Methods and Instruments for Fast Neutron Detection

Abstract

Pacific Northwest National Laboratory evaluated the performance of a large-area (~0.7 m 2 ) plastic scintillator time-of-flight (TOF) sensor for direct detection of fast neutrons. This type of sensor is a readily area-scalable technology that provides broad-area geometrical coverage at a reasonably low cost. It can yield intrinsic detection efficiencies that compare favorably with moderator-based detection methods. The timing resolution achievable should permit substantially more precise time windowing of return neutron flux than would otherwise be possible with moderated detectors. The energy-deposition threshold imposed on each scintillator contributing to the event-definition trigger in a TOF system can be set to blind the sensor to direct emission from the neutron generator. The primary technical challenge addressed in the project was to understand the capabilities of a neutron TOF sensor in the limit of large scintillator area and small scintillator separation, a size regime in which the neutral particle's flight path between the two scintillators is not tightly constrained.The project comprised an experimental campaign and a modeling campaign. The experimental campaign focused on measuring the response of an existing, dual-sheet scintillator sensor to mono-energetic gamma sources (including 137 Cs and 54 Mn) and a 252 Cf gamma + neutron source. The sensor's intrinsic gamma and neutron detection efficiencies were mapped as a function of the TOF threshold above which the sensor's response is integrated. A fast neutron intrinsic detection efficiency of approximately 2.5%, averaged over the 252 Cf neutron spectrum, was obtained at a TOF threshold that permits approximately 10,000:1 gamma rejection. An additional contribution to the sensor's total intrinsic neutron detection efficiency of roughly 1.3% results from including the TOF response to neutron flux reflected from a 5.08 cm (2 in.) layer of Pb shielding surrounding the sensor on four sides. In addition to the TOF measurements, the report presents preliminary investigations of the supplementary neutron spectroscopic information content available in scintillator pulse-height measurements.The modeling campaign consisted of development of a simulation code based upon the Geant4 radiation transport framework. A prescription for calculating effective particle interaction times in each scintillator of the TOF sensor is described. This prescription avoids the significant computational overhead associated with tracking large ensembles of scintillation photons, at the cost of sacrificing a realistic model of the sensor's photomultiplier tube (PMT) signal pulse development with time. The model predicts the qualitative features of the neutron TOF distribution (although it underpredicts the Pbreflected component) and provides a consistent description of the measured neutron and gamma detection efficiencies. The model fails to predict the shapes of the gamma and neutron responses accurately enough to permit reliable prediction of the details of the gamma reje...

Advanced Large Area Plastic Scintillator Project (ALPS): Final Report

“…It is also worth pointing out in passing that dual-sheet geometry may represent a promising approach to achieving cost-effective, large-area fast neutron spectroscopy by an appropriate combination of time-of-flight (TOF) and recoil-proton pulse height methods. Follow-on activities to the current project have, in fact, successfully demonstrated use of the ALPS II apparatus for fast neutron detection by the TOF method; this research is described elsewhere [Jordan et al, 2005].…”