{"id":11612,"date":"2021-05-18T10:37:53","date_gmt":"2021-05-18T09:37:53","guid":{"rendered":"https:\/\/www.innovationnewsnetwork.com\/?p=11612"},"modified":"2021-05-18T10:37:53","modified_gmt":"2021-05-18T09:37:53","slug":"observing-nuclear-reactions-formation-heavy-elements","status":"publish","type":"post","link":"https:\/\/www.innovationnewsnetwork.com\/observing-nuclear-reactions-formation-heavy-elements\/11612\/","title":{"rendered":"Observing nuclear reactions in the formation of heavy elements"},"content":{"rendered":"

Professor Gabriel Mart\u00ednez-Pinedo from GSI Helmholtzzentrum f\u00fcr Schwerionenforschung<\/a> discusses the r-process and its role in the formation of heavy elements.<\/h2>\n

Around 98% of all the observable matter in the Universe consists of hydrogen and helium. These elements were created in the hot, early phase of the Universe, about a minute after the Big Bang. Heavier elements including carbon, oxygen and iron are produced by fusion processes in stars and finally ejected to the interstellar medium by stellar winds and supernova explosions.<\/p>\n

The production of heavier nuclei requires free neutrons, as processes involving charged particles are suppressed due to the increase in coulomb barrier with atomic number. Neutron-captures lead to the production of new isotopes of the same element that are unstable to beta-decay. After the beta-decay<\/a>, a new element is produced with a higher atomic number. Following this sequence of neutron captures and beta-decays, heavy elements are synthesised. Depending on the relative magnitude of the time scales for successive neutron captures and beta-decay, two possibilities emerge that depend on the neutron density of the astrophysical environment. At low neutron densities, the beta-decay time scale is shorter than the neutron capture time scale and we deal with the \u2018slow\u2019 or \u2018s\u2019 process that is responsible for the production of half of the elements between iron and lead. For high neutron densities, the timescale for neutron captures becomes shorter than the one for beta-decay. The resulting sequence of reactions constitutes the \u2018rapid\u2019 or \u2018r\u2019 process (see figure 1).<\/p>\n

The s-process operates in stars of low and intermediate mass (less than eight solar masses) during their asymptotic giant branch phase. This is confirmed by the observation of 98<\/sup>Tc in the surface of these stars. Its half-life, 4.2 million years, is too short compared with the lifetime of the star where it is observed, and so its must be produced in situ.<\/p>\n

Compared with the s-process, the situation for the r-process is rather different. Research during the 20th century has developed a complete picture of the nuclear processes responsible for the production of energy in stars and the nucleosynthesis of elements. However, the identification of one of the sites where the r-process operates has only been possible recently. Further research is still needed to determine the extent of r-process nuclei produced in the site and the frequency with which these events occur in the Galaxy.<\/p>\n

Operation of the r-process and the role of nuclear physics<\/h3>\n

The r-process operates on a timescale of seconds, producing elements up to thorium and uranium. It involves neutron densities above 1020<\/sup> cm\u22123<\/sup> and temperatures of around 1 GK that lead to the production of very neutron-rich nuclei by a sequence of neutron-captures and beta-decays. Once the initial supply of neutrons is used, the material decays to stability, producing the final abundance distribution that can be compared with observations. The radioactive energy liberated during these decays can potentially lead to a bright electromagnetic transient similar to the decay of 56<\/sup>Ni to 56<\/sup>Co and finally 56<\/sup>Fe powering the light curve of a supernova. Models show that an r-process transient can be as bright as a thousand novae and hence the name \u2018kilonova\u2019.<\/p>\n

The r-process remains the most complex nucleosynthesis process to model from the astrophysics as well as nuclear physics points of view. From the nuclear physics point of view, the r-process requires the properties of nuclei with extreme neutron excess far from the valley of stability. Most of these nuclei have not yet been produced in the laboratory and their properties (masses, neutron-capture rates, beta-decay half-lives, and fission rates) have to be computed theoretically. These three reactions (neutron-captures, beta-decays, and fission) are fundamental to describe r-process nucleosynthesis.<\/p>\n

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Fig. 1: Nuclear chart showing the nucleosynthesis processes occurring during stellar burning (yellow), the s-process (orange) and the r-process (violet) (credit: EMMI, GSI\/Different Arts)<\/figcaption><\/figure>\n

Nuclear masses<\/h4>\n

The masses of these exotic neutron-rich nuclei and their dependence on neutron excess that are particularly important for the r-process. The presence of magic neutron numbers at Nmag<\/sub>=50,82,126, and 184 results in small neutron separation energies for nuclei with neutron number Nmag<\/sub>+1. Hence, the neutron capture rates become very small at the magic neutron numbers, requiring a sequence of several neutron-captures and beta-decays for the r-process to proceed to heavier nuclei. The r-process path moves closer to the line of beta-stability, experiencing longer beta-decay half-lives than those of \u2018regular\u2019 r-process nuclei. These long beta-decay half-lives result in an accumulation of material at the magic numbers. It produces peaks corresponding to maxima in the r-process abundance pattern at A~90,130, and 195.<\/p>\n

Fission<\/h4>\n

Fission reactions during the r process involve several competing reaction channels, including neutron induced fission, beta-delayed fission, spontaneous fission, and gamma-induced fission. During the last phase of the r-process, alpha decays compete with fission in the region of translead nuclei. This competition determines the abundances of long-lived actinides at timescales of days and the final abundances of lead, uranium, and thorium. Fission is also an important source of neutrons during the last phases of the r-process. Those neutrons, together with the fission yields, shape the final abundance distribution and lead to a robust abundance distribution that is basically independent of the astrophysical conditions as suggested by astronomical observations.<\/p>\n

Beta-decay<\/h4>\n

The role of beta-decay in the r-process is twofold. First, by changing a neutron into a proton determines how fast the nucleosynthesis flow moves from one isotopic chain to the next, i.e. the speed at which heavy nuclei are produced starting from the seed nuclei. Secondly, beta-decay produces neutrons via beta-delayed neutron emission that play an important role in determining the final r-process abundances.<\/p>\n

Radioactive beam facilities have been fundamental to measuring the masses and beta-decay rates for r-process nuclei in key regions around the magic numbers N=50 and 82. These data, combined with theoretical advances, has led to the development of fully microscopic models for the description of masses and beta-decays of r-process nuclei, leading to improved predictions of r-process yields.<\/p>\n

The future FAIR facility will allow, for the first time, experimental access to heavy r-process nuclei around magic number N=126 removing one of the largest uncertainties affecting prediction of r-process yields.<\/p>\n

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Fig. 2: Simulation of a neutron star merger showing the distribution of matter shortly after the merger. The colour code shows the temperature. Courtesy of Andreas Bauswein<\/figcaption><\/figure>\n

Observational constraints in the r-process<\/h3>\n

Our present understanding of the operation of the r-process in the Galaxy is based on observations of isotopic and elemental abundances in the Solar system and stars of different ages. These stars are formed from clouds of material that were polluted by r-process nucleosynthesis events that occurred much before their formation. Observations of abundances in our Solar System provide unique information about the proportions at which the different isotopes of a given element are produced by nucleosynthesis processes.<\/p>\n

Stellar observations provide information about the abundances of elements at different locations in the Galaxy and at different times of galactic evolution. Astronomers use the metallicity of a star, i.e. the relative abundance of iron to hydrogen, as a proxy for its age. Particularly important are observations at low metallicities, as they may allow us to observe the products of individual nucleosynthesis events. These observations show that the r-process already operates at the lowest metallicities we have been able to observe, corresponding to a relative iron to hydrogen abundance 10-4<\/sup> times the one of the Sun. They show that, independently of the location in the Galaxy, the relative abundances of elements above Z\u223c50 are always very similar, indicating that there is a mechanism that leads to such a robust abundance pattern. As discussed above, fission may provide such a mechanism.<\/p>\n

Stellar observations show that the r-process operates on events that eject substantial amounts of r-process material and occur with a frequency much smaller than core-collapse supernova that are the main contributors to the production of Iron at low metallicities. As a consequence, one is left with basically two possible astrophysical sites in which the r-process can operate1<\/sup>:<\/p>\n