CBM - RICH - Workshop 6.3.06 1
The CRES RICHCErenkov Ring Electron Spectrometer at the CERN SPS
(1990-2000)
www.physi.uni-heidelberg.de/physi/ceres
Presented by Peter Glässel, Heidelberg (now ALICE)
Univ. Heidelberg, H.J. SpechtWeizmann Institute, Rehovot, I. TserruyaMPI-K Heidelberg, J.P. WurmPolitechnico di Milano, E. GattiBNL, P. RehakCERN, J. Schukraft
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OutlineCERES setup
Radiators and windows
Mirrors
UV-photon detectors
Understanding performance
Summary
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CERES RICH layoutaxially symmetric
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CERES
in 1996
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Radiator and UV windows
CH4 at ambient pressure → = 32
O2 and H2O about 1 ppmRICH1, L = 90 cm
window CaF2 in Al alloy
UV bandwidth limited by radiator gas CH4
RICH2, L = 175 cm window quartz (Haereus Suprasil) in Invar frame, sliding O-ring seal vs. radiator tankbandwidth limited by quartz
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UV1 CaF2 window
10 mm thick
10 sectors, each consisting of 4 pieces, glued
Al alloy (cast) with tuned thermal expansion coeff
border to CaF2 shaped like a bellow (1mm wall)
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CERES RICH mirrors
RICH1: 1 mm CFK, replica-molded on glas master
~60 cm Invar ring mount
Reflectivity typ > 85%, 70% @ 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
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RICH1 Mirror
1 mm CFK
replica on glas master
~ 60 cm Invar ring mount
Reflectivity: avg. 85%,
70% @ 140 nm
Optical quality does not limit RICH resolution
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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
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The CERES UV detectors, 1st try
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Assembly of UV2
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UV2 detector
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Why did 2-step (or 3-step) PPAC not work?
Avalanches ≥ 5107 … 108 e spark (‘Raether Criterion’ 1939)Independent of quencher → Fonte, Pestov, Sauli NIM A305(1991)91
Gas gain x dE/dx in typ. avalanche dimension: 105 x 1000 e/mm x 1 mm = 108
Slow knock-on protons from shower neutrons do it!CERES: in 32S+Au: 20% spark probability per interaction in UV2 (3 m2)
Solution: fundamentally different behavior of the multiplication schemes in the space-charge limit:
PPAC: gain divergesMWPC: gain saturates
New scheme worked up to Pb-Au ( 20 % dead time due to sparks)
Last stage must be a MWPC
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The CERES UV detectors from 1992 on
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UV1 multi-wire anode plane
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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 epoxyResistive layer: carbon-filled epoxy, 150 M/MWPC: anodes 30 μm , 2 (3) mm pitch53.800 (48.400) pads
He + 6% CH4 + TMAE(400 C) at ambient pressure, operated at 500 CGas gain 2 … 4105
Detectors functioned for 7 beam periods
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Event displays
p-Be
Close-up of e+e− pair
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Event displayPb-Au
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Understanding N0
from: CERES Status Report to the SPSLC, CERN SPSLC/94-02
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Understanding Resolution
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Cherenkov Ring Radial distribution
Ring center resolution: 1-dim, for tracking, momentum
c = 1.6 h / sqrt(N)
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Single hit background
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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
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