K-Shell Internal Ionization Accompanying Beta Decay

Abstract

Internal ionization in the $K$ shell during nuclear $\ensuremath{\beta}$ decay has been studied experimentally as well as theoretically. The subsequent $K$ x rays were measured in coincidence with emitted electrons for various segments of the $\ensuremath{\beta}$ spectra of $^{147}\mathrm{Pm}$ and $^{63}\mathrm{Ni}$, using $4\ensuremath{\pi}$ detection geometry for these electrons; the sources were mounted in the electron detectors. The energy-dependent ionization probability ${P}_{K}({E}_{\ensuremath{\beta}}^{0})$ was measured as a function of ${E}_{\ensuremath{\beta}}^{0}$, where the parameter ${E}_{\ensuremath{\beta}}^{0}$ is defined as a sum of energies of the $\ensuremath{\beta}$ particle and emitted $K$ electron plus the $K$-shell binding energy of the daughter atom. This energy-dependent and the simultaneously measured total ionization probabilities have been found to be in fairly good agreement with theoretical values calculated by the theory developed based on a relativistic one-step treatment of electron shakeoff. From this result it has been established that electron shakeoff is the predominant mechanism even for electrons ($\ensuremath{\beta}$ particles and $K$ electrons) emitted with very low energies. As to the total ionization probability, it is pointed out that the theoretical values calculated by the simple wave-function-overlap theory using self-consistent-field wave functions should be improved by multiplying by a correction factor, a function of ${B}_{K}$ and ${E}_{0}$, where ${B}_{K}$ is the $K$-shell binding energy of the daughter atom and ${E}_{0}$ is the ordinary maximum kinetic energy of $\ensuremath{\beta}$ rays. The theoretical treatment of the phenomenon and future fruitful experiments to be hoped for are also discussed.