R. Botha - Academia.edu (original) (raw)
Papers by R. Botha
Procedia Earth and Planetary Science, 2015
Proposed shale-gas exploration in the semi-arid Karoo, South Africa, has created the need for the... more Proposed shale-gas exploration in the semi-arid Karoo, South Africa, has created the need for the development of a robust geochemical baseline, differentiating deep and shallow groundwaters. Shallow groundwater is the main source of potable water in the Karoo, and the possibility of upwards migration of poorer quality deep groundwaters is a cause for concern. Radon concentrations of nineteen groundwater samples, from eight locations in the Karoo Basin, were determined in summer and winter. Sub-thermal waters were used as an initial proxy to define deep groundwater. Radon concentrations of < 100 Bq/L were recorded for most sites, with higher concentrations in shallow groundwaters. Seasonal and geographical variations in radon provide insight into the processes controlling radon concentrations in the Karoo groundwater. The presence of uranium and radium nuclides (possibly originating in the Karoo Uranium Province), and the presence of dolerite intrusions are important for controlling radon concentrations. Negligible uranium in deep groundwater was associated with anoxic, low alkalinity, high pH, old sub-thermal waters. Discrepancies between radon concentration and chemistry of groundwater sources defined as shallow is attested to the short half-life of 222 Rn, which records only the last stage of the water's history. Elevated radon may be linked to seismic activity, a cause for concern considering the deep formation micro-fractures associated with hydraulic fracturing.
Acta Geodaetica et Geophysica, 2015
The Hartebeesthoek Radio Astronomy Observatory of South Africa is currently developing a lunar la... more The Hartebeesthoek Radio Astronomy Observatory of South Africa is currently developing a lunar laser ranger (LLR) system based on a one metre aperture telescope in collaboration with National Aeronautics and Space Administration and the Observatoire de la Côte d'Azur. This LLR will be an addition to a limited list of operating LLR stations globally and it is expected to achieve sub-centimetre range precision to the Moon. Key to this expectation including the overall telescope operational performance is thermal analysis of the telescope structure, based on the thermal properties of component materials and their interaction with the environment through conventional heat transfer mechanisms. This paper presents transient thermal simulation results of the telescope's optical tube and one metre primary mirror in terms of thermal variations and consequent structural deformations. The results indicate that on a non-windy, cloud-free and winter day, the temperature gradients on the structure could be within 1°C with respect to the temporal ambient air temperatures at the site when these are between 9 and 23°C. Furthermore, these gradients were coupled with thermally-induced total deformations that vary between 2.9 and 40.7 lm of the assembled telescope components. In overall, these findings suggest that both the tube and especially the mirror may respond very slowly to ambient temperatures; however, correcting for structural thermal variations is imperative in maximizing the pointing accuracy of the telescope thereby increasing the chance being on-target with the retroreflectors located on the Moon surface.
South African Journal of Geology, 2011
A Lunar Laser Ranging (LLR) system is to form part of geodetic instrumentation to be located at a... more A Lunar Laser Ranging (LLR) system is to form part of geodetic instrumentation to be located at a new fundamental space geodetic observatory for South Africa. For optimal efficiency, LLR requires optical resolution or so-called astronomicai seeing conditions of -1 arc-second in order to deliver usable ranging data. Site characterisation should include a description of astronomical seeing for various locations on-site and overall atmospheric conditions. Atmospheric rurbulence degrades astronomical seeing. In-siru methods of determining astronomical seeing are difficult, time-consuming and costly. \7e propose the use of a turbulence-resolving model to determine and predict astronomical seeing at a site. Large Eddy Simulation NERSC (Nansen Environmental and Remote Sensing Centre) Improved Code (LESNIC) is a turbulence-resolving simulation code which models atmospheric turbulence. It has been used to compile a database of turbulence-resolving simulations, referred to as DAfABASE64. This database consists of a collection of LESNIC runs for a stably stratified planetary boundary layer (SBL) over a homogeneous aerodynamically rough surface. Results from DATABASE64 for the nocturnal boundary layer are employed to render profiles of the vertical distribution of optical turbulence (Cniprofiles). Seeing parameter values are aiso obtained by making use of DAIABASE64 results. The 6112 profiles and seeing parameter values obtained from DATABASE64 results are compared with general observational results that have been published in the literature. The values obtained are consistent with results from fieid campaigns as reported. Turbulence-resolving models, such as LESNIC, show potential for delivering and predicting profiles and parameters to characterise astronomical seeing, which are essential prerequisites for establishing an LLR system at the most suitable site and most suitable on-site location. A two-pronged approach is envisagedin addition to modelling, quantitative seeing measurements obtained with an on-site seeing monitor will be used to veri$' and calibrate results produced by the LESNIC model.
physica status solidi (c), 2008
ABSTRACT In this work, hydrogenated silicon nitride (SiNx:Hy) grown by chemical vapour deposition... more ABSTRACT In this work, hydrogenated silicon nitride (SiNx:Hy) grown by chemical vapour deposition as passivant layers for high electron mobility transistors (HEMT) have been studied. The film composition and bonding structure were determined by ion beam analysis and x-ray absorption spectroscopy techniques, respectively. The effects of gas precursors (SiH4/N2 and SiH4/NH3) and film/substrate interface on the film growth have been addressed. The growth on different substrates (Si, GaN, AlGaN), and the effects of plasma pre-treatments have been studied before the growth and the film growth evolution. Results yield no significant differences in all the analysed samples. This points out the relevant role of SiHn radicals as growth precursor species and that intrinsic characteristics of the SiNx:Hy layers are not affected by the film/substrate interface. Hence, improved performance of HEMT with surface plasma pre-treatments before passivation should be related to extrinsic mechanisms (such as creation of defects in AlGaN surface, removal of the surface contamination or ion-induced roughness). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c), 2008
ABSTRACT
physica status solidi (c), 2008
physica status solidi (c), 2008
Optics Letters, 2013
A 1314 nm Nd:YLF laser was designed and operated both CW and passively Q-switched. Maximum CW out... more A 1314 nm Nd:YLF laser was designed and operated both CW and passively Q-switched. Maximum CW output of 10.4 W resulted from 45.2 W of incident pump power. Passive Q-switching was obtained by inserting a V:YAG saturable absorber in the cavity. The oscillator delivered a maximum of 825 μJ energy per pulse, with a pulse duration of 135 ns at a pulse repetition frequency of 6.3 kHz, effectively delivering 5.2 W of average power.
Monitoring and managing air quality is an important activity globally. This is because polluted a... more Monitoring and managing air quality is an important activity globally. This is because polluted air can be harmful to human health and other parts of ecosystems. Many studies of air pollution have been done internationally, including South Africa. However, our literature survey shows that most of South African air pollution studies have been based on measuring the particulate matter (PM) levels and concentrations of “classical” pollutants like carbon dioxide (CO2), carbon monoxide (CO), ozone (O3), benzene (C6H6) and different compounds of nitrogen (N) and sulfur (S). These measurements are traditionally made by pumping air through filters and subsequently analyzing the filters or residual air. According to [1], some chemical elements are also known to have an adverse effect on living organisms and the worst toxicants are believed to be heavy metals. Biological monitoring with mosses and lichens is a low-cost, effective method to determine trace and other elements in the atmosphere....
Procedia Earth and Planetary Science, 2015
Proposed shale-gas exploration in the semi-arid Karoo, South Africa, has created the need for the... more Proposed shale-gas exploration in the semi-arid Karoo, South Africa, has created the need for the development of a robust geochemical baseline, differentiating deep and shallow groundwaters. Shallow groundwater is the main source of potable water in the Karoo, and the possibility of upwards migration of poorer quality deep groundwaters is a cause for concern. Radon concentrations of nineteen groundwater samples, from eight locations in the Karoo Basin, were determined in summer and winter. Sub-thermal waters were used as an initial proxy to define deep groundwater. Radon concentrations of < 100 Bq/L were recorded for most sites, with higher concentrations in shallow groundwaters. Seasonal and geographical variations in radon provide insight into the processes controlling radon concentrations in the Karoo groundwater. The presence of uranium and radium nuclides (possibly originating in the Karoo Uranium Province), and the presence of dolerite intrusions are important for controlling radon concentrations. Negligible uranium in deep groundwater was associated with anoxic, low alkalinity, high pH, old sub-thermal waters. Discrepancies between radon concentration and chemistry of groundwater sources defined as shallow is attested to the short half-life of 222 Rn, which records only the last stage of the water's history. Elevated radon may be linked to seismic activity, a cause for concern considering the deep formation micro-fractures associated with hydraulic fracturing.
Acta Geodaetica et Geophysica, 2015
The Hartebeesthoek Radio Astronomy Observatory of South Africa is currently developing a lunar la... more The Hartebeesthoek Radio Astronomy Observatory of South Africa is currently developing a lunar laser ranger (LLR) system based on a one metre aperture telescope in collaboration with National Aeronautics and Space Administration and the Observatoire de la Côte d'Azur. This LLR will be an addition to a limited list of operating LLR stations globally and it is expected to achieve sub-centimetre range precision to the Moon. Key to this expectation including the overall telescope operational performance is thermal analysis of the telescope structure, based on the thermal properties of component materials and their interaction with the environment through conventional heat transfer mechanisms. This paper presents transient thermal simulation results of the telescope's optical tube and one metre primary mirror in terms of thermal variations and consequent structural deformations. The results indicate that on a non-windy, cloud-free and winter day, the temperature gradients on the structure could be within 1°C with respect to the temporal ambient air temperatures at the site when these are between 9 and 23°C. Furthermore, these gradients were coupled with thermally-induced total deformations that vary between 2.9 and 40.7 lm of the assembled telescope components. In overall, these findings suggest that both the tube and especially the mirror may respond very slowly to ambient temperatures; however, correcting for structural thermal variations is imperative in maximizing the pointing accuracy of the telescope thereby increasing the chance being on-target with the retroreflectors located on the Moon surface.
South African Journal of Geology, 2011
A Lunar Laser Ranging (LLR) system is to form part of geodetic instrumentation to be located at a... more A Lunar Laser Ranging (LLR) system is to form part of geodetic instrumentation to be located at a new fundamental space geodetic observatory for South Africa. For optimal efficiency, LLR requires optical resolution or so-called astronomicai seeing conditions of -1 arc-second in order to deliver usable ranging data. Site characterisation should include a description of astronomical seeing for various locations on-site and overall atmospheric conditions. Atmospheric rurbulence degrades astronomical seeing. In-siru methods of determining astronomical seeing are difficult, time-consuming and costly. \7e propose the use of a turbulence-resolving model to determine and predict astronomical seeing at a site. Large Eddy Simulation NERSC (Nansen Environmental and Remote Sensing Centre) Improved Code (LESNIC) is a turbulence-resolving simulation code which models atmospheric turbulence. It has been used to compile a database of turbulence-resolving simulations, referred to as DAfABASE64. This database consists of a collection of LESNIC runs for a stably stratified planetary boundary layer (SBL) over a homogeneous aerodynamically rough surface. Results from DATABASE64 for the nocturnal boundary layer are employed to render profiles of the vertical distribution of optical turbulence (Cniprofiles). Seeing parameter values are aiso obtained by making use of DAIABASE64 results. The 6112 profiles and seeing parameter values obtained from DATABASE64 results are compared with general observational results that have been published in the literature. The values obtained are consistent with results from fieid campaigns as reported. Turbulence-resolving models, such as LESNIC, show potential for delivering and predicting profiles and parameters to characterise astronomical seeing, which are essential prerequisites for establishing an LLR system at the most suitable site and most suitable on-site location. A two-pronged approach is envisagedin addition to modelling, quantitative seeing measurements obtained with an on-site seeing monitor will be used to veri$' and calibrate results produced by the LESNIC model.
physica status solidi (c), 2008
ABSTRACT In this work, hydrogenated silicon nitride (SiNx:Hy) grown by chemical vapour deposition... more ABSTRACT In this work, hydrogenated silicon nitride (SiNx:Hy) grown by chemical vapour deposition as passivant layers for high electron mobility transistors (HEMT) have been studied. The film composition and bonding structure were determined by ion beam analysis and x-ray absorption spectroscopy techniques, respectively. The effects of gas precursors (SiH4/N2 and SiH4/NH3) and film/substrate interface on the film growth have been addressed. The growth on different substrates (Si, GaN, AlGaN), and the effects of plasma pre-treatments have been studied before the growth and the film growth evolution. Results yield no significant differences in all the analysed samples. This points out the relevant role of SiHn radicals as growth precursor species and that intrinsic characteristics of the SiNx:Hy layers are not affected by the film/substrate interface. Hence, improved performance of HEMT with surface plasma pre-treatments before passivation should be related to extrinsic mechanisms (such as creation of defects in AlGaN surface, removal of the surface contamination or ion-induced roughness). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c), 2008
ABSTRACT
physica status solidi (c), 2008
physica status solidi (c), 2008
Optics Letters, 2013
A 1314 nm Nd:YLF laser was designed and operated both CW and passively Q-switched. Maximum CW out... more A 1314 nm Nd:YLF laser was designed and operated both CW and passively Q-switched. Maximum CW output of 10.4 W resulted from 45.2 W of incident pump power. Passive Q-switching was obtained by inserting a V:YAG saturable absorber in the cavity. The oscillator delivered a maximum of 825 μJ energy per pulse, with a pulse duration of 135 ns at a pulse repetition frequency of 6.3 kHz, effectively delivering 5.2 W of average power.
Monitoring and managing air quality is an important activity globally. This is because polluted a... more Monitoring and managing air quality is an important activity globally. This is because polluted air can be harmful to human health and other parts of ecosystems. Many studies of air pollution have been done internationally, including South Africa. However, our literature survey shows that most of South African air pollution studies have been based on measuring the particulate matter (PM) levels and concentrations of “classical” pollutants like carbon dioxide (CO2), carbon monoxide (CO), ozone (O3), benzene (C6H6) and different compounds of nitrogen (N) and sulfur (S). These measurements are traditionally made by pumping air through filters and subsequently analyzing the filters or residual air. According to [1], some chemical elements are also known to have an adverse effect on living organisms and the worst toxicants are believed to be heavy metals. Biological monitoring with mosses and lichens is a low-cost, effective method to determine trace and other elements in the atmosphere....