Simulated Explosives And Several Related Substances Oxygen Detection Technology

The problem how to detect explosives rapidly, accurately and sensitively is widelyconcerned in the world. Because detection precision and environment is different, everyexistent technology for detecting explosives has its own limitation.Neutron Activation Analysis (NAA) is an effective non-destructive examinationtechnology for detecting explosives. A series of technical problems should be seriouslyresearched if Neutron Activation Analysis is employed to build a rapid sensitive accuratemethod for detecting explosives. Most explosives are composed of the elements C, H, O andN. Further more, the concentration of O and N is usually higher than commodities. Theconcentration of O and N, as well as the ratio of the concentration of O to N are used toidentify different materials. So detecting the concentration of N, C, H, and O is an importantmethod to identify explosives. Now there are many studies in detecting the concentration of Nin explosives. Detecting the concentration of H and O in several relative materials andsimulative explosives using Neutron Activation Analysis is the main work in this paper. The Oor H atoms in materials are activated by 14MeV neutrons. These atoms subsequently decayand emit gamma rays, which are directly proportional to the concentration of the respectiveelement. Different materials can be identified by calculating the concentration of O and N andtheir ratio.As the explosives are usually composed of many elements, it is difficult to detect realexplosives directly. For the purpose of this paper, we research water and alcohol whichcontain a few elements in the beginning. 14 MeV neutrons are used to irritate the elements Oand H in samples. The excited oxygen atoms emit 6129 keV gamma rays and the excitedhydrogen atoms emit 2223 keV gamma rays, which can be detected by HPGe detector. Wecan get some information of samples by analyzing the count rate of characteristic peak, singleescape peak and double escape peak of the emitting gamma rays. If the shape of the samplesis definite, we discover that the intensity of gamma rays emitted by oxygen atoms becomeslarger when the mass of the sample increases at first, but when we continue to increase themass of the sample, the intensity of gamma rays emitted by oxygen atoms becomes smaller.This rule is similar to the result of the element N using NAA method. By researching theknown samples with a definite shape but different mass, we can get a curve that the intensityof gamma rays varies with the mass. The concentration of the element O in unknown samplewith the same shape can be calculated by interpolation method. The ratio of the concentration of O to H in samples of different shape correspond to a shape correctioncoefficient curve. Combining with the curve of gamma rays count rate varied with mass, wecan calculate the concentration of O or H in samples.Subtracting background and analyzing gamma rays spectroscopy are the two key pointsof NAA. In this paper, we draw a technique for unscrambling overlapped spectrum and adynamic background analysis method.Since the sample itself is the neutron moderator, the count rate of background varieswith the shape and mass of the samples in analysis process. In this paper, background is thecount rate when the mass of sample is zero. The moderative effect of samples is considered inthe curve that the count rate of gamma rays varies with mass. Thus we can analyze therelationship between detection sensitivity and detection time. The proper detectionconditions can be chosen when detecting.For simulative explosives are composed of many elements, it is possible that thedetection peaks of gamma rays overlap. On the basis of investigating many unscramblingspectrum methods, we find that the Gauss Fit of single peak is the fittest method. So thismethod is employed to analyze data of experiments in this paper.Another important work of this paper is simulating experiments with MCNP. But for theenvironments of our experiments are too complex, our code for simulating is simplified. Wejust study the technique of simulating detection process using MCNP. By comparing theresults of simulations and experiments, we find that the elements in environment must beconsidered when simulating, and the factor that the cross sections of the elements O and Hvary with the energy of neutrons also should be considered when analyzing data ofexperiments. Moreover, the positions of neutron source, samples and detector should beconsidered in data analysis.The technology of detecting explosives using NAA and the influence factors of thismethod are studied in this paper, which can be very useful for successive research in this area.