Browsing by Author "Jivan, Harshna"

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    Proton induced radiation damage studies on plastic scintillators for the tile calorimeter of the atlas detector
    (2016) Jivan, Harshna
    Plastic scintillators play a key role in reconstructing the energy and tracks of hadronic particles that impinge the Tile Calorimeter of the ATLAS detector as a result of high energy particle collisions generated by the Large Hadron Collider of CERN. In the detector, plastic scintillators are exposed to harsh radiation environments and are therefore susceptible to radiation damage. The radiation damage effects to the optical properties and structural damage were studied for PVT based commercial scintillators EJ200, EJ208, EJ260 and BC408, as well as PS based UPS923A and scintillators manufactured for the Tile Calorimeter. Samples of dimensions 5x5x0.3 mm were subjected to irradiation using 6 MeV protons to doses of approximately 0.8 MGy, 8 MGy, 25 MGy and 25 MGy using the 6 MV tandem accelerator of iThemba LABS. Results show that damage leads to a reduced light output and loss in transmission character. Structural damage to the polymer base and the formation of free radicals occur for doses ≥ 8 MGy leading to reduced scintillation in the base and re-absorption of scintillation light respectively. Scintillators containing a larger Stokes shift, i.e. EJ260 and EJ208 exhibit the most radiation hardness. EJ208 is recommended as a candidate to be considered for the replacement of Gap scintillators in the TileCal for the 2018 upgrade.
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    Study on the influence of Nuclear Deformation on the Pygmy Dipole Resonance in Samarium isotopes
    (University of the Witwatersrand, Johannesburg, 2023) Jivan, Harshna; Sideras-Haddad, Elias; Pellegri, Luna
    The past decade has seen an increase in studies dedicated to understanding the low-lying electric dipole (E1) response, commonly referred to as the Pygmy Dipole Resonance (PDR). These studies revealed that the PDR has a mixed isospin nature, and that the use of complimentary probes is needed to fully understand this response. Since majority of studies on the PDR focused on spherical nuclei, the influence that deformation has on the PDR response is yet to be understood. Preliminary relativistic proton scattering studies on 154Sm performed at RCNP (Japan), showed potential evidence for a splitting in the PDR responses similar to that of the Giant Dipole Resonance with deformation. A tentative interpretation suggested that this splitting could be connected to the splitting of the resonance structure with respect to the K quantum number. Theoretical studies considering the deformed HFB+QRPA model however, suggest that this splitting is connected to the isospin mixed character of these states as observed in spherical nuclei. The isoscalar responses of the spherical 144Sm and axially deformed 154Sm isotopes were investigated for the first time using the inelastic scattering of alpha particles at 120 MeV. The comparative experiments were performed at iThemba LABS in South Africa, coupling together for the first time, the K600 magnetic spectrometer in zero-degree mode with the BaGeL (Ball of Germanium and LaBr3:Ce detectors) array. The particle-gamma coincidence measurement was used to obtain the cross section for the population of the pygmy states. In both nuclei, the region of the PDR was excited and the E1 multipolarity of the transitions was supported by the angular correlation between the α-particles and the co-incident γ-rays measured. The total exclusive cross section measured for 144Sm amounted to 24.3 ± 3.8 mb/sr while for 154Sm to 18.8 ± 2mb/sr. The experimental results were compared with the prediction of the RQTBA and the deformed HFB+QRPA theories, respectively. The theoretical cross sections were extracted within a semiclassical coupled-channel approach. The fragmentation observed in the experiment for the 144Sm was underestimated by the calculations, although good agreement with the total cross section measured was found. In the case of the deformed 154Sm however, the experimental cross section accounted for only 52% of the predicted cross section in the same excitation region. The isoscalar response extracted in this thesis was compared with the isovector strength obtain from an experiment performed at RCNP using the relativistic proton scattering at forward angles. The double hump observed in the isovector channel was not found in the case of the isoscalar strength. This implies that the difference obtained between these two experiments is related to the “isospin splitting” of the PDR rather than a splitting of thestrength connected with the K quantum number.