Unmixing algorithms and materials for radiation detection in harsh environments

Weiss, Matthew A.

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https://hdl.handle.net/2142/116272 Copy

Description

Title

Unmixing algorithms and materials for radiation detection in harsh environments

Author(s)

Weiss, Matthew A.

Issue Date

2022-07-20

Director of Research (if dissertation) or Advisor (if thesis)

Di Fulvio, Angela

Committee Member(s)

Sankaran, Mohan

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• radiation
• detection
• unmixing
• SNM
• neutron
• spectroscopy
• deuterated trans-stilbene

Abstract

Spectroscopy-grade radiation measurements need to be performed in harsh environments for several applications, such as radiological search, decommissioning, and remediation. In harsh environments, there may be many shielded and bare radionuclides present simultaneously, the detector may be damaged by the radiation, and other environmental effects may degrade its performance. This work focuses on tackling these issues from both a hardware and software perspective, including the development of unmixing algorithms for source identification, and a methodology for growing diamond as a radiation-hardened material for powering remote sensors and for radiation detection. Source identification is particularly challenging when using organic scintillators. These detectors can be grown in large sizes and are sensitive to both gamma-rays and neutrons, through pulse shape discrimination, but their energy resolution is limited. Hence discriminating different sources directly from their spectral differences is a challenging task. To solve this issue, we first demonstrated that Bayesian unmixing algorithms applied to organic scintillator spectra can reliably identify unshielded gamma-ray radionuclides, even with fewer than 1,000 detected counts and in the presence of two or three nuclides at the same time. We experimentally studied the robustness of a state-of-the-art unmixing algorithm to different radiation background spectra, due to varying atmospheric conditions, in the 16 $^{\circ}$C to 28 $^{\circ}$C temperature range. In the presence of background, the algorithm is able to identify the nuclides present in unknown radionuclide mixtures of three nuclides, when at least 1,000 counts from the sources are detected. With fewer counts available, we found larger differences of approximately 35.9\% between estimated nuclide fractions and actual ones. In these low count rate regimes, the uncertainty associated with the result that is provided by our algorithm with the identified fractions could be an additional valuable tool to determine whether the identification is reliable or a longer measurement to increase the signal-to-noise ratio is needed. We then applied the unmixing algorithm to identify shielding materials in the detection of a shielded source. A series of experiments was performed at the Device Assembly Facility at the Nevada National Security Site to test the usage of a deuterated trans-stilbene organic scintillation (stilbene-d$_{12}$) detector on a shielded 4.5 kg sphere of alpha-phase plutonium surrounded by stainless steel cladding and shielded by various materials, including polyethylene and lead. These measurements required a series of post-processing steps, including calibration, pile-up rejection (PUR), and pulse-shape discrimination (PSD). Optimizing the PUR parameters allowed for a neutron detection efficiency of 34.4$\pm0.2\%$ with a neutron rejection fraction of 8.02 \%. Once processed, these neutron spectra were used to validate a simulated model of the shielded SNM detection. %using a comparable MCNPX-Polimi simulation. I simulated the stilbene-d$_{12}$ light output response matrix in the 0.1 to 20 MeV range based on the stilbene-d$_{12}$ light output response measured at the Nuclear Measurement Lab. Four unmixing testing scenarios were employed to test the feasibility of using the unmixing algorithm for identifying shielding materials. The algorithm is able to identify the simulated shielding components in a mixed shielded spectra scenario when at least 50,000 counts are detected from the source. Lastly, the algorithm is able to correctly identify shielded light output spectra and unfolded neutron energy spectra with a 99.9\% certainty with the addition of Poisson noise. The second part of this thesis details the preliminary construction and development of a plasma reactor to grow diamond using the chemical vapor deposition (CVD) method. The reactor design allows for a diamond film to be formed on the surface of a substrate for various uses such as in diamond-based betavoltaic cells and diamond detector materials. Diamond is an attractive material for application in harsh environments due to its radiation hardness and large indirect band-gap of 5.5 eV. The proposed reactor design was simulated in COMSOL for verifying adequate heat dissipation from the high temperature substrate heating needed for diamond layer creation. This simulation confirms the ability for the chamber to maintain a temperature increase of 120--140 ${^\circ}$C, while the maximum temperature never exceeds 300 ${^\circ}$C. Lastly, the reactor was constructed for upcoming plasma-enhanced diamond CVD with a remote radio-frequency (RF) plasma.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Matthew Weiss

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