Studies on the atomic capture of stopped negative pions in binary mixtures of 3He with other gases

Abstract Systematic experimental study has been carried out on the atomic capture of negative pions by 3 He in binary gas mixtures of 3 He + Z , where Z is Ne, Ar, Kr, Xe, N 2 , O 2 , CO 2 and sf 6 . The results are analysed in the framework of a phenomenological model. It is shown that there is no pion transfer from the 3 H eπ − 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 concentrations, thereby excluding the possibility of a concentration dependence in the atomic capture ratio A ( Z / 3 He). 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 3 He + 4 He mixture is A( 4 He / 3 He ) = 0.75 ± 0.13 , which might be the indication of an isotopic effect. The branching ratio for the charge-exchange reaction at rest (π − + 3 He → π 0 + 3 H ) is found to be 0.128 ± 0.012.