Date of Award

9-1-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Engineering Physics

First Advisor

Buckley E. O'Day, PhD

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, based on the absorption of thermalized neutrons by the 6Li and subsequent energy release of the 6Li(n,t)4He reaction. 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. 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. After evaluation of the Eu:LiCAF, a portable ten-layer neutron spectrometer was fabricated from the rubberized wafers. A layer of high density polyethylene (HDPE) was used as a neuton moderator between each layer. Neutrons entering the spectrometer are downscattered and absorbed by the lithium in a wafer of rubberized Eu:LiCAF; the layer in which the absorption occurs is dependent on the incident neutron's energy. A library of neutron response curves was created using Geant4 and applied with the maximum entropy principle algorithm, MAXED, to unfold the experimentally acquired data. The spectrometer was commissioned using two DD generators and a 252Cf source. This research demonstrated that Eu:LiCAF is a promising potential replacement for 3He and shows excellent promise for neutron spectroscopy applications.

AFIT Designator

AFIT-ENP-DS-18-S-024

DTIC Accession Number

AD1063458

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