Energy Dependence of Auto-Ionization in Beta Decay

Auto-ionization accompanying $\beta$ decay was studied in an equilibrium mixture of ${\mathrm{Sr}}^{90}$-${\mathrm{Y}}^{90}$ by a comparison of the intensities of $K$ x rays accompanying these decays. Since both nucleides undergo $\beta$ decay of the unique first forbidden type, the main difference between the two decays is a factor of ∼ 4 in endpoint energy. The ratio of the amount of x rays accompanying the ${\mathrm{Y}}^{90}$ decay to the amount accompanying ${\mathrm{Sr}}^{90}$ decay was found to be 1.3. This is interpreted to mean that the direct collision process plays an unimportant role in auto-ionization in these two isotopes and that it is most likely that the probability of auto-ionization in $\beta$ decay is independent of the energy of the $\beta$ particle in decays with endpoint energies high compared with the $K$-shell ionization energy.