Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by Author "Brummer, Johann Wiggert"

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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.