DSpace Community:http://hdl.handle.net/10386/82014-07-13T20:02:03Z2014-07-13T20:02:03ZComparison of measured photon and electron beam dose distributions between 3D water phanton and profiler 2 scanning system, South AfricaMoji, Kabelo McDonaldhttp://hdl.handle.net/10386/10862014-06-07T22:01:38Z2014-01-01T00:00:00ZTitle: Comparison of measured photon and electron beam dose distributions between 3D water phanton and profiler 2 scanning system, South Africa
Authors: Moji, Kabelo McDonald
Abstract: Background and Objectives: To establish whether the profiler 2 scanning system can be used
as a substitute for the 3D-water phantom, by comparing the percentage depth doses and beam
profiles for both the photons and electron beams, and validating the results using CMS XiO
treatment planning system.
Methods: Beam data (profiles, percentage depth doses and absolute dosimetry) were acquired
for the two systems: (3D-water phantom and profiler 2 scanning system) for beam energies
6 MV and 15 MV photon beams, and 4, 6, 8, 10, 12 and 15 MeV electron beams generated by
the Elekta Synergy linear accelerator (linac) for the field sizes of 6 × 6 cm2, 10 × 10 cm2, 14 × 14
cm2, 20 × 20 cm2, and 25 × 25 cm2 at depths of 0.5 cm, 1.0 cm, 2.0 cm, and 5.0 cm respectively.
These measurements were acquired using ionization chambers in water and diode detectors in
Perspex. The acquired data was sent to CMS XiO treatment planning system for validation.
Results: In general, the dose distributions for both systems compared very well with
uncertainties within recommended limits. The largest maximum difference in symmetry was
1.6 % for a 6 MV photon beam defined at 25 × 25 cm2 field size. The largest maximum
difference in flatness was 2.77 % for a 4 MeV electron beam defined at 10 × 10 cm2 applicator
size. The penumbra largest maximum difference was 1.708 cm for 8 MeV electron beam defined
at 25 × 25 cm2 applicator size, which was outside the recommended limit of 1.2 cm. The largest
maximum difference in field size was 2.388 cm for a 6 MeV electron beam defined at 20 × 20
cm2 applicator size, which was outside the recommended limit of 0.4 cm.
The largest maximum difference in percentage depth dose at 10 cm depth was 1.69 % for the
6 MV photon beam. The absolute dose output measurements showed a very good agreement
between the two systems to a maximum percentage difference and highest standard deviation of
-0.99 % and 0.69 % respectively for the 6 MV photon beam. Validation measurements showed
an agreement to less than 1 % and 2 mm for percentage depth doses and beam profiles
respectively.
Conclusion and recommendation: From the results obtained, it is evident that the profiler 2
scanning system can be used as a substitute for the 3D-water phantom beam data acquisitions
during linear accelerator commissioning. The future work based on this study could be to study
the limitations involved with the profiler 2 scanning system when used during measurements for
commissioning of a linear accelerator. Limitations like field size (maximum field size of
20 × 30 cm2 at SSD = 100 cm), number of Perspex slabs to be used on top of the profiler 2
scanning system and diagonal profile measurements.
Description: Thesis ( MSc ( Physics) ) -- University of Limpopo, 2013.2014-01-01T00:00:00ZStatistical modelling of the relationship between learner support intervention and matric pass rates in Letlhabile area, North-West Province of South AfricaMatshego, Joseph Nkashehttp://hdl.handle.net/10386/10662014-05-31T22:01:10Z2014-05-29T00:00:00ZTitle: Statistical modelling of the relationship between learner support intervention and matric pass rates in Letlhabile area, North-West Province of South Africa
Authors: Matshego, Joseph Nkashe
Abstract: This study used statistical methods to determine the relationship between matric pass rates and interventions in the schools in the Letlhabile area. Nine schools were identified in this area. Five years (2007 to 2011) were looked at. Regressions methods were tried in which various forms of relationships were compared. The methods were linear, curvilinear (quadratic to polynomial of 4th power), exponential and power regressions were used in the tentative models investigated. The measures of bias and precision were used to compare the
models. Multicollinearity was also investigated where it was possible. Time series analysis was used to illustrate the trend patterns of the pass rates in the various schools as well as the pattern of the numbers of interventions. In most of these schools the numbers of interventions increased over the five years and in only a few schools the number of interventions decreased over these years. . A highlight of this study is that more interventions enhanced the matric pass rates. It was also evident that in the years in which the interventions decreased, the pass rates also de.creased. The regression methods investigated were all showing to be applicable in the prediction of pass rates from the numbers of interventions. This was concluded from realising that thelneasures of bias, precision and quality all ratified them. The methods were compared in order to select the best one based on the measures. The linear regression in which the pass
rates.areregressed on the number of interventions came out as the leading model in terms of all the criteria used.
Description: Thesis (MSc(Statistics)) -- University of Limpopo (Medunsa Campus), 2012.2014-05-29T00:00:00ZComputational modeling studies of cobalt pentlandite (Co₉S₈)Mehlape, Mofuti Amoshttp://hdl.handle.net/10386/9842013-11-02T22:01:06Z2013-01-01T00:00:00ZTitle: Computational modeling studies of cobalt pentlandite (Co₉S₈)
Authors: Mehlape, Mofuti Amos
Abstract: The intention of the current study is to investigate structure, ion transport and reactivity of various forms of the cobalt pentlandite, Co9S8, at different temperatures using atomistic simulation methods with the support of electronic structure calculations. The first interatomic potentials of Co9S8 were derived with input data as structure and elastic properties from experiment and electronic structure calculations respectively. The potentials were validated by running energy minimization and molecular dynamics calculations. Structure, elastic properties and phonon spectra were well reproduced, together with the complex high temperature transformations and melting of Co9S8 as deduced from crystal structure, radial distribution functions, density profiles and diffusion coefficients.
Amongst the high symmetry surfaces {111}, {101} and {101} atomistic surface energy calculations proposed the {111} surface of Co9S8 as the most stable in agreement with experimental morphologies, and water adsorption energies on the such surfaces which mostly agreed with those from electronic structure calculations. The structural and ion transport variations with temperature were investigated and predicted surface melting at lower temperatures than the bulk. The effects of hydration on the surfaces at low and high temperatures were also studied.
The structural and ion transport properties of Co9S8 nanoparticles of varying sizes, covered by high symmetry surfaces {111}, {101} and {100} were predicted using molecular dynamics method based on our derived interatomic potentials. The structural and ion transport properties of Co9S8 nanoparticles of varying sizes, covered by high symmetry surfaces {111}, {101} and {100} were predicted using molecular dynamics method based on our derived interatomic potentials. Generally for {111}, {101} nanoparticles, high temperature transitions were abrupt for smaller nanoparticles and these tended to disintegrate and form voids. However, for larger nanoparticles the transitions were more gradual. Transitions in the {100} bound nanoparticles were less dramatic for all sizes and the formation of voids was reduced at high temperatures. Generally, the melting temperatures of different sizes of nanoparticles increases with the particle size hence approach the bulk limit. The interaction of nanoparticles with water was investigated.
Description: Thesis (Ph.D. (Physics)) --University of Limpopo, 20132013-01-01T00:00:00ZComputer simulation study of apatite mineral surfaces and interfaces with silicatesMkhonto, Donaldhttp://hdl.handle.net/10386/9402013-08-24T22:01:17Z2005-01-01T00:00:00ZTitle: Computer simulation study of apatite mineral surfaces and interfaces with silicates
Authors: Mkhonto, Donald
Abstract: We have derived a potential model for °uorapatite Ca10(PO4)6F2, ¯tted
to structure, elastic constants and vibrational frequencies of the phosphate
groups, which is compatible with existing calcite and °uorite potential mod-
els. We then modelled the structure and stabilities of the dry and hydrated
f0 0 0 1g, f1 0 1 0g, f1 0 1 1g, f1 1 2 0g, f1 0 1 3g and f1 1 2 1g surfaces,
which calculations con¯rmed the experimental dominance of the f0 0 0 1g
surface, which is prominently expressed in the calculated thermodynamic
morphologies. The dehydrated morphology further shows the experimental
f1 1 2 1g twinning plane, while the f1 0 1 0g cleavage plane is expressed in
the hydrated morphology. Molecular adsorption of water has a stabilising
e®ect on all six surfaces, where the surfaces generally show Langmuir be-
haviour and the calculated hydration energies indicate physisorption
(73 - 88 kJ mol¡1). The chains of °uoride ions surrounded by hexagonal
calcium channels can become distorted in two major ways during relaxation:
either by a shortening/lengthening of the FF distances, when the channel is
perpendicular to the surface, or by distortion of the CaF bonds when the
channel is parallel to the surface. Both distortions occur when the channel
runs at an angle to the surface. Other relaxations include compression of the
calcium sub-lattice and rotation of surface phosphate groups.
We have modelled adsorption of a range of organic molecules onto dif-
ferent °uorapatite surfaces, due to the importance of organic/ inorganic in-
teractions in biological situations. We have selected organic molecules that
represent a model for the carboxylic acids, alkyl hydroxamates and those
3
that contain both the aldehyde and hydroxyl functional groups. Adhesion of
these organic molecules on the surfaces has shown strong interaction between
the surface's Ca ions and the molecule's oxygens, more especially the car-
bonyl oxygens than any other interactions. It was found that the number of
interactions between the ions of adsorbate molecule and the mineral surfaces
thus contribute signi¯cantly to the exothemicity of adsorption.
Further more, simulations of apatite thin ¯lms at a range of ®-quartz
surfaces have shown how the strength of adhesion between thin ¯lms of ap-
atite material and ceramic silica surfaces is crucially dependent upon both
the orientation of the ¯lm relative to the substrate and the nature of the
silica surfaces, a ¯nding that is important in a wide number of applications,
from basic geological research on intergrowth of phosphate and silicate rock
minerals to the search for more e®ective surgical implant materials. It was
shown that although the unrelaxed quartz surface is more reactive toward the
apatite ¯lm, the more regular thin ¯lm structures grown at the pre-relaxed
quartz surfaces lead to more stable interfaces. Film growth at the unrelaxed
quartz surface is energetically increasingly unfavorable, whereas growth at
the pre-relaxed surface is calculated to continue beyond the ¯rst layer, where
the adhesion energy is convergent with the layer growth of the thin ¯lm. Ad-
hesion of apatite thin ¯lm on hydroxylated surfaces of ®-quartz has shown
to be energetically less favourable than at dry surfaces. This was because
the thin ¯lm interact mainly with the hydroxyl ions on the surface of quartz.
However, the adhesion energy is still convergent with layer growth of the thin
¯lm on the hydroxylated surfaces.
Description: Thesis (Ph.D (Physics)) --University of Limpopo, 20052005-01-01T00:00:00Z