Novel Concept and Applications for a Directional Fast Neutron Detector

This paper describes a novel concept, theory, and preliminary results for a directional fast neutron detector (US Patent 7,521,686 B2) with applications in international safeguards. The proposed detector has alternating layers of a hydrogenous non-scintillating material for generating recoil protons, a non-hydrogenous scintillating material for generating scintillation photons, and a nonhydrogenous non-scintillating barrier material. Neutrons are detected if the recoil protons they create deposit part of their energy in a layer of scintillating material, but are not detected if the recoil proton loses all its energy in the non-scintillating hydrogenous layer and/or barrier layer. The composition and thickness of the layered materials can be varied in order to reduce the probability that neutrons from large angles will be detected or to prevent their detection all together. The effects of this layered composition, when combined with the overall size and shape of the detector, results in a detector whose light output is a strong function of the direction of the incident neutron. Selecting layered materials that have similar indices of refraction and are transparent to the scintillation light produced facilitates light collection and minimizes the requirements for signal processing and data analysis. The use of two different types of scintillator materials with different decay constants, in alternating scintillator material layers could provide differentiation of gamma rays of fast neutrons. Compton scattered electrons, with their longer range, would deposit energy in both types of scintillator materials, while the recoil protons with their shorter range would deposit their energy in only one type of scintillator material. Pulse shape discrimination based on the differing decay constants could then be used. This concept, if proven feasible, could lead to the development of compact, relative easy to use, directional neutron detectors. Potential applications include locating sources of fast neutrons and monitoring sources of fast neutrons; monitoring of SNM at nuclear facilities under safeguards regimes; and detecting sources of fast neutrons, including SNM, in containers and packages being scanned. Additionally, an array of these detectors, each optimized for different neutron energy, could be used for fast neutron spectrometr