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

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,γ) activation measurements, related by detailed balance to the respective photodisintegrations, were carried out at the Karlsruhe Van de Graaff accelerator using the ⁷Li(p,n)⁷Be source for simulating a Maxwellian neutron distribution of kT = 25 keV. First results for the experimental (n,γ) cross sections of the light p nuclei ⁷⁴Se and ⁸⁴Sr are reported. These experimental values were used for an extrapolation to the Maxwellian averaged cross section at 30 keV, <σ>₃₀, yielding 271 ± 15 mb for ⁷⁴Se, and 300 ± 17 mb for the total capture cross section of ⁸⁴Sr. The partial cross section to the isomer in ⁸⁵Sr was found to be 190 ± 10 mb.