Seed nuclei

The data available so far show no evidence for a dependence of the s-process efficiency (traced by [Pb/Ba]) from metallicity (see Figure 1, left panel). Conversely, the state-of the art models (e.g. Busso et al. 2001) predict that, as the number of seed nuclei decreases with decreasing metallicity, the path of the s-process shifts more toward the third peak (e.g. Pb) with respect to the second peak (e.g. Ba). Thus an increase of [Pb/Ba] is expected as the metallicity lowers. 

A possible path for the r-process is shown in Fig. 6.9. The iron seed nuclei need to capture many neutrons leading to unstable nuclei with very short /f-decay half-lives, demanding a high neutron density ... 

The r-process path is terminated by (neutron-induced or yd-delayed) fission near A max = 270, feeding matter back into the process at around Amax/2, followed by recycling as long as the neutron supply lasts, assuming sufficient seed nuclei to start the process off. The number of heavy nuclei is thus doubled at each cycle, which could take place in a period of a few seconds, yd-delayed fission also occurs after freeze-out, when the yd-decay leaves nuclei with A > 256 or so with an excessive positive charge (see Eq. 2.90). 

Given that seed nuclei are exposed to a flux of neutrons at T = 1.5 x 108 K, nn = 109 cm-3 for 20 years during a pulse, and r0 = 0.3 mb-1, estimate the overlap fraction r between successive inter-shell convective zones. 

Hydrogen atoms and part of He are believed to have been created during the Big Bang by proton-electron combinations. Most nuclides lighter than iron were created by nuclear fusion reactions in stellar interiors (cf table 11.1). Nuclides heavier than the Fe-group elements (V, Cr, Mn, Fe, Co, Ni) were formed by neutron capture on Fe-group seed nuclei. Two types of neutron capture are possible slow (s-process) and rapid (r-process). 

However, care must be taken to ensure that seed nuclei are present on which condensation can occur. If a very clean system is used in which nuclei are not present, spontaneous nucleation may occur this process is such that nuclei do not appear uniformly either in space or in time, and the initial particle growth rate depends on the degree of supersaturation. As a result, a polydisperse aerosol is produced under these conditions. 

The formation of primary nanometer-sized subunits, as an intermediate step in the growth reaction, can also account for the observed induction period that was observed in the seeded growth experiments (13,38,56,57). The condensation reaction can proceed parallel to the hydrolysis reaction from the very first beginning, and still it will take a certain time (induction time) to produce the primary subunits, which then may agglomerate to form the new seed nuclei or, in the case of seeded growth, adhere to the surface of already existing particles. 

If the time scale of neutron capture reactions is very much less than 3 -decay lifetimes, then rapid neutron capture or the r process occurs. For r-process nucleosynthesis, one needs large neutron densities, 1028/m3, which lead to capture times of the order of fractions of a second. The astrophysical environment where such processes can occur is now thought to be in supernovas. In the r process, a large number of sequential captures will occur until the process is terminated by neutron emission or, in the case of the heavy elements, fission or (3-delayed fission. The lighter seed nuclei capture neutrons until they reach the point where (3 -decay lifetimes have... 

Multicircuit installations tend to dampen the effects of surging since the peak in production from one circuit may balance a valley in production from another. Surging can also be reduced by the use of a two-drum circuit [5] in which the correct proportion of seed nuclei is formed under controlled conditions in the first drum and then passed on to a second where more feed is added to grow the seeds to product size by layering. 

The y-process involves the photodisintegration of heavy elements. Obviously, this process is not very efficient, as the effective seed nuclei are the primordial heavy-element constituents of the star. Most of the heavy p-elements with atomic mass number greater than 100 that are produced in massive stars (see Figure 5) are formed by this mechanism. It is primarily for this reason that these proton-rich heavy isotopes are rare in nature. 

For exposures equivalent to about 20 neutrons per Fe seed nucleus (r 0.5), the seed nuclei are converted principally to elements of atomic mass A from 70 to 85 with very little synthesis of nuclei beyond the neutron magic number N = 50. At about 50 neutrons per Fe seed nucleus, the Fe seeds have been flushed to nuclei between the magic numbers N = 50 and N = 82. And at exposures of 130 neutrons per Fe seed nucleus, the greatest crmnm occur between N = 82 and Pb. Finally for higher exposures the Fe seeds are predominantly converted to Pb nuclei. A recent discovery of lead stars shows that a high neutron to seed ratio is possible for metal-poor stars (Van Eck et al. 2001). 

Investigation Techniques. DSC measurements were carried out under nitrogen atmosphere. In order to destroy the self-seeding nuclei in the components, the samples were preheated for 5 min at least 35°C above the melting points of the higher melting component. Then, several crystallization and reheating runs were performed at a standard rate of 10°C/min. In some cases, other rates were used too. 

The MES relies on a superposition of a given number of canonical events, each of them being defined by a neutron irradiation on the 56Fe seed nuclei during a time f rr at a constant temperature T and a constant neutron density Nn. In contrast to the canonical model, no hypothesis is made concerning any particular distribution of the neutron exposures. Only a set of canonical events that are considered as astrophysically plausible is selected a priori. We adopt here about 500 s-process canonical events covering ranges of astrophysical conditions that are identified as relevant by the canonical model, that is, 1.5 x 108 < T < 4 x 108 K, 7.5 < log./Vn[cm 3] < 10, and 40 chosen t rr-values,... 

Fig. 7.3 Average pellet diameter as a function of agglomeration time (measured in drum revolutions). (A) Region of seed (nuclei) formation (B) transition region (C) agglomerate growth region.

Howard, W. M. Arnett, W. D. Clayton, D. D. Woosley, S. E. Nucleosynthesis of Rare Nuclei from Seed Nuclei in Explosive Carbon Burning. Astrophys. J. 1972,175,201-216. 

Carbon. Carbon has two stable isotopes, and which have distinct nucleosynthetic origins. The formerj abundant, isotope is produced during He-buming by the triple-alpha reaction ( 3 He + y) whereas requires seed nuclei and hence... 

In the condensation method, a sample of vapour-saturated gas is subjected to a rapid volume expansion. This lowers the temperature and causes a state of supersaturation. Condensation of the vapour will then take place on any particles or ions present in the sample. If the sample is free of particles and ions, there will still be collisions of vapour molecules that create clusters, called embryos, which can serve as nuclei for condensation. Although the seeding nuclei maybe of different sizes, they grow by diffusion of the condensable vapour to ultimate diameters that are almost independent of the original nuclei sizes. The condensation method can be used to make aerosols having diameters from about 36 nm to just over 1 pm and concentrations from about 10 to 10 particles/cm [64]. 

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