STOPPING MECHANISMS OF NEGATIVE HEAVY-PARTICLES IN GAS TARGETS

The mechanisms responsible for the stopping of negatively charged, hea vy particles in gas targets are discussed. Experimentally determined s topping powers of antiprotons and negative muons in He and H-2 targets are compared to the results of various theoretical calculations for t he electronic and nuclear contributions to the stopping power. Target excitation and ionization via direct (first-order) electronic transiti ons are found to dominate the stopping in the range of high projectile velocities. Higher-order effects are revealed for intermediate veloci ties, while near-adiabatic dynamics is shown to prevail in the electro nic stopping power at low velocities. The threshold behavior of the el ectronic stopping power in heavy rare-gas targets is discussed as a si gnature of the near-adiabatic stopping mechanism.