A 150 MHz all sky survey with the Precision Array to Probe the Epoch of Reionization
- Authors: Chege, James Kariuki
- Date: 2020
- Subjects: Epoch of reionization -- Research , Astronomy -- Observations , Radio interferometers
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/117733 , vital:34556
- Description: The Precision Array to Probe the Epoch of Reionization (PAPER) was built to measure the redshifted 21 cm line of hydrogen from cosmic reionization. Such low frequency observations promise to be the best means of understanding the cosmic dawn; when the first galaxies in the universe formed, and also the Epoch of Reionization; when the intergalactic medium changed from neutral to ionized. The major challenges to these observations is the presence of astrophysical foregrounds that are much brighter than the cosmological signal. Here, I present an all-sky survey at 150 MHz obtained from the analysis of 300 hours of PAPER observations. Particular focus is given to the calibration and imaging techniques that need to deal with the wide field of view of a non-tracking instrument. The survey covers ~ 7000 square degrees of the southern sky. From a sky area of 4400 square degrees out of the total survey area, I extract a catalogue of sources brighter than 4 Jy whose accuracy was tested against the published GLEAM catalogue, leading to a fractional difference rms better than 20%. The catalogue provides an all-sky accurate model of the extragalactic foreground to be used for the calibration of future Epoch of Reionization observations and to be subtracted from the PAPER observations themselves in order to mitigate the foreground contamination.
- Full Text:
- Date Issued: 2020
The variability and predictability of the IRI shape parameters over Grahamstown, South Africa
- Authors: Chimidza, Oyapo
- Date: 2008
- Subjects: Ionosphere -- Mathematical models -- South Africa , Atmosphere, Upper -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5496 , http://hdl.handle.net/10962/d1005282 , Ionosphere -- Mathematical models -- South Africa , Atmosphere, Upper -- South Africa
- Description: The International Reference Ionosphere (IRI) shape parameters B0, B1, and D1 provide a representation of the shape of the F2 layer, the thickness of the F2 layer and the shape of the F1 layer of the ionosphere respectively. The aim of this study was to examine the variability of these parameters using Grahamstown, South Africa (33.3±S, 26.5±E) ionosonde data and determine their predictability by the IRI-2001 model. A further aim of this study was to investigate developing an alternative model for predicting these parameters. These parameters can be determined from electron density profiles that are inverted from ionograms recorded with an ionosonde. Data representing the B0, B1 and D1 parameters, with half hourly or hourly intervals, were scaled and deduced from the digital pulse sounder (DPS) ionosonde for the period April 1996 to December 2006. An analysis of the diurnal, seasonal, and solar variations of the behaviour of these parameters was undertaken for the years 2000, 2004 and 2005 using monthly medians. Comparisons between the observational results and that of the IRI model (IRI 2001 version) indicate that the IRI-2001 model does not accurately represent the diurnal and seasonal variation of the parameters. A preliminary model was thus developed using the technique of Neural Networks (NNs). All available data from the Grahamstown ionosonde from 1996 to 2006 were used in the training of the NNs and the prediction of the variation of the shape parameters. Inputs to the model were the day number, the hour of day, the solar activity and the magnetic index. Comparisons between the preliminary NN model and the IRI-2001 model indicated that the preliminary model was more accurate at the prediction of the parameters than the IRI-2001 model. This analysis showed the need to improve the existing IRI model or develop a new model for the South African region. This thesis describes the results from this feasibility study which show the variability and predictability of the IRI shape parameters.
- Full Text:
- Date Issued: 2008
A study of why some physic concepts in the South African Physical Science curriculum are poorly understood in order to develop a targeted action-research intervention for Newton’s second law
- Authors: Cobbing, Kathleen Margaret
- Date: 2020
- Subjects: Physics -- Study and teaching (Secondary) -- South Africa , Physics -- Examinations, questions, etc. -- South Africa , Motion -- Study and teaching (Secondary) -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/146903 , vital:38575
- Description: Globally, many students show a poor understanding of concepts in high school physics and lack the necessary problem-solving skills that the course demands. The application of Newton’s second law was found to be particularly problematic through document analysis of South African examination feedback reports, as well as from an analysis of the physics examinations at a pair of well-resourced South African independent schools that follow the Independent Examination Board curriculum. Through an action-research approach, a resource for use by students was designed and modified to improve students’ understanding of this concept, while modelling problemsolving methods. The resource consisted of brief revision notes, worked examples and scaffolded exercises. The design of the resource was influenced by the theory of cognitive apprenticeship, cognitive load theory and conceptual change theory. One of the aims of the resource was to encourage students to translate between the different representations of a problem situation: symbolic, abstract, model and concrete. The impact of this resource was evaluated at a pair of schools using a mixed methods approach. This incorporated pre- and post-tests for a quantitative assessment, qualitative student evaluations and the analysis of examination scripts. There was an improvement from pre- to post-test for all four iterations of the intervention and these improvements were shown to be significant. The use of the resource led to an increase in the quality and quantity of diagrams drawn by students in subsequent assessments.
- Full Text:
- Date Issued: 2020
A feasibility study into the possibility of ionospheric propagation of low VHF (30-35 MHZ) signals between South Africa and Central Africa
- Authors: Coetzee, Petrus Johannes
- Date: 2009
- Subjects: Communications, Military -- South Africa , Communications, Military -- Africa, Central , Digital communications -- South Africa , Digital communications -- Africa, Central , Signals and signaling -- South Africa , Signals and signaling -- Africa, Central , Artificial satellites in telecommunication -- South Africa , Artificial satellites in telecommunication -- Africa, Central , Shortwave radio
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5465 , http://hdl.handle.net/10962/d1005250 , Communications, Military -- South Africa , Communications, Military -- Africa, Central , Digital communications -- South Africa , Digital communications -- Africa, Central , Signals and signaling -- South Africa , Signals and signaling -- Africa, Central , Artificial satellites in telecommunication -- South Africa , Artificial satellites in telecommunication -- Africa, Central , Shortwave radio
- Description: The role of the South African National Defence Force (SANDF) has changed considerably in the last decade. The emphasis has moved from protecting the country's borders to peacekeeping duties in Central Africa and even further North. Communications between the peacekeeping missions and the military bases back in South Africa is vital to ensure the success of these missions. Currently use is made of satellite as well as High Frequency (HF) communications. There are drawbacks associated with these technologies (high cost and low data rates/interference respectively). Successful long distance ionospheric propagation in the low Very High Frequency (VHF) range will complement the existing infrastructure and enhance the success rate of these missions. This thesis presents a feasibility study to determine under what ionospheric conditions such low VHF communications will be possible. The International Reference Ionosphere (IRI) was used to generate ionospheric data for the reflection point(s) of the signal. The peak height of the ionospheric F2 layer (hmF2) was used to calculate the required antenna elevation angle. Once the elevation angle is known it is possible to calculate the required F2 layer critical frequency (foF2). The required foF2 value was calculated by assuming a Maximum Useable Frequency (MUF) of 20% higher than the planned operational frequency. It was determined that single hop propagation is possible during the daytime if the smoothed sunspot number (SSN) exceeds 15. The most challenging requirement for successful single hop propagation is the need of an antenna height of 23 m. For rapid deployment and semi-mobile operations within a jungle environment it may prove to be a formidable obstacle.
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- Date Issued: 2009
Temperature dependence of the HartRAO pointing model
- Authors: Copley, Charles Judd
- Date: 2008
- Subjects: Astronomical instruments , Observatories -- South Africa , Telescopes , Astronomical observatories , Astronomy -- Data processing , Antennas (Electronics)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5491 , http://hdl.handle.net/10962/d1005277 , Astronomical instruments , Observatories -- South Africa , Telescopes , Astronomical observatories , Astronomy -- Data processing , Antennas (Electronics)
- Description: This thesis investigates control aspects of the Hartebeeshoek Radio Astronomy Observatory (HartRAO) antenna. The installation of a new 22 GHz receiver has required the pointing accuracy to be improved to less than 4 mdeg. The effect of thermal conditions on the the HartRAO antenna pointing offset is investigated using a variety of modelling techniques including simple geometric modelling, neural networks and Principal Component Analysis (PCA). Convincing results were obtained for the Declination pointing offset, where applying certain model predictions to observations resulted in an improvement in Declination pointing offset from 5.5 mdeg to 3.2 mdeg (≈50%). The Right Ascension pointing model was considerably less convincing with an improvement of approximately from 5.5 mdeg to 4.5 mdeg (≈20%) in the Right Ascension pointing offset. The Declination pointing offset can be modelled sufficiently well to reduce the pointing offset to less than 4 mdeg, however further investigation of the underlying causes is required for the Right Ascension pointing offset.
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- Date Issued: 2008
The development of an improved coded-pulse, vertical-incidence ionosonde
- Authors: Cretchley, Brian Brind
- Date: 1979
- Subjects: Ionosondes Ionospheric sounds Computer simulation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5436 , http://hdl.handle.net/10962/d1001989
- Description: This thesis describes the theoretical development of a new ionospheric sounding system. The different types of ionosonde, their prime objectives, and their relative merits and demerits are discussed. The various types of code and their correlation functions are described. The essential requirements of the new system are listed, and suitable codes are found for it. Computer calculations and mathematical derivations demonstrate the (theoretical) suitability of these codes under all conditions. Essentials of the mode of operation of the system and details of its design are specified, and computer simulations are used to examine relevant aspects of its operation. Finally, since the construction of the system is not complete and results cannot therefore be presented, the present state of construction of the system is described
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- Date Issued: 1979
Thermoluminescence and phototransferred phermoluminescence of synthetic quartz
- Authors: Dawam, Robert Rangmou
- Date: 2020
- Subjects: Thermoluminescence , Quartz
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/145849 , vital:38472
- Description: The main aim of this investigation is on thermoluminescence and phototransferred thermoluminescence of synthetic quartz. Thermoluminescence was one of the tools used in characterising the electron traps parameters. The samples of quartz annealed at various temperatures up to 900̊C and the unannealed were used. The thermoluminescence glow curve was measured at 1̊C s~ 1 following beta irradiation to 40 Gy from the samples annealed at 500̊C and the unannealed consist of main peak at 70̊C and secondary peaks at 110, 180 and 310̊C. In comparison, the thermoluminescence glow curve for the sample annealed at 900̊C have main peak at 86̊C and the secondary ones at 170 and 310̊C. The kinetic analysis was carried out only on the main peak in each case. The activation energy was found to be decreasing with increase in annealing temperatures. The samples annealed at 500̊C and the unannealed were found to be affected by thermal quenching while sample annealed at 900̊C shows an inverse quenching for irradiation dose of 40 Gy. However, when the dose was reduce to 3 Gy the effects of thermal quenching was manifested. The activation energy of thermal quenching was also found to decrease with increase in annealing temperature. Thermally assisted optically stimulated luminescence measurement was carried out using continuous wave optical stimulated luminescence (CW-OSL). The samples studied were those annealed at 500̊C for 10 minutes, 900̊C for 10, 30, 60 minutes and 1000̊C for 10 minutes prior to use. The CW-OSL is stimulated using 470 nm blue LEDs at sample temperatures between 30 and 200̊C. It is measured after preheating to either 300 and 500̊C. When the integrated OSL intensity is plotted as a function of measurement temperature, the intensity goes through a peak. The increase in OSL intensity as a function of temperature is associated to thermal assistance and the decrease to thermal quenching. The kinetic parameters were evaluated by fitting the experimental data. The values of activation energies of thermal quenching are the same within experimental uncertainties for all the experimental conditions. This shows that annealing temperature, duration of annealing and irradiation dose have a negligible influence on the recombination site of luminescence using OSL. Phototransferred thermoluminescence (PTTL) induced from annealed samples using 470 nm blue light was also investigated. The quartz were annealed at 500 _C for 10 minutes, 900̊C for 10, 30, 60 minutes and 1000̊C for 10 minutes prior to use. The glow curves of conventional TL measured at 1 _C s1 following irradiation to 200 Gy shows six peaks in each case labelled I-VI for ease of reference whereas peaks observed under PTTL are referred to as A1 onwards. Only the first three peaks were reproduced under phototransfer for the sample annealed at 900̊C for 60 minutes and 1000̊C C for 10 minutes. Interestingly, for the intermediate duration of annealing of 30 minutes, the only peak that appears under phototransfer is the A1. For quartz annealed at 900̊C for 10 minutes, the PTTL appears as long as the preheating temperature does not exceed 560̊C. All other annealing temperatures, PTTL only appears for preheating to 450 and below. This shows that the occupancy of deep electron traps at temperatures beyond 450̊C or 560̊C is low. The activation energy for peaks A1, A2 and A3 were calculated. The PTTL peaks were studied for thermal quenching and peaks A1 and A3 were found to be affected. The activation energies for thermal quenching were determined as 0.62 ± 0.04 eV and 0.65 ± 0.02 eV for peaks A1 and A3 respectively. The experimental dependence of PTTL intensity on illumination time is modelled using sets of coupled linear differential equations based on systems of donors and acceptors whose number is determined by preheating temperature.
- Full Text:
- Date Issued: 2020
First H F Doppler soundings of the ionosphere at SANAE
- Authors: De Kock, Errol James
- Date: 1980
- Subjects: Ionosphere
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5500 , http://hdl.handle.net/10962/d1006869 , Ionosphere
- Full Text:
- Date Issued: 1980
The development and evaluation of a Nd:YAG laser incorporating an unstable resonator
- Authors: De Kock, Trevor Neil
- Date: 1986
- Subjects: Nd-YAG lasers , Lasers -- Resonators , Laser beams
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5512 , http://hdl.handle.net/10962/d1008566 , Nd-YAG lasers , Lasers -- Resonators , Laser beams
- Description: Introduction: For approximately the last eight years the Laser Section of the National Physical Research Laboratory (NPRL) has been interested in inter alia, pulsed solid-state lasers and in particular, Nd:YAG. Investigations of various resonator types were undertaken with a view to the improvement of the laser parameters such as output energy, pulse width, beam quality and sensitivity to mirror misalignment. In 1980 a Nd: YAG laser employing a rotating prism Q-switch was constructed (Preussler (1980)). It involves rotating one of the two cavity reflectors so that they are parallel for only a brief instant in time. Typically the prism must rotate at a speed of 20 000 r.p.m. to ensure a single pulse output. Such lasers suffer from the tendency to emit multiple pulses, they are very noisy and they require frequent maintenance because of the short lifetime of the bearings. A resonator employing conventional curved mirrors and an electro-optical Q-switch was constructed in 1980 (Robertson & Preussler (1982)). In 1981 an electro-optically Q-swi tched laser making use of a crossed Porro-prism resonator was investigated due to its relative insensitivity to misalignment of the reflectors compared with the conventional mirror resonator (Nortier (1981)). Improvements in terms of output power, beam divergence and beam quality can be achieved by making use of a so-called unstable resonator. Such a laser has been investigated and is reported on in this study. Chapter 2 provides some background into laser theory and operation while chapter 3 deals with the theory of the unstable resonator. Chapter 4 provides details of the experimental equipment and techniques used in the work and chapter 5 discusses the evaluation of the project and results obtained.
- Full Text:
- Date Issued: 1986
The utilization of tilting-filter photometry in airglow and auroral research
- Authors: Dore, Ian Stuart
- Date: 1992
- Subjects: Photometry -- Research , Airglow -- Research , Auroras -- Research , Auroral photography -- Research
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5494 , http://hdl.handle.net/10962/d1005280 , Photometry -- Research , Airglow -- Research , Auroras -- Research , Auroral photography -- Research
- Description: This thesis describes the application of tilting-filter photometry to the study of the airglow and aurora. Previous South African photometric research is reviewed. Optical instrumentation and techniques used in airglow and auroral research are reviewed. The transmission characteristics of narrowband interference filters are discussed. The analogue meridian-scanning tilting-filter photometry system used at Sanae, Antarctica is described. Shortcomings of this system have been identified, and modifications have been made to improve its spatial and temporal resolution. Details are given of the computer-controlled digital photometry system which replaced the analogue system. Equations are derived for the conversion of raw photometric data (analogue chart deflections or digital photon counts) to absolute emission intensities. The accuracy of the intensities obtained depends on the absolute calibration of the photometer, the transmission characteristics of the filter used to isolate the spectral feature of interest, and the effects of atmospheric extinction and scattering. The influence of these factors on observed emission intensities is discussed. Various models used to determine atmospheric correction factors are reviewed. It is shown that atmospheric correction factors can have a significant effect on both emission intensities and intensity ratios. The procedure used to determine the transmission characteristics of interference filters is described, as is the procedure used to cross-calibrate secondary light sources. The transmission characteristics of the filters and the brightnesses of the light sources were both found to have changed appreciably with age. The observation of a magnetospheric substorm at Sanae (L ≃ 4) is used to illustrate the use of a meridian-scanning tilting-filter photometer system in auroral research. The ratio I(557.7)/ I(391.4) observed at Sanae was found to be lower than expected, as were the OJ airglow emission intensities. A prototype digital photometer system was used aboard a ship, to observe the airglow in the region of the South Atlantic Anomaly. Significant N₂⁺ lNG emissions at 391.4 nm were measured, confirming the presence of discernable particle precipitation in the region. The 0I557.7 and 630.0 nm intensities measured from the ship were found to be lower than expected. This, combined with low airglow and auroral intensities measured at Sanae, is a cause for concern. It is recommended that further checks be made regarding the brightness of the calibration sources.
- Full Text:
- Date Issued: 1992
A study of the fluorescence excitation spectrum of crystalline anthracene
- Authors: Driver, Adrian Stanford
- Date: 1960
- Subjects: Fluorimetry , Anthracene crystals -- Spectra
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5518 , http://hdl.handle.net/10962/d1011463 , Fluorimetry , Anthracene crystals -- Spectra
- Description: The work described in this thesis was performed at the Physics Department, Rhodes University during 1958 and 1959 under the supervision of Professor J.A. Gledhill. Use was made of a vacuum ultra-violet spectrograph which had been constructed in the Physics Department (1.1) and modifications to be described were made to this instrument. The instrument was used for studying the effects of oxygen on the fluorescence excitation spectrum of Anthracene.
- Full Text:
- Date Issued: 1960
Modelling Ionospheric vertical drifts over the African low latitude region
- Authors: Dubazane, Makhosonke Berthwell
- Date: 2018
- Subjects: Ionospheric drift , Magnetometers , Functions, Orthogonal , Neural networks (Computer science) , Ionospheric electron density -- Africa , Communication and Navigation Outage Forecasting Systems (C/NOFS)
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/63356 , vital:28396
- Description: Low/equatorial latitudes vertical plasma drifts and electric fields govern the formation and changes of ionospheric density structures which affect space-based systems such as communications, navigation and positioning. Dynamical and electrodynamical processes play important roles in plasma distribution at different altitudes. Because of the high variability of E × B drift in low latitude regions, coupled with various processes that sometimes originate from high latitudes especially during geomagnetic storm conditions, it is challenging to develop accurate vertical drift models. This is despite the fact that there are very few instruments dedicated to provide electric field and hence E × B drift data in low/equatorial latitude regions. To this effect, there exists no ground-based instrument for direct measurements of E×B drift data in the African sector. This study presents the first time investigation aimed at modelling the long-term variability of low latitude vertical E × B drift over the African sector using a combination of Communication and Navigation Outage Forecasting Systems (C/NOFS) and ground-based magnetometer observations/measurements during 2008-2013. Because the approach is based on the estimation of equatorial electrojet from ground-based magnetometer observations, the developed models are only valid for local daytime. Three modelling techniques have been considered. The application of Empirical Orthogonal Functions and partial least squares has been performed on vertical E × B drift modelling for the first time. The artificial neural networks that have the advantage of learning underlying changes between a set of inputs and known output were also used in vertical E × B drift modelling. Due to lack of E×B drift data over the African sector, the developed models were validated using satellite data and the climatological Scherliess-Fejer model incorporated within the International Reference Ionosphere model. Maximum correlation coefficient of ∼ 0.8 was achieved when validating the developed models with C/NOFS E × B drift observations that were not used in any model development. For most of the time, the climatological model overestimates the local daytime vertical E × B drift velocities. The methods and approach presented in this study provide a background for constructing vertical E ×B drift databases in longitude sectors that do not have radar instrumentation. This will in turn make it possible to study day-to-day variability of vertical E×B drift and hopefully lead to the development of regional and global models that will incorporate local time information in different longitude sectors.
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- Date Issued: 2018
Computer control of a barry research chirpsounder
- Authors: Evans, Geoffrey Philip
- Date: 1985
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5507 , http://hdl.handle.net/10962/d1007495
- Description: This thesis describes the design and development of a computer-based controller together with additional hardware that greatly extends the capabilities of a Barry Research VOS-1 Chirpsounder. The measurement of the virtual height of the ionosphere as a function of frequency using pulse- and frequency-modulated carrier wave (FMCW techniques is described and the concept of the so called "digital" ionosonde is introduced. The modifications required for the standard Chirpsounder to perform as a versatile digital chirp ionosonde are discussed. Simplified block diagrams are used to describe the Controller hardware which is fully described in two comprehensive service manuals which have been included as appendices. Important aspects of the Controller software and data storage formats are described in detail. The emphasis is then placed on system capabilities. An operators' software manual which describes system initialization and operation in terms of system commands is included as an appendix. Results of tests at both Grahamstown, South Africa , and at the SANAE base in the Antarctic are presented.
- Full Text:
- Date Issued: 1985
A real time Fast Fourier Transform analyser
- Authors: Fisher, John Stanley
- Date: 1980
- Subjects: Fourier transformations Ionosondes Algorithms Computer simulation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5439 , http://hdl.handle.net/10962/d1001992
- Description: From the requirements of the Ionosonde digitisation project, undertaken by Rhodes University Antarctic Research Group, it was decided to use the Fast Fourier Transform to compute the spectrum analysis. Several FFT algorithms are reviewed and properties discussed, and the Ccoley Tukey algorithm chosen for utilization. The hardware implementation of this algorithm, and the microprogram control of the whole system are discussed in detail, and such design aspects that required computer simulation are also treated in detail. The final testing of the analyser is shown, and includes a test using data from an ionosonde sounding. The conclusions contain details of extensions to the analysers present operation, required by plans to place the whole Chirpsounder under microprocessor control
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- Date Issued: 1980
Observations of glitches in PSR 0833-45 and 1641-45
- Authors: Flanagan, Claire Susan
- Date: 1996
- Subjects: Pulsars Neutron stars
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5480 , http://hdl.handle.net/10962/d1005266
- Description: An eleven-year series of radio timing observations of 0833- 45 (Vela) and PSR 1641- 45 is presented. During this time, five large spin-ups ("glitches") were observed in 0833- 45 and one in 1641-45. The stellar response to these events is investigated, and the three relat ively long complete inter-glitch intervals in 0833-45 are modeled. The results are of relevance to studies of the interiors of neutron stars. The initial aim of the project - to obtain good observational coverage of large glitches in the Vela pulsar - was successfully achieved, and high quality observations of the periods between glitches were obtained as a by-product. The results of the analysis presented here provide support for the existence of both linear and non-linear coupling in the Vela pulsar, and put a limit on the former in PSR 1641- 45. The recently observed existence of a rapidly recovering component of part of a glitch in Vela was verified in the subsequent glitch, although there is now evidence to contradict the suggestion that this component involves a particular region of the star that is implicated in every glitch. Observations of a recent glitch in the same pulsar have resolved a small component of the spin-up; such a component has not been reported for any other large glitch.
- Full Text:
- Date Issued: 1996
Dynamics of stimulated luminescence in natural quartz: Thermoluminescence and phototransferred thermoluminescence
- Authors: Folley, Damilola Esther
- Date: 2020
- Subjects: Thermoluminescence , Quartz
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/146255 , vital:38509
- Description: Natural quartz has remained an important mineral that is of topical interest in luminescence and dosimetry-related research. We investigate the dynamics of stimulated luminescence on this material through thermoluminescence (TL) and phototransferred thermoluminescence (PTTL). Measurements were made on unannealed natural quartz as well as quartz annealed at 800 and 1000̊C. The samples were annealed for 10 minutes and for 1 hour. The material, in its un- and annealed state has its main peak between 68 and 72̊C when measured at 1Cs ̃1 after a dose of 50 Gy. A study of dosimetric features and kinetic analysis was carried out on two prominent peaks, peak I and III for all the samples. The peaks show a sublinear dose response for irradiation doses between 10 and 300 Gy. Kinetic analysis shows that peak I is a first-order peak and peak III a general-order peak. Interestingly, we observe for peak I for the sample annealed at 800̊C for 1 hour an inverse thermal quenching behaviour. We demonstrate that a peak affected with an inverse thermal quenching-like behaviour can still show effect of thermal quenching when the dose the sample is irradiated to is significantly reduced. We ascribe the apparent dependence of thermal quenching on dose to competition between radiative and non-radiative transitions at the recombination centre. Peaks I, II, and III for all the samples were reproduced under phototransfer when the peaks, initially removed by preheating to a certain temperature are exposed to 470 and 525 nm light. The infuence of duration of illumination on the PTTL intensity of these peaks corresponding to various preheating temperatures is modelled using coupled first-order dfferential equations. The model is based on systems of acceptors and donors whose number and role depends on preheating temperature
- Full Text:
- Date Issued: 2020
Radio studies of ionized hydrogen in the southern Milky Way
- Authors: Gaylard, Michael John
- Date: 1990
- Subjects: Interstellar hydrogen Milky Way Astrophysics
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5440 , http://hdl.handle.net/10962/d1001993
- Description: This thesis describes the results of a survey of the HI42ɑ recombination line emission at 2.3 GHz from HII regions in the Southern Milky Way, carried out with the 26 m diameter Hartebeesthoek radio telescope. The Galactic Longitude range covered was 290° to 40°. Single recombination lines were detected from 375 positions. Multiple lines were observed towards 90 positions in the inner Galaxy. No line emission could be detected in 28 positions. Continuum antenna temperatures were estimated from drift scans or radio maps observed for the purpose. LTE electron temperatures and turbulent velocities of the HII regions were calculated where possible. The properties of the sample were compared to those observed in HI09ɑ surveys. The lines observed from over 50 positions were first detections, of which half were associated with optically-identified HII regions. In about 150 cases the lines were only the second to be detected from those HII regions. The processes of the radio emission, detection, and analysis were simulated numerically. The detectability of the emission and the magnitude of non-LTE effects and pressure-broadening in multi-component HII regions was predicted and compared to observations. The radio luminosity function of the HII regions was determined over a range of three orders of magnitude in intrinsic brightness for the first time, using techniques which corrected for different types of incompleteness in the samples. The luminosity function was compared to those in five selected spiral galaxies, and shown to lie between those of M33 and M81. An alternate form of the luminosity function was developed for use with a numerical model of the spiral arm structure of the Milky Way. The physical parameters defining the major spiral arms were established by comparing synthesized diagrams of radial velocity versus Galactic Longitude with those actually observed. The faint, extended HII regions S9 and RCW129 in Scorpius, the Barnard Loop in Orion, and S296 in Canis Major were analyzed, using all available data. All the recombination lines from these HII regions were first detections
- Full Text:
- Date Issued: 1990
Tomographic imaging of East African equatorial ionosphere and study of equatorial plasma bubbles
- Authors: Giday, Nigussie Mezgebe
- Date: 2018
- Subjects: Ionosphere -- Africa, Central , Tomography -- Africa, Central , Global Positioning System , Neural networks (Computer science) , Space environment , Multi-Instrument Data Analysis System (MIDAS) , Equatorial plasma bubbles
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/63980 , vital:28516
- Description: In spite of the fact that the African ionospheric equatorial region has the largest ground footprint along the geomagnetic equator, it has not been well studied due to the absence of adequate ground-based instruments. This thesis presents research on both tomographic imaging of the African equatorial ionosphere and the study of the ionospheric irregularities/equatorial plasma bubbles (EPBs) under varying geomagnetic conditions. The Multi-Instrument Data Analysis System (MIDAS), an inversion algorithm, was investigated for its validity and ability as a tool to reconstruct multi-scaled ionospheric structures for different geomagnetic conditions. This was done for the narrow East African longitude sector with data from the available ground Global Positioning Sys-tem (GPS) receivers. The MIDAS results were compared to the results of two models, namely the IRI and GIM. MIDAS results compared more favourably with the observation vertical total electron content (VTEC), with a computed maximum correlation coefficient (r) of 0.99 and minimum root-mean-square error (RMSE) of 2.91 TECU, than did the results of the IRI-2012 and GIM models with maximum r of 0.93 and 0.99, and minimum RMSE of 13.03 TECU and 6.52 TECU, respectively, over all the test stations and validation days. The ability of MIDAS to reconstruct storm-time TEC was also compared with the results produced by the use of a Artificial Neural Net-work (ANN) for the African low- and mid-latitude regions. In terms of latitude, on average,MIDAS performed 13.44 % better than ANN in the African mid-latitudes, while MIDAS under performed in low-latitudes. This thesis also reports on the effects of moderate geomagnetic conditions on the evolution of EPBs and/or ionospheric irregularities during their season of occurrence using data from (or measurements by) space- and ground-based instruments for the east African equatorial sector. The study showed that the strength of daytime equatorial electrojet (EEJ), the steepness of the TEC peak-to-trough gradient and/or the meridional/transequatorial thermospheric winds sometimes have collective/interwoven effects, while at other times one mechanism dominates. In summary, this research offered tomographic results that outperform the results of the commonly used (“standard”) global models (i.e. IRI and GIM) for a longitude sector of importance to space weather, which has not been adequately studied due to a lack of sufficient instrumentation.
- Full Text:
- Date Issued: 2018
Optimizing MIDAS III over South Africa
- Authors: Giday, Nigussie Mezgebe
- Date: 2014
- Subjects: Multi-Instrument Data Analysis System (MIDAS) , Global Positioning System , Ionosphere -- South Africa , Ionospheric electron density -- South Africa , Ionosondes -- South Africa , Tomography -- Scientific applications -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5517 , http://hdl.handle.net/10962/d1011277 , Multi-Instrument Data Analysis System (MIDAS) , Global Positioning System , Ionosphere -- South Africa , Ionospheric electron density -- South Africa , Ionosondes -- South Africa , Tomography -- Scientific applications -- South Africa
- Description: In this thesis an ionospheric tomographic algorithm called Multi-Instrument Data Anal- ysis System (MIDAS) is used to reconstruct electron density profiles using the Global Positioning System (GPS) data recorded from 53 GPS receivers over the South African region. MIDAS, developed by the Invert group at the University of Bath in the UK, is an inversion algorithm that produces a time dependent 3D image of the electron density of the ionosphere. GPS receivers record the time delay and phase advance of the trans- ionospheric GPS signals that traverse through the ionosphere from which the ionospheric parameter called Total Electron Content (TEC) can be computed. TEC, the line integral of the electron density along the satellite-receiver signal path, is ingested by ionospheric tomographic algorithms such as MIDAS to produce a time dependent 3D electron density profile. In order to validate electron density profiles from MIDAS, MIDAS derived NmF2 values were compared with ionosonde derived NmF2 values extracted from their respective 1D electron density profiles at 15 minute intervals for all four South African ionosonde stations (Grahamstown, Hermanus, Louisvale, and Madimbo). MIDAS 2D images of the electron density showed good diurnal and seasonal patterns; where a comparison of the 2D images at 12h00 UT for all the validation days exhibited maximum electron concentration during the autumn and summer and a minimum during the winter. A root mean square error (rmse) value as small as 0.88x 10¹¹[el=m³] was calculated for the Louisvale ionosonde station during the winter season and a maximum rmse value of 1.92x 10¹¹[el=m³] was ob- tained during the autumn season. The r² values were the least during the autumn and relatively large during summer and winter; similarly the rmse values were found to be a maximum during the autumn and a minimum during the winter indicating that MIDAS performs better during the winter than during the autumn and spring seasons. It is also observed that MIDAS performs better at Louisvale and Madimbo than at Grahamstown and Hermanus. In conclusion, the MIDAS reconstruction has showed good agreement with the ionosonde measurements; therefore, MIDAS can be considered a useful tool to study the ionosphere over the South African region.
- Full Text:
- Date Issued: 2014
The establishment of a Lidar facility at Rhodes University
- Authors: Grant, Richard Peter James Seton
- Date: 1988
- Subjects: Optical radar , LIDAR , Receiver , Transmitter , Photon counting electronics , Aerosol scattering , Temperature profiles , Stratosphere
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5445 , http://hdl.handle.net/10962/d1001998
- Description: LIDAR is the optical equivalent of RADAR. A LIDAR facility has been established at Rhodes University using a flashlamp-pumped dye laser as the transmitter and a photomultiplier tube at the focus of a searchlight mirror as the receiver. The setting up of the receiver and transmitter as well as the design and construction of the photon counting electronics is described. The LIDAR has been used to measure aerosol scattering ratios and temperature profiles in the stratosphere and these results are presented with the algorithms and software used to reduce the data. Finally some recommendations are made for future work
- Full Text:
- Date Issued: 1988