Using the shell-model Lanczos method, we calculate the Gamow-Teller matrix elements for beta transitions of astrophysical interest from excited states of ${}_{}{}^{99}{}_{}{}^{}\mathrm{Tc}$ ${\mathrm{}\left(\right(7/2\mathrm{})\mathrm{}}^{+}$,141 keV; ${\mathrm{}(5/2)\mathrm{}}^{+}$,181 keV) to the ground and excited states of ${}_{}{}^{99}{}_{}{}^{}\mathrm{Ru}$. The level schemes of low-lying positive-parity states in these nuclei and in the analogous isotonic nuclei ${}_{}{}^{97}{}_{}{}^{}\mathrm{-}_{}^{97}{}_{}{}^{}$Mo are reproduced fairly well within a model space consisting of low-seniority excitations in the 1g2d shell, which are mixed via the Kallio-Kolltveit effective interaction. The calculations lead to a ${}_{}{}^{99}{}_{}{}^{}\mathrm{Tc}$ half-life of ∼20 yr at a stellar temperature of 3×${10}^8$ K typical for the s process with a ${}_{}{}^{22}{}_{}{}^{}(\mathit{\alpha }$,n${)}^{25}$Mg neutron source. Referring to recent studies of thermal-pulse s-process models, we stress that a substantial amount of ${}_{}{}^{99}{}_{}{}^{}\mathrm{Tc}$ can survive the s process in spite of its enormously enhanced decay rate. The factual observation of ${}_{}{}^{99}{}_{}{}^{}\mathrm{Tc}$ at the surface of certain stars, therefore, does not necessarily contradict an s-process scenario with ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}$ as a neutron source.

• Received 26 August 1985

DOI:https://doi.org/10.1103/PhysRevC.33.296