AGATA (Advanced GAmma Tracking Array) represents the state-of-the-art in gamma-ray spectroscopy and is an essential precision tool underpinning a broad program of studies in nuclear structure, nuclear astrophysics and nuclear reactions. Consisting of a full shell of segmented germanium detectors instrumented with digital electronics its projected detection sensitivity rises much higher than for precedent shielded germanium array and up to 1000 times in specific configurations.
\end{abstract}
\section{The GAMMA experiment and the AGATA array}
The strong interaction described by quantum chromodynamics (QCD) is responsible for binding neutrons and protons into nuclei and for the many facets of nuclear structure and reaction physics. Combined with the electroweak interaction, it determines the properties of all nuclei in a similar way as quantum electrodynamics shapes the periodic table of elements. While the latter is well understood, it is still unclear how the nuclear chart emerges from the underlying strong interactions. This requires the development of a unified description of all nuclei based on systematic theories of strong interactions at low energies, advanced few- and many-body methods, as well as a consistent description of nuclear reactions. Nuclear structure and dynamics have not reached the discovery frontier yet (e.g., new isotopes, new elements, …), and a high precision frontier is also being approached with higher beam intensities and purity, along with better efficiency and sensitivity of instruments. The access to new and complementary experiments combined with theoretical advances allows key questions to be addressed such as:
How does the nuclear chart emerge from the underlying fundamental interactions?
Where are the limits of stability and what is the heaviest element that can be created?
How does nuclear structure evolve across the nuclear landscape and what shapes can nuclei adopt?
How does nuclear structure change with temperature and angular momentum?
How can nuclear structure and reaction approaches be unified?
How complex are nuclear excitations?
How do correlations appear in dilute neutron matter, both in structure and reactions?
What is the density and isospin dependence of the nuclear equation of state?
\noindent The experiment GAMMA is addressing most of these questions through gamma-ray spectroscopy measurements, in particular studying: properties of nuclei far from stability by measuring observables (energy, spin, transition probabilities, beta-decay lifetimes) to constrain theoretical models in exotic regions of the nuclear chart; collective modes like Pygmy Dipole Resonances, Giant Resonances; symmetries, including dynamical symmetries; Nuclear shapes; Magnetic moments, as a test of the nature of specific isomeric configurations; conversion electrons (EC), probing electromagnetic decays and E0 transitions. The experimental activity is based on the use of multi-detector arrays at stable (LNL Legnaro, GANIL Caen) and radioactive (GSI Darmstadt, RIKEN, SPES/LNL in future) ion beam facilities. The experimental setups involve the use of Ge detector arrays coupled to complementary detectors such as large scintillators (for high-energy gamma-rays), small scintillators (for multiplicity measurements or fast timing techniques), charged particle detectors, neutron detectors and magnetic spectrometers. The main activities involving CNAF are carried out with AGATA, presently installed at GANIL and coupled now to the VAMOS spectrometer and the segmented charged particle detector MUGAST, and previously to the NEDA neutron array.
\noindent AGATA \cite{ref:gamma_first,ref:gamma_second} is the European Advanced Gamma Tracking Array for nuclear spectroscopy project consisting of a full shell of high purity segmented germanium detectors. Being fully instrumented with digital electronics it exploits the novel technique of gamma-ray tracking. AGATA will be employed at all the large-scale radioactive and stable beam facilities and in the long-term will be fully completed in 60 detectors unit geometry, in order to realize the envisaged scientific program. AGATA is being realized in phases with the goal of completing the first phase with 20 units by 2020. AGATA has been successfully operated since 2009 at LNL, GSI and GANIL, taking advantage of different beams and powerful ancillary detector systems. It will be used in LNL again in 2022, with stable beams and later with SPES radioactive beams, and in future years is planned to be installed in GSI/FAIR, Jyvaskyla, GANIL again, and HIE-ISOLDE.
\section{AGATA computing model and the role of CNAF}
At present the array consists of 15 units, each composed by a cluster of 3 HPGe crystals. Each individual crystal is composed of 36 segments for a total of 38 associated electronics channels/crystal. The data acquisition rate, including Pulse Shape Analysis, can stand up to 4/5 kHz events per crystal. The bottleneck is presently the Pulse Shape Analysis procedure to extract the interaction positions from the HPGe detectors traces. With future faster processor one expects to be able to process the PSA at 10 kHz/crystal. The amount of raw data per experiment, including traces, is about 20 TB for a standard data taking of about 1 week and can increase to 50 TB for specific experimental configuration. The collaboration is thus acquiring locally about 250 TB of data per year. During data-taking raw data is temporarily stored in a computer farm located at the experimental site and, later on, it is dispatched on the GRID in two different centers, CCIN2P3 (Lyon) and CNAF (INFN Bologna), used as TIER1: the duplication process is a security in case of failures/losses of one of the TIER1.
The GRID itself is seldom used to re-process the data and the users usually download their data set to local storage where they can run emulators able to manage part or the full workflow.
\section{References}
\begin{thebibliography}{9}
\bibitem{ref:gamma_first}
S. Akkoyun et al., Nucl. Instrum. Methods A 668, 26 (2012).
\bibitem{ref:gamma_second} A. Gadea et al., Nucl. Instrum. Methods A 654, 88 (2011).
