b w kennedy, cclrc rutherford appleton laboratory vpts for the cms experiment b w kennedy cclrc...
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B W Kennedy, CCLRC Rutherford Appleton Laboratory
VPTs for the CMS experiment
B W KennedyCCLRC Rutherford Appleton Laboratory
St Petersburg, 26 April 2005
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Outline of talk
1. Description of CMS Electromagnetic Calorimeter
2. Experimental conditions and challenges3. Selection of Vacuum PhotoTriodes (VPTs)4. Experience with VPTs from RIE5. Conclusion
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”)
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Details of the SuperCrystal
Basic unit –Supercrystal55 array of crystalsCarbon-fibre alveolar
PbWO4 crystalsRadiation hardX0 = 8.9 mm
Fast scintillation 90% light in
100ns
Mechanically fragile Low light yield
~50 /MeV
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Challenges for ECAL photodetector
High radiation environmentDose is strong function of angle to proton beamsBarrel: Up to 4 kGy in 10 years of LHC runningEndcap: 4-200 kGy in 10 years
Fast response requiredInterval between LHC beam crossings is only 25ns
Low light yield from PbWO4
Approximately 50 photons/MeV need device with gain
CMS choicesBarrel – Avalanche PhotoDiodesEndcap – Vacuum PhotoTriodes (VPTs)
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Structure of Vacuum PhotoTriode
Single-stage photomultiplier
Gain ~ 10
Light
0V
1000V
800V
Photocathode
Grid anode
Dynode
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Specification for CMS VPTs
Radiation toleranceLess than 10% loss in output after 20 kGy
Quantum efficiency pp 15% for light of wavelength 420nm
Gain g at zero magnetic fieldg 7
Magnetic field responseLoss in yield at 4T < 15% with respect to 0TStable (10%) at angles up to 26° to magnetic
field
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Selection of VPTs
Prototypes supplied by several manufacturersElectron TubesHamamatsuMeltzPhotonisResearch Institute Electron
Testing and evaluation up to 1999Most manufacturers were able to meet
specifications
Tendering for contract in 1999Successful tender from RIE to supply 15500 VPTs
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Typical response of prototype VPTs
Typical magnetic response
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 0.5 1 1.5 2
Magnetic Field (Tesla)
Re
l. A
no
de
Re
sp
on
se
0.0
0.2
0.4
0.6
0.8
1.0
1.2
-90 -60 -30 0 30 60 90VPT angle (deg.)
Rel.
An
od
e R
esp
on
se
VPT response vs angle in 1.8T magnetic field
VPT response vs field at 15°
0
2
4
6
8
10
12
0 200 400 600 800 1000
Dynode Voltage
Gai
n
V(A)=1000V
V(A)=800V
Variation of gain with applied voltage
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Radiation tolerance of faceplate glass
dose varies strongly with angle.
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
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Delivery of VPTs
VPT progress
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
01/01/2000 01/01/2001 01/01/2002 01/01/2003 01/01/2004 01/01/2005 01/01/2006 01/01/2007
Date
Nu
mb
er
of
VP
Ts
Total deliveredTotal scheduledTested at 1.8TTested at 4T
B W Kennedy, CCLRC Rutherford Appleton Laboratory
VPT testing procedure
Two UK testing facilitiesUp to 1.8T at RAL
All VPTs, variable angle
4T system at Brunel University Sample testing (10%), fixed angle
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Measurements on 10000 VPTs
Response measured at RAL in 1.8T magnetic field.
Consistent VPT response from batch to batch.
Correlation between 0T measurements at RIE and 1.8T measurements at RAL
B W Kennedy, CCLRC Rutherford Appleton Laboratory
4T to 0T relative response
Measured at Brunel University. VPT at 15° to magnetic field.
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Resolution of problems
Regular meetings between RIE and CMS groups at CERN, RAL, RIE.
Some problems inevitable in long production run.Most significant – discharges
seen in small fraction VPTs.Exchange of information
between CMS groups and RIE, and careful investigation by RIE experts
improvement in manufacturing procedure to reduce number of VPTs with discharges.
VPT 790
Initial spike
Example of discharge in VPT response
Fraction of VPTs showing discharge
Change to procedure
B W Kennedy, CCLRC Rutherford Appleton Laboratory
Conclusions
• VPTs supplied by RIE meet needs of the CMS experiment.
• Delivery of VPTs is on schedule.• Regular and open meetings between CMS
and RIE enable problems to be solved.• We look forward to the successful
conclusion of the contract in February 2006.