Disintegration ofCe139

Abstract

The decay of ${\mathrm{Ce}}^{139}$ is shown to proceed by $s$-electron capture to a single excited state of ${\mathrm{La}}^{139}$ with the subsequent emission of an internally converted gamma ray. Magnetic beta-ray spectrometry and scintillation coincidence techniques have been used to show that the 166-kev gamma ray is magnetic dipole in character. The measured $\frac{K}{\mathrm{}(L+M)\mathrm{}}$ ratio is 6.6±0.3. An estimation of the $K$ internal conversion coefficient yields 0.20±0.05 and the lifetime of the excited level is found to be less than ${10}^{-8\mathrm{}}$ sec. An upper limit of 315 kev is found for the total disintegration energy. Although no estimate of orbital electron capture to the ground state can be made, a small amount is probable. A value of 0.94±0.02 for the $K$ fluorescent yield at $Z=57\mathrm{}$ is obtained from the experiment.