The rad vault on Clipper is an aluminum-zinc alloy, not lead. There are different kinds of radiation to worry about (alpha, beta, gamma, neutron, protons, heavy

The rad vault on Clipper is an aluminum-zinc alloy, not lead. There are different kinds of radiation to worry about (alpha, beta, gamma, neutron, protons, heavy ions), and I think certain shielding approaches good for one aren't always good for the others. Different sources of radiation interact with electrons or nuclei (1:1 with number of atoms) or nucleons (individual protons/neutrons, 1:1 with the mass). For instance, neutrons bounce off nuclei in nuclear reactors, and the lighter they are, the more energy the bounce can siphon off from the neutron. So having more, lighter (low-Z) nuclei (hydrogen in water and carbon in graphite are commonly used) provides better slowing of the neutrons vs. heavier (high-Z) elements, like lead. Smashing ions (alpha, protons, heavy ions) into materials can also cause a https://en.wikipedia.org/wiki/Particle_shower https://en.wikipedia.org/wiki/Particle_shower