Nuclear reaction network

The yields of r-process nucleosynthesis species are obtained by application of an extensive nuclear reaction network code that consists of 4000 species. The mass-integrated abundances from the surface (zone 1) to the zones 83 (a), 92 (b), 95 (c), 98 (d), 105 (e), and 132 (f) are compared with the solar r-process abundances in Fig. 1. A solar r-process pattern for A>130 is naturally reproduced in cases c-f, while cases a-b fail to reproduce the third abundance... 

Fig. 4.3. Nuclear reaction network. As an example, this figure shows the hnks established using nuclear reactions as a go-between during silicon fusion. Black dots indicate stable isotopes. The notation (x, y) is equivalent to x+A B + y, where x is the initial particle and y the final particle. (After Clayton 1983.)...

By coupling the nuclear reaction network to stellar models, we may calculate the compositions resulting from these nuclear processes, under any imaginable... 

As the temperature decreases further, the QSE clusters fragment more and more into smaller clusters until total breakdown of the QSE approximation, at which point the abundances of all nuclides have to be calculated from a full nuclear reaction network. In the relevant a-particfe-rich environment, the reaction flows are dominated by (a, 7) and (a, n) reactions with the addition of radiative neutron captures. Nuclei as heavy as Fe or even beyond may result. For a low enough temperature, all charged-particle-induced reactions freeze-out, only neutron captures being still possible. This freeze-out is made even more efficient if the temperature decrease is accompanied with a drop of the density p, which is especialiy efficient in bringing the operation of the p2-dependent a - a + n reaction to an end. 

The nuclear generation processes in stars, however, do not follow simple fusion of nuclei one at a time. There are preferential networks of nuclear reactions called cycles that produce different atomic nuclei, but only some cycles are triggered in heavier stars. 

On the basis of these estimates, we can identify the flow of nuclear reactions and plot the rivers they follow on the (A, Z) map. By coupling this network of nuclear reactions with models of stars or the Big Bang, which predict temperature and density variations in space and time, we may hope to identify the nature of the elements and isotopes produced, as well as their relative proportions. 

Hydrogen-burning (in particular) does not proceed via a single reaction, but through a chain or cycle involving several different nuclear reactions. In this case, the reaction rate of the network is governed by the rate of the slowest reaction in the network, but the energy released per product nucleon will be that for the entire cycle, thus ... 

As the hydrogen-burning reaction chains suggest, a complex web of nuclear reactions will occur at the same time. Yields are determined by the branching ratios and rates of individual reactions. The chemical composition of this thermonuclear soup can be obtained, at least for steady-state burning, by setting up a system of simultaneous equations that model the entire reaction network and solving for the equilibrium abundances of each chemical species. 

In addition to the above tools, we are developing other tools related to Galactic chemical evolution. These include a Nuclear Reactions Tool, a Nuclear Network Tool, and a Stellar Ejecta Tool. The Nuclear Reactions Tool will help users calculate nuclear reaction rates and help organize, view, and sort many of the common parameters need for these calculations. The Nuclear Network Tool will provide an easy way to evolve a system of species through time for a given environment s temperature and pressure. The features of the Stellar Ejecta Tool are designed to help a user understand the isotopic anomalies found in primitive meteorites or presolar grains. The Stellar Ejecta Tool will provide an easy way to view the isotopic abundance of a star s ejecta, run a nuclear decay network on this material, and then mix it with a second distribution of isotopic abundances. In this way it can simulate systems such as a late injection of material into the early solar nebula. When these tools are released, we will announce them over the Webnucleo mail list (see below). 

The reliable long-term safety assessment of a nuclear waste repository requires the quantification of all processes that may affect the isolation of the nuclear waste from the biosphere. The colloid-mediated radionuclide migration is discussed as a possible pathway for radionuclide release. As soon as groundwater has access to the nuclear waste, a complicated interactive network of physical and chemical reactions is initiated, and may lead to (1) radionuclide mobilization (2) radionuclide retardation by surface sorption and co-precipitation reactions and (3) radionuclide immobilization by mineralization reactions, that is, the inclusion of radionuclides into thermodynamically or kinetically stabilized solid host matrices. 

The position of the methyl substituent of 1,4-bis(tolylthio)butane plays also a major role for the control of the nuclearity. This was evidenced by mixing Cul with l,4-bis(o-tolylthio)butane (Scheme 10). The 2D network of the reaction product [(CuI)2 p,-o-TolS(CH2)4STol-o 2]ra 20 (Fig. 35) contains discrete dinuclear Cu(p2-I)2Cu rhomboids as connecting nodes, similar to the bonding situation encountered in [(CuBr)2 p,-p-TolS(CH2)4STol-p 2] 19. The Cu-Cu separation of 2.796(3) A matches closely with the sum of the van der Waals radii of two Cu atoms. 

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