Diffusion of muonic deuterium and hydrogen atoms

Diffusion of muonic deuterium μd and muonic hydrogen μp atoms produced following the stopping of negative muons in ${\mathrm{D}}_2$ or ${\mathrm{H}}_2$ at 300 K was studied at pressures of 47–750 mbar (${\mathrm{H}}_2$) and 94–1520 mbar (${\mathrm{D}}_2$) in two distinct target geometries. Time intervals were recorded between entry of negative muons into the gas and arrival of each resulting μd or μp atom at one of 50 foils immersed in the gas, and spaced regularly along the muon beam axis. The results of such measurements were fitted to time distributions generated by Monte Carlo methods, using theoretical scattering predictions and empirically chosen forms for the initial energy distributions of the muonic atoms in the 1S state. Results indicate muonic atom energy distributions which (a) are different for μd and μp and (b) vary with pressure. The best-fit energy distributions have mean energies ranging from 1.5 eV for μd at 94 mbar to ⩾9 eV for μp at 750 mbar. The data are also sensitive to scattering cross sections for μd and μp, and are consistent with current theoretical calculations for the μd+${\mathrm{D}}_2$ cross sections. In the case of μp+${\mathrm{H}}_2$ scattering, the experimental data suggest discrepancies with the theoretical predictions.