Focussing DIRC - A new compact Cherenkov ring imaging device

We present a new device for particle identification, the Focussing DIRC that fits into a tight detector configuration and fully exploits the potentiality of Cherenkov ring imaging technique in the momentum range of high-energy fixed-target and collider experiments. In the device, internally reflected Cherenkov rings in a thin quartz radiator plate are focussed by a toroidal mirror surface to arc-fike loci at the circumference of the plate. The image is taken on the focal surface by segmented photon detectors or by tightly packed optical fibers of diameter around 1 mm. The focussing optics is implemented as extension of the radiator plate and occupies no more than about 10 sm by 12 cm in cross section. The image width of the Cherenkov ring is mostly determined by the chromatic dependence of the Cherenkov angle and the multiple Coulomb scattering in the radiator plate. We expect to separate pions and kaons by similar to 3.5-4 sigma up to 3.5 GeV/e and by similar to 3 sigma up al 4 GeV/e with presently available photon sensors. With higher quantum efficiency sensors such as VLPCs, we will be able to separate kaons from pions up to similar to 10 GeV/c with a 2 cm thick quartz radiator.