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Master of Science / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / Fission chambers are a common type of detector used to determine the neutron flux and power of a nuclear reactor. Due to the limited space and high neutron flux in a reactor core, it is difficult to perform real-time flux measurements with present-day in-core instrumentation. Micro-pocket fission detectors, or MPFDs, are relatively small in size and have low neutron sensitivity while retaining a large neutron to gamma ray discrimination ratio, thereby, allowing them to be used as active neutron flux monitors inside a nuclear reactor core. The micro-pocket fission chamber allows for multiple detectors to be inserted into a flux port or other available openings within the nuclear reactor core. Any material used to construct the MPFD must be rugged and capable of sustaining radiation damage for long periods of time. Each calibrated MPFD provides measurements of the flux for a discrete location. The size of these detectors allows for a spatial map of the flux to be developed, enabling real-time analysis of core burnup, power peaking, and rod shadowing. Small diameter thermocouples can be included with the array to also measure the temperature at each location. The following document details the research and development of MPFDs for long term use in nuclear power reactors. Previous MPFD designs were improved, miniaturized, and optimized for long term operations in reactor test ports designed for passive measurements of fluence using iron wires. Detector chambers with dimensions of 0.08 in x 0.06 in x 0.04 in were attached to a common cathode and individual anodes to construct an array of the MPFDs. Each array was tested at the Kansas State University TRIGA Mark II nuclear reactor to demonstrate functionality. The linear response in reactor power was measured. These arrays have also demonstrated reactor power tracking by following reactivity changes in steady state operations and reactor pulsing events. Stability testing showed consistent operation at 100 kW for several hours. The MPFDs have been demonstrated to be a viable technology for in-core measurements.

Work has been carried out to investigate the applicability of Neutron Induced Emission Tomography (NIET) to areas of non-destructive testing within the nuclear industry. The principal application was the scanning of irradiated nuclear fuel rods. In addition to its medical uses, tomography has been employed in many areas of non-destructive testing. However, the applications have generally used transmission tomography to examine structure. NIET provides a method of determining the spatial distribution of radionuclides produced by the action of neutrons by activation or fission processes. This has applications as an extension of neutron activation analysis and as a method of non-destructive testing for use in the nuclear industry. The work has involved the use of a number of experimental tomography scanners. Various test objects have been scanned using both emission and transmission tomography and computer programs were developed to collect, process and reconstruct the data. High resolution detectors were used to scan single and multi-energetic radionuclides in test objects designed to model various characteristics of nuclear fuel rods. The effects of scattering on image quality were examined and a method of scattering correction based upon the use of an additional energy window was applied. The work showed the viability of using NIET to study the distribution of radionuclides within objects such as irradiated fuel pellets. It also demonstrated the need for reducing the scattering component within images. The use of narrow energy windows and a high resolution detector were shown to be succesful in reducing the effects of scattering. The employment of scattering correction using additional energy windows was shown to be necessary when scanning multi-energetic radionuclides.

The main objective of this research project was to investigate the suitability of the silicon photodiode as a light sensor for scintillation detection of ionising radiation. The type of instrument originally envisaged by Fisher would be particularly applicable to measurement of gamma-ray dose rates. During the course of the research, it was found that some modern silicon photodiodes are very effective as direct semiconductor detectors for both charged particles, and also for photons in the energy range 8 - 140 keV. A summary of this section of the work was published (GOODA and GILBOY, 1987) and has generated considerable interest in the subject. An associated medical physics project in Denmark was developed on the basis of these observations. The development of the photodiode as a scintillation light detector also proved highly successful. Using a CsI(Tl) scintillator and commercially available photodiodes, pulse height energy spectra rivalling those obtainable from conventional photomultiplier-NaI(Tl) assemblies were achieved. By comparing scintillation pulses with direct gamma absorption events in the photodiode, the light output of CsI(Tl) was determined to be significantly higher than that of NaI(Tl), which is usually accepted as the most efficient scintillator at room temperature. The detector assembly developed was successfully employed in the acquisition of data for a gamma ray transmission computer tomography system. A gamma dose rate instrument based on the CsI(Tl)-photodiode combination is also clearly feasible, but more work needs to be done to ascertain the range and sensitivity of this device. In addition to the developmental side of the project, some investigations were made into scintillation pulse shapes induced by gamma rays and alpha particles in CsI(Tl), with particular attention paid to afterglow. The Bollinger-Thomas method employed was modified by the inclusion of a spectrophotometer to investigate the wavelength dependence of pulse shapes.

Investigations have been made on the minimisation of the background count rate detected in proportional counters, while maximising the signal detection efficiency. Two methods of background rejection have been developed, based on the time profile of the shaped detector pulses and on the lateral extent of the induced charge distribution on the cathode. Both these systems have proved highly efficient, and the former has been applied to a multiwire proportional chamber designed as a monitor for plutonium lung contamination, an application where high efficiency detection is critical. This instrument, which incorporates full six-sided "massless" guard cells, and utilises real-time data acquisition and handling by microcomputer, has been developed, optimised, and assessed as a possible competitor to the phoswich detectors in use at present and the solid state detectors under development. Results indicate that the counter is likely to offer no more than a marginal improvement in sensitivity, insufficient to justify its increased compexity and higher costs.

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Uncrewed aerial systems (UAS) have steadily become more prevalent in both defense and industrial applications. Nuclear detection and deterrence is one such field that has given rise to many new opportunities for UAS operations. There is a need to research and develop methods to integrate existing radiation detection technology with UAS capable of flying low-altitude missions. This low-altitude scanning can be achieved by combining small and lightweight radiation detectors and state-of-the-art aircraft and avionics. High resolution mapping can then be conducted using the results of these scans. Significant work has been conducted in this field by both private industry and academic institutions, including the Uncrewed Systems Lab (USL) at Virginia Tech. This work seeks to expand this body of knowledge and provide practical experimental information to showcase and validate the efficacy of radiation detection via UAS. Multiple missions were conducted using samples of 137Cs and 60Co as a radioactive source. Various filtering methods were applied to the results of these missions to produce visual maps that aid in the localization of an unknown source to compare various flight parameters. In addition, significant work was conducted to characterize two radiation detectors available to the USL to provide metrics to assist in the UAS design and flight planning. Finally, the detectors were taken to Savannah River National Laboratories to conduct experiments to provide information to aid future designs and missions that wish to detect a wider variety of radioactive sources. / Master of Science / Drones are becoming more common in many applications for both industry and defense. One of these applications is in the field of nuclear detection which seeks to both regulate the shipping of radioactive material as well as aid response to nuclear disasters. Methods need to be researched to combine existing radiation detectors with new drone technology. These new state-of-the-art drones are capable of flying at very low altitudes which can allow for the use of small and lightweight radiation detectors. Past work in this area, including at the Uncrewed Systems Lab (USL) at Virginia Tech, has explored larger scale aircraft as well as simulated radioactive sources. This work expands the existing knowledge of this field by providing scan results from real radioactive sources and drone flights. Multiple search flights were conducted using small quantities of radioactive cesium and cobalt. Maps were then produced using the information from these flights to showcase the system's ability to quickly locate the areas of high radioactivity. Flights were flown with different altitudes and speeds to determine the effects on mapping accuracy. Finally, experiments were conducted at Savannah River National Laboratories on a variety of more controlled nuclear materials to help inform future drone designs and mission planning.

Analysis of the radiation produced by Z-machine nuclear experiments at Sandia National Laboratory and the materials irradiated indicate that the majority of produced radionuclides can easily be detected. One significant exception is volumetric contamination of stainless steel by iron-55. Detecting iron-55 in Z-machine components presents a particular problem due to its low-abundance and the low-energy (5.9 keV) xray it emits. The nuclide is often below the minimum detectable activity (MDA) threshold and resolution criteria of many standard radiation detection devices. Liquid scintillation has proven useful in determining iron-55 presence in loose contamination at concentrations below that of regulatory guidelines, but determination of volumetric iron- 55 contamination remains a significant challenge. Due to this difficulty, an alternate method of detection is needed. The use of radioactive surrogates correlating to iron-55 production is proposed in order to establish an estimate of iron-55 abundance. The primary interaction pathways and interaction probabilities for all likely radionuclide production in the Z-machine were tabulated and radionuclides with production pathways matching those of iron-55 production were noted. For purposes of nuclide identification and adequate detection, abundant gamma emitters with half-lives on the order of days were selected for use as surrogates. Interaction probabilities were compared between that of iron-55 production and a chosen surrogate. Weighting factors were developed to account for the differences in the interaction probabilities over the range of the known energy spectra produced on the device. The selection process resulted in cobalt-55, cobalt-57 and chromium-51 as optimal surrogates for iron-55 detection in both deuterium and non-deuterium loaded interactions. A decay corrected correlation of the surrogates (chromium-51, cobalt-57 and cobalt-55) to iron-55 for deuterium and non-deuterium loaded Z-machine driven reactions was derived. The weighting factors presented here are estimates which are based on rough comparisons of cross-section graphs. Analysis considering factors such as energy spectrum criteria to provide refined weighting factors may be utilized in future work.