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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Vacuum Phototriodes for the CMS Electromagnetic Calorimeter Endcaps
K.W.Bell, R.M.Brown, D.J.A.Cockerill, K.W.Bell, R.M.Brown, D.J.A.Cockerill, P.S.Flower,P.S.Flower,P.R.HobsonP.R.Hobson, B.W.Kennedy, A.L.Lintern, , B.W.Kennedy, A.L.Lintern, C.S.Selby, O.SharifC.S.Selby, O.Sharif, M.Sproston, J.H.Williams, M.Sproston, J.H.Williams
CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, UKCCLRC Rutherford Appleton Laboratory, Chilton, Didcot, UKBrunel University, Uxbridge, UKBrunel University, Uxbridge, UK
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
The CMS Experiment
ECAL
Tracker
HCAL
4T solenoid
Muon chambers
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
The CMS Electromagnetic calorimeter
PbWO4 crystals
~61000 in barrel~15000 in endcaps
Pointing geometry
Length: 6m
Radius: 1.75m
Depth: 24-25 X0
Target energy resolution:<1% at E = 100 GeV
Endcap detail
(supercrystals)
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
The electromagnetic endcap
• Basic unit –Supercrystal 55 array of crystals Carbon-fibre alveolar
• PbWO4 crystals Radiation hard X0 = 8.9 mm
Mechanically fragile Fast scintillation
90% light in 100ns Low light yield
~50 /MeV
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Challenges for ECAL photodetector
• High radiation environment Dose is strong function of rapidity (ie angle to p
beams) Barrel: Up to 4 kGy in 10 years LHC running Endcap: 4-200 kGy
• Fast response required LHC beam crossing time = 25ns
• Low light yield from PbWO4
~50 photons/MeV need internal gain
• CMS choices Barrel – Avalanche PhotoDiodes Endcap – Vacuum PhotoTriodes (VPTs)
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Structure of VPT
• Single-stage photomultiplier
• Gain ~ 10
Light
0V
1000V
800V
Photocathode
Grid anode
Dynode
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Specification for CMS VPTs
• Quantum efficiency p15% at 420nm (mean of delivered VPTs =
22%)
• Gain g at zero magnetic field7 (mean of delivered VPTs = 10.3)
• Radiation tolerance<10% loss in output after 20 kGy
• Magnetic field responseLoss in yield at 4T < 15% with respect to 0T
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
VPT delivery schedule
• Manufactured by Research Institute Electron, St Petersburg, Russia Developed in collaboration with PNPI Gatchina, Russia
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Measurements of VPT characteristics
• Two UK testing facilities Up to 1.8T at RAL
All VPTs, variable angle 4T system at Brunel Univ
Sample testing, fixed angle
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Sensitivity to operating voltage
• Typical variation of gain with applied voltageOperating voltage in CMS: Va=1000V, Vd=800V
Close to plateau
0
2
4
6
8
10
12
0 200 400 600 800 1000
Dynode Voltage
Ga
in
V(A)=1000V
V(A)=800V
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Radiation tolerance
dose varies strongly with rapidity
• All VPT faceplateglass tested to 20 kGy at Brunel University
• Glass batch accepted if <10% transmission loss (convoluted over PbWO4 spectrum) after 20kGy
Nb-doped Lead tungstate
0.000
0.050
0.100
0.150
0.200
0.250
300 350 400 450 500 550 600 650
Wavelength (nm)
Rela
tive e
mis
sio
n i
nte
nsit
y
YT-49 Batch 30832a28.2 kGy dose
-0.02
0
0.02
0.04
0.06
0.08
0.1
0.12
300 400 500 600 700 800
Wavelength (nm)
Ind
uc
ed
ab
so
rba
nc
e
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Photocathode response
• Uniformity of photocathode response Measured by
colleagues in Split, Croatia
Flat response over photocathode area
• Quantum efficiency typically 22% Thanks are due to N.Godinovic, I.Puljak, and
I.Soric, University of Split, Croatia, for the use of this data.
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
VPT response in magnetic field
• Characteristic response of VPT vs angle at B = 1.8T vs magnetic field at angle of 15 nb: flat response v field above 1T
8-24 in CMS
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Measured yield in 1.8T field
• Measured yield In 1.8T fieldMean over 8-24Converted to
electrons/MeV
• VPTs meet CMS requirements
• Good correlation with measurements at 0T and 4T
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Summary and conclusion
• 6200 (of total 15500) VPTs delivered to dateDelivery follows agreed schedule closely
• Measured performance of production devices meets CMS requirementsAll VPTs tested at 1.8TSample testing (about 10%) at 4T
• Radiation tolerance assured by testing (in UK) of all faceplate glass batches
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Measurements in different B fields
• Manufacturers measure p & g at 0Tpg well correlated
with yield at 1.8T
• Sample testing at 4T using Brunel Univ superconducting solenoidGood correlation
with 1.8T data
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Relative gain at 4T
Production tubes
0
10
20
30
40
50
60
70
0.8 0.85 0.9 0.95 1 1.05 1.1 >1.1
Relative 4T/0T pulsed gain (upper bin edge)
Nu
mb
er
in b
in
Passed
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
Dose rate and neutron fluence in CMS
0.20.350.5
3
2050
1.22
5
70
HCAL Barrel
ECAL Barrel
ECAL Endcap
10-year dose in italicsNeutron fluence in red