Charlie Paxman

The 25 Al(p, γ) reaction has long been highlighted as a possible means to bypass the production of 26 Al cosmic γ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in 26 Si have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed γ-ray spectroscopy study of 26 Si and present evidence for the existence of a new, likely ℓ = 1, resonance in the 25 Al + p system at Er = 153.9(15) keV. This state is now expected to provide the dominant contribution to the 25 Al(p, γ) stellar reaction rate over the temperature range, T ∼ 0.1 − 0.2 GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of 26 Al from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, Galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed Galactic abundance of 26 Al is ∼0.2 M⊙. Finally, we briefly highlight the important role that Super-AGB stars may play in the production of 26 Al.

A high-precision branching ratio measurement for the superallowed Fermi beta(+) emitter Ga-62 was performed with the Gamma-Ray Infrastructure for Fundamental Investigations of Nuclei (GRIFFIN) spectrometer at the Isotope Separator and Accelerator (ISAC) radioactive ion beam facility at TRIUMF. The high efficiency of the GRIFFIN spectrometer allowed 63 gamma -ray transitions, with intensities down to approximate to 1 part per million (ppm) per Ga-62 beta(+) decay, to be placed in the level scheme of the daughter nucleus Zn-62, establishing the superallowed beta branching ratio for Ga-62 decay to be 99.8577(-0.0029)(+0.0023)%, a factor of 4 more precise than the previous world average. For several cascades, gamma-gamma angular correlation measurements were performed to assign spins and/or determine the mixing ratios of transitions. In particular, the spin of the 2.342 MeV excited state in the daughter nucleus Zn-62 was definitively assigned as J = 0. This assignment resolves a discrepancy between previous measurements and has important implications for the isospin symmetry breaking correction, delta(C1), in Ga-62 superallowed Fermi beta decay.

Physical Review Letters 127, 112701 (2021) We have performed the first direct measurement of the 83Rb(p,g) radiative capture reaction cross section in inverse kinematics using a radioactive beam of 83Rb at incident energies of 2.4 and 2.7 A MeV. The measured cross section at an effective relative kinetic energy of Ecm = 2.393 MeV, which lies within the relevant energy window for core collapse supernovae, is smaller than the prediction of statistical model calculations. This leads to the abundance of 84Sr produced in the astrophysical p process being higher than previously calculated. Moreover, the discrepancy of the present data with theoretical predictions indicates that further experimental investigation of p-process reactions involving unstable projectiles is clearly warranted.