First ALICE detectors installed!

Detectors to track down penetrating muon particles are the first to be placed in their final position in the ALICE cavern.


The Alice muon spectrometer: in the foreground the trigger chamber is positioned in front of the muon wall, with the dipole magnet in the background.

After the impressive transport of its dipole magnet, ALICE has begun to fill the spectrometer with detectors. In mid-July, the ALICE muon spectrometer team achieved important milestones with the installation of the trigger and the tracking chambers of the muon spectrometer. They are the first detectors to be installed in their final position in the cavern.

All of the eight half planes of the RPCs (resistive plate chambers) have been installed in their final position behind the muon filter. The role of the trigger detector is to select events containing a muon pair coming, for instance, from the decay of J/ or Y resonances. The selection is made on the transverse momentum of the two individual muons. The internal parts of the RPCs, made of bakelite, have been fabricated by the company General Tecnica. The readout chambers are being constructed by groups from INFN Torino and Alessandria, while the front-end electronics have been developed at the IN2P3 Laboratory in Clermont-Ferrand and the readout electronics by Subatech Nantes. Thanks to a very special integrated circuit called 'Adult', the RPCs can operate in two different modes: streamer or saturated avalanche.

At the same time, the first half station of the tracking system has been installed a few metres before the muon wall. The main task of the tracking system is to sample the trajectory of muons with a resolution better than 100 µm. The tracking system is made of cathode pad/strip chambers, among the first of this kind, made from composite material. Extremely thin but still very rigid, the composite material helps to minimize the scattering of the muons. Altogether the chambers represent more than 1 million channels. The big chambers have been constructed by INFN Cagliari, PNPI Gatchina, Subatech Nantes and CEA Saclay, while the smallest were made by IPN Orsay and Saha Kolkata. The electronics coordination was the responsibility of the laboratories of IPN Orsay. The Saha Laboratory in Kolkata (India) has contributed to the electronics with the fabrication of the chip MANAS, derived from the GASSIPLEX chip designed at CERN, while the chip MARC has been fabricated at INFN in Cagliari. The chambers and the electronics have been shipped to CERN and are now being assembled and tested on the surface at Point 2 before installation in the cavern. The final installation should be completed and fully tested in Spring 2007.

Did you know?

Muons are very penetrating particles and difficult to stop. For this reason, muon detectors are placed beyond several metres of material which absorb most of the particles except muons. A magnetic field bending the trajectory of the muons and a set of detectors sampling the trajectory complete the muon spectrometer set-up.