Measurement of the ${}^{77}\mathrm{Se}(n,\gamma )$ cross section up to 200 keV at the n_TOF facility at CERN

The ${}^{77}\mathrm{Se}(n,\gamma )$ reaction is of importance for ${}^{77}\mathrm{Se}$ abundance during the slow neutron capture process in massive stars. We have performed a new measurement of the ${}^{77}\mathrm{Se}$ radiative neutron capture cross section at the Neutron Time-of-Flight facility at CERN. Resonance capture kernels were derived up to 51 keV and cross sections up to 200 keV. Maxwellian-averaged cross sections were calculated for stellar temperatures between $kT=5\mathrm{keV}$ and $kT=100\mathrm{keV}$, with uncertainties between 4.2% and 5.7%. Our results lead to substantial decreases of 14% and 19% in ${}^{77}\mathrm{Se}$ abundances produced through the slow neutron capture process in selected stellar models of $15M_{\odot }$ and $2M_{\odot }$, respectively, compared to using previous recommendation of the cross section.

Figure 1

Weighted counts as a function of neutron energy for the ${}^{77}\mathrm{Se}$ sample (black), empty sample holder (blue), and ambient experimental area background (red).

Figure 2

Examples of sammy fits (red) to ${}^{77}\mathrm{Se}(n,\gamma$) yield data (black).

Figure 3

(a) Deposited energy spectra for one $0^{-}$ resonance and a sum of three $1^{-}$ resonances. (b) Fraction of counts for deposited energy $E_D=8–10.5\mathrm{MeV}$ to the total number of counts for resonances with sufficient statistics. The line indicates the adopted threshold to differentiate between the two spin assignments ($J^{\pi }=0^{-}$ and $1^{-}$ below and above the line, respectively) used in resonance fits with sammy. Empty squares indicate resonances where no measured spin data are available in Refs. [6, 7, 8].

Figure 4

A comparison of the ${}^{77}\mathrm{Se}(n,\gamma$) URR cross section obtained in this work with evaluations [12, 25, 27, 28] and fitacs [29] fit to our data.