Slow neutron captur

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

Middle-sized stars, between about 1 and 8 M , undergo complicated mixing processes and mass loss in advanced stages of evolution, culminating in the ejection of a planetary nebula while the core becomes a white dwarf. Such stars are important sources of fresh carbon, nitrogen and heavy elements formed by the slow neutron capture (s-) process (see Chapter 6). Finally, small stars below 1 M have lifetimes comparable to the age of the Universe and contribute little to chemical enrichment or gas recycling and merely serve to lock up material. 

Beyond iron lies a first population of so-called s-process nuclei, which includes among others barium and lead. This population has an abundance distribution with peaks around mass numbers 87, 138 and 208. These nuclei are produced by slow neutron capture, referred to as the s process. A second population, slightly shifted from the first, including gold, platinum and uranium, is imputed to the process of rapid neutron capture, referred to as the r process. 

The term s process is an abbreviation for slow neutron capture process . Here, capture is slow relative to the characteristic time for internal transformation of the neutron into a proton (radioactive decay). Between two neutron captures, there is ample time for () decay to occur. The r process represents quite the opposite situation. Neutron capture is not interrupted by () decay. 

For the slow neutron capture process, there is an equilibrium between the production and loss of adjacent nuclei. Stable nuclei are only destroyed by neutron capture. For such nuclei, we can write for the rate of change of a nucleus with mass number A ... 

Gamma-ray spectrometry is a probe of nudear rather than chemical processes, but its high specificity and sensitivity have applications in analysis of materials (286). It is especially suited for activation analysis. Unstable nudides produced by nudear bombardment can be identified by their characteristic gamma-ray decay emissions. An important example is slow neutron capture by nitrogen with subsequent decay of 15 detected from its 1.7—10.8 MeV gamma lines, a signature useful for remote, nondestmctive detection of possible hidden explosives (see Explosives and propet. t.ents). Gamma-ray... 

Intermediate-mass red giant stars are understood to be the primary source both of and of the heavy s-process (slow neutron capture) elements, as well as a significant source of and other less abundant CNO isotopes. Their contributions to galactic nucleosynthesis are... 

Figure 5 A display of prominent exotic (presolar) noble-gas compositions (from Anders and Zinner, 1993). In the left two panels, for each isotope on the abscissa the ordinate is the ratio (to °Xe) in the HL component (left panel) or the G (formerly termed Xe-S) component (center panel), divided by the equivalent ratio in solar xenon (i.e., solar xenon would plot with all isotopes at unity on the ordinate). The HL component shows the defining characteristics of enriched heavy and light isotopes. For the G-component, the pattern is that expected for s-process (slow neutron capture) nucleosynthesis. The right panel is a three-isotope diagram analogous to Figure 4, except that both scales are logarithmic. It shows experimental limits for the R-component (formerly Ne-E(L)) and the G-component (formerly...

As an example of (i) gold has a cross-section of 96 barns for slow-neutron capture, lead of only 5 X 10 barns. Irradiation, while hardly affecting the lead, changes the gold into an active form. The gold content of lead can thus be found. 

Neutron absorption processes occur at different times and places in the course of the evolution of massive stars. The S-process (slow neutron capture) occurs in the He burning region (state of a red giant). The R-process (rapid neutron capture) occurs during the super nova explosion, either within a short distance of the forming neutron star or in the shell where He burning took place prior to the blast when the shock wave hits this area. 

This pathway cannot continue indefinitely, however. What limits the process is the relationship between neutron capture time and the half-life of isotopes produced by neutron capture. The reactions just described, for example, which successively change iron to cobalt and nickel, take place very slowly and are, therefore, known as slow neutron capture reactions or, more simply, as s reactions. They are called "slow" because, on average, hundreds to thousands of years may pass before any given nucleus absorbs a neutron. These reactions can occur because they all involve the presence of a stable isotope at some point, for instance, iron-57, iron-58, or cobalt-59. As long as these isotopes are present—or as long as isotopes with half-lives greater than a few hundreds or thousands of years are present—there is enough time for neutron capture to occur. 

The book has been organized into three parts to address the major issues in cosmochemistry. Part I of the book deals with stellar structure, nucleosynthesis and evolution of low and intermediate-mass stars. The lectures by Simon Jeffery outline stellar evolution with discussion on the basic equations, elementary solutions and numerical methods. Amanda Karakas s lectures discuss nucleosynthesis of low and intermediate-mass stars covering nucleosynthesis prior to the Asymptotic Giant Branch (AGB) phase, evolution during the AGB, nucleosynthesis during the AGB phase, evolution after the AGB and massive AGB stars. The slow neutron-capture process and yields from AGB stars are also discussed in detail by Karakas. The lectures by S Giridhar provide some necessary background on stellar classification. 

Libby, W. F. 1940. Reactions of high energy atoms produced by slow neutron capture. J Am ChemSoc 62, 1930-1943. 

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