Transcript The Lead Tungstate Calorimeter for CMS

The response to high magnetic
fields of the Vacuum Phototriodes
for the Compact Muon Solenoid
endcap electromagnetic calorimeter
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
Poster presented at the 3rd Beaune Conference on New Developments in Photodetection
Beaune, 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
Electromagnetic
Calorimeter
Superconducting coil
Total mass :
12,500t
Overall Diameter: 15.0m
Overall Length:
21.6m
Magnetic field:
4T
7 TeV protons
ECAL design objectives
Benchmark 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%
Constant term, b:
5.7%
(non-uniformities, shower leakage)
0.55%
Noise term, c:
0.55%
(Electronic noise, event pile-up)
Low L
High L
155 MeV 205 MeV
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
12
10
Gain
8
6
V(A)=1000V
V(A)=800V
4
2
0
0
200
400
600
Dynode Voltage
800
1000
RIE
production
VPT
Principles of VPT operation
Light
Primary
photoelectron
Photocathode
10 µm pitch mesh anode (1000V)
Dynode (800V)
Dynode gain is ~ 20 but collection
efficiency is about 50%
Typical tube gain is ~10
Magnet characterisation
Multiple VPT holders inserted into
full field of warm iron magnet
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
4.0T Superconducting
solenoid at Brunel
B =4.0T and  = 15o at Brunel
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
IEEE 488.2
HV Anode
HV Dynode
SR PS310
SR PS310
PC
NT4 WorkStn
Cathode current
Anode current
RS 232
NI 6033E
Step Motor
Maclennan
GPIB Bus
Isolator
DVM
(General)
Keithley
2000
NI GBIB120A
Keithley
2700
Floating unit
1.5 kV
maximum
DAQ Card
PCI
bus
DVM
(Floating)
VPT and
Pre-amp
Non-Floating
0.5 kV
maximum
Automated VPT tester
at Brunel. Mainly uses
IEEE488.2 instruments.
ADC is mulitplexed and
PCI based.
Response vs Angle at B=1.8T
Very reproducible
distribution
1.2
Rel. Anode Response
1.0
0.8
Arrows indicate
angular regions
of end caps
0.6
0.4
0.2
0.0
-90
-60
-30
0
30
VPT angle (deg.)
60
90
Response vs B-Field Strength
VPT Axis at 15o angle
to the magnetic field
Typical magnetic response
Rel. Anode Response
1.2
1.0
Note: the precise details of
how much reduction in relative
response depends on the
uniformity of photocathode
illumination.
0.8
0.6
0.4
0.2
0.0
0
0.5
1
1.5
Magnetic Field (Tesla)
2
Response at 4T and 15°
Relative Pulsed Gain
Distribution of norm a lise d diffe re nce s for tube s m e a sure d tw ice
Production tubes
10
Mean = -0.0055
Std Dev = 0.027
9
8
N umber in bi n
18
16
14
Fail
7
6
5
4
3
2
Number in bin
1
0
12
-0.1
-0.075
-0.05
-0.025
0
0.025
0.05
0.075
0.1
Normalise d diffe re nce s
10
Measurement repeatability
Of 4T/0T gain ratio on
pre-production tubes was good
8
6
4
2
0
< 0.8
0.85
0.9
0.95
1
1.05
Relative 4T/0T pulsed gain (upper bin edge)
> 1.05
Anode Response Distribution
Data plotted to show
expected response to
scintillation light as a
function of incident
electron energy on the
PbWO4 crystal
Production tubes which
will be used in CMS
Mean anode pulse height over the angular range 8-25 in a 1.8T magnetic field.
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).