Performance of a forward neutron calorimeter for the ALICE experiment

R. Arnaldi, E. Chiavassa, C. Cicalo, P. Cortese, A. De Falco, G. Dellacasa, N. De Marco, A. Ferretti, M. Gallio, P. Macciotta, A. Masoni, P. Mereu (+10 others)
2000 IEEE Transactions on Nuclear Science  
The aim of the ALICE experiment [l] is the study of ultrarelativistic heavy ion collisions in which nuclear matter at high temperatures and energy densities can be produced. In these conditions the formation of the state of matter where quarks and gluons are deconfined, i.e. the Quark Gluon Plasma (QGP), could be possible. ALICE is planned to start taking data in 2005, at the LHC facility at CERN, with two colliding lead ion beams at a c.m. energy of fi = 5.5 TeV per nucleon. The centrality of
more » ... he collision is defined by the impact parameter, which can be estimated by measuring the energy carried away by the non-interacting nucleons. The spectator protons and neutrons will be separated from the ion beams, using the separator magnet (Dl) of the LHC beam optics [2] 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 T O , 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 N 115 meters away from the beam intersection point. The calorimeters will be placed in a radiation rich environment, of the order of lo4 Gylday at a luminosity of 2 . 1027~m-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 [3] . This technique fulfills the above requirements. In addition, quartz-fiber calorimeters are intrinsically insensitive to radio-activation background, which produces particles below the Cherenkov threshold. The ALICE ZDCs should have an energy resolution comparable with the spectator energy fluctuations, for a given impact parameter, which ranges from M 20% for central events to M 5% for peripheral ones, according to simulations that use HIJING as event generator [4] . The fiber-to-absorber filling ratio must be chosen as a compromise between the required energy resolution and the fiber cost. Two sets of calorimeters will be placed on the opposite sides with respect to the beam intersection point. The proton calorimeters (ZP) will be made of brass as absorber material and its dimensions will be 2 0 . 8~1 2~1 5 0 cm3 [5] . Two prototypes of these calorimeters have been already studied [6] . The necessity of containing the shower together with geometrical constraints imposed by the beam pipes, require for the neutron calorimeters (ZN) small transverse dimensions (7 x 7 cm2) and the use of a dense material.
doi:10.1109/23.873005 fatcat:dkx357fjxbfyrkwxxbjyk3dlna