FACT - First GAPD imaging air shower Cherenkov Telescope – electronics systems

1. FACT - First GAPD imaging air shower Cherenkov Telescope – electronics systems NEC2013 – XXV International Symposium on Nuclear Electronics and Computing 9-16 Sept 2013, Varna, Bulgaria W. Lustermann, ETH Zurich for the FACT collaboration TU Dortmund, ISDC Geneva, University of Geneva, EPFL Lausanne, University of Würzburg, ETH Zurich

2. Detection > 100 GeV gamma rays Signal amplitude: 200 photons / m2 (1 TeVγ-ray) Spectrum: (300 – 600) nm Duration: few ns Night sky: up to several GHz Optical imaging system (causes losses) Mirror  light concentrators  photo-detectors Cherenkov spectrum 2.2 km altitude Cut off ~320 nm Showers can as well originate from protons or electrons – requires a selection

3. Gamma induced air shower detection • GOAL: detection of gamma induced air showers, measurement of the energy and the source position at the sky • The image parameters including the shower shape, position, photon arrive times allow the reconstruction of energy and source position • Air showers induced by gammas, muons and electrons are short ~few ns • Air showers induced by protons are rather long ~(30 – 100) ns • The lower the detectable light level the lower the energy threshold for photons Operation at high night sky background (~1GHz) and at moon light conditions, should be possible – extending observation time

4. The Telescope • European Northern Observatory • Roquede los Muchachos, altitude: 2200 m • Canary Island La Palma • Refurbished HEGRA CT3 • Mirror area: 9.5 m2 • New drive system • New counting hut and electrical installation • New camera • G-APDs (SiPMs. …) • Solid light guides • Fully integrated electronics • Using DRS4 • Operational since October 2011 • Monitoring of bright Blazars • Evaluation of stability and performance

5. Camera features/requirements • Camera • Dim: Length 812 mm, diameter 532 mm, • Weight: ~ 150 kg • 1440 pixels (G-APDs) • FOV: 0.11 deg / pixel (4.5deg total) • Water cooled 4.5 deg • Requirements for the readout electronics: • Dynamic range: ~200 photons / pixel • Resolution: < 0.5 photons (for less than 10 photons) – this will allow to measure the gain of the GAPDs from single photon spectra • Timing resolution ~500 ps • Typical trigger rate of ~50 Hz (~ 350 Hz sustainable trigger rate) • Synchronous trigger distribution ~50ps • Low power consumption • additional electronics: • G-APD bias supply • Low voltage power conversion system • Light monitoring system • Slow control system

6. Telescope Systems Overview

7. Electronics Systems Overview 2 GHz FLV: low voltage conversion FSC slow control (Temp., rel Humidity, voltages) FLP light pulser FDC drive calibration

8. Photo Detectors (G-APDs) • Easy to use • As good as best PMTs (PDE) • Cheaper than PMTs • Dark counts, cross talk and after pulse are no problem for IACTs • Voltage and Temperature dependence can be kept under control rather easily MPPC glued to solid light concentrator: Increase of the sensitive area Limiting angular (watch only the mirror) Single photon resolution

9. Electronics Systems Overview 2 GHz FLV: low voltage conversion FSC slow control (Temp., rel Humidity, voltages) FLP light pulser FDC drive calibration

10. FACT Pre-Amplifier (FPA) • Two different functionalities: • Pre-amplification of signals for later digitization • Summing of signals for the trigger primitives generation • Pre-amplifier • 36 pre-amplifiers channels • Input: AC coupling with npn transistor in base configuration • Input impedance: 25 ohm • Followed by an OPA with gain ~10 • gain: 45 mV / µA • Bandwidth: 200 MHz • Single avalanche signal 2.5 mV at the FAD input of 50 ohm design: U. Roeser, layout: L.Djambazov

11. Electronics Systems Overview 2 GHz FLV: low voltage conversion FSC slow control (Temp., rel Humidity, voltages) FLP light pulser FDC drive calibration

12. FACT Digitization (FAD) • DRS4 (Domino Ring Sampler) – PSI (S. Ritt) • Analog switched capacitor array • 9 channels, 1024 time slices per channel • Operated at 2 GHz (500ps / slice) • ROI: 300 slices (150 ns) • Serial readout • Digitization 12 bit ADC running at 20 MHz • FAD board (in total 40) • 36 channels, four DRS4 with input buffers • 2 dual 12bit ADC (AD9238) • Ethernet interface (Wiznet W5300) • FPGA (Xilinx Spartan-3) • Internal PLL of DRS4 used, locked on clock of the trigger master • Relative timing of all channels of all boards to 300ps is possible (requires calibration) • Controllable voltage source for amplitude calibration DRS4 data: permit and require digital signal processing

13. Arrangement of PCBs FAD’s in one crate are booted in sequence: limit the startup power (required for FPGA booting) Pre-amplifier board (FPA) and analog pipeline ASIC (DRS4) & digitization board (FAD) connected via the mid plane (FMP) distributing power and slow control signals FAD FTU FMP FPA FMP FAD FPA

14. Electronics Systems Overview 2 GHz FLV: low voltage conversion FSC slow control (Temp., rel Humidity, voltages) FLP light pulser FDC drive calibration

15. Trigger system (1) • Trigger on analog sums of non overlapping patches: 9 pixel • Functionality spread over several components: FPA – pre-amplifier, FTU – trigger unit and FTM – trigger master • Rate control system (software) maintains a constant trigger rate (70 Hz) under varying conditions • Counter for all discriminator outputs and the majority coincidence output are implemented  automatic adjustment of discriminator thresholds stabilize trigger rates under varying conditions

16. Electronics Systems Overview 2 GHz FLV: low voltage conversion FSC slow control (Temp., rel Humidity, voltages) FLP light pulser FDC drive calibration

17. Trigger system – Trigger master Note: the trigger part of the VHDL code was purchased from a company

18. Electronics Systems Overview 2 GHz FLV: low voltage conversion FSC slow control (Temp., rel Humidity, voltages) FLP light pulser FDC drive calibration

19. FTM continued and fast control (FFC)

20. Power conversion system

21. Slow Control • The slow control board measures: • all Voltages and currents of the DC-DC converters • 31 temperatures close to the G-APDs in the sensor plane • 24 temperatures for the electronics compartment (crates, DC-DC conv.) • 4 times humidity Arduino Temperature probes: PT1000

22. GAPD bias supply system G-APDs are sorted in groups of 4/5 according to their operation voltage  320 bias channels • HV crate: 320 channels • 1 crate controller with USB interface • 10 HV mother boards • power conversion /distribution and control bus wired in the back of the crate • primary power source: Agilent N5769A • Single channel board • HV operational amplifier OPA454 • controlled by a 12 bit serial DAC (DA8034U) • output voltage adjustable (0 – 90) V • calibration using trim potentiometer • voltage set precision 22 mV • High side current monitor (HV7800) • Over current protection, limit (1-5)mA 32 channel HV mother boards

23. Sensor Plane Assembly 1) MPPCs – cone gluing 2) cone gluing to front window 3) connector cable soldering to MPPC Completed sensor plane

24. Images

25. Control Software (C++, boost) Qt4 DIM: Distributed Information Management System (CERN)

26. Single Photons, Time Resolution • Digitized data allow post-processing – increasing understanding and performance Oversampling allows noise reduction • Excellent single photon resolution allows precise inter-calibration • Excellent timing resolution 1pe single photon spectrum of one channel 2pe gain fit Timing resolution obtained from the differences of the photons arrival times in muon rings: 600 ps

27. Single p.e. spectrum all pixels 1pe • Dark count spectrum (calirated) • Closed shutter • 1440 pixel • 180k events • All gains normalized to 1 • Gain variations: • < 6% (temp/time) • < 4 % pixel to pixel 2pe 3pe 4pe fit 5pe 6pe gain 7pe 8pe

28. Gain stabilization • 2) Compensation for changing NSB conditions • Measure bias currents • Calculate voltage drops and compensate • 1) Compensation for temperature changes • Measure temperatures near the GAPDs • Correct Vbias for the change of Vbd to maintain Vover constant Vs Vbias with compensation Vbias= Vs – R * I(NSB) 3) Verify the stability using the temperature stabilized light pulser installed in the center of the mirror dish without compensation Achieved gain stability: ~6% Conclusion: light pulsernot required, temperature and bias current based feedback sufficient

29. Trigger Rate Scans Trigger rates as function of Vbias • Trigger rate scans: • Varying the discriminator thresholds of the trigger patches • 26 trigger rate scans (Mar – Jul 2012) Vbias: (0.8 – 1.6) V Nominal: 1.2 V 90% full moon Digital noise NSB air showers Gains and trigger system are very stable Dark night air showers Observations with high night sky background (NSB) are possible (full moon)  increase of observation time

30. Crab nebula Hubble: Optical FACT: CRAB PWN 14.3 h (19.5-29.6.2012) - ‘standard candle’ Chandra: X-ray Significance: 20.8σ ton / toff = 0.2 N excess = 328.8 N background = 102.2 Courtesy of NASA/ESA

31. Singnals from Markarians FACT: Mrk 501 – 35.1 h (19.5-29.6.2012) Significance: 37.9σ ton / toff = 0.2 N excess = 1009.4 N background = 269.6 FACT: Mrk 421 – 23.4 h (28.2-9.5.2012) Significance: 6.6σ ton / toff = 0.2 N excess = 101.4 N background = 162.6

32. Mrk501 rate Mrk 501 flare (observed by FACT) – send alert to Magic About 5 min. of observation would have been sufficient to detect the flare! Monitoring of bright sources with small telescope is possible rate / hour excess x 7 alert stable background Day (MJD)

33. Summary/Conclusion • Electronics system works reliably since 2 years • Signals from CRAB, Mrk421, Mrk501 observed • Excellent performance permit bright blazar monitoring • Minor problems (one DC-DC converter failure, one cooling pump failure solved on site) Join us during observation at: www.fact-project.org/smartfact