First measurement of the partial widths of $^{209}$Bi decay to the ground and to the first excited states

$^{209}$Bi alpha decay to the ground and to the first excited state have been contemporary observed for the first time with a large BGO scintillating bolometer. The half-life of $^{209}$Bi is determined to be $tau_{1/2}$=(2.01$pm$0.08)$cdot10^{19}$ years while the branching ratio for the ground-state to ground-state transition is (98.8$pm$0.3)%.