Muonium formation and the “missing fraction” in vapors

Abstract

The vapor phase fractional polarizations of positive muons thermalizing as the muonium atom (PM) and in diamagnetic environments (PD) has been measured in H2O, CH3OH, C6H14, C6H12, CCl4, CHCl3, CH2Cl2 and TMS, in order to compare with the corresponding fractions measured in the condensed phases. There is a marked contrast in every case, with the vapor phase results being largely understandable in terms of a charge exchange/hot atom model. Unlike the situation in the corresponding liquids, there is no permanent lost fraction in the vapor phase in the limit of even moderately high pressures (≈1 atm); at lower pressures, depolarization is due to hyperfine mixing and is believed to be well understood. For vapor phase CH3OH, C6H14, C6H12, and TMS therelative fractions are found to be pressure dependent, suggesting the importance of termolecular hot atom (or ion) reactions in the slowing down process. For vapor phase H2O and the chloromethanes, the relative fractions are pressure independent. For CCl4,PM=PD≈0.5 in the vapor phase vs.PD=1.0 in the liquid phase; fast thermal reactions of Mu likely contribute significantly to this difference in the liquid phase. For H2O,PM ≈ 0.9 andPD≈0.1 in the vapor phase vs.PD≈ 0.6 andPM≈0.2 in the liquid phase. Water appears to be the one unequivocal case where the basic charge exchange/hot atom model is inappropriate in the condensed phase, suggesting, therefore, that radiation induced “spur” effects play a major role.