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Abstract With the increasing cost and decreasing availability of 3He, there have been many efforts to find alternative neutron detection materials. Lithium calcium aluminum fluoride (LiCAF) enriched to 95% 6Li doped with europium was evaluated here as a replacement material for 3He. Wafers 0.5 cm thick, consisting of LiCAF crystals in a rubberized matrix, were embedded with wavelength shifting fibers (WSF) and mated to silicon photo-multipliers (SiPMs) to measure the photon response in a flux of neutrons from a DD neutron generator. Excellent discrimination was realized between neutrons and gammas, and both pulse-height discrimination and pulse-shape analysis were explored. A Figure of Merit (FoM) of 1.03 was achieved. By applying pulse-shape analysis, a simple neutron count output was generated by utilizing a low-pass filter to suppress fast pulses from the SiPM output and subsequently applying a threshold to the remaining signal. Custom electronics were built to bias the SiPMs, then amplify, filter, discriminate, and digitize the LiCAF/WSF scintillation photons, resulting in a digital pulse that can easily be counted with any microcontroller or field programmable gate array. A significant advantage of LiCAF is that it can be fabricated into any shape/size (when embedded in a rubberized matrix), and the light output and transparency is sufficient to allow for thicker scintillators which enable detection of both thermal and epithermal neutrons. This work demonstrated that Eu:LiCAF is capable of discriminating gammas from neutrons and is a potential replacement material for 3He, especially for nuclear security applications and neutron spectroscopy. [ABSTRACT FROM AUTHOR]

A portable ten-layer neutron spectrometer has been developed using a novel material, LiCAF. The spectrometer consists of rubber matrix wafers with embedded europium doped lithium calcium aluminum fluoride (Eu:LiCAF) crystals and crosshatched wave shifting fibers (WSF). Layers of high density polyethylene (HDPE) separating each Eu:LiCAF wafer serve as neutron moderating material. Neutrons entering the spectrometer react in the Eu:LiCAF crystal, creating scintillation photons. The photons travel down the WSFs to silicon photomultipliers (SiPMs) that convert the photons to an electrical signal. Custom electronics are used to gather, amplify, process, and readout the signal. A library of several neutron response curves was created in Geant4 and MCNP. The experimental output of the spectrometer was unfolded using a maximum entropy algorithm, MAXED. The spectrometer was tested using two separate DD generators. These two tests resulted in unfolded neutron spectra with average neutron energies of 2.71 and 2.78 MeV and chi-squared values of 0.88 and 1.51 χ 2 /D.O.F. respectively, when fit to a 2.45 MeV monoenergetic energy spectrum. [ABSTRACT FROM AUTHOR]

The Rotating Scatter Mask system is a low cost, directional radiation detection system with a nearly 4 π field-of-view over a broad range of photon energies. However, the original mask design is limited by numerous similarities in the detector response directional modes. These similarities introduce potential misidentification errors when determining a source's direction. Previous studies identified a better mask design, the Mace, which significantly reduced the similarities between the modes. In this work, a new design class was simulated and compared to the Mace mask design using the modal assurance criterion to assess the differentiability between directional modes. At the expense of a reduced field-of-view, 93% of a full 4 π steradians, these novel mask designs were shown to successfully decouple the angular components of the source's direction, improving the average criterion value by up to 66%. The new designs also significantly improved the system's detection efficiency, reducing the time to identify the source's direction by up to 60%, while enabling a simplified, alternative algorithm for identifying the source direction. This alternative approach, called the geometric correlation method, further improved detection efficiency leading to a near-real time analysis for locating a source direction with the Rotating Scatter Mask. [ABSTRACT FROM AUTHOR]

Rotating scattering masks have shown promise as an inexpensive, lightweight method with a large field-of-view for identifying the direction of a gamma emitting source or sources. However, further examination of the current rotating scattering mask design shows that changing the geometry may improve the identification by reducing or eliminating degenerate solutions and lower required count times. These changes should produce more linearly independent characteristics for the mask, resulting in a decrease in the mis-identification probability. Three approaches are introduced to generate alternative mask geometries. The eigenvector method uses a spring–mass system to create a geometry basis. The binary approach uses ones and zeros to represent the geometry with many possible combinations allowing for additional design flexibility. Finally, a Hadamard matrix is modified to examine a decoupled geometric solution. Four criteria are proposed for evaluating these methodologies. An analysis of the resulting detector response matrices demonstrates that these methodologies produced masks with superior identification characteristics than the original design. The eigenvector approach produces the least linearly dependent results, but exhibits a decrease in average efficiency. The binary results are more linearly dependent than the eigenvector approach, but this design achieves a higher average efficiency than original. The Hadamard-based method produced a lower maximum, but a higher average linear dependence than the original design. Further possible design enhancements are discussed. [ABSTRACT FROM AUTHOR]