  Experiment 910 has measured slow protons and deuterons from collisions of 18
GeV/c protons with Be, Cu, and Au targets at the BNL AGS. These correspond to
the ``grey tracks'' first observed in emulsion experiments. We report on their
momentum and angular distributions and investigate their use in measuring the
centrality of a collision, as defined by the mean number of projectile-nucleon
interactions. The relation between the measured Ngrey and the mean number of
interactions, nu(Ngrey), is studied using several simple models, one newly
proposed, as well as the RQMD event generator. RQMD is shown to reproduce the
Ngrey distribution, and exhibits a dependence of Ngrey on centrality that is
similar to the behavior of the simple models. We find a strong linear
dependence of average Ngrey on nu, with a constant of proportionality that
varies with target. For the Au target, we report a relative systematic error
for extracting nu(Ngrey) that lies between 10% and 20% over all Ngrey.
