Coarasa, J A

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29th International Cosmic Ray Conference Pune (2005) 3, 281284The Data Acquisition of the MAGIC II Telescope.J.A. Coarasa , J. Cortina, M. Barcelo, M. Martinez, I. Martinez, E. Ona-Wilhelmi,R. Paoletti, R. Pegna, A. Piccioliand N. Turinion behalf of the MAGIC collaboration.(a) Max Planck Institut f¤ur Physik, F¤ohringer Ring 6, 80805 M¤unchen, Germany(b) IFAE, Edici Cn, Facultat de Ciencies, UAB, 08193 Bellaterra (Barcelona), Spain(c) University of Siena and I.N.F.N. Sezione di Pisa, ItalyPresenter: J.A. Coarasa (tcoarasa@mppmu.mpg.de), ger-coarasa-perez-JA-abs1-og15-posterThe MAGIC telescope is the largest 	 -ray Imaging Cherenkov telescope in the world. It is operatingsince 2004 at the Roque de Los Muchachos observatory, La Palma, Canary islands. MAGIC-II is the up-grade of this project, consisting of a twin telescope frame with innovative features like new photon detectors tolower the threshold energy further, and an ultrafast signal sampling to reduce the effect of the diffuse night skybackground. The new acquisition system is based upon a low power analog sampler (Domino Ring Sampler)with frequency ranging from 1.5 to 4.5 GHz and data are digitized with a 12 bits resolution ADC. The analogsampler, originally designed for the MEG experiment, has been successfully tested on site and showed a verygood linearity and single photon discrimination capability. Data management is performed by 9U VME digitalboards which handle the data compression and reformatting as well. Every board hosts 32 analog channelsplus auxiliary digital signals for trigger and monitor purposes. For a 1 kHz trigger rate and a 2 GHz frequencysampling, the data throughput can be as high as 100 MBytes/s, thus being a challenge for modern data trans-mission and storage solutions. The data are transferred to PCI memory via Gbit optical links using the CERNS-link protocol and to the mass storage system consisting of a RAID system and tape minilibraries. The dataacquisition system design and performance will be described in detail.1. IntroductionThe MAGIC telescope aims at measuring low energy gamma-initiated showers (  ) by looking atthe Cherenkov light emitted in the atmosphere [1]. This signal is contaminated by background events due tovarious sources: hadronic showers, local muons, Light of the Night Sky Background (NSB), moon light andbright stars in the field of view of the camera.The upgrade program of the MAGIC telescope involves the construction of a second telescope (MAGIC-II)which is mechanically identical. Several technological updates are foreseen for MAGIC-II, namely: newphoton detectors, either high quantum efficiency photomultipliers or new generation silicon photomultipliers(APD or SiPM); and a new data acquisition system, based on fast (2 GHz) analog samplers, coupled to highbandwidth data transmission. These two changes will allow a more effective suppression of the light of thenight sky and a stronger rejection of hadron induced air showers, while efficiently selecting  -ray showersthrough using the more accurate timing information.The Data taking chain works the following way. The new photon detectors collect the 1-2 nsec full width halfmaximum Cherenkov light flashes. Their ouput signals are amplified by ultrafast and low-noise transimpedancepre-amplifiers in the camera housing. The amplified analog signals are transmitted over 170 m long opticalfibers. In the electronics hut the signals are split. One branch goes to a discriminator with a software adjustablethreshold that generates a signal for a 2 level trigger system [2] capable of limiting the trigger rate to ﬀ 1kHz. The signal in the second branch goes to the new analog sampler described in the next section. Thedigitized signals are stored by a data acquisition system capable to process a sustained data rate of  100 MB/s.Immediately after the data is taken, thanks to an independent network for the storage, an Online Analysis startson the data.282 J.A. Coarasa et al.2. Sampling ElectronicsThe digitizing core of the Data Acquisition for the MAGIC-II telescope is constituted by the analog samplerdeveloped at the Paul Scherrer Institute (PSI, Villigen-Switzerland) [3], called Domino Ring Sampler (DRS)and now at a second version (DRS2). It is an analog sampler designed with 0.25 ﬁ m CMOS technology. Anextensive work of characterization has been performed on the Domino chip [4] whose main characteristicsare: integration of at least 10 analog channels in one chip; analog bandwidth of around 1 GHz; excellenttime resolution, less than 80 picosecond with a sampling frequency of 4 GHz; very low power consumption,around 35 mW per chip; the response of the sampler cells can be calibrated and the useful input voltage can beextended to 2 Volts (twice the range for commercial Flash ADCs).Figure 1. Schematic picture of the Domino sampler. Shown on top is theinverters sequence that originates the Domino wave. The shift register onthe bottom enables serially the single capacitors connecting them to theoutput stage.The sampler consists of two sections, ananalog one for the signal sampling and adigital for control and multiplexing. Theanalog signal is stored in a multi capaci-tor bank (1024 cell in DRS2) that is orga-nized as a ring buffer, in which the singlecapacitors are sequentially enabled by ashift register driven by a high frequencyclock internally generated (see Figure 1),called Domino wave. At a 2.5 GHz fre-quency the sampling window is 400 nswide. The speed of the domino waveis controlled by an external voltage suchthat the domino inverters can be seen as aVoltage Controlled Oscillator (VCO). Ev-ery turn a pulse is produced for synchro-nization and monitoring. The phase andspeed of the domino wave are synchronized to an external common reference clock by an on-board PLL,designed around the domino VCO. The sampling signal jitter is less than 200 ps (see Figure 2a).Input DAC (V)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2ADCcount220024002600280030003200340036003800Linearity plot for channel 0 cell 0(a) (b)Figure 2. (a) Domino wave frequency jitter. (b) Domino response as a function of the input voltage.The chip is housed in a PLCC package and mounted on a mezzanine card, also called CRAB (Capacitor RingAnalog Board). Once the external trigger has been received, the sampled signal in the ring buffer is put on anThe Data Acquisition of the MAGIC II Telescope 283output stage by a multiplexer and digitized at high resolution (12 bits) at lower frequency (40 MHz). The digitalconversion is done by an external ADC. The transfer curve of the output stage is typical of a MOS transistor,with an active region quite linear up to 800 mV, that tends to saturate for larger signals (see Figure 2b). Thischaracteristics turns out to be a useful one since it is a natural signal compression and results in an extensionof the response dynamic range. With a calibration procedure that is implemented in the analog mezzanine it ispossible to calibrate input signals up to 2 Volt, about twice the range of commercial Flash ADCs.By using fast sampling frequency and large dynamic range it is possible to reconstruct the shape of individualphotons impinging on the photon detector and measure the signal with good precision, allowing for furtherbackground rejection.3. Data ReadoutThe readout system is based on the so called Pulsar board (see Figure 3) that can host up to 4 analog mezzanineboards, for a total of 32 analog channels. Three FPGAs (ALTERA EP20k400) are mounted on each board; theyare responsible for interfacing to the CRAB boards and data communication with the transmission board lo-cated on the rear VME backplane. Additional digital signals are interfaced via front panel connectors and enterdirectly in the data acquisition stream. The external trigger signal is sampled together with the analog channelsgiving a precise time reference, useful for synchronization of different devices or precise time measurement.Figure 3. Schematic drawing of the data readout board.The whole readout system consists of 18 Pulsar boardsthat are accommodated in two VME-9U crates, interfacedto the telescope control system by an embedded CPU.The data transmission is performed by optical link drivers(called HOLA), specifically designed for LHC experi-ments [5]. The optical fibers are interfaced to a PC mem-ory by dedicated PCI cards named FILAR, also designedfor LHC purposes.The usage of an ultrafast sampler in Cherenkov telescopeswith a large number of channels brings to systems that,at 1 kHz trigger rate, produce a data flow of the orderof 100 MBytes/s, the data volume to store being of theorder of several Terabytes per night. To cope with suchlarge throughput rates and data volumes a RAID systembased on a separate network (probably fiberchannel) is be-ing tested. Such a network (allowing up to 4 Gbit/s access)together with a file locking filesystem will allow true Online access to the just recorded data, for analysis andarchiving. A cluster of computers will start an Online Analysis immediately after the files are closed.4. Performance testThe readout system has been used to measure the night sky background with standard photomultipliers in theMAGIC telescope framework and a work on the characterization of SiPM has just started.Tests performed on the MAGIC telescope site have shown that it is possible to disentangle the single pho-toelectrons (phe) and to measure the diffuse light background (40 phe in a 400 ns window) and to obtain aphysics meaningful measurement, provided that the photomultiplier signals are sampled at 2.5 GHz at least. InFigure 4a a typical image of the incoming photons is shown. The poissonian nature of this process is verifiedby the arrival time distribution of single peaks that shows a typical exponential trend.284 J.A. Coarasa et al.(a) (b)Figure 4. (a) Typical sampling of a MAGIC photomultiplier signal during a 400 nsec interval. (b) Charge spectrum onSiPM with simple integrating algorithm.The Domino readout can be effectively used for the characterization of very fast pulses, such as those producedby Gieger-mode, APD-based, silicon photomultipliers (SiPM) [6]. Figure 4b shows the preliminary chargespectrum obtained by illuminating a SiPM with a fast-pulsed LED at a low light level. The peaks due to theindividual photolectrons are clearly resolved.5. ConclusionsThe upgrade program of the MAGIC telescope is currently under way. Together with the adoption of newphoton detectors, the analog signals are going to be sampled by the Domino ring sampling chip. For a 1 kHztrigger rate and a sampling frequency of 2 GHz, the data throughput is higher than 100 MBytes/s, being achallenge for modern data transmission and storage solutions. The data are transferred to PCI memory viaGbit optical links using the CERN S-link protocol and to the mass storage system.6. AcknowledgementsWe would like to thank the IAC for excellent working conditions. The support of the German BMBF andMPG, the Italian INFN and the Spanish CICYT is gratefully acknowledged.References[1] C. Baixeras et al. Nucl. Instr. and Meth. A 518 (2004) 188.[2] R. Paoletti for the MAGIC Collaboration, ”The Trigger System of the MAGIC Experiment”, to appear onthe IEEE Trans. Nucl. Sci., 2004[3] S. Ritt, Nucl. Instrum. Meth. A 518 (2004) 470.S. Ritt, ”The DRS2 Chip: A 4.5 GHz Waveform Digitizing Chip for the MEG Experiment”, to appear onthe IEEE Trans. Nucl. Sci., 2004.[4] R. Pegna et al., The DRS VME Board: a Low Power Digitizing System in the GHz Range, Proceedingsof the 9th Topical Seminar on Innovative Particle and Radiation Detectors, 23 - 26 May 2004, Siena, Italy.[5] H.C. van der Bij et al., S-LINK, a Data Link Interface Specification for the LHC Era, Proceedings of theBeaune 97 Xth IEEE Real Time Conference.[6] V. Savelev, Nucl. Instrum. Meth. A 535 (2004) 528.

The Data Acquisition of the MAGIC II Telescope

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