The experimental research of the group in the field of nuclear structure is mainly conducted through the study of the gamma decay of nuclei excited in different types of reactions: Coulomb and nuclear inelastic scattering, fragmentation reactions, transfer or fusion-evaporation reactions. The experiments are carried out within large international collaborations in national (Laboratori Nazionali di Legnaro and del Sud) or in foreign laboratories such as GSI (Germany) and RIKEN (Japan)through the use of advanced detector arrays like AGATA , based on HPGe detectors with tracking capabilities, or with new generation scintillators like LaBr3:Ce.
The physical problems presently under analysis reflect the data-taking carried out during several experiments. They mainly concern isospin mixing at zero and finite temperature or isospin-dependent entrance effects (dynamic dipole), the study of the collective properties (rotations and vibrations) of the nucleus, and effects induced by the nuclear deformation. Very recently data have been acquired on the gamma decay of the collective giant dipole and quadrupole resonance state in various isotopes from Zr to Pb. The collective states have been excited through inelastic scattering reactions allowing the direct measurement of their wave function and identification of the isoscalar and isovector nature of the observed states. In the very next future such states (in particular the Pygmy Dipole Resonance of astrophysical interest) will be studied using Coulomb excitation with relativistic radioactive exotic beams. The experimental data will allow the study of the structure of exotic neutron-rich nuclei and the measurements of their ‘neutron skin’ providing additional important information concerning the isospin dependence of the nuclear equation of state.
Detectors and instrumentations are a key factor in such experimental research activity. A very active line of research focuses on the development of new instrumentation and experimental techniques based on pulse shape analysis for the imaging of the gamma source. Related topics range from the development of electronics (analog and digital) and algorithms for segmented HPGe tracking detectors and highly performing scintillators detectors (LaBr3:Ce) to gamma imaging. Furthermore, the group pursues R&D activity focused on silicon based detectors for the measurement of x-rays and charged particles together with the development of dedicated and optimized electronics.
The Milano nuclear physics group is also active in the LUNA (Laboratory for Underground Nuclear Astrophysics) experiment. This experiment makes use of an electrostatic accelerator, placed at the Gran Sasso Underground Laboratory, and delivering intense beams of protons and alpha particles. The accelerator, coupled to gas or solid targets and to particle or photon detection apparata, allows to reproduce in the laboratory the thermonuclear fusion reactions powering stars. These reactions are characterized by extremely low cross sections at astrophysically relevant energies, and their measurement is only possible due to the extremely high reduction of the cosmic background provided by the Gran Sasso underground laboratory. So far LUNA has measured a few key reactions belonging to the hydrogen burning cycle, which transforms 4 protons into an alpha particle with a net energy release. The Milano group is highly involved in this experimental activity and can offer exciting Ph.D theses, aiming at the preparation of new measurements and data taking to be performed at Gran Sasso, or dealing with data analysis which can also be carried out in Milano. Click here for further details about LUNA.
Another activity concerns the Aegis experiment, currently in the commissioning phase at Cern at the Antiproton Decelerator. The experiment aims at testing fundamental laws such as the Weak Equivalence Principle (WEP) and the CPT symmetry. In the first phase of Aegis, a beam of antihydrogen will be formed whose fall in the gravitational field is measured in a MoirÃ¨ deflectometer; this will constitute the first test of the WEP with antimatter. The activity of the Milano group focuses on the laser system for positronium excitation, the positronium production and spectroscopy and the antiproton beam monitoring.
There is also a substantial theoretical activity concerning the structure of atomic nuclei and the application of nuclear techniques to other many-body systems. The properties of collective states of different nature – density, spin and isospin modes of excitation – are calculated with state-of-the-art effective interactions, at the mean field level and beyond. The group has a long tradition in the study of the interplay between single-particle and collective degrees of freedom, and played a key role in the development of nuclear field theory. Recent progress has concerned the treatment of ultraviolet divergences associated with calculations beyond mean field using zero-range effective interactions, a better treatment of the continuum and the consequences of particle-vibration coupling on the low-energy spectra as well as on superfluid properties in open shell nuclei. Different aspects of the structure of exotic nuclei – light as well as heavy – are actively studied, in connection with experimental activity carried out with radioactive beams. Recently, a systematic analysis of two-nucleon transfer reactions has been carried out, leading to a calculation of the absolute cross sections which turns out to be in good agreement with experimental findings. The implications of nuclear structure on neutron stars – concerning the equations of state as well as the superfluid character of the inner crust and the microscopic structure of vortices induced by the rotation of the star – are also actively studied.