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The X-HPD : A modern Implementation of a SMART Concept

The X-HPD : A modern Implementation of a SMART Concept. Braem + , C. Joram + , J. Séguinot + , L. Pierre * and P. Solevi # + CERN, Geneva (CH) * Photonis SAS, Brive (F) # ETH Zürich (CH). History: C2GT , requirements, Hybrid tubes The X-HPD concept

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The X-HPD : A modern Implementation of a SMART Concept

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  1. The X-HPD: A modern Implementation of a SMART Concept • Braem +, C. Joram+, J. Séguinot +, L. Pierre * and P. Solevi # • + CERN, Geneva (CH) * Photonis SAS, Brive (F) # ETH Zürich (CH) • History: C2GT, requirements, Hybrid tubes • The X-HPD concept • Proto 0: A first sealed X-HPD prototype with metal anode • Proto 1 (PC119): Towards a full X-HPD • Proto 2 (PC120): The first operational X-HPD with LYSO crystal • Summary and outlook C. Joram, VLVnT08, Toulon, 23 April 2008

  2. Principle of neutrino detection by Cherenkov effect in C2GT (CERN ToGulf of Taranto) A. Ball et al., Eur. Phys. J. C (2007) 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 mass ~ 1.5 Mt

  3. 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°

  4. 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

  5. C2GT project initiative was stopped in 2006. But photodetector development had in the meantime stimulated enough interest (e.g. at Photonis) to be continued.  Partnership agreement CERN / Photonis.

  6. T ~ 0.4 QE QE Concept of a spherical tube with central spacial anode • Radial electric field • negligible transit time spread • ~100% collection efficiency • no magnetic shielding required • Large viewing angle (dW ~ 3p) • Possibility of anode segmentation • imaging capability (limited!) • Sensitivity gain through • ‘Double-cathode effect’

  7. 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. • Silicon is a good anode material for a HPD • Gain = e·UC-A / 3.6 eV ~ 5000 for 20 kV • Back scattering rel. small eBS = 0.18 (20 kV) • However: large tube requires large Si sensor •  large capacitance (35 pF/cm2) • ns timing only with segmented sensor (few pF) Scintillator as alternative anode ? Readout by conv. PMT. • Idea is not new ! Philips SMART tube, Lake Baikal QUASAR tube. both tubes used thin disks of P47 phosphor (YSO:Ce powder) 200 tubes in operation since 1998 Philips made ~30 tubes (1980s-1992)

  8. The Philips SMART Tube The QUASAR 370 Tube pressure sphere R. Bagduev et al., Nucl. Instr. Meth. A 420 (1999) 138 PMT XP2982 G. van Aller et al. A "smart" 35cm Diameter Photomultiplier. Helvetia Physica Acta, 59, 1119 ff., 1986. gprimary ~ 35, st ~ 2.5 ns / Npe

  9. SMART and Quasar tubes were the first X-HPDs • however the use of a disk shaped scintillator didn’t allow to exploit full potential. • need to use fast scintillator, e.g. cube or cylinder. • Require • high light yield  high gain • short decay time • Low Z preferable  low back scattering coefficient • Emission around l = 400 nm LaBr3 is quite hygroscopic • side and top require conductive and reflective coating  define el. potential, avoid light leaking out. • Problem: crystals have high refr. index (~1.8) •  large fraction of light is trapped inside crystal due to total internal refection.

  10. X-HPD ½ scale prototype (208 mm Ø) LYSO 12 Ø × 18 mm

  11. Electrostatics(SIMION 3D) • Expected performance • Viewing angle 120° • X-tal hit probability ~ 100% • flight time spread ~0.4 ns (RMS) Can be further improved by optimizing bulb geometry.

  12. First build a HPD with metal-cube anode (1 cm3) ‘Proto 0’ measured at Photonis Built in CERN plant. A. Braem et al., NIM A 570 (2007) 467-474

  13. Prepare and characterize components for real X-HPD ‘Proto 1’ Anode consists of cylindrical LYSO crystal :12 mm Ø × 18 mm electrical feedthrough Al coated PMT Photonis XP3102 (25 mm Ø)

  14. PMT XP3102 has very good signal properties. ENF ~ 1.15 Calibration: 1 p.e. = 0.0514·109 Vs

  15. mirror MgF2 collimator sapphire window Pulsed H2 flash lamp (VUV) vacuum pump (turbo) P < 10-5 mbar -UPC = 0 -30 kV CsI photocathode Test set-up Lab e- accelerator based on pulsed photoelectrons from a CsI cathode. X-HPD anode DAQ on digital scope. LeCroy Waverunner LT344.

  16. Primary Gain and timing resolution measurements (in pumped set-up, using single incident photoelectrons) = primary gain of X-HPD A. Braem et al., NIM A 581 (2007) 469–472 However: Fully processed and sealed X-HPD (Proto-1, PC119) showed HV problems above ~14 kV  Tube not exploitable for measurements.

  17. Improvements for Proto-2 • Suspect HV instabilities on non-coated glass surfaces •  Treat these surfaces with special UHV compatible coating (secret, Photonis) 2) Try to increase light output of LYSO crystal.  Shape upper part of crystal conically. 33º Expect significant Increase in light output… Non-coated glass surface. High electric field. ~ 5kV/cm Solid angle limited to ~2 · 2p(1-cos(33º) ) ~ 2 sr dW / 4p ~ 16% 0 kV

  18. Cylindrical crystal (12 Ø x 18 mm) Geant4 simulations of a 20 keV electron hitting a LYSO crystal (light yield scaled down for clarity) • Qualitative results : • (simulations are 3 days old) • Conical geometry gives more light cylindrical geometry (~factor 2). • There is a strong difference between electrons hitting the flat top and the conical part. • This leads to a • large spread in • the light output. Cylindrical crystal with conical part and flat top (~3 Ø) Backscattered electron

  19. Light beam for QE online monitoring X-HPD processing in CERN transfer plant Evaporation head (Sb, K, Cs) Base plate with LYSO anode

  20. Cylindrical crystal (PC119)

  21. X-HPD (PC120)

  22. Charge distributions (1 ADC bin = 50 fC) Test with single photons from flash lamp Uc = -20 kV Log scale 1 pe signal Waveforms from the scope tLYSO Dark noise Trigger: pick-up from flash lamp 50 fC / ADC count Linear scale 50 fC / ADC count

  23. Relative light output of LSO W. Moses et al. NIM A487 (2002), 123 – 128. Amplitude attenuated by factor of 8 (18 dB) ~20 p.e. (E/s)2 ~ 23.5 400 fC / bin ADC 50 fC / bin ADC

  24. Photocathode (?) dark noise at 20 kV (Temperature ~ 25ºC, PC120) Cathode surface ~ 1020 cm2 Normalized Noise rate (20 kV, 0.5 p.e.) ~ 400 Hz / cm2

  25. Simple timing studies with single photons stop Single p.e. time resolution limited by signal amplitude fluctuations st ~ 3.6 ns Np.e. = 20 st ~ 0.8 ns, confirming sn ~ s1 / √n start stop start

  26. Summary… • X-HPD • is an attractive concept for large area photodetection • promises higher performance than classical PMT (QE, collection efficiency, TTS) • proof of principle OK • First operational X-HPD with LYSO crystal produced. • Very preliminary analysis performed (2 days).

  27. … and Outlook • X-HPD • More measurements and finer analysis to follow. • Crystal shape critical for light output and amplitude resolution. Shape and optical coupling to PMT to be optimized. • Study of alternative anode configurations, e.g. based on phosphor. Ideas exist. • HV performance improved, but not yet optimal. Needs another iteration. • Study of industrialization with aim of cost effective mass production. Many thanks to our technicians Claude David and Miranda v. Stenis (both CERN) for their very competent support in producing the X-HPD.

  28. Back-up Transparencies

  29. LSO crystal coupled via sapphire window to PMT. Use of optical grease. A. Braem et al., NIM A 570 (2007) 467-474

  30. q ~ 35° q ~ 30° q ~ 35° Photonis XP 1807 q ~ 31° q ~ 31° q ~ 35° Photonis XP 1804 Photonis XP 1805 Photonis XP 1806

  31. CERN photocathode ‘transfer’ plant

  32. Pulse shape on scope. Contributions from individual photoelectrons clearly visible

  33. Single photoelectron spectra are well described by Gaussian + Exponential. Where does the non-Gaussian part come from ? LYSO, Ee = 27.5 keV Fit = Gauss + Exp. e- back-scattering from LYSO e- Ee 1-eBS eBS E’e lost ? Wrote a very simple M.C.: Ingredients: eBS + E’e spectrum + statistics Input: <Np.e.>, mPoisson Ee-E’e E.H. Darlington, J. Phys. D, Vol. 8 (1975) 85 PMT Expect to loose <10% of single photoelectrons by a 3s pedestal cut.  single p.e. detection efficiency of X-HPD ~90%

  34. What happens to back scattered electrons in radial E-field of X-HPD ? • Low energy electrons (E < 5 keV) are reabsorbed within << 1 ns. • For E > 5 keV re-absorption (within ~1 ns) or loss. Depends on hit position and emission angle. X-tal top X-tal side

  35. Timing studies with first electron method clock start clock stop

  36. Include time calculation in M.C. Needed 1 ns overall jitter to describe data. M.C. shows stronger tails than actually measured, because all BS electrons are assumed to be lost.

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