M -Subshell Fluorescence Yields and the L 1 − L 3 Radiative Transition at Z = 93 and 96 from Am 241 and 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 ($Z=96$) following $\ensuremath{\alpha}$ 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: ${\ensuremath{\nu}}_{1}^{M}=0.065\ifmmode\pm\else\textpm\fi{}0.014$, ${\ensuremath{\nu}}_{2}^{M}=0.080\ifmmode\pm\else\textpm\fi{}0.029$, ${\ensuremath{\nu}}_{4}^{M}=0.062\ifmmode\pm\else\textpm\fi{}0.005$, ${\ensuremath{\nu}}_{4,5}^{M}=0.065\ifmmode\pm\else\textpm\fi{}0.012\ensuremath{\approx}{\ensuremath{\omega}}_{5}^{M}$ at $Z=93$; and ${\ensuremath{\nu}}_{1}^{M}=0.081\ifmmode\pm\else\textpm\fi{}0.016$, ${\ensuremath{\nu}}_{2}^{M}=0.068\ifmmode\pm\else\textpm\fi{}0.023$, ${\ensuremath{\nu}}_{3}^{M}=0.062\ifmmode\pm\else\textpm\fi{}0.019$, ${\ensuremath{\nu}}_{4}^{M}=0.080\ifmmode\pm\else\textpm\fi{}0.006$, and ${\ensuremath{\nu}}_{4,5}^{M}=0.075\ifmmode\pm\else\textpm\fi{}0.012\ensuremath{\approx}{\ensuremath{\omega}}_{5}^{M}$ at $Z=96$. 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 $Z=76 \mathrm{to} 96$. 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 different values of $Z$ indicates that the ratio of radiative to Auger widths decreases from the $1{s}_{\frac{1}{2}}$ to $2{p}_{\frac{3}{2}}$ state, but remains constant from $2{p}_{\frac{3}{2}}$ to $3{d}_{\frac{5}{2}}$. The resolution of the Si(Li) detector was sufficient to separate the radiative transitions from fillings of the ${M}_{1,2}$- and ${M}_{3,4,5}$-subshell groups, so that from $K$ and $L$ x-ray coincidences with the ${M}_{1,2}$ group of x rays, it was possible to determine the following quantities: ${\ensuremath{\omega}}_{1}^{M}+{f}_{12}^{M}{\ensuremath{\omega}}_{2}^{M}=({2.0}_{\ensuremath{-}2.0}^{+3.1})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} \mathrm{and} ({7.5}_{\ensuremath{-}7.5}^{+8.9})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at $Z=93 \mathrm{and} 96$, respectively, and ${\ensuremath{\omega}}_{2}^{M}=({4.6}_{\ensuremath{-}4.6}^{+5.1})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at $Z=96$. Thus, about 94% of ${M}_{2}$-subshell vacancies undergo Coster-Kronig shifts to higher subshells before filling from $N$ and higher major shells occurs. Observation of the radiative ${L}_{1}\ensuremath{-}{L}_{3}$ transition was made for the first time in the high-$Z$ region, and its intensity is in reasonable agreement with extrapolated theoretical predictions (6-7% of x rays filling ${L}_{1}$-subshell vacancies). The radiative fraction ${\ensuremath{\omega}}_{13}^{L}$ in the Coster-Kronig yield ${f}_{13}^{L}$ is found to be about 2% in the region of $Z=93\ensuremath{-}96$, and ${\ensuremath{\omega}}_{13}^{L}=0.011\ifmmode\pm\else\textpm\fi{}0.005 \mathrm{and} 0.009\ifmmode\pm\else\textpm\fi{}0.005$ at $Z=93 \mathrm{and} 96$, respectively. The absolute emission rate of $M$ x rays from ${\mathrm{Am}}^{241}$ was measured with multiwire and single-wire proportional counters, and values are found of (6.35 \ifmmode\pm\else\textpm\fi{} 0.60) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$ $M$ x rays per $\ensuremath{\alpha}$ decay and 0.470 \ifmmode\pm\else\textpm\fi{} 0.045 $M$ x rays per ${L}_{\ensuremath{\alpha}}$ x ray. These intensities were used in determinations of efficiency at 3.25 keV of the semiconductor x-ray detectors.