Stellar krypton cross sections atkT=25and 52 keV

Studies of neutron capture nucleosynthesis in the s process usually refer to a thermal energy of kT=30 keV, corresponding to a temperature of T=3.5×${10}^8$ K. This convention is justified as stellar neutron capture rates are in general not sensitive to temperature because of the approximate $E_{\mathrm{n}{}^{\mathrm{-}1/2}}$ dependence of most capture cross sections. But several important exceptions of this rule require more detailed information on the variation of the average stellar cross section with temperature. We show that activation measurements can be performed in a quasistellar neutron spectrum for kT=52 keV, using kinematically collimated neutrons from the ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(p,n) reaction. Neutron capture cross sections were measured in this spectrum and at kT=25 keV for ${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}$(n,γ) and for the reaction ${}_{}{}^{84}{}_{}{}^{}\mathrm{Kr}$(n,γ${)\mathrm{}}^{85}$ ${\mathrm{Kr}}^{\mathrm{m}}$, which populates the isomeric state in ${}_{}{}^{85}{}_{}{}^{}\mathrm{Kr}$. In this way, the respective 30 keV cross sections of 3.5±0.3 and 16.7±1.2 mb could be derived by interpolation.