JP3709340B2 - Radiation measurement equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a radiation measuring apparatus, and more particularly to an energy calibration process of the radiation measuring apparatus.
[0002]
[Prior art]
Some detectors used in radiation measuring devices output a detection pulse signal that is proportional to the energy lost by incident radiation in the detector. For example, scintillator detectors and semiconductor detectors are typical examples. Radiation measurement equipment using this type of detector obtains the energy spectrum of the detected radiation by analyzing the pulse height of the detected pulse using a multi-channel analyzer, and counts the detected pulse with a weight corresponding to the wave height. In some cases, the dose rate or dose equivalent rate is obtained by doing so.
[0003]
However, in this type of apparatus, the pulse height of the detection pulse with respect to incident radiation of the same energy may change due to a change in the characteristics of the detector or the like due to a change in temperature or aging. For example, in an environmental monitoring device installed outdoors, the temperature of the device changes in accordance with changes in temperature, changes in solar radiation, etc., and this temperature change causes the characteristics of the NaI scintillator and photomultiplier tube used in the detector. Change and change the pulse height. Such a change in the pulse wave height causes a shift in the energy evaluation of the detected radiation, which may cause an error in the calculation results of the energy spectrum, the dose equivalent rate, and the like.
[0004]
For this reason, in this type of apparatus, energy calibration processing has been performed conventionally. In a conventional general energy calibration process, a calibration radiation source is measured by the apparatus, a photoelectric peak or a Compton edge corresponding to the radiation source on the obtained energy spectrum is detected, and those peaks are detected as the radiation source. The gain of the amplifier and the voltage applied to the detector were adjusted so that the correct energy value (channel) was located. However, this method has a problem that a manual or dedicated mechanism is required to hold and manage the calibration radiation source and to set the radiation source in the apparatus every time calibration is performed. In particular, in an environmental area monitor that performs automatic measurement over a long period of time, a method using a calibration radiation source has not been realistic.
[0005]
As an apparatus for solving this problem, there is an apparatus disclosed in Japanese Patent Laid-Open No. 9-304542 by the present applicant. This device performs energy calibration based on the photoelectric peaks and Compton edges of ⁴⁰ K natural radionuclides widely present in the environment.
[0006]
[Problems to be solved by the invention]
However, since the detected amount of radiation of a natural radionuclide dynamically changes according to changes in environmental conditions, the peak of the nuclide and the Compton edge do not always appear prominently on the energy spectrum. For example, a photoelectric peak on the energy spectrum can be automatically detected using a well-known smoothed second-order differential method. However, if the natural radionuclide of interest is not sufficiently counted, automatic detection by this method becomes impossible. . For example, in an environmental monitoring device, when snow is piled up, ⁴⁰ K γ rays radiated from the soil are blocked by the snow, and the count is lowered and automatic peak detection becomes impossible. Conventionally, when automatic detection of photoelectric peaks and Compton edges is not possible, energy calibration based on peaks and the like is impossible, and automatic energy calibration is performed using other calibration methods that are inferior in accuracy, such as calibration based on device temperature. It was.
[0007]
The present invention has been made in order to solve such a problem, and enables energy calibration based on index points such as photoelectric peaks as much as possible even when automatic detection of photoelectric peaks or the like has not been possible. An object is to provide a radiation measurement apparatus.
[0010]
[Means for Solving the Problems]
In addition, the apparatus according to the present invention includes a spectrum measuring unit that detects radiation and obtains an energy spectrum of the detected radiation for each unit measurement period, and a spectrum storage unit that stores energy spectrum data for each unit measurement period in time series. Determining whether or not there is a predetermined index point on the current energy spectrum obtained by the spectrum measuring means, and if the index point can be detected, the basic determining means for specifying the energy value; If the index point cannot be detected in the above, the energy spectrum stored in the spectrum storage means is sequentially integrated retroactively with the current energy spectrum until the predetermined point is detected. Extended judgment method for identifying the energy value of the index point on the accumulated spectrum When, and a calibration means for performing an energy calibration of the device based on the energy value of the index point.
[0011]
In this device, when the index point cannot be detected on the energy spectrum of the unit measurement period, the stored past energy spectrum is added to the current spectrum for a predetermined period, and the index point is added to the obtained integrated spectrum. Perform detection processing. If the index point cannot be detected even in the integrated spectrum, the spectrum for a predetermined period is further added retroactively and the index point detection is repeated. And if an index point can be detected, an energy calibration process will be performed based on it. According to this configuration, it is only necessary to perform the minimum spectrum integration necessary to detect the index point, so that the calibration process can be performed based on the measurement result as close as possible to the present.
[0012]
In the present invention , if the index point cannot be detected by the extension determination means even if the energy spectrum is integrated retroactively to a predetermined retroactive upper limit, the calibration by the calibration means is canceled based on the index point. In this aspect, it is possible to set an upper limit of retroactively when spectrum is integrated, and it is possible to prevent calibration using data that is too old.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.
[0014]
FIG. 1 is a block diagram showing an example of a hardware configuration of a radiation measuring apparatus to which the present invention is applied. This example illustrates an environmental monitoring device as an example of a radiation measurement device. The detector 10 of this environmental monitoring apparatus includes a NaI scintillator 10a that emits light in response to the incidence of radiation (mainly γ rays), a photomultiplier tube 10b that converts the emitted light into an electrical detection pulse signal, and photomultiplier. A preamplifier 10c for amplifying the output of the tube 10c is included. The detection pulse signal output from the preamplifier 10c is converted into a digital signal having a value corresponding to the pulse wave height by an analog-digital converter (ADC) 12, and the multi-channel analyzer controller (MCAC) 14 and the digital signal are converted. Input to the processor (DSP) 28.
[0015]
The DSP 28 obtains and outputs a weight value corresponding to the level of the digital signal (corresponding to the detected pulse wave height) using a pre-implemented arithmetic algorithm or conversion table. This weight value is supplied to a dose measuring device connected to the subsequent stage. The dose measuring device calculates various measured values such as a dose rate and a dose equivalent rate by adding the weight values.
[0016]
On the other hand, the MCAC 14 inputs an input digital signal into the spectrum memory 16a or 16b and performs control for creating an energy spectrum on the memory 16a or 16b. That is, the spectrum memories 16a and 16b are memories having the same structure having a plurality of channels corresponding to the respective peak levels. Each time the digital signal corresponding to each detection pulse is input from the ADC 12, the MCAC 14 increases the count of the channel corresponding to the value of the digital signal in the spectrum memory 16 a or 16 b. By performing this process over a predetermined unit measurement period (hereinafter simply referred to as “measurement period”), a wave height distribution of radiation incident on the detector 10, that is, an energy spectrum, is formed in the spectrum memory 16a or 16b. In this configuration, two spectrum memories are provided because, while the energy spectrum of a certain measurement period is formed in one spectrum memory 16a or 16b, the other spectrum memory 16b or This is because various processes can be performed using the energy spectrum of the previous measurement period already formed in 16a.
[0017]
The processor 18 executes processing for storing the energy spectrum stored in the spectrum memories 16a and 16b in the RAM disk 26 and energy calibration processing of the present apparatus. Programs and data for these processes are stored in a ROM (Read Only Memory) 20. In the figure, among various programs and data for the processing of the processor 18, a calibration processing program 22 for energy calibration and data of a reference value 24 used for the calibration are illustrated. The reference value 24 is in principle the energy value of the photoelectric peak of the radiation (γ rays) emitted by the natural radionuclide used for the energy calibration reference, and actually the channel corresponding to the energy value on the energy spectrum. The number is used. For example, when ⁴⁰ K (potassium 40) that exists relatively widely in soil is selected as the calibration standard, the channel number corresponding to the energy 1.462 MeV of γ rays emitted by ⁴⁰ K is stored in the ROM 20 as the reference value 24. Is done. Needless to say, a rewritable recording medium such as an EEPROM may be used instead of the ROM 20.
[0018]
The RAM disk 26 is a semiconductor memory used for storing an energy spectrum. For example, the RAM disk 26 has a capacity for storing energy spectrum data formed every 10 minutes for several months. Instead of the RAM disk 26, other recording media such as a hard disk can be used.
[0019]
At the end of one measurement period, the processor 18 reads the value of the energy spectrum formed during that period from the spectrum memory 16a or 16b and stores it in the RAM disk 26 in chronological order. The energy spectrum stored in the RAM disk 26 for each measurement period can be read periodically or as needed for analysis.
[0020]
In addition, the processor 18 executes the calibration processing program 22 to perform energy calibration processing every time one measurement period ends. For example, if the measurement period is 10 minutes, energy calibration is performed every 10 minutes. In the present embodiment, energy calibration is performed by adjusting the gain of the preamplifier 10c. An example of the processing procedure of this energy calibration process performed for each measurement period will be described with reference to FIG.
[0021]
When one measurement period is completed and the energy spectrum of the detected radiation during that period is formed in the spectrum memory 16a or 16b, the processor 18 uses the RAM (not shown) as a work area from the spectrum memory 16a or 16b. The energy spectrum data is read above (S10). Next, from the energy spectrum, a spectrum within ROI (Region Of Interest) for detecting a ⁴⁰ K photoelectric peak is cut out, and the count value (or count rate) of that portion exceeds a predetermined allowable range. It is determined whether or not (S12).
[0022]
The ROI used here is a region having a predetermined channel width centered on the correct channel (ie, the reference value 24) corresponding to the energy value of ⁴⁰ K γ-rays. By limiting the target range (ROI) for peak detection in this way, it is possible to prevent an erroneous peak from being detected as a ⁴⁰ K photoelectric peak. For example, ⁶⁰ Co, an artificial radionuclide, emits 1.33 MeV gamma rays, so if ⁶⁰ Co is present in the environment, the ⁶⁰ Co peak appears very close to the ⁴⁰ K photoelectric peak. May be misrecognized as a ⁴⁰ K peak, but such misrecognition can be prevented by setting the ROI so as not to include the ⁶⁰ Co peak.
[0023]
When there are too many count values (or count rates) in this ROI (that is, the determination result of S12 is Y), there is a possibility that some kind of abnormality has occurred and the radiation in the environment has increased. The peak obtained from such data cannot be used for energy calibration. In such a case, energy calibration based on the ⁴⁰ K photoelectric peak is canceled and calibration processing based on the apparatus temperature is performed (S30). Since the calibration process based on the apparatus temperature is a conventionally known technique, only a brief description will be given here. That is, in S30, the processor 18 sends a gain command corresponding to the temperature detected by a temperature sensor (not shown) provided in the vicinity of the detector 10 to the preamplifier 10c to adjust the gain. For this reason, for example, an appropriate gain for each temperature is stored in the ROM 20. However, this stored information is a value based on a test result in a specific environment at a specific time such as at the time of factory shipment, and is likely to be slightly different from the current optimum value. Therefore, the calibration process based on such temperature is inferior in accuracy to the ⁴⁰ K peak standard calibration process based on the actual measurement data, but the temperature standard calibration is not performed at all. In many cases, it is better to perform the calibration.
[0024]
If the determination result in S12 is N, it is next determined whether or not the count value (or count rate) of the ⁴⁰ K ROI is less than the allowable range (S14). If the count value (or count rate) in the ROI is too small, the photoelectric peak obtained from the data will be statistically unreliable, so here whether the ROI count is sufficient for peak judgment Are inspected.
[0025]
When the determination result of S14 is N, the ROI count value (or count rate) is within the allowable range for the detection of the ⁴⁰ K photoelectric peak. In this case, the processor 18 performs arithmetic processing for peak detection on the energy spectrum data in the ROI. A known algorithm such as a smoothed second-order differential method can be used for this calculation process.
[0026]
Next, it is determined whether or not a ⁴⁰ K photoelectric peak can be detected by this peak detection process (S18). If it can be detected, energy calibration based on the peak is executed (S20). That is, the processor 18 obtains the channel of the photoelectric peak (corresponding to the energy value), and issues a command to decrease the gain of the preamplifier 10c if the channel is larger than the reference value (that is, the channel that the peak should originally be). (This reduces the pulse wave height), and if the channel is smaller than the reference value, a command is issued to increase the gain of the print amplifier. At this time, the gain adjustment amount is determined according to the difference between the obtained peak channel and the reference value.
[0027]
When the ROI count value (or count rate) is less than the allowable range in the determination of S14 and when it is determined that the peak cannot be detected in S18, in the present embodiment, 1 stored in the RAM disk 26 is stored. The energy spectrum of the previous measurement period is taken out (S24), integrated with the energy spectrum of the current measurement period (S28), and the processes from S14 onward are repeated.
[0028]
By integrating the spectra, an energy spectrum for a longer period is obtained, and a ⁴⁰ K photoelectric peak becomes statistically significant. For example, even if the number of counts near the ⁴⁰ K photoelectric peak in the current measurement period was accidentally large and the peak could not be detected, the statistical fluctuation part was leveled by the addition to the previous spectrum, and the ⁴⁰ K The photo peak stands out. For example, in the energy spectrum of one measurement period (10 minutes) shown in FIG. 3 (a), the number of ⁴⁰ K photoelectric peaks is small, and the difference between the counts of the preceding and following channels is not statistically sufficient. The photoelectric peak cannot be detected by an automatic peak detection method such as a smoothed secondary differential method. On the other hand, FIG.3 (b) has shown the energy spectrum of a total of 1 hour which integrated the spectrum for the past 50 minutes on this energy spectrum. In this one-hour energy spectrum, a ⁴⁰ K photoelectric peak clearly rises from the surroundings and can be detected by an automatic peak detection method.
[0029]
Here, the energy spectrum is accumulated in order from the newest in time series. And the process of S14, S16, and S18 is performed with respect to the energy spectrum (it calls an integration spectrum) obtained as a result of integration (S28). If the ROI count does not reach the allowable range even after integration (the result of S14 is Y), or the photoelectric peak cannot be detected (the result of S18 is N), the energy spectrum is further integrated retroactively. (S28). Although FIG. 2 shows an example in which the energy spectrum is integrated for one measurement period, it is also possible to integrate for a plurality of measurement periods. If a ⁴⁰ K photoelectric peak is found on the integrated spectrum at any point in time while the integration is repeated, an automatic energy calibration process based on the photoelectric peak is performed in S20.
[0030]
Although spectrum integration has the merit of making the photoelectric peak stand out, on the other hand, there is a demerit that the influence of past data is strengthened by the integration. Therefore, if accumulated to the far past, even if a photoelectric peak is obtained, it will be strongly influenced by the past rather than the current device status, and the significance of device calibration based on the current status May fade. Therefore, in this embodiment, it is checked in S22 whether or not the number of integrated spectra has reached a predetermined number, and even when a photoelectric peak is not found, the integration is cut off at a preset position and calibration processing based on temperature is performed. (S30). Thereby, energy calibration can be performed based on information as close as possible to the present. Thereby, for example, it is possible to realize a limitation that the integration is limited to past data within 40 minutes from the current measurement period.
[0031]
Also, if the ⁴⁰ K ROI count value (or count rate) of the energy spectrum extracted from the RAM disk 26 is larger than the allowable range, the spectrum is not suitable for peak detection (and energy calibration based on it) for the reasons described above. In this case, S28 is skipped so that the spectrum is not integrated (S26).
[0032]
The preferred embodiments of the present invention have been described above. As described above, according to the present embodiment, even when a ⁴⁰ K photoelectric peak cannot be automatically detected in the energy spectrum obtained by measurement, the possibility that the peak can be detected by spectrum integration can be increased. Opportunities for performing energy calibration processing based on accurate measured photoelectric peaks with high accuracy can be increased. Further, in the present embodiment, by providing an upper limit for spectrum integration (how far back in the past), it is possible to prevent automatic calibration based on data that is too old.
[0033]
In the above embodiment has used the photoelectric peak of ⁴⁰ K as an index point for the calibration based on the measured energy spectrum, it may be used as the index point Compton edge instead of the photoelectric peak, Nature than ⁴⁰ K A radionuclide may be used. For example, when a plastic scintillator is used for the detector, it is preferable to use the Compton end rather than the photoelectric peak. Which natural radionuclide is used may be determined according to the environmental conditions of the installation location of the apparatus.
[0034]
In the above example, the gain of the preamplifier 10c is adjusted for energy calibration. Of course, other than this, for example, the voltage of the high-voltage power supply applied to the photomultiplier tube 10b may be adjusted. In the case where the main amplifier is arranged between the preamplifier 10c and the ADC 12, energy calibration may be performed by adjusting the gain of the main amplifier.
[0035]
【The invention's effect】
As described above, according to the present invention, when an index point such as a photoelectric peak is not found from the energy spectrum of the unit measurement period, the index is displayed on the spectrum of a longer period by adding past spectra. Since the point is searched, the possibility of finding the index point is further improved. Thereby, the effect that the opportunity which can perform highly accurate energy calibration based on the index point of a measurement spectrum increases is acquired. Further, in the present invention, if the index point cannot be detected from the integrated spectrum, the spectrum is further integrated retroactively, and the process is stopped when the index point is detected, so that the spectrum data at the time as close as possible to the present is obtained. Based on the energy calibration.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a hardware configuration of a radiation measuring apparatus to which the present invention is applied.
FIG. 2 is a flowchart showing a flow of energy calibration processing in the embodiment.
FIG. 3 is a diagram for explaining an effect of energy spectrum integration;
[Explanation of symbols]
10 detector, 10a NaI scintillator, 10b photomultiplier tube, 10c preamplifier, 12 ADC (analog-digital converter), 14 MCAC (multi-channel analyzer controller), 16a, 16b spectrum memory, 18 processor, 20 ROM, 22 Calibration processing program, 24 reference values, 26 RAM disk, 28 DSP (digital signal processor).

Claims (2)

Spectral measurement means that detects radiation from natural radionuclides present in the environment and obtains the energy spectrum of the detected radiation every unit measurement period;
  Spectrum storage means for storing energy spectrum data for each unit measurement period in chronological order;
  Basic determination means for determining the presence or absence of a predetermined index point on the current energy spectrum obtained by the spectrum measurement means, and specifying the energy value if the index point can be detected;
  If the index point is not detected on the current energy spectrum in the basic determination unit, the energy spectrum stored in the spectrum storage unit is stored in the past until the index point is detected. And an extended determination unit that sequentially integrates retrospectively and identifies the energy value of the index point on the obtained integrated spectrum;
  Calibration means for calibrating the energy of the apparatus based on the energy value of the index point;
  With
  In the extended determination means, when the index point cannot be detected even if the energy spectrum is integrated retroactively to a predetermined retroactive upper limit, the energy measurement based on the index point by the calibration means is canceled. apparatus. The radiation measurement apparatus according to claim 1,
  The extended determination means corresponds to an index point to be detected in the energy spectrum extracted from the spectrum storage means for integration when sequentially integrating the energy spectrum stored in the spectrum storage means retroactively. A radiation measurement apparatus characterized in that, when a count value of a predetermined attention range exceeds an allowable range, the integration is canceled for the extracted energy spectrum.

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