6 Cross sections
A cross section tells us basically the probability of an interaction (with something). This could be fission, absorption, scattering, etc. It is not a probability in the formal sense in that the range of cross section values does not rang from 0 to 1, but the higher the number, the more probable the reaction. For example, [latex]^{10}B[/latex] has a neutron absorption cross section of 2 million barns (more on the units later). That means it is really likely the neutron is going to be absorbed by the boron.
The unit of the cross section is [latex]barns[/latex], where [latex]1 \; b = 10^{-24} \; cm^2[/latex]. So, the cross section is effectively an area.
Since there is a lot of space in between nuclei in a particular medium, some neutrons just fly by or through the material.
The radius of a nucleus [latex]\sim 10^{-12} \; cm^2[/latex], so the cross sectional area is [latex]\sim 10^{-24} \; cm^2[/latex]. The probability of interaction is the ratio of the total surface area of the atoms to the total area of the medium, which is then defined as the cross section ([latex]\sigma[/latex]).
Additional notes
- MIT Open Courseware – Neutron interactions and applications
- MIT Open Courseware – Neutron science and reactor physics
- NIST – Summer school on methods and applications of neutron spectroscopy
- University of Illinois – Dr. Magdi Ragheb
- Oregon State University – Nuclear chemistry 418/518
- University of Rochester – CHM465/PHY465 – Nuclear structure and reactions
- MIT Open Courseware – Applied nuclear physics