1. Vacuum Phototriodes for the CMS Electromagnetic Calorimeter Endcap*
07/16/96
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, UK Brunel University, Uxbridge, UK
IEEE IMTC Conference – Como – May 2004 *
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

2. Ken Bell - CCLRC Rutherford Appleton Laboratory*
07/16/96
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 *
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

3. The Compact Muon Solenoid Experiment at the CERN LHC
7 TeV
protons
B=4T Superconducting coil
Electromagnetic Calorimeter
Total mass : ,500t
Overall Diameter: 15.0m
Overall Length: m
Magnetic field: T
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

4. The CMS Electromagnetic Calorimeter
Hermetic calorimeter
of scintillating crystals
Quasi-Pointing geometry
PbWO4 crystals (~430nm)
61,200 in barrel
14,648 in endcaps
Length: 6 m
Diameter: 3.5 m
Depth: ~25 X0
Pb/Si preshower detector
in front of endcap crystals
Target energy resolution
<1% at E = 100 GeV
Endcap geometry based on array of identical 5x5 modules (supercrystals)
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

5. The Endcap Electromagnetic Calorimeter
Basic endcap unit – Supercrystal
55 array of tapered crystals, each ~3030220 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)
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

6. 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
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

7. Vacuum Phototriodes for CMS from Research Institute Electron (St Petersburg)
Single-gain-stage mesh photomultiplier
Light 0V
1000V
800V
Photocathode
Grid anode
Dynode
Bi-alkali
No EB effects if VPT axis
aligned with magnetic field
10um pitch mesh Anode
Mesh transparency ~50%
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

8. Ken Bell - CCLRC Rutherford Appleton Laboratory
Vacuum Phototriodes for CMS from Research Institute Electron (St Petersburg)
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
Quantum efficiency typically 22% at 430nm, flat over photocathode area
Mean VPT gain is 10.2 at B=0T
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

9. 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%
4.0T Superconducting Solenoid at Brunel
1.8T Dipole Magnet at RAL
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

10. VPT Response v/s Angle at B=1.8T
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, EB effects and probability of electron capture on anode grid  characteristic periodic behaviour
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

11. VPT Response v/s Magnetic Field Strength
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
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

12. 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
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

13. 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
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

14. 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
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

15. 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
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

16. Ken Bell - CCLRC Rutherford Appleton Laboratory
Back-up Slides…
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

17. Radiation Environment
0.2
0.35
0.5 3 20 50
1.2 2 5 70
HCAL Barrel
ECAL Barrel
ECAL Endcap
10-year  dose in italics (black: at shower maximum inside the crystals)
Neutron fluence in red
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

18. 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
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

19. Comparisons of B=0 and B=1.8T and of B=1.8T and B=4T
B=1.8T at RAL (Y) v/s B=0 at Research Institute Electron (X)
B=4T at Brunel (Y) v/s B=1.8T at RAL (X)
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

20. VPT Gain as a function of applied HV
Operating voltage in CMS: VA=1000V, VD=800V
Close to plateau
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory

21. Ensuring the Gamma Radiation Tolerance
Typical PbWO4
emission spectrum
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
Typical VPT faceplate
absorbance spectrum
20-May-04
Ken Bell - CCLRC Rutherford Appleton Laboratory