There is a worldwide renaissance in nuclear science, driven by new and unexpected experimental results from improved experimental techniques, theoretical breakthroughs, and expanded applications. On the experimental side, two major milestones have advanced the research: large arrays of detectors with extraordinary data-collecting power, and accelerator facilities which provide experimental access to intense mass-selected (isotope-selected) beams with lifetimes down to the microsecond scale. The combination of these two capabilities marks a unique, major advance in nuclear physics that has not been seen in a number of decades.
“What is the structure of nuclear matter?” is a central question that touches upon many different areas. The central goal of nuclear physics is to explain the properties of nucleons, nuclei, and nuclear matter. Ultimately, it is desirable to attain this goal, starting from an understanding of the nucleon-nucleon interaction based on the fundamental theory of quantum chromodynamics. Indeed, considerable progress is being made in this direction with the development of ab initio methods, a more sophisticated understanding of the nucleon-nucleon interaction, and the application of advanced many-body techniques to nuclear physics. But connecting the fundamental theory to the nucleon-nucleon interaction poses a severe challenge, and the complexity of the strongly interacting many-body system requires that the study of detailed properties of nuclei rely on both ab initio calculations and more phenomenological models for the foreseeable future.
We cannot at this point give a satisfactory answer to the question posed above. To make progress, we need to break the question down into smaller, more manageable questions. While there are many questions that can be asked in relation to “What is the structure of nuclear matter?” TRIUMF and its user community have concentrated on the following:
- How do the properties of nuclei evolve as a function of the neutron-to-proton asymmetry?
- How do the properties of nuclei evolve as a function of proton and neutron number?
- What are the mechanisms responsible for the organization of individual nucleons into the collective motions that are observed?
Structure of Halo Nuclei
A halo nucleus is oversized and fragile, the exact opposite of a stable atomic nucleus, which is small and dense. The outermost neutrons, called the halo neutrons, are found an unusual distance away from the core nucleus, forming a halo around it. 11Li is a 9Li nucleus with two additional halo neutrons, making the nucleus as large as a 208Pb nucleus, having 208 protons and neutrons compared to 11 in the lithium isotope.
Lithium-11 is a unique three-body quantum system composed of 9Li+n+n, known as a Borromean system, where any two of the subsystems taken together are unbound, meaning there is not sufficient energy present to hold them together. In the vast sea of nuclear species these objects are located at the extreme edge of existence, far away from the valley of stable nuclei.
Structure of Heavy Nuclei
Research into the structure of heavy nuclei, which in the present context implies nuclei with mass greater than 20 (the sum of the numbers of protons and neutrons), is focused on two main themes. The first theme is the evolution of shell structure where neutrons and protons orbit the centre of a nucleus analogous to the orbiting of electrons in shells about the centre of the atom. The second theme is the development and evolution of collective excitations in nuclei, which depends on the composition of the nuclei (the number of protons and neutrons) and how these particles interact with each other. The two themes, evolution of shell structure and the development and evolution of collective excitations in nuclei, are intimately connected because collective modes are emergent only when there are a sufficient number of valence nucleons that will support the degrees of freedom required. A prime example of this is the development of the rotational degree of freedom: a number of valence nucleons of both protons and neutrons must be present, obtained either by the filling of orbitals in open shells, or by the breaking of pairs in closed shells, such that the quadrupole-quadrupole interaction causes the onset of a static deformation required to break spherical symmetry.
Nuclear astrophysics brings together the latest developments in astronomy and theoretical and experimental nuclear physics in a quest to understand the origins and evolution of all the naturally occurring chemical elements in the universe, without which the world as we know it would not exist. Nuclear astrophysics requires an intimate knowledge of the inner workings of stars, particularly either those that die in energetic explosions such as supernovae or undergo cataclysmic thermonuclear blasts, such as novae and X-ray bursts. All the chemical elements except the very light hydrogen, helium, and lithium were created in nucleosynthesis processes in hot stellar environments such as stars, novae, and supernovae. The underlying processes that govern the evolution of these objects are the interactions between atoms, and the microscopic properties of individual nuclei.
The field of nuclear astrophysics aims to solve the mystery of the origins of the chemical elements and to understand the physics and evolution of cataclysmic variable stellar systems such as novae and X-ray bursts. Sophisticated models are used to predict and reproduce the observations seen with the latest generation of astronomical observational tools. Crucially, the nuclear physics input to the models is based on laboratory measurements, making these models as close to reality as current technology and techniques allow. Most of the key nuclear reactions that are important to the study of these environments involve short-lived radioactive nuclei.
The ISAC facility at TRIUMF is the ideal location to study these nuclei and their reactions because of its combination of beams of short-lived nuclei, variable-energy accelerators, and a suite of world-class experimental facilities. The nuclear beams, the accelerators, and experimental facilities have been optimized for studying reactions of astrophysics interest.
One of the great challenges in science is to explain the amount of matter we see around us. Evidence for some kind of Big Bang is overwhelming, but any cosmological model using the physics that we presently know produces a photon-to-baryon ratio much higher than the billion-to-one ratio observed. To correct the discrepancy requires processes that create a larger matter-to-antimatter asymmetry. The discovery and subsequent measurement of electric dipole moments (EDMs) potentially provides an experimental tool to constrain theoretical models that address this issue.
The world around us is governed by a series of fundamental laws and symmetries. The standard model has provided a precise framework for calculation and prediction, the results of which agree with experimental data remarkably well. Many extensions to the standard model have been proposed. TRIUMF has a long history of performing high-precision experiments that both test the standard model of particles and forces and look for physics beyond it. In recent years, this testing has centred on different aspects of the charged weak current as measured in the first generation of particles in nuclear beta decay.
Superallowed β-Decay Studies
β-decay occurs when there are either too many protons or too many neutrons in a nucleus, making the system unstable. One or more of the excess protons or neutrons is transformed into the other so the nucleus can move to a more stable state with a more balanced number of protons and neutrons. Although the numbers of protons and neutrons in an atom’s nucleus change during β-decay, their sum remains the same.
For some β-decays, the structure of the nucleus is very similar before and after the β-decay and the decay happens faster than for most other β-decays. This special type of decay is a “superallowed β-decay.” Because nuclear structure uncertainties are small for these decays, precision measurements of them can be used to test the hypothesis for the nature of the weak interaction and determine the properties of quarks.
To describe the decays, three types of measurements are necessary. The first is a measurement of the masses of the atom before and after the decay. The second is the time it takes for the decay to occur. The third is the fraction of the decays that go to the final state of interest. At TRIUMF, we are able to do all three types of measurements and independently determine the decay properties. TRIUMF’s recent high-precision lifetime measurements have contributed significantly to the understanding of the decay properties.
Exotic Physics Searches
TRIUMF’s ISAC-I facility provides the facilities for many experiments that either test the standard model or are in search of physics beyond it. In addition to the program based around superallowed β-decays, there is an extensive research program encompassing several groups in search of exotic particles and couplings that lie outside the standard model. These include scalar bosons, right-handed currents, tensor interactions, axions, permanent electric dipole moments, and nuclear anapole moments.