Typescript (photocopy). / Digitized by Kansas Correctional Industries

Digitized by Kansas Correctional Industries

Call number: LD2668 .T4 1979 N37 / Master of Science

In this study variations in Ortec and Hamner equipment with different types of detectors were utilized in coincidence circuits to receive the smallest lifetime values of nuclear states. Results from Ge(Li)-NaI(Tl) detectors were compared to results received when NaI(Tl)-NaI(Tl) detectors were combined in a coincidence circuit.Four different isotopes were investigated. Samples of 22Na and 60Co were utilized as prompt sources. The lifetimes of the 0.81 MeV energy level of i33Ba and the 0.124 MeV energy level of 154Eu were studied. These lifetimes have been reported as being 6.3 nsec and 1.18 nsec respectively.The best value received for the 154Eu energy state was a slope measurement of 5.85 ± 3.15 nsec. The smallest value for the 133Ba energy state was a centroid shift measurement of 6.93 nsec.

Call number: LD2668 .T4 1985 T53 / Master of Science

The Anger camera has been used for the last quarter century in many areas of science to image gamma radiation. Some typical applications include medicine, where functionality of organs are studied in vivo, and industrial inspection of fuel rods for nuclear reactors. The standard Anger geometry includes a large scintillation crystal, light guide, photomultiplier array, and analog processing electronics. Even the most modern gamma cameras built today still use the standard Anger design. The work presented here describes an alternative to the standard gamma-camera design that is flexible enough to be used in a wide variety of applications. Especially in single-photon emmission computed tomography (SPECT) applications, the new design has the potential to be more efficient than the standard design. The new design is modular, that is, several small, separate units comprise a system. Each unit consists of a small gamma camera that is optically and electronically independent from other units. The units, called "modular cameras," can be configured around the region of interest so as to provide the maximum amount of information for reconstruction algorithms or direct information to the operator. The theoretical and experimental investigation of this report focuses on the design and construction of the modular cameras. Each modular camera is, in esscence, a small Anger camera. Components of each module include a scintillation crystal, a light guide, and an array of four photomultiplier tubes. Instead of an analog processing network, each module utilizes fast digital circuitry which includes direct analog-to-digital conversion of the photomultiplier signals, a lookup table which maps detector responses to position estimates of the scintillation flashes in the crystal, and an image memory which accumulates the position estimates and forms an image of the radiation incident on the faceplate of the camera. The digital electronics are necessary because analog techniques fail to give satisfactory estimates of scintillation position when the flashes occur near the sides of the crystal. The contents of the lookup table are determined from the statistical properties of the detected signals as a function of scintillation position. Experiments are described in which "best" estimates of position are found by processing data collected from an array of point-source positions in contact with the crystal. Alternative methods for construction of the lookup table are also discussed, which involve computer generation of the estimates. Both maximum-likelihood and mimimum-mean-square-error estimation rules are used, and the results are compared. A mathematical bound on the performance of the estimators is calculated assuming Poisson statistics for the detection process. The bound, which is a Cramer-Rao lower bound, is used to compare module geometries before lookup tables are constructed. A one-dimensional module, which accumulates information along one axis of the faceplate, is designed first. The one-dimensional module provides proof-of-principle evidence for the estimation techniques and is used to determine critical parameters for modular-camera design. The results of the experiments with the one-dimensional camera are extended to two-dimensional designs, which yield position estimates along both axes of the camera faceplate. Several two-dimensional cameras are tested, and an optimum geometry is constructed and tested for spatial resolution and bias of the estimators.

Worldwide measurement of radon flux on mine tailing dams has been performed using various instruments. Some of the methods used in South Africa are electrets, alpha tracks, accumulator cans etc. Although these techniques and methods have been used for many years, a number of shortcomings are still evident. The major shortcomings are that, the methods lack spatial representivity that is, they only measure the radon flux at a point where they' are placed and not the whole site in that way, the spatial variation is not shown in a site which is not homogeneous. Another shortcoming is that, they do not show seasonal variation and some have a back diffusion problem, and the time required for the result to be known is too long. For· example it takes several days for electrets to gather sufficient information required, yet it is a single point result. This makes it difficult to steer the measurement. Furthermore, the moisture and atmospheric pressure on the mine dump influence some of the measurements. The above shortcomings led to the investigation of a new· technique based on gamma ray spectrometry to quantitatively assess the radon flux from the mine tailings dam. The system is called Multi Element Detector for Underwater Sediment Activity (MEDUSA). Initially, this technique was uniquely designed to measure the radioactivity on the sea floor where it proved to be successful. The major focus of this research study was, therefore, to critically assess the MEDUSA gamma ray detector system for measurement of radon flux on a tailings dam. The process of determining the radon flux in this work involved field measurements using MEDUSA and laboratory measurements using Hyper Pure Germanium (HPGe} detector. The laboratory measurements were for correcting the field measurements. The HPGe has better advantage over MEDUSA in terms of resolution and this means that the gamma ray energy peaks have better visibility than on the MEDUSA. The field measurements on the tailings dam were done using the MEDUSA mounted on a 4x4 vehicle, and simultaneously sample points were identified and samples collected. The samples were measured for specific activity in the laboratory using gamma spectrometer with HPGe detector. These measurements enabled the determination of a factor that was used to calculate the activity of radium in the field. This activity was found to have an average of 309 Bq.kg"1 with data range of 60 -540 Bq.kg-1.A radon flux equation was then derived and used to calculate the radon flux on the field. Based on the radium content, the radon flux was calculated to average about 0.105 ± 0.023 Bq.m-2 .s-1 The results are within the same range as the previous flux measurement on the same tailings dam but with better statistics. This research work has demonstrated that the MEDUSA can be adapted for radon flux determination from tailings dam. The method promises to address some of the key shortcomings of existing techniques and the usefulness of this method can be extended to measuring radioactivity on contaminated sites for rehabilitation purposes. / Thesis (MSc. ARST) North-West University, Mafikeng Campus, 2003

Errata inserted inside back cover. / Bibliography: p. 155-162. / vii, 162 p. : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1994

This work aims to contribute to the study of the origins of cosmic rays, and broadly, to the advancement of both data analysis methods and instrumentation for very-high-energy γ-ray astronomy. First, reviewing the state of γ-ray astronomy, we show how gains in sensitivity can be achieved through sophisticated data analyses and improved instrumental designs. We then develop such an improved analysis method for the Very Energetic Radiation Imaging Telescope Array System (VERITAS) by combining Image Template Method (ITM) with Boosted Decision Trees (BDT), and study its performance, attaining a 30-50% improvement in integral sensitivity over the instrument’s standard analysis. Systematic issues in spectral reconstruction that the analysis displays are resolved satisfactorily by imposing a more stringent condition on the selection of its energy threshold. We employ the newly developed analysis to measure the γ-ray energy spectrum of the starburst galaxy M82, and combining our result with a measurement from the Fermi Large Area Telescope (Fermi-LAT), we find that a single power law fits the spectrum well between 100 GeV and 10 TeV, with no evidence for a spectral break or a cutoff. We conclude that this is in line with the current understanding that M82 is not a good proton calorimeter. Finally, we detail the design, implementation, and performance of the optical alignment system of the prototypeSchwarzschild-Couder Telescope (pSCT) for the Cherenkov Telescope Array (CTA), a novel two-mirror design that addresses many shortcomings of current instruments.

A new generation of medical grade Cf-252 sources was developed in 2002 at the Oak Ridge National Laboratory (ORNL). The combination of small size and large activity of these Cf-252 sources makes them suitable to be used with the conventional high-dose-rate (HDR) remote afterloading systems for interstitial brachytherapy. A recent in-water calibration experiment showed that the measured gamma dose rates near the new source are slightly greater than the neutron dose rates; contradicting the well established neutron-to-gamma dose ratio of approximately 2:1 at locations near a Cf-252 brachytherapy source. Specifically, the MCNP-predicted gamma dose rate is a factor of two higher than the measured gamma dose rate at the distance of 1 cm, and the differences between the two results gradually diminish at distances farther away from the source. To resolve this discrepancy, we updated the source gamma spectrum by including in the ORIGEN-S data library the experimentally measured Cf-252 prompt gamma spectrum as well as the true Cf-252 spontaneous fission yield data to explicitly model delayed gamma emissions from fission products. We also investigated the bremsstrahlung x-rays produced by the beta particles emitted from fission-product decays. The results show that the discrepancy of gamma dose rates is mainly caused by the omission of the bremsstrahlung x-rays in the MCNP runs. By including the bremsstrahlung x-rays, the MCNP results show that the gamma dose rates near a new Cf-252 source agree well with the measured results and that the gamma dose rates are indeed greater than the neutron dose rates. The calibration experiment also showed discrepancies between the experimental and computational neutron dose profiles obtained. Specifically the MCNP-predicted neutron dose rates were ~25% higher than the measured neutron dose rates at all distances. In attempting to resolve this discrepancy the neutron emission rate was verified by the National Institute of Standards and Technology (NIST) and an experiment was performed to explore the effects of bias voltage on ion chamber charge collection. So far the discrepancies between the computational and experimental neutron dose profiles have not been resolved. Further study is needed to completely resolve this issue and some suggestions on how to move forward are given.