Post on 01-Jan-2016
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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
220-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
320-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
420-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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)
520-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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)
620-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
720-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
820-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
920-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
1020-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
1120-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
1220-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
1320-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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)
1420-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
1520-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
1620-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
Back-up Slides…
1720-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
1820-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
1920-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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)
2020-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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
2120-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory
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