DELAYED ANNIHILATION OF ANTIPROTONS IN HELIUM GAS

The delayed annihilation of antiprotons, which was recently discovered in liquid He-4 at KEK, has been studied at CERN in gas-phase He-4 and He-3. The annihilation time spectra in gas He-4 at various pressures were found to be similar to that for liquid He-4. The observed average lifetime in the region t > 1 musec for 3 atm He-4 was about 3.2 musec, while for 3 atm He-3 gas it was 2.8 musec, i.e., shorter by 15%. The time spectra show a growth-decay-type function, which is indicative of the presence of a series of metastable states. For He-4 and He-3 they have nearly identical shapes, differing only in the time scale by (14 +/- 3)%. These observations are qualitatively consistent with the atomic model of pe-He-2+BAR proposed by Condo [Phys. Lett. 9, 65 (1964)]. The time spectra were found to be sensitive to the presence of small amounts (as small as 20 ppm) of H-2. No evidence was seen for delayed annihilation in gaseous Ne.