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Background: Because of its half-life of about 35 million years, 92Nb isconsidered as a chronometer for nucleosynthesis events prior to the birth ofour sun. The abundance of 92Nb in the early solar system can be derived frommeteoritic data. It has to be compared to theoretical estimates for theproduction of 92Nb to determine the time between the last nucleosynthesis eventbefore the formation of the early solar system.  Purpose: The influence of a low-lying short-lived isomer on thenucleosynthesis of 92Nb is analyzed. The thermal coupling between the groundstate and the isomer via so-called intermediate states affects the productionand survival of 92Nb.  Method: The properties of the lowest intermediate state in 92Nb are knownfrom experiment. From the lifetime of the intermediate state and from its decaybranchings, the transition rate from the ground state to the isomer and theeffective half-life of 92Nb are calculated as a function of the temperature.  Results: The coupling between the ground state and the isomer is strong. Thisleads to thermalization of ground state and isomer in the nucleosynthesis of92Nb in any explosive production scenario and almost 100% survival of 92Nb inits ground state. However, the strong coupling leads to a temperature-dependenteffective half-life of 92Nb which makes the 92Nb survival very sensitive totemperatures as low as about 8 keV, thus turning 92Nb at least partly into athermometer.  Conclusions: The low-lying isomer in 92Nb does not affect the production of92Nb in explosive scenarios. In retrospect this validates all previous studieswhere the isomer was not taken into account. However, the dramatic reduction ofthe effective half-life at temperatures below 10 keV may affect the survival of92Nb after its synthesis in supernovae which are the most likely astrophysicalsite for the nucleosynthesis of 92Nb.

Nucleosynthesis of $^{92}$Nb and the relevance of the low-lying isomer at 135.5 keV