Branching ratio to the 803 keV level in Po210α decay

Precise knowledge of the branching ratio in the \ensuremath{\alpha} decay of $^{210}\mathrm{Po}$ is important for accurate measurement of the $^{209}\mathrm{Bi}(n,\ensuremath{\gamma})^{210\mathrm{g}}\mathrm{Bi}$ cross section, the reaction involved in the termination of the astrophysical $s$ process. The branching ratio was determined from independent measurements of \ensuremath{\alpha} and \ensuremath{\gamma} spectra of bismuth samples simultaneously irradiated by neutrons near the core of the Soreq research reactor (IRR1). The branching ratio was found to be $(1.15\phantom{\rule{4pt}{0ex}}\ifmmode\pm\else\textpm\fi{}0.09)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5\phantom{\rule{4pt}{0ex}}}$, consistent with the results of several measurements performed six decades ago. As a by-product value the $^{209}\mathrm{Bi}(n,\ensuremath{\gamma})^{210\mathrm{g}}\mathrm{Bi}$ thermal cross section was measured to be $21.6\ifmmode\pm\else\textpm\fi{}1.1\phantom{\rule{4pt}{0ex}}$ mb.