### Browsing by Subject "Physics"

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- ItemOpen AccessA new compact neutron spectrometer(2016) Comrie, Angus; Buffler, Andrew; Smit, RickyA new compact neutron spectrometer has been designed, developed and characterized. The detector is based on EJ299-33 plastic scintillator coupled to silicon photomultipliers, and a digital implementation of pulse shape discrimination is used to separate events associated with neutrons from those associated with gamma-rays. The spectrometer is suitable over the neutron energy range 1 – 100 MeV, and the development illustrated with measurements made using an Am-Be radioisotopic source, a D-T sealed tube neutron generator and quasi-monoenergetic neutron beams produced using the iThemba LABS cyclotron. A segmented variation of the spectrometer is capable of providing directional information through the comparison of count rates between scintillator cells.
- ItemOpen AccessA New Digital Data Acquisition System for Neutron Metrology(2022) Sole, Chloe; Buffler, Andrew; Hutton,TanyaWithin the neutron metrology and spectrometry community digital pulse processing systems are being developed for measurements of Fast neutron fields in a wide variety of contexts, for example at accelerator and medical radiation facilities, around nuclear power plants, in airplanes in flight and space stations. These fields often vary widely with respect to both energy and intensity, which complicates measurements of energy dependent fluence. Investigations have been completed into the suitability of a CAEN DT5730 digitiser unit as a viable alternative to a traditional analogue system for data acquisition for fast neutron metrology. Experiments were completed at the fast neutron facilities of AMANDE (IRSN, Cadarache) using a BC501A scintillation detector and both the DT5730 digitiser and an analogue system based on NIM electronics and an MPA-3 multichannel analyser acquisition unit. Follow-up measurements were made at the n-lab facility at the University of Cape Town. The measurements covered an energy range from 0.5 MeV to 20 MeV, over a large range of intensities. The energy and intensity response for both systems and digital configurations were investigated based on the unfolding of measured light output spectra using an existing neutron response matrix for the detectors. Deadtime behaviour, rate dependent losses and linearity over the energy range were explored. The quality of the measured neutron spectra was compared through both uncertainty budgets and shape comparison analyses. Factors to consider when migrating from analogue signal processing to digital signal processing are discussed for the measured energy and intensity range, including advantages and disadvantages using digital pulse processing for metrology both in the laboratory and in the field. While there is more research required for a thorough bench-marking, the present results indicate that digital data acquisition technology has matured to the point where it can now be considered for use within neutron metrology.
- ItemOpen AccessA search for tWZ production in the Full Run 2 ATLAS dataset using events with four leptons(2021) Reich, Jake; Keaveney, James; Yacoob, Sahal
- ItemOpen AccessA search for tWZ production in the trilepton channel using Run 2 data from the ATLAS experiment(2021) Warren, Benjamin; Keaveney, JamesThis dissertation describes an analysis of events containing three leptons from 136 fb−1 of proton-proton collision data, with a centre of mass energy of 13 TeV, recorded by the ATLAS detector between 2016 and 2018. The aim of the analysis was to search for evidence of a top quark produced in association with a W boson and a Z boson (tWZ). An event selection scheme was developed using simulation to broadly suppress background events and to select signal events. Events were separated into mutually-exclusive regions of phase space to increase the ratio of signal to backgrounds and to calibrate the modelling of the backgrounds. Background events were further suppressed through the use of the Gradient Boosted Decision Tree (GBDT) machine learning algorithm. First, a hadronically-decaying W boson candidate was identified using a GBDT; this was used to suppress WZ background events. Then, an event-level GBDT was used to suppress all background events. A maximum likelihood fit was used to estimate the signal strength µ of tWZ production, where nuisance parameters were assigned to theoretical and experimental systematic uncertainties. The work presented here forms the basis of an official ATLAS experiment analysis, thus, the signal region was blinded to avoid potential biases in the future development of the official ATLAS analysis. The best fit value of µ resulting from a fit to a modified Asimov dataset was µˆ = 2.08+1.48 −1.45. This corresponds to an expected significance of Z exp µ = 0.72 σ. An expected upper limit µ exp up = 2.77+2.39 −1.28 was also determined from the fit to the modified Asimov dataset. Thus, this analysis has the potential to put the strongest ever constraint on tWZ production, but does not have the potential to observe tWZ production as predicted in the Standard Model. These constraints are limited by statistical uncertainties.
- ItemOpen AccessAlpha decay and alpha elastic scattering by heavy nuclei(1993) Tripe, Peter; Perez, Sandro MA simple three parameter cluster model has previously been developed by Buck, Merchant and Perez to successfully describe alpha decay half lives for more than 400 nuclei. An important feature of this model is that it envisages preformed (preformation factor, P=1.0) alpha particles in the parent nuclei to be moving in orbits with a large value of a global quantum number, G. The discontinuity in decay half-lives at the N=l26 neutron shell closure is then naturally explained by an increase of G as the alpha particle is forced into a higher orbit by the shell closure. We consider alternative approaches to this model and extend it to consider different values of P, and different changes in G at shell closures. We find the original approach of Buck et al., a radius fit with ΔG=2, is still the most successful, but that a potential fit with ΔG=4 turns out to be competitive, lending support to the suggestion that the proton shell closure is also felt. We have also analysed low energy (24.7MeV) scattering of alpha particles from a number of heavy nuclei in an attempt to find a common set of potential parameters that adequately reproduces both the decay and scattering data. Although not completely successful in this attempt, we find that the potential parameters obtained in the decay calculations provide a good first guess at the scattering potential parameters. The above analysis constrains P to the range 0.01≤P≤0.1, with the value of P=1.0 not ruled out.
- ItemOpen AccessAlpha decay of excited states of carbon-12(1970) Shackleton, David; Brooks, F DA crystal of the carbon-rich phosphor, anthracene, has been bombarded by monoenergetic fast neutrons. Alpha-particle decays of carbon nuclei have been separated from other events by a specialised application of the pulse shape discrimination technique. A numerical analysis of the data has yielded alpha-particle energy spectra over a wide energy range. A counter has been assembled to detect neutrons scattered by carbon nuclei in the crystal, and the measurements repeated in coincidence with these neutrons. The energy range of particle resolving power has been extended. Structure in the resulting alpha-particle and proton energy spectra is attributed to particle decays of specific levels in carbon-12. It is shown how the experiment may be modified and extended to measure decay parameters in absolute terms.
- ItemOpen AccessAlpha-radioactive isotopes in the marine environment.(1975) Higgo, Jennifer Joan Wynne; Cherry, R DVarious marine organisms collected from southern hemisphere waters around Cape Town were analysed by radiochemistry and alpha-spectrometry for Pu-239, 240. Many of these organisms were also analysed for Pu-238, Po-210 and - those isotopes of thorium and uranium that are alpha-emitters. In some samples an estimate of the Ra-226 content was also made. The Pu-239 concentrations are compared with the values reported in the literature for similar organisms found in the northern-hemisphere and an attempt is made to relate the differences in concentration to differences in the quantity of fall-out Pu-239 delivered to the sea-surface in the two hemispheres. Within the limits of experimental error it appears that the Pu-239 content of recent fall-out is reflected in the Pu-239 concentration in many marine organisms.
- ItemOpen AccessAn uncertainty budget for the precursor Watt balance for South Africa(2019) Mametja, Thapelo Given; Buffler, Andrew; Karsten, AlettaThe 26th General Conference on Weights and Measures (CGPM) held on the 16th November 2018 has adopted the revision of the International system of units (SI) to be based on the fundamental physical constants.
- ItemOpen AccessAnalysis of a deep neural network for missing transverse momentum reconstruction in ATLAS(2020) Leigh, Matthew; Yacoob, Sahal; Young, ChristopherThe ATLAS detector is a multipurpose particle detector built to record almost all possible decay products of the high energy proton-proton collisions provided by the Large Hadron Collider. The presence and combined kinematics of unobserved particles can be inferred by the observed momentum imbalance in the transverse plane. In this work, a deep neural network was trained using supervised learning to measure this imbalance. The performance of this network was evaluated in MC simulation and in 43 fb⁻¹ of data recorded at ATLAS. The network offered superior resolution and significantly better pileup resistance than all other pre-existing algorithms in every tested topology. The network also provided the best discriminator between events that did and did not contain neutrinos. The potential gain insensitivity to new physics was demonstrated by using this network in a search for the electroweak production of supersymmetric particles. The expected sensitivity to observe the production of said particles was increased by up to 26%.
- ItemOpen AccessAnalysis of combined isotropic and anisotropic line-width data with a view to improved rotational CARS thermometry(2002) Horner, Mark John Newlyn Horner; Robertson, GThe development of a new line-width model and recently measured rotational (anisotropic) line-widths has allowed for a critical analysis of the various basis rates models. These basis rates will be analysed using both vibrational (isotropic) and rotational line-width data. The data sets are analysed individually and in combination. For all fits, parameter correlations are determined. In addition, the sensitivity of the goodness of fit to variations in the effective interaction length for collisions, le, is investigated. To complete the investigation full rotational spectra are calculated and compared with experimental spectra.
- ItemOpen AccessAnalytical application of proton-induced prompt photon specrometry(1982) Gihwala, Dherendra; Peisach, MaxPrompt gamma-rays emitted tinder bombardment with 3.5 to 6.0MeV protons from the 77 stable non-gaseous elements were evaluated for analytical application. The compilation included the yields of about 2200 gamma-rays and their detection limit for analysis. Because the data were measured under identical experimental conditions, the relative values may be generally applicable. For each element, the gamma-ray spectrum and the numerical data are presented in tables listing identified gamma-rays with yields and detection limits for Ep = 4.5 MeV, the gamma yields as a function of proton energy for the most intense gamma-prays, and, the polynomial coefficients of the function relating the detection limit to the bombarding energy. An Atlas of Spectra and a Catalogue of gamma-rays summarised the data.
- ItemOpen AccessThe anomalous magnetic moment of baryons in cavity QCD(1992) Lindebaum, R J; Viollier, Raoul DUsing a generalised form of the Gell-Mann and Low theorem, all the diagrams in cavity QCD to order as that contribute to the magnetic moment are calculated. The calculations are performed for massive quarks so a mass renormalisation scheme has been developed to cope with the new divergences this brings into the self-energy insert diagrams. The results of this work show that no improvement on the simple SU(3) model is made by including these corrections. These calculations point to a smaller value of αs than that which is usually used.
- ItemOpen AccessThe anomalous magnetic moment of the nucleon in cavity QCD(1991) O'Connor, M SPerturbative quantum chromodynamics is developed in a spherical cavity using a symmetric form of the Gell-Mann and Low theorem. This formalism allows one to generate any desired term in the perturbation series, in a manner which is similar to the familiar Feynman rules in free space. All corrections to order eg² in the electromagnetic and strong coupling constants which contribute to the magnetic moment of a baryon are generated using this formalism. The O(eg²) radiative corrections to the magnetic moment of the nucleon are calculated here in an arbitrary covariant gauge. The gauge-dependent parts are found to vanish identically, and the divergences arising from the loop diagrams cancel amongst each other, making renormalization unnecessary. However, it is shown here that one can, if it is necessary, remove the divergences from the cavity diagrams by subtracting from them a singular factor which is found using dimensional regularization in the analogous free-space diagrams.
- ItemOpen AccessApplication of a Bragg curve detector to inclusive (p, α) reactions to the continuum(2003) Thovhogi, Tshilidzi; Fearick, Roger; Fortsch, S VThe Bragg curve spc
- ItemOpen AccessThe application of Bayesian statistics and maximum entropy to Ion beam analysis techniques(1997) Padayachee, Jayanethie; Prozesky, Victor M; Britton, David TThe elimination of some blurring property, such as the detector response function, from spectra has received a considerable amount of attention. The problem is usually complicated by the presence of noise in the data, and in general, there exists an infinite set of possible solutions which are consistent with the data within the bounds imposed by the noise. Such a problem is known, generally, as an ill-defined inverse problem. Many techniques have been developed in an attempt to solve inverse problems, for example the problem of deconvolution, but these techniques employ ad hoc modifications to solve different problems. Bayesian Statistics has been proved to be the only consistent method for solving inverse problems of the type where the information is expressed in terms of probability distributions. This dissertation presents results of applying the Bayesian formalism, together with the concepts of maximum information entropy and multiresolution pixons, to various inverse problems in ion beam analysis; The results of this method of deconvoluting Rutherford Backscattering Spectrometry (RBS) and Proton Induced X-ray Emission (PIXE) spectra are compared to the results from other deconvolution techniques, namely Fourier Transforms, Jansson's method and maximum entropy (MaxEnt) without pixons. All the deconvolution techniques show an improvement in the resolution of the RBS spectra but only the MaxEnt techniques show a significant improvement in the resolution of the PIXE spectra. The MaxEnt methods also produce physically acceptable results. The MaxEnt formalism was applied to the extraction of depth profiles from RBS and PIXE spectra and yielded good results. The technique was also used to deconvolute the beam profile from one-dimensional nuclear microprobe scans.
- ItemOpen AccessAtomic mobility in thin solid Pa2Si films(1985) Zingu, Edmund Charles; Comrie, Craig MA theory for the growth kinetics of planar silicide formation in single- and bi-layer metal silicon systems has been developed on the basis that the chemical potential gradient in the growing layer is the driving force for diffusion. The predictions of the theory, when applied to single layer metal-silicon systems, is in agreement with other theories and with experimental results. Planar growth of the outer silicide layer in bilayer metal-silicon systems is predicted to proceed linearly with time, both when controlled by an interfacial reaction and when limited by diffusion through the interposed silicide layer (when this layer is sufficiently thick). In the latter case it is predicted that the growth rate of the outer silicide layer is inversely proportional to the thickness of the interposed layer.
- ItemOpen AccessAxial segregation of granular flows in rotating drums(2017) Ahmed, Elbasher M E; Govender, IndresanA mechanistic model of axial segregation in rotating drums is presented for mixtures of granular material. We show that grains composed of different material properties which are heuristically argued to manifest as differences in frictional properties at the continuum scale-diffuse into axial bands as a consequence of concentration fluctuations in the free surface layer caused by friction- limited mobility. The model is composed of two new ingredients that success- fully recover the well-known phenomenon of axial banding and subsequent band coarsening in the long-time evolution: (1) A Bagnoldian stress assumption facilitated band formation at drum fill levels < 50% that have hitherto not been possible with a Newtonian shear stress ansatz, through the formation of an asymmetric free surface profile that is experimentally verified by the nuclear imaging technique of Positron Emission Particle Tracking. (2) The geometric slope corresponding to the gravity-driven axial flux between adjacent bands is (binomially) expanded to second order thereby ensuring band coarsening in the long-time limit for all boundary conditions investigated. Numerical implementation of the axial diffusion model is shown to be sensitive to the boundary conditions of the tumbling mill (particle size, volume concentration, mill speed, average friction, end wall friction, drum length and drum diameter). The explicit solutions for a binary-, ternary-, quaternary- and n-species granular mixture was developed with numerical implementation up to n = 4. Consistent with experimental observations in the literature, the 3-species mixture successfully captures bands within bands, while the 4-species mixture successfully recovers bands within bands, within bands.
- ItemOpen AccessBoltzmann equation studies of the off-equilibrium QCD phenomena(2016) Viljoen, Brandon; Peshier, AndréMuch of the evolution of the quark-gluon plasma (QGP) produced in heavy-ion collisions can be modeled by relativistic (viscous) hydrodynamics, which assumes that the partons are sufficiently close to equilibrium. We would like to explore the dynamics of QGP, even before this (quasi) equilibrium stage is reached. To that end, a useful tool from the stage where we can assume the partons to be on-shell is the relativistic Boltzmann equation. We develop parallel code to solve the relativistic Boltzmann equation in the relaxation time approximation in 3 + 1 dimensions (without simplifying assumptions on possible symmetries of the dynamics). Our approach, solving for the distribution function, will allow us to obtain detailed information about the dynamics of heavy ion collisions beyond hydrodynamics, which specify only bulk properties of the medium. Following recent work, we also explore the possibility of forming a transient Bose-Einstein condensate in a dense system of gluons, such as those found in the early stage of a heavy ion collision. For simplicity, we focus here on purely gluonic systems (without quark degrees of freedom). We first use our code to describe a system undergoing 0 + 1 longitudinal Bjorken expansion, after which we present some first numerical results for a system in the full 3 + 1 dynamics.
- ItemOpen AccessBose-Einstein condensation from a gluon transport equation(2018) Harrison, Brent; Peshier, AndreIn this thesis we investigate the evolution of the quark gluon plasma, as produced in the early stages of a relativistic heavy ion collision, towards equilibrium. To this end we put forward a new numerical scheme to solve the QCD Boltzmann equation in the small-scattering angle approximation, which we develop here for the quenched limit of QCD. We initially restrict our analysis to spatially homogeneous systems of gluons distributed isotropically in momentum space. With our scheme we confirm results of Blaizot et al. [1], in particular that for certain “overpopulated” initial conditions, a transient Bose-Einstein condensate emerges during equilibriation in a finite time. We further analyse the dynamics of the formation of this condensate. We then extend our scheme to systems with cylindrically symmetric momentum distributions, in order to investigate the effects of anisotropy. In particular we compare the rates at which isotropization and equilibriation occur. We also compare our results from the small-scattering angle scheme to the relaxation-time approximation.
- ItemOpen AccessBound states of heavy and light quarks in the framework of quantum chromodynamics(1985) Gavin, Elizabeth Jane Ogle; Viollier, Raoul DThe spectra of the D, F, B and E mesons have been calculated using the MIT bag model together with a static potential related to the Fourier transform of the "dressed" gluon propagator. The heavy quark has been assumed to coincide with the centre of the bag, while the Light antiquark was treated relativistically using the Dirac equation. The spectra obtained are compared with experimental data as well as with the results of other models of these Qq mesons. The ratio mb/mc obtained in the fit to experimentally known states is compared with the result expected from the hyperfine splitting of the D and B mesons. It appears that tris ratio is model dependent. More experimental data are required to further evaluate the validity of this model.