BOX 1.1

Exotic Nuclei, Rare Isotopes, and Radioactive Nuclear Beams

The terms exotic nuclei, rare isotopes, and radioactive nuclear beams all refer to essentially the same sector of study, an area to which this report refers as rare-isotope science. The field of rare-isotope science can be characterized in the following way.

Atoms that make up everyday matter on Earth are predominantly stable; that is, they retain their identity in terms of their elemental nature (the numbers of protons and neutrons remain constant over time). The nuclei located at the center of each atom comprise over 99.9 percent of the mass of the visible universe. However, in the broader cosmos, many other nuclei exist and play an important role in the evolution of the universe. These nuclei are exotic (they occur only rarely on Earth) and, in terms of chemistry, are isotopes of the stable atoms on Earth. By vast majority, these rare isotopes are radioactively unstable, meaning that, when left alone on the shelf, they undergo spontaneous decay and transform into different nuclei. Figure 1.1.1 depicts the standard organization of knowledge of rare isotopes.

Nuclear physics is the general study of the principles that govern phenomena of the nucleus, and rare-isotope science is the study of the behavior and interactions of those nuclei that are unstable, exotic, and rare. By studying physical processes that transform nuclei into other nuclei (with the emission of residual particles and energy), scientists learn not only how to control and predict these phenomena, but they also learn about the origins of the chemical elements in the universe.

In particular, the study of rare isotopes allows scientists to expand the basic understanding of nuclear physics in two general ways: (1) rare isotopes present “extremes” to physicists and thereby offer leverage on testing the basic understanding of nuclear physics, and (2) rare isotopes themselves play an important role in physical environments that are hot, dense, or highly interacting, such as those within neutron stars, stellar fusion cycles, nuclear reactions in reactor fuel cycles, and so on.

communities proposed that such 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. Over the years, studies by the joint Department of Energy/National Science Foundation (DOE/NSF) Nuclear Science Advisory Committee supported the need for such a facility. In a landmark report published in 1999,2 a formal concept was envisioned for achieving these capabilities: it was termed the Rare Isotope Accelerator (RIA). To obtain an inde-

2

DOE/NSF Nuclear Science Advisory Committee, ISOL Task Force Report, Washington, D.C., 1999.



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