FAQ - Neutron activation - what happens in the moderator?

[Richard Hull]

Regarding neutron reflection in a moderator:

Fast neutrons in a moderator can reflect based on impacts with H atoms. However, it is only slowed neutrons that can interact to activate via resonances with the item to be activated. Often, resonances for slow neutrons can have tremendous capture cross sections! However, due to the spike like nature of multiple resonance energies, one must never rely on these huge bands of super capture cross sections to activate an item as they are rare and by total chance.

Once fully thermalized, the 1/V law takes over and applies to all neutrons as being totally activation ready when in the item to be activated. After this idealized thermalization, reflection and further activation at higher cross sections afforded by continued slowing down is desired. There is an optimum thickness for a moderator which will allow most of the slower thermal neutrons to escape the moderator altogether. (Lost activation ready neutrons) In theory, an infinite volume of moderator is ideal, but who can do that.

We are limited to placing our physically large neutrons source, (fusor), next to the moderator with the enclosed activation material in the moderator.
Thus, we suffer a large loss of our fast neutrons to the surrounding unmoderated air space around the fusor.

The ideal is to place the fast neutron source in the moderator in contact with the material to be activated with moderator surrounding both the fast source and the touching item to be activated. This is easily realized only when the fast source is typically a very small radioisotope-beryllium button or capsule. Both the small fast source and the item to be activated are dropped into a well-hole in a solid cube or spherical moderator block.

There are charts found in these forums for the ideal thickness of a moderator based on the hydrogenous materials available.

Richard Hull