Findings and Conclusions
A new era is beginning in low-energy nuclear physics research with the advent of facilities capable of providing beams of radioactive, or unstable, atomic nuclei. These exotic nuclear species can be studied themselves or used to induce nuclear reactions to access still-more-exotic nuclei. These new developments can open up new frontiers in nuclear physics research—both basic and applied.
The Rare-Isotope Science Assessment Committee (RISAC) was charged by the National Research Council, the Department of Energy (DOE), and the National Science Foundation (NSF) to define a scientific agenda for a U.S.-sited facility for rare-isotope beams (see Appendix A for the charge). A U.S. Facility for Rare-Isotope Beams (FRIB) was identified as a priority in the 2002 long-range plan of the DOE/NSF Nuclear Science Advisory Committee (NSAC), in which it was further ranked as the “highest priority for new construction” and the second overall (after support of the operating facilities, the Relativistic Heavy Ion Collider [RHIC], the CEBAF Large Acceptance Spectrometer Detector at the Thomas Jefferson National Accelerator Facility, and the National Superconducting Cyclotron Laboratory, and the university research programs). A large and active segment of the nuclear physics community has worked to develop a scientific case in support of a version of a FRIB called the Rare Isotope Accelerator (RIA). Two
strong efforts by groups interested in hosting RIA have developed facility plans and the required technology for a U.S. FRIB. These groups had developed impressive technical plans with significant similarities, each incorporated a 400 MeV/A superconducting radio-frequency linear accelerator driver and capabilities to produce rare isotopes by in-flight fragmentation, the traditional Isotope Separator On-Line (ISOL) technique, and gas stopping and reacceleration. The expected cost of either facility was about $1.1 billion.
After RISAC began its work, the DOE announced that it intended to pursue a FRIB at about half the cost, with funds for project-engineering definition not to begin until 2011. In response to these new guidelines for a U.S. FRIB, both groups pursuing a FRIB presented the committee with new plans for a smaller facility based on a 200 MeV/A linear accelerator (linac) and somewhat reduced experimental capabilities. Although the committee could not review these preliminary design concepts in detail, it is important to note that both plans significantly scaled back the multiuser capabilities of the facility in order retain as much of the intensity and diversity of rare isotopes as possible. Thus, the suggested designs for a FRIB would have much reduced access compared with that of the earlier RIA proposals. However, this revised approach could engender a useful series of upgrades. While arguments can be mustered about the dire consequences of delay, experience shows that it is not always a bad choice, especially when accounting for the uncertainties in any predictions about the future of science. For these reasons and because it lay outside the charge, the committee chose not to specifically evaluate the consequences of the proposed change in schedule. Healthy stewardship of the U.S. nuclear science community and continued exploitation of the key scientific opportunities will be matters that NSAC will need to consider carefully in its next long-range plan.
In response to these events and the charge, the committee proceeded to assess the science that could be accomplished with a reduced-scope FRIB as described by the proponents, taking account of the time frame consistent with a 2011 start for engineering definition. The committee was not charged to recommend a specific facility or to make recommendations about the utility of a FRIB in comparison with other possible initiatives for U.S. nuclear science. Indeed, a new long-range planning process for nuclear science will begin in 2007, and the community will have the opportunity to assert its priorities.
Nuclear structure physics as pursued at a FRIB aims to describe nuclei as a collection of neutrons and protons. Current theoretical approaches are much more powerful than the pioneering models developed in the 1940s and 1950s. The
nuclear structure approach is still the most appropriate way to understand much of nuclear physics from ordinary nuclei to neutron stars. Understanding nuclear matter in this regime is of great interest to nuclear astrophysicists and to experimentalists who attempt to exploit the atomic nucleus as a laboratory for fundamental interactions. For instance, a better characterization of nuclear structure will play an essential role in correctly extracting the true nature of the neutrino’s mass from neutrinoless double-beta-decay experiments now in development. This is a fundamental issue with significant implication for physics beyond the Standard Model. Beginning more than a decade ago, the U.S. nuclear structure community, along with colleagues interested in important problems in nuclear astrophysics and the fundamental interactions, proposed that a new rare-isotope accelerator be built in the United States. This facility would produce a wide variety of high-quality beams of unstable isotopes at unprecedented intensities. The proponents of a FRIB argue that the science goals driving these subjects, and nuclear structure in particular, require a new class of experiments to elucidate the structure of exotic, unstable nuclei to complement the studies of stable nuclei that had been the primary focus of the subject in the past century. A facility with this capability could also provide critical information on the very unstable nuclei that must be understood in order to help explain the origin of the nuclear abundance observed in the universe. This facility would produce abundant samples of specific isotopes, which can serve as laboratories for studying fundamental symmetries and for applications.
RESPONSE TO THE CHARGE
As stated in its charge, the committee was asked to “define a scientific agenda for a U.S. domestic rare-isotope facility taking into account current government plans.”
The committee concludes that a next-generation, radioactive-beam facility of the type embodied in the U.S. FRIB concept represents a unique opportunity to explore the nature of nuclei under conditions that previously only existed in supernovae and to challenge our knowledge of nuclear structure by exploring new forms of nuclear matter. While a facility capable of intense beams of a wide variety of radioactive nuclei will clearly impact many areas of science and technology, the committee identified several key science drivers.
Nuclear structure. A FRIB would offer a laboratory for exploring the limits of nuclear existence and identifying new phenomena, with the possibility that a more broadly applicable theory of nuclei will emerge. A FRIB would allow the investigation of new forms of nuclear matter such as the large
neutron excesses occurring on the surfaces of nuclei near the neutron drip line, thus offering the only laboratory access to matter made essentially of pure neutrons. A FRIB might lead to breakthroughs in the ability to fabricate the neutron-rich superheavy elements that are expected to exhibit unusual stability in spite of huge electrostatic repulsion.
Nuclear astrophysics. A FRIB would lead to a better understanding of nuclear astrophysics by creating exotic nuclei that, until now, have existed only in nature’s most spectacular explosion, the supernova. A FRIB would offer new glimpses into the origin of the elements, which are mostly produced in processes very far from nuclear stability and which are barely within reach of present facilities. A FRIB would also probe properties of nuclear matter at extreme neutron richness similar to that found in neutron star crusts.
Fundamental symmetries of nature. Experiments addressing questions of the fundamental symmetries of nature could likewise be explored at a FRIB through the creation and study of certain exotic isotopes. These nuclei could be important laboratories for basic interactions because aspects of their structure greatly magnify the size of the symmetry-breaking processes being probed. For example, a possible explanation for the observed dominance of matter over antimatter in the universe could be studied in experiments seeking to detect a permanent electric dipole moment in heavy radioactive nuclei.
A successful scientific program in these areas would require significant theoretical input from nuclear structure physicists.
Last but not least, a U.S.-based FRIB facility, capable of producing high-specific-activity samples of exotic isotopes, could contribute to research in the national interest. The applications of rare-isotope technology could influence many areas, including medical research, national security, energy production, materials science, and industrial processes. It would provide an important contribution to the education and training of future U.S. scientists in the physics of nuclei. The aspects of nuclear physics addressed by the FRIB community directly impact the basic-science knowledge base relevant for nuclear reactors and nuclear weapons.
As part of the overall strategy for nuclear science in the United States, the committee believes that the United States should plan for and develop the technologies for a national facility for rare-isotope science of the type embodied in the FRIB concept. The overall scientific priority for this facility will be evaluated in a forthcoming NSAC study developing a long-range plan for the field.
The committee was asked to address the importance that a FRIB would have “in the future of nuclear physics, considering the field broadly.”
It is useful to recall the primary mission of nuclear science: “To explain the origin, evolution and structure of the baryonic matter of the universe.”1 Clearly restrained by its charge (see Appendix A), the committee did not evaluate the relative importance of a FRIB compared with other major initiatives in nuclear physics. However, the committee does comment here on the role that a FRIB would play in the future of the field.
Nuclear science of the 21st century tackles this question through three broad and complementary research frontiers: (1) the exploration of quantum chromo-dynamics and its implications and predictions for the origin of matter in the early universe, quark confinement, the structure of hadrons, and the nature of strong force; (2) the study of nuclei and nuclear astrophysics, which explores the structure and limits of nuclei, the origin of the elements, and the evolution of the cosmos; and (3) the formulation of the Standard Model and its possible extensions as they are manifested in the properties of neutrinos, neutrons, and other subatomic particles. These three frontiers, and the facilities that explore them, are the pillars of the field. In order to make progress on a broad front, investments are needed in all three areas. The modern nuclear physics facilities RHIC and CEBAF provide the state-of-the-art experimental tools for addressing the first of these nuclear science frontiers; FRIB, with its ability to produce groundbreaking research on nuclei far from stability, would provide similar world-class opportunities for the second. Thus, by creating and characterizing a broad range of exotic nuclei, a FRIB would contribute directly to the quest of nuclear physics to understand the multibody phenomena that underpin all nuclei. A variety of instruments and experiments under way or planned will address the third frontier.
The committee believes that studies of nuclei and nuclear astrophysics constitute a vital component of the nuclear science portfolio in the United States. Failure to pursue such a capability will not only lead to the forfeiture of U.S. leadership but will likely erode the nation’s current capability and curtail the training of future U.S. nuclear scientists. The federal research agencies (primarily DOE’s Office of Science and the National Science Foundation) have a responsibility to address the major science questions that the committee has identified; in particular, DOE and NSF as a whole have the responsibility to ensure a competence in nuclear science necessary to support the national interests of the United States.
The committee was asked to address the role of a U.S. FRIB “in the context of international efforts in this area.”
Other countries throughout the world are aggressively pursuing rare-isotope science, often as their highest priority in nuclear science, attesting to the significance accorded internationally to this exciting area of research. The remarkable technical innovations developed for RIA appear to be directly applicable to the FRIB concept and could enable the United States to maintain its position among the leaders in this highly competitive field.
The committee concludes that a U.S. facility for rare-isotope beams along the lines presented to the committee would be complementary to existing and planned international efforts. A FRIB would offer unique technical capabilities to the American region. As a partner among equals, a U.S. rare-isotope facility constructed in the next decade could be well matched to compete with the new initiatives in Asia and Europe and would support world-leading scientific thrusts within the United States. Additionally, the committee heard testimony that global “demand” for radioactive beams exceeds projected “supply.”
The committee concludes that the science addressed by a rare-isotope facility, most likely based on a heavy-ion driver using a linear accelerator, should be a high priority for the United States. The facility for rare-isotope beams envisaged for the United States would provide capabilities, unmatched elsewhere, that will directly address the key science of exotic nuclei.