JP4828962B2 - Radioactivity inspection method and apparatus - Google Patents

Description

  The present invention relates to a radioactivity inspection method and apparatus for inspecting radioactive contamination of articles and the like used in nuclear fuel facilities and the like.

  In order to reduce the effects of background radiation, such as radiation from natural radioactivity present in the surrounding environment, when measuring radiation for the purpose of inspecting radioactive contamination of articles used in nuclear fuel facilities, etc. A method is adopted in which the background radiation is measured in the absence of the measurement object, then the radiation of the measurement object is measured, and the net radiation value is calculated by subtracting the previously measured background radiation value. (Patent Document 1).

  A typical example of such background radiation is radiation from K-40 contained in concrete or the like, but it is also emitted from radontrons in the air, and also due to cosmic rays. The amount of background radiation varies on a daily basis due to influences and the like. For this reason, a commercially available radiation monitor measures the background between measurements and uses the previous value in order to cope with such background fluctuations.

  In the stationary type radiation monitor, as a classification based on the presence or absence of background compensation at the time of measurement in JIS Z4337, the background is always measured by each detector while the measurement article is not installed on the monitor. A monitor that subtracts the background count rate immediately before measurement from the count rate when the article is installed is defined as a background continuous subtraction type.

In addition, in order to remove a fluctuation portion having a large background, a method of excluding a setting error (generally 3σ to 5σ of a count value) or more from background variations measured several times may be used. Furthermore, since the distribution of the radiation count values follows a normal distribution, there is a method of removing background fluctuations by monitoring such deviation from the statistical model.
Japanese Examined Patent Publication No. 4-26073

  Now, in the case of a conventional radiation monitor, when a strong radiation source such as a sample to be measured is placed near the monitor, it detects that the background has risen and measures the background again. It is not determined whether it is a background fluctuation or a fluctuation caused by a radiation source placed near the monitor.

The detection sensitivity of the radiation monitor is represented by the detection efficiency and the minimum detection count rate. Among these, the minimum detection count rate is also called a detection limit count rate, and is generally calculated using the following equation (1).

  That is, since the detection limit count rate depends on the background count rate, when the background rises, measurement is performed in a state where detection sensitivity is low due to high background without giving an alarm to the measurer. Therefore, there is a problem that even if contamination due to radioactivity exists, it is overlooked.

  Furthermore, simply monitoring the amount of fluctuation of the background level has the problem that it cannot be determined whether it is due to radiation fluctuations or due to detector deterioration or failure.

  Therefore, an object of the present invention is to provide a radioactivity inspection method and apparatus capable of preventing erroneous measurement with a low detection sensitivity even when the background changes.

  The radioactivity inspection apparatus according to the first aspect of the present invention comprises a radiation detection means for detecting radiation emitted from an object whose radioactivity is to be examined, a radiation counting means for measuring the number of radiation detected by the radiation detection means, and a back BG / main measurement switching means for switching between ground measurement and measurement on the inspection object, BG storage means for storing the background measurement values counted for each designated time in time series, and the stored time series back BG fluctuation amount calculation means for calculating the background fluctuation amount from the ground measurement value, and fluctuations for the background fluctuation cause stored in advance for the fluctuation pattern of the background measurement value obtained by the BG fluctuation amount calculation means. A variation cause estimating means for estimating a cause of fluctuation of a background measurement value with reference to a pattern, and the estimated original In accordance with the measurement stop instruction or warning display, BG count calculation means for obtaining a value necessary for background subtraction from the background measurement value stored in the BG storage means, and the radiation measurement value from the inspection object BG subtraction means for subtracting the background measurement value from the net radiation measurement value and conversion means for obtaining the radioactivity amount of the inspection object from the net radiation measurement value.

In the radioactivity inspection method of the invention according to claim 4 , when inspecting the radioactivity of the inspection object, the background is measured every specified time, and the background fluctuation amount is calculated from the time series background measurement value. The background measurement value fluctuation pattern is estimated with reference to the fluctuation pattern of the background measurement value obtained in advance with respect to the background measurement value fluctuation pattern, and the measurement stop instruction is issued according to the estimated cause. Or, a warning is displayed.

  According to the present invention, it is possible to prevent erroneous measurement with a low detection sensitivity even when the background changes.

Hereinafter, first and second embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
As shown in the block diagram of FIG. 1, the radioactivity inspection apparatus according to the first embodiment of the present invention has one γ-ray detector represented by, for example, a plastic scintillation detector or a NaI (Tl) detector. Or n number of arranged radiation detection means 1 and, for example, an amplifier, a counter, and a computer or multi-channel scaler for reading and storing the count value, and having a plurality of systems corresponding to the number of detectors The radiation counting means 2, the BG / main measurement switching means 3 for automatically switching between the background measurement and the main measurement when the main measurement of the inspection object is performed, and the background is counted at a preset measurement time, BG storage means 4 for sequentially storing count values, and background count per specified time stored in BG storage means 4 BG count calculation means 5 for obtaining a background count rate by adding together, BG fluctuation amount calculation means 6 for calculating a background fluctuation amount per designated measurement time, background fluctuation amount, statistical error, and inspection apparatus Fluctuation level comparison means 7 for detecting the presence or absence of background fluctuation compared to the set level defined by the systematic error, and how the background fluctuation of the plurality of detectors 1 to n is known in advance. By referring to the variation pattern of each detector corresponding to the background variation cause, the variation cause estimation means 8 for estimating the variation cause, and when there is a variation exceeding the allowable value, or depending on the radiation source in the variation pattern If background rise is observed, stop the background measurement or reset the background data. Measurement stop means 9 to perform, warning display means 10 to display a warning, BG subtraction means 11 to subtract the background from the count value in the main measurement for the inspection object, and to obtain by performing calibration in advance A constant for converting into the obtained radioactivity amount is stored, and the radioactivity amount conversion means 12 converts the net count rate measured by referring to the conversion constant into the radioactivity amount.

  Note that the fluctuation level comparison means 7 and the fluctuation cause estimation means 8 need only include at least one of them. Further, when the cause of variation is not estimated from the pattern of background change amounts of a plurality of detectors, only one radiation detector 1 to n may be used.

  The radioactivity inspection apparatus of the present embodiment configured as described above operates as follows. That is, in the normal state, the background radiation is first detected by the radiation detection means 1 in the absence of the inspection object. At this time, a signal of detected radiation is output from n radiation detectors which are each element of the radiation detection means 1, and radiation counting means 2 having n systems connected corresponding to each radiation detector. Thus, counting is repeated at a predetermined time. The n background count data counted by the radiation counting means 2 are sequentially sent to and stored in the BG storage means 4 by the BG / main measurement switching means 3. The stored data is sequentially read out to the BG fluctuation amount calculating means 6 and the background data for a certain time and the amount of change of the next background data are obtained for each of the detectors 1 to n. When the fluctuation of the background count for each time element is large, an arbitrary interval is provided to perform smoothing by moving average, and the fluctuation amount can be obtained using the result.

  Since the generation of radiation is a random phenomenon and follows a Poisson distribution, the statistical error is the square root of the count value. The error of the background count rate of the time element following the time element is normal to the statistical error obtained by the square root of the background count rate of the time element if the measuring apparatus is normal and the radiation source is not varied. 99% or more of the total error added by the system error of the measuring device is entered with a probability of 99% or more.

  If the measurement error is a variation of n background measurement values, which is the conventional method, the measurement error is (background statistical error) + (systematic error of the measuring device) + (background level error). Variation error), the background variation is incorporated into the measurement error, and the measurement error σ becomes large.

  Therefore, only the systematic error of the measuring device is evaluated in advance under conditions that are not easily affected by fluctuations in the background level, and (statistical error of the reference background count rate at the time of determination) + (preliminary evaluation) If the measurement system error) is set as the measurement error, the determination level can be set without being affected by the fluctuation of the background level.

  Accordingly, when (statistical error obtained from background count) + (normal system error) = (measurement error σ), for example, when ± 3σ is used as a criterion as an example, the BG variation calculation means When a fluctuation amount exceeding 6 is output from 6, the fluctuation level comparison means 7 detects the fluctuation of the background radiation.

  FIG. 2 is a graph illustrating fluctuations when four detectors are arranged in parallel and the background count rate is measured every 300 seconds.

  Focusing on the fluctuation pattern of the background count rate for each detector, the background rises uniformly when a radiation source is placed in the vicinity of the detector. In other words, any detector is affected by fluctuations caused by external radiation. The degree of influence of each detector is affected by the structure of the shielding body and the direction of the radiation source. Therefore, by measuring the pattern in advance, the natural background such as radontron is used. It is possible to discriminate between fluctuations and fluctuations due to the proximity of a high-dose sample.

  It can be determined that the fluctuation that occurs only in a specific channel is caused by the trouble of the detector. When a long-term fluctuation pattern of several hours to several days or more is referred to, it is possible to grasp the abnormality of the detector. Furthermore, a background count is stopped by outputting a signal to the measurement stop means 9 and the warning display means 10 due to an abnormality such as detection of a large fluctuation or detection of a radiation source near the detector, etc. A warning is displayed.

  According to the present embodiment, when the background rises due to the radiation source being placed in the vicinity of the radiation detection means 1 or the like, an erroneous value is prevented from being output, and the background has changed. Also, it is possible to prevent erroneous measurement with a low detection sensitivity.

(Second Embodiment)
FIG. 3 is a block diagram showing the configuration of the radioactivity inspection apparatus according to the second embodiment of the present invention. The radioactivity test apparatus according to the present embodiment calculates and sets a detection limit on the output side of the BG count calculation means 5 on the configuration of the radioactivity test apparatus according to the first embodiment shown in FIG. The detection limit calculation / comparison means 13 for comparing with the reference level is provided, and the output is input to the warning display means 10 and the measurement stop means.

  In the radioactivity inspection apparatus of the present embodiment, radiation detection means 1, radiation counting means 2, BG / main measurement switching means 3, BG storage means 4, BG count calculation means 5, BG fluctuation amount calculation means 6, fluctuation level comparison The means 7, the fluctuation cause estimation means 8, the BG subtraction means 11 and the radioactivity amount conversion means 12 operate in the same manner as in the radioactivity inspection apparatus of the first embodiment.

  In the detection limit calculation comparison means 13, when the background count rate Nb obtained by the BG count calculation means 5 and the background count per unit time stored in the BG storage means 4 for obtaining the background count rate are added up From the total counting time tb and the measurement time ts set for measuring the measurement object, the detection limit count rate is obtained using the equation (1).

  The detection limit calculating / comparing means 13 further compares the measurement limit level with a predetermined measurement reference level according to the object to be measured and the purpose, and when the detection limit count rate is equal to or higher than the measurement determination reference level, the measurement stop means 9 and the warning display means 10, stop the background measurement, and display a warning to the operator or the like.

  According to the present embodiment, the measurement is performed in a state in which the required detection sensitivity cannot be obtained because the dose increases due to the radiation source being placed in the vicinity of the radiation detection means 1 and the background level increases. Can be prevented.

The block diagram which shows the structure of the radioactivity inspection apparatus of the 1st Embodiment of this invention. The graph which illustrates the fluctuation | variation pattern of a background count rate, and demonstrates operation | movement of the radioactivity inspection apparatus of the 1st Embodiment of this invention. The block diagram which shows the structure of the radioactivity inspection apparatus of the 2nd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Radiation detection means, 2 ... Radiation counting means, 3 ... BG / main measurement switching means, 4 ... BG storage means, 5 ... BG count calculation means, 6 ... BG fluctuation amount calculation means, 7 ... Fluctuation level comparison means, 8 ... fluctuation cause estimation means, 9 ... measurement stop means, 10 ... warning display means, 11 ... BG subtraction means, 12 ... radioactivity amount conversion means, 13 ... detection limit calculation comparison means.

Claims (6)

  Radiation detection means for detecting radiation radiated from an object to be examined for radioactivity, radiation counting means for measuring the number of radiation detected by the radiation detection means, BG / which switches between background measurement and measurement for the inspection object The main measurement switching means, the BG storage means for storing the background measurement values counted every designated time in time series, and the background fluctuation amount is calculated from the stored time series background measurement values. BG fluctuation amount calculation means, and the background measurement value fluctuation cause by referring to the fluctuation pattern for the background fluctuation cause stored in advance with respect to the fluctuation pattern of the background measurement value obtained by the BG fluctuation amount calculation means Variation cause estimation means for estimating the measurement stop instruction or warning table according to the estimated cause BG count calculating means for obtaining a value necessary for background subtraction from the background measurement value stored in the BG storage means, and subtracting the background measurement value from the radiation measurement value from the inspection object A radioactivity inspection apparatus comprising: BG subtraction means for obtaining a net radiation measurement value; and conversion means for obtaining a radioactivity amount of an inspection object from the net radiation measurement value. The radioactivity test apparatus according to claim 1, further comprising a moving average unit that performs a moving average of background measurement values in a designated time interval after the BG storage unit. A total measurement time when the background measurement value obtained by the BG count calculation means and the background count per specified time stored in the BG storage means for obtaining the background measurement value are added together; A detection limit calculation / comparison means for determining whether measurement is possible or not with reference to a predetermined measurement reference level for a detection limit count rate obtained from a measurement time set for measuring an inspection object. The radioactivity inspection apparatus according to claim 1, wherein:   When inspecting the radioactivity of an inspection object, the background is measured every specified time, the amount of background fluctuation is calculated from the time-series background measurement value, and the background measurement value fluctuation pattern is calculated in advance. A radioactivity characterized by referring to a fluctuation pattern for a requested background fluctuation cause to estimate a fluctuation cause of a background measurement value, and giving a measurement stop instruction or a warning display according to the estimated cause. Inspection method. The radioactivity test method according to claim 4, wherein the background measurement value is subjected to a moving average in a specified time interval when the background fluctuation amount is calculated from the time-series background measurement value. Arbitrary measurement criteria set in advance for the detection limit count rate obtained from the background measurement value, the total measurement time of the background measurement value, and the measurement time set for measuring the inspection object The radioactivity test method according to claim 4, wherein whether or not measurement is possible is determined with reference to a level.

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