Rare isotopes in the cosmos

© 2008 American Institute of Physics, S-0031-9228-0811-020-1 Radioactive nuclei with extreme neutron-to-proton ratios—rare isotopes—often decay within fractions of a second. Typically, they are not found on Earth unless produced at an accelerator. Yet nature produces copious amounts of them in supernovae and other stellar explosions in which the rare isotopes, despite their fleeting existence and small scale, imprint their properties. Large amounts of them also exist as stable layers in the crusts of neutron stars. Rare isotopes are therefore intimately linked to fundamental questions in astrophysics. An example is the origin of the 50-odd naturally occurring elements between the iron region and uranium in the periodic table. As figure 1 shows, with recent progress in stellar spectroscopy and the continuing discovery of very old and chemically primitive stars, a “fossil record” of chemical evolution is now emerging. Nuclear science needs to make its own progress to match specific events to the observed elemental abundance patterns produced in stellar explosions. That has turned out to be a tremendous challenge. Nuclear theory is still far from being able to predict the properties of rare isotopes. On the contrary, the limited experimental data obtained so far have produced many surprises that have forced theorists to adjust or to rethink nuclear theory (see the article by David Dean, PHYSICS TODAY, November 2007, page 48). In some cases, such as certain nuclear masses or the excitation energies of resonant states in nuclear reactions, theory is not likely in the foreseeable future to reach the precision needed for reliable astrophysical models. If scientists are to make further inroads, they must produce and study in the laboratory the rare isotopes that exist in astrophysical environments. That task, too, has proved to be enormously difficult. To date, the vast majority of the astrophysically important rare isotopes are still out of reach of even the most advanced rare-isotope production facilities, and, more often than not, the isotopes that can be made cannot be produced in the quantities needed to extract the necessary information. Nevertheless, impressive progress has been achieved. A number of rare-isotope facilities now operate worldwide, and nuclear astrophysics has been an important motivation for many of them. In recent years, centers and networks for nuclear astrophysics have emerged that facilitate interdisciplinary connections and integrate experiments at rareisotope and other experimental facilities with astronomical observations and astrophysical and nuclear theory to address open questions in nuclear astrophysics. The Joint Institute for Nuclear Astrophysics in the US exemplifies such a center. Across the Atlantic, Europe is witnessing such initiatives as the Extreme Matter Institute and the Munich Cluster of Excellence on the Origin and Structure of the Universe, both in Germany, and the international Challenges and Advanced Research in Nuclear Astrophysics network.