Speaker
Description
During stellar evolution, nuclear reactions predominantly occur at energies significantly lower than the Coulomb barrier, highlighting the role of neutron-induced reactions in the synthesis of chemical elements, especially those heavier than iron. By employing the evaporation technique, neutron-induced reactions can also be utilized to extract nuclear level densities (NLD) of unstable isotopes, which are essential for accurate reaction rate calculations. The main idea of the evaporation technique is that the differential cross section for the emission of a particle from a compound nucleus is proportional to the appropriate transmission coefficient and NLD. Therefore, the detailed shape of the particle spectrum is determined by the energy dependence of the level density. Improved experimental NLD can be obtained by comparing the experimental spectra with those calculated using the Hauser-Feshbach theory and adjusting the theory parameters to more accurately reproduce the experimental spectra.
To probe nuclear level densities in the Ni region, particularly relevant to the i-process in metal-poor stars, cross sections measurements for the $^{68}Zn(n,p)^{68}$Cu and $^{ 68}Zn(n,a)^{65}Ni$ reactions were carried out at the Weapon Neutron Research (WNR) facility of the Los Alamos Neutron Science Center (LANSCE). A "white" neutron beam ranging in energy between 0.1 and 100 MeV impinged on a highly enriched $^{68}$Zn target located at the center of the Low Energy NZ (LENZ) detection system. The reaction products were detected using annular S1 DSSD telescopes upstream and downstream of the target.
Utilizing the wide energy spectrum of the neutron beam, an appropriate incident neutron energy range (10-13 MeV) was selected for the level density analysis so that both the direct and non-primary mechanism components in the evaporation spectra are minimal.
Our study aims to deduce nuclear level densities for the neutron-rich isotopes of $^{68}Cu$ and $^{65}Ni$ via the evaporation technique using a white neutron beam for the first time. In the future, we plan to expand the technique to heavier nuclei that may be relevant to stellar environments characterized by i-process neutron densities.
In this talk, we present details on the experimental setup, analysis, and preliminary results for the extraction of the nuclear level density of $^{68}Cu$ and $^{65}Ni$.
This work is supported by the U.S. Department of Energy, Office of Science, Nuclear Physics program under the award number DE-SC0022538
