COMPREHENSIVE ANALYSIS OF THE STOPPING POWER OF ANTIPROTONS AND NEGATIVE MUONS IN HE AND H-2 GAS TARGETS

A comprehensive analysis of the stopping power of antiprotons and nega tive muons in He and H-2 gas targets for projectile velocities (equiva lent antiproton energies) ranging from about 0.1 to 10 au (0.25 keV to 2.5 MeV) is performed, Recent experimental data are contrasted with t heoretical results obtained from different approaches. The electronic stopping power is evaluated within the coupled-state atomic-orbital me thod and the distorted-wave Born approximation as well as, for low pro jectile velocities, within a generalized adiabatic-ionization model th at takes into account collisional-broadening effects. The departure of the antiproton stopping power from the proton stopping power ('Barkas effect'), observed for intermediate projectile velocities, is discuss ed. The contribution to the stopping power arising from energy transfe r to the translational degrees of freedom of the target system ('nucle ar stopping') is evaluated. Our analysis results in a good understandi ng of the stopping mechanisms of negative heavy particles in gases, in particular in He. Discrepancies between theory and experiment in the H-2 case are attributed to effects of the molecular structure of the t arget.