Monte-Carlo simulations of timeresolved, optical readout detector for pulsed, fast-neutron transmission spectroscopy (PFNTS)

Abstract

Over the past two years, we have developed and tested an efficient, large-area, sub-mm spatialresolution, fast-neutron imaging system with time-of-flight spectroscopic capability. The detector is based on a 30 mm thick scintillating fiber screen viewed by a time-gated optical readout, described in another contribution to this conference. In order to analyze key parameters affecting detector performance, Monte-Carlo simulations using the GEANT 3.21 code were performed. To characterize the intrinsic spatial resolution of the scintillating fiber screen, a neutron transmission image of a steel mask containing a series of slits with various widths, pitch and thicknesses was simulated. The point spread function of the scintillating fiber screen was determined by exposure to an infinitesimally narrow neutron beam, incident perpendicular to the surface, calculating the spatial distribution of the energy deposited by the protons in the screen fibers. The energy distribution of the (n,p) protons produced in the screen and the amount of scintillation light subsequently created were also calculated. All the above simulations were performed for 3 neutron energies (2, 7.5 and 14 MeV). For the detector tests performed at the PTB cyclotron, the neutron beam-line geometry was simulated as accurately as possible, in order to calculate the contribution of neutron scatter as well as gamma rays and to enable a comparison with the experimental results.