As we know, the relationship between atomic weight and neutron energy loss can give the number of collision between a neutron and nucleus to slow down (average logarithmic energy decrement.)
I was wondering if this can be a good factor of selecting materials for fast and thermal reactor. I have not found the any relevancy between the relationship mentioned above and the material selection of the reactors.
Can anyone please explain how the relationship between atomic weight and neutron energy loss can be relevant to the selection of materials between fast and thermal reactor? Or, can anyone give me any link/sites to know about it more?