# What are the natural forces that slow the neutron down?

In a nuclear bomb, I've noticed that slow neutrons don't affect plutonium 239, if we're using this kind of atoms (plutonium 239), we need fast neutrons to make a sustainable chain reaction.

Also I've read that neutrons emitted by a fission are very fast, but they lose power while they're moving, So I'm wondering of what kind of forces slow down the neutrons while they're scattering? Please help with details.

Basically, when a neutron overlaps with another nucleus, there is a large interaction in which the neutron and the nucleus can trade energy and momentum (scattering) or in which the neutron can be absorbed by the nucleus. The absorption reactions are more probable for slow neutrons than for fast neutrons. In fact, once the neutron is too slow to excite any of the vibrational modes in the nucleus, the capture cross section becomes proportional to the neutron's "dwell time" near the nucleus. In the business this is called a "one over vee" (or $1/v$, where $v$ is velocity) cross section.