HEP2001, Budapest, July 2001 R M Brown - RAL 1

The Vacuum Phototriodes for the CMS Electromagnetic Calorimeter

P R Hobson, D C Imrie, O Sharif

Brunel University, UK

K W Bell, R M Brown, D J A Cockerill, P S Flower, B W Kennedy, A L Lintern, M Sproston, J H Williams

CLRC - Rutherford Appleton Laboratory, UK

(With acknowledgements to H F Heath and colleagues at Bristol University, UK and D Seliverstov and colleagues at PNPI, Russia)

HEP2001Budapest - Hungary

July 2001

HEP2001, Budapest, July 2001 R M Brown - RAL 2

Outline of Talk Overview of CMS The Electromagnetic Calorimeter (ECAL) Properties of Lead Tungstate Radiation levels VPT Performance (End cap) APD Performance (Barrel) Status summary

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Compact Muon Solenoid

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

ECAL

Superconducting coil

HCAL

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ECAL design objectivesBenchmark physics process:

Search for ~130 GeV Higgs via H (Sensitivity depends critically on mass resoln)

m / m = 0.5 [E1/ E1 E2

/ E2 / tan( / 2 )]

Where E / E = a / E b c/ E

Performance Aims: Barrel End cap

Stochastic term, a:(p.e. statistics/shower fluctuation)

2.7% 5.7%

Constant term, b:(non-uniformities, shower leakage)

0.55% 0.55%

Noise term, c:(Electronic noise, event pile-up)

Low L 155 MeV 205 MeV

High L 210 MeV 245 MeV

(Angular resolution limited by uncertainty in position of interaction vertex)

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Lead Tungstate PropertiesAdvantages:• Fast• Dense• Radiation hard• Emission in visible

Disadvantages:• Temperature dependence• Low light yield Photodetector with gain (in a strong magnetic field)

PWO Radioluminescence Spectrum

0

5

10

15

20

340 380 420 460 500 540 580 620

Wavelength (nm)

Inte

ns

ity

Density [g/cm3] 8.28

Radn length,X0 [mm] 8.9

Interaction length [mm] 224

Molière radius [mm] 21.9

Decay time [ns] 5(39%)15(60%)100 (1%)

Refractive index 2.30

Max emission [nm] 425

Light yield [photon/MeV] ~50

Temp coeff [%/ºC] -2

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CMS ECAL Layout

3170 mm

1290 mm

Parameter Barrel End caps

Xtal size (mm3)Depth in X0

21.8 × 21.8 × 23025.8

30.0 × 30.0 × 22024.7

No. crystalsVolume (m3)Xtal mass (t)

612008.1467.4

146642.7722.9

Full projective geometry

(‘Off-pointing’ by 3o)

Barrel: 17x2 Crystal types

End cap: 1 Crystal type

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Doses and neutron fluences

Integrated dose (kGy) and neutron fluence (x1013 cm-2) for L = 5x105 pb-1 (~10 yrs)

Black: Dose in the Crystals at the position of the shower maximumBlue: Dose behind the crystals at the position of the photodetectorsRed: Neutron fluences behind the crystals

0.20.350.5

3

2050

1.22

5

70

HCAL Barrel

ECAL Barrel

ECAL Endcap

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Photodetectors: end caps

•B-field orientation favourable for VPTs (Axes: 8.5o < || < 25.5o wrt to field)

•More radiation hard than Si diodes (with UV glass window)

• Gain 8 -10 at B = 4 T

• Active area of ~ 280 mm2/crystal

• Q.E. ~ 20% at 420 nm

= 26.5 mm

MESH ANODE

Vacuum Phototriode (VPT):Single stage photomultiplier tube with fine metal grid anode

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VPT Gain vs Dynode Voltage

0

2

4

6

8

10

12

0 200 400 600 800 1000

Dynode Voltage

Gai

n

V(A)=1000V

V(A)=800V

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VACUUM PHOTOTRIODE

HV FILTERING ELECTRONICS

CRYSTAL

‘Supercrystal’ Layout

‘Supercrystal’: carbon-fibre alveola containing 5x5 tapered crystals + VPTs + passive HV filter (160 Identical Supercrystals per Dee)

Signals fed via 600 mm cable to Preamplifier + Front End electronics behind Dee Backplate

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Characterisation of VPTs

4.0T Solenoid at Brunel 1.8T Dipole Magnet at RAL

Detail of RAL test Cell

Perspex diffuser plate with LEDs at corners. (Red circle indicates effective VPT diameter)

All VPTs are measured at 0 B 1.8T and -30o 30o at RALSample VPTs are measured at B =4.0T and = 15o at Brunel

500 ‘Preproduction’ VPTs delivered by RIE (St Petersburg)

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Response vs 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.)

Rel

. An

od

e R

esp

on

se

Arrows indicate angular regions of

end caps

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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

Response vs B-Field Strength

VPT Axis at 15o w.r.t. Magnetic Field

Gain Ratio (4T/0T)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

1 2 3 4 5 6 7 8 9 10 11 12 13

Relative Gain

Nu

mb

er

pe

r b

in

Batch 101-300

0.80 0.85 0.90 0.95 1.00 1.05 1.10

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Distributions of Gain (B=0)

and Quantum Efficiency

0.0

5.0

10.0

15.0

20.0

15.0 17.5 20.0 22.5 25.0 27.5 30.0

Quantum Efficiency (%)

Gai

n

Batch 301-500

0

20

40

60

80

100

120

140

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Gain

Batch 301-500

Batch 101-300

6 8 10 12 14 16 18 20

Gain and quantum efficiency are uncorrelated0

10

20

30

40

50

60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Quantum Efficiency

Batch 301-500

Batch 101-300

16 18 20 22 24 26 28 30 (%)

Taken from the ‘passport’ supplied with each tube

by the manufacturer

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0

5

10

15

20

25

30

35

40

45

50

1 11 21 31 41

Anode pulse height (Arb. unit)

Post-irradation

Prototypes

Batch 301-500

Batch 101-300

Batch 1-100

0 10 20 30 40 50 60 70 80 90 100

Anode Response Distribution

B=1.8T

=15o

Spread in anode response Some sorting of VPTs necessary

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Test beam:Energy Resolution

No preshower detector With preshower detector

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0

1

2

3

4

5

350 375 400 425 450 475 500 525 550 575 600 625 650 675 700Wavelength (nm)

Tran

smis

sio

n L

oss

(%

)

Sample 1

Sample 2

Faceplate optical transmission Post-irradiation

Loss in optical transmission of 2 faceplate samples

after 25 kGy 60Co irradiation (380Gy/hour)

(approx 10 yrs LHC at = 2.6)

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VPT Behaviour Under Irradiation

60Co Irradiation (58 Gy/hr)

Photocurrent produced by Cerenkov light in VPT window.(Vertical lines correspond to pauses in irradiation)

0.0

5.0

10.0

15.0

0 5000 10000 15000 20000

Dose (Gy)

Ph

oto

curr

ent

(nA

)

CathodeAnode

Missing Data

VA=1000 VD= 800

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VPT Summary

A new generation of fine-mesh VPTs has been developed to satisfy the high magnetic field/radiation hardness requirements of CMS

An automated characterisation facility has been commissioned to handle 15000 devices

The performance of 500 pre-production VPTs from RIE meets CMS requirements