JP5376968B2 - Radiation monitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To shorten the total time of background measurement time and sample measurement time. <P>SOLUTION: A radiation monitor is provided with both a radiation detector for outputting pulse signals according to the detection of radiation and a data processing device for counting pulse output from the radiation detector, measuring radiation, and determining the net amount of radiation of a sample to be measured on the basis of results of each measurement in each case that the sample to be measured is not opposed to the radiation detector and that the sample to be measured is opposed to the radiation detector. The data processing device sets both a preset value (background measurement time Tb) of background measurement time for counting with the sample to be measured not opposed to the radiation detector and a preset value (sample measurement time Ts) of sample measurement time for counting with the sample to be measured opposed to the radiation detector in such a way as to satisfy the previously set minimum detectable level of radiation and that the total (total measurement time Tall) of both preset values of measurement time may be minimum. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a radiation monitor, and more particularly, to a technique for shortening radiation measurement time.

  The radiation monitor is used for examining radioactive contamination of solid objects such as articles and clothes used in a nuclear facility or the like, and examining the radiation dose in a semiconductor device material such as a memory.

  This type of radiation monitor has a radiation detector that outputs a pulse signal in response to the detection of radiation, and each of the state in which the sample to be measured is not placed opposite the radiation detector and the state placed oppositely The radiation counting is performed for a predetermined time. That is, the sample to be measured is not placed opposite to the radiation detector from the apparent radiation count rate (radiation count / radiation measurement time) when the sample to be measured is placed opposite to the radiation detector. The net radiation count rate of the sample to be measured is obtained by subtracting the background radiation count rate at.

  By the way, in general, a statistical error occurs in radiation detection by the radiation detector, and this statistical error corresponds to a minimum level at which radiation can be detected (minimum level at which contamination can be determined). The minimum detectable level of radiation is set in advance according to the amount of radiation to be detected by the sample to be measured. For example, in order to detect a very small amount of radiation, it is necessary to reduce the minimum detectable level of radiation, and to that end, it is necessary to increase the measurement time of radiation.

  In this regard, Patent Document 1 captures the background radiation measurement result, and the measured sample faces the radiation detector based on the captured measurement result and the minimum detectable level of radiation set in advance as a target. It describes that the measurement time of radiation in the installed state (hereinafter referred to as “sample measurement time” as appropriate) is obtained to minimize the sample measurement time.

JP-A-6-148334

  However, although the technique described in Patent Document 1 takes into account minimizing the sample measurement time, the measurement time of radiation in a state where the sample to be measured is not placed opposite to the radiation detector. No consideration is given to minimizing the total time of the sample measurement time (hereinafter referred to as background measurement time as appropriate).

  That is, in Patent Document 1, the background measurement time is set as a fixed value, the background measurement time set in advance and the minimum level of radiation that can be detected in advance set as a target, the background The sample measurement time is obtained based on the ground radiation measurement result and the like.

  For this reason, even if the sample measurement time relative to the preset background measurement time can be minimized, the total measurement time of the total of the background measurement time and the sample measurement time is not necessarily This is not a necessary minimum and may cause a reduction in processing capacity.

  Accordingly, an object of the present invention is to reduce the total time of the background measurement time and the sample measurement time.

  The radiation monitor according to the present invention includes a radiation detector that outputs a pulse signal in response to detection of radiation, and a radiation detector in each of a state in which the sample to be measured is not installed opposite to the radiation detector And a data processing device that measures the radiation by counting the pulse output from the device and obtains the net radiation dose of the sample to be measured based on each measurement result.

  In particular, in order to solve the above-described problem, the data processing apparatus performs counting in a state where the measurement target sample is disposed opposite to the set value of the background measurement time for performing counting in a state where the sample is not disposed opposite to the radiation detector. It is characterized in that the set value of the sample measurement time is set so as to satisfy a preset minimum detectable level of radiation and to minimize the total of the set values of both measurement times.

  In other words, there are multiple combinations of background measurement time and sample measurement time that satisfy the preset minimum detectable level of radiation, but from these, find the combination that minimizes the total measurement time of both. This is the set value for each measurement time. Therefore, it is possible to measure the radiation of the sample to be measured in the minimum measurement time that satisfies the minimum detectable level of radiation set in advance according to the radiation dose to be detected. As a result, the total time of the background measurement time and the sample measurement time can be shortened, and the processing capacity of the radiation monitor can be improved.

  More specifically, the data processing apparatus counts the pulse output from the radiation detector based on the set value of the background measurement time in a state where the sample to be measured is not placed opposite to the radiation detector, and the background radiation. A background measurement unit that calculates the counting rate, and the radiation output of the sample to be measured by counting the pulse output from the radiation detector based on the set value of the sample measurement time while the sample to be measured is placed facing the radiation detector. A sample measurement unit for obtaining a rate, and a net radiation count rate of the sample to be measured can be obtained based on the background radiation count rate and the radiation count rate of the sample to be measured.

Further, it captures the radiation counting and real measurement time during radiation measurement of background, captured radiation counter and on the basis of the actual measurement time, detectable minimum level of radiation in much many very radiation detector sample measurement time The minimum background measurement time that cannot be satisfied is Tbmin = 4nb / (2A / aK) ² (1) (where A is the minimum detectable level of radiation, a is the detection efficiency, K is the reliability, nb is calculated from the background count rate, Tbmin is the minimum background measurement time), and when the actual measurement time has passed the minimum background measurement time, Ts = (4nb−4A / a) / {(2A / aK) ^{2 -4nb / Tb} ... (2} ) ( provided that, Ts is the time sample measurement, Tb background measurement time) and Tall = Ts + b ... (3) (however, Tall the total measurement time) determine the Tall from ^{= (4nb-4A / a)} / {(2A / aK) 2 -4nb / Tb} + Tb ... (4), In Equation (4) The Tall value and the Tb value at that time are obtained, and Ts is obtained based on Equation (3) . The obtained Tb and Ts are used as the set value for the background measurement time and the set value for the sample measurement time, respectively. It can be configured to have a measuring time determining unit for determining.

  According to this, during the background radiation measurement, the background radiation count value and the background actual measurement time are sequentially input to the measurement time determination unit, and the time and sample required for the background measurement in the measurement time determination unit The time required for measurement is calculated. This necessary time is input to the background measurement unit and the sample measurement unit, respectively, and the measurement time is controlled. Accordingly, the total measurement time of the background measurement time and the sample measurement time can be shortened according to the actual measurement state of the background.

  Also, a mounting table for mounting the sample to be measured, and a drive mechanism for moving the mounting table between a state in which the sample to be measured is not installed facing the radiation detector and a state in which the sample is placed facing the radiation detector. A sample placement device is provided, and when the background radiation measurement is started and the set value of the background measurement time has elapsed, the drive mechanism is placed opposite from the state in which the sample to be measured is not placed opposite the radiation detector. After moving the mounting table so that the measurement sample is ready and radiation measurement of the sample to be measured is started and the set value of the sample measurement time has elapsed, the sample to be measured is not placed opposite to the radiation detector. It can comprise so that a mounting base may be moved so that it may be in a state.

  According to the present invention, the total time of the background measurement time and the sample measurement time can be shortened.

It is the schematic of the whole structure of the radiation monitor of this embodiment. It is a block diagram of the characteristic part of the radiation monitor of this embodiment. It is a graph which shows the relationship between background measurement time Tb, sample measurement time Ts, and total measurement time Tall.

  Hereinafter, embodiments of a radiation monitor to which the present invention is applied will be described. In the following description, the same functional parts are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a schematic diagram of the overall configuration of the radiation monitor of the present embodiment. FIG. 2 is a block diagram of the characteristic part of the radiation monitor of this embodiment. As shown in FIG. 1, the radiation monitor 10 of the present embodiment includes a radiation detector 12 that outputs a pulse signal in response to detection of radiation, and a state in which the sample 14 to be measured is not placed opposite to the radiation detector 12. A data processing device 16 that performs radiation measurement by counting the pulse output from the radiation detector 12 in each of the opposingly installed states, and obtains the net radiation dose of the sample 14 to be measured based on each measurement result. It is prepared for.

  Further, the radiation monitor 10 includes a mounting table 18 on which the sample 14 to be measured is mounted, and a state in which the sample 14 is not placed opposite to the radiation detector 12 by moving the mounting table 18 via the rotary arm 20. It comprises a sample placement device 24 having a drive mechanism 22 such as a motor that moves between the opposing placement states.

  As the radiation detector 12, for example, a proportional counter, a GM counter, or a scintillation detector that outputs the number of detected radiation as a pulse signal can be employed. Further, the radiation monitor 10 is used as a sample 14 to be measured, for example, articles used in nuclear facilities, clothes, filters that collect dust in the air, or semiconductor device materials such as memories. For example, the present invention can be applied to a contamination detection monitor, a dust radiation monitor, a minute amount α-ray measuring apparatus, and the like.

  As shown in FIG. 2, the data processing device 16 counts the pulse output from the radiation detector 12 based on the set value of the background measurement time in a state where the sample 14 to be measured is not placed opposite to the radiation detector 12. The background measurement unit 26 for obtaining the background radiation count rate, and the pulse output from the radiation detector is counted based on the set value of the sample measurement time while the sample to be measured is placed opposite to the radiation detector. And a sample measuring unit 28 for obtaining a radiation count rate of the sample 14 to be measured (apparent radiation count rate of the sample 14 to be measured).

  The data processing device 16 uses the radiation count rate of the sample 14 measured by the sample measurement unit 28 (the apparent radiation count rate of the sample 14 to be measured) as a background radiation count rate determined by the background measurement unit 26. Is subtracted to obtain the net radiation count rate of the sample 14 to be measured.

  In the radiation monitor 10 of this embodiment, background radiation measurement is started in a state where the sample 14 to be measured is not placed opposite to the radiation detector 12. When the background radiation measurement is started and the set value of the background measurement time elapses, the drive mechanism 22 changes the state in which the measured sample 14 is not opposed to the radiation detector 12 to the opposed state. The mounting table 18 is moved. Then, when radiation measurement of the sample 14 to be measured is started and the set value of the sample measurement time has elapsed, the state in which the sample 14 is opposed to the radiation detector 12 by the drive mechanism 22 is changed from being opposed to the radiation detector 12. It is comprised so that the mounting base 18 may be moved.

  By the way, in general, a statistical error occurs in radiation detection by the radiation detector 12, and this statistical error corresponds to the minimum level at which radiation can be detected (minimum level at which contamination can be determined). That is, the minimum net radiation count rate of the sample 14 to be measured when (net radiation count rate of the sample 14 to be measured)> (error of the net radiation count rate of the sample 14 to be measured) can detect radiation. The minimum level. In other words, the error range of the net radiation count rate of the sample 14 to be measured should not overlap with zero. The minimum detectable level of radiation is set in advance according to the amount of radiation to be detected by the sample to be measured. For example, in order to detect a very small amount of radiation, it is necessary to reduce the minimum detectable level of radiation, and to that end, it is necessary to increase the measurement time of radiation.

  On the other hand, it is desirable to improve the processing capability (processing efficiency) by shortening the radiation measurement time as much as possible after satisfying the minimum detectable level of radiation. Therefore, the data processing device 16 of the radiation monitor 10 of the present embodiment is placed opposite to the set value of the background measurement time for counting in a state where the measured sample 14 is not placed opposite to the radiation detector 12. The set value of the sample measurement time for counting is set so as to satisfy a preset minimum detectable level of radiation and to minimize the total of the set values of both measurement times.

  More specifically, the data processing device 16 includes a measurement time determination unit 30 that determines a set value for the sample measurement time (sample measurement time Ts) and a set value for the background measurement time (background measurement time Tb). Yes. The measurement time determination unit 30 takes in a radiation count during background radiation measurement (background count value) and an actual measurement time (background actual measurement time), and sets in advance based on the acquired radiation count and actual measurement time. Obtain the correlation between the background measurement time and the sample measurement time that satisfy the minimum detectable level of the emitted radiation, satisfy the required correlation, and minimize the total of both measurement times and sample measurement Time is obtained, and the obtained background measurement time and sample measurement time are determined as a set value for the background measurement time and a set value for the sample measurement time, respectively.

  Hereinafter, the principle of determining the optimum background measurement time Tb and sample measurement time Ts in the measurement time determination unit 30 will be described. Statistical errors occur when radiation is counted. This statistical error is the minimum level that can be detected. This minimum level that can be detected is expressed by Equation (1).

(Expression 1) A = a × K / 2 [K / Ts + {(K / Ts) ² +4 nb (1 / Ts + 1 / Tb)} ^1/2 ]
Here, A is the minimum detectable level of radiation, a is the detection efficiency, K is the reliability, nb is the background count rate (background count value / background actual measurement time), Ts is the sample measurement time, and Tb is This is the background measurement time.

When Equation 1 is solved for the sample measurement time Ts, Equation 2 is obtained.
(Expression 2) Ts = (4nb-4A / a) / {(2A / aK) ² -4nb / Tb}

When the total value obtained by adding the sample measurement time Ts and the background measurement time Tb is the total measurement time Tall, Tall is expressed by the following equation (3).
(Expression 3) Tall = Ts + Tb

By substituting equation (2) into equation (3), equation (4) is established.
(Expression 4) Tall = (4nb−4A / a) / {(2A / aK) ² −4nb / Tb} + Tb

  In Equation 4, since A, a, K, and nb can be treated as constants, the relationship between Tb, Tall, and Ts is as shown in FIG. FIG. 3 is a graph showing the relationship between the background measurement time Tb, the sample measurement time Ts, and the total measurement time Tall. In FIG. 3, the horizontal axis represents the background measurement time Tb, and the vertical axis represents the sample measurement time Ts and the total measurement time Tall.

As shown in FIG. 3, in the background measurement time shortage region below the background minimum measurement time Tbmin, the target radiation minimum detectable level A cannot be satisfied no matter how much the sample measurement time Ts is increased. The minimum background measurement time Tbmin is obtained as the value of Tb when the denominator in Equation 2 is zero. That is, when Equation 5 is solved for Tb, the minimum background measurement time Tbmin is obtained as shown in Equation 6.
(Formula 5) (2A / aK) ² -4nb / Tb = 0
(Expression 6) Tbmin = 4 nb / (2A / aK) ²

  Further, as shown in FIG. 3, since the total measurement time Tall has a minimum value in a region equal to or greater than the background minimum measurement time Tbmin, the background minimum measurement time Tbmin is obtained by Equation 6 to obtain the background minimum measurement time. For example, the minimum value of Tall and the value of the background measurement time Tb when Tall becomes the minimum value can be obtained by the steepest descent method using Tbmin as an initial value. The minimum value of Ts can be obtained from the value of Tb, the minimum value of Tall, and Equation 3. In addition, you may use the method of calculating | requiring not only the steepest descent method but the minimum value of Tall based on Formula 4.

  When the background radiation measurement is started, the measurement time determination unit 30 sequentially fetches the background count value and the background actual measurement time from the background measurement unit 26, executes the above calculation, and performs the background measurement time Tb. The sample measurement time Ts is obtained and output to the background measurement unit 26 and the sample measurement unit 28.

  It should be noted that the background measurement time Tb and the sample measurement time Ts obtained by executing the above calculation by taking the background count value and the background actual measurement time, that is, the background count rate nb once, are set to the background measurement time. And it can also be set as a set value of the sample measurement time. In addition, the setting value of the background measurement time and the setting of the sample measurement time are obtained based on the background measurement time Tb and the sample measurement time Ts obtained by taking the background count rate nb multiple times and executing the above calculation each time. The value can also be determined.

  The background measurement unit 26 ends the measurement when the background measurement time Tb obtained by the calculation is reached, and notifies the sample placement unit 24 that the measurement is completed. When the background measurement is completed, the sample placement device 24 places the measurement sample 14 so as to face the radiation detector 12, and notifies the sample measurement unit 28 that the measurement sample has been placed. When the sample to be measured is installed, the sample measurement unit 28 performs radiation counting of the sample 14 to be measured at the sample measurement time Ts obtained by calculation.

  As described above, according to the radiation monitor of this embodiment, the sum of the sample measurement time Ts and the background measurement time Tb necessary to achieve the target minimum detectable level according to the changing background count value. Can be a minimum value. For this reason, the background level rises more than expected and the target detectable minimum level cannot be reached no matter how long the sample measurement time Ts is taken (becomes the background measurement time shortage region in FIG. 3), or the total measurement. It is possible to avoid that the time Tall is not necessarily minimized. As a result, the processing amount per unit time can be reliably increased as compared with the conventional apparatus, and the processing capacity can be improved.

  That is, the conventional technique calculates the sample measurement time Ts using the preset background measurement time Tb, the preset minimum level A at which radiation can be detected, the background count rate nb, and the like in Formula 2. Is. Therefore, when there are many background counts, the sample measurement time may not be less than the target measurement lower limit, or the sample measurement time becomes too long and the total measurement time is not necessarily the minimum. There is a risk that processing capacity will be reduced.

  In this regard, according to the radiation monitor of the present embodiment, the background measurement state is set so as to satisfy the preset minimum detectable level A of radiation, instead of setting the background measurement time Tb in advance. At the same time, the sample measurement time Ts and the background measurement time Tb are automatically determined. That is, during the background radiation measurement, the background radiation count value and the background actual measurement time are sequentially input to the measurement time determination unit 30, and the time required for the background measurement by the measurement time determination unit 30 and the sample measurement The time required for is calculated. The necessary times are input to the background measurement unit 26 and the sample measurement unit 28, respectively, and the measurement time is controlled. Therefore, the total measurement time Tall of the total of the background measurement time Tb and the sample measurement time Ts can be shortened according to the actual measurement state of the background.

DESCRIPTION OF SYMBOLS 10 Radiation monitor 12 Radiation detector 14 Sample to be measured 16 Data processing device 18 Mounting table 22 Drive mechanism 24 Sample setting device 26 Background measurement unit 28 Sample measurement unit 30 Measurement time determination unit A Minimum level nb that can detect radiation nb Background Count rate Ts Sample measurement time Tb Background measurement time Tall Total measurement time

Claims (2)

A radiation detector that outputs a pulse signal in response to detection of radiation, and a pulse output from the radiation detector in each of a state in which the sample to be measured is not placed opposite to the radiation detector and a state in which the sample is placed opposite to the radiation detector. A radiation monitor comprising a data processing device that performs radiation measurement by counting and calculates a net radiation dose of the sample to be measured based on each measurement result,
The data processing device counts a pulse output from the radiation detector based on a set value of a background measurement time for counting in a state where the sample to be measured is not placed opposite to the radiation detector and A background measurement unit for obtaining a radiation count rate of the sample and a pulse output from the radiation detector based on a set value of a sample measurement time for counting in a state where the sample to be measured is placed facing the radiation detector And a sample measurement unit for obtaining a radiation count rate of the sample to be measured, and obtaining a net radiation count rate of the sample to be measured based on the background radiation count rate and the radiation count rate of the sample to be measured. Is,
Capture the radiation count and actual measurement time during the background radiation measurement, and based on the captured radiation count and actual measurement time,
Background minimum measurement time can not satisfy the detectable minimum level of radiation in much more very the radiation detector through the sample measurement time,
Tbmin = 4nb / (2A / aK) ² (1)
(However, A is the minimum detectable level of radiation, a is the detection efficiency, K is the reliability, nb is the background count rate, and Tbmin is the minimum background measurement time)
Calculated from the case where the actual measuring time has elapsed the background minimum measurement time,
Ts = (4nb-4A / a) / {(2A / aK ) ² -4nb / Tb} (2)
(However, Ts is sample measurement time, Tb is background measurement time)
as well as
Tall = Ts + Tb (3)
(However, Tall is the total measurement time.)
From
Tall = (4nb-4A / a) / {(2A / aK ) ² -4nb / Tb} + Tb (4)
The Tall that is the minimum value in the equation (4) and the value of Tb at that time are obtained, and Ts is obtained based on the equation (3) , and the obtained Tb and Ts are respectively measured in the background measurement. A radiation monitor comprising a measurement time determination unit that determines a set value for time and a set value for the sample measurement time. A mounting table on which the sample to be measured is mounted; and a drive mechanism that moves the mounting table to move the sample to be measured between a state in which the sample is not disposed facing the radiation detector and a state in which the sample is disposed facing the radiation detector. A sample placement device having
When the background radiation measurement is started and the set value of the background measurement time has elapsed, the drive mechanism changes from a state in which the measured sample is not opposed to the radiation detector to a state in which it is opposed to the radiation detector. When the measurement sample is moved and the radiation measurement of the sample to be measured is started and the set value of the sample measurement time elapses, the sample to be measured is opposed to the radiation detector. The radiation monitor according to claim 1, wherein the mounting table is moved so as to be in a state where it is not performed.

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