Speaker
Description
Short-lived radionuclides (SLRs) with million-year lifetimes are powerful chronometers of Solar System formation. Among the 19 known SLRs, $^{92}$Nb is one of the few confirmed proton-rich SLRs and is of particular interest because the meteoritic $^{92}$Nb/$^{92}$Mo ratio provides a sensitive probe of proton-rich nucleosynthesis. However, its interpretation remains limited by both astrophysical and nuclear-physics uncertainties. Because $^{92}$Nb is shielded by the stable isobars $^{92}$Zr and $^{92}$Mo, it cannot be produced through $\beta$-decay chains and must instead be synthesized through direct reactions in explosive supernova environments. The leading production scenarios are the $\gamma$-process in core-collapse supernovae (CCSNe) and Type Ia supernovae (SNe Ia), together with the neutrino-process in CCSNe. For the $\gamma$-process, the $^{91}$Nb($n,\gamma$)$^{92}$Nb reaction and its inverse are key regulators, yet direct measurements are not presently feasible. Their recommended values therefore rely on Hauser-Feshbach calculations whose accuracy depends on the nuclear level density (NLD) and $\gamma$-ray strength function ($\gamma$SF), both of which remain uncertain for the odd-odd nucleus $^{92}$Nb because of its complex nuclear structure and limited spectroscopic information. Presented here are the first experimental constraints on the NLD and $\gamma$SF of $^{92}$Nb, extracted with the Oslo method from particle-$\gamma$ coincidences measured in the $^{90}$Zr($\alpha$,d$+\gamma$)$^{92}$Nb reaction at OCL using a 30 MeV $^{4}$He beam, with the SiRi particle telescope and the OSCAR array. These newly constrained quantities yield a $^{91}$Nb($n,\gamma$)$^{92}$Nb reaction rate lower by a factor of $\sim$2-5 than the recommended NON-SMOKER value. The revised rate increases the predicted $^{92}$Nb abundance by 50-90% in both CCSNe and SNe Ia $\gamma$-process models, thereby easing the long-standing Early Solar System $^{92}$Nb/$^{92}$Mo discrepancy. By reducing one of the dominant nuclear uncertainties in the $^{92}$Nb production network, this work shows that resolving the remaining discrepancy will require improved astrophysical models, revised Galactic Chemical Evolution (GCE) calculations, and possibly additional contributors such as neutrino-driven processes. The neutrino-process contribution to $^{92}$Nb production in CCSNe is also being investigated experimentally. This contribution is governed primarily by the charge-current (CC) $^{92}$Zr($\nu_e,e^{-}$)$^{92}$Nb reaction, for which Gamow-Teller (GT) and Fermi transitions from the ground state of $^{92}$Zr are the dominant nuclear inputs at supernova energies. The first experimentally constrained CC $^{92}$Zr($\nu_e,e^{-}$)$^{92}$Nb cross sections have been derived from GT and Fermi strengths extracted through multipole-decomposition analysis (MDA) of high-resolution $^{92}$Zr($^{3}$He,t)$^{92}$Nb data taken at RCNP with a 420 MeV $^{3}$He beam and the Grand Raiden spectrometer. Results based on these new CC cross sections will also be presented. As an independent verification, both the $\gamma$- and neutrino-process contributions are planned to be extracted in a single nuclear experiment through the newly developed charge-exchange Oslo method and MDA in the proposed $^{92}$Zr($^{3}$He,t$+\gamma$)$^{92}$Nb experiment at RCNP. This will establish a new experimentally anchored framework for reducing nuclear uncertainties in astrophysical models and, consequently, in GCE calculations.
This research is supported by the "U.S. National Science Foundation (NSF), the Norwegian Nuclear Research Center (NNRC), and the International Research Network for Nuclear Astrophysics (IReNA)''.
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