The electromagnetic calorimeter (ECAL) is one of the most important components of the CMS detector, and has been optimised to detect the decay of the Higgs boson into a pair of photons. The calorimeter comprises a barrel section and two endcaps, and is constructed from approximately 80000 crystals of lead tungstate (link opens in a new window).
RAL physicists and engineers have played a major role in designing the ECAL (link opens in a new window) from the inception of the CMS collaboration, and have the lead responsibility for the construction of the ECAL endcap detector; a PPD physicist (David Petyt) is the ECAL endcap project leader.
The endcaps are built from 5x5 arrays of crystals, known as "supercrystals (link opens in a new window)", mounted on an aluminium backplate (link opens in a new window). Each endcap contains 7324 lead tungstate crystals, producing blue-green scintillation light when energy is deposited in the material by charged particles or photons. Each crystal is equipped with a Vacuum Phototriode (VPT) (link opens in a new window) (essentially a single-stage photomultiplier) for light detection.
The VPTs were designed by the RAL group, in collaboration with the manufacturer, Research Institute Electron of St Petersburg, Russia, to operate efficiently in the solenoidal magnetic field of CMS.. A total of 16100 VPTs were delivered to RAL and tested in a 1.8T magnetic field (link opens in a new window) before being shipped on to CERN for incorporation into the endcaps.
CMS ECAL Endcap Crystals