Nuclear Structure

The atomic nucleus is a unique many-body system that display a large array of properties and phenomena. It is the goal of nuclear structure to understand all these facets from simple underlying principles. This is a lofty goal, but we are making strides on various fronts from understanding novel structures such as alpha-cluster structures to the evolution of deformation and collectivity in medium mass nuclei to understanding the equation of state of nuclear matter through the study of giant resonances. Novel structures and collective motion in nuclei all stem from correlations between nuclei and the underlying shell structure. As this shell structure changes with the nuclear environment changes, for example when there is a large excess of neutrons, we expect even more novel structures to appear. The study of nuclear structure is both the study of the interaction between nucleons and the many-body aspects of this quantum system that spans the areas of few-body to statistical physics.

Collective phenomena: Nuclei exhibit a large array of collective phenomena from the formation of rotational bands to surface vibrations. As a function of nucleon number, more specifically the number of valence nucleons, the onset of collective features and nuclear deformation gives a very important constraint on how collectivity is built up from individual nucleons. The onset of quadrupole and octupole deformations, the nature of 0+ states, nature of vibrational modes in deformed nuclei, the impact of deformation on nuclear masses and mass models. In addition, phenomena such as exotic quantal rotation (i.e wobbling, chirality, tidal waves) and octupole condensation, applying supersymmetry to nuclei using algebraic models are actively being pursued at Notre Dame.

Nuclear incompressibility: The nuclear equation of state, the relation between pressure, energy, and temperature, for nuclear matter is rooted in nuclear interactions and has large implications for astrophysical objects such as neutron stars. Two of crucial parameters of the nuclear equation of state is the strength of the nuclear imcompressibility and symmetry energy, but is yet to be fully constrained by experiment. One of the leading methods for determining these parameters is to study the properties of giant resonances, very collective oscillations that involve a large number of nucleons moving coherently.

Alpha clusterization: Nuclear correlations give rise to the formation of states that resemble alpha clusters in nuclei. The most famous example of this is the Hoyle state in 12C, where the three-alpha structure is responsible for the production of 12C when two alpha particles come together to make 8Be and then capture a third to make 12C. Although there have been predictions for more alpha cluster states in nuclei, the extent of their existence is still an open question. We are searching for more experimental evidence of alpha-cluster states in light unstable nuclei to understand where these states exist and start to understand the conditions necessary for their formation. We use the radioactive beam capability at Notre Dame and active-target detectors to carry out such studies.

Evolution of shell structure: The nuclear binding energy is a fundamental property of the nucleus that allows us to probe the interactions at play in the nucleus. Hence, changes in the structure of the nucleus including the degradation to quenching of shell closure as well as the emergence of new magic number can be observed in a striking and model-independent way from differences in atomic masses, which are directly related to binding energies. Penning trap mass spectrometry as proven to be the most precise and accurate way of measuring atomic masses. We are currently involved with various groups world-wide in experiments aiming at looking for changes in the nuclear structure. For instance, the high-precision mass measurement of 20,21Mg and TRIUMF have resulted in a breakdown of the isobaric mass multiplet equation, a breakdown that cannot be explain neither by shell model calculations or ab-initio theory.

Our research group in nuclear structure utilizes accelerator based facilities and state of the art instruments both at Notre Dame and around the world.