Advances in organic scintillators for fast and thermal neutron detection*

Detection of special nuclear materials requires instruments which can detect uranium and plutonium isotopes, having at the same time the ability to discriminate among different types of radiation. For many decades, neutron detection has been based on ³He proportional counters sensitive primarily to thermal neutrons. The most common methods for detection of fast neutrons have been based on liquid scintillators with pulse shape discrimination (PSD). The shortage of ³He and handling issues with liquid scintillators stimulated a search for efficient solid-state PSD materials.
Studies conducted at LLNL led to development of new materials, among which are organic crystals and first PSD plastics for fast neutron detection. More advantages have been introduced by ¹⁰B and ⁶Li plastics that can be used for combined detection of both thermal and fast neutrons, offering, in addition, a unique “triple” PSD for signal separation between fast neutrons, thermal neutrons, and gamma-rays. More recent studies have been focused on scaling new compositions with increased performance stability and development of deuterated scintillators that can be used for neutron spectroscopy without time-of-flight assemblies. Among commercially produced materials are large-scale (>10 cm) stilbene crystals grown by the inexpensive solution technique, and different types of PSD plastics which, due to the deployment advantages and ease of fabrication, create a basis for the widespread use of solid-state scintillators as large-volume and low-cost neutron detectors.