[Wolfman77]

Neutron radiation seems to be the most penetrating. They can pass through quite a bit, because of their weight. Not the most abundant type of radiation, but one of the more dangerous. When it is absorbed, it typicaly releases gamma radiation, which must then be shielded against. Earths atmosphere absorb most of it, in fact background radiation at ground level is primarily from the Earth itself. The human body itself does not always stop it, so if you shield stoped the same ammount some would still get through, which could then be absorbed by a human on the other side. Although it does seem water is good at absorbing it. Light atoms absorb more of it, hydrogen is very light, and makes up 2/3 of a water molecule (by number, 11.19% by mass).

Still can't find any good sites on the subject though.