The Vacuum Phototriodes for the CMS Electromagnetic Calorimeter

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• The Vacuum Phototriodes for the CMS
  Electromagnetic Calorimeter
• P R Hobson, D C Imrie, O Sharif
• Brunel University, UK
• K W Bell, R M Brown, D J A Cockerill, P S Flower,
  B W Kennedy,
• A L Lintern, M Sproston, J H Williams
• CLRC - Rutherford Appleton Laboratory, UK
• (With acknowledgements to H F Heath and
  colleagues at Bristol University, UK and D
  Seliverstov and colleagues at PNPI, Russia)
• HEP2001
• Budapest - Hungary
• July 2001

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Outline of Talk

• Overview of CMS
• The Electromagnetic Calorimeter (ECAL)
• Properties of Lead Tungstate
• Radiation levels
• VPT Performance (End cap)
• APD Performance (Barrel)
• Status summary

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Compact Muon Solenoid
ECAL
HCAL
Superconducting coil
Total mass 12,500t Overall Diameter
15.0m Overall Length 21.6m Magnetic field
4T
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ECAL design objectives
Benchmark physics process Search for 130 GeV
Higgs via H ? ? ? (Sensitivity depends critically
on mass resoln) ?m / m 0.5 ?E1/ E1 ? ?E2/ E2 ?
??/ tan(?? / 2 ) Where ?E / E a / ? E
? b ? c/ E
Performance Aims Barrel
End cap Stochastic term, a (p.e.
statistics/shower fluctuation)
2.7 5.7 Constant term,
b (non-uniformities, shower leakage)
0.55 0.55 Noise term,
c (Electronic noise, event pile-up) Low L
155 MeV 205 MeV High L 210 MeV 245
MeV (Angular resolution limited by uncertainty
in position of interaction vertex)
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Lead Tungstate Properties

• Advantages
• Fast
• Dense
• Radiation hard
• Emission in visible

• Disadvantages
• Temperature dependence
• Low light yield
• ? Photodetector with gain
• (in a strong magnetic field)

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CMS ECAL Layout
Full projective geometry (Off-pointing by 3o)
Barrel 17x2 Crystal types End cap 1 Crystal type
1290 mm
3170 mm
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Doses and neutron fluences
Integrated dose (kGy) and neutron fluence (x1013
cm-2) for ? L 5x105 pb-1 (10 yrs) Black Dose
in the Crystals at the position of the shower
maximum Blue Dose behind the crystals at the
position of the photodetectors Red Neutron
fluences behind the crystals
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Photodetectors end caps

• B-field orientation favourable for VPTs
• (Axes 8.5o lt ? lt 25.5o wrt to field)
• More radiation hard than Si diodes
• (with UV glass window)
• Gain 8 -10 at B 4 T
• Active area of 280 mm2/crystal
• Q.E. 20 at 420 nm

• Vacuum Phototriode (VPT)
• Single stage photomultiplier tube with fine metal
  grid anode

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VPT Gain vs Dynode Voltage
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Supercrystal Layout
Supercrystal carbon-fibre alveola containing
5x5 tapered crystals VPTs passive HV filter
(160 Identical Supercrystals per Dee) Signals fed
via 600 mm cable to Preamplifier Front End
electronics behind Dee Backplate