The 26mAl(p,γ)27Si reaction rate in ONe novae

Catherine M. Deibel, Yale University
Jason A. Clark, Yale University
Rachel Lewis, Yale University
Anuj Parikh, Yale University
Peter D. Parker, Yale University
Christopher Wrede, Yale University


Evidence of the ongoing nucleosynthesis of 26Al in our galaxy has been found in presolar grains and in observations of the 1.809-MeV γ ray, which results from the β decay of 26gAl (t1/2 = 7.2×105 yrs) to an excited state in 26Mg. The nucleosynthesis of 26Al is complicated by the existence of the isomeric state, 26mAl, at 228 keV (t1/2 = 6.3 s), which must be treated independently from 26gAl in certain stellar environments, such as ONe novae, where temperatures are below 0.4 GK. 26mAl β decays directly to 26gMg, bypassing the emission of the 1.809-MeV γ ray. The 26gAl(p,γ) 27Si and 26mAl(p,γ)27Si reactions destroy 26Al in novae and have a direct impact on the net amount of 26Al produced. While the 26gAl(p,γ)27Si reaction rate has been studied extensively, there has been virtually no information published on resonances of the 26mAl(p,γ) 27Si reaction and previously published reaction rates [1] have been based on 26gAl + p resonances and Hauser-Feshbach calculations. The 27Al(3He,t)27Si*(p) 26Al and 28Si(3He,α)27Si *(p)26Al reactions have been studied at the Wright Nuclear Structure Laboratory at Yale University using 3He beams of 25 and 17.5 MeV, respectively, produced by the Tandem Van de Graaff accelerator. Reaction products were momentum analyzed by the Yale Enge magnetic spectrograph and tritons and alpha particles were detected at the focal plane with a position-sensitive, gas filled ionization detector backed by a plastic scintillator to determine energies of states in 27Si from the two transfer reactions. Proton decays from these excited states in 27Si were detected by the Yale Lamp Shade Array of silicon strip detectors in coincidence with the reaction products of interest detected at the focal plane. Angular correlations were measured to constrain spins and proton branching ratios for 26mAl + p resonances with Ecm > 450 keV, and a reaction rate has been calculated for 26mAl(p,γ) 27Si. The extreme differences in proton decays from excited states in 27Si to 26gAl and 26mAl prove that 26mAl(p,γ)27Si reaction rates found using experimental data for 26gAl + p resonances are not valid, and a direct 26mAl(p,γ)27Si measurement must be made to reliably determine the strengths of low-energy resonances which most likely dominate the reaction rate. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlikeLicence.