Ultra-fast sampling and readout for the MAGIC-II telescope Data Acquisition system

Since a couple of decades the astronomical and astrophysical exploration has been proceeding through two major streams. The former in the study and observation of far and weak sources in the Universe, with the consequent need to develop new technologies to increase the instruments sensitivity in order to explore the Universe, the latter in the observation and study of the cosmic radiation, composed by charged cosmic particles, neutrinos and high energy photons, at different energies in order to investigate their sources and the related physical phenomena. Such stream of research is called Astroparticle physics, being strictly connected to many issues, items and instrumental technologies common to particle physics. The γ-astronomy is, therefore, a branch of astroparticle physics whose target is to study all those astrophysical sources responsible for the emission of High Energy (HE) radiation. Several kind of sources are responsible for the radiation emission, both in the galactic environment and at extragalactic distances. γ radiation can be studied from few tens of keV up to several tens of TeV, covering an energy range of nine orders of magnitude. The related physical phenomena involved in the production of γ radiation can be very different and the experimental techniques to detect the γ particles can vary a lot. This is the main reason behind the efforts of the astronomical community for the so-called Multi Wave Lenght (MWL) campaigns, observational campaigns on sources at different wavelength (energy). The γ cosmic radiation is mainly detected by using two techniques. The space-based technique, that consists in the direct detection of the primary γ by instruments installed onboard of a satellite. This technique has been used for the last three decades, and until the end of ’80s it has been the only possible technique suited for the detection of the γ cosmic radiation. Space-based instruments for γ radiation usually consist in particle detectors, while rejecting the charged ones by using anti-coincidence techniques. Space-based instruments can observe the low energy γ radiation, from few tens of keV up to few GeV. Only recently new instruments capable to reach hundreds of GeV have been built1. This kind of instruments typically have a large Field of View (FoV) and therefore can perform a sky-survey activity, becoming then well suited for transient phenomena search like Gamma-Ray Bursts. On the other hand, the effective area available on such kind of detectors is very low, of the order of 1 m2, which makes the observation of sources beyond GeV energies very difficult or even impossible because of the steep decrease of the γ radiation flux. The second technique, used by ground-based experiments, consists in the indirect observation of the primary γ thanks to the atmosphere which acts as a calorimeter. The primary γ, in fact, when interacting with the hadronic nuclei of the atmosphere, decays into an electron-positron pair that starts to develop a particle shower. All the charged particles of the shower are moving relativistically through the atmosphere, thus producing light by Cherenkov effect [1]. Information about the primary γ photon and the direction of incidence can be retrieved by the analysis of the particle shower shape by using two techniques. The one used by Extensive Air Shower (EAS) arrays, which simply collects the charged particles of the shower reaching the ground, and reconstruct the shape and direction of the primary γ by looking at their arrival time and distribution at ground. The other technique, used by Imaging Air Cherenkov Telescopes (IACT) instruments, reconstructs the image of the shower from the Cherenkov light produced in the atmosphere by the shower itself. Groundbased instruments are thus complementary to space-based ones in γ astronomy since they cover a different energy range extending from hundreds of GeV up to several TeV. Low γ fluxes at high energies become observable thanks to the greater sensitivity available for ground based telescopes, of the order of 105-106 m2 i.e. five or six orders of magnitude bigger than that one available on a space-based instruments. On the other hand, EAS arrays can perform a sky survey while IACT can only perform a follow-up observation because of their small Field of View. Moreover local ground conditions strongly constrains the IACT duty cycle, affecting then the observation schedule as well as the search for transient phenomena. A detailed description of the ground based IACT technique can be found in [1]. The Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC Telescope) is a new generation of IACT. It is operated since 2004 at the Roque de Los Muchachos, on the Canary Island of La Palma, Spain. The site has been chosen according to some requirements that are very important for an optimal Cherenkov observation [1]: the climate is very dry, in normal conditions is common to have the relative humidity of the air below 10%, and the air is cleaned from heavier particles, so there is a low Rayleigh and Mie diffusion very important point for observations of Cherenkov light in the 290-700 nm wave length range; there is a low natural diffuse background light, like auroras, as well as low artificial diffuse light, like urban luminosity. Moreover the cloud coverage of the sky is less than 15% during the year. The MAGIC Telescope exploits several new technologies for the observation of gamma cosmic radiation, to be applied in the context of IACT technique. A complete technical description of the MAGIC Telescope can be found in [2]. The general design aims to reduce the energy threshold down to few tens of GeV and to reduce the slewing time to few tens of seconds, in order to observe the prompt emission of transient phenomena like Gamma-Ray Bursts [3], an important phenomenon in astroparticle. With its large reflective surface, consisting in a 17 m Ø dish tasselled with square aluminium mirrors, the MAGIC telescope can achieve a 30 GeV energy threshold at trigger level, the lowest energy threshold currently available on a IACT, covering the energy gap between the observations by satellites and previous generation Cherenkov detectors. Moreover the extremely light carbon fibre structure reduces the total weight of the telescope as well as its inertial moment. These last two features provide the lowest slewing time currently available on a IACT, less than 30 s to point any position in the sky. The camera of the MAGIC telescope consists of 576 photomultiplier tubes, shortly PMTs, which record the fast pulses from the γ-ray air shower Cherenkov light about 2 ns full width at half maximum to the experimental control house. There, they are converted to digital data by the data acquisition system. Until 2006 the PMT signals were digitised with 300 MSamples/s Flash analog-todigital Converters (FADCs). On February 2007 the MAGIC Telescope data acquisition system was upgraded with ultra-fast FADCs capable to digitize at 2 GSamples/s [4], [5], [6] The upgrade resulted in an improvement of the telescope performance for two reasons: a reduction in the amount of night sky background light integrated with the real signal, and an improvement in the reconstruction of the timing characteristics of the detected events. A good precision in the measurement of the arrival time of the signal allows to exploit the timing characteristics of the shower particles more scattered when produced by hadrons than hen they are produced by γ particles. The image cleaning procedure can be made more efficient thanks to the introduction of time-constrains, and the background rejection can be made more effective by introducing new time-related image parameters in addition to the standard set commonly used [1]. Previous studies [7] reported the possibility to use effectively the timing information to improve the analysis performance. In particular, from a study based only on simulated data, sensitivity improvement of the order of 15 - 20 % in sensitivity was expected also with the 300 MSamples/s MAGIC readout configuration [8]. Currently, the MAGIC collaboration is building a second telescope, MAGIC-II, a mechanical clone of the first telescope with innovative features, such as a camera [9] composed by an array of 1039 new photon detectors, high quantum efficiency photomultipliers that will be upgraded by new generation silicon photomultipliers and a new ultrafast signal sampling to improve the time signal resolution and reduce the effect of the diffuse night sky background. The camera signals, generated by photon detectors, are transported to Receiver boards2 [10] by using optical fibers. Here they are discriminated and logical signals for the trigger are produced [11]. If a trigger occurs the camera signals are recorded by the Data Acquisition System (DAQ) and digitized. The design and development of the DAQ for the MAGIC-II telescope is the subject of this PhD thesis. The new DAQ is based on an innovative analog sampler called Domino Ring Sampler Version 2, or shortly DRS2 [12], designed for sampling high speed signals, and on a 9U VME digital motherboard, called PULSAR board3, explicitly developed for high energy and astroparticle experiments. Furthermore, the future upgrade of the DAQ system, based on the new Domino Ring Sampler version 4 (DRS4) [13], is presented. The DRS2 sampler is a 0.25 μm CMOS chip developed at the Paul Scherrer Institute (PSI), Villigen, Switzerland. It is especially suited for the modern physics experiment where often it is required to sample ultra fast signals at high frequency. An extensive work of characterization has been performed on the DRS2 chip [14] whose innovative characteristics are available frequency ranging from 0.5 to 5 GHz, integration of at least 10 analog channels in one chip, analog bandwidth around 250 MHz and a very low power consumption of about 35 mW per chip. The chip is housed in a PLCC package and mounted on a mezzanine board, called DRS2 Mezzanine [15]. Each DRS2 Mezzanine can house up to 2 DRS2 chip for a total of 20 analog channels, while the data are digitized with a 12-bit resolution ADCs at 40 MHz placed on the same Mezzanine. The data handling and reformatting are performed by the PULSAR board [16], [17]. The PULSAR board can host up to 4 DRS2 Mezzanine, for a total of 80 analog channels. Three FPGAs (ALTERA EP20K400BC652-1XV) are mounted on each board; they are responsible for interfacing the DRS2 Mezzanine and data communication with the transmission board located on the rear VME backplane. Additional digital signals are interfaced via front panel connectors of the board and enter directly in the data acquisition stream. This work is organized as follows. In chapter 2 a short introduction and description both of the Gamma Ray astronomy and the MAGIC experiment is given, including a description of the Cherenkov effect and the Imaging Air Cherenkov Technique. Furthermore, the novelties introduced in the new MAGIC-II telescope are presented and discussed. In chapter 3 the MAGIC-II DAQ is presented: in order to increase the sensitivity of the MAGIC telescope a new ultra-fast data acquisition system is developed, based on the innovative DRS2 chip. The DRS2 Mezzanine board, that hosts the sampler is developed and discussed in detail. Furthermore, the complex firmware to format and control the data flow and the Mezzanine is explained. The DAQ system is being commissioned in these months and already taking data while this thesis is being written. In chapter 4 the upgrade of the DAQ system is presented and discussed, the new system is based on the new DRS4 chip, that is now being tested and characterized. Finally, the results of the work will be summarized in the concluding chapter, together with a short description of future developments based on this knowledge. New concepts and trends for modern astroparticle experiments will be presented. Chapters 3 and 4 are the main work of the candidate during his PhD course. The chapter 4 and the Appendix B cannot be published for disclosure agreements.