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Why is neutron radiation best shielded with materials made from light elements?

Hi all, 

I know that since neutrons are not charged, they tend to be shielded through capture by nuclei.  This seems to often create unstable isotopes that emit secondary radiation.  
Why do I hear that the best materials for neutron shielding are derived from elements with low atomic number (such as even water)?  Is it because these materials tend to have a higher volume density of nuclei due to fewer electrons in high level orbitals?  In that case, how is the secondary radiation dealt with?

Thanks!
gpt
If you work through the math, you will find that a neutron will give up more energy (per collision) if it is running into nuclei that have lower mass.  Ever play pool (billiards)?  The balls all weigh the same - if you hit another ball dead-on with the cue ball, the cue ball comes to a complete stop.  If the balls weighed 10 times as much as the cue ball, it becomes much harder to transfer any energy to them.

Water is mostly hydrogen atoms (protons), which have basically the same mass as neutrons.  It only takes an average of 18 scattering events with protons to slow a fission neutron completely.  It takes over 2000 scattering events with uranium to do the same thing. 

Why do we want to slow them?  Neutrons are easier to capture the lower their velocity.  So an effective neutron shield will be able to 1) slow neutrons down and 2) capture them.  Hydrogen also has a decently large capture cross section, so it can both slow and absorb them.  Also, if H-1 absorbs a neutron, it becomes H-2 - which isn't radioactive.  If you want to get fancy, you can mix in a strong absorber like helium-3 or cadmium.  To shield the secondary radiation, you can throw on another shielding layer (like lead).

One of the best neutron shields is just plain old concrete.  It's low-atomic-number, dense, fairly cheap, and you can mix in other components if you want to get fancy.