Decay Scheme ofLa138

The methods of scintillation spectroscopy have been applied to the naturally occurring radionuclide ${\mathrm{La}}^{138}$. It is shown that ${\mathrm{La}}^{138}$ emits gamma radiation of energies 1.43±0.01 and 0.81±0.01 Mev. Coincidence studies show that the 1.43-Mev radiation is correlated in time with an observed Ba $K$ x-ray (32±1 kev), but not with the 0.81-Mev gamma ray. A decay scheme is proposed in which ${\mathrm{La}}^{138}$ undergoes electron capture to the first excited level of ${\mathrm{Ba}}^{138}$ at 1.43±0.01 Mev with a partial half-life of (2.1±0.1)×${10}^{11}$ years and an $\frac{L}{K}$ ratio of 1.4±0.25, and negatron decay to an excited level of ${\mathrm{Ce}}^{138}$ at 0.81 Mev with a partial half-life of (2.4±0.2)×${10}^{11}$ years. Consideration of beta decay systematics suggest that in each case the transition energy is low. The total half-life is estimated to be (1.0±0.1)×${10}^{11}$ years.