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

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

• Introduction• Camera systems• Electronics• Control software• Results• Summary/Conclusion

Content

NEC2013, 8-16 Sept 2013, Varna, Bulgaria 2W. Lustermann, ETH Zurich (for the FACT collaboration)

Detection > 100 GeV gamma rays

Cherenkov spectrum 2.2 km altitude Cut off ~320 nm

Signal amplitude: 200 photons / m2 (1 TeV γ-ray)Spectrum: (300 – 600) nmDuration: few nsNight sky: up to several GHz

Optical imaging system (causes losses)Mirror light concentrators photo-detectors

Showers can as well originate from protons or electrons – requires a selection


Gamma induced air shower detectionGOAL: 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) nsThe 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


The TelescopeRefurbished HEGRA CT3• Mirror area: 9.5 m2

• New drive system• New counting hut and

electrical installation

European Northern Observatory• Roque de los Muchachos, altitude: 2200 m• Canary Island La Palma

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


Camera features/requirementsCamera• Dim: Length 812 mm, diameter 532 mm,• Weight: ~ 150 kg• 1440 pixels (G-APDs)• FOV: 0.11 deg / pixel (4.5 deg total)• Water cooled4.5

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


Telescope Systems Overview


Electronics Systems Overview

FLV: low voltage conversionFSC slow control (Temp., rel Humidity, voltages)FLP light pulserFDC drive calibration

2 GHz


Photo Detectors (G-APDs)

Photo detectors: Hamamatsu MPPC• active area: 3 x 3 mm2

• 3600 pixels of (50 m)2

Operation voltage: ~70 V

Gain (nominal): 7.5 x 105

Photon detection efficiency (peak): ~35%

Single photon resolution

• 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 areaLimiting angular (watch only the mirror)




2 GHz


FACT Pre-Amplifier (FPA)

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

Two different functionalities:• Pre-amplification of signals for later digitization• Summing of signals for the trigger primitives generation

design: U. Roeser, layout: L.Djambazov




2 GHz


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


Arrangement of PCBs

Pre-amplifier board (FPA) and analog pipeline ASIC (DRS4) & digitization board (FAD) connected via the mid plane (FMP) distributing power and slow control signals

4 water cooled costume crates:• 10 FAD boards• 10 FPA + FTU boards• Heat spreading planes in the pcb’s• Wedge locks as thermal interface

Mid plane (FMP): • press fit connectors with pins mate able on both side for

analog signal passage• RS485 buses• Power distribution

FAD’s in one crate are booted in sequence: limit the startup power (required for FPGA booting)

FTU

FPA

FAD

FMP

FPA

FMP

FAD




2 GHz


Trigger system (1)

Trigger unit (FTU) – 40 pieces• Mezzanine card on the pre-amplifier card• 12 bit DACs for discriminators thresholds: 15 DAC counts / p.e.• Majority coincidence N-out-of-4 logic, combine trigger signals

(practically an OR is used)• RS485 interface to trigger master

FPA: pre-amplifier• analog summing of signals of patches (9 channels)• Masking of individual channels (noisy)• Clipping of trigger sum to 10 ns• Discrimination of the sums: 4 trigger signals

• 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




2 GHz


Trigger system – Trigger master

Trigger master (FTM) – 1 piece• receives 40 FTU signals• Trigger: N out of 40 logic, for an

adjustable time window of (8-64)ns• Special triggers: light pulser (N=25)

random (clock)• Interface for external trigger• Triggers can be individually enabled

and run simultaneously • Ethernet interface for control and

setup• Control of all sub units FAD and FTU

via RS485

Note: the trigger part of the VHDL code was purchased from a company




2 GHz


FTM continued and fast control (FFC)4 fast control signals: CLOCK, RESET, TRIGGER, TIMEMARKER

• CLOCK: used to synchronize all DRS4 (<100ps jitter)• RESET: reset all DRS4• Trigger:

• upon detection of a trigger condition a rectangular pulse (TIMEMARKER) is emitted (with a customizable delay) which is coupled into channel 9 of all DRS4 chips

• After another customizable delay the TRIGGER signal is send• A bit pattern containing the event ID and the trigger type is distributed to all FAD boards via

RS485 and included into the digitized data• While digitizing the FAD boards send a busy signal blocking the generation new triggers

Tow dedicated fast control pcbs: • 1 to 10 fold fan-outs (ON Semiconductor

MC100LVEP111• WireWin Cat. 6 slim LVDS cables with RJ45

connectors• Test results: jitter < 20ps and skew<250 ps


Power conversion system

three Agilent AC-DC supplies: • G-APD bias (85V) • interlock system and heaters (24V)• Camera (48V)• two 45m long cables provide power and ten

G-APD bias voltages to the patch panel

Power conversion inside the camera:• DC-DC converters (VICOR VI-J300 series)• Adapted filter: mainly a common mode choke

and a Tantalum capacitor• step-down converters on the FAD

Power consumption:• Total inside 570W• 100W in the cables• 100W DC-DC converter• Outside: 100W G-APD bias


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

Slow control board:• Atmel ATmega32L micro-controller (on Arduino board)• Wiznet W5300 Ethernet interface for data transmission. • 148 channels multiplexed onto a 24 bit ADC, AD7719 with integrated

current sourcing for temperature probes.

Temperature probes: PT1000

Arduino


GAPD bias supply system

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

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

G-APDs are sorted in groups of 4/5 according to their operation voltage 320 bias channels


Sensor Plane Assembly1) MPPCs – cone gluing

2) cone gluing to front window

Completed sensor plane 3) connector cable soldering to MPPC


Images


Control Software (C++, boost)

DIM: Distributed Information Management System (CERN)

Qt4


Single Photons, Time Resolution

single photon spectrum of one channel

Timing resolution obtained from the differences of the photons arrival times in muon rings: 600 ps

• Digitized data allow post-processing – increasing understanding and performance Oversampling allows noise reduction

• Excellent single photon resolution allows precise inter-calibration

• Excellent timing resolutionfit

1pe

2pe

gain


Single p.e. spectrum all pixels

Dark count spectrum (calirated)• Closed shutter• 1440 pixel• 180k events• All gains normalized to 1• Gain variations: < 6% (temp/time) < 4 % pixel to pixelgain

fit

1pe

2pe

3pe

4pe

5pe

6pe

7pe

8pe


Gain stabilization

1) Compensation for temperature changes• Measure temperatures near the GAPDs• Correct Vbias for the change of Vbd to

maintain Vover constant

2) Compensation for changing NSB conditions• Measure bias currents• Calculate voltage drops and compensate

VbiasVs

Vbias = Vs – R * I(NSB)

3) Verify the stability using the temperature stabilized light pulser installed in the center of the mirror dish

with compensation

without compensation

Achieved gain stability: ~6%

Conclusion: light pulser not required, temperature and bias current based feedback sufficient


Trigger Rate Scans

air showers

Dark night

90% full moon

NSB

Trigger rate scans:• Varying the discriminator thresholds

of the trigger patches• 26 trigger rate scans (Mar – Jul 2012)

Observations with high night sky background (NSB) are possible (full moon) increase of observation time

Trigger rates as function of Vbias

Vbias: (0.8 – 1.6) VNominal: 1.2 V

Digital noise

Gains and trigger system are very stable

air showers


Crab nebulaFACT: CRAB PWN 14.3 h (19.5-29.6.2012) - ‘standard candle’

Significance: 20.8σton / toff = 0.2N excess = 328.8N background = 102.2

Courtesy of NASA/ESA

Hubble: Optical

Chandra: X-ray


Singnals from Markarians

FACT: Mrk 501 – 35.1 h (19.5-29.6.2012)

Significance: 6.6σ ton / toff = 0.2N excess = 101.4N background = 162.6

FACT: Mrk 421 – 23.4 h (28.2-9.5.2012)

Significance: 37.9σ ton / toff = 0.2N excess = 1009.4N background = 269.6


Mrk501 rate

Day (MJD)

rate

/ ho

ur

excess x 7

Mrk 501 flare (observed by FACT) – send alert to MagicAbout 5 min. of observation would have been sufficient to detect the flare!Monitoring of bright sources with small telescope is possible

alert

stablebackground


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

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

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