In order to build up a picture of events occurring in the LHC, CMS must find the energies of emerging particles. Of particular interest are electrons and photons, because of their use in finding the Higgs boson and other new physics.
These particles are measured using an electromagnetic calorimeter (ECAL). But to find them with the necessary precision in the very strict conditions of the LHC - a high magnetic field, high levels of radiation and only 25 nanoseconds between collisions - required very particular detector materials.
Lead tungstate crystal is made primarily of metal and is heavier than stainless steel, but with a touch of oxygen in this crystalline form it is highly transparent and “ scintillates ” when electrons and photons pass through it. This means it produces light in proportion to the particle’s energy. These high-density crystals produce light in fast, short, well-defined photon bursts that allow for a precise, fast and fairly compact detector.
Photodetectors that have been especially designed to work within the high magnetic field, are also glued onto the back of each of the crystals to detect the scintillation light and convert it to an electrical signal that is amplified and sent for analysis.
The ECAL, made up of a barrel section and two ”endcaps”, forms a layer between the tracker and the HCAL. The cylindrical “barrel” consists of 61,200 crystals formed into 36 “supermodules”, each weighing around three tonnes and containing 1700 crystals. The flat ECAL endcaps seal off the barrel at either end and are made up of almost 15,000 further crystals.
For extra spatial precision, the ECAL also contains Preshower detectors that sit in front of the endcaps. These allow CMS to distinguish between single high-energy photons (often signs of exciting physics) and the less interesting close pairs of low-energy photons.
The CMS ECAL…
- crystals each weigh 1.5kg but with a volume roughly equal to that of a small coffee cup,
- contains nearly 80,000 such crystals, each of which took two days to grow.
CMS ECAL Technical Design Report (though be aware that significant changes have been made since the report's writing in 1997)
Production Story ....
A Russian factory in a former military complex took on the job of producing most of the crystals, whilst the remainder were produced in China. It took about ten years to grow all 78,000 crystals to stringent specifications, taking around two days to artificially grow each one. Inside the detector they also face high radiation, so the lead tungstate material had to be carefully chosen and developed…