The Advanced GAmma Tracking Array (AGATA) is a European project to develop and operate the next generation γ-ray spectrometer. AGATA is based on the technique of γ-ray energy tracking in electrically segmented high-purity germanium crystals. This technique requires the accurate determination of the energy, time and position of every interaction as a γ ray deposits its energy within the detector volume. Reconstruction of the full interaction path results in a detector with very high efficiency and excellent spectral response. The realisation of γ-ray tracking and AGATA is a result of many technical advances. These include the development of encapsulated highly segmented germanium detectors assembled in a triple cluster detector cryostat, an electronics system with fast digital sampling and a data acquisition system to process the data at a high rate. The full characterisation of the crystals was measured and compared with detector-response simulations. This enabled pulse-shape analysis algorithms, to extract energy, time and position, to be employed. In addition, tracking algorithms for event reconstruction were developed. The first phase of AGATA is now complete and operational in its first physics campaign. In the future AGATA will be moved between laboratories in Europe and operated in a series of campaigns to take advantage of the different beams and facilities available to maximise its science output. The paper reviews all the achievements made in the AGATA project including all the necessary infrastructure to operate and support the spectrometer

No abstract

3The general phenomenon of shell structure in atomic nuclei has been understood since the pioneering work of Goeppert-Mayer, Haxel, Jensen and Suess [1].They realized that the experimental evidence for nuclear magic numbers could be explained by introducing a strong spin-orbit interaction in the nuclear shell model potential.However, our detailed knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), the unique nature of the atomic nucleus as an object composed of two distinct types of fermions can be expressed as enhanced correlations arising between neutrons and protons occupying orbitals with the same quantum numbers. Such correlations have been predicted to favor a new type of nuclear superfluidity; isoscalar neutron-proton pairing [2][3][4][5][6], in addition to normal isovector pairing (see Fig. 1). Despite many experimental efforts these predictions have not been confirmed. Here, we report on the first observation of excited states in N = Z = 46 nucleus 92 Pd. Gamma rays emitted following the 58 Ni( 36 Ar,2n) 92 Pd fusionevaporation reaction were identified using a combination of state-of-the-art highresolution -ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutronproton coupling scheme, different from the previous prediction [2][3][4][5][6]. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling [7,8]) in the ground and low-lying excited states of the heaviest N = Z nuclei. The strong isoscalar neutron-proton correlations in these N = Z nuclei are predicted to have a considerable impact on their level structures, and to influence the dynamics of the stellar rapid proton capture nucleosynthesis process.For all known nuclei, including those residing along the N = Z line up to around mass 80, a detailed analysis of their properties such as binding energies [9] and the spectroscopy of the excited states [10] strongly suggests that normal isovector (T = 1) pairing is dominant at low excitation energies. On the other hand, there are long standing predictions for a change in the heavier N = Z nuclei from a nuclear superfluid dominated by isovector pairing to a structure where isoscalar (T = 0) neutron-proton (np) pairing has a major influence as the mass number increases towards the exotic doubly magic nucleus 100 Sn [2-6], the heaviest N = Z nucleus to be bound. N = Z nuclei with mass number > 90 can only be produced in the laboratory with very low The two-neutron (2n) evaporation reaction channel following formation of the 94 Pd compound nucleus, leading to 92 Pd, was very weakly populated with a relative yield of less than 10 −5 of the total fusion cross section. Gamma rays from decays of excited states in 92 Pd were identified by comparing γ-ray spectra in coincidence with two emitted neutrons and no charged particles with γ-ray spectra in coincidence with oth...

No abstract