  First results are presented from BNL experiment E910 on pion production and
stopping in proton-Be, Cu, and Au collisions at a beam momentum of 18 GeV/c. We
characterize the centrality of the collisions using the measured number of
"grey" tracks, Ngrey,and a derived quantity, nu, the number of inelastic
nucleon-nucleon scatterings suffered by the projectile during the collision. We
find that for the three targets the average backward rapidity shift of the
leading proton follows a common trend versus nu with the projectile losing, on
average, 2 units of rapidity in the first 2-3 scatterings. The average rapidity
shift increases more slowly with subsequent scatterings reaching a maximum of
2.5 units. The pi- multiplicity measured within the E910 acceptance saturates
with increasing nu in p-Au collisions while the pi- multiplicity in p-Be
collisions increases faster with nu than expected from the wounded-nucleon
model. Comparisons of our data with the RQMD cascade model suggest that in very
central p-Au collisions most of the pions are produced near zero rapidity in
the lab.
