Browsing by Author "Bark, R A"

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    Exploring the spectroscopy of vibrational levels in the 160 mass region
    (2016) Majola, Siyabonga Ntokozo Thandoluhle; Aschman, David; Bark, R A; Jones, P
    Vibrational levels are well known in atomic nuclei but despite many decades of research, some of their properties still remain elusive. In particular, low-lying rotational bands based on the first excited 0+ state, which are traditionally understood as β vibrational bands nevertheless show properties at odds with this interpretation, more especially in the transitional rare earth region with N~ 90 [Gar01]. An alternative is that they can be better be described as a "second vacuum", or coexisting minimum in the pairing degree of freedom [Sch11a]. In order to produce a complete and definitive microscopic picture of the so-called β and γ bands, an extensive systematic review is performed for nuclides in the 160 mass region, between N = 88 and 92 and Sm to Yb. The data are explained using a five dimensional collective Hamiltonian for quadrupole rotational and vibrational degrees of freedom [Li09, Nik09]. A good qualitative agreement is obtained between measured energies and electromagnetic transition rates across the entire A ~ 160 mass region. The implication of these findings on the interpretation the first excited 0+ states is there from discussed. In this study, the γ-ray spectroscopy of 156Dy and 157Dy are also revised. Three experiments with different reactions to produce 156Dy have been analyzed. The data were acquired using the 155Gd (α, 3n) 156Dy and 148Nd (12C, 4n) 156Dy and 124Sn (36S, 4n) 156Dy reactions at beam energies of 25, 65 and 165 MeV, respectively. The 155Gd (α, 3n) 156Dy experiment was performed with aid of the JUROGAM II array. The experiments that used the 148Nd (12C, 4n) 156Dy and 124Sn (36S, 4n) reactions were both conducted using the GAMMASPHERE array. In total, the current work reports the observation of about 30 bands in 156Dy. The spins and parities for the majority of new rotational structures have been successfully assigned using the Directional Correlation from Oriented States (DCO ratios) and polarization anisotropy (Ap). In order to meaningfully describe the quantum behavior of the newly established structures, the Nilsson and Cranked Shell Models have been used. The results also test the latest and most sophisticated quantum theories of nuclear structure and also point to inadequacies of these models, where they exist. Amongst important spectroscopic features, is the identification of the γ wobbling candidate, which implies in some way that 156Dy is γ deformed. Extensions have been made on the γ bands and they are now observed to spins 31+ and 32+, which marks the highest spin to be ever observed for both the odd and even spin members of the γ bands in any nucleus. The current study also reports the first candidate for the aligned γ bands based on the two-quasiparticle configurations (S-band). This is the first time "γ bands" have been identified on both the ground state band (continuation) and on the yrast aligned S-band. Finally, an odd spin negative parity sequence, namely band 9, has been identified as a candidate of the K = 1- octupole excitation. [Please note: the full text of the thesis is deferred until January 2018]
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    Measurement of neutron production from an ¹⁸O target for the production of radioactive-ion beams at iThemba LABS
    (2018) Ngcobo, Phumlani Zipho; Buffler, Andy; Bark, R A
    iThemba LABS, in the South African Isotope Facility (SAIF) project, proposes to produce radioactive ion beams by the fission of uranium. A natural way to fission uranium is to use high-intensity proton beams from a cyclotron. However, neutron-induced fission gives enhanced production of neutron-rich fission fragments compared to proton-induced fission, thus there is a need to find efficient ways of producing neutrons from protons. Beryllium targets have been suggested, but increasing the proton beam current to achieve higher fission rates could lead to cooling problems. Cooling might be achieved in a natural way by using enriched water as a converter, but ¹⁶O is a poor neutron converter compared with ⁹Be. An alternative would be to use water enriched in ¹⁸O. With no data available for ¹⁸O above 25 MeV, this necessitated the measurement of neutron yields from the ¹⁸O(p,xn) reactions between 30 and 66 MeV. Quasi-monoenergetic neutron spectra were therefore measured at proton energies of 66; 54; 42 and 30 MeV on a 2.03 mm thick H₂¹⁸O water target using the time of flight (ToF) technique at 0˚ and 16˚. Neutron energy spectra from a ⁷Li (2.5 ± 0.5 mm thick) target were also measured at the same energies to validate the H₂¹⁸O measurements. The spectra deduced at these energies were used to simulate the neutron fluence spectra from a stopping-length H₂¹⁸O target and were compared with the measured neutron fluence spectra from a thick target of 40.0 ± 0.1 mm at 0˚ and 16˚, using a proton beam of 62 MeV. The neutron differential cross sections, dσ(E)/dΩ for the ¹⁸O(p,n) reactions at 66; 54; 42 and 30 MeV were derived and compared to those of ⁷Li(p,n) and ⁹Be(p,n). The cross sections of ⁷Li and ⁹Be are higher than those of ¹⁸O between 20 MeV and 50 MeV at 0°. At above 60 MeV, the ⁷Li target produces double the amount of neutrons produced by ¹⁸O and ⁹Be targets, but importantly ¹⁸O produce neutrons more than ⁹Be. The ¹⁸O(p,xn) neutron cross sections (d²σ(E)/dΩdE in mb/MeV/sr) from the thick stopping-length target (40 mm) were compared to the corresponding cross sections of ⁹Be thick stopping-length target (24.1mm). Conclusively, on average, the ¹⁸O neutrons cross sections are half those of ⁹Be.