Antiproton Stopping Power In Hydrogen Below 120 Kev and the Barkas Effect

The simultaneous measurement of the spatial coordinates and times of ps annihilating at rest in a H 2 target at very low density ρ (ρ/ρ 0 <10 -2 , ρ 0 being the STP density) gives the possibility of evaluating the behavior of the p stopping power in H 2 at low energies (below 120 keV). It is different from that of protons (the Barkas effect). Moreover, it is shown that a rise at low-energy values (≤1 keV) is needed to agree with experimental data