Determination of the impact vector in intermediate energy heavy ion collisions

We examine a variety of methods for determining both the impact parameter and the direction of the impact vector in symmetric nuclear collisions at intermediate energies. Two quantities, the particle multiplicity and the midrapidity charge, retain their dependence on the impact parameter after filtering through the acceptance of a typical 4π detector. By gating on these quantities we can select four ranges of impact parameters. There is some overlap of these ranges. It is noted that this is dependent on the model used to simulate the collisions. The midrapidity charge has the advantage that it integrates over the final fragments and should be less sensitive to how the collision zone disassembles. The angle of the impact vector is well reproduced with the method developed by Danielewicz and Odyniec. The difference between known and determined reaction plane has a half-width at half-maximum of less than 70°. Some comparisons are made to experimental data.