The purpose of this research is to design and optimize the performance of a unique, inexpensive 6Li neutron-capture pulse-mode ion chamber (LiPMIC) for neutron detection that overcomes the fill-gas contamination stemming from outgas of detector components. This research also provides a demonstration of performance of LiPMICs. Simulations performed with GARFIELD, a drift-chamber simulation package for ion transport in an electrostatic field, have shown that argon-methane mixtures of fill-gas allow maintenance of electron drift velocity through a surprisingly wide range of fill-gas content. During the design stage of LiPMIC development, the thicknesses of lithium metallization layer, the neutron energy conversion site of the detector, and the thickness of neutron moderator, the high-density polyethylene body, are optimized through analytical and MCNPX calculations. Also, a methodology of obtaining the suitable combination of electric field strength, electron drift velocity, and fill-gas mixtures has been tested and simulated using argon-methane gas mixtures. The LiPMIC is shown to have comparable efficiency to 3He proportional counters at a fraction of cost. Six-month long baseline measurements of overall detector performance shows there is a 3% reduction in total counts for 252Cf sources, which provides a good indicator for the longevity of the detector.

Determination of the spatial distribution and quantification of concentration of pure beta- and photon-emitting radionuclides in absorbing media by external measurements is the subject of this study. Measurements of radiation and the operation of radiation detectors are based on the radiation interactions with matter and the theory governing these interactions has been discussed. Various techniques for localising pure beta- and photon-emitting radionuclides situated inside attenuating media have been suggested on a theoretical basis, and have been experimentally shown to work successfully. Most of the work is on single photon emission computed tomography (SPECT). The theory of mathematical reconstruction of a two dimensional distribution from its projections is discussed and reconstruction techniques and their relative merits and demerits have been reviewed. SPECT seeks the determination of absolute regional radionuclide concentrations as a function of time. A SPECT system has been developed by modifying an existing transmission CT scanner and the reconstruction algorithms. The performance of the SPECT system has been tested for a number of point sources and various extended sources in gas, liquid and solid forms. The SPECT scanner in its present design is capable of performing in both the transmission and emission modes. The characteristics of the SPECT scanner, including the detector efficiency, spatial resolution and the effect of collimator size, have been studied experimentally. The major problems faced by SPECT include the solid angle effect, which influences the collection efficiency, in scattered radiation, and attenuation of photons inside the surrounding medium. These problems together with their various possible solutions have been discussed in detail. Methods for compensation for solid angle variation, in scattered radiation and photon attenuation have been devised and used successfully to compensate the projection data.

This thesis is missing page 36, all other versions of the thesis are missing this page as well. -Digitization Centre / This thesis deals with the theory, fabrication and application of semi-conductor detectors to gamma ray spectroscopy. The theory section discusses the semi-conductor lattice with respect to the incoming gamma ray and the subsequent hole-electron pairs formed in the diode depletion region. The fabrication section describes the materials and apparatus used in this work to produce the Ge(Li) detectors. A table is given showing the characteristics of these devices as they proceed through the process. The section on the characteristics of these detectors gives information as to how a good detector should operate under certain tests., The detectors made in this laboratory were used to investigate a well known "fission product" nuclide and a neutron capture reaction on a rare earth element. However, these investigations were not exhaustive but only show how the improved resolution of the Ge(Li) detectors has opened up new areas of gamma ray studies. / Thesis / Master of Science (MS)

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.

Made available in DSpace on 2014-10-09T12:34:35Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:07Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:34:35Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:07Z (GMT). No. of bitstreams: 0 / Neste trabalho, cristais de TlBr foram crescidos e purificados pelo método de Bridgman Repetido a partir de sais comerciais de TlBr e caracterizados para serem usados como detectores de radiação à temperatura ambiente. Para avaliar a eficiência de purificação, estudos da diminuição da concentração de impurezas foram feitos após cada crescimento, analisando as impurezas traços por Espectrometria de Massas com Plasma (ICP-MS). Um decréscimo significativo da concentração de impurezas em função do número de purificações foi observado. Os cristais crescidos apresentaram boa qualidade cristalina de acordo com os resultados de análise por Difração de Raios X (DRX). Para avaliar os cristais a serem usados como detectores de radiação, medidas de suas resistividades e resposta à incidência de radiação gama da fonte de 241Am (59,5 keV) foram realizadas. Essa resposta foi dependente da pureza do cristal. Um modelo compartimental foi proposto para avaliar as concentrações de impurezas no cristal de TlBr e suas segregações em função do número de crescimentos pelo método de Bridgman. Este modelo compartimental definido por equações diferenciais pode ser usado para calcular o coeficiente de migração das impurezas e mostrou ser útil para prever o número necessário de repetições de crescimento Bridgman para atingir nível de pureza adequado para assegurar a qualidade do cristal como detector de radiação. A diferença dos valores dos coeficientes de migração das impurezas nos cristais entre os sais, de duas procedências diferentes, utilizados foi significativa. Portanto, a escolha do sal de partida deve ser realizada experimentalmente, independente da declaração nominal da sua pureza. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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As the Earth's atmosphere evolves due to human activity, today's modern industrial society relies significantly on the scientific community to foresee possible atmospheric complications such as the celebrated greenhouse effect. Scientists, in turn, rely on accurate measurements of the Earth Radiation Budget (ERB) in order to quantify changes in the atmosphere. The Thermal Radiation Group (TRG), a laboratory in the Department of Mechanical Engineering at Virginia Polytechnic Institute and State University, has been at the edge of technology designing and modeling ERB instruments. TRG is currently developing a new generation of thermoelectric detectors for ERB applications. These detectors consist of an array of thermocouple junction pairs that are based on a new thermopile technology using materials whose electro-thermophysical properties are not completely characterized. The objective of this investigation is to design experiments aimed at determining the electro-thermophysical properties of the detector materials. These properties are the thermal conductivity and diffusivity of the materials and the Seebeck coefficient of the thermocouple junctions. Knowledge of these properties will provide fundamental information needed for the development of optimally designed detectors that rigorously meet required design specifications. / Master of Science

The importance of gamma-ray spectroscopy – the science of determining the distribution of energy in a gamma field – can rarely be overstated. High performance scintillators for gamma-ray spectroscopy in Nuclear Nonproliferation applications and homeland security require excellent energy resolution to distinguish neighboring element and isotope lines while minimizing the time and exposure to do so. Semiconductor detectors operate by converting incident photons directly into electrical pulses, but often have problems of high costs due to constituent segregation and surface states as is the case for Cadmium Zinc Telluride. The ideal scintillator material for gamma spectrometer will therefore requires high light yield, excellent proportionality between light yield and gamma photon energy, and material uniformity. A scintillator should possess the following properties; it should convert the kinetic energy of the generated charged particles (typically K-shell electrons) into detectable visible light. This conversion should be linear-the light yield should be proportional to deposited energy over as wide a range as possible. For good light collection, the medium should be transparent to the wavelength of its own emission. The decay time of the induced luminescence should be short so that fast signal pulses can be generated. The medium should be of good optical quality and subject to manufacture in sizes large enough to be of interest as a practical detector. Its index of refraction should be near that of glass (~1.5) to permit efficient coupling of scintillation light to a photomultiplier tube or other photo-sensor. In the past decade, inorganic scintillator research has focused less on improving the characteristics of known scintillators, but rather on the search for new hosts capable of fast response and high energy resolution. Extensive searches have been made for hosts doped with lanthanide activators utilizing the allowed 5d-4f transition. These 5d-4f transitions are dipole-allowed and thus are about 106 times stronger than the more frequently observed 4f-4f transition in the trivalent rare earth ions. Ce3+, Nd3+ and Pr3+ have been investigated for fast response applications while Ce3+, Eu2+ and Yb2+ stand out as the most promising activators offering high light yield, and high energy resolution. Using a modified Bridgman growth technique we have grown crystals with a low energy resolution of 2.6% at 662 keV, which is lower than the previous 2.8% reported for SrI2:Eu2+. The modified technique (called so for its vertical crystal growth orientation) is necessary due to the anisotropic thermal expansion coefficient of Strontium Iodide. The problem plaguing the growth of the crystal is spontaneous cracking, which usually appear during cooling in the bulk. With the use of a zone separating shield, one can achieve more control of the temperature gradient between the two zones without compromising the actual temperature of the two zones. Additionally the use of codopants, in particular divalent magnesium improved the crystalline quality by acting as a gathering for iodine ions, which led to reduction of defect density.