γ spectroscopy of states in ~(32)Cl relevant for the ~(31)S( p,γ )~(32)Cl reaction rate

摘要

Background: The~(31)S(p,γ ) ~(32)Cl reaction becomes important for sulfur production in novae if the 31P(p,α)28Si reaction rate is somewhat greater than currently accepted. The rate of the~(31)S(p,γ ) ~(32)Cl reaction is uncertain, primarily due to the properties of resonances at E_(c.m.)= 156 and 549 keV. Purpose:We precisely determined the excitation energies of states in ~(32)Cl through high-resolution γ spectroscopy including the two states most important for the~(31)S(p,γ ) ~(32)Cl reaction at nova temperatures. Method: Excited states in 32Cl were populated using the ~(10)B(~(24)Mg,2n) ~(32)Cl reaction with a ~(24)Mg beam from the ATLAS facility at Argonne National Laboratory. The reaction channel of interest was selected using recoils in the Fragment Mass Analyzer, and precise level energies were determined by detecting γ rays with Gammasphere. Results:We observed γ rays from the decay of six excited states in ~(32)Cl. The excitation energies for two unbound levels at E_x= 1738.1 (6) keV and 2130.5 (10) keV were determined and found to be in agreement with a previous high-precision measurement of the ~(32)S(~3He,t )~(32)Cl reaction [1]. Conclusions: An updated~(31)S(p,γ ) ~(32)Cl reaction rate is presented. With the excitation energies of important levels firmly established, the dominant uncertainty in the reaction rate at nova temperatures is due to the strength of the resonance corresponding to the 2131-keV state in ~(32)Cl.