Secondary nuclei

Growth of secondary nuclei, small crystals and attrition fragments... 

Garside, J. and Larson, M.A., 1978. Direct observation of secondary nuclei production. Journal of Crystal Growth, 43, 694. 

The development of the freeze concentration process for fruit juices has been hampered by the fact that solute concentrate is entrained by the ice crystals. This incomplete separation of the entrained concentrate from the ice results in a considerable increase of the cost of the process. In this investigation sucrose solutions were concentrated by the formation of an ice layer on the externally cooled walls of the crystallizer. The formation of the layer was initiated by secondary nuclei induced by rotating ice seeds, at subcoolings smaller than the critical subcooling needed for spontaneous nucleation. A minimum in the amount of sucrose entrapped in the ice layer was observed at a subcooling smaller than the critical subcooling for spontaneous nucleation. The effect of soluble pectins on the minimum was also studied. 

Variation of Shape Factor. Three typical variations In the shape factor of MgSOi 71120 crystal are shown In Figure 1. The factors show the results obtained from three experiments of crystal growth In the solution without gas admittance at a supersaturation degree (AC) of 1.78 kg m , and a temperature of 298.2 K. In this solution, secondary nuclei(4) were hardly found. The curves connecting the points In the figure were determined by Fourier series equations to... 

McBride and Carter [139] have videotaped the collisions associated with these crystallizations, and they observed that once the first crystal was formed the stirring bar produces secondary nuclei. These nuclei, of the same handedness as the Adam crystal, are formed in overwhelming numbers and dispersed through the entire solution, serving as seeds for the formation of fresh crystals of the same handedness. 

Secondary nuclei tend to be neutron-rich isotopes of the alpha elements (O, Ne, Mg, Si, S, Ar, and Ca). To make neutron-rich isotopes in stars requires a source of neutrons, and this in turn requires that the gas in bulk contain more neutrons than protons by a small amount. This requires that the star contain an abundant... 

Table 8.7 The contents of PolyPs and P in Acetabularia crenulata at different stages of development (Kulaev et al., 1975). The stages of growth were as follows (1) young cells, 1.5-2 cm long (2) cells 2.5-3 cm long, up to 2 mm in diameter (3) cells with umbellulles tilled with secondary nuclei (4) cells with mature umbellulles tilled with cysts.

Secondary nucleation may also occur in static conditions under certain circumstances (11). In lipid systems, needle-like or dendritic crystals that form under certain conditions may lead to the formation of secondary nuclei. Heat dissipation and/ or concentration of noncrystallizing species in certain regions may lead to melting/ dissolution at the base of the branches of dendritic crystals and result in the formation of numerous nuclei centers. Although the exact mechanisms for this type of secondary nucleation are not fully understood, it is undoubtedly important for... 

In this picture, the amount of secondary nuclei is a measure of the characteristic time for propagation of cosmic rays before they escape from the galaxy into inter-galactic space. A simplified version of the diffusion equation that relates the observed abundances and spectra to initial values is... 

The thickness of the disk of the galaxy is of order 300 pc = 1000 light years, which is much shorter than the characteristic propagation time of 10 million years. The explanation is that the charged particles are trapped in the turbulent magnetized plasma of the interstellar medium and only diffuse slowly away from the disk, which is assumed to be where the sources are located. Measurements of the ratio of unstable to stable secondary nuclei (especially 10Be/9 lie) are used to determine resc independently of the product np resc and hence to constrain further the models of cosmic-ray propagation. 

The primary cosmic rays propagate through the interstellar medium (ISM) until they either escape into extragalactic space, or are removed by interaction or energy losses in the ISM. Their interstellar equilibrium intensity may be recorded with a detector which is usually carried above the earth s atmosphere on spacecraft or balloon. Secondary cosmic rays are those that are generated as products from interactions of the primaries in the ISM positrons and antiprotons mostly come from interactions of primary protons, while the secondary nuclei such as Li, Be, B, and the elements just below iron, which cannot be produced by primary nucleosynthesis, are the products of spallation reactions of heavier primaries in the ISM. The overall arriving cosmic-ray intensity represents a mix of primary and secondary particles. 

It is only a relatively small energy interval below 100 GeV/nucleon, where the spectra and relative elemental abundances at the cosmic-ray source have been inferred from measurements of the intensities of primary and secondary nuclei. Extending this energy range is one of the most important, but unfortunately, also most difficult tasks in cosmic-ray astrophysics. 

Contact nucleation is the most common mechanism of secondary nucleation. Crystal-crystal-, crystal-impeller, and crystal-wall collisions are involved. Secondary nuclei arise from microabrasion (crystal surface damage) or ordered cluster removal by fluid shear forces, as noted above. Figure 4-12 shows that for a given substance, impeller speed and material of construction can both play a role. 

PRIMARY GROWING PRIMARY SECONDARY SECONDARY NUCLEI NUCLEI MICROGEL. NUCLEI MICROGEL MACROGEL X/ Z l L X Z ... 

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