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Name: Martin S.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 2001-2002

Whilst reading a textbook as part of my physics course, I discovered a reference to the fact that the neutron capture cross-section of the nucleus can be smaller than the actual geometric cross-section. How is this possible?


An interaction cross section is more a measure of probability than of size. If touching the nucleus always means capture and not touching the nucleus always means freedom, then the capture cross section will be the same as the geometric cross section. When the capture cross section is less than the geometric cross section, the neutron is less likely to be captured than to make contact. Sometimes it will make contact without being captured. The cross section for neutrino capture (which is even less likely) would be smaller. Some think of the cross section as "how big the nucleus looks to the neutron".

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

It simply means that a neutron is not necessarily captured every time it crosses the particular nucleus.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

You can think of this as a neutron hitting a nucleus and not sticking to it but scattering off of it.

Tim Mooney

The concept of a "cross section" is somewhat of a mis-nomer. It is a measure of the ability of a target to interact with some incident energy or particle. It is defined as the RATIO of the total energy per second [Energy / sec] scattered by a target (which is measured by some array of appropriate detectors) to the total incident energy per second per square meter [Energy/ (m)^2 * sec]. This ratio is then: [Energy / sec] / [Energy / (m)^2 * sec]. This ratio then has the units of (m)^2, meters squared. It is in this sense that the process is said to have a "cross section". It is a convenient way to compare the strengths of interactions, rather than a measure of geometric area. See: "Lectures on Physics" by R. Feynman, Vol. 1, Ch. 32 for a lucid detailed treatment.

Vince Calder

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