The Scientific Working Group "Nuclear Structure" provides a forum for all Canadian scientists working in nuclear structure research. The working group facilitates communication and collaboration within the community by organizing workshops and topical sessions at conferences and fosters the interaction with closely related communities. The recent achievements and activities as well as the future perspectives of this community are described in detail in the CINP Brief to the Sub-atomic Physics Long Range Plan.
I'm a beginner, what is a nucleus and what does nuclear structure mean?
An atom consists of a cloud of electrons around a central nucleus. The nucleus is a collection of protons and neutrons. The number of protons determines the type of element, for example Carbon has six protons. The sum of the protons and neutrons determines in the isotope number. Continuing out example of Carbon, 12C (Carbon-12) has six neutrons with the six protons, whereas 14C (Carbon-14) has eight neutrons with the six protons.
12C is stable meaning it will never undergo radioactive decay. In contrast, 14C is radioactive with a half life of more than 5000 years. Being radioactive means that at some point one of the neutrons in this 14C nucleus will change into a proton which will transform this nucleus into 14N (Nitrogen-14) with seven protons and seven neutrons. During the decay event two other elementary particles are emitted, a beta electron and a neutrino. This radioactive decay of 14C in organic matter is the basis of the Carbon dating technique.
The interactions between the constituent particles (protons and neutrons) creates a wide range of fascinating behaviors and structural features. Some nuclei behave a bit like a bag of marbles where the overall features can be described by the behavior of just one proton or neutron. At the other end of the spectrum, the nucleus is much better described as one lump of nuclear matter in the shape of a rugby ball. In this situation, the excitations of the nucleus are created by the whole lump rotating faster and faster. These analogies are of course far too simplistic and in reality quantum mechanics must play a key role in any description.
These nuclear structure features can be explored experimentally through a range of techniques, such as detecting radiation emitted from nuclei as they decay, probing the nucleus with magnetic or electric fields, smashing nuclei together to form different ones and a whole host of others. All experimental observations made by experimental physicists are used to examine the accuracy of the predictions made by theoretical physicists using various theoretical models and techniques. The most advanced nuclear structure calculations today are only possible using large supercomputers.
Nuclear Structure is an interesting and exciting field of research!
The nucleus is one of the most challenging quantum many-body systems to describe. This is largely due to the fact that the primary modes of excitation (single-particle excitation, spherical vibrations, rotations of a deformed shape) all act on a very similar energy scale, meaning that all are observed in nature and become mixed together. The key to describing the diverse features of nuclear structure is to develop a robust and complete understanding of the nuclear force which acts between the constituent nucleons. This is a major challenge because of the enormous computational power required to describe the behaviour of tens or hundreds of nucleons, and the many contributions to the nuclear force which subtly change as a function of neutron and proton number, neutron-proton ratio, and excitation energy. Nonetheless, tremendous progress has been made in recent years in both developing the theoretical tools and frameworks which can make this link from QCD to nuclei, and in the acquirement of pertinent nuclear data that can challenge and drive forward the development of these theoretical calculations. Canadian researchers are at the forefront of this field of research and the many contributions and activities are described in this Chapter. With continued support and strategic investment, Canada is well positioned to make key contributions to the field of nuclear structure research in the coming years.
Where can I study Nuclear Structure in Canada?
Canadian Researchers working in theoretical Nuclear Structure are primarily located at the University of Guelph and TRIUMF. Canadian Researchers working in experimental Nuclear Structure are working at the following places:
- University of British Columbia
- University of Guelph
- University of Manitoba
- McGill University
- University of Regina
- Saint Mary's University
- Simon Fraser University
Where are the experiments performed?
The experiments performed in nuclear structure by Canadian researchers make use of the best equipment and facilities available world-wide, many of which are located in Canada at the TRIUMF-ISAC and ARIEL facilities for rare-isotope beams. Most Canadian researchers are involved in research collaborations which span the globe. Some example are provided here.
What are the main questions being looked at in Nuclear Structure research in Canada?
Studies of neutron halos and skins
Halo nuclei probe some of the most urgent questions in nuclear physics. The classical example of a halo nucleus is 11Li where the two neutrons nearest the Fermi surface are weakly bound, so that their spatial wave functions are diffuse compared to stable nuclei. These properties are confirmed by a very small two-neutron separation energy and a large inclusive reaction cross section. Many of these details were revealed by experiments led by Canadian researchers and studying the details of halo nuclei remains an insightful pursuit with new experimental results continuing to challenge the most sophisticated ab-initio theories.
Tests of ab-initio theories in light- to medium-mass systems
Understanding the strong nuclear force binding the protons and neutrons to form the wide variety of complex nuclei in the Universe has been a century long challenge. The shell model is, in many ways, the standard model of low-energy nuclear structure. Phenomenological shell models with valence particles (or holes) coupled to an otherwise inert core have been successful at explaining a wide range of nuclear observables such as ground-state spins, parities, binding energies, and charge radii, and excited-state properties such as excitation energies. The desire is to obtain these descriptions from first principles, rather than using phenomenology. The chiral effective field theory enables a link for a description of the nuclear force connected with the theory of quantum chromodynamics but requires certain parameters that are not uniquely defined. Strategic measurements of certain nuclear properties are necessary in order to refine the theory.
Evolution of nuclear shell structure
In the last decade, a significant fraction of the research programs at radioactive ion beam facilities has been driven by the observation that the traditional nuclear shell gaps, or 'magic numbers', are modified in light neutron-rich nuclei and new shell gaps such as N=16 and 32 appear with large neutron excess. First indications of this evolution of nuclear shell structure came from anomalies in the masses of neutron-rich Na and Mg isotopes and subsequent data on excitation energies, transition strengths and spectroscopic factors have confirmed the picture of a weakening of the N=20 shell gap and the generation of a so-called 'island of inversion' around 32Mg in which deformed configurations involving 2p-2h and 4p-4h excitations across the shell gap become the ground state. The evolution of the sub-shell gap at N=34 established in neutron-rich 54Ca remains the subject of intense study, while strong deformation of 64Cr has been interpreted in terms of the disappearance of the harmonic oscillator (sub-)shell gap at N=40 in neutron-rich nuclei. From the theoretical perspective, the microscopic origins of the evolution of shell structure in light and intermediate mass nuclei has been linked to shifts of the effective single-particle energies associated with the central, tensor, and three-nucleon (3N) components of the monopole interaction between valence nucleons, while modifications of the spin-orbit interaction, pairing correlations, and coupling to continuum states have all been discussed in the context of shell structure evolution in heavy nuclei approaching the neutron dripline.
Studies of nuclear collectivity, shape coexistence, and shape transitions
A key question in nuclear physics is how simple patterns emerge in complex nuclei. To put this another way, how do macroscopic behaviours manifest themselves from the microscopic proton and neutron interactions. In reality, it is a complex combination of excitation modes that is typically observed in nuclei and, with very similar excitation energy scales, it is often difficult to disentangle the coupled excitation modes. Many collective phenomena cannot yet be predicted from the microscopic interactions of the individual nucleons, but theoretical developments towards this ultimate goal are advancing quickly. Experimental studies are essential to elucidate the nature of excitation modes and drive theoretical advancements. A number of important contributions have been made from experiments performed at TRIUMF-ISAC.