New measurement of the Sm-144(alpha, gamma) Gd-148 reaction rate for the gamma process

Background: Most of the heavier p isotopes are believed to be produced in the gamma process whose reaction path crucially depends on the proton and alpha-particle penetrability at sub-Coulomb energies. Both nuclei of the samarium p-process chronometer, Sm-146 and Sm-144, are produced in the. process, and their initial abundance ratio is very sensitive to the (gamma, n) and (gamma, alpha) branching ratio on Gd-148. The Gd-148(gamma, alpha) Sm-144 reaction rate was measured roughly 20 years ago by means of the activation technique and its surprising results triggered adjustments to the global low-energy a+nucleus optical-model potentials (OMPs). Purpose: We want to take advantage of modern alpha-particle spectroscopy techniques in order to constrain the controversial previous results on the Gd-148(gamma, alpha) Sm-144 reaction rate. Method: The Gd-148(gamma, alpha) Sm-144 reaction rate has been determined by measuring the cross section of the reverse reaction Sm-144(alpha, gamma) Gd-148, applying the activation technique to the alpha decay of Gd-148. Targets have been irradiated at the cyclotron of the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany. The alpha-particle spectroscopy has been carried out with a state-of-the-art low-background ionization chamber of the Technische Universitat Dresden, Germany. Results: Cross sections for the Sm-144(alpha, gamma) Gd-148 reaction have been measured between 10.66 and 12.66 MeV with much higher precision than in the previous measurement. The results agree with earlier results within their uncertainties. The statistical-model analysis has been carried out using the TALYS code on the basis of the latest parametrizations of alpha-OMPs. The best reproductions of the experimental results within the statistical model have been used to calculate the reaction rates. Conclusion: The values presented here suggest a steeper increase in the astrophysical S factor towards lower center-of-mass energies. Different parametriza