Nuclear structure physics aims to describe nuclei as collections of neutrons and protons. Nuclear structure is the traditional core of nuclear science, and it has been able to describe a broad range of phenomena, from normal nuclei to neutron stars. The understanding of nuclei in this regime provides critical support for important research in nuclear astrophysics and for efforts to exploit nuclei as laboratories for exploring fundamental symmetries.
More than a decade ago, the U.S. nuclear structure and nuclear astrophysics communities proposed that a new rare-isotope accelerator be built in the United States. Such a facility would produce a wide variety of high-quality beams of unstable isotopes at unprecedented intensities. It would enable a new class of experiments to elucidate the structure of exotic, unstable nuclei to complement the studies of stable nuclei that have been the primary focus of nuclear physics 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 explain nuclear abundances observed in the universe. This facility would also produce large samples of specific isotopes that could enable a new class of experiments for the study of fundamental symmetries. A series of studies by the joint Department of Energy/National Science Foundation (DOE/NSF) Nuclear Science Advisory Committee have supported the need for such a facility, initially termed the Rare Isotope Accelerator (RIA).
To obtain an independent scientific assessment of the scientific agenda for such a facility, the National Research Council convened the Rare-Isotope Science
Assessment Committee (RISAC). The committee was charged by the Department of Energy and the National Science Foundation to define the science agenda for a next-generation U.S. Facility for Rare-Isotope Beams (U.S. FRIB). RISAC members included several experts in rare-isotope science, but the committee consisted largely of scientists from outside the rare-isotope science community; it also included members from Canada, Europe, and Asia. Soon after RISAC was formed, the DOE announced that the budget of what was then understood as RIA would be reduced by about half. In response to this announcement and the charge, the committee focused on articulating the science that could be accomplished at a rare-isotope facility of reduced scope, referred to as a FRIB or a U.S. FRIB in this report. The charge also directed the committee to evaluate the scientific impact of a FRIB in the overall context of the national and international nuclear physics programs.
The committee heard presentations about applications of a FRIB for nuclear physics studies and also about applications in areas of medical research and stockpile stewardship. RISAC was not asked to give advice on whether a facility should be constructed or to compare the relative merits of various possibilities. For its analysis, the committee interpreted “U.S. FRIB” as a general-purpose rare-isotope production facility with a cost about half that of the earlier RIA concept. To gain a better understanding of the potential impact on the scientific agenda of such a cost reduction, the committee heard views from some of the proponents of a U.S. FRIB in a public meeting; these individuals gave the committee their views on production techniques and beam intensities that they judged to be technically feasible. As indicated in these presentations, the primary trade-off expected from such a decrease in cost would be a modest reduction in the quantity and diversity of possible isotopes and a significant reduction in the multiuser aspects of the facility.
In developing its conclusions regarding a FRIB, the committee took into account the worldwide portfolio of related experiments and the likely time frame in which the facility might begin operations (2016, according to current DOE plans). Despite the uncertainty inherent in predicting what will be the important scientific questions in the far future, a powerful new rare-isotope facility could resolve scientific issues of clear importance. Arguments from the groups that have conducted the research and development for a FRIB convinced the committee that most of the major technical issues have been addressed. The committee concluded that the case for 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 only exist otherwise in supernovae and to challenge current understanding of nuclear structure through the exploration of new forms of nuclear matter and the development of a more robust quantitative description.
A rare-isotope facility produces beams of unstable atomic nuclei for direct
study or for use in subsequent reactions to produce even more exotic nuclear species. Thus, a FRIB could impact the study of the origin of the elements and the evolution of the cosmos as well as the Standard Model of elementary particle physics with groundbreaking research on nuclei far from stability. 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 produced mostly 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 conducted 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.
The committee concluded that nuclear structure and nuclear astrophysics constitute a vital component of the nuclear science portfolio in the United States. Moreover, nuclear-structure-related research provides the scientific basis for important advances in medical research, national security, energy production, and industrial processing. Historically, scientific and technological developments in nuclear science have had extremely broad impact—for example, in the development of nuclear magnetic resonance imaging and the fabrication of more-robust
electronics. Failure to pursue a U.S. FRIB would likely lead to a forfeiture of U.S. leadership in nuclear-structure-related physics and would curtail the training of future U.S. nuclear scientists.
The committee concluded that a U.S. facility for rare-isotope beams of the kind described to it would be complementary to existing and planned international efforts, particularly if based on a heavy-ion linear accelerator. With such a facility, the United States would be a partner among equals in the exploration of the world-leading scientific thrusts listed above.
The committee concluded 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 would help to provide answers to the key science topics outlined above.