Experimental (n,gamma) cross sections of the p-process nuclei Se-74 and Sr-84

The nucleosynthesis of elements beyond iron is dominated by the s and r processes. However, a small amount of stable isotopes on the proton-rich side cannot be made by neutron capture and are thought to be produced by photodisintegration reactions on existing seed nuclei in the so-called "p process." So far most of the p-process reactions are not yet accessible by experimental techniques and have to be inferred from statistical Hauser-Feshbach model calculations. The parametrization of these models has to be constrained by measurements on stable proton-rich nuclei. A series of (n,gamma) activation measurements, related by detailed balance to the respective photodisintegrations, were carried out at the Karlsruhe Van de Graaff accelerator using the Li-7(p,n)Be-7 source for simulating a Maxwellian neutron distribution of kT = 25 keV. First results for the experimental (n,gamma) cross sections of the light p nuclei Se-74 and Sr-84 are reported. These experimental values were used for an extrapolation to the Maxwellian averaged cross section at 30 keV, (30), yielding 271 +/- 15 mb for Se-74, and 300 +/- 17 mb for the total capture cross section of Sr-84. The partial cross section to the isomer in Sr-85 was found to be 190 +/- 10 mb.