Development and Operation of a Li6F:ZnS(Ag) Scintillating Plastic Capture-Gated Detector

Open Access
Author:
Wilhelm, Kyle Jacob
Graduate Program:
Nuclear Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 07, 2017
Committee Members:
  • Igor Jovanovic, Thesis Advisor
  • Marek Flaska, Committee Member
  • Arthur Thompson Motta, Committee Member
Keywords:
  • Neutron scintillation
  • Composite detector
  • Capture-gated spectroscopy
  • Fast neutron detection
  • Pulse shape discrimination
  • Li6F:ZnS(Ag) scintillator
Abstract:
Efforts to detect special nuclear materials (SNM), including those that are heavily shielded and in transit, have resulted in a need to develop improved radiation detectors, including fast neutron detectors that can be readily scaled to achieve large area coverage. Among the various neutron detection technologies, 3He detectors combined with moderating materials still serve as a benchmark. Costs associated with the use of 3He have become prohibitively expensive, as a result of the scarcity of this isotope, which precipitates the need for the development of cost effective and efficient alternative technologies. A critical design feature of any replacement technology includes the ability to effectively detect and separate neutron and gamma events for independent analysis. Active and passive interrogation techniques result in SNM exhibiting various modes of decay, giving off a spectroscopic fingerprint for detection and identification. Harnessing the signatures of both gamma and neutron emissions reduces the limitations associated with the shielding effects of the surrounding media, and inherent to the penetrability of each type of radiation. Additionally, ascertaining the incident neutron energy spectrum would further enhance detection capabilities by helping to determine the type of radioactive material in question. In this work, the design, construction, and operation of a detector capable of both separating neutron and gamma events to a high degree and performing fast neutron spectroscopy is reported. Pulse shape discrimination (PSD), by way of charge integration, provides excellent neutron/gamma separation. Furthermore, use of the capture-gated technique identifies neutron thermalization events occurring within the detector for further spectroscopic analysis. This detector is readily scaled to larger volumes for increased efficiency and large area coverage. Monte Carlo simulations were performed for scalability analysis and to determine the thermalization-to-capture time acceptance windows to be used for capture-gated iii spectroscopy. The detector design assessed in this report is comprised of a flat, 500 μm thick sheet composed of a mixture of lithium-6-fluoride capture agent, 6LiF, and zinc sulfide phosphor, ZnS(Ag), wrapped around scintillating polyvinyl toluene (PVT) in the form of a cylinder. The 6LiF:ZnS(Ag) sheet uses an aluminum foil backing as a support for the scintillating material and as an optical reflector, and its optical properties have been characterized independently. The composite scintillator was tested using 252Cf, DD fusion, 137Cs, and 60Co sources. The intrinsic detection efficiency for neutrons from an unmoderated 252Cf source and rejection of gammas from 137Cs were measured to be 3.6% and E−6, respectively. Furthermore, capture-gated spectroscopic analysis has been demonstrated.