Speaker
Description
The extinct radionuclide $^{92}$Nb (half-life $\sim$ 34.7 Myr) is a sensitive tracer of proton-rich nucleosynthesis and a chronometer for the early Solar System. Interpretation of meteoritic $^{92}$Nb/$^{92}$Mo ratios is currently limited by both astrophysical and nuclear-physics uncertainties. In particular, the origin of $^{92}$Nb remains uncertain because it is shielded by the stable isobars $^{92}$Zr and $^{92}$Mo and therefore must be synthesized through direct nuclear reactions in explosive supernova environments. The $^{91}$Nb($n$,$\gamma$)$^{92}$Nb reaction rate is among the most influential inputs for the production of $^{92}$Nb under such conditions. However, due to the instability of $^{91}$Nb, direct measurements of the $^{91}$Nb($n$,$\gamma$)$^{92}$Nb cross section and thus the reaction rate are not currently feasible. As a result, recommended rates in commonly used libraries rely heavily on Hauser-Feshbach calculations, whose accuracy depends on nuclear statistical properties, such as the nuclear level density (NLD) and $\gamma$-ray strength function ($\gamma$SF) of $^{92}$Nb, for which no experimental constraints have previously been available. Here, we report an experimental constraint on the $^{91}$Nb($n$,$\gamma$)$^{92}$Nb rate by extracting the NLD and $\gamma$SF of $^{92}$Nb using the Oslo method applied to particle-$\gamma$ coincidences measured in the $^{90}$Zr($\alpha$,d$+\gamma$)$^{92}$Nb reaction at the Oslo Cyclotron Laboratory using a $^{4}$He beam at 30 MeV, with the SiRi particle telescope in coincidence with the OSCAR LaBr$_3$(Ce) array. The extracted NLD and $\gamma$SF were propagated through Hauser-Feshbach calculations with TALYS to obtain an experimentally constrained $^{91}$Nb($n$,$\gamma$)$^{92}$Nb reaction rate. The resulting band is a factor of $\sim$ 2-3 below the recommended NON-SMOKER rate over the supernova relevant temperature window. The astrophysical impact of the improved rate was evaluated using NuGrid one-zone post-processing calculations for core-collapse and thermonuclear supernova conditions. The results show that the yield response can differ in sign between stellar environments, demonstrating that a single experimentally constrained reaction rate can shift inferred $^{92}$Nb production and, consequently, early Solar System chronology, highlighting the leverage of Oslo-type inputs for reducing nuclear uncertainties in proton-rich nucleosynthesis. An independent verification of the $^{91}$Nb($n$,$\gamma$)$^{92}$Nb reaction rate is also planned through extraction of the NLD and $\gamma$SF of $^{92}$Nb using the Charge-Exchange (CE) Oslo method, which will be applied to particle-$\gamma$ coincidence data from the planned $^{92}$Zr($^{3}$He,t$+\gamma$)$^{92}$Nb experiment at RCNP using a 420 MeV $^{3}$He beam, with the Grand Raiden spectrometer in coincidence with a scintillation $\gamma$-ray detector array. The CE-Oslo method was first tested using $^{93}$Nb($t$,$^{3}$He$+\gamma$) data taken with the S800 spectrometer in coincidence with the GRETINA $\gamma$-ray detector at the National Superconducting Cyclotron Laboratory (NSCL). Using the constructed particle-$\gamma$ coincidence matrix for $^{93}$Zr, the NLD and $\gamma$SF of $^{93}$Zr were extracted and then propagated through Hauser-Feshbach calculations with TALYS to estimate the $^{92}$Zr($n$,$\gamma$)$^{93}$Zr cross section. The resulting cross section is in good agreement with direct measurements, thereby validating the CE-Oslo method, and has been published as its first demonstration.
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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