Polarization measurement of L -shell radiative recombination x rays from highly charged bismuth ions

We report a polarization measurement of $L$-shell radiative recombination (RR) x rays emitted from He- to F-like bismuth ions. The polarization feature was obtained from the azimuthal angular distribution of the Compton-scattered RR x rays detected with a recently developed state-of-the-art Compton polarimeter installed on an electron beam ion trap. This is one of a very few polarization measurements of RR x rays ever conducted and the first one with bismuth ions. We derived the degree of linear polarization averaged over the charge states to be $0.666\\ifmmode\\pm\\else\\textpm\\fi{}0.014$. By contrast, a theoretical estimation of the degree of polarization with the Flexible Atomic Code (fac) yields $0.716\\ifmmode\\pm\\else\\textpm\\fi{}0.023$. The discrepancy between the observed and theoretical values is likely to be due to the electron correlation effects.