Abstract
π+π+ correlations from S + Pb collisions at 200 GeV/c per nucleon, measured by the focusing spectrometer of the NA44 experiment at CERN, are presented. The large data set, particularly at small values of Qinv, allows determination of the detailed shape of the correlation function. We construct the background by event mixing, and correct for resolution, Coulomb interaction, and distortion of single-particle spectra which produce effects large compared to the statistical errors at small Qinv. An exponential, rather than gaussian, source distribution is slightly favoured, but both are statistically acceptable.