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

The M4 isomeric transition (E/sub gamma /-238 keV) of the 13/2/sup +/ state in /sup 199/PO to the 5/2/sup -/ level, with a half life of a 4.17 min, has been observed in mass separated sources. An approximately 72 keV transition in coincidence with the isomeric transition suggests the placement the 5/2/sup -/ level above the 3/2 /sup -/, and when combined with alpha -decay Q values, this results in a placement of the 13/2/sup +/ isomeric state in /sup 195/Pb at approximately 200 keV. The transition rate for the M4 transition in /sup 199/PO is similar to those for other M4 transitions in this region. Contrastingly, a M4 transition in /sup 199/Bi, which is highly likely to occur on the basis of systematics of /sup 1///sub 2//sup +/ and 9/2/sup -/ states and a sequence of gamma rays among low spin states observed in the authors' data, is highly retarded. Although, this M4 transition is not observable in their data, an upper limit for its strength is established at approximately 1/1000 the intensity of typical M4 transitions in this region. Possible interpretations of this retardation are discussed. (23 refs).