The RHIC Zero Degree Calorimeters

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 θ ≤ 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.