Department of Physics

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Browsing Department of Physics by browse.metadata.advisor "Adsley, Philip"

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    Characterization of the pre-eminent 4-α cluster state candidate in 16O
    (Stellenbosch : Stellenbosch University, 2015-12) Li, Kevin Ching Wei; Papka, Paul; Neveling, Retief; Adsley, Philip; Stellenbosch University. Faculty of Sciences. Dept. of Physics.
    ENGLISH ABSTRACT: Please see full text for abstract
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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.
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    Spectroscopy of 22Mg relevant to explosive nucleosynthesis in classical novae and X-ray bursts
    (Stellenbosch : Stellenbosch University., 2020-04) Brummer, Johann Wiggert; Adsley, Philip; Papka, Paul; Smit, F. D.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: This thesis discusses the spectroscopy of 22Mg. This was done by performing the 24Mg(p,t) 22Mg reaction at iThemba LABS using the K600 magnetic spectrometer in coincidence with the CAKE silicon-detector array. Using this experimental setup, resonances in 22Mg were studied and proton decays from this nucleus to the ground state and excited states in 21Na were detected with the CAKE. The 22Mg nucleus was studied for two different reasons. Firstly, it is the compound nucleus in the 18Ne(α, p) 21Na HCNO breakout reaction in X-ray bursts (XRBs). Secondly, the 24Mg(p,t) 22Mg dataset also revealed that it is possible to use the excitation energy spectrum below the α-particle separation threshold, Sα, at 8.142 MeV to study the 21Na(p, γ) 22Mg reaction. The 18Ne(α, p) 21Na reaction is one of a number of crucially-important reactions that greatly influence t he e nergy g eneration i n X RBs. T herefore, it also affects the shape of the lightcurve considerably which is the only realistic observable of these events and only means to detect and study stellar XRBs. Direct measurements of this reaction are challenging and, to date, many studies have performed a variety of different types of measurements to determine the thermonuclear reaction rate. In particular, one such study performed the time-reversed reaction. The data from that study were used and reevaluated in this thesis. The resultant excitation energy spectrum from the 24Mg(p,t) 22Mg experiment was analysed in two sections. Above Sα the spectrum was analysed in 100-keV bins up to an excitation energy of 13 MeV. The proton branching ratios Bpi (i ∈ [0, 4]) were calculated for each bin. A moving-average function was determined for the proton branching ratio to the ground state, Bp0 , to calculate the cross section as a function of energy, σ(E), using cross section data from the previous study of this reaction. This was used to recalculate the thermonuclear reaction rate of 18Ne(α, p) 21Na. From this study it is concluded that the reaction rate varied by no more than a factor of 2.3 higher as compared to the average reaction rate from the previous study that performed the time-reversed reaction. The recalculated rate is in good agreement, within uncertainties, with another study that made an estimate of this rate by means of evaluating all pre-existing data. In that study, the reaction rate was determined by evaluation of the uncertainties from previous 18Ne(α, p) 21Na reaction-rate measurements and determining the regions where those uncertainties overlap. Results from this thesis were used with a stellar simulation code, MESA, to simulate the lightcurve of a particular XRB event. Clear changes to the lightcurve, due to the recalculated reaction rate, were apparent. The 21Na(p, γ) 22Mg thermonuclear reaction rate is linked to the production of 22Na by means of β emission from the 22Mg nucleus. The production of 22Na plays an important role in the ability of astronomers to detect and study the astrophysical phenomenon of classical novae. For the 21Na(p, γ) 22Mg reaction the compound nucleus is also 22Mg. The region of the excitation energy spectrum below Sα was resolved and as a result isolated, narrow resonances were studied. The resonance energies and widths were determined by fitting the spectrum using Gaussian and Voigt functions. The proton branching ratios were calculated and possible Jπ assignments were discussed. However, owing to the physical and electronic thresholds of the silicondetector array, the protons from the decay of resonances in the astrophysicallyrelevant Gamow energy region were too low in energy to detect successfully. Therefore, it was not possible to use their branching ratios to calculate the thermonuclear reaction rate of 21Na(p, γ) 22Mg. To achieve this, subsequent experiments that aim to calculate this rate may need to use silicon detectors that can operate at lower thresholds. Alternatively, active-target detectors can be used as they have very low energy thresholds coupled with high detection efficiencies.