Dissipative ^209Bi+^136Xe Collisions at E/A=55 MeV

Projectile--like fragments from the ^209Bi+^136Xe reaction at E/A=55 MeV have been measured in coincidence with neutrons, as well as light-- and intermediate--mass charged products, using a highly efficient detector setup including two 4π devices --- the Rochester SuperBall neutron detector and the combination of the St. Louis MiniWall and MSU MiniBall with a Si--CsI forward array. The observed joint distribution of neutron and charged--particle multiplicities and the emission patterns of charged particles and projectile--like fragments are indicative of dissipative collisions. In addition to two excited heavy fragments, a neck--like reaction zone is formed. Up to m_IMF≈ 5, the intermediate mass fragment multiplicity m_IMF is correlated with thermal excitation. However, ``thermal observables'' reach a plateau and no longer change with m_IMF, for m_IMF>= 5. This work was supported by the U. S. Dept. of Energy under Grant Nos. DE--FG02--88ER40414 and DE--FG02--87ER40316.