First detector installed inside the ALICE solenoid...

ALICE's emblematic red magnet welcomed its first detector on 23 September, when the array of seven Cherenkov detectors, named HMPID, was successfully installed.

After the installation of the ACORDE scintillator array and the muon trigger and tracking chambers, the ALICE collaboration fitted the first detector inside the solenoid. The HMPID, for High Momentum Particle Identification, was installed at the 2 o'clock position in the central and most external region of the space frame, just below the solenoid yoke. It will be used to extend the hadron identification capability of the ALICE experiment up to 5 GeV/c, thus complementing the reach of the other particle identification systems (ITS, TPC and TOF).

The HMPID is a Ring Imaging Cherenkov (RICH) detector in a proximity-focusing configuration, employing liquid C₆F₁₄, circulated and purified in a closed loop as radiator medium. The Cherenkov photons are converted into electrons by a thin (300 nm) layer of caesium iodide (CsI), sensitive to UV photons. This layer is deposited on the cathode, segmented into 8x8 mm², of a multi wire proportional chamber. The 161280 pads of the detector are individually read out by two ASIC chips, GASSIPLEX and DILOGIC, developed under Jean Claude Santiard's (CERN Microelectronics Department) supervision.

The particle identification system, realised by the Bari University and INFN, CERN and INR-Moscow, is approximately 8 metres wide by 8 metres tall, and weighs about 5 tonnes. It has been segmented into seven identical modules and shaped to fit against two sides of ALICE's eight-sided octagonal magnet.

Each of the seven RICH modules, fully equipped with electronics, was separately transported to Point 2 in Saint-Genis-Pouilly, where ALICE is based, and then mounted on the support structure (cradle). After assembly, the completed HMPID was lowered down the ALICE cavern and inserted into its final position inside the magnet. Three months of planning ensured that the transport and installation processes were accomplished smoothly within a few hours.

Caesium iodide is a highly hygroscopic chemical compound and any contact with moisture would destroy the thin photosensitive layer. Therefore, during the production and assembly phases, as well as during the transport, manipulation and installation, the detector was constantly flushed with high purity argon under controlled conditions. Methane will be used during the operation of the detector.

With an active area of about 11 m² covered with CsI, the HMPID is the largest application of this technology so far. This development started at CERN within the RD26 project under the leadership of Francois Piuz (CERN), now retired. It took fifteen years of research and development to master the method to the current scale and efficiency. During the construction phase the project was led by Eugenio Nappi (INFN Bari, Italy). In August 2006 Paolo Martinengo (CERN) took over the leadership and he will lead the project during the commissioning and exploitation phase.

In the meantime the first modules of the Time of Flight (TOF) and Transition Radiation Detector (TRD) have been lowered into and installed in the ALICE cavern. The Photon spectrometer (PHOS) will soon be installed, before the spectacular displacement of the Time Projection Chamber (TPC) takes place at the end of November.

See also the article about the history of this detector (Bulletin No. 40/2002).