1. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 1 Development of spherical HPDs for a Water Cherenkov Detector (initiated by C2GT study) A. Braem, E. Chesi, Christian Joram, J. Séguinot, P. Weilhammer CERN / PH representing the C2GT team www.cern.ch/ssd/HPD

2. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 2 Hybrid Photon Detectors (HPD)  V photocathode focusing electrodes silicon sensor + FE electronics e - light quantum segmented silicon sensor Combination of sensitivity of PMT with excellent spatial and energy resolution of silicon sensor Gain: Gain is achieved in a single dissipative step ! U C = 20 kV  G ~ 5000 small compared to  electronics

3. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 3 Classical HPD designs Proximity focused 1:1 imaging Operates in axial magnetic fields. ‘Fountain’ focused Demagnification D No real focusing  ballistic point spread Intolerant to magnetic fields ‘Cross’ focused Demagnification D Focusing leads to small point spread Intolerant to magnetic fields

4. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 4 Main advantages of HPD technology Excellent signal definition Allows for photon counting Free choice of segmentation (50  m - 10 mm) Uniform sensitivity and gain no dead zones between pixels 2 p.e. Drawbacks Rel. low gain (3000 - 8000)  low noise electronics required Expressed sensitivity to magnetic fields CMS HCAL, 19-pixel HPD (DEP, NL) Pad HPD Single pad S/N up to 20 1 p.e. 2 p.e. 3 p.e. Viking VA3 chip (  peak = 1.3  s, 300 e - ENC) U C = -26 kV electronic noise cut e- backscattering from Si surface  continuous ‘background’   det (p.e.) ~ 85-95 % pulse height (ADC counts)

5. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 5 Some prototype developments for physics experiments developed and built at CERN NIM A 442 (2000) 128-135 NIM A 478 (2002) 400-403 www.cern.ch/ssd/HPD 5-inch  10-inch 127 mm Ø, D ~ 2.5, 2048 channels 1mm 2 bialkali photocathode NIM A 504 (2003) 19 NIM A 518 (2004) 574-578 254 mm Ø, D ~ 4, 2048 channels 1mm 2 bialkali photocathode

6. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 6 e, ,  CC reactions in H 2 O segmented photosensitive ‘wall’ about 250 × 250 m 2 Fiducial detector volume ~ 1.5 Mt (E below threshold for  production) ~ 50 m e ±,  ± 42° Cherenkov light Principle of neutrino detection by Cherenkov effect in C2GT (CERN To Gulf of Taranto) The wall is made of ~600 mechanical modules (10 x 10 m 2 ), each carrying 49 optical modules. 10 m A. Ball et al., C2GT, Memorandum, CERN-SPSC-2004-025, SPSC-M-723 A. Ball et al., Proc. of the RICH2004 conference, subm. to NIM A

7. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 7 The ideal photodetector for C2GT large size (>10”) spherical shape, must fit in a pressure sphere ± 120° angle of acceptance optimized QE for 300 < < 600 nm single photon sensitive timing resolution 1-2 ns no spatial resolution required electronics included cost-effective (need ~32.000 !) adapted to industrial fabrication 120° 42°

8. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 8 Silicon or dynodes ? Design considerations Silicon or dynodes ? silicondynodes Angular coverage > ± 90°Angular coverage < ± 60° TTS < 1 ns (FWHM) ?TTS ~ 3 ns  det (1 p.e.) ~93% (back scattering)  det (1 p.e.) ~90% ‘delicate’ components (Si, ceramics)only metal inside tube no magnetic shielding needed (to be verified !) mu-metal shielding needed Uniform response over ± 90°Uniform response over ± 25° p-i-n diode or APD ? If silicon, p-i-n diode or APD ? p-i-nAPD Signal at 20 kV: 5·10 3 e, G = 12-3 ·10 5 e, G ~ 50 C d = 35 pF/cm 2, ENF ~ 1C d = 300 - 1500 pF/cm 2, ENF = 2 - 5 detector size ‘arbitrary’few sizes available (1,3,5 mm Ø, 5x5 mm 2 )

9. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 9 432 mm (17”) 380 mm PAHV optical gel (refr. index matching + insulation) benthos sphere (432 / 404) electrical feed-throughs valve standard base plate of HPD 10” (prel. version). C2GT Optical Module 15 joint Si sensor ceramic support

10. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 10 ElectrostaticsSimulations with SIMION 3D 110º Potential and field distribution similar to a point charge. Low field at photocathode (~100 V/cm), Very high field close to Si sensor (~10,000 V/cm). Try to reduce by a grounded field cage around Si sensor. - 20 kV 120º - 20.3 kV

11. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 11 0 <  < 120° Transit Time

12. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 12 Effect of angular spread and magnetic field E kin (t 0 ) = 2 eV (conservative) -40 º   em  40º (rel. to surface) 0.45 Gauss B E~1/r 2 produces a focusing effect effect of earth magentic field seems to be marginal

13. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 13 0 V ~23 mm 500 V/mm 1000 V/mm 1500 V/mm 2000 V/mm Is E-field around Si sensor too high ? A grounded grid could help 0 V Gradients at Si sensor reduced to ~500 V/mm. High field around grid, f(diam.). -11.000 V Gradients up to 2000 V/mm at edges of Si sensor 500 V/mm Grid at 0 V Grid at ‘natural’ potential: -11 kV Grid at 0 V

14. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 14 A ‘half-scale’ prototype 208 mm (~8-inch) Al coating 2 rings Development in collaboration with Photonis-DEP, C. Fontaine et al.

15. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 15 Enevlope fabricated by SVT, France. Sphere blown from a glass cylinder !

16. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 16 Current status: All components ready  ‘dry assembly’. Processing soon. First tests will be done with a metal cube rather than Si sensors

17. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 17 Fabrication Large number of tubes needed  industrial fabrication  internal process (as for large PMTs). However, this would lead to a ‘pollution’ of Si-sensor and high field region with Cs/K/Sb.  protect by mask (difficult!)  use ‘hybrid’ process ? Q.E. monitoring

18. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 18 Processing in the existing set-up at CERN, used for 5” and 10” HPDs 10”Ø Only minor mechanical adaptations required. 5”Ø heating element glass envelope Sb source K/Cs sources base plate with pre-mounted sensor

19. C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 19 Summary A spherical HPD has been designed which promises to meet the C2GT requirements Strong features Large acceptance Low TTS Good signal definitionIssues Low noise and short peaking time for large detector capacitance Industrialization of production A half scale propotype tube is under construction in collaboration with Photonis to prove the basic characteristics of this design.