JPS6127716B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a radiation contamination determination method for determining whether each of measurement objects or contamination degree measuring instruments having different degrees of contamination has exceeded a contamination alarm level.

A gate monitor is a contamination degree measuring device that determines whether the degree of contamination of the human body surface by radiation exceeds a contamination alarm level that requires vigilance. When sequentially measuring a large number of subjects with different contamination levels using this gate monitor, first measure the background radiation for the minimum measurement time required to maintain the detection sensitivity of the gate monitor (hereinafter referred to as Furthermore, the degree of contamination of the person to be measured is measured within this minimum measurement time. Further, the background measurement is performed on the next person to be measured in exactly the same manner, and then the degree of contamination of the person to be measured is measured.

This is because contamination of the gate monitor caused by measuring a highly contaminated person will have a direct effect on the measured values of the next person. Before measuring the degree of contamination, be sure to measure the background, and compare it with the previous background measurement. If there is a large difference between these values, it is determined that the gate monitor is contaminated. This is done to immediately decontaminate the gate monitor and accurately measure the degree of contamination of the subject.

In this way, it can be easily determined whether the subject or the gate monitor exceeds the contamination control level from the indicated value of the gate monitor or the difference thereof. However, in cases where a small number of contaminated subjects are found among a large number of subjects, the measurement time may be shorter than that of the majority of non-contaminated subjects. This means that the processing capacity per unit time is naturally limited, and this method is said to be less efficient when the proportion of contaminated persons is smaller compared to the increase in the number of subjects. Must be.

The present invention has been made in view of the above points, and the processing capacity per unit time increases as the proportion of non-contaminated substances included in the measurement object increases, without reducing the accuracy of determining the degree of contamination of the measurement object or measuring instrument. The aim is to provide a method for determining radiological contamination, which is increasing in number.

Hereinafter, one embodiment of the present invention will be described with reference to the accompanying drawings.

First, the radiation pulse follows a Gaussian distribution,
If the counting rate is γ and the measurement time is T, the standard deviation σ of the counting rate γ is It is expressed as As is clear from this equation (1), in order to maintain a predetermined measurement accuracy, if the measurement sensitivity of the radiation detector is the same, a certain amount of measurement time, that is, the minimum measurement time is required.
Approximately 20 seconds was used as this time.

In this example, the minimum measurement time T is t ₁ (=10 seconds)
and t ₂ (=10 seconds), and the contamination judgment level A ₁ corresponding to measurement time t ₁ and measurement time t ₁ + t ₂ (=T)
The contamination determination level _A2 corresponding to the contamination determination level A2 is set respectively, but the contamination determination level _A2 is the contamination alarm level itself of the conventional contamination degree measuring instrument, whereas the contamination determination level _A1 is the contamination determination level _A2. is set considerably lower than .

Figure 1 shows the distribution of measurements with different measurement times,
R ₁ is the distribution at the measurement time t ₁ , and the contamination determination level A ₁ is determined at a point related to the standard deviation σ 1 with respect to the true counting rate N ₁ of the measurement target. Similarly, since R ₂ is at the measurement time t ₁ + t ₂ , the contamination judgment level A ₂ is the standard deviation σ2 for the true counting rate N ₂ of the measurement target.
It is determined at the lower point in relation to. That is,
The contamination judgment level _A1 is not the contamination alarm level _A2 of the minimum measurement time T divided by the ratio of the measurement time _t1 and _t1 + _t2 , but the measurement time _t1 and the measurement time _t1 + _t2.
It is set to be lowered by the difference in standard deviation in the measurement of .

If the measured value at measurement time t ₁ is clearly lower than the contamination determination level A ₁ set as described above, the measurement is stopped at this point and the measurement target is determined to be non-contaminated. Further, if the measured value at measurement time t ₁ is approximately equal to the contamination determination level A ₁ , the measurement is further continued, and the measurement value at measurement time t ₁ +t ₂ is compared with the contamination determination level A ₂ for determination. on the other hand,
The measurement value before measurement time t ₁ has passed is the contamination determination level.
If A ₁ is exceeded, the contamination is determined to have exceeded the contamination warning level at that point, and if this situation occurs during background measurement, it is immediately determined to be equipment contamination and necessary decontamination is carried out. shall be taken as a thing.

The above describes the case where contamination is determined using the count rate. However, in the case of a contamination degree measuring device that uses count values, of course, the contamination determination level related to the count value is set, and from the count value at time t ₁ , the contamination determination level is set at time t. ₁ +t ₂
It is also possible to predict the count value at .

Figure 2 is a diagram for explaining an example of making a contamination judgment based on a count value, where the count value A ₂ is used as the contamination alarm level at the minimum measurement time T = t ₁ + t ₂ , and the contamination judgment level at the measurement time t ₁ is used. Set the count value A ₁ as the measured value and set value A ₁ at time t ₁
By comparing the values, it is possible to predict the radiation measurement value at the lowest measurement time and determine whether it exceeds the contamination warning level.

In the figure, ○A is an example in which the contamination determination level A ₁ as well as A 2 is exceeded before time t ₁ has passed, and contamination is determined at time _{t 2 when the contamination determination level A 2 is} reached, and subsequent measurements are performed. Abort. ○B is an example in which the count value at measurement time t ₁ exceeds the contamination determination level A ₁ , and in this case, it is determined that there is contamination at time t ₁ and subsequent measurements are discontinued. In addition, ○C is an example where the measured value at measurement time t ₁ is almost equal to the contamination judgment level A _1. In this case, it is difficult to judge whether or not it exceeds the contamination alarm level, so the measurement is continued. , measurement time t ₁ + t ₂
Items that are determined to be non-contaminated by comparing the measured value with the contamination determination level _A2 . ○D shows an example in which the measured value at measurement time t ₁ is clearly lower than the contamination determination level A ₁ , and in such a case, it is determined that there is no contamination at time t ₁ and the measurement is stopped at this point.

In this way, the gate monitor, which maintains a predetermined detection sensitivity with a minimum measurement time of about 20 seconds, can be measured without reducing the detection sensitivity by just measuring for 10 seconds as in the above embodiment. This is also clear from the fact that the contamination determination level _A1 is set at a point related to the standard deviation σ1.

In addition, in this example, the minimum measurement time T (=20 seconds) is divided into two, and the measured values for the measurement time t ₁ (=10 seconds) are determined as contaminated (or high radioactivity) or non-contaminated (or low radioactivity). The actual processing is determined according to the following formula, where n is the count rate or count value at 10 seconds.

[(dn/dt) _t=10 ×10+n]×α ₁ >A ₂ ...If (2), it is determined to be contaminated, and [(dn/dt) _t=10 ×10+n]×α ₂ <A ₂ ...In the case of (3), it is determined that there is no contamination.

However, A ₂ : Contamination alarm level A ₁ : Contamination judgment level α ₁ , α ₂ : Determination coefficients, respectively. Note that the determination was easy when α ₁ =1.2 and α ₂ =1.5.

In addition, in the above embodiment, the minimum measurement time is simply 2
Contamination was determined by comparing the measured value at the time of division with an appropriately set determination level, but this minimum measurement time was further divided into smaller pieces so that each division time was exactly 1 second. If the determination level is set for each division point, it becomes possible to determine contamination in a shorter time, and in particular, it is possible to quickly determine the contaminated measurement target and also to determine equipment contamination at an early stage. be able to.

As is clear from the above explanation, according to the radiation contamination determination method of the present invention, the smaller the proportion of contaminated persons included in the measurement object, that is, the greater the proportion of non-contaminated persons, the more measurements can be made per unit time. The number of times can be increased, and processing capacity can be significantly improved.

[Brief explanation of the drawing]

Fig. 1 is a count rate distribution diagram with two measurement times changed to explain one embodiment of the radiation contamination determination method according to the present invention, and Fig. 2 is another embodiment of the radiation contamination measurement method according to the present invention. FIG. 2 is a diagram showing the relationship between measurement time and count value to explain the above.

Claims (1)

[Scope of Claims] 1. In a radiation contamination determination method for determining whether or not a contamination degree measuring object or a contamination degree measuring device exceeds a contamination alarm level, the minimum detection sensitivity of the contamination degree measuring device can be maintained. While dividing the measurement time, contamination determination levels are set corresponding to each division time so that the measured value at the time when the minimum measurement time has elapsed becomes the contamination alarm level, and then the measurement value at the division time is set. and the corresponding contamination determination level, and unless the two are approximately equal, a contamination determination is made according to the magnitude, and subsequent measurements are stopped, and if the two are approximately equal, the minimum measurement time is A radiation contamination determination method characterized by determining contamination after a period of time has elapsed. 2. The radiation contamination measurement method according to claim 1, wherein the minimum measurement time is divided into a number of equal intervals.