Development of a combined device for the detection of unauthorized transportation of explosive, fissionable and radioactive materials

2006

The optimization is accomplished on the basis of methods used for checking statistical hypotheses. Two possible methods for processing information from γ-ray detectors are examined: setting a threshold according to a general likelihood ratio including all combinations of detectors or separate combinations of detectors. In the second case, all possible combinations of detectors must be sorted when the measurements are analyzed. Calculations for a specific setup with six detectors show that sorting detector combinations permits detection of smaller quantities of explosives than can be done with a general likelihood ratio. A sequential analysis instead of sorting detector combinations permits identifying explosives more quickly than can be done with a general likelihood ratio.Neutron-radiation analysis is one method for detecting exlosives in transportation hubs and in companies [1][2][3][4]. The method is based on the capture of thermal neutrons by nitrogen isotopes followed by the emission of γ rays, which are detected by a detector. Most explosives contain from 16 to 40% nitrogen (25% on the average) while ordinary materials in objects which are checked contain up to 28% nitrogen (10% on the average) [5]. The most informative part of the γ-ray spectrum in detecting nitrogen corresponds to the range 10-11 MeV [1,3]. Since an explosive can be found at any location of an object being checked, several detectors are used to detect γ rays. This makes it possible to indicate the relative location of an explosive and to estimate its quantity. The observable random quantities are the sums of the numbers of detector count n ik over N successive time intervals with duration ∆t:where k is the number of the successive time interval; i is the number of the detector, i = 1, 2, ..., Q; and Q is the number of detectors. The measurement time is N∆t. When analyzing measurements, the indications of each detector can be examined separately or the indications of several detectors can be summed. It is useful to divide the space of the measurement chamber into sections according to the number of detectors used. An explosive can be present in one or several of these sections. Thus, when measurements are analyzed, the relative configuration of the detectors and the explosive must be taken into account. The present paper analyzes methods of information processing that can be used to detect the smallest mass of explosives.Since γ-ray detection is of a random character, a judgement as to the presence or absence of an explosive on the basis of measurements must be regarded as a check of statistical hypotheses [6,7]. Two alternative hypotheses are checked: an explosive is present in one or several sections of the measurement chamber (hypothesis 1) and there is no explosive (hypothesis 0). An indicator that an effect is present (in this case an explosive) is that a selected threshold for the number K n i k i k N = = ∑ 1 ,

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confidence: 99%

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confidence: 99%

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confidence: 99%

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confidence: 99%

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Optimal processing of information from detectors used for detecting explosives by neutron-radiation analysis

2006

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“…Since the nitrogen content in explosives is higher than in ordinary materials, neutron-radiation analysis is performed at γ-ray energies 10-11 MeV [1,2]. To detect explosives by using inelastic fast-neutron scattering, it is desirable to use as indicators the contents of nitrogen, oxygen, and carbon.…”

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Optimal use of spectrometric information for discovering explosives by neutron-radiation analysis and inelastic neutron scattering

2011

Two methods of setting the threshold for determining the difference of one γ-ray spectrum from another in the case where these differences are related with the presence of explosives are compared. In the first method, the threshold is set according to a general likelihood function corresponding to the energy channels of the spectrum. In this method, to improve the detection characteristics the number of counts in the channels must be multiplied by weighting factors. In the second method, the threshold is set according to the channel combinations. Here it is necessary to sort through the channel combinations. The calculations of the detection characteristics are performed for two setups: a neutron-activation channel is used in one and inelastic neutron scattering is used in the other. It is established that in this case the threshold setting according to the channel combinations gives better detection characteristics than threshold setting according to a likelihood function. Detection quality increases as the number of processing channels increases.To detect explosives by means of neutron-radiation analysis of inelastic neutron scattering, the background spectrum must be distinguished from the spectrum due to the presence of explosives. Since the nitrogen content in explosives is higher than in ordinary materials, neutron-radiation analysis is performed at γ-ray energies 10-11 MeV [1, 2]. To detect explosives by using inelastic fast-neutron scattering, it is desirable to use as indicators the contents of nitrogen, oxygen, and carbon. The nitrogen and oxygen content in explosives is higher than in ordinary materials and the carbon content is lower. In this case, the γ-ray spectrum is analyzed in a wider energy range 2-7.5 MeV [3,4].The theory of checking statistical hypotheses is used to process the γ-ray spectra; this permits determining on the basis of the measurements the presence or absence of the effect with a prescribed probability of false alarms α and correct detection 1 -β. These quantities depend on, specifically, the chosen threshold, for example, the number of detector counts. An excursion above threshold attests to the presence of the effect. The detection system with an method of analysis of the measurements is characterized by the mass G of the explosive, determined with a prescribed probability of false alarms and correct detection.The optimization of the processing of information coming from many detectors with different threshold setting methods is examined in [5]. In the present work, the considerations used in [5] are applied to optimal processing of γ-ray spectra.The random quantity in detection is the number of detector counts n ik over N successive time intervals with duration Δt. The total number of counts in the ith channel over the entire measurement time is K n i k i k N = = ∑ 1 ,

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Analysis of the Probability Distribution of the Number of Detector Counts for Detecting Materials by Radiation Methods

2015