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
Background: Neutron-rich nuclei with protons in the fp shell show an onset of collectivity around N = 40. Spectroscopic information is required to understand the underlying mechanism and to determine the relevant terms of the nucleon-nucleon interaction that are responsible for the evolution of the shell structure in this mass region. Methods: We report on the lifetime measurement of the first 2 + and 4 + states in 70,72,74 Zn and the first 6 + state in 72 Zn using the recoil distance Doppler shift method. The experiment was carried out at the INFN Laboratory of Legnaro with the AGATA demonstrator, first phase of the Advanced Gamma Tracking Array of highly segmented, high-purity germanium detectors coupled to the PRISMA magnetic spectrometer. The excited states of the nuclei of interest were populated in the deep inelastic scattering of a 76 Ge beam impinging on a 238 U target. Results: The maximum of collectivity along the chain of Zn isotopes is observed for 72 Zn at N = 42. An unexpectedly long lifetime of 20 +1.8 −5.2 ps was measured for the 4 + state in 74 Zn. Conclusions: Our results lead to small values of the B(E2; 4 + 1 → 2 + 1 )/B(E2; 2 + 1 → 0 + 1 ) ratio for 72,74 Zn, suggesting a significant noncollective contribution to these excitations. These experimental results are not reproduced by state-of-the-art microscopic models and call for lifetime measurements beyond the first 2 + state in heavy zinc and nickel isotopes.
We report on the observation of a new isomeric state in 68 Ni. We suggest that the newly observed state at 168(1) keV above the first 2 + state is a π (2p-2h) 0 + state across the major Z = 28 shell gap. Comparison with theoretical calculations indicates a pure proton intruder configuration and the deduced low-lying structure of this key nucleus suggests a possible shape coexistence scenario involving a highly deformed state. The atomic nucleus is a complex quantum system consisting of two kinds of strongly interacting fermions. A direct consequence of this fermionic nature, the Pauli principle, is the shell model of the nucleus, one property of which being the existence of magic gaps. Shell structures are present in a number of systems such as atoms, metal clusters, and quantum dots and wires, for instance, and are strongly linked to the symmetries of the mean field. How the shell gaps evolve in nuclei that are further and further away from stability is one of the key questions to which the radioactive beam facilities that are currently under construction hope to bring answers. Already today, the structure of moderately exotic nuclei such as 68 Ni allows one to pave the way toward a general answer to the problem of shell evolution. Unusual configurations which are expected to dominate in the ground-state structure of very exotic nuclei can be identified as excited structures in systems not very far away from stability. The strong contribution of the spin-orbit term in the nucleon-nucleon interaction affects in a major way the single-particle levels with the largest angular momentum, pushing it down in energy. This quenches significantly the N = 40 magic gap from the spherical harmonic oscillator. The intrusion of the 1g 9/2 and the 2d 5/2 neutron orbitals brings collectivity and enhances neutron-pair excitations across N = 40 from the fp shell into the 1g 9/2 . Conversely, however, this parity change hinders quadrupole excitation and mimics some properties usually associated to magicity. In 68 Ni, the observation of a first excited 0 + 2 state at low energy [1] and the high excitation energy of the 2 + 1 state [2] are examples of such properties. These competing consequences of shell quenching make 68 Ni a particularly suited case to study the evolution of shell gaps with isospin.Reactions involving single-proton particle-hole excitations, π (1p-1h), are an ideal tool to learn about the residual interaction. Unfortunately they lie at very high excitation energy. One possibility to circumvent this obstacle is to look for π (2p-2h) states which are lowered in energy thanks to pairing correlations and proton-neutron residual interactions. Studying pair excitation across magic gaps means, therefore, studying these residual interactions. Pair excitations are revealed by the presence of excited 0 + states. In 68 Ni, two such states are reported, mainly of neutron character, originating from the scattering of pairs into the 1νg 9/2 . A state corresponding to the excitation of two protons (2p-2h) has been predicted by ...
The NEutron Detector Array (NEDA) project aims at the construction of a new highefficiency compact neutron detector array to be coupled with large γ-ray arrays such as AGATA. The application of NEDA ranges from its use as selective neutron multiplicity filter for fusionevaporation reaction to a large solid angle neutron tagging device. In the present work, possible configurations for the NEDA coupled with the Neutron Wall for the early implementation with AGATA has been simulated, using Monte Carlo techniques, in order to evaluate their performance figures. The goal of this early NEDA implementation is to improve, with respect to previous instruments, efficiency and capability to select multiplicity for fusion-evaporation reaction channels in which 1, 2 or 3 neutrons are emitted. Each NEDA detector unit has the shape of a regular hexagonal prism with a volume of about 3.23 litres and it is filled with the EJ301 liquid scintillator, that presents good neutron-γ discrimination properties. The simulations have been performed using a fusion-evaporation event generator that has been validated with a set of experimental data obtained in the 58 Ni + 56 Fe reaction measured with the Neutron Wall detector array.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.