Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by Subject "Astrophysics"

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    A ghostly tail of the Hoyle state and its breathing-mode excitation
    (Stellenbosch : Stellenbosch University., 2020-04) Li, Kevin Ching Wei; Papka, Paul; Smit, F. D.; Neveling, Retief; Adsley, Philip; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: 12C is almost exclusively produced in stars from a combination of 4He and unstable 8Be which form an excited three-alpha resonance named the Hoyle state, located just above the 3α separation energy at Ex = 7.65407(19) MeV. Whilst this resonance is understood to be the archetypal α-cluster state, 12C is understood to straddle the region between the shell-model and collective pictures as its ground state presents with a large overlap with shell-model states. The significant work by the nuclear physics community to theoretically describe 12C is therefore of both astrophysical and structural significance. An indication that an unlisted source of monopole strength may manifest at Ex ≈ 9 MeV was given by the peak-fitting and Multipole Decomposition Analysis (MDA) of M. Itoh et al. which suggested a double-peaked, broad monopole structure in the excitation-energy region of Ex ≈ 8-13 MeV just above the Hoyle state. Some recent theoretical works which employ formalisms such as AMD, 3α OCM and GCM suggest that this unestablished origin of monopole strength may be understood as the breathing-mode excitation of the 0+ 2 Hoylestate, corresponding to a low-energy fragment of the isoscalar giant monopole resonance (ISGMR). The primary motive of this thesis was to search for any unidentified sources of monopole strength in the Ex ≈ 7-16 MeV excitation-energy region and investigate if they exhibit strong collectivity. A new measurement of the 14C(p,t)12C reaction at θlab = 0◦ was performed with the K600 magnetic spectrometer and the CAKE array to detect coincident charged-particle decay. This data was combined previous datasets of 14C(p,t)12C at 21◦ and 12C(α,α0)12C at θlab = 0◦(×2), 6◦, 8.5◦ and 10◦ to be simultaneously analysed with a new multilevel multichannel R-matrix analysis fitting code. A total of 6 hypotheses were explored and the best quality model corresponded to the monopole structure in the Ex ≈ 7-16 MeV excitation-energy region being modeled by a two-level approximation with coherent interference and an additional 0+ resonance at Ex = 9.270(14) MeV with a width of Γtot = 1581(58) MeV. The extracted parameters for this resonance are in excellent agreement with both the aforementioned measurement by M. Itoh et al. as well as various theoretical predictions. There is particularly good agreement with the OCM calculation by C. Kurokawa and K. Kato¯ which yielded a resonance energy of Ex ≈ 8.95 MeV and a width of Γtot = 1.48 MeV for the 0+ 3 state [1]. The angular distribution of α0 decay in the associated interval of 8.5 MeV < Ex < 9.0 MeV was found to exhibit isotropy which implies that the strength is predominantly monopole. The identification of this new source of monopole strength may play a significant role in the 3α rate in certain astrophysical conditions.