Lifetimes of 26Al and 34Cl in an astrophysical plasma

We study here the onset of thermal equilibrium affecting the lifetimes of ²⁶Al and ³⁴Cl nuclei within a hot astrophysical photon gas. The ²⁶Al isotope is of prime interest for γ-ray astronomy with the observation of its delayed (t_1/2=0.74 My) 1.809 MeV γ-ray line. Its nucleosynthesis is complicated by the presence of a short lived (t_1/2=6.34 s) spin isomer. A similar configuration is found in ³⁴Cl where the decay of its isomer (^34mCl, t_1/2=32 m) is followed by delayed γ-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 ²⁶Al and ³⁴Cl 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.