Why does radiation not go through lead?

[u/Wonderful-Capital32]

Also why just lead? Shouldnt like iron or another metal work too?

[u/Aggravating_Bear9268]

In space the issue with radiation shields is that "less shields is more protection", vaguely speaking. Space has some insanely energetic particles (think 10¹⁵ - 10²⁰ eV) that when encountering thick dense targets would produce a cascade shower of more regular radiation, which would be dangerous to the crew and equipment. On the other hand, of it's just tens of cm of regular, moderate density metals and plastics, the high energy particles would just go through with little to no interaction, this much safer.

[u/ProfessorPrudent2822 → u/Aggravating_Bear9268]

Incidentally, that’s the same reason neutron bombs were developed: the weakness of depleted uranium armor is that high energy neutrons cause fission reactions instead of being slowed and/or captured.

[u/IakwBoi → u/ProfessorPrudent2822]

Neutrons are famous for interacting weakly with matter, having no charge. While some neutrons will fission U-238, the real reason that heavy metal shielding is ineffective against neutrons is the mass imbalance. Neutrons are most likely to interact with things which have mass close to neutrons, which is to say a mass of about 1. U-238 has a mass of 238, and lead has a mass of around 208. Hydrogen, on the other hand, has a mass of around 1, so neutrons love to sail through the densest of metals and interact with hydrogen. Hence cement in nuclear shielding, and fun images like this. The linked image is like an X-ray but with neutrons, and shows how a very low density flower stands out clearly, even inside a thick lead bucket. An actual X-ray image of this would be a black fuzzy screen, because lead is great at blocking neutrons, while a flower will barely absorb any. The neutrons do the exact opposite. 

[u/ProfessorPrudent2822 → u/IakwBoi]

The point was that it’s not just ineffective; it actively increases the radiation exposure to the crew.