ANNIHILATION DYNAMICS IN POSITRON SCATTERING: Zeff CALCULATIONS FOR N2 AND C2H2 AND ANNIHILATION PROBABILITY DENSITY (APD) FOR COLLISIONS AGAINST He, H2 AND C2H4 Marcio T. do N. Varella*, Claudia R. C. de Carvalho*, Euclimar P. da Silva**, Jose S. E. Germano*** and Marco A. P. Lima* *Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas, UNICAMP, 13083-970, Campinas, Sao Paulo, Brazil **Instituto de Fisica, Universidade Federal do Ceara, 60455--760, Fortaleza, Ceara, Brazil ***Departamento de Fisica do Instituto Tecnologico de Aeronautica - Centro Tecnico de Aeronautica, 12228-900, Sao Jose dos Campos, Sao Paulo, Brazil In order to better understand the annihilation dynamics in e+ -target collisions, we have calculated annahilation probability densities (APD) for positron scattering by He, H2 and C2H4. The APD points out where annihilation is more likely to take place. We have also obtained elastic cross sections and the annihilation parameter Zeff for positron collisions against two isoelectronic targets, N2 and C2H2. All calculations were performed with the Schwinger Multichannel Method (SMC).