Relevance of ionization and electron refilling to the observation of theM1transition (γM1:2s-1s) in light muonic atoms

The M1 transition (γM1:2s-1s) may serve as a probe to observe parity-nonconserving effects in muonic atoms with an atomic number Z≤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 $N_K$¯ amount to 0.07≤$N_K$¯≤0.68, 0.04≤$N_K$¯≤0.38, and 0.76≤$N_K$¯≤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).