Interactions of antiprotons with atoms and molecules

Abstract

Antiproton beams of relatively low energies (below hundreds of MeV) have recently become available. The present article discusses the significance of those beams in the contexts of radiation physics and of atomic and molecular physics. Studies on individual collisions of antiprotons with atoms and molecules are valuable for a better understanding of collisions of protons or electrons, a subject with many applications. An antiproton is unique as a stable, negative heavy particle without electronic structure, and it provides an excellent opportunity to study atomic collision theory. Comparison of the stopping powers of a material for an antiproton and a proton at the same speed will be the most clearcut approach to the Barkas effect. The moderation of an antiproton in matter is roughly similar to that of negative pions and muons, but some differences remain to be elucidated. Full discussion of the interactions of a low-energy antiproton with an atom or molecule must consider the adiabatic potential determined by the electronic motion in the field of the antiproton and nucleus at rest. When an antiproton approaches an atom or molecule sufficiently closely, then some of the atomic or molecular electrons no longer remain bound and ooze out with extremely low kinetic energies; thus, the atom or molecule becomes ionized, often multiply ionized. This mechanism of ionization is absent for a proton. Upon complete moderation, an antiproton is eventually captured by the Coulomb field of a nucleus and thus an antiprotonic atom is formed. The present article also touches upon some problems related to condensed matter. For instance, the possibility or chanelling of antiproton beams in a crystal is considered.