Decay of /sup 199m,g/Po: isomeric transitions in 199Po and 199Bi

The M4 isomeric transition (E/sub γ/ = 238 keV) of the 13/2+ state in 199Po to the 5/2- level, with a half life of 4.17 min, has been observed in mass separated sources. An approx. 72 keV transition in coincidence with the isomeric transition suggests the placement of the 5/2- level above the 3/2-, and when combined with α-decay Q values, this results in a placement of the 13/2+ isomeric state in 195Pb at approx. 200 keV. The transition rate for the M4 transition in 199Po is similar to those for other M4 transitions in this region. Contrastingly, a M4 transition in 199Bi, which is highly likely to occur on the basis of systematics of 1/2+ and 9.2- states and a sequence of gamma rays among low spin states observed in our data, is highly retarded. Although, this M4 transition is not observable in our data, an upper limit for its strength is established at approx. 1/1000 the intensity of typical M4 transitions in this region. Possible interpretations of this retardation are discussed