Dressing up the CMS detector
Eighty percent of the crystals for the CMS electromagnetic calorimeter barrel have been produced.
More precious than Swarovski, the truncated pyramidal-shaped crystals for the electromagnetic calorimeter (ECAL) are arguably the prettiest part of the CMS detector. Tightly packed in large aluminum boxes, more than two-thirds of the 61000 or so barrel detector crystals are ready for installation. Twenty-five of the 36 supermodules, which each contain 1700 of the fragile crystals, have been produced since assembly began in 2002.
The ECAL crystals, which are 23 cm long in the barrel and 22 cm long in the endcaps, are 86 percent metal but are completely transparent. Made from lead tungstate, they are characterized by a high density and a small radiation length. Their weight ranges from 1.3 to 1.5 kilos each.
The crystals are made in Russia and China and must pass a series of steps before they are ready for installation. Each month, CERN and the INFN/ENEA Laboratory near Rome share a batch of 1200 freshly made crystals. They are loaded into a machine that automatically measures several parameters, including dimensions, transparency and light yield. 'Usually when you have a crystal, the beauty of it is that it is coloured,' said Etiennette Auffray, leader of the crystal supermodule construction at CERN. 'But in our case if crystals are coloured it signifies defects, reducing their performance.' The small fraction of flawed crystals is weeded out and the rest receive a photodetector, which a technician glues onto the rear of the crystals while carefully avoiding air bubbles. About 120 crystals are glued per day. The glued crystals are then inserted and sealed in groups of 10 into a lightweight glass-fibre 'alveolar' box to form a 'submodule.'
'After each step of construction, we verify that everything is OK,' Auffray said. 'That's important because it's hard to get to the crystals after they are in the boxes.' Forty to 50 submodules are assembled together to make a 'module.' The modules made at the INFN/ENEA laboratory are sent to CERN, which takes over the rest of the assembly process.
The final piece, the 'supermodule,' is made from four modules. Each supermodule weighs more than 3 tonnes, heavier than a hippopotamus, and is placed in a special shock-absorbing crate for slow transport to the Prévessin site. In Building 867, the supermodules will undergo electronic integration involving several ECAL institutes under the supervision of ETH Zurich. Going just 10 km/hour to avoid damaging the delicate crystals, the drive to the site takes about an hour.
The 11-member crystal assembly team produces one supermodule per month. The last supermodule must be completed by March 2007. Half of the 36 supermodules will be installed on one side of the CMS detector this summer.
'Hidden away in their compacted boxes, blending in with the metal and wires of the rest of the detector, it's easy to forget the beauty of the crystals that lie beneath', Auffray said.
And then there was light
The crystals are an essential part of the CMS electromagnetic calorimeter. When particles collide in the centre of the CMS detector, incident electrons and photons will deposit energy in the crystals, which will emit scintillation light. The amount of light created is an approximate linear function of the particle energy. Photodetectors glued onto each of the crystals will detect this light and produce a proportional electronic signal.
