  High Energy collisions of nuclei usually lead to the emission of evaporation
neutrons from both ``beam'' and ``target'' nuclei. At the RHIC heavy ion
collider with 100GeV/u beam energy, evaporation neutrons diverge by less than
$~2$ milliradians from the beam axis Neutral beam fragments can be detected
downstream of RHIC ion collisions (and a large aperture Accelerator dipole
magnet) if $\theta\leq$ 4 mr but charged fragments in the same angular range
are usually too close to the beam trajectory.
  In this 'zero degree' region produced particles and other secondaries deposit
negligible energy when compared with that of beam fragmentation neutrons.
  The purpose of the RHIC zero degree calorimeters (ZDC's) is to detect
neutrons emitted within this cone along both beam directions and measure their
total energy (from which we calculate multiplicity). The ZDC coincidence of the
2 beam directions is a minimal bias selection of heavy ion collisions. This
makes it useful as an event trigger and a luminosity monitor\cite{baltz} and
for this reason we built identical detectors for all 4 RHIC experiments.
  The neutron multiplicity is also known to be correlated with event geometry
\cite{appel} and will be used to measure collision centrality in mutual beam
int eractions.
