On Mon, Mar 25, 2013 at 11:39 PM, David Roberson <dlrober...@aol.com> wrote:
> I was thinking that the neutrons would move relatively freely through > matter since they lack a charge to interact and the physical sizes of the > nuclei as well as the neutron are so small compared to the electron > orbitals. > That is one way in which the cross sections are a little magical. When we start talking about very large cross sections, they seem to extend beyond the actual nuclei, which is counterintuitive since the neutrons are not affected by electrostatic forces. How quickly does the cross section fall as neutron energy goes up? Can you > relate them as an inverse proportion? > When both the absorption cross section and the energy are graphed on a log scale, you get a pretty good linear fit for lower energies. Here is a typical plot, this one for the absorption cross section of 64Ni: http://i.imgur.com/EB42KRt.gif In graph for 64Ni below, the absorption cross section (green) and total cross section (red) are plotted together, with only the energy plotted on a log scale. As you can see, at higher energies, the total cross section (including elastic and inelastic collisions) is very much larger than the absorption cross section. At lower energies, the total cross section is dominated by the absorption cross section. http://i.imgur.com/rMbCX9o.png > If the neutrons become extremely cold do you see them being reflected by > the electrons of the atom? > I would have guessed that neutrons passing through a metal lattice would be pretty much ignored by the electrons, since electron clouds are diffuse whereas neutrons are very concentrated. But here I'm just going off of intuition. It would seem that the motion of the target atoms themselves would tend to > defeat the entire concept of ultra cold neutrons since their relative > motion would be so much greater. > This makes a lot of sense. Eric [1] http://en.wikipedia.org/wiki/Ultracold_neutrons
