Performance of a forward neutron calorimeter for the ALICE experiment

The aim of the ALICE experiment is the study of ultrarelativistic heavy ion collisions in which nuclear matter at high temperatures and energy densities can be produced. The spectator protons and neutrons will be separated from the ion beams, using the separator magnet (D1) of the LHC beam optics and respectively detected by a proton (ZP) and a neutron (ZN) “Zero-degree Calorimeter” (ZDC). An electromagnetic calorimeter is also foreseen for the measurement of the forward photons coming from the decay of π⁰, which is anti-correlated with the impact parameter of the collision. It could be used to make the centrality trigger more selective, and less sensitive to possible effects related to the fragmentation of the nuclei. The detectors will be placed at ~115 meters away from the beam intersection point. The calorimeters will be placed in a radiation rich environment, of the order of 10⁴ Gy/day at a luminosity of 2×10²⁷ cm^-2s^-1. The high interaction rate, corresponding to ~8000 minimum bias collisions per second, will require a fast detection method. Moreover, since geometrical constraints will limit the lateral dimensions, the transverse size of the detectable shower should be kept as small as possible. The ZDCs are quartz-fiber calorimeters that exploit the Cherenkov light produced by the shower particles in silica optical fibers. This technique fulfils the above requirements. In addition, quartz-fiber calorimeters are intrinsically insensitive to radioactivation background, which produces particles below the Cherenkov threshold