$M$-Subshell Fluorescence Yields and the $L_1-L_3$ Radiative Transition at $Z=93\mathrm{} \mathrm{and} 96$ from ${\mathrm{Am}}^{241}$ and ${\mathrm{Cf}}^{249}$ Decays

High-resolution Ge(Li) and Si(Li) x-ray spectrometers and a wall-less anticoincidence multiwire proportional counter were used for coincidence measurements between various $K$ and $L$ x rays and $M$ x rays of Np (Z=93) and Cm ($Z=96\mathrm{}$) following $\alpha$ decay of ${\mathrm{Am}}^{241}$ and ${\mathrm{Cf}}^{249}$, respectively. The effect of multiple vacancies in the $M$ shell was taken into account in the determination of the following mean $M$-subshell fluorescence yields: ${\nu }_1^M=0.065\mathrm{}\pm 0.014$, ${\nu }_2^M=0.080\mathrm{}\pm 0.029$, ${\nu }_4^M=0.062\mathrm{}\pm 0.005$, ${\nu }_{4,5}^M=0.065\mathrm{}\pm 0.012\approx {\omega }_5^M$ at $Z=93\mathrm{}$; and ${\nu }_1^M=0.081\mathrm{}\pm 0.016$, ${\nu }_2^M=0.068\mathrm{}\pm 0.023$, ${\nu }_3^M=0.062\mathrm{}\pm 0.019$, ${\nu }_4^M=0.080\mathrm{}\pm 0.006$, and ${\nu }_{4,5}^M=0.075\mathrm{}\pm 0.012\approx {\omega }_5^M$ at $Z=96\mathrm{}$. The close agreement of the various mean $M$-subshell fluorescence yields with that of the $M_{4,5}$ subshells for each element indicates that Coster-Kronig transitions are very strong in the $M$ shell. Including earlier measurements of mean $M$-shell fluorescence yields at lower $Z$, corrected for multiple vacancies, and accepting Bhalla's theoretical calculations of radiative $M_{4,5}$-subshell widths, one concludes that the nonradiative widths of the $M_{4,5}$ subshells are essentially constant from $Z=76\mathrm{} \mathrm{to} 96$. Comparison of fluorescence yields of the $K$ shell and the $L_3$ and $M_{4,5}$ subshells at the same energy of the principal radiative transition (i. e., at...