Background: The γ process in core-collapse supernova explosions is thought to explain the origin of proton-rich isotopes between Se and Hg, the so-called p nuclei. The majority of the reaction rates for γ process reaction network studies has to be predicted in Hauser-Feshbach statistical model calculations. Recent investigations have shown problems in the prediction of α widths at astrophysical energies. This impacts the reliability of abundance predictions in the upper mass range of the p nuclei. Purpose: Our purpose is to measure the 127I(α,γ)131I and 127I(α,n)130I reaction cross sections close to the astrophysically relevant energy range to test the predictions, to derive an improved reaction rate, and to extend the database required to define an improved global optical α+nucleus potential. Methods: The cross sections are derived using the activation technique and the yield of the emitted γ, and characteristic x-ray photons are measured using a LEPS and an HPGe detector. Results: Cross sections of the 127I(α,γ)131Cs reaction are determined for the first time, at energies 9.50≤Ec.m.≤15.15 MeV. The 127I(α,n)130Cs reaction is studied in the range 9.62≤Ec.m.≤15.15 MeV. Furthermore, the relative intensity of the 536.1-keV γ transition is measured precisely; its uncertainty is reduced from 13% to 4%. The results are then compared to Hauser-Feshbach calculations which are also used to extend the cross sections into the astrophysically relevant region and to compute the reaction rate. Conclusions: The comparison to statistical Hauser-Feshbach model calculations shows that the α width can be described well in the measured energy range using a standard, energy-independent global optical potential. The newly derived stellar reaction rates at γ process temperatures for 127I(α,γ)131I and its reverse reactions, nevertheless, are faster by factors of 4-10 than those from previous calculations, owing to further improvements in the reaction model. The importance of the inclusion of complete level schemes in the Hauser-Feshbach calculations is illustrated by comparing the impacts of two level schemes, one of them extending to higher excitation energies but not containing all relevant levels.
ASJC Scopus subject areas
- Nuclear and High Energy Physics