Publications

Because of their penetrating power, energetic neutrons and gamma rays ({approx}1 MeV) offer the best possibility of detecting highly shielded or distant special nuclear material (SNM). Of these, fast neutrons offer the greatest advantage due to their very low and well understood natural background. We are investigating a new approach to fast-neutron imaging - a coded aperture neutron imaging system (CANIS). Coded aperture neutron imaging should offer a highly efficient solution for improved detection speed, range, and sensitivity. We have demonstrated fast neutron and gamma ray imaging with several different configurations of coded masks patterns and detectors including an 'active' mask that is composed of neutron detectors. Here we describe our prototype detector and present some initial results from laboratory tests and demonstrations.

An anisotropy in a scintillator's response to neutron elastic scattering interactions can in principle be used to gather directional information about a neutron source using interactions in a single detector. In crystalline organic scintillators, such as anthracene, both the amplitude and the time structure of the scintillation light pulse vary with the direction of the proton recoil with respect to the crystalline axes. Therefore, we have investigated the exploitation of this effect to enable compact, high-efficiency fast neutron detectors that have directional sensitivity via a precise measurement of the pulse shape. We report measurements of the pulse height and shape dependence on proton recoil angle in anthracene, stilbene, p-terphenyl, diphenyl anthracene (DPA), and tetraphenyl butadiene (TPB). Image reconstruction for simulated neutron sources is demonstrated using maximum likelihood methods for optimal directional sensitivity.

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An anisotropy in the response of crystalline organic scintillators such as anthracene to neutron elastic scattering interactions has been known for some time. Both the amplitude and the time structure of the scintillation light pulse vary with the direction of the proton recoil with respect to the crystalline axes. In principle, this effect could be exploited to develop compact, high-efficiency fast neutron detectors that have directional sensitivity via a precise measurement of the pulse shape. We are investigating the feasibility and sensitivity of such a detector, particularly for neutrons in the fission energy spectrum. Here we will report new measurements of the pulse shape dependence on proton recoil angle in anthracene and stilbene single crystals, for proton energies in the few MeV range. Digital pulse acquisition and processing are used to allow an exploration of different pulse shape analysis techniques.

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