Target-density effects in muonic-atom cascades

Muonic x-ray spectra have been measured for N2, SF6, and Ar at pressures up to 20 atm at room temperature and in the liquid or solid state, and for O2 at 20 atm and CO, NO, and a N2+O2 mixture at 10 atm. Relative intensities of the higher members of the muonic Lyman series of N decrease monotonically with decreasing pressure (density). A similar effect is observed for F in SF6, where some effect was also noted for the S x rays. A gas-pressure dependence in Ar was not observed in the pressure range covered. The density effect can be modeled approximately by a muonic-atom cascade code in which the K-electron refilling width is related to the gas density, which determines the frequency of collision of the atom with neighboring atoms or molecules. The difference in behavior between N2 or SF6 and Ar at comparable pressures provides evidence for the ''Coulomb explosion'' that is expected to occur when a negative muon is captured by a polyatomic molecule