The ATLAS collaboration recently celebrated installing the last of the eight "big wheels" that form part of the endcap muon spectrometer of the detector. The big wheels harbour ATLAS's middle layer of muon chambers in the forward region and are one of the last large pieces to be installed. Each is 25 m across, weighs between 40 and 50 tonnes and contains around 80 precision tracking chambers or 200 trigger chambers.

Because of their sheer size, each wheel had to be made in 12 pieces for the trigger planes and 16 pieces for the tracking planes. Designing a suitable support structure was a unique challenge, and the result is a uniquely thin and light structure that is precise to less than a millimetre.

Each wheel was assembled at CERN using components from all over the world. The 100-member collaboration from China, Europe, Israel, Japan, Pakistan, Russia and the US began assembly of components in 2005 and installation in 2006. Now, just two smaller-scale wheels and the end-wall chambers remain to be installed. The big wheels have already begun to take part in test runs using cosmic-ray data that ATLAS performs on a six-weekly basis (CERN Courier September 2007 p23).