# Thermal neutrons in fission

These so-called fast neutrons do not cause fission as efficiently as slower-moving ones so they are slowed down in most reactors by the process of moderation.

Why slowly neutrons "thermal" are more efficiently in causing the fission? Is due to their larger cross section? And why the cross section of thermal neutron is larger than the faster ones?

• Check this answer physics.stackexchange.com/a/175483/203041 – sslucifer May 6 at 15:10
• Ok thank you so much – Salmone May 6 at 15:31
• Yes, their generally larger cross section covers it. Looking at neutron cross sections on ENDF (nndc.bnl.gov/exfor/endf00.jsp) might be illuminating. – Jon Custer May 6 at 15:32
• Sorry, I can't answer why the kinetic energy of the particles matters, but you should know that "cross section" and "efficiency in causing fission" both pretty much mean the same thing. – Solomon Slow May 6 at 18:21

For 235U you see that the thermal (low energy) cross section for fission is quite large (greater than $$10^{4}$$ barns, and is quite a large fraction of the overall cross section - 235U really wants that neutron to enable it to split apart. The slope of the cross section in that region is quite common, basically going as the inverse of the velocity. In simple terms it is kind of like a transit time of the nucleus. The real question is what the 'size' of the nucleus is, and that depends on just what you are asking it to do.