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,

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


The CMS Experiment

ECAL

Tracker

HCAL

4T solenoid

Muon chambers


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)


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


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)


Structure of VPT

• Single-stage photomultiplier

• Gain ~ 10

Light

0V

1000V

800V

Photocathode

Grid anode

Dynode


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


VPT delivery schedule

• Manufactured by Research Institute Electron, St Petersburg, Russia Developed in collaboration with PNPI Gatchina, Russia


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


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


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


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.


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


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


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



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


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


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

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,

Documents

cms requirementsall

t datarelative gain

t fieldmean

tsample testing

gain g

uk brunel university

brunel univ sample testing

t fieldmeasured yieldin