|
The r-process is a nucleosynthesis process that occurs in core-collapse supernovae (see also supernova nucleosynthesis) and is responsible for the creation of approximately half of the neutron-rich atomic nuclei heavier than iron. The process entails a succession of ''rapid'' neutron captures (hence the name r-process) by heavy seed nuclei, typically 56Fe or other more neutron-rich heavy isotopes. The other predominant mechanism for the production of heavy elements in the universe (and in our Solar System) is the s-process, which is nucleosynthesis by means of ''slow'' captures of neutrons, primarily occurring in AGB stars. The s-process is ''secondary'', meaning that it requires preexisting heavy isotopes as seed nuclei to be converted into other heavy nuclei. Taken together, these two processes account for a majority of galactic chemical evolution of elements heavier than iron. The r-process occurs to a slight extent in thermonuclear weapon explosions, and was responsible for the historical discovery of the elements einsteinium (element 99) and fermium (element 100). == History == The need for some kind of rapid capture of neutrons was seen from the relative abundances of isotopes of heavy elements given in a newly published table of abundances by Hans Suess and Harold Urey in 1956. Radioactive isotopes must capture another neutron faster than they can undergo beta decay in order to create abundance peaks at germanium, xenon, and platinum. According to the nuclear shell model, radioactive nuclei that would decay into isotopes of these elements have closed neutron shells near the neutron drip line, where more neutrons cannot be added. Those abundance peaks created by rapid neutron capture implied that other nuclei could be accounted for by such a process. That process of rapid neutron capture in neutron-rich isotopes is called the ''r-process''. A table apportioning the heavy isotopes phenomenologically between s-process and r-process was published in the famous B2FH review paper in 1957, which named that process and outlined the physics that guides it. B2FH also elaborated the theory of stellar nucleosynthesis and set substantial frame-work for contemporary nuclear astrophysics. The r-process described by the B2FH paper was first computed time-dependently at Caltech by Phillip Seeger, William A. Fowler and Donald D. Clayton, who achieved the first successful characterization of the r-process abundances and showed its evolution in time. They were also able using theoretical production calculations to construct more quantitative apportionment between s-process and r-process of the abundance table of heavy isotopes, thereby establishing a more reliable abundance curve for the r-process isotopes than B2FH had been able to define. Today, the r-process abundances are determined using their technique of subtracting the more reliable s-process isotopic abundances from the total isotopic abundances and attributing the remainder to the r-process nucleosynthesis. That r-process abundance curve (vs. atomic weight) gratifyingly resembles computations of abundances synthesized by the physical process. Most neutron-rich isotopes of elements heavier than nickel are produced, either exclusively or in part, by the beta decay of very radioactive matter synthesized during the r-process by rapid absorption, one after another, of free neutrons created during the explosions. The creation of free neutrons by electron capture during the rapid collapse to high density of the supernova core along with assembly of some neutron-rich seed nuclei makes the r-process a ''primary process''; namely, one that can occur even in a star of pure H and He, in contrast to the B2FH designation as a ''secondary process'' building on preexisting iron. Observational evidence of the r-process enrichment of stars, as applied to the abundance evolution of the galaxy of stars, was laid out by Truran in 1981. He and many subsequent astronomers showed that the pattern of heavy-element abundances in the earliest metal-poor stars matched that of the shape of the solar r-process curve, as if the s-process component were missing. This was consistent with the hypothesis that the s-process had not yet begun in these young stars, for it requires about 100 million years of galactic history to get started. These stars were born earlier than that, showing that the r-process emerges immediately from quickly-evolving massive stars that become supernovae. The primary nature of the r-process from observed abundance spectra in old stars born when the galactic metallicity was still small but that nonetheless contain their complement of r-process nuclei. This scenario, though generally supported by supernova experts, has yet to achieve a totally satisfactory calculation of r-process abundances because the overall problem is numerically formidable; but existing results are very supportive. The r-process is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most neutron-rich isotopes of each heavy element. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「R-process」の詳細全文を読む スポンサード リンク
|