K-Electron Capture and Internal Conversion inCr51

${\mathrm{Cr}}^{51}$ has been chemically isolated from titanium which has been bombarded with high speed alpha-particles and chromium which has been activated with deuterons. Its half-life is 26.5±1.0 days and its identification depends on its formation in the reactions ${\mathrm{Ti}}^{48}+{\mathrm{He}}^4\to {\mathrm{Cr}}^{51}+n^1; {\mathrm{Cr}}^{50}+{\mathrm{H}}^2\to {\mathrm{Cr}}^{51}+{\mathrm{H}}^1; {\mathrm{Cr}}^{50}+n^1\to {\mathrm{Cr}}^{51}+\gamma .$ ${\mathrm{Cr}}^{51}$ decays mainly by $K$-electron capture but is probably unstable against positron emission as it emits strong $\gamma$-radiation of energy 0.5 Mev and 1.0 Mev. No positrons of energy > 100,000 ev have been detected, the lower limit of the ratio $K$ capture to emission of positrons of energy > 100,000 ev being 10,000.