Lifetimes of 26Al and 34Cl in an astrophysical plasma -: art. no. 015801

We study here the onset of thermal equilibrium affecting the lifetimes of Al-26 and Cl-34 nuclei within a hot astrophysical photon gas. The Al-26 isotope is of prime interest for gamma-ray astronomy with the observation of its delayed (t(1/2) = 0.74 My) 1.809 MeV gamma-ray line. Its nucleosynthesis is complicated by the presence of a short lived (t(1/2) = 6.34 a) spin isomer. A similar configuration is found in Cl-34 where the decay of its isomer (Cl-34m, t(12) = 32 m) is followed by delayed gamma-ray emission with characteristic energies. The lifetimes of such nuclei are reduced at high temperature by the thermal population of shorter lived levels. However, thermal equilibrium within Al-26 and Cl-34 levels is delayed by the presence of the isomer. We study here the transition to thermal equilibrium where branching ratios for radiative transitions are needed in order to calculate lifetimes. Since some of these very small branching ratios are not known experimentally, we use results of shell model calculations.