Delayed annihilation of antiprotons in helium gas

The delayed annihilation of antiprotons, which was recently discovered in liquid ⁴He at KEK, has been studied at CERN in gas-phase ⁴He and ³He. The annihilation time spectra in gaseous ⁴He at various pressures were found to be similar to that for liquid ⁴He. The observed average lifetime in the region t>1 {μ}sec for 3 atm ⁴He was about 3.2 {μ}sec, while for 3 atm ³He gas it was 2.8 {μ}sec, 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 ⁴He and ³He they have nearly identical shapes, differing only in the time scale by (14 ±{}3)%. These observations are qualitatively consistent with the atomic model of p{}e^-{He}²⁺$ 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.