R-Process Nucleosynthesis in Supernovae The heaviest elements are made only in cataclysmic events. most inevitably followed by beta decay, and the path to increasingly Finding out whether supernovae are cataclysmic enough heavy nuclear species charted by suc- requires extensive astronomical observation and cessive n captures remains close to the sophisticated computer modeling. valley of beta stability in figure 1. The figure shows how beta-decay lifetimes decrease for a given Z as the increas- John J. Cowan and Friedrich-Karl Thielemann ing neutron number N carries a nu- cleus away from the valley of stability. Isotopes involved in s-process nu- lmost all of the hydrogen and helium in the cosmos, cleosynthesis are, in general, sufficiently long lived to be Aalong with some of the lithium, was created in the first studied in the laboratory. That is not so for the r-process, three minutes after the Big Bang. Two more light ele- in which a sufficient flux of neutrons makes tn much ments, beryllium and boron, are synthesized in interstel- shorter than tb. In that case, n captures will proceed into lar space by collisions between cosmic rays and gas nuclei. the very neutron-rich and unstable regions far from the All of the other elements in nature are formed by nuclear beta-stable valley. Such high neutron fluxes are only tran- reactions inside stars. sient—coming, for example, from a supernova explosion. Over the 14-billion-year history of the universe, ele- Once the flux is exhausted, the unstable nuclei produced ments made in stars have been ejected back into space to by the r-process will beta-decay back to the valley of sta- be incorporated into new stars and planets. Thus there is bility to form the so-called stable r-process nuclei. Because an intricate relationship between the life cycles of stars the r-process path (shown by the magenta line in figure 1) and the nucleosynthesis of the elements. Fusion reactions wanders through regions so neutron-rich and so far from inside stellar cores are exothermic. They release the en- stability, experimental measurement of the properties of ergy that powers stars and supports them against gravi- nuclei along the way is very difficult. tational contraction. During most of a star’s life, the prin- The r-process and the s-process contribute roughly cipal fusion process is the burning of H to form He. equally to the nucleosynthesis of heavy isotopes. The ele- But binding energy per nucleon increases with nu- ments that compose the materials of the solar system con- clear mass only up to iron-56, the most tightly bound of all tain admixtures from both—but, interestingly, nothing nuclei. The production of any heavier nucleus by direct fu- that would appear to be from any astrophysical process in- sion is endothermic. Another impediment to the produc- termediate between the two. Some of the n-capture ele- tion of heavy nuclei in stars is the growth of the Coulomb ments are produced by both processes, but some come al- barrier with increasing proton number Z. At sufficiently most exclusively from one process or the other. All the high Z, the Coulomb barrier prevents all nuclear reactions n-capture elements are rare by the standards of the lighter induced by charged particles at stellar temperatures. elements. But those you find at the jeweler’s—gold, plat- Therefore, the isotopes of elements beyond Fe are almost inum, and silver—originate almost entirely in the exclusively formed in neutron-capture processes. The r-process. products are referred to as n-capture elements. The basic ideas of how the r-process operates have been known for some time. But the specific physical con- The two main n-capture processes for astrophysical ditions and nuclear properties required for the process, nucleosynthesis were originally identified in 1957 in pio- and particularly its astrophysical sites, have not been un- neering work by Margaret and Geoffrey Burbidge, William ambiguously identified.2,3 The s-process is much better Fowler, Fred Hoyle, and Alistair Cameron.1 They are called known. Its primary sites are low- or intermediate-mass the slow (s) and rapid (r) n-capture processes. After a nu- stars (from about 0.8 to 8 solar masses [M0]) with long evo- cleus has captured a neutron to become a heavier nucleus, lutionary time scales measured in billions of years.4 the time scale t for it to capture an additional neutron is n By contrast, stars heavier than about 8 M live only a either slow or rapid on the competing time scale t for it 0 b few million years. They are thought to end up as core-col- to undergo beta decay. Whereas tb, the mean beta-decay lapse (type II) supernovae when their thermonuclear fusion lifetime, depends only on the nuclear species, tn depends fuel is exhausted. The products of the s-process take longer crucially on the ambient neutron flux. to be produced and ejected into the galaxy. That is, they ar- When a stable nucleus has captured enough neutrons rive later in galactic history than the r-process elements.5 to leave the valley of stability, it becomes unstable. Even- The density of free neutrons required for the r-process tually it undergoes beta decay, which transforms a neutron points to explosive environments. Supernovae have long into a proton and thus increases the nucleus’s Z by 1 with- been the prime suspects. The earliest studies1 suggested out changing its mass number A. In the s-process, tn is that the edge of the collapsing core of a type II supernova, much longer than tb. Therefore, a single n capture is al- ejecting a rich flux of neutrons, might be the site of the r-process. But many difficulties arise in actually confirm- John Cowan is a professor of physics and astronomy at the Uni- ing the supernova connection. Not enough is known about versity of Oklahoma in Norman. Friedrich-Karl Thielemann is a professor of physics at the University of Basel in Switzerland. the detailed physics—for example, the explosion mecha- nism, the role of neutrino interactions in the explosion, the © 2004 American Institute of Physics, S-0031-9228-0410-020-3 October 2004 Physics Today 47 120 neutron density. The maxi- mum occurs at a specific neu- tron separation energy Sn, the energy released in a neu- 100 tron capture. At a given temperature and neutron density, the abundance- maximum value of S is the Z n 80 same for all isotope chains, irrespective of Z. The r-process path in the NZ- plane is then determined; it 60 connects the maximum-abun- log(tb /s) dance isotopes of all the iso- 1.0 topic chains. Beta decay PROTON NUMBER 40 0.5 (Z, A) O (Z+1, A)+e– + n 0.0 e –0.5 transfers nuclei from one iso- Beta decay –1.0 topic chain to the next and de- 20 –1.5 termines the speed with –2.0 which heavy nuclei are Neutron capture –2.5 formed. The thin magenta line 0 traversing the nuclide chart of 0 20 40 60 80 100 120 140 160 figure 1 illustrates an r- NEUTRON NUMBER N process path with Sn between 2 and 3 MeV. Such a path re- Figure 1. The stable and neutron-rich unstable nuclides.3 Isotopes stable against beta quires a synthesis time on the decay, indicated by black and magenta boxes, form the valley of stability that runs along order of seconds to form the the top edge of the band. (Proton-rich isotopes on the valley’s other side are not shown.) heaviest elements, such as thorium, uranium, and pluto- Colored bands indicate decreasing measured or predicted lifetimes tb with increasing dis- tance from the valley. The jagged black line is the limit of laboratory information. The nium. During an r-process jagged magenta line shows a typical path of rapid (r-process) neutron captures. Such paths event, temperature and neu- tend to turn vertical at the double vertical lines that mark neutron numbers corresponding tron density—and therefore to closed neutron shells. (The horizontal double lines indicate closed proton shells.) A nu- the path’s Sn—change with cleus on an r-process path eventually beta decays up to the valley to become one of the r- time. Thus, very unstable nu- process stable nuclei indicated by the magenta squares. (Courtesy of Peter Möller.) clei with neutron separation energies ranging from about 4 MeV all the way down to zero treatment of hydrodynamic instabilities in three-dimen- can be involved in the r-process. The condition Sn = 0 defines sional simulations, and the equation of state of ultradense the so-called neutron drip line, at which nuclei become un- matter—required to create realistic supernova models stable to neutron emission. that actually yield explosions. Furthermore, imprecise nu- When the intense neutron flux ends, a nucleus on the clear data are lacking on the very unstable nuclei involved r-process path will beta decay back up to the valley of sta- in the r-process. bility and produce one of the stable nuclei indicated by the The situation has, however, been improving rapidly. magenta boxes in the figure. For example, the stable r- There are new high-resolution abundance observations of process nucleus platinum-198 is originally formed as an n-capture elements in halo stars that surround the galac- unstable lower-Z nucleus of the same A but with more neu- 198 tic disk. Models of core-collapse supernovae are becoming trons. A sequence of beta decays then converts it to 78Pt.