A study has been made of the external conversion spectrum of gamma-rays of less than 200 kev. A set of semi-empirical curves relating the photoelectric peak height to the gamma-ray intensity has been established using antimony radiators. These results will be discussed. A method of preparing thin beta sources will also be described. / Thesis / Master of Science (MSc)

In this thesis I describe the development of a compact camera for ground-based multi TeV gamma-ray astronomy, using the Imaging Atmospheric Cherenkov Telescope (IACT) technique. The camera is based on multi-anode photomultipliers (MAPM) and is designed for use on the Gamma Cherenkov Telescope (GCT), which is proposed to be part of the Small Size Telescope (SST) array of the Cherenkov Telescope Array (CTA). GCT achieves high performance with a compact and cost efficient design via a Schwarzschild-Couder (SC) dual-mirror optical system. The GCT optical design allows the use of a compact camera of diameter roughly 0.5 m. The curved focal plane is equipped with 32 tiles of 64-pixels MAPM for a total of 2048 pixels of ~0.2° angular size, resulting in a field of view of ~9°. The GCT camera is designed to record the flashes of Cherenkov light from electromagnetic cascades, which last only a few tens of nanoseconds. I give a detailed description of the design, the challenges encountered during testing in the lab, and the performance of the most critical components. I give details on the custom front-end electronics modules that provide the required fast electronics, facilitating sampling and digitization, as well as first level of triggering. The camera-level triggering system is a custom backplane, developed to reject spurious triggers on the night sky background, which typically is of the order of few tens of millions of photons per pixel per second. This is to be compared with the rate of the astrophysical signal, which is of the order of few hundreds of events per second at the relevant energies. Additionally I provide a detailed description of all the software needed for the data acquisition and control of the camera, from the very low level drivers to high level and user friendly processes. I follow the commissioning of the camera, from the individual core components to the integration of the system. I then describe the integration of the camera on the GCT prototype telescope structure, and the achievement of "first light", validating for the first time the full proof-of-concept of an IACT with SC optics. I also report a study I performed on expectations for an extragalactic survey for blazars with CTA. The cumulative source count distribution of blazars is presented, including implications from two different phenomena: axion-like particle (ALP) to gamma-ray oscillations in the intergalactic magnetic field, and secondary gamma rays from hadronic origins. I conclude that a shallow and wide survey will provide the best science return for CTA, that the impact of ALP is modest and that the secondary mechanism of gamma-ray production would allow detection of blazars up to redshift of 1 in the multi-TeV energy band.

Call number: LD2668 .T4 1956 B87 / Master of Science

Call number: LD2668 .T4 1958 T75

Call number: LD2668 .T4 1964 D75 / Master of Science

Black hole-neutron star (BH-NS) mergers are promising candidates for the progenitors of short-duration gamma-ray bursts (GRBs). With the right initial conditions, the neutron star becomes tidally disrupted, eventually forming a dense, accreting disk around the black hole. The thermal energy of this black hole-disk system can be extracted via neutrino processes, while the spin energy of the black hole can be extracted via magnetic processes. Either (or even a combination of these) processes could feasibly power a relativistic jet with energy ≥~ 10 49 erg and duration ≤~ 2 s, hence producing a short-duration GRB. In this thesis, we investigate BH-NS mergers with three-dimensional, pseudo-Newtonian simulations. We use the simulation code Charybdis, which uses a dimensionally-split, reconstruct-solve-average scheme (i.e. using Riemann solvers) to solve the Euler equations of hydrodynamics. Although the code is based on a Newtonian framework, it includes pseudo- Newtonian approximations of local gravitational wave effects and the innermost stable circular orbit of the BH, which are both general relativistic phenomena. The code also includes the effects of global neutrino emission, shear viscosity and self-gravity. This thesis comprises two main projects. The first project is a parameter study of the equation of state, which encapsulates the relationship between the pressure of a fluid and its other thermodynamic properties. Although the EOS is well understood at low densities, it is yet to be constrained at supranuclear densities, and so must be treated as a parameter in numerical studies of BH-NS mergers. We present simulations using three existing EOSs, in order to investigate their effect on the merger dynamics. We find that the EOS strongly influences the fate of the NS, the properties of the accretion disk, and the neutrino emission. In the second project, we begin upgrading Charybdis to include magnetic field effects, in order to investigate the magnetic processes described above. We implement existing reconstruction and Riemann solver algorithms for the equations of magnetohydrodynamics, and present 1D tests to compare them. When modelling magnetic fields in more than one dimension, we must also deal with the divergence-free condition, ∇. B = 0. We develop a new constrained transport algorithm to ensure our code maintains this condition, and present 2D tests to confirm its accuracy. This algorithm has many advantages over existing ones, including easier implementation, greater computational efficiency and better parallelisation. Finally, we present preliminary tests that use these algorithms in simulations of BH-NS mergers.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The coincident measurement of gamma-rays originating from nuclei excited in light-ion reactions and scattered alpha-particles presents interesting opportunities to characterize the decay path for low multi-polarity nuclear excitations. This coincident measurement at θlab = 0◦ was shown to be a powerful tool for the spectroscopy of isoscalar E1 transitions, being selective to distinct modes of bulk nuclear motions. Aided with the high resolution capabilities provided by the HPGe Clover detectors as well as the K600 magnetic spectrometer for both γ and charged particle detection, these experiments are motivated by the need to investigate E1 strength in stable nuclei around the particle emission threshold, more commonly referred to as the Pygmy Dipole Resonance (PDR). However, due to our detector configuration we find that higher effi- ciency detectors offer a better overall functionality in these α − γ coincidence measurements which specifically suffer from low statistics. The feasibility of the (α, α 0γ) reaction using high resolution Clover detectors and the high efficiency HAGAR detector is characterized through their effi- ciency to detect γ rays in coincidence with the K600 magnetic spectrometer at θlab = 0◦ . Through the construction of Eγ − EX coincidence matrices, decay transitions of closely lying nuclear excitations can be studied on a stateby-state basis. The result is a quantified representation of γ yield, of which optimization is the key focus in order to extract meaningful nuclear structure information. / AFRIKAANSE OPSOMMING: Die samevallende meting van gammastrale van kerne opgewek in die lig-ion reaksies en verspreide alfa-deeljties, bied interessante geleenthede aan om die verval pad vir lae multi-polariteit kern opwekkings te karakteriseer. Hierdie samevallende meting by θlab = 0◦ was getoon om ‘n kragtige instrument vir die spektroskopie van isoscalar E1 oorgange te wees, as gevolg van die vermoë om selektief te onderskei tussen modes van kollektiewe kern ossillasies. Aangehelp met die hoë resolusie moontlikhede wat deur die HPGe Clover detektors asook die K600 magnetiese spektrometer vir beide γ en gelaaide deeltjie opsporing, hierdie eksperimente is gemotiveer deur die behoefte om E1 krag te ondersoek in stabiele kerne rondom die deeltjie emissie drumpel, meer algemeen na verwys as die Pygmy Dipool Resonansie (PDR). Maar as gevolg van die detektor opset, vind ons dat ’n hoër doeltreffendheid detector ’n beter algehele funksionaliteit aanbied wat in hierdie samevallende metings veral belangrik is weens lae statistiek. Die haalbaarheid van die reaksie wat gebruik maak van hoë resolusie Clover detektors en die hoë doeltreffendheid HAGAR detektor word gekenmerk deur hul doeltreffendheid om gammastrale optespoor in toeval met die K600 magnetiese spektrometer by θlab = 0◦ . Deur die konstruksie van toeval Eγ − EX matrikse, verval oorgange van kern opwekkings kan op ’n staat-by-staat basis bestudeer word. Die resultaat is ’n gekwantifiseerde verteenwoordiging van γ opbrengs, waarvan optimalisering die sleutel fokus is om betekenisvolle kernstruktuur inligting te kan onttrek

The purpose of this research is to determine the feasibility of creating an affordable and durable neutron detector for usage in the field surveys, site inspections, and transportation hub monitoring. Currently, organic scintillating detectors are an established method of detecting neutrons but are either costly, fragile solids like stilbene, or flammable liquids like benzene. In this work, several scintillation mixtures were tested with a PuBe source, which emits both neutrons and gamma rays. The pulse shape discrimination method was utilized to separate the signal pulses created from the mixed radiation field of the PuBe source. Two candidate mixtures were selected for solidification with elastomers for their verified neutron detection capabilities. The solid detectors measured high energy neutrons and gamma rays from the PuBe source. The solidified detectors have a Figure of Merit for separating neutrons of 0.859 ±0.419 and cost $0.13 per gram, while commercially available stilbene separates neutrons from gammas with a Figure of Merit of 4.70 and costs $64.36 per gram. This research shows that it is feasible to create affordable solid organic scintillators sensitive to high energy neutrons.

samples were irradiated for approximately 45 minutes to allow for the saturation of fission products. The first method used the beam port shutter and allowed for longer counting and irradiation times, but was unsuitable for examining fission products with half-lives below 10 seconds. The on/off method used a cycle of equal irradiation and counting times of one minute. The second method is able to measure track the production of fission products with half-lives of less than 10 seconds. This method used a borated aluminum wheel beam chopper to stop the irradiation of the sample during counting. The beam chopper was set to cycle for approximately one second of counting following half a second of irradiation. The spectra from both methods were analyzed and the peaks were assigned to the appropriate fission products. The majority of the peaks were composed of gamma-rays from multiple nuclides. The peaks created by gamma-rays from decays of a single nuclide were used to calculate the detection limits of the system. Using the beam chopper system, 21 peaks would be above the detection limits of our system 95% of the time for uranium samples of less than one kilogram. / text

Gamma-ray bursts (GRBs) are the mysterious, short and intense flashes of gamma-rays in the space, and are believed to originate from the rare, explosively devastating, stellar events that happens at cosmological distances. Enormous progress has been made from four decades of GRB research endeavor but the ultimate understanding of their origins has yet to arrive. Recently revealed features in their early afterglows broadened the opportunity space for exploration. We have carried out extensive studies on various physical processes in GRBs. We showed that the distribution of electrons' energy spectral index in GRBs and other relativistic sources is inconsistent with the prediction from the first-order Fermi theory of the shock particle acceleration. We investigated the photon scattering processes within the relativistic outflow that produces the GRB and calculated the resultant emission flux from it. We showed the scattering of the GRB prompt photons by the circum-burst dust, although an attractive possibility, can not explain the puzzling plateau component in the GRB afterglow light curve. We made meaningful constraint on the GRB prompt emission radius, R [greater-than or equal to] 10¹⁴, by studying the synchrotron self absorption for a small sample of bursts with good data. We showed that a late jet, which is thought to be producing the late X-ray flares in GRB afterglows, will produce detectable emissions from its interactions with other components in the explosive event of GRB, and identification of these emissions could verify the existence of the late jet and further prove the massive star origin of long-duration GRBs. / text