by Chan Yiu Nam. / Chinese title: / Includes bibliographical references / Thesis (M.Phil.) -- Chinese University of Hong Kong, 1983

Graduation date: 1997

Thesis (M.S.)--University of Michigan, 1984.

Thesis (Ph.D.)--University of California, Berkeley, 1960. / "Physics & Mathematics, UC-34" -t.p. "TID-4500 (15th Ed.)" -t.p. Includes bibliographical references.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / Interest in high-efficiency replacements for thin-film-coated thermal neutron detectors led to the development of single-sided microstructured semiconductor neutron detectors (MSNDs). MSNDs are designed with micro-sized trench structures that are etched into a vertically-oriented pvn-junction diode, and backfilled with a neutron converting material, such as ⁶LiF. Neutrons absorbed by the converting material produce a pair of charged-particle reaction products that can be measured by the diode substrate. MSNDs have higher neutron-absorption and reaction-product counting efficiencies than their thin-film-coated counterparts, resulting in up to a 10x increase in intrinsic thermal neutron detection efficiency. The detection efficiency for a single-sided MSND is reduced by neutron streaming paths between the conversion-material filled regions that consequently allow neutrons to pass undetected through the detector. Previously, the highest reported intrinsic thermal neutron detection efficiency for a single MSND was approximately 30%. Methods for double-stacking and aligning MSNDs to reduce neutron streaming produced devices with an intrinsic thermal neutron detection efficiency of 42%. Presented here is a new type of MSND that features a complementary second set of trenches that are etched into the back-side of the detector substrate. These dual-sided microstructured semiconductor neutron detectors (DS-MSNDs) have the ability to absorb and detect neutrons that stream through the front-side, effectively doubling the detection efficiency of a single-sided device. DS-MSND sensors are theoretically capable of achieving greater than 80% intrinsic thermal neutron detection efficiency for a 1-mm thick device. Prototype DS-MSNDs with diffused pvp-junction operated at 0-V applied bias have achieved 53.54±0.61%, exceeding that of the single-sided MSNDs and double-stacked MSNDs to represent a new record for detection efficiency for such solid-state devices.

<div> <div> <div> <p>This dissertation describes work conducted in pursuit of interests in adapting Tension Metastable Fluid Detectors (TMFDs) for dosimetry-related applications with the specific intent of engineering a neutron ambient dose spectrometer. TMFDs possess several charac- teristics desirable for neutron spectrometry, including high efficiencies, complete blindness to gamma and beta radiation, and tailorable-threshold response functions. Prior spectro- scopic work with TMFDs, aptly named Single Atom Spectroscopy (SAS), was constrained to a specific subset of detection fluids who’s composition includes hydrogen and only one other higher Z element (e.g. hydrocarbons), where only one element is assumed capable of initiating a cavitation detection event (CDE). The present work alleviates these restrictions, enabling spectroscopy in detection fluids with multiple constituent elements. </p> <p>Simulating the detector’s response predicates knowledge of the energy necessary for ra- diation induced nucleation, which has been theoretically derived with nucleation theory for superheated fluids, but remains unbeknownst for tensioned metastable states. This limi- tation was overcome using MCNPX-PoliMI to model the spatial recoil nuclei spectra from isotope sources and coupled with SRIM to generate the ion energy deposition probabil- ity density within a critical length scale of each interaction event. Thereafter, the energy deposition threshold necessary to generate a detection event, and corresponding response matrix, was derived empirically by solving for the solution curve that minimizes the residual difference between the measured and simulated count rates. </p> <p>The accuracy of the derived response matrix was evaluated through comparisons with a 6LiI Bonner Sphere Spectrometer in which, for 252Cf and 239PuBe/241AmBe isotope source neutron spectra, the two systems offered results within ±10% of each other for ambient equivalent fluences on the order of 100 μRem/hr fields. Notably, when under ultra-low (10 μRem/hr) fields the Bonner spectrometer and other traditional detectors proved impractical. In contrast, the TMFD system was capable of resolving underlying spectral features and corresponding ambient dose rates within ±5% of MCNP predictions. </p> </div> </div> </div>

Isotopic abundances of the elements xenon, krypton, and cesium formed in the 14-Mev neutron fission of 238U have been measured using the mass-spectrometric method. The relative yields of some isotopes of krypton, strontium, zirconium, molybdenum, ruthenium, iodine, xenon, barium, cerium and neodymium were measured using a Ge(Li) detector. The ratios were normalized through isobaric nuclides, and absolute yields were obtained by normalizing the sum of the heavy-mass yields to 100%. A semi-empirical method has been developed for constructing neutron yield curves. Such a curve was used to obtain a primary-yield curve from the cumulative yields reported here. The results indicate that considerable structure might exist in the primary-yield curve at the higher excitation energy. / Thesis / Doctor of Philosophy (PhD)

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The Best PhD Thesis in the Faculties of Dentistry, Engineering, Medicine and Science (University of Hong Kong), Li Ka Shing Prize,1991-1993 / published_or_final_version / Physics / Doctoral / Doctor of Philosophy