Detection of Explosives by Neutron-Radiation Analysis

Abstract

The possibility of using in practice the analytical methods of computational efficiency and minimization of the directed discrepancy in the algorithm of the working program for a setup for detecting explosives by neutron-radiation analysis is discussed. The captured-radiation spectra, which are recorded on a model of a real setup, and the computed response functions for γ-ray energies 6 and 10.9 MeV taking account of the energy resolution of the detector are presented. The possibility of applying these methods to the problem of detecting explosives is briefly described. Difficulties arising in the practical implementation of the methods are noted.A large number of technical means for detecting explosives have been developed and tested in Russia and other countries using different methods [1-3]. One method is neutron-radiation analysis, based on the rate of capture of thermal neutrons by a natural mixture of nitrogen isotopes. The nitrogen content in the most often used explosives is on the average 25%. Since nitrogen is a widely occurring element in nature, for example, its content in air is 78%, it is also present in various household objects, clothes, shoes, and food. Since a necessary condition for detonating an explosive is that the explosive must be in a compact form, making a decision as to whether or not an explosive is present must be based not on the fact that nitrogen is present but rather that the nitrogen concentration is elevated in individual parts of the object being inspected. A characteristic feature of the capture radiation spectrum of nitrogen is that it contains a 10.83 MeV γ-ray line. Analysis of the γ radiation capture spectra of various elements [4] has shown that γ rays with such energy are very rarely encountered. The main difficulty in using the method of neutron-radiation analysis in practice is that the resulting γ spectrum is a superposition of a large number of γ lines in a wide energy range 100 keV-12 MeV, distorted by the energy resolution of the detector. Consequently, in the 10-11 MeV nitrogen detection range the neighboring section of the spectrum 8-10 MeV makes a contribution, which increases the background component. In this connection, it should be noted that in the 8-10 MeV range the capture radiation of elements such as iron, aluminum, chromium, silicon, nickel, and chlorine makes a substantial contribution. Since the composition of the inspected object is unknown beforehand, the background component in the range 10-11 MeV is continually changing, as a result of which the nitrogen detection threshold is not constant. Thus, to distinguish events associated directly with nitrogen and to ensure that the detection threshold is independent of the composition of the object being inspected, the initial γ distribution must be reproduced at least at the high-energy part of the spectrum.Neutron (d, d) or (d, t) generators and isotopic sources, specifically, 252 Cf, are used as neutron sources in neutron-radiation analysis setups to soften the spectrum of the neutrons from the source...

Customs monitoring of nuclear

“…Consequently, most methods proposed for detecting explosives are based on measuring the energy spectra of secondary γ radiation from C, N, and O arising under irradiation by neutrons [1,20]. Most explosives consist of elements with a low atomic number.…”