The COMPASS collaboration, which will have a technical set-up run this year, took an important step forward on March 10 when a major component of its detector was installed in hall 888 (EHN2) in the North Area. The new component, known as the RICH-1 vessel, is the outer vessel of the first of two Ring Imaging Cerenkov detectors for the COMPASS experiment. Its job will be to identify individual particles coming from muon and hadron collisions with the COMPASS target, and in particular to distinguish pions and kaons up to 60 GeV/c. RICH-1 joins the experiment's two spectrometer magnets and a large hadron calorimeter. The remaining large components, another hadron calorimeter and the muon filters, will follow shortly.
RICH-1 is one of the largest new elements of the COMPASS apparatus. It works, as its name suggests, by forming an image of the ring of light emitted by fast moving particles passing through it. Because there will be several detectors down stream of RICH-1, it has been designed to present the minimum possible impedance to passing particles. The radiator medium, which is where particles emit their ring of light, is three metres long along the beam direction and is made up of C4F10 gas held at atmospheric pressure and a stable temperature of 25 degrees Celsius. The total volume of gas contained in the RICH-1 vessel is some 80 metres cubed to maximise the number of particles coming from the collision that pass through it - its acceptance in physics parlance - and this gas must be kept extremely pure. Particles passing through the detector will emit light and part of the ultraviolet (UV) portion of the emission spectrum will be detected, so the gas must be transparent to UV. That means that traces of oxygen and water vapour, which absorb UV, must be kept below five parts per million.
Light emitted by passing particles will be focused by two spherical mirror surfaces made up of 120 hexagonal and pentagonal reflecting surfaces onto two sets of detectors. Each of these reflecting pieces will have individual angular regulation with the goal of aligning each one with an angular accuracy of about 0.2 mrad (about 0.01 degrees). The detectors are multi-wire proportional counters (MWPCs) equipped with Caesium Iodide photocathodes, developed at CERN in the context ot the RD26 project and EP/TA1, to convert the light into an electrical pulse. In total, they will cover a total active area of 5.3 metres squared. With a photocathode pixel segmentation of eight by eight millimetres, RICH-1 will have some 80 000 channels, and with around 5% of channels registering light, the detector will generate a maximum data flow of 2.5 Gbytes per second during a typical burst of beam particles from the SPS.
RICH-1 is a project led by the Italian INFN's Trieste and Turin centres with Bielefeld contributing to the radiator gas system. The vessel was designed at CERN in collaboration with the EST/ESI group and the production was closely followed up by the CERN workshops (EST/MF), where parts of the mirror support wall were also produced. The RICH-1 gas system is carried out in collaboration with CERN's EST/SM group. Assembling the complete RICH-1 detector for the technical run is a complex operation that started in January this year and will continue until May. CERN groups EP/TA2, EST/MF and EST/ESI are assisting the COMPASS collaboration in this task. The arrival of the huge RICH-1 vessel and its installation, on the same day, in hall 888 is an important milestone in the assembly exercise. It was made possible by the enthusiasm and dedication of all involved.