JP4197727B2 - αβ detector - Google Patents

Description

  The present invention relates to a radiation measurement technique used in a radioactive material handling facility such as a nuclear power plant, for example. In particular, the α ray and the β ray can be measured independently at the same position at the same time. The present invention relates to a suitable αβ detection apparatus.

  FIG. 4 shows a hoswitch type detection device for the purpose of simultaneous detection of α rays and β rays as a conventional example.

  In this apparatus, the first scintillator 2 and the second scintillator 3 are superposed on the lower layer of the light-shielding film 1 that can transmit α and β rays and can block light from the outside. ZnS (Ag) or the like that detects α rays is applied to the first scintillator 2, and plastic or the like is often applied to the second scintillator 2 to detect β rays. The first and second scintillators 2 and 3 stacked on the two layers are directly mounted on the photodetector 5 and stored in the case 6. As the photodetector 5, a photomultiplier tube capable of high-speed response and high sensitivity is generally used.

  The light emission decay time constant of ZnS (Ag) constituting the first scintillator 2 is on the order of μsec, but the light emission decay time constant of the plastic constituting the second scintillator 3 is on the order of several tens of nsec. The decay time constant is much shorter. When the output current signal of the photodetector 5 is converted into a voltage signal by an RC integration circuit having a time constant sufficiently longer than the light emission decay time of each scintillator 2, 3, the rise time of the pulse becomes substantially equal to the light emission decay time, An exponential decay waveform with a time constant determined by a resistance R and a capacitance C is shown. This signal conversion process can be performed by, for example, a preamplifier unit included in the photodetector 5 and attached to the photomultiplier tube.

  If necessary, this signal is amplified to a voltage level that can be analyzed by the waveform discrimination processing device 7. When the signal is input, the waveform discrimination processing device 7 outputs a pulse wave height signal proportional to the rise time, so that the pulse wave height is converted into a digital value by an analog-digital converter, and a general multiple wave height analysis is performed. The wave height distribution is measured by an apparatus.

  From the spectrum data representing the rise time obtained by the waveform discrimination processing device 7, it is possible to distinguish light emission from the first scintillator 2 from light emission from the second scintillator 2.

  FIG. 5 shows an αβ detector using an energy spectrum measuring sensor 8 as another conventional example.

  For example, a Si semiconductor sensor or the like is used as the energy spectrum measurement sensor 8 of this apparatus. However, since this sensor is sensitive to room light other than radiation, the case with the light-shielding film 1 attached as described above is used. 6.

  The output signal of the energy spectrum measuring sensor 8 is signal-processed by the pulse height analysis processing system 9 and measured as an energy spectrum. The wave height analysis processing system 9 generally includes a charge-sensitive preamplifier, a linear amplifier, an analog-digital converter, a multiple wave height analyzer, and the like that process the sensor output signal. On the energy spectrum obtained by the wave height analysis processing system 9, the α-ray signal and the β-ray signal have different distributions and peak shapes, and the data processing of these signals enables the discrimination between the α-ray and the β-ray. It can be carried out.

  However, the waveform discrimination processing device 7 required for the conventional hop-switch type detection device shown in FIG. 4 is a waveform discrimination processing device for rising analysis, which is very expensive. For this reason, it is useful for research at a laboratory level, but there is a problem in terms of cost as a detection device mounted on a monitor device used in an actual nuclear facility or the like. In addition, it is originally intended to analyze the rise time itself, and it is also over-spec for the purpose of simply discriminating those with different rise times.

  From a more fundamental viewpoint, in order to obtain the rise time, it is necessary to detect signals at 10% and 90% levels of the input peak value, for example, and analysis / measurement for signals with low peak values. There is a problem that can not be. This is a problem related to the dynamic range of the peak value of the signal. For example, the amount of light emitted from the ZnS (Ag) α-ray is considerably larger than the amount of light emitted from the plastic scintillator β-ray. Then, ZnS (Ag) is larger by 10 times or more in comparison at the time of conversion to a voltage signal.

  For this reason, it is disadvantageous in measurement for β-ray signals that have a small peak value and are continuously distributed on the low energy side, and in particular, components with small peak values are not analyzed or measured. Has the serious disadvantage of being low. In particular, when the thickness of the plastic scintillator is reduced in order to suppress the γ-ray sensitivity, the amount of light emission is further reduced, so that the above phenomenon is further accelerated.

  Also in the case of the apparatus using the energy spectrum measuring sensor 8 shown in FIG. 5, there is a problem that the same wave height analyzing apparatus as described above is required and the cost is increased, and further, the base material of the energy spectrum measuring sensor 8 is increased. Since the effective atomic weight is larger than that of a plastic scintillator, the γ-ray sensitivity is high and the γ-ray signal is mixed into the β-ray signal.

  Furthermore, when the measurement is not in a vacuum or when the α ray from the α ray emitting nuclide adsorbed on the filter paper is measured, the energy loss of the α ray is large and the range fluctuation is also large. For this reason, the Gaussian peak that can be obtained in a vacuum cannot be obtained and may overlap with the energy spectrum of β-rays. Even though the energy spectrum was measured, the α-rays and β-rays were clearly separated. May be difficult.

  The present invention has been made in view of such circumstances, can be practically used as a radiation monitor detector and can be manufactured at low cost, and sufficiently suppresses γ-ray sensitivity, It is an object of the present invention to provide an αβ detector capable of detecting both α rays and β rays independently and simultaneously while ensuring the maximum sensitivity, and an αβ detector using the detector.

  In the αβ detector, as described above, the light emitted from the first scintillator for α rays passes through the second scintillator for β rays and is guided to the photodetector by the condensing means. In this case, conventionally, a waveform discrimination processing device for rising analysis has been applied by paying attention to the pulse rising time of the converted signal by the RC integration circuit which is substantially equal to the light emission decay time of each scintillator.

  In this regard, the inventor eliminates the need for a waveform discrimination processing device for rising analysis, which has been conventionally required, by adjusting and optimizing the scintillator used, the light emission wavelength, and the light emission amount. The idea of being able to do something.

  That is, as the photodetector, a photomultiplier tube is preferably used in terms of response speed and sensitivity. In this case, the photomultiplier tube should be selected so that the maximum sensitivity can be obtained at the emission wavelength of the first scintillator. For example, the sensitivity of the second scintillator in a longer wavelength band is lowered. On the other hand, when a photomultiplier tube optimized for the second scintillator is used, the quantum efficiency in the shorter wavelength band of the first scintillator decreases. In other words, if the light emission wavelength of the first scintillator and the light emission wavelength of the second scintillator are set differently, they are adjusted and optimized according to the scintillator to be used, the light emission wavelength, and the light emission amount. It can be done. Furthermore, by constructing the sensor by intentionally changing the emission wavelength as well as the emission decay time, it is possible to use a means for optically identifying the wavelength.

  In addition, in the inventor, in combination with the above points, the arrangement of the first scintillator and the second scintillator as means for detecting both the α ray and the β ray independently and simultaneously while ensuring the maximum sensitivity. Based on the relationship, the inventors conceived that light can be easily confined inside the scintillator and the light collection density is increased. That is, the first scintillator that emits light by α rays is extremely thin in order to suppress the sensitivity of β rays and γ rays, and is often composed of, for example, powder or a sintered body. Therefore, in this first scintillator, light is emitted as a result of diffuse reflection within itself. When this light emission is transmitted through the second scintillator for β rays and air is interposed between the first scintillator and the second scintillator guided to the photodetector by the condensing means, the first scintillator Although the probability of causing Fresnel reflection when the light of the scintillator passes through the second scintillator increases, the second scintillator is surrounded by air having a lower value than its own refractive index, so that the inside of the scintillator Easy to confine light. For this reason, it is possible to obtain an advantage that it is easy to apply a method using light collected at high density on the edge side as the light condensing means for the second scintillator.

Based on the above idea, in the invention of claim 1 , the β-ray detection unit, the air layer having a constant volume secured on the β-ray incident surface side, and means for sucking ionized air by α-ray at a constant flow rate or a constant volume And an electrode for measuring the ionization current of the sucked air, a power supply device for applying an applied voltage to the electrode, and a minute current measuring means for measuring the ionization current itself. Providing equipment.

In the present invention, consider that no place is obstructing the entrance of β-rays as possible in front of the β ray detection unit that optimizes, deployed ahead of the ionizing current measuring electrodes to a separate vessel, the The container is connected to the sealed space in front of the β-ray detector, and the ionization current is measured while sucking air in the sealed space. According to this configuration, in addition to the fact that there is no need to provide any additional material on the β-ray incident surface, the collected charge is always integrated while being sucked during the measurement time, thereby reducing the leakage current of the current measurement system. An effect of suppressing the influence is obtained.

According to a second aspect of the present invention, there is provided the αβ detection device according to the first aspect, further comprising a filter that adsorbs dust to an air ventilation port newly introduced from the outside into a fixed volume forming an air layer serving as an α-ray detection layer. An αβ detector characterized by the above is provided.

In the present invention, in the configuration of claim 1 , when diurnal variation, seasonal variation, etc. of natural radionuclide radon and thoron contained in external air ventilated by suction becomes a problem, a filter is attached to the ventilation opening. In addition, by adsorbing the floating dust to which radon, thoron, and the like are adsorbed so as not to be taken into the sealed space, an effect of suppressing the influence of the α ray and β ray contribution from the radon thoron can be obtained.

According to the first and second aspects of the present invention, the structure is such that the α-ray detection layer, through which β-rays must be transmitted for independent measurement at the same time, is air and the method of detecting α-rays by air ionization is combined. By doing so, the excess energy loss of β rays can be eliminated. In addition, since the sensitive portions of the α ray and β ray can be handled independently, the β ray sensitivity maintenance and the γ ray sensitivity suppression can be optimized without considering the influence of the α ray.

  Hereinafter, embodiments of an αβ detector and an αβ detection device using the detector according to the present invention will be described with reference to the drawings.

[First Embodiment (FIG. 1)]
FIG. 1 shows an αβ detector according to a first embodiment of the present invention.

  In the present embodiment, the β-ray detector 30 is housed in the case 13, the light-shielding film 12 is mounted on the incident surface of the α-ray and β-ray of the case 13, and the β-ray detector 30, the light-shielding film 12, and the case 13 are mounted. It is the structure which provided the closed space formed by. This space is filled with air 31, and two or more electrodes 32 are set.

  A voltage is applied to these electrodes 32 by a bias power source 33 to form a polarity necessary for collecting positive ionization ions, ionization electrons or negative ionization ions. Then, ions or electrons are collected on the electrode 33, and the induced current is measured by the microammeter 34.

  The air ionization ability of β-rays is extremely low, and only an induced current below the leakage current level of a general microammeter 34 is generated. However, since the ionization ability of α-rays is much larger than that of β-rays, 34 can be measured. In the case of two, the electrode 32 is unipolar with respect to the ground potential, but in the case of three, the electrode 32 can act as a positive and negative collecting electrode.

  According to the present embodiment, the α-ray can be measured from the ionization current of the air on the incident surface side of the β-ray detection unit 30, and the β-ray detection can be measured independently by a dedicated detector. Further, since the α-ray detection layer is basically made of air, the energy lost when β-rays pass through the α-ray detection layer can be kept to a minimum.

[Second Embodiment (FIG. 2)]
FIG. 2 shows an αβ detector according to a second embodiment of the present invention.

  In this embodiment, the ionization current in a constant volume of the closed space is measured in the first embodiment, and the measurement is performed outside the case 13 while sucking air. .

  That is, also in this embodiment, some β-ray detection unit 30 is accommodated in the case 13, the light shielding film 12 is attached to the incident surface of the α ray and β ray of the case 13, and the β-ray detection unit 30 and the light shielding film 12 are attached. A space formed by the film and the case 13 is provided. The case 13 is provided with a ventilation port 35 through which air can be taken in from the outside. The suction device 36 disposed outside and the case 13 are connected by an intake line 37. Between the suction device 36 and the intake line 37, two or more electrodes 38 for measuring the ionization current of air are arranged, and a voltage having a necessary polarity is applied to the electrodes 38 from a bias power source 39. A microammeter 34 for measuring the flowing ionization current is connected.

  According to such a configuration, since the ionization current of air exceeding the volume of the closed space can be measured, the effective α-ray detection limit value can be improved. Compared to the first embodiment, although an additional device is required, there is no electrode in the β-ray passage portion, so that energy loss when β rays pass through the α-ray detection layer is minimal due to air. You can keep it in value.

[Third Embodiment (FIG. 3)]
In the present embodiment, a replaceable adsorption filter 40 for adsorbing particulates is attached to the ventilation port 35 in the third embodiment.

  As this adsorption filter 40, a common filter for adsorbing floating dust similar to that used in a dust collector for radiation dust can be used. Other configurations are the same as those in FIG.

  According to such a configuration, the adsorption filter 40 removes the daughter nuclide Radon Tron, which originates from the radionuclide from the natural uranium thorium series contained in the air, or the dust ionized and adsorbed by the adsorption filter 40. It can be filtered.

  Therefore, according to this embodiment, the influence by the fluctuation | variation of the alpha ray background in the air layer of the inhaling side can be suppressed.

The figure which shows the (alpha) (beta) detection apparatus by 1st Embodiment of this invention. The figure which shows the (alpha) (beta) detection apparatus by 2nd Embodiment of this invention. The figure which shows the (alpha) (beta) detection apparatus by 3rd Embodiment of this invention. The figure which shows an example of a prior art. The figure which shows the other example of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 (alpha) (beta) detector 12 Light shielding film 13 Case 30 (beta) ray detection part 31 Air 32 Electrode 33 Bias power supply 34 Microammeter 35 Ventilation port 36 Suction device 37 Intake line 38 Electrode 39 Bias power supply 40 Adsorption filter

Claims (2)

β-ray detector, a fixed-volume air layer secured on the β-ray incident surface side, means for sucking ionized air by α-ray at a constant flow rate or constant volume, and for measuring the ionization current of the sucked air An αβ detection device comprising an electrode, a power supply device for applying an applied voltage to the electrode, and a minute current measuring means for measuring the ionization current itself. The αβ detection device according to claim 1, further comprising a filter that adsorbs dust to a ventilation port of air newly introduced from the outside in a fixed volume forming an air layer serving as an α-ray detection layer. Detection device.

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