PRINCIPLES OF TAKING INTO ACCOUNT THE KEY CHARACTERISTICS OF RADIATION MONITORING DEVICES WHEN DECISION-MAKING ON THE POSSIBILITY OF THEIR USE FOR DETECTING RADIOACTIVE MATERIALS AT THE STATE BORDER.

BASICS OF ACCOUNTING FOR KEY CHARACTERISTICS OF RADIATION MONITORING INSTRUMENTS WHEN DECISION-MAKING ON THE POSSIBILITY OF THEIR USE TO DETECT RADIOACTIVE MATERIALS AT THE STATE BORDER..

Makhnev I.A., Lyamzenko K.V.
Scientific and production small joint venture «EXPERIENCE».
Reshetnikov E.V.
State Administration of Ecology and Natural Resources in Luhansk Region
Ukraine, Luhansk.
 

BASICS OF ACCOUNTING FOR KEY CHARACTERISTICS OF RADIATION MONITORING DEVICES WHEN DECISION-MAKING ON THE POSSIBILITY OF THEIR USE TO DETECT RADIOACTIVE MATERIALS AT THE STATE BORDER.

In August 2003, the IAEA, together with the WTO, Europol and Interpol, issued the document «Detection of Radioactive Materials at the Border» (TEC DOC-1312/R), which is intended to serve as a methodological aid in organizing radiation monitoring at the border and is of a recommendatory nature. Taking the provisions of the aforementioned document as a basis, the authors of this article propose simple methods for assessing the necessary key characteristics of devices. Descriptions of devices and systems are given, which in their characteristics largely correspond to and even exceed the recommendations of TEC DOC-1312/R.

Section 5 of TEC DOC -1312/R provides the concepts of investigation levels and alarm triggering settings for the device.  

In the figure on the left is the curve of the distribution of the background frequency of the device's reports, on the right is the curve of the distribution of the frequency of the device's reports under additional irradiation. Under background irradiation, reports exceeding the threshold (to the right of point C) will cause false alarms. The frequency of these alarms is proportional to the area of ​​the background peak (shaded yellow). Under irradiation, device reports that have not reached the threshold (to the left of point C) will mean missing anomalies. The frequency of missing is proportional to the area of ​​the right peak (shaded blue). The device's response threshold can be set not only at point C, as shown in the figure. By increasing the threshold, we reduce the frequency of false alarms and increase the probability of missing.

The distributions of the device's reports are in most cases described by the Gaussian distribution. The parameters of this distribution depend on the detector count rate of the device (on sensitivity) and on the result accumulation time.

For example, we present the curve of the distribution of the device's reports with an accumulation time of 1 s, having a detector count rate of 100 s-1 at a background of 0.1 μSv/h.

If we follow the recommendations of TEC DOC -1312/R, to achieve a false alarm rate of 1 in 10,000 (which corresponds to approximately 1 false alarm every 3 hours), the alarm threshold should be set at +3.5 sigm, which corresponds to an DER level of 0.135 μSv/hour, i.e. +35% of the background level. Further, to achieve the recommended pass rate of 0.1% (detection probability of 99.9%), it will be necessary to set the investigation level equal to 0.175 μSv/hour.

Recommendations TEC DOC -1312/R are developed for 3 categories of devices and equipment: pocket, portable, stationary. For each category, its own requirements for sensitivity, frequency of false alarms and probability of detection have been developed. Let us summarize these recommendations in a table.