Remeasurement of the half-life of the 615 keV level in 181Ta

The lifetimes of the 615 and 619 keV levels in Ta-181 have been remeasured by using a beta-gamma delayed coincidence set up with fast plastic (Pilot U) and BaF2 detectors. The presently measured values for both levels have been found to be in strong disagreement with results reported earlier. For the 615 keV level, the half-life has been found to be T-1/2 = 12.1 +/- 0.1 ns in striking contrast to the earlier reported value of 17.6 mus. The half-life of the 619 keV level has been found to be T-1/2 less than or equal to 230 ps, whereas values of 0.87 ns and 2.4 ns were reported previously. The nanosecond half-life of the 615 keV level has been confirmed from a separate measurement by employing a NaI(Tl)-Pilot U detector combination. The importance of lifetime measurements for the 615 keV level in perturbed angular correlation studies is discussed.