RECENT RESULTS FROM OBELIX IN ATOMIC PHYSICS

The experimental behaviors of the stopping power for ($) over bar p in H-2 and in He in the energy range similar to 0.5 keV - 3.3 MeV obtain ed by the OBELIX spectrometer are reported. The differences from the p stopping power are evident (the Barkas effect), while the comparison with the IT stopping power indicates similar behaviors. Furthermore, t he measurements of the effect that the contaminants (H-2, N-2, Ne, Ar, and Xe) have on the time distribution of delayed annihilations of ($) over bar ps stopped in He gas at 3 atm and at room temperature are pr esented. As the pressure value of the contaminants increases, the perc entage of the delayed annihilations remains constant (3.0 +/- 0.3%), w hile the average time decreases (the effect is stronger for H-2 and Xe ). With the exception of Ne, a fast delayed annihilation component is also observed.