Nuclear physics research aims to explore the properties of sub-atomic matter and, in particular, to study the properties of atomic nuclei—where approximately 99% of the mass of the visible Universe is concentrated—and their interactions. Nuclei are highly complex objects composed of protons and neutrons, the fundamental constituents of all matter surrounding us, which interact via nuclear forces whose nature and characteristics are not known in detail.
To study the properties of this microscopic world, physicists have developed particle accelerators that allow, through the collision between the accelerated nuclei of the beam and those of the target, to "see" the nuclei and study their properties.
The Laboratori Nazionali del Sud (LNS) are equipped with two particle accelerators, the TANDEM and the Superconducting Cyclotron, which are used to accelerate ions with masses ranging from that of a proton to lead. These facilities are employed to seek answers to some of the fundamental questions of modern nuclear physics, including:
- How are elements synthesized in stars?
- How is matter organized at the sub-atomic scale?
- What combinations of protons and neutrons give rise to a bound system, and why?
- What is the maximum thermal energy that a nucleus can sustain before fragmenting?
Research activities are carried out by various experimental groups at LNS, using experimental apparatus designed and built for the study of specific topics in nuclear physics. The research topics pursued at LNS concern nuclear astrophysics, nuclear structure and reaction mechanisms, the phase transition of nuclear matter, quarks, and hadron dynamics. The role of nuclear physics is set to grow in the coming years thanks to ongoing upgrades, which include increasing the intensities extracted from the superconducting cyclotron and installing a new magnetic separator for the production of radioactive ion beams.
Nuclear Astrophysics with Indirect Methods: The ASFIN2 Experiment
The ASFIN2 collaboration investigates fusion reactions fundamental to stellar evolution and stellar and primordial nucleosynthesis. In most astrophysical contexts, nuclear processes occur at very low energies that are difficult to access in the laboratory. For this reason, ASFIN2 has developed specific measurement techniques over the years: from the use of indirect methods (such as the Trojan Horse Method, THM, or the Asymptotic Normalization Coefficient, ANC) for the measurement of cross-sections, to the use of high-intensity gamma beams, and the production of plasmas using high-intensity lasers. Thanks to the Trojan Horse Method, ASFIN2 has been able to study key astrophysical reactions in the laboratory without distortion effects due to Coulomb repulsion and electron screening. Furthermore, the method, applied at RIB (Radioactive Ion Beam) facilities, also allows the study of neutron-induced reactions on radioactive nuclei, thus exploring both the quiescent phenomena characteristic of nucleosynthesis in the early stages of stellar evolution and the more exotic ones typical of the final, more catastrophic phases.
Additionally, ASFIN2 is involved in studying the effects of nuclear structure on astrophysics. For all these purposes, innovative position-sensitive silicon detector arrays (e.g., the NEFASTA multi-detector, see figure) are used to investigate nucleon cluster structures or the presence of neutron halos in light nuclei, as well as collective nuclear motions, such as the giant dipole resonance or the dynamic dipole. An important result of the group is the measurement of the 12C+12C cross-section carried out at LNS (A.Tumino et al., Nature, 557, 687–690 (2018)).
The Study of the Phase Transition of Nuclear Matter: The CHIRONE Experiment
The study of the phase transition of nuclear matter is one of the main research themes of the CHIRONE experiment. CHIRONE utilizes the high granularity of the CHIMERA detector and its capabilities in identifying reaction products by mass and charge to study the effects of isospin (a quantity associated with the ratio between the number of neutrons and protons in the nucleus) on the reaction mechanism and the density dependence of the symmetry energy term of the nuclear equation of state. Other research topics include the study of cluster structure in exotic nuclei and that of "pygmy resonances," manifestations of a particular collective nuclear motion, which are carried out using exotic beams produced by fragmentation (FRIBS, and in the near future, FRAISE) at LNS.
The Study of Quark Dynamics and Structure of Nuclear Constituents: The JLAB12 Experiment
The study of the dynamics and quark structure of nuclear constituents represents the research objective of the JLAB12 experiment. JLAB12 is an experiment that uses a high-energy electron beam (approximately 10 GeV) produced by the accelerator at Jefferson Laboratory in Virginia (USA) to study the dynamics and internal structure of nucleons and nuclei through electron scattering. Other research topics concern the study of physics beyond the Standard Model and dark matter, about which very little is known. LNS activities focus on the HPS experiment, which aims to find traces of the so-called "dark photon" or "heavy photon," a massive photon whose detection would be a first sign of the existence of a hidden world.
Between Nuclear Astrophysics and Applied Physics: The n_TOF Experiment
The n_TOF (neutron Time-Of-Flight) facility at CERN is an international experiment dedicated to the study of neutron-induced nuclear reactions, which are fundamental in numerous fields: from basic nuclear physics to astrophysics, from nuclear safety to medicine. The heart of the experiment is a high-intensity pulsed neutron source (up to 106 neutrons/cm2 per pulse) with energies ranging from 25 meV to 1 GeV, produced by bombarding a lead target with the proton beam from the Proton Synchrotron. The neutron energy is measured with high precision through the time they take to reach the experimental areas, where their reactions with different materials are studied.
Researchers from INFN (National Institute for Nuclear Physics) play a leading role in the collaboration. In particular, the group from the Laboratori Nazionali del Sud has made a fundamental contribution to the development of detectors, the execution of complex measurements, and their data analysis. LNS researchers have led important experimental campaigns, including the measurement of the cross-section of the 140Ce(n, γ) reaction, of interest for the nucleosynthesis of heavy elements in stars, and the fission of Uranium-235. Currently, the group is developing a new measurement apparatus, based on silicon detectors, for the study of reactions on structural materials of fusion reactors.
From Nuclear Physics to Particle Physics: The NUMEN Experiment
NUMEN's activity is focused on the study of double charge exchange reactions, predominantly carried out using the MAGNEX spectrometer. The main purpose of these studies is the measurement of nuclear matrix elements relevant to neutrinoless double beta decay, an extremely rare decay whose observation would allow the neutrino to be identified as a Majorana particle, i.e., a particle that is its own antiparticle, the antineutrino. NUMEN also aims to identify the best candidate nuclei for neutrinoless double beta decay experiments.
PANDORA: A New Experiment to Study β Decays in Magnetized Plasmas
The PANDORA experiment aims to build a magnetic trap capable of confining high-temperature plasmas (up to 108 K) and densities of the order of 1011-13 cm-3, containing multi-ionized radioactive isotopes in order to study their β decay. The measurement of beta decay in plasma is of fundamental relevance in the study of the s-process, a nucleosynthesis process based on neutron capture in which the created radioisotope undergoes beta decay before another neutron can be captured. The predicted modification in the beta decay rate linked to the opening of a new decay channel, the bound-state β decay—possible thanks to the ionization of plasma atoms, and preliminarily observed in a limited number of isotopes exclusively under conditions of maximum ionization (in some experiments at the GSI Storage Ring)—has so far never been investigated in plasma.