Why is FRIB crucial?
FRIB-based science is extremely broad and diverse. It spans the gamut from nuclear structure to astrophysics, tests of fundamental laws of nature, and myriad applications. Nevertheless, it is characterized by several encompassing themes that reflect the major challenges facing modern science today, and it has deep links to many other fields. In this Introduction, we will briefly touch on these themes and relate them to specific areas of FRIB research. The sections that follow will be couched in terms of this framework.
The study of exotic nuclei opens new opportunities, excitement, and challenges. The opportunities arise because we now (with FRIB) will have the ability to select specific nuclei from a greatly enhanced "gene pool" in order to isolate and/or amplify specific interactions, nucleonic correlations, excitation modes, and symmetries. The challenges and the excitement arise because exotic nuclei will present new and radically different manifestations of nucleonic matter that arise near the bounds of nuclear existence, where the special features of weakly bound, quantal systems come into prominence, and because these nuclei are key to understanding the cosmos. We already see glimpses of the exciting physics, for example, in the appearance of Borromean halo nuclei, and in the breakdown of the long cherished magic numbers as benchmarks for structural evolution, but a much broader range of new phenomena is expected to emerge beyond the present limits of experimental accessibility.
Figure 1: On this chart of the nuclides, black squares represent stable nuclei and the yellow squares indicate unstable nuclei that have been produced and studied in the laboratory. The many thousands of these unstable nuclei yet to be explored are indicated in green (Terra incognita). The red vertical and horizontal lines show the magic numbers, reflecting regions where nuclei are expected to be more tightly bound and have longer half-lives. The anticipated paths of astrophysical processes for nucleosynthesis (r-process, purple line; rp-process, turquoise line) are also shown.
The nuclear landscape (Figure 1) defines the territory of FRIB research. Most of what we know about nuclei today comes from studies with stable nuclei: these are the black squares on the figure. By adding either protons or neutrons to one of these stable nuclei, one moves away from the line of stability, first producing unstable nuclei and finally reaching the drip lines where nuclear binding forces are no longer strong enough to hold nuclei together. The yellow squares indicate unstable nuclei that have been produced and studied in the laboratory. But many thousands of radioactive nuclei have yet to be explored: this nuclear "terra incognita" is indicated in green. FRIB will expand our investigations into the nature of nucleonic matter by providing experimental access to these nuclei. It will define and map the limits of nuclear existence and allow us to explore the structure of the exotic systems that inhabit these boundaries.
How Complex Systems Emerge from Simple Ingredients
The world around us, ranging from microscopic matter to the cosmos, seems, and often is, incredibly complex. Yet it is constructed from a small number of entities that obey simple physical laws and interact with only a small number of forces. It is a remarkable achievement and an on-going challenge of modern science to understand the immense diversity of nature in terms of elemental particles, a set of conservation laws (energy, angular momentum, and the like), four forces and a framework of fundamental physical Principles (the laws of quantum mechanics, quantum statistics, and the like).
The atomic nucleus is a finite, 2-fluid (protons and neutrons) many-body laboratory that manifests this challenge in unique ways. With FRIB, we will have the capability to specify, control, and vary precisely the number of nucleonic bodies over wide ranges so that we can study, not only the structure of individual nuclei, but the evolution of that structure across the nuclear chart. The goal of nuclear physics with FRIB is to achieve a comprehensive, unified theory of nuclear structure across the entire nuclear landscape.
Simplicities and Regularities in Complex Systems
Despite the complexity of nucleonic matter, the nuclear systems that emerge display astonishing regularities and simple excitation patterns, at least in those systems near stability that have been accessible to date.
This brings us to the second principle theme and challenge, namely, understanding how such complex nuclear systems, with up to hundreds of interacting nucleons, can display these elegant simplicities. This challenge is nothing less than the elucidation of paradigms of structure that allow the classification of wide varieties of nuclei under the umbrella of a single unified conceptual framework. These paradigms often involve a geometrical perspective on structure and therefore lend themselves to the concept of symmetries and, especially, dynamical symmetries, and even supersymmetries. In weakly bound systems the dramatic changes in nuclear structure may lead to entirely different manifestations of symmetries in nuclei and possibly to the emergence of new symmetries and new structural paradigms. With FRIB we will, for the first time, have broad access to this physics.
Note that these first two themes are beautifully complementary near-mirror images of each other: how can complex systems be constructed from basic ingredients, and how can the resulting complex many-body systems display such elegant regularities and symmetries.
Understanding the Nature of the Physical Universe
Clearly, one of the goals of modern science is to understand the nature of the physical universe. Here, nuclear physics, and in particular nuclear structure, plays an especially central role, since so much of the energy generation in stars involves the release of nuclear energy, either through fusion or fission processes, or in radioactive decay. All of the nucleosynthesis of the elements in our world (and ourselves) involves nuclear reactions. Given the temperatures and particle densities in stellar objects and in cataclysmic stellar explosions, these reactions often occur in unstable nuclei. (Figure 1 shows the anticipated paths of astrophysical processes for the formation of the heaviest elements.) This makes a facility such as FRIB critical to advancing our understanding in this field. This is also an area of optimal cross fertilization between disciplines—optical (especially modern satellite) observational astronomy on the one hand, provides windows of observation for celestial phenomena originating in nuclear processes, while FRIB provides the tool for the terrestrial study of the nuclear reactions that drive these events. The link between FRIB physics and astrophysics runs even deeper than this since the study of neutron rich nuclei provides the tools for understanding the properties of such important objects as neutron stars, and the physics of nuclear phase evolution in the realm of extreme densities.
Testing Fundamental Laws of Physics
FRIB will provide us with opportunities to test fundamental conservation laws. Its unique capabilities, including the ability to create large quantities of specific exotic nuclei which can then be trapped, will permit sensitive tests of basic laws of nature, of basic symmetries and other important aspects of, in particular, the weak interaction (one of the four fundamental forces, together with gravitational strong, and electromagnetic interactions). The ability to access long sequences of isotopes is crucial to eliminating sources of theoretical uncertainty in carrying out these tests. Moreover, often specific nuclei, such as the rare examples with octupole deformed (pear) shapes, enhance certain sought- after effects, such as violations of time reversal symmetry.
It is interesting that two such very different frontiers of FRIB research as the stability of the heavy elements and tests of fundamental symmetries both depend on inherently nuclear structure effects, specifically, the non-uniform distribution of single particle levels which has its origin in geometric symmetries of classical orbits.
Interdisciplinary Aspects
In the past, nuclear physics has often borrowed from other fields of physics. The concepts of the mean field and the importance of pairing are two examples, as are techniques such as the BCS formalism and the RPA approach to the microscopic understanding of collective modes. Now the flow of interdisciplinary cross fertilization proceeds actively in both directions. In recent years, ideas from nuclear physics have found applications in other mesoscopic systems such as quantum dots, metallic clusters, and molecular systems, in astrophysical environments such as neutron stars, and in the exploitation of the concepts of dynamical symmetries and supersymmetries. After decades in which the disciplines of physics (and of science) have tended to diverge, there is a growing interlinkage between fields. Exploitation of this will surely enhance all disciplines. Discussions of interdisciplinary aspects of FRIB science permeate this White Paper.
Science for the Betterment of Mankind
In addition to its basic research agenda, FRIB will provide huge opportunities for potential applications of nuclear physics. Often, these applications rely, as above, on our ability to select specific nuclei with particular decay modes, half-lives, and energies. This is especially true in medical (diagnostic and therapeutic) applications, in waste management, and for National Security. In other fields (e.g., in condensed matter research) the availability of time-delayed (by radioactive decay) chemical changes in an implanted atom may be critical to enhancing the performance of electronic devices, or of rendering them feasible to begin with. For medicine, FRIB's high intensity beams and isotope separation capabilities will provide opportunities to generate a wide variety of isotopes not currently available. This will lead to the development of materials best suited for diagnostic and therapeutic procedures. Another application in the health area is the use of implanted species for studying the wear of artificial joints. In the National Security arena, FRIB will provide significant new capabilities for the Science-Based Stockpile Stewardship program by its ability to measure important reactions involving unstable nuclei. FRIB will also provide important information needed to determine the most effective means to transform nuclear waste. Perhaps most importantly, FRIB will also provide a superb venue for the important mission to educate and train the next generation of nuclear scientists, who will play key roles not only in basic research itself but in myriad allied fields.