Portal Monitor for Diversion Safeguards

Sampson, T.E.; Fehlau, P.E.; Worth, G.M.; Henry, C.N.

doi:10.13182/nt74-a31445

Cited by 5 publications

(1 citation statement)

References 4 publications

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“…The method of the moving interval of the exposure time or the moving average method is often used in radiation monitoring practice [8,9]. The essence of this method is that the signal from the detector is measured over an exposure time T, equal to the time during which the background is exceeded, not in succession but rather in parallel with some delay τ being imposed.…”

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Comparison of statistical methods of radiation monitoring

Shumakov

Kasatkin

2009

At Energy

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The results of a Monte-Carlo simulation of algorithms for statistical analysis of information from detectors in radiation monitors, recording the presence of random, short-time, weak radioactive radiation, are presented. The Neyman-Pearson test and likelihood ratio, moving average, digital recursive filter, a priori probability, half-sums, and relative dispersion methods are examined. The results of a comparative analysis of the algorithms and the investigation of the possibilities and conditions for realizing their maximum possibilities are presented. The analysis has shown that the moving average method has advantages. It can be replaced by the similarity ratio method in the event of a Poisson distribution and the a priori probability method in the event of a Gaussian distribution. For an average background of not more than 5 with a large separation the moving average and similarity ratio methods are best. Otherwise the digital recursive filter method is best. The best results are obtained by combining the detection method with the moving average method together with likelihood ratio and a priori probability methods.The objective of radiation monitoring and some other applied problems is to detect weak addition background radiation. This radiation can be short-time and random, and its presence must be detected in real-time. The measurement conditions can vary over a wide range. There can be a many-fold variation of the absolute background and its temporal instability. Information on the radiation is limited to several measurements whose results can vary over a wide range -from several or more impulses. The additional signal can be comparable to the background or equal to a fraction of the background. The confidence probability with which a decision concerning the presence of additional radiation is made and the probability of a wrong decision can likewise vary over a wide range.The following criteria and statistical processing methods are often used in radiation monitoring practice to make a decision about the presence of additional radiation: Neyman-Pearson test, moving average method, and likelihood ratio test as well as digital recursive filter, a priori probability, half-sums, and relative variance methods. The results of a comparison to these criteria by means of the Monte Carlo method are presented in the present article.Algorithms for temporal processing of counts were not considered because they are rarely used in practice. For example, two methods of temporal processing of signals with detection of a moving source of radiation exceeding the background signal are compared in [1]. The first method is the conventional Neyman-Pearson test and the second is the Cox-Lewis test based on an analysis of a risk function.The problem formulated consists in choosing one of two hypotheses -the zeroth H 0 and an alternative H 1 . The first one means that the detector records only the background and the second one supposes the detection of radiation (the effect) in addition to the background. Both hypotheses are simpl...

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Comparison of statistical methods of radiation monitoring

Shumakov

Kasatkin

2009

At Energy

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Computational algorithms in vehicle radiation monitors

Skorik¹

1998

At Energy

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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...

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