1. HEP2001, Budapest, July 2001 R M Brown - RAL 1 The Vacuum Phototriodes for the CMS Electromagnetic Calorimeter P R Hobson, D C Imrie, O Sharif Brunel University, UK K W Bell, R M Brown, D J A Cockerill, P S Flower, B W Kennedy, A L Lintern, M Sproston, J H Williams CLRC - Rutherford Appleton Laboratory, UK (With acknowledgements to H F Heath and colleagues at Bristol University, UK and D Seliverstov and colleagues at PNPI, Russia) HEP2001 Budapest - Hungary July 2001

2. HEP2001, Budapest, July 2001 R M Brown - RAL 2 Outline of Talk  Overview of CMS  The Electromagnetic Calorimeter (ECAL)  Properties of Lead Tungstate  Radiation levels  VPT Performance (End cap)  APD Performance (Barrel)  Status summary

3. HEP2001, Budapest, July 2001 R M Brown - RAL 3 Compact Muon Solenoid Total mass : 12,500t Overall Diameter: 15.0m Overall Length: 21.6m Magnetic field: 4T ECAL Superconducting coil HCAL

4. HEP2001, Budapest, July 2001 R M Brown - RAL 4 ECAL design objectives Benchmark physics process: Search for ~130 GeV Higgs via H    (Sensitivity depends critically on mass resol n )  m / m = 0.5 [  E 1 / E 1   E 2 / E 2    / 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% 5.7% Constant term, b: (non-uniformities, shower leakage) 0.55% 0.55% Noise term, c: (Electronic noise, event pile-up) Low L 155 MeV 205 MeV High L 210 MeV 245 MeV (Angular resolution limited by uncertainty in position of interaction vertex)

5. HEP2001, Budapest, July 2001 R M Brown - RAL 5 Lead Tungstate Properties Advantages: Fast Dense Radiation hard Emission in visible Disadvantages: Temperature dependence Low light yield  Photodetector with gain (in a strong magnetic field)

6. HEP2001, Budapest, July 2001 R M Brown - RAL 6 CMS ECAL Layout 3170 mm 1290 mm Full projective geometry (‘Off-pointing’ by 3 o ) Barrel: 17x2 Crystal types End cap: 1 Crystal type

7. HEP2001, Budapest, July 2001 R M Brown - RAL 7 Doses and neutron fluences Integrated dose (kGy) and neutron fluence (x10 13 cm -2 ) for  L = 5x10 5 pb -1 (~10 yrs) Black: Dose in the Crystals at the position of the shower maximum Blue: Dose behind the crystals at the position of the photodetectors Red: Neutron fluences behind the crystals 0.2 0.35 0.5 3 20 50 1.2 2 5 70 HCAL Barrel ECAL Barrel ECAL Endcap

8. HEP2001, Budapest, July 2001 R M Brown - RAL 8 Photodetectors: end caps B-field orientation favourable for VPTs (Axes: 8.5 o < |  | < 25.5 o wrt to field) More radiation hard than Si diodes (with UV glass window) Gain 8 -10 at B = 4 T Active area of ~ 280 mm 2 /crystal Q.E. ~ 20% at 420 nm  = 26.5 mm MESH ANODE Vacuum Phototriode (VPT): Single stage photomultiplier tube with fine metal grid anode

9. HEP2001, Budapest, July 2001 R M Brown - RAL 9 VPT Gain vs Dynode Voltage

10. HEP2001, Budapest, July 2001 R M Brown - RAL 10 VACUUM PHOTOTRIODE HV FILTERING ELECTRONICS CRYSTAL ‘Supercrystal’ Layout ‘Supercrystal’: carbon-fibre alveola containing 5x5 tapered crystals + VPTs + passive HV filter (160 Identical Supercrystals per Dee) Signals fed via 600 mm cable to Preamplifier + Front End electronics behind Dee Backplate

11. HEP2001, Budapest, July 2001 R M Brown - RAL 11 Characterisation of VPTs 4.0T Solenoid at Brunel1.8T Dipole Magnet at RAL Detail of RAL test Cell Perspex diffuser plate with LEDs at corners. (Red circle indicates effective VPT diameter) All VPTs are measured at 0  B  1.8T and -30 o    30 o at RAL Sample VPTs are measured at B =4.0T and  = 15 o at Brunel 500 ‘Preproduction’ VPTs delivered by RIE (St Petersburg)

12. HEP2001, Budapest, July 2001 R M Brown - RAL 12 Response vs Angle at B=1.8T Arrows indicate angular regions of end caps

13. HEP2001, Budapest, July 2001 R M Brown - RAL 13 Response vs B-Field Strength VPT Axis at 15 o w.r.t. Magnetic Field 0.80 0.85 0.90 0.95 1.00 1.05 1.10

14. HEP2001, Budapest, July 2001 R M Brown - RAL 14 Distributions of Gain (B=0) and Quantum Efficiency 6 8 10 12 14 16 18 20 Gain and quantum efficiency are uncorrelated 16 18 20 22 24 26 28 30 (%) Taken from the ‘passport’ supplied with each tube by the manufacturer

15. HEP2001, Budapest, July 2001 R M Brown - RAL 15 0 10 20 30 40 50 60 70 80 90 100 Anode Response Distribution B=1.8T  =15 o Spread in anode response  Some sorting of VPTs necessary

16. HEP2001, Budapest, July 2001 R M Brown - RAL 16 Test beam:Energy Resolution No preshower detector With preshower detector

17. HEP2001, Budapest, July 2001 R M Brown - RAL 17 Faceplate optical transmission Post-irradiation Loss in optical transmission of 2 faceplate samples after 25 kGy 60 Co irradiation (380Gy/hour) (approx 10 yrs LHC at  = 2.6)

18. HEP2001, Budapest, July 2001 R M Brown - RAL 18 VPT Behaviour Under Irradiation 60 Co Irradiation (58 Gy/hr) Photocurrent produced by Cerenkov light in VPT window. (Vertical lines correspond to pauses in irradiation) V A =1000 V D = 800

19. HEP2001, Budapest, July 2001 R M Brown - RAL 19 VPT Summary  A new generation of fine-mesh VPTs has been developed to satisfy the high magnetic field/radiation hardness requirements of CMS  An automated characterisation facility has been commissioned to handle 15000 devices  The performance of 500 pre-production VPTs from RIE meets CMS requirements