Coulomb effects on pions produced in heavy ion reactions

Double differential cross sections for the production of π+ and π- near the velocity of the incident beam for pion lab angles less than 40 degrees are presented. The experimental apparatus and the techniques are discussed. Beams of 20Ne with E/A from 80 to 655 MeV and 40Ar with E/A = 535 MeV incident on Be, C, NaF, KCl, Cu, and U targets were used. A sharp peak in the π- spectrum and a depression in the π+ spectrum were observed at zero degrees near the incident beam velocity. The effect is explained in terms of Coulomb interactions between the pions and fragments of the incident beam. Least squares fits to the data using the Coulomb correction formulas of Gyulassy and Kauffmann and an effective projectile fragment charge are made. The relationship between these data and previously measured pion production and projectile fragmentation data is discussed. The data are also compared to some theoretical models. A simple expression is given for the differential cross section as a function of the projectile mass, target mass, and beam energy