Bismuth activation with quasi-Maxwellian neutrons at kT ∼ 30 keV

Bismuth capture of neutrons is the termination point of the $s$-process cycle of nucleosynthesis in stellar environments. A new measurement is reported here for neutron activation of bismuth with an intense quasi-Maxwellian neutron source at $kT\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$. The measurement was performed at the SARAF phase I accelerator facility by bombarding a 1.5-mA proton beam on the liquid-lithium larget. The cross section of the $^{209}\mathrm{Bi}(n,\ensuremath{\gamma})$ capture reaction leading to the $^{210}\mathrm{Bi}$ ground state was determined by combining $\ensuremath{\beta}$ measurements from the $^{210\mathrm{g}}\mathrm{Bi}$ decay and $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ from the subsequent $^{210}\mathrm{Po}$ decay, along with detailed Monte Carlo simulations of the $^{7}\mathrm{Li}(p,n)$ reaction kinematics and the activation experimental setup. Deduced Maxwellian averaged cross sections (MACS) for $^{209}\mathrm{Bi}(n,\ensuremath{\gamma})^{210\mathrm{g}}\mathrm{Bi}$ at $kT=30\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ using the ENDF, JEFF, and JENDL databases for the corrections and extrapolations yielded a value of $1.84\ifmmode\pm\else\textpm\fi{}0.09\phantom{\rule{0.16em}{0ex}}\mathrm{mb}$. A comparison is made with previous measurements, including time-of-flight (TOF) measurements of the total bismuth capture cross section. Plans for obtaining the MACS for capture to the bismuth-210 metastable state in the reaction $^{209}\mathrm{Bi}(n,\ensuremath{\gamma})^{210\mathrm{m}}\mathrm{Bi}$ are discussed, along with estimates based on our results in comparison with TOF measurements. The bismuth neutron activation cross section is also of importance for design of GenIV reactor coolant and subcritical accelerator driven systems, especially in light of the 3 million year half-life of the $^{210\mathrm{m}}\mathrm{Bi}$ isomer.