Measurement of Absolute Electron Capture Rates with an Application to the Decay ofNi57

Observation of radiations from 36-hour ${\mathrm{Ni}}^{57}$ with a proportional counter and pulse-height analyzer shows that cobalt x-rays are emitted. Decay is divided equally between $K$-capture and emission of 845-kev positrons. The positron spectrum has the allowed shape. Coincidence measurements involving the comparison of ${\mathrm{Ni}}^{57}$ and ${\mathrm{Na}}^{22}$ give the result that both $K$-capture and positron emission in ${\mathrm{Ni}}^{57}$ are followed by emission of a 1.9-Mev gamma-quantum. In the $K$ branch there are some 120-kev gamma-rays and possibly others of energy less than 0.5 Mev in cascade with the hard gammas.