by Yuri Kharlov. Published: 17 May 2013

 The Photon Spectrometer PHOS is a high-resolution photon detector; its primary aim is to measure photon radiation from the hot QCD matter. PHOS has been designed to detect photons produced at angles around 90º in the center-of-mass system in a wide energy range up to 100 GeV and reconstruct neutral mesons decaying to photons.

The PHOS detector has been operated in the ALICE experiment since 2009. PHOS consisted  of 3 modules each containing 64x56 lead tungstate crystals. These inorganic scintillating crystals have a strong temperature dependence of the light yield; in order to achieve a high energy resolution, the operating temperature of the PHOS is -25ºC.

PHOS module seen from front side, showing the lead tungstate crystals.

The PHOS modules are divided into two parts. The crystal matrix is located in the cold volume at -25ºC, and the readout electronics is in the warm volume. To avoid condensation of air humidity in the cold volume, the PHOS modules with their cold and warm volumes were immersed into the air-tight boxes completely insulated from the outer ALICE environment and were blown by a flux of dry nitrogen. Before starting cooling, it takes a few months to reach low humidity inside the PHOS modules, which ensures a dew point below the operation temperature of -25ºC. Because of this fact, access to the PHOS electronics was impossible during the three years of ALICE running. The health of the PHOS systems was monitored by the detector control system, but if anything happened with the front-end cards, they could not be replaced nor repaired. It was like a satellite experiment: once launched in 2009, PHOS was working without any human access and was controlled only remotely via telemetry.

PHOS module seen from the rear side with front-end electronics. It is currently installed in the lab to repair electronics.

Several problems and pending tasks were accumulated over the three years of operation. Some front-end cards went out of order and needed repair. Readout time of 850 μs was adequate to cope with the low luminosity in the beginning of the ALICE operation in 2010. However, when the 2011 high-luminosity runs started it was realized that the readout time was rather long. This readout time can be improved by almost a factor of 3 by reprogramming the firmware of the front-end cards.

 Other subsystems of the PHOS detector also required maintenance, repair or reprogramming, such as monitoring system, trigger, cooling system. All these tasks can be fulfilled only with access to inner parts of the PHOS. That is why we profit from the current long shutdown to repair broken front-end cards, reprogram their firmware, and to improve remote control of the internal PHOS systems.

 The time of the current long shutdown is also being used to assemble and commission the new, fourth PHOS module and one module of the Charge-Particle Veto detector (CPV). In the upcoming run-2 of the LHC, the one CPV and the four PHOS modules will be installed together with the new electromagnetic calorimeter DCAL, which will be sitting on the new support structure. New rails and a cradle of this support structure are being installed inside the ALICE solenoid magnet while the PHOS detector is absent there.

Fitting test of the PHOS module to the new DCAL-PHOS cradle which was built recently and is now waiting for both detector installation in 2014.

 The three old PHOS modules were extracted from the ALICE pit at the end of March 2013 and elevated to the surface. Two modules have been already transported to the Meyrin site in the PHOS lab for repair. The third module was left at the ALICE site for fitting test of the new support structure, i.e. its fitting to the new cradle will be tested on the surface in order to make sure that it will be installed in the pit without any problems afterwards. At the moment one PHOS module is disassembled, its repair is going on.

A case and mechanics for the new 4th PHOS module which will be added to the existing three modules.

In total 17 front-end cards were repaired and returned back to their normal operation. To test and commission the renovated modules, the PHOS lab is equipped with all ALICE systems (detector control and data acquisition systems and the central trigger processor). All this allows working in the PHOS lab in the same conditions as those found in ALICE.

 Installation of the upgraded PHOS, along with the DCAL, is scheduled for the beginning of 2014. This time is rather tight, taking into account the amount of work to be done. Repair of each PHOS module is allocated only 2 months. Then the whole of 2014 will be devoted to integration of the PHOS and CPV detectors to the new ALICE environment, in order to be ready for physics data taking after the LHC restart in 2015.