Delayed annihilation of antiprotons in helium gas

The delayed annihilation of antiprotons, which was recently discovered in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at KEK, has been studied at CERN in gas-phase ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. The annihilation time spectra in gaseous ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at various pressures were found to be similar to that for liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The observed average lifetime in the region t>1 μsec for 3 atm ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ was about 3.2 μsec, while for 3 atm ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ 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 ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ 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¯${\mathit{e}}^{\mathrm{-}}$ ${\mathrm{He}}^{2+}$ 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 ${\mathrm{H}}_2$. No evidence was seen for delayed annihilation in gaseous Ne.