This thesis presents MCNPX simulations of the SCANDAL set-up used at the Theodor Svedberg Laboratory for neutron scattering cross-section measurements. The thesis describes processes and data important for the upcoming off-line data analysis. In the experiment, neutrons scattered off the target are converted to protons which are stopped in scintillator crystals. The results of presented simulations include a description of the proton spectra in dependence of the neutron-to-proton conversion angle, calculation of the hit position gates and a study of the converter describing the role of its chemical composition and also the role of other plastic scintillator on the proton spectra. / QC 20100520

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in ful filment of the requirements for the degree of Master of Science in Engineering. Johannesburg, May 2018 / This work was aimed at the advancement of the Laser Shock Peening (LSP) process for aeronautical applications. This involved developing a robust strategy for residual stress measurements to various depths in LSP treated samples with different thicknesses and then to perform a parametric analysis of the LSP process. For the residual stress measurement part of the study, aluminium alloy 7075 samples with thicknesses of 10, 6, 3 and 1.6 mm were treated with LSP and the residual stresses were measured using various complementary techniques: Incremental Hole Drilling (IHD), Neutron Diffraction (ND), Synchrotron Energy-Dispersive X-Ray Diffraction (ED-XRD), Laboratory XRD (L-XRD) and Synchrotron Angle-Dispersive XRD (AD-XRD). The results compared favourably and it was concluded that to obtain a complete depth profile in the subsequent work, the residual stresses would need to be measured using at least three methods: one for near the surface, one at intermediate depths and one at greater depths. For the parametric analysis phase of the study, aluminium alloy 7075 samples with thicknesses of 10 and 1.6 mm were treated with LSP; the following LSP parameters were varied: Power Intensity (PI), Spot Size (SS) and %Overlap. The residual stresses were measured using L-XRD, IHD and ED-XRD. In addition to this, the following were investigated: the sample deformation, the surface integrity, the microhardness, and the microstructure under a Scanning Electron Microscope (SEM). A clear trend in the residual stress depth profile as well as the additional characterisations was observed as the parameters varied. This work will form part of a database of LSP results for various alloys that can be used for engineering residual stress profiles using optimal parameter selection for specific industrial applications and as benchmark for the development of LSP Finite Element tools. / MT 2018

First we shall consider the rare-earth titanate Yb2Ti2O7 which has been extensively studied within the past two decades but is still not very well understood. There has been much debate over the nature of the magnetic ground state of this system with studies reporting the development of long-range order below 250mK to the absence of long-range order below 250mK. One culprit suspected of generating these differences in behaviour of the ground state is the effect of crystal growth on sample quality. In order to address this discrepancy, measurements of the crystal-field levels of Yb2Ti2O7 were conducted on two powder samples of different composition since the exact ground state selection depends on size and anisotropy of the magnetic moment. The second focus of this thesis concerns the family of rock-salt ordered double perovskites of the form Ba2YXO6 in which X is either a 4d or 5d magnetic ion. The interplay between the strong spin orbit coupling expected for such heavy magnetic ions and frustration has been theoretically predicted to exhibit a variety of exotic phenomena. Neutron scattering is utilized to investigate the low temperature dynamics of the system Ba2YOsO6 to compare with that observed in the 4d analog, Ba2YRuO6. / Thesis / Master of Science (MSc)

β decay studies of nuclei at the limits of stability are essential in evaluating the physical aspects behind the structural changes, particle configurations and interactions in neutronor proton-rich systems. Isobarically purified beams were used at the Holifeld Radioactive Ion Beam Facility at Oak Ridge National Laboratory to study the β decays of 75Cu and 77Cu. Two different experiments were performed. In the first study, only concernig the decay of 77Cu, the 25-MV tandem accelerated ions were time-tagged using a micro-channel plate detector, passed through a six-segment ion chamber, and implanted on the tape of a moving tape collector. The passage through an ion chamber insured the ion identification by energy loss in the six segments. The Low Energy Radioactive Ion Beam Spectroscopy Station consisting of a universal detector support with four Ge clover detectors, two â detectors and a moving tape collector, was used in the second experiment. Bypassing of the tandem accelerator gave a factor of 10 gain in beam intensity for both 75Cu and 77Cu. These experiments resulted in considerable information on the previously unknown level structure of 75Zn with some 120 γ-ray transitions placed in a level scheme containing 59 levels including two above the neutron separation energy. We have also identified the previously unknown 1/2− isomeric state at 127 keV. A total of 64 γ rays were placed in a level scheme for 77Zn containing 35 excited states including one state above the neutron separation energy, while two γ rays were observed for the βn branch to states in 76Zn. The growth and decay curves of some prominent γ rays indicate a single β-decaying state with a half-life of 480(9) ms. The decay pattern for 77Cu, with observed feeding of 8(3)% to the 7/2+ 77Zng and 6(3)% to the 1/2− 77Znm, in contrast to the large feeding observed for decay of πp3/2 73Cug to 1/2− 73Zng, strongly suggests a πf5/2 ground state for the studied 77Cu activity. Results will be presented and the prospects for future possible studies will also be discussed.

No description available.

Neutron bombardment of bipolar transistors creates cluster defects in semiconductor material. The clusters are small volumes of semiconductor material containing several hundred atoms displaced from their proper lattice sites owing to collision processes. They act as recombination centers in transistor bases, reducing minority carrier lifetime and consequently reducing transistor current gain. The damage is permanent to the semiconductor device and can only be corrected by thermally annealing the transistor. Copious test data available on bipolar transistors d.c. gain (hFE) response to incident fast neutron fluences confirm their mathematically derivable functional relationships. This report develops a neutron fluence detector system based upon the current gain (hFE) degradation. An approximate model extending these results to include the effects of temperature is developed. A probe containing an npn silicon planar transistor with associated components to allow hFE measurements is designed. A thermal sensor is also designed. More precise neutron data is obtained by correcting for d.c. current gain versus ambient temperature error. The design of the probe is the major contribution in this report. In addition to the computer simulation of the probe model a system architecture and implementation is presented. The detector system is comprised of the probe and associated data acquisition I/O circuitry. A microcomputer processes the probe data to calculate the neutron fluence received.

No description available.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / A novel design of neutron sensor was investigated and developed. The perforated, or micro-structured, diode neutron sensor is a concept that has the potential to enhance neutron sensitivity of a common solid-state sensor configuration. The common thin-film coated diode neutron sensor is the only semiconductor-based neutron sensor that has proven feasible for commercial use. However, the thin-film coating restricts neutron counting efficiency and severely limits the usefulness of the sensor. This research has shown that the perforated design, when properly implemented, can increase the neutron counting efficiency by greater than a factor of 4. Methods developed in this work enable detectors to be fabricated to meet needs such as miniaturization, portability, ruggedness, and adaptability. The new detectors may be used for unique applications such as neutron imaging or the search for special nuclear materials. The research and developments described in the work include the successful fabrication of variant perforated diode neutron detector designs, general explanations of fundamental radiation detector design (with added focus on neutron detection and compactness), as well as descriptive theory and sensor design modeling useful in predicting performance of these unique solid-state radiation sensors. Several aspects in design, fabrication, and operational performance have been considered and tested including neutron counting efficiency, gamma-ray response, perforation shapes and depths, and silicon processing variations. Finally, the successfully proven technology was applied to a 1-dimensional neutron sensor array system.

Measurements of neutron emission from a fusion plasma can provide a wealth of information on the underlying temporal, spatial and energy distributions of reacting ions and how they are affected by a wide range of magneto-hydro-dynamic (MHD) instabilities. This thesis focuses on the interpretation of the experimental measurements recorded by neutron flux monitors with and without spectroscopic capabilities installed on the Mega Ampere Spherical Tokamak (MAST). In particular, the temporally and spatially resolved measurements of the neutron rate measured by the neutron camera, which also possesses spectroscopic capabilities, are combined with the temporally resolved measurements of the total neutron rate provided by the absolutely calibrated fission chamber in order to study the properties of the fast ion distributions in different plasma scenarios. The first part of the thesis describes in detail the two forward modelling methods, which employ the set of interconnected codes developed to interpret experimental observations such as neutron count rate profiles and recoil proton pulse height spectra provided by the neutron camera. In the second part of the thesis the developed methods are applied to model the neutron camera observations performed in a variety of plasma scenarios. The first method, which involves only TRANSP/NUBEAM and LINE2 codes, was used to validate the neutron count rate profiles measured by the neutron camera in three different plasma scenarios covering the wide range of total neutron rate typically observed on MAST. In addition, the first framework was applied to model the changes in the total and local neutron rates caused by fishbone instability as well as to estimate the Hydrogen and Deuterium ion ratio. The second modelling method, which involves TRANSP/NUBEAM, LINE2, DRESS and NRESP, was used to validate the measured recoil proton pulse height spectra in a MHD-quiescent plasma scenario.

A fast pneumatic transfer facility was installed at the Nuclear Engineering Teaching Laboratory (NETL) of the University of Texas at Austin for the purpose of cyclic thermal and epithermal neutron activation analysis. In this work efforts were focused on the evaluation of cyclic epithermal neutron activation analysis (CENAA). Various NIST and CANMET certified materials were analyzed by the system. Experiment results showed 110Ag as one of the isotopes favored by the system. Thus, the system was put into practical application for identifying silver concentration in the Arctic atmospheres in air filters collected in 2009, and traces in metallic ores Comparison of silver concentrations via CENAA with the CANMET certified reference materials gave very good results. / text