New neutron-deficient isotopes from Kr-78 fragmentation

Blank, B. and Goigoux, T. and Ascher, P. and Gerbaux, M. and Giovinazzo, J. and Algora, Alejandro and Kiss, Gábor Gyula (2016) New neutron-deficient isotopes from Kr-78 fragmentation. PHYSICAL REVIEW C, 93 (6). ISSN 2469-9985

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Abstract

Background: Because of its half-life of about 35 million years, 92Nb is considered as a chronometer for nucle-osynthesis events prior to the birth of our sun. The abundance of 92Nb in the early solar system can be derivedfrom meteoritic data. It has to be compared to theoretical estimates for the production of 92Nb to determine thetime between the last nucleosynthesis event before the formation of the early solar system. Purpose: The influence of a low-lying short-lived isomer on the nucleosynthesis of 92Nb is analyzed. The thermalcoupling between the ground state and the isomer via so-called intermediate states affects the production andsurvival of 92Nb. Method: The properties of the lowest intermediate state in 92Nb are known from experiment. From the lifetimeof the intermediate state and from its decay branchings, the transition rate from the ground state to the isomerand the effective half-life of 92Nb are calculated as a function of the temperature. Results: The coupling between the ground state and the isomer is strong. This leads to thermalization of groundstate and isomer in the nucleosynthesis of 92Nb in any explosive production scenario and almost 100 % survivalof 92Nb in its ground state. However, the strong coupling leads to a temperature-dependent effective half-life of92Nb which makes the 92Nb survival very sensitive to temperatures as low as about 8 keV, thus turning 92Nb atleast partly into a thermometer. Conclusions: The low-lying isomer in 92 Nb does not affect the production of 92Nb in explosive scenarios. Inretrospect this validates all previous studies where the isomer was not taken into account. However, the dramaticreduction of the effective half-life at temperatures below 10 keV may affect the survival of 92Nb after its synthesisin supernovae which are the most likely astrophysical site for the nucleosynthesis of 92Nb.

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