[SnuffaBolshevik]

Thanks to you as well, WW. Sorry to veer OT.

"The most effective "slowing down" material is one that has a high percentage of hydrogen."

So a large quantity of diesel fuel or a big pile of coal would work pretty well? How about boron?

Just trying to figure out where to stand if a neutron bomb goes off in the vicinity. ;)

[chimera]

Boron is a little different because it has an isotope (10B) that has a strong thermal neutron capture cross-section. So it is a question of balance between scattering and absorption, you have to look at both. And as noted with the heavier nuclei it is not just a question of scattering. Heavy nuclei will scatter neutrons. But you want to maximize energy exchange without losing the particle being scattered (neutron). There is a calculation of a (dimensionless) quantity we use in reactor physics (lethargy). Where lethargy gained per collision is large, you have a good moderator.

[Wonder Warthog]

"So a large quantity of diesel fuel or a big pile of coal would work pretty well? How about boron?"

Most often used materials are simply things like paraffin wax and various C-H polymers (polyethylene,polypropylene). Cheap, easy to obtain, easy to fabricate and many other advantages. A layer of that covered by some lead (to soak up the resulting X and gamma radiation), and you're good.

Boron you don't want around at all.....it soaks up neutrons like a sponge and will stop your chain reaction dead in it's tracks. For that reason it is used on control rods, and as the "control of last resort".....flooding the reactor with water loaded with sodium borate.