Reactor Neutrons: Seeing Surface Area not Projected Cross Section?

In summary: For example, the neutron is in an L=0 (spherically symmetric) state with respect to the nucleus, so it interacts with the entire surface. The boron-10 thermal neutron x-section (over 10,000 barns) relates to the nuclear surface area because the cross section is proportional to the surface area.
  • #1
nuclear420
17
0
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?
 
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  • #2
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
 
  • #3
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?
 
  • #4
Anyone?
 
  • #5
nuclear420 said:
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
 
  • #6
nuclear420 said:
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.
 
  • #7
clem said:
\
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
 
  • #8
The size of the cross section depends on other things than the area an incident neutron encounters.
 

1. What are reactor neutrons?

Reactor neutrons are particles that are produced in nuclear reactors through the process of nuclear fission. They are electrically neutral and have a mass similar to that of protons and electrons.

2. How are reactor neutrons used to measure surface area?

Reactor neutrons can be used to measure surface area by measuring the number of neutrons that are scattered or reflected off the surface of a material. This measurement can then be used to calculate the surface area of the material.

3. What is the difference between surface area and projected cross section?

Surface area refers to the total area of the surface of an object, while projected cross section is the area that is projected when an object is viewed from a certain angle. In the context of reactor neutrons, surface area takes into account the scattering of neutrons in all directions, while projected cross section only considers those scattered in a specific direction.

4. Why is it important to use reactor neutrons to measure surface area?

Using reactor neutrons to measure surface area is important because it provides a more accurate and precise measurement compared to other methods. It also allows for non-destructive testing, which is crucial in industries such as nuclear power and materials science.

5. Are there any limitations to using reactor neutrons for surface area measurements?

While reactor neutrons are a powerful tool for surface area measurements, there are some limitations. These include the availability and access to nuclear reactors, as well as the cost and expertise required to use them. Additionally, samples may need to be specially prepared for neutron scattering measurements, which can be time-consuming and may alter the surface properties of the material.

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