Nuclear fission

In physics, fission is a nuclear process, meaning it occurs in the nucleus of an atom. Fission is when the nucleus splits into two or more smaller nuclei plus some by-products. These by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy (the strong nuclear force binding energy).

Moderators

Thermal neutrons (that is, slow neutrons) have the highest probability of producing fission of U-235 but the neutrons emitted in the process of fission have high speeds (they are not thermal). It is an oversimplification to say that the chain reaction might maintain itself if more neutrons were created by fission than were absorbed, because the probability both of fission capture and of non-fission capture depends on the speed of the neutrons. Unfortunately, the speed at which non-fission capture is most probable is intermediate between the average speed of neutrons emitted in the fission process and the speed at which fission capture is most probable.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

For some years before the discovery of fission, the customary way of slowing down neutrons was to cause them to pass through material of low atomic weight, such as hydrogenous material. The process of slowing down or moderation is simply one of elastic collisions between high speed particles and particles practically at rest. The more nearly identical the masses of neutron and struck particle, the greater the loss of kinetic energy by the neutron. Therefore light elements are most effective as neutron moderators.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

It occurred to a number of physicists that it might be possible to mix uranium with a moderator in such a way that the high speed fission neutrons, after being ejected from uranium and before re-encountering uranium nuclei, would have their speeds reduced below the speeds for which non-fission capture is highly probable. The characteristics of a good moderator are that it should be of low atomic weight and that it should have little or no tendency to absorb neutrons. Lithium and boron are excluded on the latter count. Helium is difficult to use because it is a gas and forms no stable compounds. The choice of moderator therefore lay (and still may lie) among hydrogen, deuterium, beryllium, and carbon. It was Enrico Fermi and Leó Szilárd who first proposed the use of graphite (a form of carbon) as a moderator for a chain reaction.

Related Topics:
Lithium - Boron - Helium - Hydrogen - Deuterium - Beryllium - Carbon - Enrico Fermi - Leó Szilárd - Graphite

~ ~ ~ ~ ~ ~ ~ ~ ~ ~