Cross Section general trends

We should note that the absolute value of the 170 production factor depends on the still uncertain 17O I p nuclear cross section (cf., Aubert et al. 1996) however, the general trend discussed here is likely to be independent of that. 

X-rays are scattered by electron density - the more electrons an atom has, the more intensely it scatters. Neutrons, however, are scattered by a parameter of the atomic nucleus, which is different for different isotopes (but shows no general trend with nuclear mass). Hydrogen and deuterium have very different (but large) neutron scattering cross-sections and therefore neutron diffraction, which requires access to an atomic reactor, is used where location of hydrogen atoms is critical. 

Direct collisions have been independently investigated by Brown and Muir-head who have found rather higher cross sections than those obtained by the authors first mentioned. For the (w, p) reactions with 14 Mev neutrons the calculated cross sections which always contain a considerable direct interaction component, follow the general trend found by Paul and Clarke (Sect. 28) the direct component varies from a few mb in the heaviest elements to some 40 mb or more in light elements. For proton induced reactions at 18 Mev, their theoretical cross sections do not seem quite adequate to account for the direct component observed. 

There are certain exceptions where the cross section is much lower than that expected from the general trend. These exceptions occur at, or near, the closed shells and reflect a reduction in level density in these regions. (Similar results have been obtained by Newson and Rohrer by a boron absorption method.) When plotted... 

In the region from 37Rb to ggCe, with a few exceptions, the general trend with increasing A seems to be a slow decrease in the cross sections. Both... 

Fig. 15.10 The thickness of the continental crust of Antarctica ranges from 20 to 40 km in West Antarctica and from 40 to 60 km in East Antarctica. In general, the thickness of the crust of West Antarctica is at least S km less than the crust that underlies the Transantarctic Mountains. The boundary between East and West Antarctica is sharp and partiUels the trend of the Transantarctic Mountains. The step-like increase of the crustal thickness suggests that the boundary between East and West Antarctica is a fault zone. The line A-B is the cross-section of crustal thickness shown in Fig. 15.11 (Adapted from Bentley (1983))...

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