JP4669939B2 - Background-compensated α-ray radioactivity measurement system - Google Patents

Description

  The present invention relates to an α-ray radioactivity measuring apparatus capable of measuring and evaluating an artificial radionuclide that emits α-rays. More specifically, the present invention relates to a natural radioactivity having a short half-life that has been an obstacle when measuring α-rays such as plutonium. The present invention relates to a background-compensated α-ray radioactivity measuring apparatus that reduces the background caused by nuclides by using the principle of the time interval analysis method, and can quickly separate and measure radioactivity such as plutonium.

  In nuclear facilities that handle plutonium, such as reprocessing facilities, it is necessary to monitor the concentration of radioactive materials in the air. At present, a general management method is a combination of a continuous monitoring method using a dust monitor and a batch method for measuring α rays of filter paper obtained by collecting dust contained in air in a work environment for a certain period of time.

  A general α-ray radioactivity measuring device according to the prior art detects α-rays emitted from a filter paper from which air dust has been collected for a certain period of time by means of a Si semiconductor detector, etc. The voltage is converted into a digital signal, a voltage value in a range converted into plutonium energy set in advance by an SCA (single channel analyzer) is measured, and the data is displayed. However, when an air dust sample in a work environment is measured by such a conventional apparatus, as shown in FIG. 10, it is released from natural radionuclides (particularly radon progeny) in the energy region of plutonium by the effect of self-absorption. Accurate plutonium measurement is not possible because the energy produced is a hindrance.

  FIG. 11 shows the natural radionuclide destruction series (uranium series) after radon. Among these progeny nuclides after radon, the nuclide that becomes the background (releases alpha rays that interfere with the measurement) when performing energy fractionation measurement of artificial radionuclides from air dust samples in the environment is Po-214. is there. This Po-214 has a very short half-life of 164 μs. In order to reduce the effect of this nuclide, it was necessary to leave the sample for a maximum of about 3 days after sampling.

  As a method for measuring the artificial radionuclide such as plutonium by reducing the influence of the background caused by the progeny of Rn-222, there is a method using the β-α correlation between Bi-214 and Po-214. Since Po-214 has a half-life of 164 μs, it is considered to be a short time until Bi-214 decays (α-ray emission) after Bi-214 decays (β-ray emission). Therefore, using this relationship, a background-compensated α-ray radioactivity measuring apparatus has been studied which has been devised such as providing β-rays as a start pulse and providing a dead time of several hundred μs on a pulse measurement circuit. However, due to the amplification time of the pulse, etc., due to the time delay on the circuit and the low counting efficiency, the ratio that can be measured is low, and it has not been put into practical use.

By the way, as a prior art, there is a measurement method called “time interval analysis method” (Non-patent Document 1). This measures the time interval of electrical signals (pulses) by radionuclides and creates a time interval spectrum to selectively extract only the correlation event relationship (a pair of radionuclides with lower nuclides with a short half-life). It is a method to do. This measurement technique is used to measure the radioactivity of progeny nuclides with a short half-life such as the thorium series.
Tetsuo Hashimoto, Tomoaki Kubota: “Absolute measurement of thorium series nuclides in Tamagawa hot spring water using time interval analysis”, RADIOISOTOPES, 38, 415-420 (1989)

  The problem to be solved by the present invention is to remove the influence of the background of natural radionuclides such as radon (222Rn) progeny that exist in nature, and to measure and evaluate artificial radionuclides such as plutonium quickly and accurately. It is to realize such an α-ray radioactivity measuring apparatus.

  As described above, Po-214 is a nuclide that becomes a background when performing energy fractionation measurement of an artificial radionuclide such as plutonium from an air dust sample in the environment. Therefore, if the α-ray pulse derived from Po-214 can be extracted and measured, the background when measuring the artificial radionuclide can be compensated. Therefore, the present invention obtains a background of a natural radionuclide using a time interval analysis method, which is a method for selectively measuring nuclides having a short half-life, and quickly compensates for it to obtain radioactivity such as plutonium. It is an α-ray radioactivity measuring device devised so that it can be measured separately.

  That is, the present invention provides a detection unit that detects α-rays and β-rays emitted from a sample and transmits them as electrical signals, a data processing unit that processes electrical signals transmitted from the detection units, A personal computer is provided for analyzing and displaying the results, and the data processing unit measures the time interval of a digital signal obtained by digitally converting an analog pulse transmitted from the detection unit and exceeding a predetermined classification level by an AD converter. Two systems are provided: an energy and time interval measurement system, and a high-speed time interval measurement system that measures time intervals of analog pulses exceeding a predetermined classification level without digital conversion, and the measurement data is buffered in a FIFO memory The data in the FIFO memory is sent to a personal computer and recorded by a recording device. In this case, the recorded data is used to selectively extract α-rays derived from Po-214, which is a progeny of Rn-222, by capturing β-α correlation events by time interval analysis. It is a background-compensated α-ray radioactivity measuring apparatus characterized by measuring the α-rays derived from the artificial radionuclide while reducing the interference caused by Po-214.

  The sample is a filter paper to which air dust is attached, and the detection unit arranges a combination of a detector and a preamplifier on both sides of the filter paper as a pair, and outputs of both preamplifiers are input to a summing amplifier. A structure is preferred. Both the energy and time interval measurement system and the high-speed time interval measurement system of the data processing unit share, for example, a timer for measuring the time interval, a changeover switch and a FIFO memory, and the changeover switch allows measurement data of one system to be measured. Select to be buffered in FIFO memory. Further, in the personal computer, based on the data in the recording apparatus, a count that falls within the range of 0-1 ms is used as the previously determined β-α correlation event measurement efficiency (Po− having a time interval of 0-1 ms). The net count of the α-rays derived from Po-214 is calculated by correction by the pulse count derived from 214 / total count of α-rays derived from Po-214 measured by the detector. Here, a Th-230 radiation source can be used as a radiation source for obtaining in advance the measurement efficiency of the β-α correlation event.

  The background compensated α-ray radioactivity measuring apparatus of the present invention measures the energy channel count of the artificial radionuclide to be measured by removing the count of radon progeny nuclides that are the background, so in the conventional method, Early qualification and quantification of artificial alpha-emitting radionuclides (plutonium, etc.) that could not be evaluated until radon progeny decayed. In addition, since the present invention can directly process the signal from the detector, the conventional sampling method and detector need not be changed, and it is not necessary to consider the applicability depending on the method of the detector. The lower limit of detection is expected to be about 1/3 higher than the conventional method of measuring only α rays. However, the lower limit of detection is sufficient for exposure management by changing the measurement conditions such as increasing the measurement time. The value can be compensated.

  First, the concept and method of the time interval analysis method used in the present invention will be described with reference to FIG. In the time interval analysis method, (1) the maximum peak value H of the electric signal generated by the incidence of radiation on the detector and the time interval T between pulses are measured, and these data are stored. (2) After that, a pulse is identified by the peak value H corresponding to the target energy range, and a time interval spectrum is created with the time interval of the identified pulse taken on the horizontal axis and counted on the vertical axis. (3) When a nuclide with a short half-life is present in the measurement target compared to the overall radioactivity, the time interval between the higher-order nuclide is shortened. Here, a radionuclide pair having a lower nuclide with a short half-life is called a correlation event relationship. The radionuclide time interval count, which is a correlated event relationship, shows an exponential decrease with increasing half-life as the time interval increases on the time interval spectrum. On the other hand, since nuclides with a long half-life decay according to radioactivity, there is a significant difference in slope compared with nuclides related to correlation events. (4) From this result, it is possible to selectively extract only the correlation event relationship count on the time interval spectrum.

  Looking at the natural radionuclide decay series after Radon (see FIG. 11), Po-214 has a half-life of 164 μs, so that Po-214 decays (alpha rays after Bi-214 decay (β-ray emission)). It is considered to be a short time until release. As described above, Po-214 is the background nuclide when performing energy fractionation measurement of artificial radionuclides from air dust samples in the environment. If Po-214 can be extracted and measured, then artificial radionuclides can be measured. It is possible to compensate for the background when measuring. Therefore, the background-compensated α-ray radioactivity measuring apparatus of the present invention extracts and removes components having a short half-life (β-α correlation event derived from Po-214), and a component having a long half-life (random event). The radioactivity by artificial radionuclide is measured from

  FIG. 2 is an apparatus configuration diagram showing an embodiment of a background compensation type α-ray radioactivity measuring apparatus according to the present invention for realizing such a technique. As a sample, a filter paper 10 from which air dust is collected is measured by the detection unit 12. In the detection unit 12, two sets of a detector 14 such as a Si semiconductor detector and a preamplifier (PreAMP) 16, which are set on both the front and back surfaces of the filter paper 10, are arranged so that the detector 14 is close to the filter paper 10. To place. Radiation-derived electrical signals detected by both detectors 14 are amplified by a preamplifier 16 and input to a summing amplifier (SumAMP) 18 for amplification. That is, the time-series signals from both the preamplifiers 16 become one time-series signal by the summing amplifier (SumAMP) 18. The amplified signal is subjected to data processing in the data processing unit 20.

  In the data processing unit 20, the input electrical signal exceeds the predetermined separation level without digital conversion with energy and time interval measurement system (hereinafter referred to as “A system”) for digitally converting and measuring time intervals. The processing is branched into two systems, that is, a high-speed time interval measurement system (hereinafter referred to as “B system”) that measures the time interval of analog pulses.

  In system A (energy and time interval measurement system), a signal that exceeds a predetermined classification level in order to remove noise by the first classifier (DISC1) 22 is a successive approximation type high-speed AD (analog-digital) converter. 24 is converted into a digital signal. The time interval information is measured by the programmable timer 26 for the digitally converted signal, and the time interval information is stored in the FIFO memory 28 together with the peak value corresponding to the energy of the radiation. The crest value and time interval data are sent to a personal computer (PC) 32 via the CPU bus 30 and analyzed, and the results are displayed in real time.

  In the B system (high-speed time interval measurement system), the signal exceeding the classification level for removing noise by the second classifier (DISC2) 34 remains an analog signal, and the time interval information is measured by the programmable timer 26. And stored in the FIFO memory 28. The time interval data is sent to the personal computer (PC) 32 via the CPU bus 30 and analyzed, and the result is displayed in real time.

  These two systems are configured to be selectively controlled from the personal computer 32 by the changeover switch 36. When energy information (crest value data) is also required, the A system is selected by the changeover switch 36 in response to a changeover signal from the personal computer (PC) 32. When energy information is not required and measurement is to be performed with a reduced dead time, the B system is selected by the changeover switch 36 in response to a changeover signal from the personal computer (PC) 32. Therefore, in terms of apparatus, the programmable timer 26, the FIFO memory 28, etc. are shared by both systems.

  The data in the FIFO memory 28 goes out in the order of entry (first-in, first-out), is sent to the personal computer 32 via the CPU bus 30, and is recorded by a recording device such as a hard disk device. In the personal computer 32, α-rays derived from Po-214, which is a descendant nuclide of Rn-222, are selectively extracted by capturing β-α correlation events by the time interval analysis method using the recorded data. By removing from the whole, the interference caused by Po-214 is reduced, and α rays derived from the artificial radionuclide are measured. Specifically, the selective extraction of α-rays derived from Po-214 is based on the measurement efficiency of a β-α correlation event (a time of 0-1 ms calculated in advance) by counting the time interval within the range of 0-1 ms. The calculation is carried out by calculating the net count of the α-rays derived from Po-214, corrected by the pulse count derived from Po-214 with intervals / the total count of the α-rays derived from Po-214 measured by the detector.

  FIG. 3 shows a detailed process flowchart of the background compensated α-ray radioactivity measuring apparatus according to the present invention. In FIG. 3, the thick frame portion indicates processing to be displayed by the personal computer (PC) 32 at any time. In this apparatus, the A system and the B system are properly used as follows. An outline of a normal measurement procedure is shown in FIG. At the time of measurement, since the magnitude of radiation between the measurement object (Pu: plutonium) and the background (Rn: radon progeny) in the sample is unknown, measurement of the A system (energy spectrum and time interval analysis) is first performed. The dead time for the measurement of the A system is a little longer, about 19 μs. In this measurement, the radiation dose is compared between the measurement object and the background, and the subsequent processing is determined based on the result. If Pu >> Rn (about 10 times or more), there is a clear difference in the measurement results, so the measurement may be completed only with the measurement of the A system. Otherwise, the reliability of the measurement results cannot be ensured unless precise measurement is performed. Therefore, only the time interval analysis is performed in the B system, and the measurement result is displayed. The dead time for the measurement of the B system is as short as about 2 μs, so that the counting down is reduced. This also makes it possible to distinguish Pu <Rn or Pu≈0.

In order to accurately reduce radon progeny nuclides that become a background during α-ray measurement, the α-ray radioactivity measuring apparatus of the present invention is devised particularly in the following points.
(1) The combination of the detector and the preamplifier is arranged as a pair on the front and back of the filter paper as a sample, and the measurement efficiency of radiation emitted from the sample is improved by measuring from both sides of the sample.
(2) Use a timer with good time resolution to enable measurement at short time intervals.
(3) In order to enable measurement at a short time interval, the dead time is shortened to about 10 μs by using a high-speed AD converter and a FIFO memory.
(4) By providing a changeover switch, when the energy of α rays is not measured, the dead time is further shortened by measuring time intervals for analog pulses exceeding the classification level.
(5) The pulse energy and time interval are recorded on the hard disk in the apparatus as paired data. Taking into account the half-life of the decay of Po-214, a count that falls within the range of 0-1 ms is taken as the predetermined correlation event measurement efficiency (number of pulses with a time interval of 0-1 ms / detector). The net count is calculated by correcting with the measured α-ray).
(6) By arranging all the devices used for the data processing unit on the substrate, the device is further reduced in size and weight.

  In order to confirm that the α-ray radioactivity measuring apparatus according to the present invention can actually measure a Po-214-derived β-α correlation event, measurement was performed using a Th-230 electrodeposition source. An example of the energy spectrum of a Th-230 radiation source is shown in FIG. Since the Th-230 radiation source includes a β-ray derived from Bi-214 and an α-ray of 7.69 MeV derived from Po-214, the α-radioactivity of the present invention is measured by measuring the Th-230 radiation source. Whether or not the correlation event can be measured by the measuring device can be confirmed.

  In actual measurement, the time interval of all pulses is measured, and only the result of the energy channel corresponding to Bi-214 and Po-214 is identified from the result, and the time interval spectrum is created. An example of the time interval spectrum of Po-214 by the Th-230 radiation source is shown in FIG. Thus, the time interval count at which the β-α correlation event was measured shows an exponential decrease with a slope corresponding to the half-life to the range of 1000 μs. In addition, it is thought that the count after 1000 microseconds is counting off of a detector.

  The ratio between the total count of α-rays derived from Po-214 incident on the detector and the count of α-rays derived from Po-214 at a time interval of 0-1000 μs is defined as “correlation event measurement efficiency”. FIG. 7 shows the relationship between the total count of Po-214-derived α rays and the measurement efficiency of correlation events when measuring a Th-230 radiation source. Thus, when the absolute count of Po-214 is small, the measurement efficiency of the correlation event varies, but when there are 300 counts or more, it is constant in the range of 0.3-0.35. From this result, it can be evaluated that the α-ray radioactivity measuring apparatus of the present example was able to count 30% of the correlation events incident on the detector. Therefore, the net count of the α-ray derived from Po-214 can be calculated using the measurement efficiency of the correlation event thus obtained.

  An example of measurement of an actual sample by the α-ray radioactivity measuring apparatus of the present invention will be shown. Dust in the air was collected on glass fiber filter paper (HE40T, pore diameter 8 μm) at 70 L / min for 2 hours, and immediately after collection, measurement was performed using the α-ray radioactivity measuring apparatus of the present invention. The measurement time was 2 hours every 100 seconds. FIG. 8 shows an α-ray spectrum of a general air dust sample. Thus, the energy of the α-ray derived from Po-214 shifts to the low energy side due to the self-absorption of the filter paper and the distance to the detector. As a result, it can be seen that this is an obstacle to the measurement of artificial radionuclides such as plutonium (energy region is approximately 5 MeV).

  In air dust collected in a normal environment, it is assumed that the amount of Bi-214 and Po-214 is the dominant nuclide in the background, and the measurement efficiency of the correlation event is between the total α-ray count and 0-1000 μs. The ratio to the count of pulses with time intervals. FIG. 9 shows the change over time in the measurement efficiency of the correlation event when the air dust sample is measured. Although the total α-ray count gradually decreases, the measurement efficiency of correlation events does not vary greatly over time.

  Based on the above results, the apparatus of the present embodiment can realize high-speed processing, can extract correlation events according to the theory of the time interval analysis method, compensates the background using its properties, and generates artificial radionuclides. It was confirmed that the α-ray radioactivity can be measured by.

  In addition, the background compensation type | mold alpha ray radioactivity measuring apparatus based on this invention is the measurement of B system other than the method of measuring only A system as mentioned above, and the method of measuring B system after measuring A system. It is also possible to use the method for measuring only the energy spectrum or the method for measuring only the energy spectrum.

Explanatory drawing of the concept and method of the time interval analysis method utilized by this invention. The equipment block diagram which shows one Example of the background compensation type | mold alpha ray radioactivity measuring apparatus which concerns on this invention. The detailed process flowchart in this measuring apparatus. The figure which shows the outline of the normal measurement procedure by this measuring apparatus. Energy spectrum of Th-230 source. The time interval spectrum of Po-214 by a present Example. The related figure of the total count of Po-214 by a present Example, and the measurement efficiency of a correlation event. The energy spectrum of the air dust sample by a present Example. The figure which shows the time-dependent change of the total count of Po-214 and the measurement efficiency of a correlation event by a present Example. Energy spectrum of air dust sample according to the prior art. The figure which shows the natural radionuclide destruction series (uranium series) after radon.

Explanation of symbols

10 Filter paper 12 Detector 14 Detector 16 Preamplifier (PreAMP)
18 Summing amplifier (SumAMP)
20 Data processor 22 First separator (DISC1)
24 High-speed AD converter 26 Programmable timer 28 FIFO memory 30 CPU bus 32 Personal computer (PC)
34 Second sorter (DISC2)
36 selector switch

Claims (4)

  A detection unit that detects α-rays and β-rays emitted from the sample and transmits them as electrical signals, a data processing unit that processes the electrical signals transmitted from the detection units, and analyzes the results of the data processing unit An energy and time interval measurement for measuring a time interval of a digital signal obtained by digitally converting an analog pulse transmitted from the detection portion and exceeding a predetermined classification level by an AD converter. System and a high-speed time interval measurement system for measuring time intervals of analog pulses exceeding a predetermined classification level without digital conversion, and the measurement data is buffered in a FIFO memory. The data in the memory is sent to a personal computer and recorded by a recording device. By extracting β-α correlation events by the time interval analysis method using the obtained data and selectively extracting α-rays derived from Rn-222 progeny, Po-214, and removing them from the whole A background-compensated α-ray radioactivity measuring apparatus that measures α-rays derived from artificial radionuclides while reducing interference by Po-214.   The sample is filter paper to which air dust is adhered, and the detection unit is arranged on both sides of the filter paper as a combination of a detector and a preamplifier, and the outputs of both preamplifiers are input to the summing amplifier. The background-compensated α-ray radiation measuring apparatus according to claim 1, wherein   Both the energy and time interval measurement system of the data processing unit and the high-speed time interval measurement system share a timer for measuring the time interval, a changeover switch and a FIFO memory, and the changeover switch selects measurement data of one of the systems. The background compensated α-ray radiation measuring apparatus according to claim 1 or 2, wherein the apparatus is buffered by a FIFO memory. In the personal computer, based on the data in the recording device, the count of the time interval falling within the range of 0-1 ms is measured by the Po-214-derived pulse counter / detector having a time interval of 0-1 ms. Any one of claims 1 to 3, wherein a net count of α-rays derived from Po-214 is calculated by correcting with a measurement efficiency of β-α-correlated events obtained in advance by calculation of total count of 214-derived α-rays. The background-compensated α-ray radioactivity measurement apparatus described in 1.

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