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 endash 3 μs. 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 (open-quote open-quote quenching close-quote close-quote) cross section σq was extracted. No evidence for a delayed annihilation of antiprotons in pure Ne, Kr, and Xe was observed. copyright 1996 The American Physical Society