Isomeric Transition inPb205

Abstract

A (26±1)-kev transition in ${\mathrm{Pb}}^{205}$ occurring in the electron capture decay of ${\mathrm{Bi}}^{205}$ has been identified from its $L$, $M$, and $N$ internal conversion electrons measured in an intermediate-image beta-ray spectrometer. By using NaI scintillation detectors behind the source in the spectrometer the $M$ and $N$ lines of the 26-kev transition are found to be not in coincidence with electron-capture $K$ x rays but they are in coincidence with a principal gamma ray of 1 Mev and weak components of ∼0.7 and ∼0.3 Mev. Since the ${\mathrm{Bi}}^{205}$ gamma rays of 987.8 and 284.2 kev and a fraction of the 703.3-kev gamma rays have been shown by Vegors and Heath to be delayed with respect to electron-capture $K$ x rays with a half-life of 4.8 msec, the present coincidence results indicate that the delayed radiation is associated with the 26-kev transition originating from an isomeric state in ${\mathrm{Pb}}^{205}$ at 1013.8 kev. The 26-kev transition is probably a quadrupole and a possible assignment is $M2\mathrm{}$ if the spin-parity assignment of the 987.8-kev level is $\frac{9}{2}-$ and if the 1013.8-kev level is the $\frac{13}{2}+$ state predicted at 1.1 Mev by Pryce. The possibility that all or part of the known 1014.2-kev gamma radiation constitutes the "missing" $M4\mathrm{}$ transition in ${\mathrm{Pb}}^{205}$ is discussed.