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The Doppler-broadening of Gamma-ray Spectra in Neutron-Based Explosives Detection Systems
Phillip C. Womble, Alexander Barzilov, Ivan Novikov, and Joseph Howard
Applied Physics Institute, Western Kentucky University, USA
Corresponding Author: womble@wku.edu
Neutron-induced gamma-ray reactions are the primary means used in the nondestructive analysis of materials. Since 9/11, there is an expanded interest in using these reactions to detect explosives. Simultaneously, there have been great advances in the cooling systems of semiconductor gamma ray detectors which make their deployment more practical. The high resolution of this type of gamma ray detectors shows great promise in increasing the signal-to-noise ratio which has plagued neutron-based explosives detection systems in the past. Gamma-ray analysis using these high-resolution detectors relies heavily on dependable resolution curves so that the software can calculate the full-width half-maximum for any peak based on a given energy. These curves fail if the gamma ray peak is Doppler-broadened. Due to momentum considerations, Doppler-broadening occurs primarily with gamma-rays from neutron-induced inelastic scattering. The recoiling nucleus of interest must have excited states whose lifetimes are much smaller than the time of flight in the material. We have been examining C, N, and O nuclei when bombarded by 14 MeV neutrons. We have calculated the period of time for the kinetic energy to drop from its maximum value to a value under 100 keV. We are utilizing this slowing period to predict if the peak shape of a gamma ray emitted from these nuclei under these conditions will be Doppler-broadened. With this knowledge, we can more accurately measure the intensity of the gamma ray in question and employ this information in determining the threat potential of a given object.
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