CBM - RICH - Workshop The CRES RICH CErenkov Ring Electron Spectrometer at the CERN SPS ( ) Presented by Peter Glässel, Heidelberg (now ALICE) Univ. Heidelberg, H.J. Specht Weizmann Institute, Rehovot, I. Tserruya MPI-K Heidelberg, J.P. Wurm Politechnico di Milano, E. Gatti BNL, P. Rehak CERN, J. Schukraft

CBM - RICH - Workshop Outline CERES setup Radiators and windows Mirrors UV-photon detectors Understanding performance Summary

CBM - RICH - Workshop CERES RICH layout axially symmetric

CBM - RICH - Workshop CERES in 1996

CBM - RICH - Workshop Radiator and UV windows CH 4 at ambient pressure →  = 32 O 2 and H 2 O about 1 ppm RICH1, L = 90 cm window CaF 2 in Al alloy UV bandwidth limited by radiator gas CH 4 RICH2, L = 175 cm window quartz (Haereus Suprasil) in Invar frame, sliding O-ring seal vs. radiator tank bandwidth limited by quartz

CBM - RICH - Workshop UV1 CaF 2 window 10 mm thick 10 sectors, each consisting of 4 pieces, glued Al alloy (cast) with tuned thermal expansion coeff border to CaF 2 shaped like a bellow (1mm wall)

CBM - RICH - Workshop CERES RICH mirrors RICH1: 1 mm CFK, replica- molded on glas master ~60 cm  Invar ring mount Reflectivity typ > 85%, 140 nm Optical quality does not limit RICH resolution RICH2: ring-shaped, 10 sectors ~ 1.8 m  6 mm glas, slumping method from flat glass (Andre Braem/ CERN) After software-correction for focal length variations: does not limit RICH resolution

CBM - RICH - Workshop RICH1 Mirror 1 mm CFK replica on glas master ~ 60 cm  Invar ring mount Reflectivity: avg. 85%, 140 nm Optical quality does not limit RICH resolution

CBM - RICH - Workshop RICH2 Mirror ring-shaped 10 sectors ~ 1.8 m  6 mm glas, slumping method from flat glass (Andre Braem/ CERN) After offline-correction for focal length variations: does not limit RICH resolution

CBM - RICH - Workshop The CERES UV detectors, 1 st try

CBM - RICH - Workshop Assembly of UV2

CBM - RICH - Workshop UV2 detector

CBM - RICH - Workshop Why did 2-step (or 3-step) PPAC not work? Avalanches ≥ 5  10 7 … 10 8 e spark (‘Raether Criterion’ 1939) Independent of quencher → Fonte, Pestov, Sauli NIM A305(1991)91 Gas gain x dE/dx in typ. avalanche dimension: 10 5 x 1000 e/mm x 1 mm = 10 8 Slow knock-on protons from shower neutrons do it! CERES: in 32 S+Au: 20% spark probability per interaction in UV2 (3 m 2 ) Solution: fundamentally different behavior of the multiplication schemes in the space-charge limit: PPAC: gain diverges MWPC: gain saturates New scheme worked up to Pb-Au (  20 % dead time due to sparks) Last stage must be a MWPC

CBM - RICH - Workshop The CERES UV detectors from 1992 on

CBM - RICH - Workshop UV1 multi-wire anode plane

CBM - RICH - Workshop Other UV detector specs Meshes stainless steel, 50 μm , 500x500 μm pitch, except first mesh (30 μm  for higher transparency) Materials: G10 frames, stainless steel, teflon, delrin, Cu, selected epoxy Resistive layer: carbon-filled epoxy, 150 M  / MWPC: anodes 30 μm , 2 (3) mm pitch (48.400) pads He + 6% CH 4 + TMAE(40 0 C) at ambient pressure, operated at 50 0 C Gas gain 2 … 4  10 5 Detectors functioned for 7 beam periods

CBM - RICH - Workshop Event displays p-Be Close-up of e + e − pair

CBM - RICH - Workshop Event display Pb-Au

CBM - RICH - Workshop Understanding N 0 from: CERES Status Report to the SPSLC, CERN SPSLC/94-02

CBM - RICH - Workshop Understanding Resolution

CBM - RICH - Workshop Cherenkov Ring Radial distribution Ring center resolution: 1-dim, for tracking, momentum  c = 1.6  h / sqrt(N)

CBM - RICH - Workshop Single hit background

CBM - RICH - Workshop Summary TMAE is a manageable UV-converter Beware of spark limit for any gas multiplication scheme Theoretical performance limits can be closely reached in practice