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

Abstract

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 ,