The delayed annihilation of antiprotons, which was recently discovered in liquid He4 at KEK, has been studied at CERN in gas-phase He4 and He3. The annihilation time spectra in gaseous He4 at various pressures were found to be similar to that for liquid He4. The observed average lifetime in the region t>1 μsec for 3 atm He4 was about 3.2 μsec, while for 3 atm He3 gas it was 2.8 μsec, 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 He4 and He3 they have nearly identical shapes, differing only in the time scale by (14 ±3)%. These observations are qualitatively consistent with the atomic model of p̄e-He2+ 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 H2. No evidence was seen for delayed annihilation in gaseous Ne.
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