JP7352963B2 - radiation detector - 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 radiation detector that detects multiple types of radiation.

To detect α-rays as radiation, a detector using a ZnS (Ag) scintillator, for example, is used as a detector that can detect α-rays with high efficiency, and to detect β-rays, a detector that uses a ZnS (Ag) scintillator is used. For example, a GM tube is used as an efficient detector. Therefore, in order to detect alpha rays and beta rays under a certain environment, it is often necessary to measure them separately, and it often takes time to measure them together.

On the other hand, for example, when detecting a nuclide that emits alpha and beta rays during the decay process, it is desirable to be able to detect alpha and beta rays simultaneously, so a single detector can distinguish between them. Preferably, it can be detected. Patent Document 1 describes a detector capable of distinguishing and detecting α-rays and β-rays as described above. This detector consists of a scintillator that absorbs radiation and emits visible fluorescence, and a photomultiplier tube that detects this fluorescence. Here, a scintillator made of ZnS (Ag) for detecting alpha rays and a scintillator made of plastic for detecting beta rays are stacked in the direction of incidence of alpha and beta rays. It is used as Since the range of alpha rays in ZnS(Ag) is short, and ZnS(Ag) has low sensitivity to beta rays (does not emit fluorescence due to beta rays), a ZnS(Ag) scintillator and a plastic scintillator are sequentially stacked from the top. By installing a photomultiplier tube below it, 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 photomultiplier tube. .

Here, 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 both in the form of a pulse, and thereby a pulsed output is obtained from the photomultiplier tube. However, these pulse shapes (rise time, fall time, etc.) are significantly different. Therefore, the two can be distinguished from each other by the pulse waveform of the output signal of the photomultiplier tube, and α-rays and β-rays can be detected (counted) separately. That is, by using a scintillator with a two-layer structure in this way, α-rays and β-rays can be detected separately using a single detector.

Japanese Patent Application Publication No. 5-341047

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 nuclide that emitted radiation (α rays, β rays), it is necessary to recognize not only the radiation count but also the energy of the detected radiation. It is preferable that the energy spectrum of the radiation emitted is also recognizable. Fluorescence in a scintillator is emitted when the scintillator material absorbs radiation energy, so 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, almost all of the radiation energy needs to be absorbed by the scintillator and contribute to fluorescence.

On the other hand, when a two-layered scintillator is used as in the technology described in Patent Document 1, the entire two-layered scintillator is made thicker, thereby absorbing almost all of the energy of α-rays and β-rays. Even if the structure is such that a scintillator for α rays (ZnS (Ag)) absorbs the entire energy of α rays, and a scintillator for β rays (plastic material) absorbs the entire energy of β rays, it is difficult. be. In this case, for example, the component of the energy of the β rays that is 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, their peak values do not necessarily reflect the energy of α rays and β rays. For this reason, in the detector described in Patent Document 1, it was difficult to obtain an energy spectrum with high resolution.

In addition, in such a two-layered scintillator, not only is the energy not recognized properly, but if the energy is small, it may be absorbed by the scintillator but no fluorescence is generated. For example, a situation arises in which β-rays with low energy are not detected. Therefore, in the detector described in Patent Document 1, sufficient detection performance could not be obtained.

For this reason, a radiation detector that can distinguish between alpha rays and beta rays and detect them with high sensitivity has been desired.

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 radiation detector of the present invention includes 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 made of a plate-shaped material that has a different fluorescence decay time constant when absorbing alpha rays and a decay time constant of fluorescence when absorbing beta rays, and is suitable for the sample to be measured. The position detection type photodetector is installed close to the scintillator and detects the fluorescence at each position on the surface of the scintillator, and in the output for each position obtained by the photodetector, the output signal is I1 is an integral value of the output signal within a first period including at least a period in which the output signal attenuates after the peak; and I1 is a period in which the output signal attenuates after the peak in the output signal. I2, which is an integral value of the output signal within a second period whose starting point is later than the first period, is calculated, and in the magnitude relationship between I2/I1 and a preset threshold value, When I2/I1 is large, the output signal is recognized to be due to α rays, when I2/I1 is small, the output signal is recognized to be due to β rays, and based on the peak value of the output signal or I1. , comprising an analysis unit that recognizes the distribution of nuclides that emit the α rays and nuclides that emit the β rays in the sample by calculating the energy of the α rays or β rays corresponding to the output signal. Features.
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.
The radiation detector of the present invention is characterized in that the photodetector is a silicon photomultiplier tube.

Since the present invention is configured as described above, it is possible to obtain a radiation detector that can distinguish alpha rays and beta rays and detect them with high sensitivity.

It is a figure which shows the pulse waveform of the output signal by alpha rays and beta rays when the material of the scintillator used in the radiation detector based on embodiment of this invention is used. FIG. 3 is a diagram illustrating a method of discriminating between α rays and β rays using an output signal. 1 is a diagram showing the configuration of a radiation detector according to an embodiment of the present invention. It is a flow chart showing operation in a radiation detector concerning an embodiment of the present invention. It is an energy spectrum of alpha rays (a) and beta rays (b) measured by the first example of the present invention. FIG. 7 is a diagram showing pulse waveforms of output signals due to α rays and β rays when using the scintillator material used in the second embodiment of the present invention. It is an energy spectrum of alpha rays (a) and beta rays (b) measured by the second example of the present invention.

Hereinafter, a radioactive detector according to an embodiment of the present invention will be described. This radiation detector uses a scintillator that absorbs alpha and beta rays and emits visible fluorescence. This fluorescence is detected with a photomultiplier tube, similar to a conventional radiation detector.

First, the principle of this radiation detector will be explained. The material constituting the scintillator used here is an organic compound, such as stilbene. FIG. 1 shows the pulse waveform of an output signal obtained by detecting fluorescence with a photomultiplier tube when alpha and beta rays are incident on stilbene. 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. 2 is a diagram illustrating a method for this purpose. FIG. 2 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 is, for example, a period of 0 to 1200 ns in the example of FIG.

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. 1, 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. 1 can be identified by the value of I2/I1.

Further, in FIG. 1, 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. In this respect, the detector is significantly different from the detector described in Patent Document 1, which uses a scintillator with a two-layer structure. Therefore, the energy of the detected α-rays and β-rays can be appropriately recognized based on the peak value of the pulse output. Alternatively, even if the energies of α-rays and β-rays are small, they can be properly detected. 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 pulse output from the photomultiplier tube. At this time, the starting time of T1 in FIG. 2 can be set by detecting α rays and β rays generated from the standard sample, for example, so that such discrimination based on I2/I1 can be easily performed.

FIG. 3 is a diagram showing the configuration of a radiation detector 1 that operates according to the above principle. Here, the scintillator 10 made 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 energy to be measured, and is approximately 6 mm when targeting α-rays and β-rays of 2 MeV or less, for example.

The photodetector 20 is one that can recognize the intensity of weak visible light incident on the surface, and a silicon photomultiplier is particularly preferably used. Although the photodetector 20 and the scintillator 10 are shown separated in FIG. 3 for convenience, they can actually be provided close to each other. A silicon photomultiplier tube is a photomultiplier tube constructed using silicon. The photodetector 20 emits a pulse output as shown in FIG. 1 every time α rays and β rays are incident on the scintillator 10. Although not shown, the photodetector 20 is appropriately provided with a preamplifier and the like. Further, a light guiding section may be provided between the scintillator 10 and the photodetector 20. In addition, in order to measure the two-dimensional distribution of α rays and β rays, the scintillator 10 is made into a plate, and the photodetector 20 is of a position detection type corresponding to the surface of the scintillator 10 (for example, a position detection type silicon photomultiplier). Pipe: Position Sensitive Si Photomultiplier). The photodetector 20 is driven by a voltage supplied by a 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 analysis on the digitized output signal (pulse signal). 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 integral values I1 and I2 of this pulse signal are calculated for each pulse, and I2/I1 is calculated. Calculated. At this time, the peak value of the pulse output is also recognized at the same time. Further, when the photodetector 20 is of the 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 on the photodetector 20. can.

FIG. 4 is a flowchart showing the operation of the analysis section 41 at this time. First, the analysis unit 41 recognizes I1, I2, and the peak value of the pulse in this way (S1). As a result, if the value of I2/I1 is larger than the threshold 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 lower than the peak value. is calculated (S3). On the other hand, if the value of I2/I1 is less than or equal to the threshold value K (S2: No), this output is counted as being due to β rays, and the energy is calculated from its 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 performed appropriately. Further, the specific relationship between the peak value and the energy (proportionality constant) is set individually for α rays and β rays, and is similarly set in advance.

Note that the operation in FIG. 4 is performed for each pulse output from the photodetector 20 (one detection of α rays or β rays), but the storage unit 42 stores all outputs from the digitizer 30. After being stored 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 of the above data obtained during the measurement is stored in the storage section 42, the analysis section 41 can display the results on the display section 43. Here, for example, if the photodetector is a 20g position detection type, if the detection results for each position information are mapped for α rays and β rays, this result will be a two-dimensional intensity distribution of α rays and β rays. This result corresponds to the two-dimensional distribution of the nuclides that emitted these radiations.

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

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

FIG. 5 shows the energy spectra of the α-rays (a) and β-rays (b) obtained as a result. Here, the horizontal axis is the energy channel (corresponding to the peak value), which is a value proportional to energy. Appropriate results corresponding to the characteristics of the radiation source were obtained for each method, and the energy resolution for α-ray measurement was 23.6% in FWHM.

In the example of FIG. 5, stilbene was used as the scintillator material, but other organic compounds with similar properties can also be used. FIG. 6 shows a pulse waveform corresponding to FIG. 1 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.

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

Therefore, as the material for the scintillator 10 described above, various scintillator materials having similar characteristics to stilbene, particularly organic compounds, can be used.

In this radiation detector 1, unlike the technique described in Patent Document 1, a single-layer scintillator having the above characteristics can be used. Further, as the photodetector, one similar to a 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.

In addition, in the above example, an organic compound was used as the material constituting the scintillator, but if other materials have different fluorescence decay time constants for α-rays and β-rays, the above structure may be used in the same way. It can be applied to

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

Claims (4)

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 radiation detector comprising:
The scintillator is made of a plate-shaped material that has a different decay time constant of the fluorescence when absorbing α rays and a decay time constant of the fluorescence when absorbing β rays, and is arranged in close proximity to the sample to be measured. installed,
The position detection type photodetector that detects the fluorescence at each position on the surface of the scintillator is used,
In the output for each position obtained by the photodetector,
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;
I2, which is an integral value of the output signal within a second period in which the output signal attenuates after the peak in the output signal and whose start period is later than the first period;
Calculate,
Regarding the magnitude relationship between I2/I1 and a preset threshold value, when I2/I1 is large, the output signal is recognized to be due to α rays, and when I2/I1 is small, the output signal is recognized to be due to β rays. By recognizing the energy of the α-ray or β-ray corresponding to the output signal based on the peak value or I1 of the output signal , the nuclide that emitted the α-ray in the sample, the β-ray A radiation detector characterized by comprising an analysis section that recognizes the distribution of nuclides that emit radiation. The radiation detector according to claim 1, wherein the material is an organic compound. 3. A radiation detector according to claim 2, wherein the material is stilbene. The radiation detector according to any one of claims 1 to 3, wherein the photodetector is a silicon photomultiplier tube.

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