Reactor Neutrons: Seeing Surface Area not Projected Cross Section?

[nuclear420]

My professor told me that the neutrons in a reactor see the surface area of nuclei, and not the the projected cross section. That really doesn't make intuitive sense. Is he right?

 

[Bob S]

See the plot of the thermal neutron capture cross sections for boron-10 and boron-11 in the section on enriched boron in
http://en.wikipedia.org/wiki/Boron
The cross section is ~1,000,000 times higher for boron-10 than for boron-10. This not the projected geometric ratio.

Bob S

 

[nuclear420]

Talked to my professor again, he said that it is only true for scattering cross sections. If you look at the math, you will notice it is dependant upon the surface area. I'm guessing its some sort of quantumn mechanical phenomena?

 

[nuclear420]

Anyone?

 

[Astronuc]

My professor told me that the neutrons in a reactor see the surface area of nuclei, and not the the projected cross section. That really doesn't make intuitive sense. Is he right?

That really depends on the energy of the neutron. For high energy (MeV range) neutrons, e.g., those emitted during fission, the one considers 'potential scattering'. At much lower (thermal) energies, < 1 eV, and particularly in the 0.025 eV range, then the cross-section is more or less determined by the nature of the particular nucleus encountered. That's were QM play becomes dominant. In between though (1 eV-10 keV range), QM is a factor in resonance scattering/absorption.

http://www.nndc.bnl.gov/sigma/index.jsp

http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=4551&mf=3&mt=1&nsub=10

 

[Meir Achuz]

My professor told me that the neutrons in a reactor see the surface area of nuclei, and not the the projected cross section. That really doesn't make intuitive sense. Is he right?

\
I don't think he was talking about the size of the cross section, which depends on details other than the effective area. Possibly what he meant was that the slow neutrons are in an L=0 (spherically symmetric) state with respect to the nucleus so they interact at the entire surface.

 

[Bob S]

\
I don't think he was talking about the size of the cross section, which depends on details other than the effective area. Possibly what he meant was that the slow neutrons are in an L=0 (spherically symmetric) state with respect to the nucleus so they interact at the entire surface.

Clem-
Please look up the (n.total) cross sections on boron (Z = 5) in

http://www.nndc.bnl.gov/sigma/index.jsp

and tell me why the cross sections for boron-10 and boron-11 (click on PLOT) are so different. How does the boron-10 thermal neutron x-section (over 10,000 barns) relate to the nuclear surface area?

Bob S

 

[Meir Achuz]

The size of the cross section depends on other things than the area an incident neutron encounters.