Feasibility of bulk explosives detection using photoneutron spectroscopy

Abstract

Research has been conducted since the 1950s on nuclear methods to confirm the presence of bulk explosives by detecting characteristic emitted radiation. In most practical situations, penetrating radiation is required, which restricts the problem to gamma rays and neutrons. The most successful reactions to date has been radiative thermal neutron capture (thermal neutron analysis) and prompt radiative emission following inelastic fast neutron scattering (fast neutron analysis). An alternative to these neutron-in, gamma ray-out reactions is photoneutron production. A gamma ray whose energy exceeds the threshold for neutron production in a particular atomic nucleus can cause a neutron to be emitted. For a given isotope and assuming monoenergetic photons, the emitted neutrons will have a spectrum consisting of one or more discrete energies. If the gamma ray source and neutron spectrometer are appropriately chosen, the neutron spectrum can be used as a fingerprint to identify the isotope. This photoneutron spectroscopy method has a number of potential advantages over thermal and fast neutron analysis, such as generally simpler spectra and low inherent natural neutron background. It also has drawbacks, such as possible induced neutron background and a present lack of suitable fieldable photon sources. This paper will describe the method and preliminary simulations and calculations to examine its feasibility. Possible sources, detectors and geometries will be discussed.