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CMS-Crystal Clear

CMS is a high-performance general-purpose detector whose goal is to seek out new physics, whatever form it may take. The detector, weighing 12 500 tons, was designed and is currently being built by a world-wide collaboration of 1800 physicists from 32 countries.

CMS is a large, technologically-advanced detector made up of many layers. Each of these is designed to perform a specific task and together they will allow CMS to identify and precisely measure the energy of all the particles produced in LHC proton-proton collisions. The layers of the CMS detector are arranged in concentric cylinders around the collision point.

Central to the design of CMS is the biggest superconducting solenoid magnet with the highest field and stored energy ever. The solenoid will be 6 metres in diameter and 13 metres long. It will generate a field of 4 Tesla, which means that the energy stored will be 2.5 Giga Joules, enough to melt 18 tons of gold.

All in all, CMS will have 15 000 000 individual electronics channels, controlled by powerful computers, from which information from the detector is readout.

The sheer size of CMS can be gauged from the first Magnet Barrel Yoke Wheel assembled in the IWE factory in Deggendorf, Germany on 29 September 1999.

A high precision electromagnetic calorimeter made of Lead Tungstate (PbWO4) crystals optimised for the detection of key processes sits inside the magnetic field. CMS will need 80 000 blocks of these crystals for a total volume of 11m3 and a weight of 90 tons. In order to produce these over the five year timescale required, the world's production rate would have to be substantially increased. This will be made possible thanks to expertise gained in China through work on crystals for L3, existing expertise and impressive crystal growing infrastructure in Russia and financial support from the ISTC through a joint venture with CERN.

Members of the CMS crystal team with some of the finished product.

A pre-production phase of 6000 crystals is coming to an end in Russia and will be followed by the production of about 40 000 crystals. Pre-production will also start soon in China. This should see production rates rising to 2000 crystals per month when full-scale production will be in place. All the crystals are scheduled to be ready by 2004, one year ahead of LHC start-up.

As with BGO before it, industry around the world is keeping a close eye on Lead Tungstate development. These crystals, as well as other crystals resulting from the large development effort made for CMS, could be used in medical imaging, where in conjunction with other technologies developed for physics, they could offer sensitivity 10 times better than today's imaging devices.

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