Target-density effects in muonic-atom cascades

Muonic x-ray spectra have been measured for ${\mathrm{N}}_2$, S${\mathrm{F}}_6$, and Ar at pressures up to 20 atm at room temperature and in the liquid or solid state, and for ${\mathrm{O}}_2$ at 20 atm and CO, NO, and a ${\mathrm{N}}_2$+${\mathrm{O}}_2$ 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 S${\mathrm{F}}_6$, 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 ${\mathrm{N}}_2$ or S${\mathrm{F}}_6$ 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.