Why Neutrons Find Nuclei Easily on Atomic Level

[1907Quarter]

and nuclei are so small(on an atomic level!). is it that there are SO many atoms around and that thermal neutrons are still incredibly fast, that if there is a reflector surrounding(lets say we're talking about a critical mass of uranium and an uncontrolled chain reaction), that the neutron will inevitably find a nucleus?

 

[phyzguy]

The other important point is that, since the DeBroglie wavelength of the neutron increases as the energy and momentum decreases, the capture cross-section gets much larger at low energies. This is why moderators are used to slow down the neutrons so that they are more likely to be absorbed.

 

[Naty1]

use lots of slow neutrons...lots of uranium [critical mass]..use U-235...some explanations here:
http://en.wikipedia.org/wiki/Neutron_cross_section

 

[Naty1]

good discussion here: http://en.wikipedia.org/wiki/U-235

 

[1907Quarter]

so, assuming a critical mass of pure u235, and modifiers to slow down the neutron what is the AVERAGE distance a neutron will travel before it fissions an atom? (in picometers)

 

[Astronuc]

so, assuming a critical mass of pure u235, and modifiers to slow down the neutron what is the AVERAGE distance a neutron will travel before it fissions an atom? (in picometers)

It's on the order of cm. In a critical mass of pure U-235, there would be no moderation or very little slowing down. Such systems are usually designed for prompt criticality, i.e., the fission is induced by fast (fission) neutrons. Many neutrons however would escape the mass.

 

[Doofy]

is this not just like asking how come asteroids hit planets?

 

[1907Quarter]

starting to put some numbers together, I will share some thoughts but I'm curious...within what percent off the equator of a nucleus will a fast neutron fission be possible? 5%? 10%? 20%?

 

[Drakkith]

is this not just like asking how come asteroids hit planets?

No. A neutron is a tiny subatomic particle and we must account for quantum effects, which are very dominant at that scale.