γ spectroscopy of states in Cl32 relevant for the S31

Abstract

Background:The 31 S(p, γ) 32 Cl reaction becomes important for sulfur production in novae if the 31 P(p, α) 28 Si 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 Ecm = 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 32 Cl 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 6 excited states in 32 Cl. The excitation energies for two unbound levels at Ex = 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( 3 He,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.