• handling
• cherenkov
• astroparticle
• acquisition
• sampling
• trigger

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.