Fast Neutron Detection with Varying-Length Trans-Stilbene Scintillators

Abstract

In recent years, solution grown trans-stilbene crystals have become widely used for gamma ray and fast neutron detection in many applications, including nuclear safeguards and nonproliferation. These crystals exhibit excellent pulse shape discrimination (PSD), enabling simultaneous detection and identification of gamma rays and fast neutrons. Stilbene crystals have been made widely available in 5 cm by 5 cm cylinders by Inrad Optics; production of cylinders up to 10 cm long have been demonstrated. Time-of-flight experiments using a 252Cf source have been performed to characterize 5 cm diameter stilbene crystals with 2.5 cm, 5 cm, 7.6 cm, and 10 cm lengths: all four crystals were coupled to photomultiplier tubes and symmetrically placed around the source, 1 m away. These data have been analyzed to obtain detector response functions (scintillation light output as a function of neutron energy deposited) for each of the four crystals. These functions can be used in Monte Carlo simulations to model the absolute detector response. Results will compare the performance of these stilbene cylinders as a function of length using the following quantities: neutron detection efficiency, gamma-ray detection efficiency, PSD figure of merit, and the total detector response function. Additionally, the measured detector response will be compared to Monte Carlo simulations using the MCNPX-PoliMi code.