Phase and density dependence of the delayed annihilation of metastable antiprotonic helium atoms in gas, liquid, and solid helium.

Systematic studies of the phase and density dependence of the delayed annihilation time spectra of antiprotons (DATS) in gaseous, liquid and solid helium have been performed using the Low Energy Antiproton Ring (LEAR) at CERN. The results show a small but signiicant dependence of DATS on the phase and density of the helium medium. The average lifetime of antiprotons (T av) in gas at 10-30 K was found to be 4.03 0.02 s. In liquid helium T av decreases with density from 3.0 s at 1 bar to 2.5 s at 60 bar, while in solid helium its value is 2.14 0.03 s, 20 % shorter than that in liquid helium of the same density. There is no change of DATS between normal 1-bar liquid and superruid helium. An isotope eeect between 4 He and 3 He similar to the one previously observed in the gas phase has also been found for liquid helium. All the data can be tted fairly well with a simple three-level chain decay model which describes the general structure of DATS, characterized by downward curving behavior on a logarithmic scale at later times and by the presence of a short-lived component. No delayed annihilation was observed in lithium.