Projectile rapidity pions in 775 MeV/nucleon [sup 139]La+[sup 12]C and [sup 139]La+[sup 139]La reactions

Negative pion spectra emitted in the reactions of 775 MeV/nucleon [sup 139]La+[sup 12]C and [sup 139]La+[sup 139]La reactions have been measured in coincidence with the projectile fragments using the HISS spectrometer at the Bevalac. Prominent peaks near the beam velocity were observed in the pion spectra. Position and widths of the peaks were studied as a function of the sum charge'' of projectile fragments which is a good measure of impact parameter; the smaller the sum charge,'' the smaller the impact parameter. The peak position down shifts with the smaller sum charge.'' The pion peak is wider in the transverse than in the longitudinal direction, possibly mirroring the velocity dispersions of projectile fragments in the early stage of reactions.