Direction Sensitive Neutron Detectors for Border Monitoring

Abstract:

Detection of neutrons in the field typically consists of measurements of charge deposition in a gaseous detector system. This method of detection results in relatively high efficiency, but pulse height analysis for neutron energy distributions and incident neutron direction is difficult if not impossible. In previous work, the preliminary analysis of a neutron sensor system which can measure neutron count rates as well as incident neutron energy and direction was explored. This resulted in proving the fundamental feasibility of using all of the following reactions as indicators of neutron direction: (1) the (n,a) reaction in B-10, (2) inelastic scattering reactions, and (3) the (n,p) reaction in He-3 inside a cloud chamber. The first two of these reactions produce characteristics gamma rays as well as charged particles (in the form of alpha particles and recoil nuclei). The gamma rays produced from these reactions can be measured in a high-resolution gamma spectroscopy system (such as an HPGe) and will have characteristic gamma lines that are produced from the de-excitation of recoil nuclei. These nuclei decay in flight (i.e., while moving either toward or away from the detector) and produced Doppler broadened gamma lines. The differential between the observed gamma-ray energy and the nonrelativistic gamma-ray energy is a function of the velocity vector of the recoil nuclei with respect to an interaction point in the gamma ray detector. If the energy of the reaction products, the energy of the characteristic gamma line, and the coordinates of the interaction point can be measured, then the direction of the incident neutron (actually a cone of directions) may be determinable via simple kinematics. Simulations of these systems demonstrated the feasibility of the measurement method; however, analysis of the effects of uncertainties and construction of prototype devices is needed to generate a preliminary design.

System Schematic