1 20-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory Vacuum Phototriodes for the CMS...

120-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory

Vacuum Phototriodes for the CMS Electromagnetic Calorimeter Endcap

Ken Bell, R.M.Brown, D.J.A.Cockerill, P.S.Flower, P.R.Hobson, B.W.Kennedy, A.L.Lintern, C.W.Selby,

O.Sharif, M.Sproston, J.H.Williams

CCLRC Rutherford Appleton Laboratory, Didcot, UKBrunel University, Uxbridge, UK

IEEE IMTC Conference – Como – May 2004


Outline

Introduction to CMS Electromagnetic Calorimeter Endcap

Description of CMS Vacuum Phototriodes Measurements of VPT Performance in a Magnetic Field Ensuring the Radiation Tolerance Summary


The Compact Muon Solenoid Experiment at the CERN LHC

7 TeV protons

7 TeV protons

B=4T Superconducting coil

Electromagnetic Calorimeter

Total mass : 12,500tOverall Diameter: 15.0mOverall Length: 21.6mMagnetic field: 4T


The CMS Electromagnetic Calorimeter

Hermetic calorimeter

of scintillating crystalsQuasi-Pointing geometry

PbWO4 crystals (~430nm)

61,200 in barrel

14,648 in endcapsLength: 6 mDiameter: 3.5 m

Depth: ~25 X0

Pb/Si preshower detector

in front of endcap crystalsTarget energy resolution

<1% at E = 100 GeV

Endcap geometry based on array of identical 5x5 modules (supercrystals)


The Endcap Electromagnetic Calorimeter

Basic endcap unit – Supercrystal 55 array of tapered crystals,

each ~3030220 mm3

Carbon-fibre alveolar support

PbWO4 crystals

Dense (X0 = 8.9 mm) Radiation hard Fast scintillator

(90% of light in 100 ns) Mechanically fragile Low light yield

(~50 photons / MeV)


Challenges for the Photodetectors

Operation in magnetic field of 4T High radiation environment

Dose is strong function of angle wrt incoming proton beams Barrel: Up to 4 kGy and 1013 n cm-2 in 10 years of LHC running Endcap: 4–200 kGy and up to 1015 neutrons cm-2

Fast response required LHC beam crossing time = 25 ns

Low light yield from PbWO4

~50 photons / MeV need photodetectors with internal gain CMS choices

Barrel – Avalanche PhotoDiodes Endcap – Vacuum PhotoTriodes (VPTs) Both custom developed for CMS, in collaboration with industry

VPTs previously developed in HEP for the endcap electromagnetic calorimeters of OPAL and DELPHI at LEP, but radiation levels at LEP much lower and eg OPAL magnetic field 10 lower


Vacuum Phototriodes for CMS from Research Institute Electron (St Petersburg)

Light

0V

1000V

800V

Photocathode

Grid anode

Dynode

Bi-alkali

10um pitch mesh Anode Mesh transparency ~50%

Single-gain-stage mesh photomultiplier

No EB effects if VPT axisaligned with magnetic field


Vacuum Phototriodes for CMS from Research Institute Electron (St Petersburg)

Quantum efficiency typically 22% at 430nm, flat over photocathode area

Mean VPT gain is 10.2 at B=0T

Pre-production order: 500 Production order: 15,000 VPTs Delivery spread over 4 years 7,900 devices presently

delivered and tested at B=1.8T


Automated VPT Characterisation in a Magnetic Field via 420nm LEDs

Every VPT is measured at

0 B 1.8T and -30o 30o at RAL A 10% sample of VPTs are measured at

B = 4.0T and = 15o at Brunel Dark currents and noise also measured Reproducibility of measurements 2%

1.8T Dipole Magnet at RAL4.0T Superconducting

Solenoid at Brunel


VPT Response v/s Angle at B=1.8T

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-90 -60 -30 0 30 60 90VPT angle (deg.)

Re

l. A

no

de

Re

sp

on

se

Arrows indicate angular coverage of CMS end caps

VPT response is maintained over -40o 40o

VPT axis in CMS: 8o < || < 24o wrt to magnetic field (pointing geom)

For VPT axis -30o 30o, EB effects and probability of electron capture on anode grid characteristic periodic behaviour


VPT Response v/s Magnetic Field Strength

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

VPT axis at angle of 15o to the magnetic field

Response ~flat for B>0.8T Satisfactory performance

at B=1.8T is very reliable indicator that VPT will operate well at B=4T

Note: the precise details of the reduction in relative response depends on the uniformity of photocathode illumination


Relative VPT Response B=4T/B=0T at = 15o

Results on 652 VPTs – note logarithmic Y axis ! Mean ratio is 0.95 – only 1 VPT has failed our >0.75 requirement

0.1

1

10

100

1000

<0.7 0.70-0.75

0.75-0.80

0.80-0.85

0.85-0.90

0.90-0.95

0.95-1.00

1.00-1.05

1.05-1.10

1.10-1.15

1.15-1.20

>1.2

Relative 4T/0T Pulsed Gain

Nu

mb

er o

f tu

bes

in b

in


VPT Response v/s Angle at B=4T

1-off hand angle scan of VPT at B=4T Periodicity now 5o (cf was 10o at B=1.8T) Ratio consistent with inverse ratio of magnetic field, as expected

0

10

20

30

40

50

60

0 5 10 15Angle (deg)

VP

T r

es

po

ns

e (

arb

itra

ry u

nit

s)


Ensuring the Radiation Tolerance

Have verified that VPTs are resistant to 1015 n cm-2 fluence expected

VPT faceplates made of rad-hard UV-transmitting borosilicate glass, manufactured in small batches (concern of batch-to-batch variation)

Before each batch is certified for use, several faceplates are irradiated to 20kGy using Co60 source at Brunel. Transmission loss (convoluting with PbWO4 emission spectrum) required to be <10%

Also important that instantaneous dose & neutron fluence don’t upset the performance of the VPTs

Operated VPTs at Co60 source at Brunel and at Cf252 neutron source at University of Minnesota

Saw increase in noise, attributed to photo-electrons liberated by Cerenkov light from relativistic electrons traversing the faceplate

Scaling the rates to the LHC, the effects should be negligible, except very close to the beam pipe


Summary

We have developed a new generation of fine-mesh VPTs to meet the demanding requirements of CMS Endcap ECAL

CMS requires 15,000 VPTs. 7,900 devices already delivered

Performance of all delivered VPTs already measured at B=1.8T at RAL >10% of VPTs also measured at B=4T at Brunel These measurements correlate well, confirming that VPTs which pass

at B=1.8T will operate satisfactorily in CMS at B=4T

VPT radiation tolerance ensured by testing all batches of faceplates Have verified that instantaneous dose and fluence in CMS will not

significantly degrade the VPT performance


Back-up Slides…


Radiation Environment

0.20.350.5

3

2050

1.22

5

70

HCAL Barrel

ECAL Barrel

ECAL Endcap

10-year dose in italics (black: at shower maximum inside the crystals) Neutron fluence in red


VPT Photocathode Response

Uniformity of

photocathode response Measured by CMS colleagues

in Split, Croatia Response flat response over

photocathode area

Quantum efficiency

typically 22% at 430nm

With thanks to N.Godinovic, I.Puljak, and I.Soric, University of Split, Croatia, for the use of this data


Comparisons of B=0 and B=1.8Tand of B=1.8T and B=4T

B=4T at Brunel (Y) v/s B=1.8T at RAL (X)

B=1.8T at RAL (Y) v/s B=0 at Research Institute Electron (X)


VPT Gain as a function of applied HV

Operating 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

Gai

n

V(A)=1000V

V(A)=800V


Ensuring the Gamma Radiation Tolerance

Gamma dose varies strongly across the endcaps

Samples from all batches of VPT faceplate glass first tested to 20 kGy at Brunel

Glass batch only 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)

Rel

ativ

e em

issi

on

in

ten

sity

Typical PbWO4

emission spectrum

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

Typical VPT faceplate

absorbance spectrum

1 20-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory Vacuum Phototriodes for the CMS...

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