Gamma spectroscopy with AGATA in its first phases: New insights in nuclear excitations along the nuclear chart

The Advanced GAmma Tracking Array (AGATA), the new generation high-resolution $\gamma$-ray spectrometer, has seen the realization of the first phases of its construction and exploitation. A number of nuclear structure studies based on experiments utilizing the principle of $\gamma$-ray tracking were carried out in this decade. The combination of highest detection efficiency and position sensitivity allowed very selective spectroscopic studies with stable beams and the use of instable ion beams with the lowest intensities. Nuclear-structure studies commenced already at INFN-LNL (Legnaro, Italy) with a first implementation of the array consisting of five AGATA modules. A larger array of AGATA modules was used at GSI (Darmstadt, Germany) for experiments with unstable ion beams at relativistic energies. The spectrometer was then mounted in a beam line at GANIL (Caen, France). This review discusses several of the obtained results, underlying the progress made and future perspectives. The performed experiments give insights into nuclear structure issues which are connected to single particles, collective degrees of freedom, nucleon interactions and symmetries. Most of the investigated nuclei are located outside the stability line and for stable nuclei the investigations concern unexplored configurations. Altogether the obtained results represent advances which could test theory in exclusive way and motivate new theoretical developments. Opportunities for further $\gamma$-ray spectroscopy with the foreseen more advanced phase of the AGATA emerge in the discussions of the presented data.