Development of Hybrid Photon Detectors for a Novel Deep Sea Neutrino Experiment

1. Development of Hybrid Photon Detectors for a Novel Deep Sea Neutrino Experiment • C2GT to measure Neutrino Oscillations • The Photodetector • requirements • design considerations • Concept of a spherical HPD • electrostatics • signal characteristics • fabrication issues • A half scale prototype A.E. Ball, A. Braem, L. Camilleri, A. Catinaccio, G. Chelkov, F. Dydak, A. Elagin, P. Frandsen, A. Grant, M. Gostkin, A. Guskov, C. Joram, Z. Krumshteyn, H. Postema, M. Price, T. Rovelli, D. Schinzel, J. Seguinot, G. Valenti, R. Voss, J. Wotschack, A. Zhemchugov

2. Reminder: CNGS • CERN • Neutrinos • toGran Sasso • Under • construction • Beam in 2007

3. C2GT Continuation of CNGS beam to Gulf of Taranto (Ionian Sea) Gulf of Taranto Deep trench allows to perform neutrino experiments in a depth > 1000 m

4. C2GT – what and why ? The concept of C2GT consists of a planar Cherenkov underwater detector, operated at a depth of ~1000m, and at a baseline around 1200 km. The detector can be displaced to assess baselines from 1100 – 1700 km. The CNGS neutrino beam could be converted with modest effort (no civil engineering!) to a quasi-monoenergetic off-axis neutrino beam, delivering nm of En = 0.8 GeV to the gulf of Taranto (radial distance from CNGS Beam axis: 44 km) Under certain (reasonable) assumptions, neutrino oscillations at large distances can be described by only 3 parameters: The experiment allows to measure all 3 ! ( q13 is small  cos4q13 ~ 1) Assuming ~ 2.5×10-3eV2 (Super-K), a 0.8 GeV nm beam would have its first and second oscillation maximum at L = 400 and L = 1200 km. A scan at 3 different baselines (1+1+5 yrs) leads to a precision in sin2q23 of 8% and in of 1%.

5. Nature seems to prefer bimaximal mixing, i.e |q23|~ |q12| ~ 45° At the previously established baseline with maximum oscillation, the nenm probability becomes C2GT could establish a non-zero value of sin2q13 with a 3s discovery potential for a value of 0.0039 or could improve the current upper limit of sin2q13 < 0.05 by about a factor 30.

6. Principle of neutrino detection by Cherenkov effect in C2GT segmented photosensitive ‘wall’ about 250 × 250 m2 Fiducial detector volume ~ 1.5 Mt Cherenkov light (En below threshold for t production) ne, nm, nt e±, m± 42° CC reactions in H2O Cherenkov light ~ 50 m Light absorption length in sea water

7. ‘muon event’ ‘electron event’ Use ‘amplitude’ information and fuzziness to distinguish between muon and electron events. The method require a certain granularity of the photosensor plane. M.C. predicts muon misidentification of 1×10-3 at 90% electron efficiency.

8. The detector - basic ideas: The detector plane, fixed at the sea bed. A mechanical module (~10 x 10 m) with 49 photosensors

9. 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 • dark counts <0.1 per 100 ns • no spatial resolution required • electronics included • cost-effective (need ~32.000 !) • adapted to industrial fabrication 120° 42°

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

11. 380 mm C2GT Photodetector 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

12. 120º Electrostatics 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). • Too high ?

13. Transit Time 0 < f < 120°

14. 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

15. Is E-field around Si sensor too high ? A grounded grid could help Grid at ‘natural’ potential: -11 kV Grid at 0 V -11.000 V 0 V 500 V/mm 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

16. Signal Characteristics • Signal amplitude • UC = 20 kV  signal 1 p.e. ~ 5000 e- • Design of Si sensors • 15 x 15 mm2, mini guard ring, aim for > 90% active area • Electronics • fast (tpeak = 100 ns) and low noise (500 e-) - impossible to • reach for CD = 70-80 pF  segment sensor (e.g. 4 fold) • Timing characteristics • Need st ~ 2 ns for single photons • Requires most likely waveform digitization • Electronics + readout need much more work ! • Problem ? • Electronics reads signal over a 20 cm long distance

17. InterFET J310 Ortec 450 +Vb 2 or 20 cm wire (non-coax) Si 300 mm thick CD = 36 pF cm 2 cm readout wire 241Am 241Am 20 cm readout wire ts = 2 ms ts = 2 ms Big thank you to Alan Rudge and Peter Weilhammer

18. 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

19. A ‘half-scale’ prototype 208 mm (~8-inch) Al coating 2 rings

20. ‘artistic view’ of the half-scale prototype Si sensors 5 pieces 12 x 13.2 mm2 in a grounded field cage standard 5” baseplate

21. Enevlope under fabrication at SVT, France. Sphere blown from a glass cylinder ! Expect 3 envelopes with flange before end ’04.

22. 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

23. Summary and conclusions • C2GT is a conceptual study of an n experiment to precisely measure q13 • A spherical HPD has been designed which promises to meet the C2GT requirements • Strong features • Large acceptance • Low TTS • Good signal definition • Critical issues • Low noise and short peaking time for large detector capacitance • Industrialization of production • A half scale propotype tube is under construction to prove the basic characteristics of this design. • If successful, the prototype could be used to for deep sea site exploration studies.

24. Back-up stuff …

25. fit = 1/sqrt(ts)

26. Calculations take into account • refraction (3D) at all interfaces • Fresnel reflection/transmission as f(q) • bulk absorption Full Detector Geometry n_sph = 1.472 sphere refr. index n_win = 1.47 window refr. index n_gel = 1.404 optical gel: refr. index n_wat = 1.345 water refr. index l_sph = 300 mm absorption length l_win = 500 mm “ l_gel = 600 mm “ 45º

27. Prototype Detector Geometry 45º

28. Dark count rate Rough estimate Looks too optimistic. Varies strongly with Wth. Literature (Philips PM handbook) Tambient: Rdark = 10-1000 Hz/cm2 Expect to gain a factor 5-10 by cooling T4C: Rdark = 1 - 200 Hz/cm2 R4C,total(4000cm2) = 4·103 - 8·105 Hz Prob for 1 single photon hit in 20 ms = 8% - 1600% Prob for 1 single photon hit in 100 ns = 0.04% - 8%

29. Basic considerations (3) Fabrication process:

30. The Hamamatsu 13” prototype (from A. Kusuka, Master thesis, U. Tokyo, Feb 2004)

31. ‘out’ -20 kV 0 kV Optics / HV not optimized ? Hamamatsu 13” prototype Timing looks ok. < 1 ns TTS