Effects of impurity atoms and molecules on the lifetime of antiprotonic helium atoms

Delayed annihilation time spectra (DATS) of antiprotons in room-temperature helium gas have been studied as a function of the concentration of admixed noble gases (Ne,Ar,Kr,Xe) as well as molecular gases (N₂,O₂,H₂) at the low-energy antiproton ring at CERN. The DATS were a superposition of two exponential components, one with a lifetime of several 100 ns and the other with a lifetime 1--3 {μ}s. They showed a shorter average lifetime (Tₐᵥ) than DATS of pure helium. Ne, Ar, and Kr were found to affect Tₐᵥ only slightly even in concentrations up to 20%, while Xe showed a much stronger influence. In the case of molecular gases, the presence of N₂ influenced the DATS much less than O₂ and H₂, which destroyed the metastability almost completely in concentrations of 100 ppm and less. The decay rate of the slow component of DATS was found to exhibit a linear relation to the number density of the admixture. From this a collisional destruction (``quenching'') cross section σq was extracted. No evidence for a delayed annihilation of antiprotons in pure Ne, Kr, and Xe was observed. {} 1996 The American Physical Society.