The production of the 26 Al radioisotope in astrophysical environments is not understood, in part, because of large uncertainties in key nuclear reaction rates. The 25 Al(p,␥) 26 Si reaction is one of the most important, but its rate is very uncertain as a result of the lack of information on the 26 Si level structure above the proton threshold. To reduce these uncertainties, we have measured differential cross sections for the 28 Si(p,t) 26 Si reaction and determined excitation energies for states in 26 Si. A total of 21 states in 26 Si were observed, including ten above the proton threshold. One new state at 7019 keV was observed, the excitation energies of several states were corrected, and the uncertainties in the excitation energies of other states were significantly reduced. Spins and parities of several states above the proton threshold were determined for the first time through a distorted-wave Born approximation analysis of the angular distributions. These results substantially clarify the level structure of 26 Si.

The proton drip-line defines the limit at which nuclei become unbound to the emission of a proton from their ground states. Low-Z nuclei lying beyond this limit only exist as short-lived resonances and cannot be detected directly. The location of the drip-line constrains the path of nucleosynthesis in explosive astrophysical scenarios such as novae and X-ray bursters, and consequently controls the rate of energy generation. In higher-Z regions of the drip-line, the potential energy barrier resulting from the mutual electrostatic interaction between the unbound proton and the core can cause nuclei to survive long enough to be detected. This review describes the recent major advances in the study of these exotic nuclei. Particular emphasis is placed on understanding the phenomenon of proton radioactivity and the unique insights it offers into the structure of nuclei lying beyond one of nature's fundamental limits to stability.

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This is an accepted version of a paper published in Nature. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the published paper: Hinke, C., Boehmer, M., Boutachkov, P., Faestermann, T., Geissel, H. et al. (2012) "Superallowed Gamow-Teller decay of the doubly magic nucleus 100 Sn" Nature, 486 (7403): [341][342][343][344][345] Access to the published version may require subscription.