1. ALCPG, UT-Arlington January 10th 2003 Preliminary Investigations of Geiger-mode Avalanche Photodiodes for use in HEP Detectors David Warner, Robert J. Wilson Department of Physics Colorado State University

2. R.J.Wilson, Colorado State University Outline Motivation Avalanche Photodiodes Characteristics R&D Plans Conclusions

3. R.J.Wilson, Colorado State University Motivation Scintillating fiber, or WLS readout of scintillator strips basic component of several existing detectors (MINOS, CMS-HCAL) Standard photodetector – photomultiplier tubes, great devices but… –“ Expensive ” (including electronics etc.), –Bulky, magnetic field sensitive… For the next generation would like a photon detector to be: –Cheaper –Compact? Low mass? Magnetic field insensitive? Radiation hard? Future experiments –BaBar upgrade - endcap? –Future e+e- Linear Collider? LHC? –Nuclear physics? Space-based (NASA)?

4. R.J.Wilson, Colorado State University Silicon Avalanche Photodiodes (APD) Solid state detector with internal gain. Avalanche multiplication –initiated by electron-hole free carriers, thermally or optically generated within the APD –accelerated in the high electric field at the APD junction. Proportional Mode –bias voltage below the breakdown voltage, low gain –avalanche photocurrent is proportional to the photon flux and the gain Geiger Mode –bias voltage higher than the breakdown voltage, gain up to 10 8 from single carrier –avalanche triggered either by single photon generated carriers or thermally generated carriers –signal is not proportional to the incident photon flux. –high detection efficiency of single carriers  single photon counter –to quench Geiger mode avalanche bias has to be decreased below the breakdown voltage

5. R.J.Wilson, Colorado State University UV Enhanced Avalanche Photodiodes Development by Stefan Vasile et al, Radiation Monitoring Devices, Inc. Cambridge, Massachusetts, USA. (Now at aPeak, Newton, Mass.) Small Business Innovative Research (SBIR) award motivated by an imaging Cerenkov device application (focusing DIRC). c. 1996/97-98 Design and fabrication of silicon micro-APD (  APD) pixels –20-180 µm pixels, single photon sensitivity in the 200-600 nm wavelength range. –Q.E.= 59% at 254 nm (arsenic doping, thermal annealing) –very high gain > 10 8 –Geiger mode APD array with integrated readout designed but process/funding problems. blue-infraredUV-blue

6. R.J.Wilson, Colorado State University Geiger Avalanche Characteristics Thermal carriers trigger avalanche –dark count rate decreased using small APD space charge region generation volume Compatible with 5 volt logic –strong noise rate dependence Temperature dependence  factor 3 decrease for 25°C to 0°C  factor 20 decrease for 25°C to -25°C Size dependence –roughly linear with effective avalanche region area –at room temp. predict few kHz for 100  m,  100 kHz for 500  m Characteristics measured on a small number of samples RMD Inc. 20  m diameter pixel, room temp.

7. R.J.Wilson, Colorado State University Photon Detection Efficiency RMD Inc.

8. R.J.Wilson, Colorado State University

9. Prototype  APD Array APD active area is 150  m x 150  m on 300  m pitch Compatible with CMOS process  potential for low cost large-scale production 70% photon collection efficiency with fused silica micro-mirrors (for f-DIRC) Fabrication attempt failed 1998/99. RMD claims to have solved the problems but no funds for a fabrication run. RMD Inc.

10. R.J.Wilson, Colorado State University Uses a large volume of cheap co-extruded scintillator bars (8m x 4cm x 1cm) with a single 1.2mmØ Y11-175 multiclad WLS fiber epoxied in extruded groove WLS fiber is coupled to a long clear fiber and readout with a pixelated pmt ~3-4 pe/fiber at ~3.7 m including connections and pmt QE Several production facilities still operational MINOS Scintillation System Source: BaBar IFR Upgrade Status Report III

11. R.J.Wilson, Colorado State University Short (3.7m vrs 8m) version of MINOS system with Time to the get the second coordinate Replace the pmt with (low gain) APD : 4X higher QE Increase number of fibers to 4 : ~2X more light Increase scintillator thickness to ~2cm : ~1.5X more light Project ~ 50-60 pe at 3.7m for min. ion. BaBar Modifications (SLAC/CalTech) Source: BaBar IFR Upgrade Status Report III

12. R.J.Wilson, Colorado State University CSU+SLAC Commissioned R&D at aPeak P.o. placed December 2002 3.1. Package GPD pixels –Wire bonding; –Breadboard passive quenching circuitry and GPD pixels. 3.2. Reliability evaluation –Bias several pixels at 1.1V above breakdown for 1,000 hours, document changes in dark count rate, and failure modes, if any. 3.3. GPD performance evaluation –dark count rate vs. T–40 to 30 °C –recovery time vs. pixel area: determine if one microsecond recovery time can be achieved with passive quenching –Gain vs. Temp. and bias Voltage –Detection Efficiency @ Room Temp. 3.4. Optical interface fabrication and assembly –Fab. and evaluate 4x1 beam couplers using GRIN and/or tapered fibers 3.5. Test GPD in Cosmic Ray Setup

13. R.J.Wilson, Colorado State University 50  m diameter GPD layout Proprietary. Do not distribute.

14. R.J.Wilson, Colorado State University Recovery Time with Passive Quenching. 1 x 10  m GPD Simple electronics -limiting resistor 10  s quench time 475 mV 10  s

15. R.J.Wilson, Colorado State University Recovery Time - Active Quenching Design 1: Design 2: Trade off pulse amplitude with pulse width (quench the avalanche sooner) 1  s 0.5  s 2.75 V 325 mV

16. R.J.Wilson, Colorado State University Active Quenching - New Design Preliminary Design 3: 1.2 V 100 ns

17. R.J.Wilson, Colorado State University Temperature Dependence

18. R.J.Wilson, Colorado State University 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -43 -32 -30 -24 -20 -13 2 9 23 T (°C) Preliminary Detection Efficiency Nominal operating voltage 02004006008001000120014001600 Dark Count Rate (Hz) -43 -20 2 23 T (°C) 10  m  gAPD 550 nm, 150 ns laser, 10 kHz Avg. ~7 photons/pulse DE = (Illuminated Rate - Dark Rate)/10 kHz DE Preliminary

19. R.J.Wilson, Colorado State University Optical coupling to small diameter pixels Couple 4 x 1.2 mm WLS fibers to 4 x 1mm glass fibers Draw 4 glass fiber into single fiber, various exit diameters Investigate light transmission efficiency a A D d Concentration Factor, CF = Area of input aperture (A) / Area of photodetector (a) Coupler Transmission Factor, TF = Intensity at input aperture / Intensity at output aperture

20. R.J.Wilson, Colorado State University Optical couplers – area reduction Benefit from tapered fibers compared to ratio of areas is not dramatic  50-200% Preliminary measurements at aPeak are in general agreement with the model We expect to get samples at CSU soon ratio of areas Concentration Factor, CF Transmission Factor

21. R.J.Wilson, Colorado State University Test Setup at CSU Portable dark box Initial Tests Cosmics rays Calibrated with well-understood PMT at CSU Measure efficiency with gAPD+couplers

22. R.J.Wilson, Colorado State University gAPD Progress Summary SLAC+CSU initiated a p.o. to jumpstart further gAPD work at aPeak. New design from aPeak claims to be a more reliable process than the old one. Detection efficiency in 10 micron pixels  15% at room temp.,  25% at –40°C (~kHz dark count rate). Only modest dark count reduction with lower temperature; expected to be better in next batch. Active quenching circuitry provides 1  s-0.1  s pulse widths, no additional deadtime. Successful fabrication of 4x1 tapered couplers – complexity trade-off unclear. 50  m diameter gAPDs breakdown; occurs predominantly at the surface. Due to suspected design sensitivity to humidity. New run, with better control of the surface breakdown is being fabricated. Added backup design to layout. Larger, 150  m devices by early February, 2003.

23. R.J.Wilson, Colorado State University Motivation for Geiger-mode APDs - Recap High gain (~10 9 ), > 1 volt pulses –Minimizes required electronics Good detection efficiency in WLS range (>20%? At 550 nm) –Efficient for low light output from WLS fibers Low supply voltage requirements (~10-40V) –Simplifies wiring harness Minimal cooling requirements –Simplifies mechanical plant CMOS process –“simple” –on-chip integration of readout -> cost-savings

24. R.J.Wilson, Colorado State University Next Steps Many unanswered questions. Need to get the devices in our own lab! Assisting aPeak with SBIR proposal. CSU proposal to DoE Advanced Detector R&D. Hope to provide a real HEP demonstration of utility for broad range of fiber applications.