Toward an experimentally determined 26mAl(p,γ) 27Si reaction rate in ONe novae

C. M. Deibel, Yale University
J. A. Clark, Yale University
R. Lewis, Yale University
A. Parikh, Yale University
P. D. Parker, Yale University
C. 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 the ground state of 26Al (26gA1; t1/2 = 7.2×l05 yrs) to an excited state in 26Mg. The nucleosynthesis of 26Al is complicated by the existence of the isomeric state, 26mAl, at 228 keV (t 1/2 = 6.3 s), which must be treated independently from 26gA1 in certain stellar environments, such as ONe novae, where temperatures are below 0.4 GK [1]. 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 26 Al 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 [2] have been based on 26gA1 + 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. Proton decays from the excited states in 27Si populated via the 27Al(3He,t)27Si and 28Si( 3He,α)27Si transfer reactions were detected in coincidence with the reaction products of interest, which were detected at the focal plane of the Yale Enge spectrograph. Angular correlations were measured to constrain spins and determine proton branching ratios for 26mAl + p resonances with Ecm > 450 keV and excitation energies of 27 Si were measured. Using this information a 26mAl(p, γ)27Si reaction rate based on experimental information has been calculated for the first time. The extreme differences in proton decays from excited states in 27 Si to 26gA1 and 26mAl show that 26mAl(p,γ)27Si reaction rates found using experimental data for 26gA1 + 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. © 2009 American Institute of Physics.