Mirror, mirror on the wall

RICH 2, one of the two Ring Imaging Cherenkov detectors of the LHCb experiment, is being prepared to join the other detector elements ready for the first proton-proton collisions at LHC.

In a large dark room, men in white move around an immense structure some 7 metres high, 10 metres wide and nearly 2.5 metres deep. Apparently effortlessly, they are installing the two large high-precision spherical mirrors. These mirrors will focus Cherenkov light, created by the charged particles that will traverse this detector, onto the photon detectors.

Each spherical mirror wall is made up of facets like a fly's eye. Twenty-eight individual thin glass mirrors will all point to the same point in space to within a few micro-radians. The development of these mirrors has been technically demanding : Ideally they should be massless, sturdy, precise and have high reflectivity. In practice, though not massless, they are made from a mere 6 mm thin glass substrate covered with multilayer dielectric coatings of nanometre precision. A support structure, holds the mirrors without deforming them and enables high-precision alignment.

Once flat mirrors have also been installed, the detector will be closed and transported to pit 8 at the end of August. Transporting this 30-tonne beast safely, equipped as it is with precise and fragile optical components, will be another technical challenge of the kind routinely undertaken by the CERN transport services.

In the LHCb experiment, the RICH 2 detector is similar to its upstream partner, RICH 1. RICH 1 is currently in construction and will sport even thinner spherical mirrors made from beryllium with a flash of glass. The two detectors together will enable positive kaon identification up to a momentum of 100 GeV/c over the full acceptance of the experiment.