Improved $^{192,194,195,196}$Pt($n,\gamma $) and $^{192}$Ir($n,\gamma $) astrophysical reactions rates

摘要

$^{192}$Pt is produced solely by the slow neutron capture (\textit{s})nucleosynthesis process and hence an accurate ($n$,$\gamma $) reaction rate forthis nuclide would allow its use as an important calibration point near thetermination of the \textit{s}-process nucleosynthesis flow. For this reason, wehave measured neutron capture and total cross sections for $%^{192,194,195,196,nat}$Pt in the energy range from 10 eV to several hundred keVat the Oak Ridge Electron Linear Accelerator. Measurements on the other Ptisotopes were, in part, necessitated by the fact that only a relatively small$^{192}$Pt sample of modest enrichment was available. Astrophysical $%^{192,194,195,196}$Pt($n,\gamma $) reaction rates, accurate to approximately3%--5 %, were calculated from these data. No accurate reaction rates have beenpublished previously for any of these isotopes. At \textit{s}-processtemperatures, previously recommended rates are larger (by as much as 35%) andhave significantly different shapes as functions of temperature, than our newrates. We used our new Pt results, together with $^{191,193}$Ir(n,$% \gamma $)data, to calibrate nuclear statistical model calculations and hence obtain animproved rate for the unmeasured \textit{s}-process branching-point isotope$^{192}$Ir.