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

B W Kennedy, CCLRC Rutherford Appleton Laboratory

The CMS Experiment

ECAL

Tracker

HCAL

4T solenoid

Muon chambers

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

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

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)

B W Kennedy, CCLRC Rutherford Appleton Laboratory

Structure of VPT

• Single-stage photomultiplier

• Gain ~ 10

Light

0V

1000V

800V

Photocathode

Grid anode

Dynode

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

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

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

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

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

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.

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

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

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

B W Kennedy, CCLRC Rutherford Appleton Laboratory

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

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

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