1 The X-HPD: Development of a large spherical hybrid photodetector A.Braem +, C. Joram +, J. Séguinot +, L. Pierre *, P. Lavoute * + CERN, Geneva (CH) * Photonis SAS, Brive (F) Introduction: C2GT, requirements, Hybrid tubes The X-HPD concept Proto 0: A first sealed prototype with metal anode Proto 1: Towards a full X-HPD Summary and Outlook

2 e, ,  CC reactions in H 2 O segmented photosensitive ‘wall’ about 250 × 250 m 2 Fiducial detector mass ~ 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., Eur. Phys. J. C (2007)

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 < < 600 nm single photon sensitive timing resolution 1-2 ns no spatial resolution required electronics included cost-effective (need ~ !) adapted to industrial fabrication 120° 42°

4 432 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

5 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 (d  ~ 3  ) Possibility of anode segmentation  imaging capability (limited!) Sensitivity gain through ‘Double-cathode effect’ T ~ 0.4 QE

6 Silicon is a good anode material for a HPD Gain = e·U C-A / 3.6 eV ~ 5000 for 20 kV Back scattering rel. small  BS = 0.18 (20 kV) However: large tube requires large Si sensor  large capacitance (35 pF/cm 2 )  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. 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 both tubes used thin disks of P47 phosphor (YSO:Ce powder) Philips made ~30 tubes (1980s-1992) 200 tubes in operation since 1998

7 PMT XP2982 pressure sphere The Philips SMART TubeThe QUASAR 370 Tube G. van Aller et al. A "smart" 35cm Diameter Photomultiplier. Helvetia Physica Acta, 59, 1119 ff., g primary ~ 35,  t ~ 2.5 ns /  N pe R. Bagduev et al., Nucl. Instr. Meth. A 420 (1999) 138

8 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 = 400 nm LY (  / keV  ns  Z eff  BS emission (nm) emission (nm)YAP:Ce182732~ LYSO:Ce25~4064~ LaBr 3 :Ce ~ LaBr 3 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.

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

10 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.

11 First build a HPD with metal-cube anode (1 cm 3 ) ‘Proto 0’ Built in CERN plant. measured at Photonis A. Braem et al., NIM A 570 (2007)

12 Prepare and characterize components for real X-HPD ‘Proto 1’ PMT Photonis XP3102 (25 mm Ø) electrical feedthrough Al coated

13 Calibration: 1 p.e. = ·10 9 Vs ENF ~ 1.15 PMT XP3102 has very good signal properties.

14 Test set-up Lab e - accelerator based on pulsed photoelectrons from a CsI cathode. DAQ on digital scope. LeCroy Waverunner LT344. vacuum pump (turbo) P < mbar Pulsed H 2 flash lamp (VUV) MgF 2 sapphire window mirror -U PC = kV collimator CsI photocathode X-HPD anode

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

16 Charge amplitude for single photoelectrons… and multi-photoelectrons  modest photo counting ability

17 Calibration of charge amplitude (for single incident photoelectrons) using single photon response of PMT = primary gain of X-HPD

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

19 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

20 Timing studies with first electron method clock startclock stop

21 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.

22 Summary and Outlook X-HPD is an attractive concept for large area photodetection promises higher performance than classical PMT proof of principle OK components characterized Next steps Make X-HPD with LYSO X- anode in ~2-3 weeks Test it at CERN and at Photonis + … Improve light extraction from LYSO crystal. Ideas exist.

23 Back-up Transparencies

24 E e = 25 keV keV (E-loss in Al layer) × 25 ph/kev (scint. yield)  612 ph d  /4  = 0.16  (  crit = 33.5°)  98 ph Fresnel reflection losses ~ -10%  90 ph ~ 0.25  22.5 p.e. X-tal with Al coating MgF 2 anti-reflection coating at exit side Window (+ opt. grease) Light guide (Al coated) PMT window (+ opt. grease) Photocathode Estimate of the photoelectric yield

25 LSO crystal with PMT readout. The plot shows the reconstructed number of photoelectrons as function of the applied voltage at the CsI cathode. Full and open symbols correspond to U PMT = 1000 V and 900 V, respectively. The fits to the data are explained in the text. The relative light output of LSO [15][16] is shown as dashed line (secondary axis). A. Braem et al., NIM A 570 (2007) Crystals coupled via sapphire window to PMT. Use of optical grease.

26 LSO crystal read out with PMT. As previous figure, however the light intensity of the flash tube was increased. On average there is about 1 photoelectron. The pedestal peak is cut at 250 counts. A. Braem et al., NIM A 570 (2007) LSO crystal coupled via sapphire window to PMT. Use of optical grease.

27 Photonis XP 1807 Photonis XP 1806 Photonis XP 1805 Photonis XP 1804  ~ 30°  ~ 35°  ~ 31°

28 CERN photocathode ‘transfer’ plant