Last crystals for the CMS chandelier

In March, the last crystals for CMS’s electromagnetic calorimeter arrived from Russia and China. Like dedicated jewellers crafting an immense chandelier, the CMS ECAL collaborators are working extremely hard to install all the crystals before the start-up of the LHC.

One of the last CMS end-cap crystals, complete with identification bar code.

Nearly 10 years after the first production crystal arrived at CERN in September 1998, the very last shipment has arrived. These final crystals will be used to complete the end-caps of the electromagnetic calorimeter (ECAL) at CMS. All in all, there are more than 75,000 crystals in the ECAL.

The huge quantity of CMS lead tungstate crystals used in the ECAL corresponds to the highest volume ever produced for a single experiment. The excellent quality of the crystals, both in terms of their optical properties and their radiation resistance, is the result of intense work and collaboration between the producers, ECAL groups and a network of crystallography and solid state physics experts from the Crystal Clear collaboration.

Lead tungstate crystals mounted onto one section of the CMS ECAL end caps.

Five CMS institutes (CERN, ENEA, ETH Zurich, INP Minsk and Rome University I) have been prominently involved in the monitoring and quality control of this very long production process. The optical properties of each crystal have been measured by custom-designed automatic equipment. Radiation resistance was systematically controlled through test sampling, requiring complex logistics coordinated by E. Auffray (CERN) and F. Nessi-Tedaldi (ETHZ) for the Russian and Chinese crystals respectively. Many other institutes were also involved in the early development of the crystals.

Etiennette Auffray, leader of the CERN crystals regional centre, has been working with the CMS ECAL crystals since the conception of the design of the detector in the early nineties. How does she feel about the imminent completion of the project? "I wasn’t sure at the beginning that we would succeed, but we have indeed been extremely successful and I’m very proud to have participated in this adventure. It has been a great experience for me, but I’m also a bit nostalgic, since it has been a most essential part of my work at CERN since 1992. However, together with my colleagues from ECAL I feel confident that the future will bring many other new and exciting challenges."

The 61200 crystals of the ECAL barrel were successfully installed inside CMS last year (see Bulletin 35/2007). The last steps will be the installation of the end-caps (14648 crystals). The first end-cap is due to be lowered into the cavern in June and the second end-cap should follow later in the summer. More than 90% of the end-cap crystals have already been qualified and equipped with their photo-sensor.

What are ECAL crystals and how do they work?

ECAL crystals are transparent and made from 86 % lead and tungsten, making them very dense. Although each end-cap crystal has a volume equal to around that of a can of coke, each one weigh a hefty 1.5kg.

The ECAL detects electrons and photons that are produced from proton collisions at CMS by making use of the scintillation properties of the crystals. When particles pass through the crystals they deposit energy, which is then emitted by the crystal in the form of light. The level of scintillation light is a linear function of the energy of the particle. Therefore, when you measure the light you can calculate the energy of the particle that produced it.