Relevance of ionization and electron refilling to the observation of the<i>M1</i>transition (<i>γM1</i>:<i>2s</i>-<i>1s</i>) in light muonic atoms

The M1 transition (\ensuremath{\gamma}M1:2s-1s) may serve as a probe to observe parity-nonconserving effects in muonic atoms with an atomic number Z\ensuremath{\le}14. These atoms can be useful probes, if they are ionized of all but the two K-shell electrons and electron refilling does not occur. Our experiment shows that muonic neon, argon, and krypton atoms are highly ionized by the time the muon reaches the circular (n=5) state at pressures less than 1000 h Pa. The mean K-shell occupation numbers ${N}_{K}$\ifmmode\bar\else\textasciimacron\fi{} amount to 0.07\ensuremath{\le}${N}_{K}$\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\le}0.68, 0.04\ensuremath{\le}${N}_{K}$\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\le}0.38, and 0.76\ensuremath{\le}${N}_{K}$\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\le}0.84, respectively. For muonic neon at a pressure below 500 h Pa, electron refilling does not take place during the radiative lifetime of the 2s state (200 ps).