ENERGY-LOSS OF HYDROGEN PROJECTILES IN GASES

The stopping cross sections of H-2, D2, He, and Ne for hydrogen projec tiles in the energy range 3-20 keV per nucleon have been measured by t ime of flight. We compare our experimental result to the sum of the in dividual 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 fo r the He target and only at moderate projectile velocities, whereas fo r H-2 and D2 the calculated values are about 30% too low. A Monte Carl o program allows us to simulate the measured time-of-flight spectra an d to explain minor trends in the experimental data: for increased Ne g as pressure, an increased specific energy loss has been found that can be traced to different regions of impact parameters selected in our t ransmission geometry. This also explains, in part, the increased speci fic energy loss for deuterons compared to protons of equal velocity th at is most evident for Ne. In contrast, a decrease of the specific ene rgy loss with increasing pressure for He may be explained by impuritie s in the target gas. If we correct for the effect of impurities, the s topping cross section of He at 4 keV per nucleon is slightly smaller ( 0.60 X 10(-15) eV cm2) than published earlier (0.72 X 10(-15) eV cm2) and depends on the 3.8th power of projectile velocity.