Speaker
Description
Charge-exchange (CE) reactions are a powerful tool for probing the spin-isospin response of nuclei. Because they are not restricted to a narrow Q-value window, they provide complementary access to weak-interaction processes such as $\beta$-decay and electron-capture. In particular, the proportionality between Gamow-Teller strength, B(GT), and the CE differential cross section makes it possible to extract B(GT) distributions up to high excitation energies, providing key input for electron-capture and neutrino-nucleus reaction rates in hot and dense astrophysical environments. By combining CE measurements with coincident $\gamma$-ray spectroscopy, we have extended the Oslo method to CE reactions, establishing the Charge-Exchange Oslo (CE-Oslo) method for extracting nuclear level densities (NLDs) and $\gamma$-ray strength functions ($\gamma$SFs). These two statistical quantities are essential for indirectly constraining astrophysical neutron-capture reaction rates. 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 NSCL/FRIB. Although the primary goal of that experiment was to extract the B(GT) distribution from $^{93}$Nb to $^{93}$Zr, the particle-$\gamma$ coincidence data also enabled the extraction of the NLD and $\gamma$SF of $^{93}$Zr. These were 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. This development serves as the foundation for the planned $^{92}$Zr$({}^{3}$He,$t+\gamma)^{92}$Nb experiment at RCNP. The goal of this experiment is to constrain the $^{91}$Nb$(n,\gamma)^{92}$Nb and $^{92}$Zr$(\nu_e,e^{-})^{92}$Nb reaction rates using the CE-Oslo method and Multipole Decomposition Analysis. These two reactions, neither of which has yet been experimentally constrained, are key regulators of $^{92}$Nb production in the $\gamma$-process and neutrino-process in core-collapse and Type Ia supernovae environments. $^{92}$Nb is one of the few confirmed proton-rich short-lived radionuclides, and the meteoritic $^{92}$Nb/$^{92}$Mo ratio provides a sensitive probe of proton-rich nucleosynthesis and early Solar System formation. However, its interpretation has long been limited by both astrophysical and nuclear-physics uncertainties. Using data from two separate experiments, $^{90}$Zr$(\alpha,d+\gamma)^{92}$Nb at OCL and $^{92}$Zr$({}^{3}$He,$t)^{92}$Nb at RCNP, these two reaction rates have now been experimentally constrained and their impact on $^{92}$Nb production in supernovae environments will also be presented.
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).
References:
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| Contribution category | Experiment |
|---|---|
| Presenter status | Student |