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

Delayed annihilation time spectra of antiprotons (DATS) 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 (N_2,O_2,H_2) at the low energy antiproton ring (LEAR) at CERN. The DATS were a superposition of two exponential components, one with lifetime of several 100 ns and the other with lifetime 1 to 3 \mus. They showed a shorter average lifetime (T_{av}) than DATS of pure helium. Ne, Ar, and Kr were found to affect T_{av} only slightly even in concentrations up to 20\%, while Xe showed a much stronger influence. In the case of molecular gases the presence of N_2 influenced the DATS much less than O_2 and H_2 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 \sigma_q was extracted. No evidence for a delayed annihilation of antiprotons in pure Ne, Kr, and Xe was observed.