Stellar $s$-process neutron capture cross sections of ${}^{69,71}$Ga

Abstract

The stable gallium isotopes, ${}^{69,71}$Ga, are mostly produced by the weak slow ($s$) process in massive stars. We report here on measurements of astrophysically-relevant neutron capture cross sections of the ${}^{69,71}$Ga$(n,{\hat{I}}^3)$ reactions. The experiments were performed by the activation technique using a high-intensity ($3-5\times 10^{10}$ n/s), quasi-Maxwellian neutron beam that closely mimics conditions of stellar $s$-process nucleosynthesis at $kT\approx$ 40 keV. The neutron field was produced by a mA proton beam at $E_p=1925$ keV (beam power of 2-3 kW) from the Soreq Applied Research Accelerator Facility (SARAF), bombarding the Liquid-Lithium Target (LiLiT). A 473 mg sample of Ga${}_2$O${}_3$ of natural isotopic composition was activated in the LiLiT neutron field and the activities of ${}^{70,72}$Ga were measured by decay counting via ${\hat{I}}^3$-spectrometry with a high-purity germanium detector. The Maxwellian-averaged cross sections at $kT$ = 30 keV of ${}^{69}$Ga and ${}^{71}$Ga determined in this work are 136(8) mb and 105(7) mb, respectively, in good agreement with previous experimental values. Astrophysical implications of the measurements are discussed.