Electronic Theses and Dissertations (PhDs)

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Browsing Electronic Theses and Dissertations (PhDs) by SDG "SDG-17: Partnerships for the goals"

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    Modification of boron nitride nanostructures induced by medium energy ion irradiation
    (University of the Witwatersrand, Johannesburg, 2023-08) Lisema, Lehlohonolo Innocent; Madhuku, Morgan; Derry, Trevor
    This research focused on using Chemical Vapour Deposition (CVD) to synthesize boron nitride nanostructures, particularly nanotubes, and selectively introducing defects into them through ion implantation. Boron ion implantations were carried out at ambient temperature at 150 keV energy and fluences 1x1014 and 5x1014 ions/cm2. The synthesized samples were analyzed using scanning electron microscopy (SEM), Raman spectroscopy, and Grazing incidence X-ray diffraction (GIXRD). Ion implantation was found to introduce defects into the surface of the samples, resulting in increased stress levels and a higher local density that favoured more crystallized nanostructures. SEM images showed clear evidence of BN nanostructures and boron nitride nanotubes (BNNTs), with the latter appearing as long, thin structures with diameters ranging from ⁓30-90nm. After ion implantation, the Raman spectra of samples implanted with ion fluence 5×1014 ions/cm2 at 1000oC, show an amorphous h-BN peak, and a narrower, intense E2g vibrational mode of h-BN is observed around 1366 cm-1 for samples synthesized at 1100oC and 1200oC. Raman analysis did not show any E2g mode of vibration of h-BN for all samples at implanted with ion fluence 1×1014 ions/cm2. The samples synthesized at 900 ºC had no active 1366 cm-1 Raman peak present. Grazing incidence X-ray diffraction (GIXRD) spectra revealed a prominent peak between 54 and 56 ° 2θ, corresponding to the (004) h-BN reflection, which was used to determine the average a and c lattice parameters 0.249 ± 0.0002 nm and 0.662 ± 0.001 nm, respectively, yielding an interplanar distance of 0.166 ± 0.0001 nm representing the stacking direction of the BN layers. The majority of the samples had broad peaks, indicative of a nanocrystalline material. The only exception was the sample grown at 1200 °C, which was found to have a Scherrer crystallite size >100 nm. In contrast, the rest of the samples had an average size of 3.5 ± 0.3nm. The average crystalline domain size values confirmed that after ion implantation, the phonon lifetime would be longer due to a large domain size, indicating that the BN nanostructures were more crystallized. The fluence of 5x1014 ions/cm2 showed to be the optimal growth condition for BNNTs. Overall, BNNTs and BN nanostructures were effectively synthesized at 900°C, 1000°C, 1100°C, and 1200°C CVD temperatures, and insights into the influence of ion implantation on the composition as well as properties of BN nanostructures are presented. The most noteworthy finding of the experiment was the substantial increase in the size of the Raman derived crystallite domains in the 1100°C and 1200°C samples following ion implantation with boron ions at a fluence of 5x1014 ions/cm2.
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    Transfer reactions to populate the pygmy dipole resonance in 96Mo
    (University of the Witwatersrand, Johannesburg, 2023) Khumalo, Thuthukile Charmane; Pellegri, L.; Wiedeking, M.
    The presence of a low-lying dipole strength in neutron-rich nuclei has been established and its location in the vicinity of the neutron threshold (Sn) has implications in nucleosynthesis and specifically in neutron-capture reaction rate calculations. Additionally, a correlation of this low-lying dipole strength with neutron-skin thickness has been discussed. Since its observation, there has been a great deal of work in an attempt to understand its nature, both theoretically and experimentally. Some of the characteristics of this low-lying dipole strength include isospin mixing, which allows the use of different experimental probes to study it. In addition, compared to the IVGDR, the degree to which the low-lying dipole states are collective is under scrutiny and remains an open question of interest. This study was aimed at addressing the question of collectivity of these dipole states and one-nucleon transfer reactions were the chosen probes as they have been shown to be powerful in probing the single-particle property of nuclei. In particular the (p,d) and (d,p) reactions have been instrumental in such measurements. To allow the investigation from both neutron addition and removal, the 96Mo nucleus is particularly attractive as it can be populated via both mechanisms, with the availability of stable targets as a bonus. In addition, the (d,p) has been successfully used recently used for PDR related measurements on 120Sn and 208Pb with results alluding to a strong single-particle contribution, hence conducting the investigation on 96Mo provides access to a different mass region. 97Mo(p,d)96Mo and 95Mo(d,p)96Mo transfer reactions were performed in normal kinematics using the MAGNEX magnetic spectrometer at INFN-LNS. The 25 MeV/u proton beam and 5 MeV/u deuteron beam from the Tandem accelerator interacted with the 97Mo and 95Mo targets, respectively. The MAGNEX spectrometer was utilised to analyse the scattered particles based on their momentum prior to being detected at the focal-plane. Excitation energy spectra were obtained and angular distributions were computed for the bound states and the higher excitation energy region of interest (above Ex = 4 MeV). These were fitted, using the MDA with DWBA calculations considering different single-particle configurations from a simplistic shell model. Comparing spectra from the two reactions, same excitation energy regions were populated. The results from the MDA of the (p,d) data, show a strong single-particle component in the Ex region that was analysed, with one particular configuration that excites 1− states dominating. The QPM was used for the theoretical interpretation and below 6 MeV, the configuration ((2d5 2 )+1 N(1g9 2 )−1) that populates 2+ states dominates but in the experimental data, this configuration was found to be suppressed as the momentum matching conditions were optimized for l=1 momentum transfer. When considering the QPM predictions involving only the sp configurations of momentum transfer of l=1, 2 and 3, an agreement with the data was found. Extraction of reliable angular distributions from the (d,p) was not possible thus future (d,pγ) experiments are envisaged