MASS DEPENDENCE OF PION-PRODUCTION IN HEAVY-ION COLLISIONS NEAR, BUT BELOW THRESHOLD

We have measured the inclusive cross section for production of negativ e pions near mid-rapidity in Ne-20 + NaF, La-139 + La-139 and Au-197 Au-197 collisions at E = 183 and 236 MeV/u. Au + Au is the heaviest s ystem from which subthreshold pion production has been measured to dat e. The dependence of the pion cross section on pion energy, beam energ y and associated charged particle multiplicity is consistent with prev ious results both above and below threshold. The dependence of the cro ss section on the mass of the colliding system varies only slightly as the beam energy is reduced below threshold, in contrast to some previ ous measurements. Comparison with theory suggests that at these energi es the pion production process is still dominated by nucleon-nucleon c ollisions.