Zero Degree Calorimeter for CMS at the LHC

The Zero Degree Calorimeter (ZDC) for the CMS experiment at the Large Hadron Collider is located between the incoming and outgoing beam lines at a distance of 140 m on each side of the interaction point. It measures primarily neutral particles that go in the direction of the original proton or heavy ion beam. With p-p reactions these are mostly neutrons produced by a charge exchange of the original proton. With heavy ion reactions the ZDC measures mostly spectator neutrons. The number of neutrons provides a measure of the amount of overlap of the interacting heavy ions. Over the last year the ZDC has collected data from p-p reactions at center of mass energies of 0.9, 2.36, and 7.0 TeV and Pb-Pb reactions at center of mass energies per nucleon pair of 2.75 TeV.