Fast neutron induced depopulation of the Ta-180(m) isomer

Fast neutron-induced depopulation of the Ta-180(m) (I, K-pi=9,9(-)) isomer to the 1(+) ground state was detected by activation techniques. Natural Ta foils together with a Th-232 monitoring target were activated using the neu tron flux produced from a 7.3 MeV electron beam by a special choice of conv erters and shields. The K-alpha 2 x-ray Line (54.6 keV) of Hf was successfu lly observed in the gamma-ray spectrum from activated Ta and attributed to the decay of Ta-180(g)(T-1/2=8.15 h). The yields of three reactions Th-232( n,f), Ta-181(n, gamma)Ta-182, and Ta-180(m)(n,n')Ta-180(g), were measured i n the same conditions and compared. Finally, the mean probability for Ta-18 0(m) depopulation after MeV neutron scattering was deduced to approach abou t 0.4. This establishes almost complete K mixing above the neutron binding energy. [S0556-2813(99)02602-3].