Avalanche Photo-Diodes (APDs) for the CMS Electromagnetic Calorimeter (ECAL)

1. Avalanche Photo-Diodes (APDs) • for the CMS • Electromagnetic Calorimeter (ECAL) • K.Deiters, Q.Ingram, D.Renker, T.Sakhelashvili • Paul Scherrer Institute, Villigen, Switzerland • I.Kronqvist, R.Rusack, A.Singovski, P.Vikas • University of Minnesota, Minneapolis, USA • A.Kuznetsov, Y.Musienko, S.Reucroft, J.Swain • Northeastern University, Boston, USA • Z.Antunovic, N.Godinovic, I.Soric • University of Split, Croatia • HEP2001, Budapest. July 13th 2001.

2. APDs Outline • Requirements • Performance • Radiation Hardness • Conclusion

3. The Detector PbWO4 crystal

4. APD structure Photo-conversion electrons from the thin p-layer induce avalanche amplification at the p-n junction Electrons from ionising particles traversing the bulk are not amplified 2 APDs (each 5 x 5 mm) mounted in a capsule ready for gluing to a crystal

5. APD Requirements Operate in 4 Tesla field Radiation hard (2.1013 n/cm2 + 250 kRad) Fast ( 10 nsec) Compatible with ECAL energy resolution requirement Insensitivity to particles traversing the diode Amplification Cheap (122400 pieces) ==> 8 year R&D effort by Hamamatsu (and initially EG&G) in close collaboration with CMS ECAL

7. APD Impact on Energy Resolution • ECAL energy resolution: • CMS design goal : a ~3%, b~0.5%, c~200 MeV • APD contributions to: • a:photo statistics (area, QE) and avalanche fluctuations (excess noise factor) • b:stability (gain, sensitivity to voltage, temperature variation, aging • and radiation damage) • c: noise (low capacitance, serial resistance and dark current)

8. APD Properties • Active area (2 APDs per crystal) 5 x 5 mm (each) • Quantum efficiency 75% at 430 nm • Light collection within 20 nsec 99 ± 1% • Operating voltage ~ 380 V • Gain (M) 50 (Max >1000) • Capacitance 80 pF • Serial resistance 3 Ω • Dark Current < 50 nA(~ 10 nA typical) • Voltage sensitivity (1/M*dM/dV) 3.15 % / V • Temperature sensitivity (1/T*dM/dT) -2.2 % / V • Excess noise factor 2.1 • Thickness sensitive to ionising particles 5 μm • After radiation and accelerated aging equivalent to 10 years of LHC, ONLY quantity to change is the dark current, which rises to 5 μA

9. APD Gain, V and T sensitivity Voltage sensitivity Operating Gain 50 —> Temperature sensitivity Well behaved up to gain ~2000

10. APD Capacitance, Quantum efficiency ^ Vr ^ PbWOpeak emission Q.E. is 75% at peak emission but APD insensitive to traversing ionising particles (5μm effective thickness) APD is fully depleted at operating voltage

11. Breakdown - Operating Voltage 1999 - 2001 Vb - Vr found important indicator of radiation hardness Absolute value should be large Spread in Vb - Vr is small First production 2001

12. Reliability • Production in 2000 • Few % “died” (breakdown voltage drops below operating voltage) • 1) in accelerated aging testing (80-90 deg) • 2) in radiation testing (protons) • ==> Production stopped • 1): Origin soon traced by Hamamatsu. Problem was solved. • 2): Proved much harder. Complex with number of different causes: • • over 6 months intensive R&D by Hamamatsu • • review of radiation testing procedures at PSI • ==> Production restarted (3/2001)

13. Radiation Hardness: Conclusion • Basic APD structure is radiation hard • APDs found sensitive to Co γ-irradiation, not sensitive to neutrons: • problem at surface, not inside the silicon • Solution:modify geometry to reduce lateral fields (rounder corners, change spacings between structures, field clamps, etc.) • basic APD structure unchanged. • Plus: screening of all APDs with Co γ-irradiation (500 kRad) • Reject on lowered Vb, anomalous dark current • followed by 2 weeks annealing/aging testing at 90 deg • sampling (5%) testing with 2 x 1013 neutrons/cm2

14. Cobalt screening results Induced dark current almost completely anneals after 2 weeks at 90 deg Change in Vb after Co Irradiation ---------Vr----------------------------------------------------------------- X 1 day after Co irradiation + after 10 days at 90 deg Reject 2 APDs with change in Vb

15. Radiation Hardness: Status • Of first 2700 APDs delivered in 2001, ~ 3% failed Co irradiation • Investigation:1) All APDs from a few wafer positions are bad • (over half the failures) • 2) APDs from random wafer positions • Hamamatsu: 1) Bad position on mask. Reject all APDS from here • 2) Dust, etc Reject APDs based on screening • Other indicators :Vb-Vr is abnormally low • Discharge-like noise • ….. (still investigating) • Status: In next two Lots tested (7/01) only ~ 0.5% failed Co irradiation • Of ca 500 APDs neutron irradiated, NONE have failed

16. Neutron irradiation • Neutron irradiation of APDs with the Minnesota Cf source (4 days) All 25 APDs behave similarly Spread in induced dark current largely due to flux non-uniformity Dark current (log) vs time

17. APD Summary • The Hamamatsu APD meets all the specifications • Radiation hardness proved hardest to achieve • Now satisfactory. Expect to achieve >> 99% • Mass production rising to full rate (>1000/week)

18. Photodetectors for the CMS ECAL • Photodetectors developed for the barrel (APDs) and end-cap (VPTs) of the CMS ECAL • Fast, work in 4 T field, radiation hard • Both meet the specifications necessary for CMS to find the Higgs in the favoured mass range (and much more….)