Nuclear Physics A403 (1983) 515-524 @ Noah-Holland PubIishing Company STUDIES ON THE ATOMIC CAPTURE OF STOPPED NEGATIVE PIONS IN BINARY MIXTURES OF ‘He WITH OTHER GASES A.V. BANNIKOV, B. LEVAY’, V.I. PETRUKHIN and V.A. VASILYEV Joint Institute for Nuclear Research, Dubna, USSR L.M. KOCHENDA, A.A. MARKOV, V.I. MEDVEDEV, G.L. SOKOLOV and 1.1. STRAKOVSKY Leningrad Nuclear Physics Institute, Gatchina, USSR and D. HORVATH”” Central Research Institute for Physics, Budapest, Hungary (Received 6 January 1983 Abstract: Systematic experimental study has been carried out on the atomic capture of negative pions by 3He in binary gas mixtures of 3He +Z, where Z is Ne, Ar, Kr, Xe, Nz, 02, CO2 and SFe. The results are analysed in the framework of a phenomenological model. It is shown that there is no pion transfer from the ‘Hen- mesic atoms to the heavier Z-atoms. The probabilities of pion capture in the various atoms of the mixtures are found to be proportional to the atomic concentra- tions, thereby excluding the possibility of a con~ntration dependence in the atomic capture ratio A(Z13He). In contradiction to previous assumptions the probability of pion capture into an atomic orbit is not proportional to the stopping power of the components of the mixture. The atomic capture ratio of pions in a 3He+4He mixture is A(4He/3He) =0.75*0.13, which might be the indication of an isotopic effect. The branching ratio for the charge-exchange reaction at rest, (rrm+3He+ao+3H) is found to be 0.128+0.012. NUCLEAR REACTIONS ‘He(*-, a’), E =O; measured y-r-coin; deduced reaction E branching ratio, atomiccapture ratios for’He, Ne, Ar, Kr, Xe, N1, Oz. Co,, SF6, dependence on atomic number, concentration, relative stopping powers. Gas targets, Cerenkov detectors. Phenomenological analysis. 1. Introduction A knowledge of the processes that take place when negative particles are captured by atoms has great value for the physics of exotic atoms and for the study of elementary processes occurring when primary particles are stopped in matter. In these studies one has to separate the process of atomic capture from that of the transfer of captured mesons. Until now all investigations on negative pions, except in ref. ‘), have been carried out in hydrogen or hydrogen-containing materials 233) where the unambiguous separation of atomic capture and transfer of pions is difficult. * Present address: Department of Physical Chemistry and Radiology, Eijtvijs University of Budapest, H-1088, Budapest, Hungary. ** Address 1982-83: TRIUMF, University of British Columbia, Vancouver, BC, Canada V6T 2A3. 515