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

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

The Compact Muon Solenoid Experiment at the CERN LHC 7 TeV protons B=4T Superconducting coil Electromagnetic Calorimeter Total mass : 12,500t Overall Diameter: 15.0m Overall Length: 21.6m Magnetic field: 4T 20-May-04 Ken Bell - CCLRC Rutherford Appleton Laboratory

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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