Development of spherical HPDs for a Water Cherenkov Detector (initiated by C2GT study)

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. Hybrid Photon Detectors (HPD) light quantum photocathode e - focusing V D electrodes silicon sensor + FE electronics Combination of sensitivity of PMT with excellent spatial and energy resolution of silicon sensor segmented silicon sensor Gain: Gain is achieved in a single dissipative step ! UC = 20 kV  G ~ 5000 small compared to selectronics

3. Classical HPD designs • ‘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 • Proximity focused • 1:1 imaging • Operates in axial • magnetic fields.

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

5. Some prototype developments for physics experiments • developed and built at CERN 5-inch  10-inch 254 mm Ø, D ~ 4, 2048 channels 1mm2 bialkali photocathode NIM A 504 (2003) 19 NIM A 518 (2004) 574-578 127 mm Ø, D ~ 2.5, 2048 channels 1mm2 bialkali photocathode NIM A 442 (2000) 128-135 NIM A 478 (2002) 400-403 www.cern.ch/ssd/HPD

6. Principle of neutrino detection by Cherenkov effect in C2GT (CERN ToGulf of Taranto) 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 The wall is made of ~600 mechanical modules (10 x 10 m2), each carrying 49 optical modules. (En below threshold for t production) Cherenkov light ne, nm, nt e±, m± 42° CC reactions in H2O Cherenkov light 10 m ~ 50 m segmented photosensitive ‘wall’ about 250 × 250 m2 Fiducial detector volume ~ 1.5 Mt

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 < l < 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. Design considerations Silicon or dynodes ? If silicon,p-i-n diode or APD ?

9. 380 mm C2GT Optical Module benthos sphere (432 / 404) 15 432 mm (17”) Si sensor joint ceramic support optical gel (refr. index matching + insulation) HV PA standard base plate of HPD 10” (prel. version). valve electrical feed-throughs

10. Electrostatics Simulations with SIMION 3D 120º 110º - 20.3 kV - 20 kV • 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.

11. Transit Time 0 < f < 120°

12. Effect of angular spread and magnetic field • Ekin(t0) = 2 eV (conservative) • -40º em  40º (rel. to surface) effect of earth magentic field seems to be marginal B 0.45 Gauss E~1/r2 produces a focusing effect

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

14. A ‘half-scale’ prototype 208 mm (~8-inch) Al coating 2 rings Development in collaboration with Photonis-DEP, C. Fontaine et al.

15. Enevlope fabricated by SVT, France. Sphere blown from a glass cylinder !

16. Current status: All components ready  ‘dry assembly’. Processing soon. First tests will be done with a metal cube rather than Si sensors

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

19. Summary • A spherical HPD has been designed which promises to meet the C2GT requirements • Strong features • Large acceptance • Low TTS • Good signal definition • Issues • 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.