Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: See full text for abstract / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: In the current design of the high temperature gas cooled reactor, a small fraction of coated fuel particles will be defective. Hence, 110Ag may be released from the fuel spheres into the coolant gas (helium) and plate out on the cooler surfaces of the main power system. This poses a radiation risk to operating personnel as well as general public. The objectives of this thesis were to design and construct an apparatus in which silver-109 atoms may be produced and deflected in an inhomogeneous and homogeneous magnetic field, compare experimental and theoretical results, and make a recommendation based on the findings of this thesis to the idea of removing silver-110 atoms from the helium fluid by deflecting them with an inhomogeneous magnetic field onto target plates situated on the inner perimeter of a helium pipe. The experimental results for the deflection of the collimated Ag- atoms with the round-hole collimators showed a deflection of 1.77° and 2.05° of the Ag- atoms due to an inhomogeneous magnetic field when the target plate was positioned 13 and 30 mm away from the magnet, respectively. These values were considerably greater than 0.01° and 0.02° that were calculated for the average velocity of atoms, v = 500 m/s. The case where Ag- atoms were collimated with a pair of slits and the target plate positioned 13mm away from the magnet showed the following: An inhomogeneous magnetic field changes the rectangular shape of the beam to a roughly elliptical shape. The beam of Ag- atoms was not split into two separate beams. This was caused by the beam of Ag- atoms consisting of atoms travelling at different speeds. The maximum deflection of Ag- atoms was 1.16° in the z direction and 1.12° in the x direction. These values were also significantly greater than 0.01 mm calculated at v = 500 m/s. This huge difference between the theoretical and experimental results raised a conclusion that the size of each Ag deposit depended mostly on the exposure time that was given to it. It was noticed that the beam of Ag- atoms was not split into two separate beams, in both cases. The conclusion was that the technique of removing Ag- atoms from the helium stream by means of an inhomogeneous magnetic field may not be effective. This is due to the inability of the inhomogeneous magnetic field to split the beam of Ag- atoms into two separate beams in a vacuum of ~10-5 mbar. It would be even more difficult for an inhomogeneous magnetic field to split the beam of Ag- atoms in helium, due to the Ag- atoms having a shorter mean free path in helium compared to a vacuum. / AFRIKAANSE OPSOMMING: In die huidige ontwerp van die hoë temperatuur gas afgekoelde reaktor, is 'n klein fraksie van omhulde brandstof deeltjies foutief. 110Ag kan dus vrygestel word vanaf die brandstof sfere in die verkoelingsgas (helium) wat dan op die koeler oppervlaktes van die hoofkragstelsel presipiteer. Hierdie 110Ag deeltjies hou 'n bestraling risiko vir die bedryfpersoneel sowel as vir die algemene publiek in. Die doelwitte van hierdie verhandeling is eerstens om 'n apparaat te ontwerp en konstrueer wat silwer-109 atome produseer en nie-homogene en homogene magnetiese velde deflekteer,. Tweedens om die eksperimentele en teoretiese resultate met mekaar te vergelyk. Derdens om 'n aanbeveling te maak gebasseer op die bevindinge van hierdie verhandeling rakende die verwydering van silwer-110 atome uit die helium vloeistof deur hulle met 'n nie-homogene magneetveld te deflekteer op die teikenplate binne-in 'n helium pyp. Die eksperimentele resultate vir die defleksie van die gekollimeerde Ag-atome met die ronde gat kollimators toon ‘n defleksie van 1.77° en 2.05° van die Ag-atome as gevolg van ‘n nie-homogene magneetveld wanneer die teikenplaat 13mm en 30mm, onderskeidelik, vanaf die magneet geposisioneer is. Hierdie waardes is aansienlik groter as die teoretiese defleksies van 0.01° en 0.02o wat bereken is vir ‘n gemiddelde snelheid van 500 m/s vir die atome. Die geval waar Ag-atome met 'n paar splete gekollimeer is en die teikenplaat 13 mm weg van magneet geposisioneer is, is die volgende resultate verkry: 'n nie-homogene magneetveld verander die reghoekige vorm van die bondel na 'n rowwe elliptiese vorm. Die bondel Ag-atome is nie volkome twee afsonderlike bundels verdeel nie. Dit is omdat die bondel van Ag-atome bestaan uit atome wat teen verskillende snelhede beweeg. Die maksimum defleksie van Ag-atome is 1.16° in die z-rigting en 1.12° in die x-rigting. Hierdie waardes is ook aansienlik groter as 0.01° bereken teen 500 m/s. Hierdie groot verskil tussen die teoretiese en eksperimentele resultate dui daarop dat die grootte van elke Ag neerslag grootliks afhanklik is van die blootstellingstyd wat daaraan gegee is. Daar is vasgestel dat die straal van Ag-atome in beide gevalle nie in twee afsonderlike bondels verdeel nie. Die gevolgtrekking is dat die tegniek van die verwydering van Ag-atome uit die helium stroom deur middel van 'n nie-homogene magneetveld nie effektief is nie. Dit is te wyte aan die onvermoë van die nie-homogene magneetveld om die bondel Ag-atome te verdeel in twee afsonderlike bondels in 'n vakuum van ~ 10-5 mbar. Dit sou selfs nog moeiliker vir 'n nie-homogene magnetiese veld wees om die bundel Ag-atome in helium te verdeel, weens die korter gemiddelde beskikbare pad van Ag-atome in helium wanneer dit met 'n vakuum vergelyk word.

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Active materials are a subset of soft matter that is constantly being driven out of an equilibrium state due to the energy input from internal processes such as the hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP), as found in biological systems. Firstly, we construct and study a simple model of a flexible filament with an active crosslinker/molecular motor. We treat the system on a mesoscopic scale using a Langevin equation approach, which we analyse via a functional integral approach using the Martin-Siggia-Rose formalism. We characterise the steady state behaviour of the system up to first order in the motor force and also the autocorrelation of fluctuations of the position of the active crosslink on the filament. We find that this autocorrelation function does not depend on the motor force up to first order for the case where the crosslinker is located in the middle of the contour length of the filament. Properties that characterise the elastic response of the system are studied and found to scale with the autocorrelation of fluctuations of the active crosslink position. Secondly, we give a brief overview of the current state of dynamical polymer network theory and then propose two dynamical network models based on a Cayley-tree topology. Our first model takes a renormalisation approach and derive recurrence relations for the coupling constants of the system. The second model builds on the ideas of an Edwards type network theory where Wick’s theorem is employed to enforce the constraint conditions. Both models are examined using a functional integral approach. / AFRIKAANSE OPSOMMING: Aktiewe stelsels is ’n subveld van sagte materie fisika wat handel oor sisteme wat uit ekwilibruim gedryf word deur middel van interne prossesse, soos wat gevind word in biologiese stelsels. Eerstens konstruëer en bestudeer ons ’n model vir ’n buigbare filament met ’n aktiewe kruisskakelaar of molekulêre motor. Ons formuleer die stelsel op ’n mesoskopiese skaal deur gebruik te maak van ’n Langevin vergelyking formalisme en bestudeer die stelsel deur gebruik te maak van funksionaal integraal metodes deur middel van die Martin-Siggia-Rose formalisme. Dit laat ons in staat om die tydonafhankle gedrag van die stelsel te bestudeer tot op eerste orde in die motorkrag. Ons is ook in staat om die outokorrelasie fluktuasies van die posisie van die aktiewe kruisskakelaar te karakteriseer. Ons vind dat die outokorrelasie onafhanklink is van die motorkrag tot eerste orde in die geval waar die kruisskakelaar in die middel van die filament geleë is. Die elastiese eienksappe van die sisteem word ook ondersoek en gevind dat die skaleer soos die outokorrelasie van die fluktuasies van die aktiewe kruisskakelaar posisie. Tweedens gee ons ’n vlugtige oorsig van die huidige toestand van dinamiese polimeer netwerk teorie en stel dan ons eie twee modelle voor wat gebasseer is op ’n Caylee-boom topologie. Ons eerste model maak gebruik van ’n hernormering beginsel en dit laat ons toe om rekurrensierelasies vir die koppelingskonstates te verkry. Die tweede model bou op idees van ’n Edwards tipe netwerk teorie waar Wick se teorema ingespan word om die beperkingskondisies af te dwing. Beide modelle word met funksionaal integraal metodes bestudeer.

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Ultrafast electron diffraction is a powerful method to study atomic movement in crystals on sub-picosecond timescales. This thesis consists of three parts. In part one the ultrafast electron diffraction machine is described, followed by improvements that were made and techniques that were developed in order to bring the system to state of the art level and enable the acquisition of suffcient data to obtain information on the structural dynamics in crystals. The second part contains a description of the sample which was studied in our fi rst time-resolved measurements, the transition-metal dichalcogenide 4Hb-TaSe2. This particular crystal is an example of a strongly coupled electronic system which develops a charge density wave (CDW) accompanied by a periodic lattice distortion (PLD). An overview of the formation of electron diffraction patterns and what can be learned from them are also given, followed by the results of the ultrafast electron diffraction experiments done with 4Hb-TaSe2. Part three describes an alternative source to study dynamics in crystalline samples, namely laser plasma-based ultrafast X-ray diffraction. The ultrafast electron diffraction group functions as a unit, but my tasks ranged from sample preparation and characterisation of the electron beam to the setting up and execution of experiments. I was involved in analysing the data and contributed small parts to the data analysis software. / AFRIKAANSE OPSOMMING: Ultravinnige elektron diffraksie is a metode om die beweging van atome in kristalle op sub-pikosekonde tydskale te bestudeer. Hierdie tesis bestaan uit drie dele. In deel een van die tesis word die ultravinnige elektron diffraksie masjien beskryf, gevolg deur verbeteringe wat aangebring is en tegnieke wat ontwikkel is om die sisteem tot op 'n wêreldklas vlak te bring waar die insameling van genoegsame data om inligting oor die strukturele dinamika in kristalle te bekom, moontlik is. Die tweede deel bevat 'n beskrywing van die monster wat in ons eerste tydopgeloste eksperimente gebruik is, naamlik die oorgangsmetaaldichalkogenied 4Hb-TaSe2. Hierdie kristal is 'n voorbeeld van 'n sterk gekoppelde elektroniese sisteem wat 'n ladingsdigtheid-golf en 'n gepaardgaande periodiese versteuring van die kristalrooster ontwikkel. 'n Oorsig van die formasie van elektron diffraksiepatrone en wat ons daaruit kan leer word ook gegee. Daarna word die resultate van die ultravinnige elektron diffraksie eksperimente wat op 4Hb-TaSe2uitgevoer is beskryf en bespreek. In deel drie word 'n alternatiewe metode om die dinamika in kristalmonsters te bestudeer, naamlik laser plasma-gebaseerde ultravinnige X-straal diffraksie, beskryf. Die ultravinnige elektron diffraksie groep funksioneer as 'n eenheid, maar my verantwoordelikhede het gestrek van die voorbereiding van monsters en die karakterisering van die elektron bundel tot die opstel en uitvoer van eksperimente. Ek was ook betrokke by die analisering van data en het dele van die data analise sagteware geskryf.

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Nine 100x100x600 mm3 plastic scintillators, formerly built for the neutron time of ight measurements at iThemba LABS, were refurbished. The position resolution of these detectors was determined using muon cosmic rays and coincident measurement techniques. Average position resolution of 4.28 cm (FWHM) was found. In order to predict the time spectrum of the large-volume detector Monte Carlo simulations have been performed. These simulations aimed at anticipating the separation of statistical neutrons, prompt gamma rays and uncorrelated gamma rays from the fast neutrons emitted in the reaction of interest. One of the neutron detectors was tested using fast neutrons from the 232Th( ,xn) reaction at 42 MeV. Statistical neutrons from fusion evaporation reactions were produced in 152Sm(12C,xn) fusion evaporation reaction. Coincidences between neutrons and gamma rays were successfully identi ed. Prompt gamma rays and uncorrelated gamma rays were also identi ed. / AFRIKAANSE OPSOMMING: Nege 100x100x600 mm3 plastiese scintillators, wat aanvanklik gebou was vir neutron vlugtyds meetings by iThemba LABS, was hernu. Die posisie resolusie van die detektore was bepaal deurmiddel van muon kosmiese straling en koïnsidensie meet tegnieke. Posisie resolusie van 4.28 cm (FWHM) was verkry. Monte Carlo simulasies is gebruik om die posisie resolusie van'n groot volume detektor te voorspel. Hierdie simulasies is daarop gemik om onderskeid te maak tussen statistiese neutrone, gelyktydige gamma strale en ongekorreleerde gamma strale vanaf vinnige neutrone in die reaksie van belang uitgestraal word. Een neutron detektor was getoets deur gebruik te maak van vinnige neutrone wat uit die reaksie 232Th( ,xn) by 42 MeV ontstaan. Statistiese neutrone vanaf splitsings verdampingsreaksies, gelyktydige gamma strale en ongekorreleerde gamma strale was geidenti seer. Statistiese neutrone van samesmelting verdamping reaksies was geproduseer in die reaksie 152Sm(12C,xn). Toeval tussen neutrone en gamma strale was suksesvol geïdenti seer, gevra gamma strale en ongekorreleerd gamma strale was ook geïdenti seer.

Thesis (MSc (Physics))--Stellenbosch University, 2008. / Nuclear physics can be applied in various ways to the study of neutron stars. This thesis reports on one such application, where the relativistic mean-field approximation has been employed to calculate the equations of state of matter in the neutron star interior. In particular the equations of state of nuclear and neutron star matter of the NL3, PK1 and FSUGold parameter sets were derived. A survey of available literature on neutron stars is presented and we use the derived equations of state to reproduce the properties of saturated nuclear matter as well as the mass-radius relationship of a static, spherical symmetric neutron star. Results are compared to published values of the properties of saturated nuclear matter and to available observational data of the mass-radius relationship of neutron stars.

Thesis (MSc (Physics))--University of Stellenbosch, 2007. / A relativistic plane wave model is developed for electromagnetic production of unbound hyperons with free kaons from nuclei. The differential cross section is expressed as a contraction of leptonic and hadronic tensors. The leptonic tensor is constructed by using the helicity representation of a free Dirac spinor. A model for the corresponding elementary process is used to calculate the hadronic tensor, in which the hadronic current operator Jˆμ is written as a linear combination of six invariant amplitudes and six Lorentz and gauge invariant quantities. The kinematics for this process is assumed to be a quasi-free process i.e., the electron interacts with only one bound nucleon inside the nucleus. The bound state wavefunction of the bound nucleon is calculated within the framework of the relativistic mean-field approximation. The unpolarized differential cross section for the K+ electroproduction process, e + A −→ e + K+ + A + Aresidual is calculated as a function of the hyperon scattering angle.

Thesis (MSc (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Non-commutative quantum mechanics is a generalisation of quantum mechanics which incorporates the notion of a fundamental shortest length scale by introducing non-commuting position coordinates. Various theories of quantum gravity indicate the existence of such a shortest length scale in nature. It has furthermore been realised that certain condensed matter systems allow effective descriptions in terms of non-commuting coordinates. As a result, non-commutative quantum mechanics has received increasing attention recently. A consistent formulation and interpretation of non-commutative quantum mechanics, which unambiguously defines position measurement within the existing framework of quantum mechanics, was recently presented by Scholtz et al. This thesis builds on the latter formalism, extends it to many-particle systems and links it up with non-commutative quantum field theory via second quantisation. It is shown that interactions of particles, among themselves and with external potentials, are altered as a result of the fuzziness induced by non-commutativity. For potential scattering, generic increases are found for the differential and total scattering cross sections. Furthermore, the recovery of a scattering potential from scattering data is shown to involve a suppression of high energy contributions, disallowing divergent interaction forces. Likewise, the effective statistical interaction among fermions and bosons is modified, leading to an apparent violation of Pauli’s exclusion principle and foretelling implications for thermodynamics at high densities. / AFRIKAANSE OPSOMMING: Nie-kommutatiewe kwantummeganika is ’n veralgemening van kwantummeganika wat die idee van ’n fundamentele kortste lengteskaal invoer d.m.v. nie-kommuterende ko¨ordinate. Verskeie teorie¨e van kwantum-grawitasie dui op die bestaan van so ’n kortste lengteskaal in die natuur. Dit is verder uitgewys dat sekere gekondenseerde materie sisteme effektiewe beskrywings in terme van nie-kommuterende koordinate toelaat. Gevolglik het die veld van nie-kommutatiewe kwantummeganika onlangs toenemende aandag geniet. ’n Konsistente formulering en interpretasie van nie-kommutatiewe kwantummeganika, wat posisiemetings eenduidig binne bestaande kwantummeganika raamwerke defineer, is onlangs voorgestel deur Scholtz et al. Hierdie tesis brei uit op hierdie formalisme, veralgemeen dit tot veeldeeltjiesisteme en koppel dit aan nie-kommutatiewe kwantumveldeteorie d.m.v. tweede kwantisering. Daar word gewys dat interaksies tussen deeltjies en met eksterne potensiale verander word as gevolg van nie-kommutatiwiteit. Vir potensiale verstrooi ¨ıng verskyn generiese toenames vir die differensi¨ele and totale verstroi¨ıngskanvlak. Verder word gewys dat die herkonstruksie van ’n verstrooi¨ıngspotensiaal vanaf verstrooi¨ıngsdata ’n onderdrukking van ho¨e-energiebydrae behels, wat divergente interaksiekragte verbied. Soortgelyk word die effektiewe statistiese interaksie tussen fermione en bosone verander, wat ly tot ’n skynbare verbreking van Pauli se uitsluitingsbeginsel en dui op verdere gevolge vir termodinamika by ho¨e digthede.

Thesis (MSc)--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: We investigate the nuclei 20Ne, 44Ti, 94Mo, 136Te and 212Po using a model of an α-cluster orbiting a closed shell core. A purely phenomenological cluster-core potential is found to provide a successful description of the spectra, B(E2↓) transition strengths, and α-decay rates of the low-lying positive parity states of these nuclei. We then use the same potential as the real part of an optical model potential to describe the α elastic scattering by 16O, 40Ca, 90Zr and 208Pb. The experimental differential cross-section data are reasonably well reproduced with the imaginary potential depth as the only free parameter. The special case of the 8Be system is also analysed. / AFRIKAANSE OPSOMMING: Ons ondersoek die kerne 20Ne, 44Ti, 94Mo, 136Te en 212Po deur gebruik te maak van ’n model waar ’n α-bondel om ’n kern met ’n geslote skil wentel. ’n Suiwer fenomenologiese bondelkern potentiaal is gevind wat die energie spektra, B(E2 ↓) oorgangs sterktes, en α-verval tempo vir laagliggende positiewe pariteitstoestande vir hierdie kerne beskryf. Ons gebruik dieselfde potentiaal as die reele deel van die optiese potentiaal om die alpha elastiese verstrooiing deur die kerne 16O, 40Ca, 90Zr en 208Pb te beskryf. Die eksperimentele differensiele kansvlak data word redelik goed gereprodukseer met slegs die imaginere potensiaal diepte as die enigste vrye parameter. Die spesiale geval van 8Be is ook ondersoek.

Thesis (MSc)--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: A magneto optical trap (MOT) is capable of trapping a vapor cloud consisting of atoms cooled down to the micro Kelvin range. Three orthogonal pairs of counter-propagating laser beams of the correct circular polarisation form an optical molasses which facilitates the cooling of neutral atoms. Additionally a spatially non-uniform magnetic field produced by two current carrying coils in a Maxwell gradient configuration is used to trap the cooled atoms. In this report the effects of the trap parameters, including the laser beam intensity and frequency detuning, beam diameter and magnetic field gradient, on the number of trapped atoms are discussed. Secondly the development of an experimental setup for laser cooling and trapping of 87Rb atoms in vacuum with the aid of a MOT is presented. All trap components were implemented and characterised. The vacuum system and trapping chamber in which the cooling takes place were designed and constructed. A rubidium getter to act as a source of atoms was integrated into the vacuum system. The two external cavity diode lasers used for trapping and optical re-pumping were characterised. The optical setup required for the optical molasses was designed, constructed and characterised. Saturated absorption spectroscopy was performed to investigate the hyperfine structure of 87Rb and to frequency lock the lasers. We report on the current status of the project with regards to progress, results and future work. / AFRIKAANSE OPSOMMING: 'n Magneto-optiese val (magneto optical trap, MOT) kan 'n dampwolk van atome vang en afkoel tot in die mikro Kelvin bereik. Drie ortogonale pare laserbundels, elke paar voortplantend in teenoorgestelde rigtings, met die korrekte sirkelvormige polarisasie vorm 'n sogenaamde optiese molasse wat die afkoeling van neutrale atome moontlik maak. Bykomend word 'n ruimtelik nie-uniforme magneetveld geproduseer deur twee stroomdraende spoele in 'n Maxwell gradient-opstelling gebruik om die afgekoelde atome te vang. In hierdie verslag word die invloed van die val parameters, insluitend die laserbundel intensiteit en frekwensie afstemming, die laserbundel deursnit en magneetveld gradiënt, op die aantal atome in die val bespreek. Tweedens word die ontwikkeling van 'n eksperimentele opstelling vir laser afkoeling en vang van 87Rb atome in vakuum met die hulp van 'n MOT voorgelê. Alle komponente van die val is geïmplementeer en gekarakteriseer. Die vakuumsisteem en val-kamer waarin die afkoeling plaasvind is ontwerp en gebou. 'n Rubidium gasbinder is in die vakuumsisteem ingebou om as 'n bron van atome te dien. Die twee eksterne resonator diodelasers wat gebruik is vir die val en die optiese terugpomp is gekarakteriseer. Die optiese opstelling wat nodig is vir die optiese molasse is ontwerp, gebou en gekarakteriseer. Versadigde absorpsiespektroskopie is uitgevoer om die hiperfynstruktuur van 87Rb te ondersoek en om die lasers se frekwensies te stabiliseer. Verslag word gedoen oor die huidige stand van die projek wat betref vordering, resultate en toekomstige werk.