Magister Scientiae - MSc (Physics)

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Accelerating reionization simulations using machine learning
(University of the Western Cape, 2023) Masipa, Mosima Portia; Sultan, Hassan
Epoch of Reionization (EoR) refers to the time in the history of the universe when the appearance of the first luminous sources reionized the intergalactic medium (IGM). The EoR carries a wealth of information regarding structure formation and evolution. Ongoing and planned 21cm experiments such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA) are expected to generate huge amounts of high dimensional datasets, and hence a new generation of efficient simulations and tools are required in order to maximize their scientific return. While Convolutional neural networks (CNNs) achieve the state-of-the-art performance to extract information from large scale fields, generating large training datasets and fully exploring the cosmological and astrophysical parameter space require fast simulations.
Aluminium induced crystallization of hydrogenated amorphous silicon thin films
(University of the Western Cape, 2005) Kotsedi, Lebogang; Knoesen, Dirk; Madjoe, Reginalt
This study was carried out to crystallize hydrogenated amorphous silicon (a-Si:H) thin films using the aluminium induced crystallization (AIC) technique. This was done to investigate whether is there any lateral crystallization of the a-Si:H thin film away from the aluminium covered surface of the film. The hot wire chemical vapour deposition system (HWCVD) was used to deposit hydrogenated amorphous silicon thin films (a-Si:H) on Corning glass 7059 substrates. The substrate temperature was kept at 300oC while the filament temperature was l600oC during the deposition. The aluminium top layer was deposited at room temperature using the electron beam evaporator. The aluminium deposited, only partially covered the sample, this was done to investigate whether lateral crystallization of the uncovered part will take place. Samples were then annealed at 450oC for times ranging from 30 to 150 minutes in incremental steps of 30 minutes. A temperature series of annealings at l00oC, 150oC, 200oC,300"C and 350oC for 60 minutes were also performed. Energy Dispersive Spectroscopy (EDS) was used for elemental identification after annealing. Rutherford Backscattering Spectrometry was used for the depth profiling of the diffused species. X-ray diffraction (XRD) technique was used for crystallization studies on the aluminium covered side, transmission electron microscopy (TEM) was used to study lateral crystallization and diffraction patterns of crystallized part were taken using selected area diffraction (SAD).
Analysis and simulations to obtain the weak magnetism term in ²²Na beta decay
(University of the Western Cape, 2016) Phuthu, Lutendo; Triambak, Smarajit
The study of ²²Na beta decay offers an opportunity to test the Standard Model of Particle Physics via measurements of the β−γ angular correlation. A previous measurement of this correlation yielded a non-zero value, indicating the need for a higher-order matrix element to the decay, beyond the allowed V − A approximation. On assuming the Conserved Vector Current (CVC) hypothesis for weak interactions and using the magnetic dipole M1 width of the analog 2+ state in ²²Na, one obtains an unexpectedly large 'second-class' form factor for 22Na β decay that is in disagreement with the Standard Model prediction. This thesis describes an analysis of data obtained from a previous ²¹Ne(p, γ) experiment to obtain the M1 width of the 2+ state of interest in ²²Na. This work aims to use the M1 width and the independently measured of the β − γ angular correlation to obtain a higher-order Standard-Model-allowed weak magnetism form factor for the decay, in an attempt to explain the observed anomaly mentioned above.
Anomaly Detection With Machine Learning In Astronomical Images
(University of the Western Cape, 2020) Etsebeth, Verlon; Lochner, Michelle
Observations that push the boundaries have historically fuelled scientific breakthroughs, and these observations frequently involve phenomena that were previously unseen and unidentified. Data sets have increased in size and quality as modern technology advances at a record pace. Finding these elusive phenomena within these large data sets becomes a tougher challenge with each advancement made. Fortunately, machine learning techniques have proven to be extremely valuable in detecting outliers within data sets. Astronomaly is a framework that utilises machine learning techniques for anomaly detection in astronomy and incorporates active learning to provide target specific results. It is used here to evaluate whether machine learning techniques are suitable to detect anomalies within the optical astronomical data obtained from the Dark Energy Camera Legacy Survey. Using the machine learning algorithm isolation forest, Astronomaly is applied on subsets of the Dark Energy Camera Legacy Survey (DECaLS) data set. The pre-processing stage of Astronomaly had to be significantly extended to handle real survey data from DECaLS, with the changes made resulting in up to 10% more sources having their features extracted successfully. For the top 500 sources returned, 292 were ordinary sources, 86 artefacts and masked sources and 122 were interesting anomalous sources. A supplementary machine learning algorithm known as active learning enhances the identification probability of outliers in data sets by making it easier to identify target specific sources. The addition of active learning further increases the amount of interesting sources returned by almost 40%, with 273 ordinary sources, 56 artefacts and 171 interesting anomalous sources returned. Among the anomalies discovered are some merger events that have been successfully identified in known catalogues and several candidate merger events that have not yet been identified in the literature. The results indicate that machine learning, in combination with active learning, can be effective in detecting anomalies in actual data sets. The extensions integrated into Astronomaly pave the way for its application on future surveys like the Vera C. Rubin Observatory Legacy Survey of Space and Time.
Antimicrobial activity of South African red algal secondary metabolites
(UWC, 2001) Cameron D; Keats, DW; Cyster, LF; Leng, H; Green, I
Infectious diseases, for example, measles, scarlet fever, malaria, tuberculosis, and cholera have claimed many lives. These infections are caused by pathogenic viruses, bacteria, or fungi that invade the body's tissues and multiply. According to the Online Medical Dictionary (2000), the multiplication may be clinically barely visible or result in local cellular injury because of competitive metabolism, toxins, intracellular replication, or antigen-antibody response. An infection may remain localized, sub-clinical, and temporary if the body's defensive mechanisms are effective or it could persist, spread, and become an acute, sub-acute, or chronic clinical infection or disease state.
Antimicrobial discovery from South African marine algae
(University of the Western Cape, 2018) Mabande, Edmund Rufaro; Beukes, Denzil R
Antimicrobials are chemical compounds that destroy or inhibit the growth of microorganisms. The majority of these antimicrobials are actually natural products or natural product derived with key examples being the pioneer antibiotics penicillin and cephalosporin. Antimicrobials are an extremely important class of therapeutic agents; however, the development of drug resistance and slow pace of new antibiotic discovery is one of the major health issues facing the world today. There is therefore a crucial need to discover and develop new antibacterial agents. In this study, the potential of marine algae as a source of new antibiotics was explored. Crude organic extracts and chromatographic fractions obtained from small-scale extraction of 17 different marine algae were used to prepare a pre-fractionated library that would be tested against several disease causing microorganisms. The activity of the pre-fractionated library and purified compounds was determined against a panel of drug resistant microorganisms namely Acinetobacter baumannii ATCCBAA®-1605™, Enterococcus faecalis ATCC® 51299™, Escherichia coli ATCC® 25922™, Staphylococcus aureus subsp. aureus ATCC® 33591™ and Candida albicans ATCC® 24433™. Finally, cytotoxicity tests of 50 selected library extracts and isolated compounds were done against two cell lines namely MCF-7 (breast cancer) and HEK-293 (kidney embryonic). Based on their antimicrobial activity and interesting chemical profiles, the seaweeds Plocamium sp. and Stypopodium multipartitum were selected for further study. Three new and unusual halogenated monoterpenes (4.16, 4.17 and 4.18) were isolated from Plocamium sp., and an unusual meroditerpenoid (5.8) was isolated from Stypopodium multipartitum. The metabolites were purified using preparative (silica gel) chromatography as well as semipreparative normal phase HPLC. The structures of purified compounds were determined from spectroscopic data, including nuclear magnetic resonance (NMR) spectroscopy. A small library of 153 fractions was generated from collections of South African marine algae. Pre-fractionated crude extracts showed excellent antimicrobial activity against all microbes but particularly against Staphylococcus aureus. The compounds were generally active against the Gram positive bacteria and the yeast. In conclusion, three antimicrobial halogenated monoterpenes and an unusual monoterpene were isolated from a Plocamium sp. and Stypopodium multipartitum respectively. Antimicrobial activity of crude fractions was excellent but that of isolated compounds was not as great as anticipated.
Application of anomaly detection techniques to astrophysical transients
(University of Western Cape, 2021) Ramonyai, Malema Hendrick; Lochner, Michelle
We are fast moving into an era where data will be the primary driving factor for discovering new unknown astronomical objects and also improving our understanding of the current rare astronomical objects. Wide field survey telescopes such as the Square Kilometer Array (SKA) and Vera C. Rubin observatory will be producing enormous amounts of data over short timescales. The Rubin observatory is expected to record ∼ 15 terabytes of data every night during its ten-year Legacy Survey of Space and Time (LSST), while the SKA will collect ∼100 petabytes of data per day. Fast, automated, and datadriven techniques, such as machine learning, are required to search for anomalies in these enormous datasets, as traditional techniques such as manual inspection will take months to fully exploit such datasets.
Calibration of a NaI (Tl) detector for low level counting of naturally occurring radionuclides in soil
(University of the Western Cape, 2011) Noncolela, Sive Professor; Lindsay, R
The Physics Department at the University of the Western Cape and the Environmental Physics group at iThemba labs have been conducting radiometric studies on both land and water. In this study a 7.5 cm X 7.5 cm NaI (Tl) detector was used to study activity concentrations of primordial radionuclides in soil and sand samples. The detector and the sample were placed inside a lead castle to reduce background in the laboratory from the surroundings such as the wall and the floor. The samples were placed inside a 1 L Marinelli beaker which surrounds the detector for better relative efficiency as almost the whole sample is exposed to the detector. Additional lead bricks were placed below the detector to further reduce the background by 20%. The NaI detector is known to be prone to spectral drift caused by temperature differences inside and around the detector. The spectral drift was investigated by using a ¹³⁷Cs source to monitor the movements in the 662 keV peak. The maximum centroid shift was about 4 keV (for a period of 24 hours) which is enough to cause disturbances in spectral fitting. There was no correlation between the centroid shift and small room temperature fluctuations of 1.56 ºC. A Full Spectrum Analysis (FSA) method was used to extract the activity concentrations of ²³⁸U, ²³²Th and ⁴⁰K from the measured data. The FSA method is different from the usual Windows Analysis (WA) as it uses the whole spectrum instead of only putting a ‘window’ around the region of interest to measure the counts around a certain energy peak. The FSA method uses standard spectra corresponding to the radionuclides being investigated, and is expected to have an advantage when low-activity samples are measured. The standard spectra are multiplied by the activity concentrations and then added to fit the measured spectrum. Accurate concentrations are then extracted using a chi-squared (χ²) minimization procedure. Eight samples were measured in the laboratory using the NaI detector and analyzed using the FSA method. The samples were measured for about 24 hours for good statistics. Microsoft Excel and MATLAB were used to calculate the activity concentrations. The ²³⁸U activity concentration values varied from 14 ± 1 Bq/kg (iThemba soil, HS6) to 256 ± 10 Bq/kg (Kloof sample). The ²³²Th activity concentration values varied from 7 ± 1 Bq/kg (Anstip beach sand) to 53 ± 3 Bq/kg (Rawsonville soil #B31). The ⁴⁰K activity concentration values varied from 60 ± 20 Bq/kg (iThemba soil, HS6) to 190 ± 20 Bq/kg (Kloof sample). The χ² values also varied from sample to sample with the lowest being 12 (Anstip beach sand) and the highest (for samples without contamination of anthropogenic nuclei) being 357 (Rawsonville soil #B28). A high χ² value usually represents incomplete gain drift corrections, improper set of fitting functions, proper inclusion of coincidence summing or the presence of anthropogenic (man made) radionuclei in the source [Hen03]. Activity concentrations of ⁴⁰K, ²³²Th and ²³⁸U were measured at four stationary points on the Kloof mine dump. The fifth stationary point was located on the Southdeep mine dump. These measurements were analysed using the FSA method and fitting by "eye" the standard spectra to the measured spectra using Microsoft Excel. These values were then compared to values obtained using an automated minimization procedure in MATLAB. There was a good correlation between these results except for ²³²Th which had higher concentrations when MATLAB was used, where 16 Bq/kg was the average value in Excel and 24 Bq/kg was the average value in MATLAB.
Calibration of germanium detectors for applications of radiometric methods in South Africa.
(University of the Western Cape, 2001) Maleka, P.P; Lindsay, R.; de Meijer, R. I.
AII materials that are radioactive emit characteristic gamma-radiation. Natural radioactivity can be used in heavy minerals exploration and processing, as well as in understanding sediment transport. processes in coastal zones [Dem97][Dem98]. Differences in radionuclide concentrations between minerals, allows the mineral species to be identified and quantified by the activity concentrations of a011, 23216 and 238U using a technique called radiometric fingerprinting [Dem97]. A" important tool to determine sediment transport rates along the coastline is by the MEDUSA technology developed and tested at the Kernfysisch Versneller Instituut (KVI) [Dem98]. The data from the MEDUSA (Multi-Element Detector system for IJnderwater Sediment Activity) system needs to be calibrated first in the laboratory before field measurements can be converted to useful data. The National Accelerator Centre (NAC) is setting up a radiometric laboratory to do this by means of a high-resolution germanium detector used under low-background conditions and constant geometry.
Carbon, magnesium implantation and proton irradiation on pulsed laser deposited thermochromic thin film of VO2
(University of Western Cape, 2020) Mabakachaba, Boitumelo Mafalo; Maaza, M.; Arendse, Christopher
When the spacecrafts orbit in space, it is subjected to significant thermal cycling variation. Thermal regulation of the spacecraft temperature is required to ensure a good operation of the small crafts such as CubeSats and the on-board equipment while minimizing the weight. Three methods employed for the Smart Radiator Devices (SRD) are (i) mechanical louvers, (ii) electrochromic coatings and (iii) thermochromic coatings (which is of interest in this study). Based on the characteristics of the thermochromic coatings, the passive smart radiator device is by far the most efficient option since there are no mechanical moving components and also no electric energy needed for the craft to operate.
Characterisation of natural radioactivity in Karoo Basin groundwater prior to shale gas exploration
(University of the Western Cape, 2017) Botha, Ryno; Lindsay, R.; Newman, R.T.; Maleka, P.P.
The prospect of unconventional shale-gas development in the Karoo Basin (South Africa) has created the need to obtain baseline data on natural radioactivity in Karoo groundwaters. The Karoo Basin groundwater radiological baseline developed through this study could serve as a reference to research potential future radiological contamination effects due to hydraulic fracturing. The major naturally occurring radioactive material (NORM) studied was radon (222Rn), in particular in-water activity concentrations; however, supplementary radium (226Ra and 228Ra) in-water activity concentrations and uranium (238U) in-water concentrations measurements were also made. A total of 53 aquifers across three provinces were sampled for groundwater and measured, with three measurement series from 2014 to 2016. The aquifers were categorized as shallow, mixed, or deep source. The radon-in-water baseline of the Karoo Basin can be characterised by a minimum of 0.6 ± 0.9 Bq/L, a maximum of 183 ± 18 Bq/L and mean of 41 ± 5 Bq/L. The radon-in-water levels from shallow sources (with water temperature < 20 °C) were systematically higher (40 Bq/L) than for deep sources (with water temperature > 20 °C). The natural fluctuations in radon-in-water levels were predominantly associated with shallow aquifers compared to almost none observed in the deep sources. The uranium in-water baseline can be characterised by a minimum of below detection level, a maximum of 41 μg/L, and the mean of 5.10 ± 0.80 μg/L. Similar to radon-in-water levels, uranium in-water levels for shallow sources were systematically higher than for deep sources. The limited (six aquifers) radium (228Ra and 226Ra) in-water activity-concentration measurement results were very low, with a maximum of 0.008 Bq/L (226Ra) and 0.015 Bq/L (228Ra). The 228Ra/226Ra ratio baseline were characterised by a minimum of 0.93, a mean of 3.3 ± 1.3, and a maximum of 6.5. The radium isotopes’ activity concentration ratio is an isotopic tracer for hydraulic fracturing wastewater. Pollution and contamination (radiological), due to unconventional shale gas development, in water resources has been noticed in the Marcellus Basin (United States). Consequently, developing and improving continuous baseline monitoring are of importance to study the environmental radiological effect of hydraulic fracturing.
Characterisation of the first 1/2+ excited state in 9B and isospin symmetry breaking studies in A = 9 nuclei
(University of the Western Cape, 2019) Mukwevho, Ndinannyi Justice; Triambak, Smarajit
The 9Be - 9B isospin doublet carries fundamental significance for both nuclear structure and nuclear astrophysics studies. The first excited 1/2+ state in 9Be is already well established. However, its isobaric analogue 1/2+ state in 9B has not been unambigously determined yet. Theoretically, two popular descriptions of the 9Bnucleus either use a cluster model with two unbound alpha particles held together by a covalent proton or using the shell model, as a 8Be core + proton in the sd shell. An experimental determination of the excitation energy of the first 1/2+ state in 9B will provide valuable information in validating the theoretical model that adequately describes such light unbound nuclei. Further, it will also provide a robust test of mirror (isospin) symmetry violations via measurements of mirror energy differences in the doublet. Although there have been several experimental attempts to characterize the first 1/2+ state in 9B several discrepancies still exist in reported values of the excitation energies. This thesis describes an experiment performed at iThemba LABS using the 9Be(3He,t)9B reaction to address the above issue. As a byproduct, the thesis also describes an additional determination of the excitation energy of the second J-pi = 1/2+, T = 3/2 state in 9B from the same experiment. This was performed in order to resolve a discrepancy related to the excitation energy of this state. The consequence of this measurement related to Isobaric Multiplet Mass Equation (IMME) for the excited T = 3/2, A = 9 quartet is discussed briefly.
Charge transfer efficiency and optical properties of P3HT/PCBM spin coated thin films
(UWC, 2009) Van Heerden, Brian Abraham; Arendse, Christopher J.; Malgas, G.F.
The efficiency of organic photovoltaics is influenced by the structure of the polymer, the morphology of the film, the interfaces between the layers, the choice of electron acceptor material and the ratio between the electron acceptor material and the polymer. In this project, we have used regioregular poly (3-hexylthiophene) (rr-P3HT) as the electron donor material and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a derivative of the C60 fullerene, as the acceptor material. Different weight-ratios of rr-P3HT to PCBM were prepared by stirring for several hours in a chloroform solution and subsequently spin coated onto crystalline silicon and transparent conductive oxide/glass substrates. The effect of the PCBM concentration on the photo-induced optical emission and absorption properties of rr-P3HT was investigated by photoluminescence and ultraviolet-visible spectroscopy, respectively. Changes in the structural properties, as a function of the weight-ratio, were probed by high-resolution transmission electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. Results show that the structural integrity and crystallinity of rr-P3HT is compromised with the addition of excessive amounts of PCBM, which has a negative impact on the optical absorption of rr-P3HT and the photo-induced charge transfer mechanism between the rr-P3HT and PCBM. This work illustrates that blending rr-P3HT with an equal weight of PCBM results in an optimum configuration for improved performance in organic photovoltaic devices.
Classifying non-redundancy in the HERA array
(University of the Western Cape, 2022) Malapane, Kabelo; Santos, Mario
HERA is a highly redundant radio interferometer array, where pairs of receivers with the same position vector between them should see exactly the same signal from the sky. We can use this fact to do a really good job of calibrating them. Unfortunately, the receivers are not perfectly identical, and so they don’t see exactly the same signal. This is called "non-redundancy". This project classifies the level of redundancy using a clustering machine learning technique. The aim is to see if any particular clustering algorithm can group different segments of the array into very similar blocks, so we can at least do a good job of redundantly calibrating within those blocks. We call this new calibration method, logi_cal, while the standard calibration method used in HERA is called redcal.
Comparison of neutron fluence spectra measured with NE213 proton recoil spectrometer and NE230 deuteron recoil spectrometer
(University of the Western Cape, 2014) Masondo, Vusumuzi; Herbert, M.S.; Nchodu, M.R.
A (5 cm × 5 cm) cylindrical NE213 liquid organic scintillator and a (2.5 cm × 2.5 cm)cylindrical NE230 liquid organic scintillator were used as spectrometers. A series of measurements was made with both the NE213 and NE230 spectrometers, with the time-of- flight technique used for neutron energy selection. Pulse height spectra for quasi- monoenergetic neutron beams of ~5-64 MeV produced by bombarding either a (1 mm) lithium metal target, or a (10 mm) beryllium target, or a (10 mm) graphite target with 66 MeV proton beam were measured with both spectrometers. Deuteron events identified by pulse shape discrimination were selected for measurements with the NE230 spectrometer while proton events were selected for measurements with the NE213 spectrometer. Response of the scintillator to protons using NE213 and deuterons using NE230 were obtained from the measured pulse height spectra. Detector efficiency of the NE213 spectrometer as a function of neutron energy was determined for n-p elastic scattering. The detector efficiency of the NE230 was determined relative to the well-known efficiency of the NE213 spectrometer, selecting either all or n-d elastic events in the pulse height spectra measured with the NE230 spectrometer. The detection efficiency of the NE230 spectrometer was also determined from the available cross-section for n-d elastic scattering as exploratory work. Neutron fluence spectra could be determined using the appropriate neutron detection efficiency for each spectrometer and were compared with each other. The results showed good comparison and encouragement demonstrating the reliability of neutron fluence spectral measurements withthe NE230 spectrometer using the time-of-flight technique.
Computational modelling of a hot-wire chemical vapour deposition reactor chamber
(University of Western Cape, 2020) Fourie, Lionel Fabian; Square, L. C.; Arendse, C. J.
In this thesis, I explore the subjects of fluid dynamics and the Hot-Wire Chemical Vapour Deposition (HWCVD) process. HWCVD, in its simplicity, is one of the more powerful and elegant deposition techniques available in thin film research which allows for both the growth and post deposition treatments of functional thin films. In the HWCVD process, the quality of the final films is determined by a fixed set of deposition parameters namely: temperature, pressure and the gas flow rate. Finding the optimal combination of these parameters is key to obtaining the desired film specifications during every deposition. Conducting multiple trial experiments to determine said parameters can be expensive and time consuming, this is where simulation methods come into play. One such simulation method is Computational Fluid Dynamics (CFD) modelling
Construction and analysis of exponential time differencing methods for the robust simulation of ecological models
(University of Western Cape, 2021) Farah, Gassan Ali Mohamed Osman; Patidar, Kailash C.
In this thesis, we consider some interesting mathematical models arising in ecology. Our focus is on the exploration of robust numerical solvers which are applicable to models arising in mathematical ecology. To begin with, we consider a simple but nonlinear second-order time-dependent partial differential equation, namely, the Allen-Cahn equation. We discuss the construction of a class of exponential time differencing methods to solve this particular problem. This is then followed by a discussion on the extension of this approach to solve a more difficult fourth-order time-dependent partial differential equation, namely, Kuramoto-Sivashinsky equation. This equation is nonlinear. Further applications include the extension of this approach to solve a complex predator-prey system which is a system of fourth-order time-dependent non-linear partial differential equations. For each of these differential equation models, we presented numerical simulation results.
Cosmology with HI intensity mapping: effect of higher order corrections
(University of the Western Cape, 2020) Randrianjanahary, Liantsoa Finaritra; Santos, Mario
One of the main challenges of cosmology is to unveil the nature of dark energy and dark matter. They can be constrained with baryonic acoustic oscillations (BAO) and redshift space distortions, amongst others. Both have characteristic signatures in the dark matter power spectrum. Biased tracers of dark matter, such as neutral hydrogen, are used to quantify the underlying dark matter density field. It is generally assumed that on large scales the bias of the tracer is linear. However, there is a coupling between small and large scales of the biased tracer which gives rise to a significant non-linear contribution on linear scales in the power spectrum of the biased tracer. The Hydrogen Intensity and Real-time eXperiment (HIRAX) will map the brightness temperature of neutral hydrogen (HI) over BAO scales thanks to the intensity mapping technique. We forecasted cosmological parameters for HIRAX taking into account non-linear corrections to the HI power spectrum and compared them to the linear case. We used methods based on Fisher matrices. We found values for the bias to error ratio of the cosmological parameters as high as 1 or 7, depending on the noise level. We also investigated the change in peaks location on the baryonic acoustic oscillations signal. The value of the shift goes up to Δk = 10-2h/Mpc with a reduction of amplitude of the BAO features from 16:33% to 0:33%, depending on the scales.
Coupling of single neutron configurations to collective core excitations in 162Yb using 163Yb
(University of the Western Cape, 2017) Sithole, Makuhane Abel; Schafer, John Sharpey
In odd-nuclei the single nucleon can couple to collective excitations of its even-even core nucleus . These collective excitations lie within the pairing gap and are therefore the lowest energy excitations of the core. Our physics motivation is to search for structures where an odd neutron couples to collective excitations of the 162Yb core. We also searched for high-K structures in this nucleus. The experiment 152Sm(16O,5n)163Yb at Elab = 93 MeV was performed to study 163Yb at iThemba LABS. The gamma-decays from the reaction products have been detected using the AFRODITE (AFRican Omnipurpose Detector for Innovative Techniques and Experiments) gamma-ray spectrometer [2] equipped with 8 escape-suppressed clover detectors.
Cross-sectional transmission electron microscopy of nickel silicide formation
(University of the Western Cape, 1993) Julies, Basil Allen; Knoesen, D; Pretorius, R
Silicides play a significant role in modern device technology. The operation of electronic devices heavily relies on the specific properties of metal-semiconductor interfaces. Although semiconductor technology has proven very successful in utilizing the properties of materials the knowledge on formation, structure and electrical behaviour of interfaces is still far from complete. In this study an investigation into the Ni-Si binary system was made. Several techniques namely Rutherford Backscattering Spectroscopy, Transmission Electron Microscopy, Scanning Electron Microscopy, Auger Emission Spectroscopy and X-ray Diffraction were employed in the characterization of nickel thin films on silicon and the respective silicides which were formed. Special attention was given to the phase transition from NiSi to NiSi2. First phase formation, namely Ni2Si, was investigated at a vacuum furnace temperature of 290°C. This phase was found to be polycrystalline and grew in layers of uniform thickness with sharp Si-silicide and Ni-silicide interfaces. Growth continued until all the Ni-metal was consumed. Second phase formation (NiSi) was observed at 330°C only after the Ni2Si has grown to its full thickness. This polycrystalline phase also grows in layers. These layers however, are not of uniform thickness, the interfaces between the silicide and silicon substrate therefore being less regular. It was found that NiSi grains could assume one of two crystal structures, orthorhombic or a FeSi cubic structure. Generally it seems as if NiSi initially crystallizes into an orthorhombic crystal structure, before undergoing an allotropic transformation to the FeSi cubic structure. Micro-diffraction was used to characterize individual grains. Final phase formation (NiSi2) was mainly examined at 750°C. A scanning electron microscopy investigation showed that after 5 minutes of annealing islands of NiSi2 was observed in a NiSi matrix. With longer annealing times these islands grew laterally and eventually joined up with others. Cross-sectional transmission electron microscopy very firmly confirms the presence of NiSi2 surrounded by NiSi. Rutherford backscattering, X-Ray diffraction and Auger electron spectroscopy complement these results. Scanning electron microscopy shows that after the coalescence of individual NiSi2 islands, holes appear on the grain boundaries. These holes probably result from an accumulation of vacancies on the grain boundaries during NiSi2 formation which occurs via Ni diffusion in NiSi into the underlying silicon. As the NiSi2 phase continues to grow these holes increase in size and later take on the same crystal structure as the surrounding cubic NiSi2 grains. Although the reaction: NiSi + Si => NiSi2 is thermodynamically favourable to occur at 750°C, it was found that even after 15 minutes of annealing at 750°C, some grains were still NiSi while many others had switched to NiSi2. Identification was once again done by micro diffraction. This means that there is more than just the thermodynamic aspect involved in deciding when NiSi should transform to NiSi2. A model has therefore been proposed in which the major factors in determining the time lapse for transformation to take place are presented. This model generally presents an atomistic approach which centres around the degree of Ni diffusion across the grain boundary of two individual grains. Observations also suggests that NiSi2 results from NiSi by a diffusion process although nucleation can take place at random. This model must not be seen as contradictory to models proposed in the literature which only allow for non-uniform growth at the Si-NiSi, interface, but must rather be seen as complementing it by allowing for diffusion processes as well.

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