Nuclear astrophysics
Since the middle of the 19th century, the spectral analysis of starlight was used to investigate the elemental composition of stars. Most stars have a very similar composition to our sun and lie along the so-called main sequence in the Hertzsprung-Russel diagram, which shows the stellar luminosity versus surface temperature. There is, however, a small but distinctive group of stars off the main sequence that is characterized by red light (which means relatively cool surface temperature) and enormous brightness (luminosity). The diameter of those Red Giant stars is about 50 times bigger than the diameter of our sun. The 1952 discovery of atomic lines from the heavy element technetium in the spectra of Red Giants was the longdesired smoking gun, revealing the site of heavy-element production. The half-lives of all technetium isotopes are much shorter than the billion-year time scales of stellar evolution, which means that the technetium and therefore most heavy elements are produced inside the Red Giants. Shortly after this breakthrough discovery, an overall picture of the synthesis of the elements was developed that took into account all available scientific knowledge at that time. The main aspects of this picture are still valid nowadays.
According to our current understanding, only the very light elements (hydrogen, helium, and lithium) were produced during the big bang, the initial explosion from which our universe evolved. Those elements condensed into the first generation of stars, and the most massive ones (more than a hundred times the mass of our sun) evolved on a time scale of millions of years through different burning stages, producing heavier and heavier elements up to iron, the most stable element in the periodic table. After forming an iron core, which cannot support further fusion burning of charged particles, these massive stars suddenly collapse under their own weight and then blow off their outer layers in a supernova explosion. During that explosion, heavy elements between iron and uranium were produced in the expanding envelope and blown off into the interstellar medium. The ejected material then served as seed material for new generations of stars, such as, our sun. Only much later did the low-mass stars, which were undergoing very slow hydrogen burning over billions of years, finally evolve to the Red Giant phase and contribute to heavy-element production. Almost all the elements heavier than iron are produced through neutron-induced processes. There are two major processes, the rapid neutron-capture process (r‑process), which takes place in explosive scenarios such as a supernova explosion, and the slow neutron-capture process (s‑process), which can be found in Red Giants.
About half of the element abundances from iron to the lead-bismuth group are produced by the s-process, and the other half by the r-process. The two most-important reactions occurring during the s-process are neutron capture and beta decay. Free neutrons must be available for neutron capture to occur, whereas an unstable nucleus undergoes beta decay spontaneously. The s-process starts with an iron seed exposed to free neutrons and it builds up the elements following the neutron-rich side of the nuclear valley of stability. Following neutron capture, the new unstable isotope will almost always beta-decay back to the valley of stability before it can capture another neutron. In contrast, during the r-process, the neutron flux is so high and the neutron capture times so short that a nucleus will almost always capture several neutrons before it undergoes a beta decay. Thus, the r-process follows a path that is shifted farther toward the neutron-rich isotopes. The beta decay half-lives are much shorter for isotopes very rich in neutrons, so that shortly after the neutron source terminates, the products of the r-process will beta-decay back to the valley of stability.
The exact pathway of the s-process depends on the conditions in the star. Starting at the very abundant iron group, all elements up to bismuth could, in principle, be produced by a sequence of reactions in which each stable isotop captures a neutron until an unstable isotope is produced that quickly decays to the next higher element through beta decay and then waits for the next neutron capture. If, however, conditions in the star make the rates for neutron capture comparable to the rate of beta decay by a particular isotope, then the s-process path would branch at that isotope with some fraction of that isotope transforming via neutron capture, while another fraction transforms through beta decay. The branching ratio, or relative likelihood, for the different reactions depends on the physical conditions in the interior of the star temperature, neutron density, and electron density. At higher neutron densities with all other conditions equal, more nuclei of a given isotope would capture a neutron before having the chance to betadecay than at low neutron densities. Thus, the branching ratios deduced from the isotopic ratios observed in stellar material could provide the tools to effectively constrain modern models of the stars where the nucleosynthesis occurs, provided one knows the fundamental rates, or cross sections, for neutron capture and beta decay.
As isotopic abundances from the s-process are now observable in stellar grains, neutron capture measurements on the nuclei at the branch points of the s-process are the most-important missing experimental link to further improve our picture of the evolution of Giant stars and, hence, the history of the elements of which we and our world are made. In general, the main uncertainty in predicting heavy-element abundances arises from uncertainties in the physics of mixing, whether in the long-lasting quiescent phases of Giant stars or the terminating supernova explosions. Data from neutron capture measurements, combined with information from stellar grains, can be used to constrain models of mixing in Giant stars. Some of the required neutron-capture measurements can be carried out at the n-TOF facility. Examples of succesful measurements helping to constrain our understanding of the stellar conditions during the s-process are the successful determinations of the neutron capture cross section of samarium isotopes (including samarium-151 with an half life of 100 years) as well as osmium, zyrconium and lead isotopes.
References
- Neutron Capture Cross Section Measurement of 151Sm at the CERN Neutron Time of Flight Facility. (n_TOF)Abbondanno, U. and Aerts, G. and Alvarez-Velarde, F. and Álvarez-Pol, H. and Andriamonje, S. Physical Review Letters 93, 161103 (2004)
- Neutron physics of the Re/Os clock. I. Measurement of the (n,g) cross sections of Os186,187,188 at the CERN n_TOF facilityMosconi, M. and Fujii, K. and Mengoni, A. and Domingo-Pardo, C. and Käppeler, F.Phys. Rev. C 82, 015802-+ (2010)
- Neutron capture cross section of Zr90: Bottleneck in the s-process reaction flowTagliente, G. and Fujii, K. and Milazzo, P. M. and Moreau, C. and Aerts, G.Phys. Rev. C 77, 035802 (2008)
- Measurement of the neutron capture cross section of the s-only isotope Pb204 from 1 eV to 440 keVDomingo-Pardo, C. and Abbondanno, U. and Aerts, G. and Álvarez-Pol, H. and Alvarez-Velarde, F.Phys. Rev. C 75, 015806 (2007)