Development of HPDs for applications in physics and medical imaging A. Braem, E. Chesi, Christian Joram, J. Séguinot, P. Weilhammer CERN / PH representing the CIMA collaboration and the C2GT team Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 1
Outline Why Hybrid Photo Detectors? Some Developments for physics experiments 5-inch 10-inch A spherical HPD for neutrino detection A flat HPD for 3D-Axial PET scanner www.cern.ch/ssd/hpd Great support by our technical staff: F. Cossey, C. David, I. Mcgill, M. v. Stenis, Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 2
Hybrid Photon Detectors (HPD) photocathode light quantum focusing electrodes e - V silicon sensor + FE electronics segmented silicon sensor Combination of sensitivity of PMT with excellent spatial and energy resolution of silicon sensor Gain: G e U C 3.6 ev U C = 20 kv G ~ 5000 Gain is achieved in a single dissipative step! σ G F G small compared to σ electronics Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 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 Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 4
Main advantages of HPD technology Excellent signal definition Allows for photon counting Free choice of segmentation (50 µm - 10 mm) Uniform sensitivity and gain no dead zones between pixels CMS HCAL, 19-pixel HPD (DEP, NL) electronic noise cut 1 p.e. Viking VA3 chip (τ peak = 1.3 µs, 300 e - ENC) U C = -26 kv 2 p.e. 3 p.e. Pad HPD Single pad S/N up to 20 2 p.e. Drawbacks Rel. low gain (3000-8000) low noise electronics required Expressed sensitivity to magnetic fields pulse height (ADC counts) e- backscattering from Si surface continuous background ε det (p.e.) ~ 85-95 % Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 5
Some prototype developments for physics experiments developed and built at CERN 5-inch 10-inch 254 mm Ø, D ~ 4, 2048 channels 1mm 2 bialkali photocathode NIM A 504 (2003) 19 NIM A 518 (2004) 574-578 127 mm Ø, D ~ 2.5, 2048 channels 1mm 2 bialkali photocathode NIM A 442 (2000) 128-135 NIM A 478 (2002) 400-403 www.cern.ch/ssd/hpd Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 6
Principle of neutrino detection by Cherenkov effect in C2GT (CERN To Gulf 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 ν e, ν µ, ν τ (E ν below threshold for τ production) Cherenkov light e ±, µ ± 42 The wall is made of ~600 mechanical modules (10 x 10 m 2 ), each carrying 49 optical modules. CC reactions in H 2 O Cherenkov light segmented photosensitive wall about 250 250 m 2 Fiducial detector volume ~ 1.5 Mt ~ 50 m 10 m Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 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 < λ < 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 Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 8
C2GT Optical Module 380 mm benthos sphere (432 / 404) joint optical gel (refr. index matching + insulation) Si sensor ceramic support 15 432 mm (17 ) standard base plate of HPD 10 (prel. version). HV electrical feed-throughs PA valve Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 9
Electrostatics Simulations with SIMION 3D 120º 110º - 20.3 kv - 20 kv 15000 Φ (V) or E (V/cm) 10000 5000 0-5000 -10000-15000 E ~ 1/r 2 Φ ~ 1/r -150-100 -50 distance from centre (mm) 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. Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 10
Transit Time 12 0 < φ < 120 Transit time (ns) 11 10 9 0 20 40 60 80 100 120 polar angle (deg.) 3.0 rel. frequency (a.u.) 2.5 2.0 1.5 1.0 σ = 0.15 ns RMS = 0.18 ns 0.5 angles > 110 0.0 9.0 9.5 10.0 10.5 11.0 11.5 12.0 Transit Time (ns) Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 11
A half-scale prototype 208 mm (~8-inch) Al coating 2 rings Development in collaboration with Photonis-DEP, C. Fontaine et al. Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 12
Current status: All main components ready dry assembly First tests will be done with a metal cube rather than Si sensors Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 13
Development for HPDs for medical imaging The 3D axial PET camera Principle of a camera module y z HPD1 x Axial arrangement of camera modules based on matrices of long crystals read out on both sides by HPDs Preprint CERN PH-EP/2004-050, submitted to NIM A HPD2 Patent filed. No. WO2004008177 Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 14
Technical realization 2 VATA-GP5 ASIC Si sensor 2 x 104 pads (4 x 4 mm 2 ) 208 crystals (YAP) 3.2 3.2 100 mm 3 gap between crystals 0.8 mm HPD proximity focused ceramic body sapphire window bialkali PC ceramic PCB (4 layers) Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 15
VATA-GP5 (CERN IDE AS co-development, www.ideas.no) fast shaper 25 ns discr. OR V FP mask tresh. FOR Trigger Det. 150 ns preamp slow shaper S/H Analog Out VATA-GP5, 128 ch./chip Auto-triggering analog front-end serial and sparse readout time walk compensation 0.6 µm CMOS Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 16
VATA-GP5 ASIC + ceramic PCB + Si-sensor operational DAQ CsI photoelectrons signal (ADC counts) 120 90 60 30 Si Sensor Depletion (U PC = -15 kv) CaF 2 UV light source 0 15 20 25 30 35 40 45 50 V bias (volt) signal (ADC counts) 250 200 150 cut-off at 6 kv (too thick n + layer) signal (ADC bin) 350 300 250 200 serial readout 100 50 150 100 50 sparse readout 0 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 - U C (kv) 0 0 500 1000 1500 2000 2500 Q in (fc) Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 17
2 sealed protoype tubes produced so far both were not perfect PC109 50 mm Ø Si-sensor VA-prime electronics bialkali cathode (22%) HV problem Q.E. (%) 24 20 16 12 8 4 Photocathode 109 "PCR5" sealed Center of cathode PC113 PET Si-sensor VATA-GP5 electronics bialkali cathode electronics suffered due to accident during processing x det (pads) 14 12 10 8 6 0 250 350 450 550 650 lambda (nm) charge vs spot position (centre of gravity) m = 1.04 centre sensor 4 50 60 70 80 90 x mir (mm) Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 18
Summary and Outlook Our team is developing a flat HPD with auto-triggering electronics, aimed at medical imaging applications, in particular PET with single crystal readout. Sealed fully operational prototype imminent (days!) a spherical HPD for underwater Cherenkov detector for neutrino physics. Sealed prototype later this year. After finally having entered HEP at large scale (CMS, LHCb), HPDs may also have a bright future in some advanced applications(niches?) in other fields. Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 19
BACK-UP slides Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 20
Effect of angular spread and magnetic field E kin (t 0 ) = 2 ev (conservative) -40º θ em 40º (rel. to surface) effect of earth magentic field seems to be marginal B 0.45 Gauss E~1/r 2 produces a focusing effect Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 21
Is E-field E around Si sensor too high? A grounded grid could help Grid at natural potential: -11 kv Grid at 0 V -11.000 V 500 V/mm 0 V 500 V/mm 1000 V/mm 1500 V/mm 500 V/mm 0 V 2000 V/mm 0 V ~23 mm Gradients up to 2000 V/mm at edges of Si sensor Gradients at Si sensor reduced to ~500 V/mm. High field around grid, f(diam.). Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 22
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 Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 23
Processing in the existing set-up at CERN, used for 5 and 10 HPDs heating element glass envelope Sb source K/Cs sources 10 Ø 5 Ø Only minor mechanical adaptations required. base plate with pre-mounted sensor Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 24
110 time walk (ns) 100 90 80 Gain 0 Gain 1 70 +/- 5 ns 60 0 40 80 120 160 input charge (fc) Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 25
Experimental set-up for VATA-GP5 tests with photo-electrons DAQ Readout card VME CsI photctahode (300 nm) Deposited on a grid (Au, 80% transparency) Si sensor (300 µm) 208 pads (4 4 mm 2 ) UV H 2 flash lamp (self triggering) CaF 2 photoelectrons mask (at ground) -U PC = 15-20 kv vacuum pump (turbo) P < 10-5 mbar MgF 2 mirror MgF 2 collimator Beaune 2005 C. Joram CERN / PH Development of HPDs for applications in physics and medical imaging 26