JPH06242248A - Method for deciding contamination of radiation monitor - Google Patents

Abstract

PURPOSE:To provide a method for deciding contamination of a radiation monitor which can decide radioactivity contamination in a short time based on preset contamination control counting rate. CONSTITUTION:An upper and lower contamination decision level setter 13 for setting upper and lower contamination decision levels to be decided according to a background counting rate, a background measuring time, a contamination control counting rate and a measuring time based on a formula of assay reference for rejecting coincidence of a net counting rate obtained at an arbitrary measuring time within a predetermined measuring time with a preset contamination control counting rate with statistically sufficient reliability is provided. The two levels are compared by a comparator 18 with a radiation dose measured value to be measured by a detector 17 at an arbitrary time point within a predetermined measuring time, and when the measured value is larger than the upper level, the object to be measured is decided to have 'contamination', while when the measured value is smaller than the lower level, it is decided to have 'no contamination'.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for determining contamination of a radiation monitor for inspecting radiation contamination required when a person or an object enters or leaves the radiation controlled area.

[0002]

2. Description of the Related Art Radiation control is carried out in nuclear facilities and the like that handle radioactive substances in order to monitor and prevent contamination of the human body or environment due to radioactivity. For example, when a person enters or exits a radiation controlled area of a facility, a radiation monitor installed at an entrance / exit of a facility inspects a person or a substance carried in or out for radiation contamination.

A radiation monitor called a gate monitor installed at the entrance of such a radiation controlled area includes a large monitor device for accommodating the whole body of a person and a small monitor for partial radiation measurement of limbs and the like. There are devices, etc. These monitoring devices measure the radiation dose over a predetermined period of time, and determine whether it exceeds the contamination control level obtained from the viewpoint of radiation contamination to the human body or environmental pollution. The presence or absence and the state of contamination are known.

However, in the conventional radiation monitor, the radiation dose is measured uniformly over all the measuring objects for the same predetermined measuring time (about 20 seconds), so that especially when measuring a large number of measuring objects. Has a lot of waiting time for inspection, and it is not possible to quickly monitor the radioactive contamination. Further, it is possible to install a plurality of radiation monitors and simultaneously measure a large number of objects to be measured, but using more radiation monitors than necessary has a problem in terms of installation place and cost.

Therefore, a contamination determination method has been proposed in which a person with little radioactive contamination is determined at an early stage before the predetermined measurement time is reached, and sequential measurement is terminated (Japanese Patent Laid-Open No. 1-1874).
No. 88). Specifically, based on the limit count rate ne, that is, the minimum count rate at which the difference between the measured count rate and the background count rate nb can be determined to be statistically significant,
Assuming that the true count rate from the measurement target is nb + kne, the contamination determination level AL (t) for any measurement time t
Is set as in equation (1). At this time, the coefficient k
Indicates that the contamination determination level AL and the determination level at the predetermined measurement time match at the predetermined measurement time Tmax.
It is determined from the condition of equation (2).

[0006]

[Equation 1]

However, this contamination determination method also has a problem. That is, although this contamination determination method employs a limit count rate as a criterion for determination, the criterion for contamination determination is actually a net count rate that is k times the limit count rate. Here, since the coefficient k becomes about 2 due to the change in the background value, there is a possibility that the object of measurement having a radioactivity level higher than the limit count rate originally intended as a reference may be determined to be “no pollution”. It is believed to be in. Further, in this contamination determination method, the same can be said even if the contamination management count rate is used instead of the limit count rate.

[0008]

As described above, in the conventional radiation monitor which measures the radiation dose uniformly over all the measuring objects over the same measuring time, when measuring a large number of measuring objects. Since the waiting time for inspection is long, it is not possible to quickly monitor the radioactive contamination, and in the method of installing multiple radiation monitors and measuring multiple objects at the same time, the radiation monitor installation location and There was a problem in terms of cost.

Further, in order to judge a person with a small amount of radioactive contamination at an early stage before the predetermined measurement time is reached and end the sequential measurement, a contamination judgment level for an arbitrary measurement time is set to judge the contamination. In the method, the limit counting rate is adopted as the criterion of judgment, so considering that the criterion of contamination judgment is actually the net counting rate which is about twice the limit counting rate, the limit counting rate which is originally the reference is desired. There is a possibility that a measurement target with a radioactivity level higher than the rate will be judged as "no pollution", and there is a problem that it is not possible to perform reliable pollution judgment.

The present invention has been made to solve the above-mentioned problems of the prior art, and based on a preset contamination control count rate, a highly reliable determination of radioactive contamination can be made in a short time. It is an object of the present invention to provide a method for determining contamination of a radiation monitor that can be performed.

[0011]

In order to achieve the above object, the present invention measures the radiation dose generated from an object to be measured by a radiation detector for a predetermined measurement time, and the radiation dose is measured from a preset pollution control level. In a method for determining contamination of a radiation monitor that determines whether the value is large or small, the net count rate obtained at any measurement time within the predetermined measurement time is set from the viewpoint of the allowable amount of radioactive contamination to the human body or environment. The upper and lower contamination determination levels for any measurement time are obtained and set in advance based on the test criteria formula for rejecting that the measured contamination control count rate agrees with statistically sufficient reliability. deep.

When the radiation dose measurement of the object to be measured is started, the measured values of the radiation dose are compared with the upper and lower contamination determination levels at any time point within the predetermined measurement time, and the measured value becomes the upper contamination determination level. When it is larger, it is determined that the radiation dose from the measurement target statistically reliably exceeds the pollution control level and is determined to be "contaminated". When the measured value is lower than the lower pollution determination level, It is judged that the radiation dose from the measurement target is statistically reliably lower than the pollution control level, and it is judged to be "no pollution". Also,
If the measured value is between the upper and lower contamination determination levels, the radiation measurement is continued and the measured value is again compared with the upper and lower contamination determination levels after a predetermined time interval.

Further, if the measured value of the radiation dose is still between the upper and lower contamination determination levels even after the predetermined measurement time is reached, it is judged that (a) the contamination control level may be exceeded. (B) Compare with the pollution control level and judge as "pollution" only when the pollution control level is exceeded, (c) Statistically exceed the pollution control level It is characterized in that one of the three final judgment criteria of judging that it is not said to be present and judging that there is no pollution is selected in advance.

[0014]

As described above, in the contamination determination method according to the present invention, a plurality of determination steps are set at arbitrary time intervals before the predetermined measurement time is reached, and the measured radiation dose is compared with the upper and lower contamination determination levels. Then, the contamination determination is performed. When the measured value is higher than the upper contamination judgment level at each judgment step, it is judged that the contamination level exceeds the contamination control level, and it is judged as "contaminated", and the measured value falls below the lower judgment level. At that point, it determines that there is no contamination and ends the measurement.

[0015] Here, when a measurement target with a significant amount of radioactive contamination is measured, it is considered that the measured value of the radiation dose exceeds the upper contamination determination level at an early stage in time, so that it is promptly "contaminated". Is judged. Also, when measuring an object with a very low degree of radioactive contamination, the measured value of the radiation dose falls below the lower contamination determination level at an early stage, and a rapid determination of "no contamination" is made.

Further, since the radiation contamination level of the measurement object is relatively close to the contamination control level, if the contamination level is between the upper and lower contamination determination levels even if the predetermined measurement time is reached, the contamination is determined from the above three criteria. From the operational viewpoint of management, the final judgment of contamination is made according to the final judgment criteria determined in advance.

In reality, it is natural and true that people and objects having a low degree of radioactive contamination occupy most of the measured population. Therefore, according to the present invention, among the upper and lower contamination determination levels, particularly, the lower contamination determination level determines "no contamination" at an early stage in order from a person or an object having a high degree of safety against radioactive contamination. It can be terminated, and rapid radioactive contamination monitoring can be performed for the entire measurement target population.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic block diagram of an embodiment of a radiation monitor to which the present invention is applied. This radiation monitor includes a pollution control count rate storage unit 11 that stores a pollution control count rate determined from the viewpoint of an allowable amount of radioactive contamination to the human body and the environment, a background count rate that is measured in advance, and the background count rate. BG value storage unit 12 that stores the background measurement time at that time, and upper and lower contamination determinations that set upper and lower contamination determination levels from the output of this BG value storage unit 12 and the output of contamination management count rate storage unit 11. Level setter 1
3, a timer 16 that outputs a signal at predetermined time intervals, a detector 17 that measures the radiation dose of each part of the human body and an object, an output of this detector 17 and an output of the upper and lower contamination determination level setting device 13. A comparator 18 for comparison and a display 1 for receiving and displaying a contamination determination result which is the output of the comparator 18.
9 and a final judgment standard setting unit 20 which selects and sets the final judgment standard in advance and gives an appropriate judgment standard to the comparator 18 when the final judgment is required.

The upper and lower contamination determination level setting device 13 is
The upper and lower pollution determination level calculation units 14 for calculating the upper and lower pollution determination levels with respect to an arbitrary time from the pollution control count rate and the BG value, and the output of the upper and lower pollution determination level calculation units 14 at each pollution determination step (each The upper and lower contamination determination levels are stored as upper and lower contamination determination level storage units 15 in the contamination determination time).

Here, what is characteristic of the present invention is that
By setting the contamination determination step at an arbitrary time interval within the predetermined measurement time, and comparing the upper and lower lower contamination determination levels at that time and the measured radiation dose at each time, the true value from the measured object is determined. This is a point to judge with statistically sufficient reliability whether the radiation dose of is larger or smaller than the contamination control level preset as a standard of radioactive contamination. In this embodiment, specifically, the upper and lower contamination determination levels are determined by the following method.

That is, the pollution control count rate (net value) stored in the pollution control count rate storage unit 11 is d, the background count rate and the background measurement time stored in the BG value storage unit 12 are nb, When Tb is set, the measurement time for the measurement target is t, the integrated count value up to the measurement time t is N, the gross count rate of the measurement target is ng = N / t, and the net count rate of the measurement target is n = ng-nb. An approximate test criterion for rejecting that the true net count rate of the measurement target is the pollution control count rate d is given by the following equation (3).

[0023]

[Equation 2]

Here, the contamination control count rate is a radioactive contamination control surface density [Bq / cm ^{2 which is} quantitatively obtained from the viewpoint of the allowable amount of radioactive contamination to the human body and the environment. ] With operational margin, and detection efficiency [cps / Bq / cm ² depending on the performance of detector 17] ] And the value determined from.

[0025] (3) is solved for n, d ₊ ≦ n or n ≦ d _- become. Here, d ₊ and d ₋ are referred to as an upper contamination determination count rate and a lower contamination determination count rate, respectively, and these are expressed as the functions of the measurement time t, respectively, in equations (4) and (5) below. Can be expressed as

[0026]

[Equation 3]

That is, when the net count rate n obtained when the radiation dose is measured for the time t is larger than the upper contamination determination count rate d ₊ (t) at the measurement time t, the value of the true net count rate. can be determined statistically reliably greater than pollution control count rate d, the lower contamination determination count rate net count rate n is d _- is smaller than (t) is the true net count rate values Can be statistically judged to be certainly smaller than the pollution control count rate d.

For example, specifically, Tb = 300 [seconds], n
The change in the upper contamination determination count rate d ₊ with respect to the measurement time t when b = 40 [cps] and d = 20 [cps] is shown by the curve 20 in FIG.
1 and the change of the lower contamination determination count rate d ₋ is shown by the curve 203 in the figure. Also, Tb = 300 [seconds],
A change in the upper contamination determination count rate d ₊ when nb = 5 [cps] and d = 20 [cps] is shown by a curve 301 in FIG. 3, and a change in the lower contamination determination count rate d ₋ is in the curve. Shown at 303.

When actually comparing and evaluating the count values, it is general to use the integrated count value (count value) from the start of measurement to the measurement time t, instead of the count rate. The above-mentioned pollution control count rate d expressed as an integrated count value is referred to as a reference pollution determination level D, and the above-mentioned upper and lower pollution judgment count rates d ₊ and d ₋ are expressed as accumulated count values respectively as upper pollution. Assuming that the determination level D ₊ and the lower side contamination determination level D ₋ , D = (nb + d) t D ₊ = (nb + d ₊ ) t (6) D ₋ = (nb + d ₋ ) t

Therefore, it is sufficient to compare these with the integrated count value which is the output from the detector 17 as a reference. For example,
Tb = 300 [sec], nb = 40 [cps], d = 20 [cp
s], these values are shown in Figure 4 and
01 represents the upper contamination level D ₊ , the straight line 402 represents the contamination management level D, and the curve 403 represents the lower contamination determination level D ₋ .

In the actual operation, when the measurement target is a person, for example, in consideration of the stability of the measurement posture of the person, the contamination determination is not performed for a few seconds after the measurement is started, and then the net is measured at a fixed time interval until that time. It is conceivable to compare the count rate with the upper and lower contamination determination levels to determine contamination. And
If the measured value is higher than the upper contamination judgment level or smaller than the lower contamination judgment level at that point, the measurement is terminated, and the upper and lower contamination is reached even if the predetermined measurement time (called maximum measurement time) is finally reached. If it is between the judgment levels, it may be possible to perform the contamination judgment (which will be called the final judgment) based on the predetermined judgment criteria from the following three selections only at that time. To be That is, (a) the measured radiation dose may exceed the pollution control level and is determined to be “contaminated”; (b) the measured radiation dose is compared with the pollution control level, and at that point If it exceeds the management level, it is judged as “contaminated”,
(c) It is judged that the measured radiation dose does not certainly exceed the pollution control level, and it is judged as “no pollution”.

Which of (a), (b) and (c) is selected with respect to the final criterion is related to the operational meaning of the pollution control level. For example, if the final determination is performed so as to comply with the conventionally used method, the determination criterion (b) should be selected. In addition, if the pollution control level has a strict meaning and there is a statistical possibility that the pollution control level may be exceeded, if the criterion of “contamination” is adopted, then the judgment criterion (a) Make a final decision. Furthermore, if it is considered that “contamination is present” only when it is statistically certain that the pollution control level is exceeded, the criterion is
(a) will be adopted. In any case, the point is how many objects to be measured can be judged only by the above-mentioned upper and lower contamination judgment level before the final judgment.

When the judgment is made based on the upper and lower pollution judgment levels set in this way, the upper and lower pollution judgment is made before any number of the measured population reaches the maximum measurement time, that is, without making the final judgment. The probability P (t) (this probability will be referred to as the lower determination rate or the lower passage rate) of whether the determination can be made only by the level is obtained by the following equation (8).

[0034]

[Equation 4]

In FIG. 5, Tb = 300 [seconds] and nb = 4.
The graphs obtained by the above equation (7) for some h when 0 [cps] and d = 20 [cps] are shown.
Is 0 [cps], the curve 505 is 5 [cps], and the curve 510 is 10.
[cps] and the curve 515 show the lower contamination determination rate in each case of 15 [cps]. According to this, at the stage of 5 seconds after the start of measurement, the average count rate is 0 [cps], that is, 100% and 5 [cps] in the population with the same contamination level as the background.
97% of the population can be judged even in the group of, and the group of 10 [cps] reaches the determination rate of 90% or more after 10 seconds. Also, the average count rate of 15 [cps], which is close to the pollution control count rate.
In the group of, the judgment of about half of the entire group is completed when 20 seconds have passed. Next, the operation of the radiation monitor of the present embodiment configured as described above will be described using the flowchart shown in FIG.

The background measurement data is stored in the BG value storage unit 12 in advance, and the upper and lower contamination determination levels are set in advance by the upper and lower contamination determination level setter 13 from the data in the BG value storage unit 12 and the like. It is assumed and sought.

After the measurement is started, the timer 16 measures the measurement time, the detector 17 measures the radiation dose of the object to be measured (step 600), and outputs the accumulated count value (count value) up to that time every time. . In step 601, it is determined whether or not the measurement time has reached the determination start time. If the determination start time has been reached, the process proceeds to determination step 602.

In the judgment step 602, it is judged by the comparator 18 whether or not the measured value of the radiation dose by the detector 17 is higher than the lower contamination judgment level. If "YES", the process moves to the next judgment step 603. In the case of “NO”, it means that it is lower than the lower contamination determination level and it is determined to be safe. Therefore, the process proceeds to step 606 and the display 1
The message “No contamination” is displayed by 9 and the measurement is completed.

In the judgment step 603, it is judged by the comparator 18 whether or not the measured value of the radiation dose by the detector 17 is smaller than the upper contamination judgment level. If "YES", the process moves to the next judgment step 604, and " In the case of "NO", it means that the contamination level is higher than the upper contamination determination level and it is determined that the contamination has occurred. Therefore, the process proceeds to step 606, and the display unit 19 displays "Contaminated" to perform the measurement. finish.

When the process proceeds to step 604, that is, when the radiation dose measured value by the detector 17 exists between the upper and lower contamination determination levels at that time, the step 6
In 04, it is determined whether or not the measurement time has reached a predetermined maximum measurement time, and if "YES", the final determination step 60
In step 5, the final contamination determination is performed according to the determination criteria determined in advance from the three items (a) to (c), the flow proceeds to step 606, the presence or absence of contamination is displayed, and the measurement ends. Step 6
If the result of the determination in 05 is "NO", the flow returns to step 600 to perform radiation measurement, and after the prescribed time interval, the 6
The determination steps 01 to 604 are repeated.

In actual radiation measurement of a human body, it is general that a plurality of detectors measure radiation doses at different parts of the body, instead of using a single detector. In such a case, in the case of contamination determination, conventionally, after measuring for a certain period of time, if even one of the detectors exceeds the control level, a warning that the place is contaminated is issued. It was a thing.

Therefore, an embodiment of the contamination judging method according to the present invention in the case where the radioactive contamination is measured with respect to the same measuring object by a plurality of detectors to judge the contamination will be described.

First, if one of the detectors is judged to be "contaminated" during the measurement, the measurement is continued until the maximum measurement time. This is because it is likely that other parts of the body will also be contaminated. Cases that are judged to be "contaminated" are not so frequent, and thus the effect of time saving is not greatly lost by continuing the measurement up to the maximum measurement time in the case of contamination.

Next, the comprehensive determination of "no contamination" is made when all the detectors have determined "no contamination". At this time, the effect of time reduction is required,
The fact that multiple detectors measure and determine multiple persons with a single detector and that multiple detectors measure and determine different regions (of multiple subjects) are statistically the same. Is. Therefore, the evaluation of statistical time reduction up to now can be applied as it is. That is, when the case where the degree of contamination on the body surface of the subject is sufficiently lower than the set contamination control level occupies most of the cases, the effect of the present invention can be expected to significantly reduce the measurement time. .

[0045]

As described above, according to the present invention, the contamination determination is performed by comparing the measured value of the radiation dose with the upper and lower contamination determination levels at any time within the predetermined measurement time. , It is not always necessary to restrain the measurement target for a predetermined measurement time, and in particular, the majority of the measurement target population is occupied by those whose radiation dose is lower than the contamination control level and whose radioactive contamination degree is low. In the situation,
The measurement time of the entire measurement target group can be significantly shortened.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a radiation monitor according to an embodiment of the present invention.

2] Tb = 300 [seconds], nb = 40 [cps], d =
The figure showing the pollution control count rate and the upper and lower pollution determination count rates in the case of 20 [cps]

FIG. 3 shows Tb = 300 [seconds], nb = 5 [cps], d = 2.
Diagram showing the pollution control count rate and the upper and lower pollution determination count rates for 0 [cps]

FIG. 4 is Tb = 300 [sec], nb = 40 [cps], d =
The figure showing the upper and lower contamination judgment level expressed by the integrated count value in the case of 20 [cps]

FIG. 5: Tb = 300 [seconds], nb = 40 [cps], d =
The figure which shows the result of having calculated | required the judgment rate by the vertical contamination judgment level in 20 [cps] with respect to the to-be-measured object group which has several different average count rates.

FIG. 6 is a flowchart showing the outline of the operation of this embodiment.

[Explanation of symbols]

11 ... Contamination management count rate storage unit 12 ... BG value storage unit 13 ... Vertical contamination determination level setting unit 14 ... Vertical contamination determination level calculation unit 15 ... Vertical contamination determination level storage unit 16 ... Timer 17 ... Detector 18 ... Comparator 19 ... Display device 20 ... Final determination standard setting device 201 ... Upper contamination determination counting rate (net value) 202 ... Contamination management counting rate (net value) 203 ... Lower contamination determination counting rate (net value) 301 ... Upper contamination determination counting rate (Net value) 302 ... Pollution control count rate (Net value) 303 ... Lower pollution determination count rate (Net value) 401 ... Upper pollution determination level (Integrated count value) 402 ... Pollution control level (Integrated count value) 403 ... Lower Side contamination determination level (integrated count value) 500 ... Lower determination rate when h = 0 [cps] 505 ... Lower determination rate when h = 5 [cps] 510 ... h = 10 [cps] Lower judgment rate 515 ... Lower determination rate when h = 15 [cps] 600 ... Radiation measurement step 601 ... Step for determining whether it is determination start time 602 ... Step for determining whether it is greater than lower contamination determination level 603 ... Step 604 for judging whether it is smaller than upper contamination judgment level 604. Step for judging whether the maximum measurement time has been reached 605. Step for making final pollution judgment 606 .. Display of contamination judgment result

Claims (2)

[Claims] 1. A radiation monitor that measures the radiation dose generated from a measurement target by a radiation detector over a predetermined measurement time and determines whether the measured value of the radiation dose is larger or smaller than a preset contamination control level. In the contamination determination method of, in order to reject with a statistically sufficient reliability that the net count rate obtained at any measurement time within the predetermined measurement time matches the preset pollution control count rate Based on the formula of the verification criteria, the background count rate, the background measurement time, the contamination control count rate and the upper and lower contamination determination levels determined by the measurement time are set, and at any time within the predetermined measurement time. Then, the measured value is compared with the upper and lower contamination determination levels, and when the measured value is higher than the upper contamination determination level, the radiation dose from the measurement object is When it is determined that the pollution control level is statistically reliably exceeded and "contamination is present", and the measured value is lower than the lower pollution determination level, the radiation dose from the measurement target is higher than the contamination control level. Is a contamination determination method for a radiation monitor, which is characterized by statistically surely determining that it is below the level, and determining "no contamination". 2. If the measured value is between the upper and lower contamination determination levels even after the predetermined measurement time is reached, (a) it is determined that the contamination control level may be exceeded. Judge as "contaminated", (b) Compare with the pollution control level, and judge as "polluted" only when the pollution control level is exceeded, (c) Statistically ensure the pollution control level 2. The contamination determination method for a radiation monitor according to claim 1, wherein any one of three final determination criteria, that is, "not contaminated" when it is determined that it is not exceeded, is selected in advance. .