Energy loss of hydrogen projectiles in gases

The stopping cross sections of ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, He, and Ne for hydrogen projectiles in the energy range 3–20 keV per nucleon have been measured by time of flight. We compare our experimental result to the sum of the individual contributions due to excitation and ionization of the target and of the projectile, respectively, and due to charge exchange, using published cross-section data. Satisfactory agreement is found only for the He target and only at moderate projectile velocities, whereas for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ the calculated values are about 30% too low. A Monte Carlo program allows us to simulate the measured time-of-flight spectra and to explain minor trends in the experimental data: for increased Ne gas pressure, an increased specific energy loss has been found that can be traced to different regions of impact parameters selected in our transmission geometry. This also explains, in part, the increased specific energy loss for deuterons compared to protons of equal velocity that is most evident for Ne. In contrast, a decrease of the specific energy loss with increasing pressure for He may be explained by impurities in the target gas. If we correct for the effect of impurities, the stopping cross section of He at 4 keV per nucleon is slightly smaller (0.60×${10}^{\mathrm{-}15}$ eV ${\mathrm{cm}}^2$) than published earlier (0.72×${10}^{\mathrm{-}15}$ eV ${\mathrm{cm}}^2$) and depends on the 3.8th power of projectile velocity.