M1−E2Mixing Ratios and Conversion-Electron Particle Parameters for the Electromagnetic Transitions inAs75

Abstract

Measurements have been made of the $\gamma -\gamma$ and the conversion-electron-$\gamma$ directional correlations of the 121-280-keV and the 136-265-keV cascades in ${\mathrm{As}}^{75}$ following the decay of ${\mathrm{Se}}^{75}$. High-resolution Ge(Li) and Si(Li) detector systems were used exclusively, providing a significant degree of independence of these results from the effects of competing cascades. The Si(Li) electron spectrometer included a cooled field-effect transistor preamplifier to enhance its operation. The data are used to obtain the $\gamma$ multipole mixing ratios $\delta$ and the internal-conversion particle parameters $b_2$ for transitions in ${\mathrm{As}}^{75}$. A precise definition of the mixing ratios in terms of reduced emission matrix elements is given in order to determine their sign unequivocally so that a comparison with theoretical predictions will be unambiguous. The results are $\delta (280-\mathrm{keV} \gamma )=-0.392\mathrm{}\pm 0.011$, $\delta (265-\mathrm{keV} \gamma )=-0.06\mathrm{}\pm 0.02$, $b_2(E1; 121-\mathrm{keV} K{e}^{-})=-1.96\mathrm{}\pm 0.04$, $b_2(E1; 136-\mathrm{keV} K{e}^{-})=-1.87\mathrm{}\pm 0.23$. These values for the mixing ratios are discussed in light of those obtained in other experiments, and a comparison is presented of the results for the particle parameters with the theoretical predictions based on the point- and the finite-size-nucleus pictures.