High-precision half-life determination for ²¹Na using a 4 π gas-proportional counter

A high-precision half-life measurement for the superallowed β⁺ transition between the isospin T = 1/2 mirror nuclei ²¹Na and ²¹Ne has been performed at the TRIUMF-ISAC radioactive ion beam facility yielding T_1/2 = 22.4506(33) s, a result that is a factor of 4 more precise than the previous world-average half-life for ²¹Na and represents the single most precisely determined half-life for a transition between mirror nuclei to date. The contribution to the uncertainty in the ²¹Na Ft(mirror) value due to the half-life is now reduced to the level of the nuclear-structure-dependent theoretical corrections, leaving the branching ratio as the dominant experimental uncertainty. (authors)