding to FIG. 1 in a plastic scintillator (trade name EJR-299-33: manufactured by Elgen Technology Co., Ltd.) as such an example. Even in this case, it can be confirmed that the pulse waveform by α rays has a longer attenuation time constant than the pulse waveform by β rays.

FIG. 7 shows the same result as in FIG. 5 when this scintillator is used. In this case as well, the same result as in FIG. 5 is obtained, and the energy resolution for the α-ray measurement is 27.4% in FWHM.

Therefore, as the material of the scintillator 10, various scintillator materials having the same characteristics as stilbene, particularly an organic compound, can be used.

In this radiation detector 1, unlike the technique described in Patent Document 1, a scintillator having a single-layer structure having the above-mentioned characteristics can be used. Further, as the photodetector, the same one as the conventional radiation detector can be used, and as the computer 40, a personal computer or the like can be used as long as the above processing is possible. Therefore, the above radiation detector can be obtained at low cost.

Further, in the above example, an organic compound was used as a material constituting the scintillator, but when the attenuation time constants of fluorescence are different between α-rays and β-rays in other materials as well, the above-mentioned configuration is the same. Can be applied to.

1 Radiation detector 10 Scintillator 20 Photodetector 30 Digitizer 40 Computer 41 Analysis unit 42 Storage unit 43 Display unit 50 Power supply

Claims (6)

A radiation detector comprising a scintillator that absorbs radiation and emits pulsed fluorescence, and a photodetector that receives the fluorescence and emits a pulsed output signal corresponding to the fluorescence.
The scintillator is composed of a material in which the decay time constant of the fluorescence when absorbing α rays and the decay time constant of the fluorescence when absorbing β rays are different.
I1 which is an integral value of the output signal in the first period including at least the period in which the output signal is attenuated after the peak in the output signal, and
I2, which is the integral value of the output signal in the second period, which is the period in which the output signal is attenuated after the peak in the output signal and the start time is later than the first period,
Is calculated,
A radiation detector comprising an analysis unit that identifies whether the output signal is due to α rays or β rays based on the value of I2 / I1. The radiation detector according to claim 1, wherein the analysis unit calculates the energy of α-rays or β-rays corresponding to the output signal based on the peak value of the output signal or I1. The radiation detector according to claim 1 or 2, wherein the material is an organic compound. The radiation detector according to claim 3, wherein the material is stilbene. In the magnitude relationship between I2 / I1 and a preset threshold value, the analysis unit recognizes that the output signal is due to α rays when I2 / I1 is large, and the output when I2 / I1 is small. The radiation detector according to claim 3 or 4, wherein the signal is recognized as being due to β rays. The radiation detector according to any one of claims 1 to 5, wherein the photodetector is a silicon photomultiplier tube.

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