Rare isotopes in the cosmos Hendrik Schatz Such stellar processes as heavy-element formation and x-ray bursts are governed by unstable nuclear isotopes that challenge theorists and experimentalists alike. Hendrik Schatz is a professor of physics and astronomy at Michigan State University’s department of physics and astronomy and National Superconducting Cyclotron Laboratory in East Lansing. He is also cofounder and associate director of the Joint Institute for Nuclear Astrophysics, an NSF Physics Frontier Center. Radioactive nuclei with extreme neutron-to-proton ra- Nuclear Astrophysics in the US exemplifies such a center. tios—rare isotopes—often decay within fractions of a second. Across the Atlantic, Europe is witnessing such initiatives as Typically, they are not found on Earth unless produced at an the Extreme Matter Institute and the Munich Cluster of Ex- accelerator. Yet nature produces copious amounts of them in cellence on the Origin and Structure of the Universe, both in supernovae and other stellar explosions in which the rare iso- Germany, and the international Challenges and Advanced topes, despite their fleeting existence and small scale, imprint Research in Nuclear Astrophysics network. their properties. Large amounts of them also exist as stable layers in the crusts of neutron stars. Origin of the elements Rare isotopes are therefore intimately linked to funda- Nuclear processes in stars and stellar explosions have forged mental questions in astrophysics. An example is the origin of nature’s chemical elements out of the hydrogen and helium the 50-odd naturally occurring elements between the iron re- left over from the Big Bang. Fusion reactions in stars drive gion and uranium in the periodic table. As figure 1 shows, the formation of elements with atomic numbers up to about with recent progress in stellar spectroscopy and the continu- that of iron. But fusion does not produce heavier elements; ing discovery of very old and chemically primitive stars, a the nuclear binding energy per nucleon is maximal for nuclei “fossil record” of chemical evolution is now emerging. Nu- around iron and nickel, so continued fusion would be clear science needs to make its own progress to match spe- endothermic. cific events to the observed elemental abundance patterns Heavier elements are thought to be built up by a neu- produced in stellar explosions. That has turned out to be a tron-capture process that iterates a two-step sequence. First, tremendous challenge. Nuclear theory is still far from being neutrons bombard a seed nucleus until a number have been able to predict the properties of rare isotopes. On the con- captured and an unstable isotope forms. Then beta decay in- trary, the limited experimental data obtained so far have pro- creases the number of nuclear protons by one and creates a duced many surprises that have forced theorists to adjust or new element. The resulting nucleus is heavier than the seed to rethink nuclear theory (see the article by David Dean, and has less binding energy per nucleon, but the additional PHYSICS TODAY, November 2007, page 48). In some cases, binding of the free neutrons makes the process exothermic. such as certain nuclear masses or the excitation energies of A slow neutron-capture process (the s-process) that has been resonant states in nuclear reactions, theory is not likely in the found to operate in red giant stars explains the origin of about foreseeable future to reach the precision needed for reliable 60% of the heavier elements. The remaining 40% are usually astrophysical models. attributed to a rapid neutron-capture process, the r-process If scientists are to make further inroads, they must pro- (see the article by John Cowan and Friedrich-Karl Thiele- duce and study in the laboratory the rare isotopes that exist mann, PHYSICS TODAY, October 2004, page 47). But we still do in astrophysical environments. That task, too, has proved to not know where that r-process takes place and what exactly be enormously difficult. To date, the vast majority of the as- the reaction sequence is that builds up the heavy elements. trophysically important rare isotopes are still out of reach of The pattern of elemental and isotopic abundances pro- even the most advanced rare-isotope production facilities, duced by the r-process can be directly related to properties and, more often than not, the isotopes that can be made can- of rare isotopes such as their shell structure. Those abun- not be produced in the quantities needed to extract the nec- dances can be extracted from solar-system data by subtract- essary information. ing the predicted contributions from the s-process. Though Nevertheless, impressive progress has been achieved. A the procedure has its uncertainties, the residual r-process number of rare-isotope facilities now operate worldwide, abundances clearly show pronounced peaks at nuclear mass and nuclear astrophysics has been an important motivation numbers A = 130 and A = 195 (see figure 1d) that corre- for many of them. In recent years, centers and networks for spond to enhancements of the stable isotopes of elements nuclear astrophysics have emerged that facilitate interdisci- near tellurium and platinum. Such peaks reflect a neutron- plinary connections and integrate experiments at rare- capture process in which the neutron number N crosses that isotope and other experimental facilities with astronomical of a closed neutron shell; for the peaks shown in figure 1d, observations and astrophysical and nuclear theory to address the neutron magic numbers are N = 82 and N = 126. Now, a open questions in nuclear astrophysics. The Joint Institute for rare isotope’s beta decay toward stability does not change the 40 November 2008 Physics Today © 2008 American Institute of Physics, S-0031-9228-0811-020-1 100 100 10–2 Big Bang 10–2 Oldest stars ON –4 ON 10–4 10 10–6 10–6 Iron-56 10–8 10–8 10–10 10–10 MASS FRACTI MASS FRACTI 10–12 10–12 10–14 a 10–14 b 0 50 100 150 200 0 50 100 150 200 MASS NUMBER MASS NUMBER 0 100 10 Iron-56 Solar system 10–2 Early r-process 10–2 ON –4 ON 10–4 10 Iron-56 130 10–6 10–6 195 10–8 10–8 10–10 10–10 MASS FRACTI MASS FRACTI 10–12 10–12 10–14 c 10–14 d 0 50 100 150 200 0 50 100 150 200 MASS NUMBER MASS NUMBER Figure 1. Chemical evolution in the Milky Way, from the Big Bang to the present. The iron abundance allows for rough dating so that one can establish a record of chemical evolution. (a) Element abundances attributed to the Big Bang. (Adapted from ref. 13.) (b) The composition of HE 1327-2326, which represents that of the oldest, most Fe-poor stars known. This panel indicates elements contributed by the first supernovae in the galaxy. (Adapted from ref. 14.) (c) The composition of CS 22892-052, a star much older and poorer in Fe than the Sun. The spectrum here is dominated by the decay products of the rare isotopes produced early in the so-called r-process discussed in the text. (Data courtesy of John Cowan; see also ref. 15.) (d) The composition of the solar system, as inferred from solar spectra and the analysis of pris- tine meteoritic material. The peaks at mass numbers 130 and 195 are signatures of the r-process. (Adapted from ref. 16.) The stellar observations reflected in panels b and c directly provide only an element’s atomic number. Atomic masses are assigned based on assumptions about the nucleosynthesis process. mass number of the nucleus by much, since only a small num- ing for those extraordinary densities is a considerable chal- ber of neutrons are emitted along the decay chain. Thus the lenge for theoretical astrophysics. Most current models in- r-process evidenced by the figure 1d peaks proceeds through volve neutron stars in some form—for example, the neutron- or near cadmium-130 (N = 82, A = 130) and thulium-195 rich outflow from a proto-neutron star that forms in a (N = 126, A = 195), as shown in figure 2. The lighter 130Cd is a core-collapse supernova or from the merging of two neutron rare isotope with a half-life of 160 ms, and 195Tm is an extremely stars into a black hole. All the proposed scenarios have issues neutron-rich rare isotope of thulium that has never been ob- that need to be addressed. Astronomical observations and ex- served in a laboratory. perimental nuclear data are needed to guide and ultimately How can such short-lived rare isotopes be produced by verify or falsify the theoretical possibilities. neutron capture, given that first the r-process has to produce isotopes that have fewer neutrons and that decay within a What needs to be measured? fraction of a second? Evidently, neutron densities are ex- The beta-decay half-lives of the nuclei along the r-process tremely large and make neutron-capture rates even faster path determine the time it takes to build up heavy elements. than beta-decay rates. In some r-process models, the neutron- After all, a beta decay occurs for each step up the element capture rate is always greater than the beta-decay rate. In chain. Even more important, as the r-process attempts to ap- those cases, it is photon bombardment in the hot stellar proach a local steady flow, beta-decay half-lives set the abun- plasma that removes neutrons and eventually halts the chain dances of the nuclides produced at the various locations in of neutron captures. The r-process then has to pause until a the path.