Multipolarity of the 2-→1-, ground-state transition in Bi 210 via multivariable angular correlation analysis

The multipolarity of the main transition leading to the ground state in Bi210 was investigated using the angular correlations of γ rays. The analyzed γ-coincidence data were obtained from the Bi209(n,γ)Bi210 experiment performed at Institut Laue-Langevin Grenoble at the PF1B cold-neutron facility. The EXILL (EXOGAM at the ILL) multidetector array, consisting of 16 high-purity germanium detectors, was used to detect γ transitions. The mixing ratio of the 320-keV γ ray was defined by minimizing a multivariable χΣ2 function constructed from the coefficients of angular correlation functions for seven pairs of strong transitions in Bi210. As a result, the almost pure M1 multipolarity of the 320-keV γ ray was obtained, with an E2 admixture of less than 0.6% only (95% confidence limit). Based on this multipolarity the neutron-capture cross section leading to the ground state in Bi210, that decays in turn to radiotoxic Po210, was determined to be within the limits 21.3(9) and 21.5(9) mb. This result is important for nuclear reactor applications.