Shell-model calculation of 99Tc beta decay in astrophysical environments

Physics – Nuclear Physics

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Shell Model, Beta Decay, Double Beta Decay, Electron And Muon Capture, 90<=A<=149, Origin, Formation, And Abundances Of The Elements

Scientific paper

Using the shell-model Lanczos method, we calculate the Gamow-Teller matrix elements for beta transitions of astrophysical interest from excited states of 99Tc ((7/2)+,141 keV; (5/2)+,181 keV) to the ground and excited states of 99Ru. The level schemes of low-lying positive-parity states in these nuclei and in the analogous isotonic nuclei 97-97Mo 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 99Tc half-life of ~20 yr at a stellar temperature of 3×108 K typical for the s process with a 22(α,n)25Mg neutron source. Referring to recent studies of thermal-pulse s-process models, we stress that a substantial amount of 99Tc can survive the s process in spite of its enormously enhanced decay rate. The factual observation of 99Tc at the surface of certain stars, therefore, does not necessarily contradict an s-process scenario with 22Ne as a neutron source.

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