DELAYED ISOMERIC STATE IN PB-205 AND ITS IMPLICATIONS FOR 4-PI-ALPHA LIQUID SCINTILLATION SPECTROMETRY OF PO-209

Assays of Po-209 by 4 pi alpha liquid scintillation (LS) measurements are confounded by an alpha-transition to a 2.3 keV (J(pi) = 1/2(-)) le vel in Pb-205 which was previously unknown to be a delayed isomeric st ate. The level is believed to be populated by a substantial fraction o f the total Po-209 decay (similar to 80%) and presumably decays predom inantly by internal conversion. The resulting conversion electrons fro m this delayed isomeric transition were identified in LS spectra. The lifetime is inferred to be at least several times 10(-5) s, although i t may very well be substantially longer lived by orders of magnitude. The implications of these findings for routine measurements of Po-209 by LS are discussed. Suggested experiments to precisely determine the lifetime of the state are also proposed.