1. B W Kennedy, CCLRC Rutherford Appleton Laboratory Vacuum Phototriodes for the CMS Electromagnetic Calorimeter Endcaps K.W.Bell, R.M.Brown, D.J.A.Cockerill, P.S.Flower,P.R.Hobson, B.W.Kennedy, A.L.Lintern, C.S.Selby, O.Sharif, M.Sproston, J.H.Williams CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, UK Brunel University, Uxbridge, UK

2. B W Kennedy, CCLRC Rutherford Appleton Laboratory The CMS Experiment ECAL Tracker HCAL 4T solenoid Muon chambers

3. B W Kennedy, CCLRC Rutherford Appleton Laboratory The CMS Electromagnetic calorimeter PbWO 4 crystals ~61000 in barrel ~15000 in endcaps Pointing geometry Length: 6m Radius: 1.75m Depth: 24-25 X 0 Target energy resolution: <1% at E = 100 GeV Endcap detail (supercrystals)

4. B W Kennedy, CCLRC Rutherford Appleton Laboratory The electromagnetic endcap Basic unit –Supercrystal  5  5 array of crystals  Carbon-fibre alveolar PbWO 4 crystals  Radiation hard  X 0 = 8.9 mm  Mechanically fragile  Fast scintillation  90% light in 100ns  Low light yield  ~50  /MeV

5. B W Kennedy, CCLRC Rutherford Appleton Laboratory Challenges for ECAL photodetector High radiation environment  Dose is strong function of rapidity (ie angle to p beams)  Barrel: Up to 4 kGy in 10 years LHC running  Endcap: 4-200 kGy Fast response required  LHC beam crossing time = 25ns Low light yield from PbWO 4  ~50 photons/MeV  need internal gain CMS choices  Barrel – Avalanche PhotoDiodes  Endcap – Vacuum PhotoTriodes (VPTs)

6. B W Kennedy, CCLRC Rutherford Appleton Laboratory Structure of VPT Single-stage photomultiplier Gain ~ 10 Light 0V 1000V 800V Photocathode Grid anode Dynode

7. B W Kennedy, CCLRC Rutherford Appleton Laboratory Specification for CMS VPTs Quantum efficiency p   15% at 420nm (mean of delivered VPTs = 22%) Gain g at zero magnetic field   7 (mean of delivered VPTs = 10.3) Radiation tolerance  <10% loss in output after 20 kGy Magnetic field response  Loss in yield at 4T < 15% with respect to 0T

8. B W Kennedy, CCLRC Rutherford Appleton Laboratory VPT delivery schedule Manufactured by Research Institute Electron, St Petersburg, Russia  Developed in collaboration with PNPI Gatchina, Russia

9. B W Kennedy, CCLRC Rutherford Appleton Laboratory Measurements of VPT characteristics Two UK testing facilities  Up to 1.8T at RAL  All VPTs, variable angle  4T system at Brunel Univ  Sample testing, fixed angle

10. B W Kennedy, CCLRC Rutherford Appleton Laboratory Sensitivity to operating voltage Typical variation of gain with applied voltage  Operating voltage in CMS: V a =1000V, V d =800V  Close to plateau

11. B W Kennedy, CCLRC Rutherford Appleton Laboratory Radiation tolerance  dose varies strongly with rapidity All VPT faceplate glass tested to 20 kGy at Brunel University Glass batch accepted if <10% transmission loss (convoluted over PbWO 4 spectrum) after 20kGy

12. B W Kennedy, CCLRC Rutherford Appleton Laboratory Photocathode response Uniformity of photocathode response  Measured by colleagues in Split, Croatia  Flat response over photocathode area Quantum efficiency typically 22% Thanks are due to N.Godinovic, I.Puljak, and I.Soric, University of Split, Croatia, for the use of this data.

13. B W Kennedy, CCLRC Rutherford Appleton Laboratory VPT response in magnetic field Characteristic response of VPT  vs angle at B = 1.8T  vs magnetic field at angle of 15   nb: flat response v field above 1T 8  -24  in CMS

14. B W Kennedy, CCLRC Rutherford Appleton Laboratory Measured yield in 1.8T field Measured yield  In 1.8T field  Mean over 8  -24   Converted to electrons/MeV VPTs meet CMS requirements Good correlation with measurements at 0T and 4T

15. B W Kennedy, CCLRC Rutherford Appleton Laboratory Summary and conclusion 6200 (of total 15500) VPTs delivered to date  Delivery follows agreed schedule closely Measured performance of production devices meets CMS requirements  All VPTs tested at 1.8T  Sample testing (about 10%) at 4T Radiation tolerance assured by testing (in UK) of all faceplate glass batches

16. B W Kennedy, CCLRC Rutherford Appleton Laboratory

17. Measurements in different B fields Manufacturers measure p & g at 0T  pg well correlated with yield at 1.8T Sample testing at 4T using Brunel Univ superconducting solenoid  Good correlation with 1.8T data

18. B W Kennedy, CCLRC Rutherford Appleton Laboratory Relative gain at 4T

19. B W Kennedy, CCLRC Rutherford Appleton Laboratory Dose rate and neutron fluence in CMS 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 Neutron fluence in red