Computational algorithms in vehicle radiation monitors

Abstract

The present paper considers the analysis of measurements performed with automatic stationary vehicle radiation monitors, which are intended for discovering radioactive materials being illegally transported in moving automobiles. The instantaneous analysis of measurements u background measurements and measurements performed while monitoring controlled objects --includes the calculation of functionals in successive exposures using rules for making decisions about the results of monitoring. The most widely used and inexpensive portal variant of monitors, which includes two ground-based stands containing y-ray detectors, is considered. The signals from all detectors are summed. The monitors are equipped with a presence detector, which automatically starts up and stops the monitoring regime.The objective of this investigation is to develop simple and convenient algorithms that require the minimal computational base and that increase the threshold sensitivity of monitors by taking account of and adequate compensation of the screening of the natural radiation background by moving objects and the dip in the background detected instrumentally. Compensation of such dip by means of logical programs is one of the most often discussed and important problems of portal vehicle monitors, since this. physical effect greatly decreases the sensitivity of a monitor [1, 2]. The threshold for adopting a decision is almost always set according to the undisturbed radiation background in the absence of objects, and therefore a large quantity of radioactive materials'is required in order to exceed this threshold under conditions when a dip is present in the background as a result of the presence of objects. The decrease in sensitivity is observed by comparing the dip in the background (~ 4-27% [1-3]) and the threshold --the excess due to the additional effect recorded by the monitor, which can reach -4% [4, 5]. The simple method for decreasing the influence of this physical effect --increasing the aperture of the monitor gates --results at the same time in a degradation of sensitivity because of the large distance between the detectors and the radiation source [ 1, 2, 6].Existing algorithmic methods for taking account of and compensating for the dip in the background are limited and ineffective because of the large variations of this effect [1, 2, 7, 8]. Even for vehicle monitors with the detectors placed under ground, where the dip in the background is much smaller (on the average ~ 3%) and substantially more stable, only ~ 1% of the effect can be compensated; this is also characteristic for pedestrian portal monitors with a dip ~ 1.5% in the background. There is no concrete information about whether or not this physical effect is compensated in commercially produced vehicle monitors. American standards documents do not stipulate the setting and regulation of the false-alarm level. This could be due to the fact that the dip in the background is taken into account. During operation, only the level can be checked [9]. On the basis of inves...