Decay of Selenium-72

${\mathrm{Se}}^{72}$ has been observed to decay by electron capture with a half-life of 8.40±0.08 days, predominantly to an excited state in ${\mathrm{As}}^{72}$ at 46.0±0.3 kev. The half-life of this state is less than 1.5×${10}^{-8\mathrm{}}$ sec. Less than 7% of the ${\mathrm{Se}}^{72}$ decays are to the ground state. The $K$-conversion coefficient of the 46-kev gamma was measured to be 0.63±0.06 and the $\frac{K}{L}$ conversion ratio to be 9.7±0.5. The upper limit, 0.1%, obtained for the abundance of positrons places a maximum value on the decay energy of 1.2 Mev, which corresponds to a maximum $log\mathrm{ft}$ of 5.7. On these grounds the excited state is assigned a 1+ spin and the gamma transition is $E1\mathrm{}$. In the course of these measurements, the ratio of intensities of x-rays to 46-kev gamma rays in the decay of RaD (${\mathrm{Pb}}^{210}$) was redetermined to be 5.38±0.50.