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...

The calibration of the energy scale of a device which is a spectrometer and is to be used for detecting explosives in various objects (baggage of airline passengers, mail, briefcases, cases, and others) is examined. The work is based on the method of neutron-radiation analysis. In the present case the calibration is of fundmental importance because the nitrogen sensitivity of the device is low. A brief description of a working model, the basic characteristics, the background spectrum of capture radiation with a sample of nitrogen-containing substance, and an example of a calibration curve are presented. The method of check sums and its advantages are examined. It is shown that it can be used in setups for detecting explosives.Today there are various methods for detecting explosives. Operational systems have been developed in our country and abroad on the basis of these methods [4][5][6][7]. Specifically, neutron-radiation analysis is used to detect explosives according to the elemental composition. This method is well known and is used successfully in various branches of science and technology [1-3], including searching for explosives. Analysis of the operation of the setups using various methods of detection [6][7][8] shows that in most cases it is difficult to satisfy all requirements which a real setup must satisfy. We note that from the standpoint of use in practice, irrespective of the method of detection, the reliability of the setup, simplicity of operation, and accessibility for repair are important. Application of the Method of Neutron-Radiation Analysis for Detecting Explosives.It is well known that the method of neutron-radiation analysis is based on irradiation of an object by a flux of thermal neutrons and detecting the spectrum of the capture γ radiation using spectrometric detectors. Since the nitrogen content in most explosives widely used today is 25% [7], the detection of a high nitrogen concentration in an object being inspected could indicate the presence of explosives in it. The difference of the spectrum of capture radiation of nitrogen from the spectra of other elements is that nitrogen contains a 10.83 MeV γ-ray line [1], which makes it possible to avoid the difficulties associated with complete interpretation of the spectrum of the object being inspected. The main requirement for the setup is detection with a prescribed probability of a definite amount of explosive in the shortest possible time time. The procedure should be conducted automatically to avoid any influence of a subjective factor on decision making. Given the presence of modern personal computers, the time required to process the information and make a decision is negligible compared with the total inspection time. Consequently, complicating the algorithm in exchange for increasing the size of the instrumental part of the system is justified, since doing so does not increase the inspection time and, aside from decreasing the cost of the system, makes it possible to satisfy the requirement of higher reliability.

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