Building a crystal palace

The end-caps of the CMS electromagnetic calorimeter (ECAL) take shape as the first quadrant was completed on Wednesday 3 October.

With the 61,200-crystal barrel of its electromagnetic calorimeter (ECAL) complete, CMS is now building the endcaps, on the tenth anniversary of their initial design. Crystals for the endcaps were the last to be made, so the race is now on to have them all in place and ready for the turn-on of the LHC next year. Assembly of the first of eight quadrants began in June and crystal mounting was completed on Wednesday 3 October.

Each crystal is transparent, has a volume just larger than a CERN coffee cup yet weighs a huge 1.5kg. 1831 of these lead tungstate crystals went into the first quadrant from a total 14,648 in the endcaps. The lead and tungsten account for 86% of each crystal’s weight, but as project leader Dave Cockerill explains, "with just a touch of oxygen it becomes a transparent medium that scintillates with the particles coming through it. Magic!"

The ECAL detects two of the main particles of interest coming out of collisions at CMS, electrons and photons, and measures their energies. Incident particles deposit their energy in the crystals, which emit a proportional amount of scintillation light that can be measured.

Each endcap is made up of two "Dee"s, so called for their resemblance to the letter, done so that the halves can be assembled around the beam pipe without needing to cut into it. Each Dee is split into two quadrants made up of five by five blocks named ‘supercrystals’, each of which weighs about 45 kg.

Their weight presented a novel assembly challenge: "We have a special loading arm, similar to the one used for the barrel. We bolt the supercrystal on the end of it and then, as far as the operator is concerned, it is weightless, fingertip steering. We can very delicately bring it into the right place and screw it on. You couldn’t do this manually or with a crane," explains Dave.

The endcaps are an essential part of CMS as they extend the coverage of the barrel part of the electromagnetic calorimeter. "As the particles can go in all directions, the more we can cover of these directions, the more chance we have of detecting the particles," explains Dave. "In the case of the Higgs, if it decays to two photons, then having the endcaps in CMS adds about 40% to our coverage and so our capability of detecting it".

CMS anticipates completely installing the first ECAL endcap by March next year and the second by June, so at least one endcap will be in place by start-up. Being one of the last pieces of CMS to be installed presents a challenge: "We are proceeding as safely but as fast as possible to ensure a fully working detector for LHC."

With crunch time drawing near, the team is tense but excited: "The overall feeling is excitement. It’s a bit like stage fright – you’re about to go on stage and you’re excited and frightened but you’re well prepared and you’re hoping it’s all going to go really well. That’s the feeling at the moment."