NSTAR—A capture gated plastic neutron detector

NSTAR (Neutron Sandwich Transmuter/Activation-γ Radiator) prototypes were developed and their performances were evaluated using radioactive sources and a pulsed neutron beam. The NSTAR operating principle is similar to that of Gd-loaded liquid scintillation detectors, where the scintillator has dual functions as neutron moderator and sensor of delayed capture γ-rays, but spatially separates scintillator from neutron converter components. The time dependent NSTAR response to neutrons consists of a prompt, energy related light flash followed by a delayed signal characteristic in both light output and delay time. This feature allows one to discriminate on average between neutrons and γ-rays and provides the basis for multiplicity determination. The detectors are scalable, economic to construct of environmentally benign components, and can be ruggedized. Prototype detector modules consist of 12×20×(50 or 100)cm3 stacks of plastic scintillator slabs (Saint Gobain BC-408) alternating with thin Gd converter films viewed by fast photomultipliers (Philips XP2041). The effective Gd/scintillator ratio is 0.5wt%. Results of tests of NSTAR with 252Cf and neutrons from the D(d,n)3He reactions are in good agreement with theoretical estimates based on neutron transport simulations. Characteristics of the detector module include an average neutron capture time of 〈tc〉=21.7±0.2μs and a detection efficiency of ɛ=26±3% for DD neutrons. The NSTAR has been applied to determine the multiplicity distribution of neutrons produced in D(d,n)3He reactions by a neutron generator.