Abstract
A short-lived nuclear state occurring during a series of radioactive transitions can be studied by the observation of delayed coincidences between the radiations preceding and following the state in question. The method is limited at present to transition periods lying between about 10-7 and 10-2 seconds. Two distinct experimental procedures are possible, one yielding the differential and the other the integrated decay curve of the short-lived state.
The method has been applied to the decay of the previously reported 22-microsecond metastable state in 181Ta, which is formed in the decay of the β-ray emitter 181Hf. It is shown that the isomeric transition takes place in two steps of 0.2 MeV. and 0.5 MeV. in instantaneous succession. The half-life of the metastable level has been redetermined as (20.1±0.7)×10-6 seconds. From a comparison of the observed and calculated internal conversion coefficients, it seems probable that the 0.2 MeV. and 0.5 MeV. transitions are of an electric octopole and electric quadrupole character respectively. This would mean that since the spin of the ground state of 181Ta is 7/2, the 0.5 MeV. state must also have a spin of 7/2 and the same parity as the ground state, while the metastable state must have a spin of 1/2 and opposite parity.