The (n,γ) cross sections of the p-process nuclei ⁷⁴Se and ⁸⁴Sr at kT=25 keV

The formation of the isotopes on the proton-rich side of the valley of stability cannot be ascribed to neutron capture reactions in the r- or s-processes. These nuclei are produced in the p process, which presumably takes place in the explosively burning O/Ne layers of SNe II. At typical temperatures of 2-3 GK, the p nuclei are produced by a complex reaction network via (gamma,n), (gamma,p), (gamma,alpha) and their inverse reactions. So far most of these reactions are not yet accessible to experimental techniques and must, therefore, be inferred by statistical model calculations. At present, the parametrization of these models has to be constrained by measurements on stable proton-rich nuclei. First results for the stellar (n,gamma) cross sections of Se-14 and Sr-84 are reported for a thermal energy of kT = 25 keV. These values were determined in a series of activation measurements at the Karlsruhe Van de Graaff accelerator using the Li-7(p,n)(7) Be source for simulating a Maxwellian neutron energy distribution.