The ALICE Photon Spectrometer (PHOS) measures the photons that fly out of the extremely hot plasma created in lead-lead collisions at the Large Hadron Collider (LHC). ALICE teams are repairing and upgrading existing modules and getting ready to install the new module in time for the next run. The upgraded PHOS detector will be faster and more stable and better able to identify photons.

The ALICE PHOS detector operates at -25°C, which makes it the second-coldest element in the LHC after the cryogenic superconducting magnets. Since it was installed in 2009, the PHOS detector, with its cold and warm volumes, has been immersed in airtight boxes to avoid condensation in the cold volumes. Its 10,752 lead tungstate crystals were completely insulated from the outer ALICE environment and its modules were kept at a stable operating temperature of -25°C with very low humidity. This made access to the PHOS electronics impossible during the three years of ALICE operation. The health of the PHOS systems was monitored by the detector control system, but if anything happened to the front-end cards, they could not be replaced or repaired. It was like a satellite experiment: once launched, PHOS operated without any human access and was controlled remotely via telemetry.

Several problems accumulated over the three years of operation. Some front-end electronics cards stopped working and needed repair. At the beginning of ALICE operation in 2010, a readout time of 850 microseconds was adequate to cope with the low luminosity. But this readout time was rather long for the high-luminosity runs in 2011. So ALICE teams will change the readout system, to improve the readout time by almost a factor of 30.

Other subsystems of the PHOS detector such as the monitoring system, the trigger and the cooling system also required maintenance, repair or reprogramming. All these tasks require access to the inner components of the PHOS. The PHOS team is therefore taking advantage of the current long shutdown to repair broken front-end cards, reprogram their firmware and improve the remote control of the internal PHOS systems.

The current long accelerator shutdown is also being used to assemble and commission the new, fourth PHOS module and one module of the Charged Particle Veto detector (CPV). During the upcoming second run of the LHC, the one CPV and the four PHOS modules will have been installed together with the new electromagnetic calorimeter DCal, which will sit on the new support structure.

The installation of the upgraded PHOS is scheduled for the autumn of 2014. Several months will be needed to complete the integration of the PHOS and CPV detectors into the new ALICE environment, in time for the LHC restart in 2015. 

Read a longer version of this article: "PHOS commissioning to start during LS1" – ALICE Matters