Optical concentrators for Čerenkov light detector

RICH1 (Ring Imaging Cherenkov) detector is an important part of COMPASS particle physics experiment in CERN. Its central area photon detection part is being upgraded from wire chambers with CsI layers to very fast UV extended Hamamatsu MAPMTs (Multi Anode Photo Multiplier Tubes) array. MAPMTs have approx. 3 times smaller active area than the covered region, thus optical concentrators transforming image from old system focal plane to the new photocathode were needed. System was expected to be efficient from 200 to 600nm with best performance at 300nm and with angular acceptance including all interesting physics processes. Several design types (including i.e. a hollow waveguide) were investigated and the “telescopic” two lens aspherical design concept was selected for its proven functionality in HERA-B experiment. Chosen material was UV grade fused silica. System consists of a field lens placed in the focal plane of the RICH mirrors and a condenser lens downstream. Designing procedure started with a high optical quality fully aspherical system and continued by a gradual decrease in imaging performance to match the budget but still satisfy the physics requirements. Particle simulations showed a necessity of the system tilt but mounting constraints didn’t fully allow it, so the field lens was made prismatic with one flat side and the condenser lens off centered and tilted. Performance of the designs was tested by Monte Carlo method allowing a better optimization. Testing by rays from complex detector simulation implied a necessity of tilt in another plane. Design ready for production satisfies all performance, dimensional, mounting and cost expectations.