ALICE upgrades its powerful eyes

The ALICE Photon Spectrometer (PHOS) is a high-resolution photon detector that measures the photons coming out of the extremely hot plasma created in the lead-lead collisions at the LHC. Taking advantage of the long accelerator shut-down, the ALICE teams are now repairing and upgrading the existing modules and getting ready to install the brand-new module in time for the next run. The upgraded PHOS detector will be faster and more stable with wider acceptance and improved photon identification.

PHOS crystal matrix during repair.

The key feature and the main complexity of the ALICE PHOS detector is that it operates at a temperature of -25°C, which makes it the second-coldest equipment element at the LHC after the cryogenic superconducting magnets. Since 2009 when it was installed, the PHOS detector, with its cold and warm volumes, has been immersed in airtight boxes to avoid condensation in the cold volumes. The 10,752 lead tungstate crystals of the PHOS were completely insulated from the outer ALICE environment and its modules were kept at a stable operating temperature of -25°C and at 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 in 2009, PHOS operated without any human access and was controlled remotely via telemetry.

Several problems and jobs to be done 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 μs was adequate to cope with the low luminosity. However, when the 2011 high-luminosity runs started, it was realised that the readout time was rather long. This readout time will be improved by almost a factor of 30 by changing the readout system. The upgraded readout system should also make the PHOS detector more reliable over the long term.

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 also the full article on the ALICE Matters publication.

by Yuri Kharlov, ALICE Collaboration