K W Bell1, R M Brown1, D J A Cockerill1, P S Flower1, P R Hobson2, D C Imrie2, B W Kennedy1, A L Lintern1, O Sharif2, M Sproston1, J H Williams1

1CLRC - Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX UK

2Brunel University, Department of Electronic and Computer Engineering, Uxbridge, UB8 3PH, UK

The response to high magnetic fields of the Vacuum Phototriodes for the Compact Muon Solenoid endcap electromagnetic calorimeter

Poster presented at the 3rd Beaune Conference on New Developments in PhotodetectionBeaune, France, 17-21 June, 2002

Motivation• The endcap electromagnetic calorimeter of the Compact

Muon Solenoid detector at the LHC needs fast, radiation tolerant photodetectors with moderate gain.

• Neutron radiation levels, particularly as , are too high for 10 year operation of silicon photodetectors.

• The photodetectors can be closely aligned with the axial 4T magnetic field inside CMS.

• Photodetectors need to cover the end face of lead tungstate scintillator crystals, these will allow 26 mm diameter devices.

• Lead tungstate emission spectrum is well matched to bialkali photocathodes.

Solution: Vacuum Phototriodes (VPT)

Compact Muon Solenoid

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

Electromagnetic Calorimeter

Superconducting coil7 TeV protons

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)

Vacuum PhotoTriodes

•Endcap 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

0

2

4

6

8

10

12

0 200 400 600 800 1000

Dynode Voltage

Gai

n

V(A)=1000V

V(A)=800V

RIEproductionVPT

Principles of VPT operation

Light

Photocathode

10 µm pitch mesh anode (1000V)Dynode (800V)

Primaryphotoelectron

Dynode gain is ~ 20 but collectionefficiency is about 50%Typical tube gain is ~10

Magnet characterisation

4.0T Superconducting solenoid at Brunel

1.8T Dipole Magnet at RAL

All VPTs are measured at

0 B 1.8T and -30o 30o at RAL

A sample of VPTs are measured at

B =4.0T and = 15o at Brunel

Multiple VPT holders inserted intofull field of warm iron magnet

0 to 1.8T System at RAL

•Tubes normally measured at angles from -30° to +30°

•Relative response measured at fields from 0T to 1.8T

•Noise and dark currents measured too.

•Great care taken to ensure uniform photocathode illumination with blue LED

•Multiple tubes measured simultaneously (currently 24 per run)

•DAQ system is RS232 based and uses LabView to provide the control and user interface

•ADC system is CAMAC based

RS 232 for most functions - slow, but cheap and easy

4T System at Brunel

Floating unit1.5 kVmaximum

HV Anode

SR PS310

HV Dynode

SR PS310

DVM(General)

Keithley2700

GPIB BusIsolator

NI GBIB-120A

DVM(Floating)

Keithley2000

PCNT4 WorkStn

IEEE 488.2

Non-Floating0.5 kVmaximumDAQ Card

NI 6033E

Step Motor

Maclennan

PCI bus

RS 232

Automated VPT testerat Brunel. Mainly uses IEEE488.2 instruments.ADC is mulitplexed and PCI based.

VPT andPre-amp

Anode current

Cathode current

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

Very reproducibledistribution

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 angle to the magnetic field

Note: the precise details ofhow much reduction in relativeresponse depends on theuniformity of photocathodeillumination.

Response at 4T and 15°

Production tubes

0

2

4

6

8

10

12

14

16

18

< 0.8 0.85 0.9 0.95 1 1.05 > 1.05

Relative 4T/0T pulsed gain (upper bin edge)

Nu

mb

er

in b

in

Fail

Relative Pulsed GainDistribution of normalised differences for tubes measured twice

0

1

2

3

4

5

6

7

8

9

10

-0.1 -0.075 -0.05 -0.025 0 0.025 0.05 0.075 0.1

Normalised differences

Nu

mb

er

in b

in

Mean = -0.0055Std Dev = 0.027

Measurement repeatabilityOf 4T/0T gain ratio on pre-production tubes was good

Anode Response Distribution

Mean anode pulse height over the angular range 8-25in a 1.8T magnetic field.

Data plotted to showexpected response toscintillation light as a function of incidentelectron energy on thePbWO4 crystal

Production tubes whichwill be used in CMS

Summary A new generation of fine-mesh VPTs has been

developed to satisfy the high magnetic field/radiation hardness requirements of CMS operating at the Large Hadron Collider, CERN

An automated characterisation facility based at Brunel and RAL has been commissioned to handle 15000 devices over three years

The performance of over 800 of the 1400 production VPTs from RIE has been measured to date.

Nearly all tubes pass the acceptance tests.

Only one tube passing the 1.8T tests failed at 4T

We would like to acknowledge support from PPARC (UK) and INTAS (EU).