  The sizes, temperatures and free neutron to proton ratios of the initial
interaction zones produced in the collisions of 40 MeV/nucleon $^{40}$Ar +
$^{112}$Sn and 55 MeV/nucleon$^{27}$Al + $^{124}$Sn are derived using total
detected neutron plus charged particle multiplicity as a measure of the impact
parameter range and number of participant nucleons. The size of the initial
interaction zone, determined from a coalescence model analysis, increases
significantly with decreasing impact parameter. The temperatures and free
neutron to proton ratios in the interaction zones are relatively similar for
different impact parameter ranges and evolve in a similar fashion.
