Measurement of the 25Mg(d, p)26Mg reaction to constrain nucleosynthesis in novae and the weak s-process

Abstract

The 25Al(p, γ) 26Si reaction rate is one of the few outstanding uncertainties in modelling the contribution from novae to the galactic budget of the long-lived radioactive isotope 26Al. The rate is dominated by three key resonances in 26Si (J π = 1+, 0+ and 3+), of which only the 3+ resonance has been directly constrained. The first experiment described in this thesis used the 25Mg(d, p) reaction to measure the spectroscopic factors of the three analog states in the mirror nucleus 26Mg, including a spectroscopic factor for the 0+ state.

The proton partial widths estimated from these spectroscopic factors established the 0+ state contributes .10% of the 25Al(p, γ) reaction rate, with the 3+ state dominating at higher temperatures. The upper limit extracted for the 1+ proton partial width, which disagreed with a previous (4He, 3He) study, found it only contributes to the reaction rate at low temperatures. Previous studies presented evidence for a negative parity state in 26Mg around 5.7 MeV, consistent with the angular distribution measured in the current work, which has not had an analog state in 26Si confirmed. Future work should focus on identifying such a state and further constraining the parameters of the dominant 3+ resonance.

The amount of neutrons available for the weak s-process depends on the 22Ne(α, n) and 22Ne(α, γ) reaction rates, which proceed through natural-parity states of 26Mg above the alpha and neutron thresholds. The second experiment in this thesis used the 25Mg(d, p) reaction to populate states above the 26Mg alpha threshold.

The shapes of the angular distributions constrained the `-transfers populating those states. This established the spin/parities of states at 10.82, 10.95, 11.08 and 11.11 MeV as 2+, 1−, 2+ and 2+ respectively. Combining these assignments with previous alpha-transfer studies allowed alpha partial widths to be extracted, which were used to calculate reaction rates for both reactions. Studies seeking to further reduce these rate uncertainties should focus on constraining the properties of the 10.95 and 11.11 MeV states, which dominate the reactions at temperatures whenever the 22Ne(α, n) rate overtakes that of the 22Ne(α, γ) reaction.