Made available in DSpace on 2014-10-09T12:28:34Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:03Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP

Made available in DSpace on 2014-10-09T12:28:34Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:03Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP

Magister Scientiae - MSc / Beams of polarized protons play an important part in the study of the spin dependence of the nuclear force by measuring the analyzing power in nuclear reactions. The source at iThemba LABS produces a beam of polarized protons that is pre-accelerated by an injector cyclotron (SPC2) to a energy of 8 MeV before acceleration by the main separated-sector cyclotron to 200 MeV for physics research. The polarized ion source is one of the two external ion sources of SPC2. Inside the ion source hydrogen molecules are dissociated into atoms in the dissociator and cooled to a temperature of approximately 30 K in the nozzle. The atoms are polarized by a pair of sextupole magnets and the nucleus is polarized by RF transitions between hyperfine levels in hydrogen atoms. The atoms are then ionized by electrons in the ionizer. The source has various sensitive devices, which influence beam intensity and polarization. Nitrogen gas is used to prevent recombination of atoms after dissociation. The amount of nitrogen and the temperature at which it is used plays a very important role in optimizing the beam current. The number of electrons released in the ionizer is influenced by the size and shape of the filament. Optimization of the source will ensure that beams of better quality (a better current and stability) are produced. / South Africa

Jet production in PbPb collisions at a per-nucleon center-of-mass energy of 2.76 TeV has been studied using the ATLAS detector at the LHC. Interactions between the high- pT partons and the hot, dense medium, produced in these ultrarelativistic collisions, are expected to cause the loss of the jet energy (jet quenching). This thesis presents results of the jet analysis done on the data taken during the 2011 heavy-ion run at the LHC as well as PYTHIA Monte Carlo reference. Jets are reconstructed using the anti-kt jet clustering algorithm and studied as a function of collision centrality and dijet energy imbalance. With increasing centrality, dijets are observed to be increasingly asymmetric, consistent with the theory of jet quenching. The study of charged particle tracks indicates the increase of the low-pT tracks in the strongly quenched jets. 1

At the Gesellschaft für Schwerionenforschung in Darmstadt a therapy unit for heavy ion cancer treatment has been established in collaboration with the Deutsches Krebsforschungszentrum Heidelberg, the Radiologische Universitätsklinik Heidelberg and the Forschungszentrum Rossendorf. For quality assurance the dual-head positron camera BASTEI (Beta Activity meaSurements at the Therapy with Energetic Ions) has been integrated into this facility. It measures ß+-activity distributions generated via nuclear fragmentation reactions within the target volume. BASTEI has about 4 million coincidence channels. The emission data are acquired in a 3D regime and stored in a list mode data format. Typically counting statstics is two to three orders of magnitude lower than those of typical PET-scans in nuclear medicine. Two iterative 3D reconstruction algorithms based on ISRA (Image Space Reconstruction Algorithm) and MLEM (Maximum Likelihood Expectation Maximization), respectively, have been adapted to this imaging geometry. The major advantage of the developed approaches are run-time Monte-Carlo simulations which are used to calculate the transition matrix. The influences of detector sensitivity variations, randoms, activity from outside of the field of view and attenuation are corrected for the individual coincidence channels. Performance studies show, that the implementation based on MLEM is the algorithm of merit. Since 1997 it has been applied sucessfully to patient data. The localization of distal and lateral gradients of the ß+-activity distribution is guaranteed in the longitudinal sections. Out of the longitudinal sections the lateral gradients of the ß+-activity distribution should be interpreted using a priori knowledge.

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No description available.

The Quark-Gluon Plasma (QGP) produced in heavy-ion collisions has been shown to behave like a nearly perfect fluid, characterized by a very low ratio of shear viscosity to entropy density. Significant measurements in large collision systems have improved the constraints on the value of 𝜂/𝑠. However, the precise temperature dependence of 𝜂/𝑠 still remains unknown. The interplay between viscous effects and initial geometry fluctuations is important that requires further investigation. Another key open question in the study of multi-particle production is the relationship between the “ridge” – observed azimuthal correlations between particles in the underlying event that extend over all rapidities – and hard or semi-hard scattering processes in small collision systems. In particular, it is not known whether jets or their soft fragments are correlated with particles in the underlying event. This dissertation presents two analyses. The first analysis measures flow harmonics 𝜈2–𝜈6 in 3 𝜇b^-1 of Xe+Xe collisions at √𝑠𝖭𝖭 = 5.44 TeV using the ATLAS detector at the LHC. The centrality, multiplicity, and 𝑝_T dependence of the 𝜈n values obtained using two-particle correlations and template-fit procedure are presented, and the measurements are compared with those in Pb+Pb collisions and 𝑝+Pb collisions at 5.02 TeV. The 𝜈n values in Xe+Xe collisions are observed to be larger than those in Pb+Pb collisions for n = 2, 3, and 4 in the most central events. However, the 𝜈n values in Xe+Xe collisions become smaller than those in Pb+Pb collisions with decreasing centrality or increasing harmonic order n. The 𝜈n in Xe+Xe and Pb+Pb collisions are also compared as a function of the mean number of participating nucleons and the measured charged-particle multiplicity in the detector. The 𝜈3 values in Xe+Xe and Pb+Pb collisions are observed to be similar at the same ⟨𝑁_part⟩ or multiplicity, but the other harmonics are significantly different. The second analysis studies two-particle correlations in pp collisions at 13 TeV using data collected by the ATLAS experiment at the LHC, with an integrated luminosity of 15.8 pb⁻¹, in two different configurations. In the first case, charged particles associated with jets are excluded from the correlation analysis, while in the second case, correlations are measured between particles within jets and charged particles from the underlying event. Second-order flow coefficients, 𝜈2, are presented as a function of event multiplicity and transverse momentum. These measurements show that excluding particles associated with jets does not affect the measured correlations. Moreover, particles associated with jets do not exhibit any significant azimuthal correlations with the underlying event, ruling out hard processes contributing to the ridge.

No description available.

Metallic bilayer samples of Mo(400 Å)/ Al(substrate) were characterized using Rutherford Backscattering Spectroscopy after first being irradiated with Xe ion beam having an energy of 1.8 MeV. The computer code RUMP was then used to simulate the RBS spectra. The interdiffusion at the interface was considered in terms of thermal spike induced atomic migration. It was found that the coupling of the chemical effect with spike is significant with regard to mixing of the bilayer samples. Furthermore, in addition to the initial contamination of carbon atoms on the surface and at the interface, more carbon atoms were found to be picked up by the surface, this carbon w.as from the vacuum pumps and tended to migrate into the surface once irradiation dose exceeded 11 x 10¹⁵cm². A semi-empirical model was developed for ion beam mixing taking into account collisional mixing and thermal spike effects, as well as the thermal spike shape. The collisional mixing part was accounted for by the Kinchin-Pease model, or, alternatively dynamic Monte Carlo simulation. For the thermal spike, the ion beam mixing parameter Dt/Φ was derived to be proportional to ( - F_D /ΔH_coh)^2+μ, where F_D is the damage energy deposited per unit path length, ΔH_coh is the cohesive energy of the target materials, and µ is a constant dependent on the spike shape and point defect density in the spike regions. The thermal spike introduces a nonlinear effect in the mixing process, distinguishing itself from the linear effect of ballistic mixing. The shape of the thermal spike that best fit the experimental results depends on the magnitude of the cascade density. For relatively high density collisional cascades, where thermal spikes start to be important, it was found that a spherical spike model was more consistent with experimental measurements at low temperatures. However, for extremely high density collisional cascade regions, a cylindrical shaped spike gave better results. The atomic migration energy in the spike regions is scaled by a factor of one out of 8.6 of cohesive energy. The migration mechanism was recognized to be interstitial-dominated one. / M.S.