Measurement of emitted neutrons and protons from 112,124Sn + 112,124Sn at Ebeam = 50 MeV/nucleon and 120 MeV/nucleon*

The nuclear symmetry energy affects many aspects of nuclear structure, nuclear astrophysics, and nuclear reactions. The spectral ratio of neutrons to protons from central heavy ion collisions is sensitive to the symmetry energy below saturation density, but previous measurements of the ratio have large uncertainties. In addition, transport model calculations of the ratio using the IBUU04 and ImQMD05 codes differ greatly, perhaps due to the effective mass splitting in the nuclear medium. A recent experiment at NSCL/MSU measured neutrons and protons emitted from central collisions of 112,124Sn + 112,124Sn at Ebeam = 50 MeV/nucleon to probe the symmetry energy, and at Ebeam = 120 MeV/nucleon to probe the mass splitting. First results will be presented and compared to transport model calculations.