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

We study here the onset of thermal equilibrium affecting the lifetimes of A l-26 and Cl-34 nuclei within a hot astrophysical photon gas. The Al-26 isot ope is of prime interest for gamma-ray astronomy with the observation of it s 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 isom er. 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 char acteristic energies. The lifetimes of such nuclei are reduced at high tempe rature 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 branchin g ratios for radiative transitions are needed in order to calculate lifetim es. Since some of these very small branching ratios are not known experimen tally, we use results of shell model calculations.