What metal is an effective radiation shield?

For the forms of radiation you are likely to encounter in the common medical and industrial settings you will find that most people talk about radiation shielding against x-ray and gamma radiation. One of the best metals to shield against radiation is depleted Uranium, but this is expensive and hard to find, so the next best cheap, common metal is lead. Tin is also widely used in dental x-ray apron shields.

So how much lead do you need? One problem is that x-rays and gamma come in different energy levels (like different colors of light). The energy is measured in keV or meV (1000 keV = 1 meV). A moderately strong medical x-ray machine might generate x-rays with energies around 80 to 100 keV. These x-ray machines are common, so a lot of shielding sold targets these energy levels. Now you need to know how much shielding you need to block that energy level. Shield effectiveness is usually measured in "half-layer thickness". The "half-layer thickness" number tells you how thick your shield must be to block at least HALF of the radiation. The half-layer thickness for some materials for x-rays at 100 keV:

Lead 0.12 mm

Copper 1.8 mm

Iron 2.6 mm

Aluminum 15.9 mm

Water 41.5 mm

Air 35550 mm

As you can see, aluminum is a terrible radiation shield. A sheet of lead as thick as a piece of paper works as well as a block of aluminum thicker than your finger. Aluminum is more than 10 times worse than lead. It's more than 6 times worse than iron.

It takes about 1mm of lead to block 99% of 100keV x-ray radiation.

You might think you would want as much radiation shielding as possible, not just half, and you would certainly want more than the half-value layer. However, one of the ways x-rays and gamma radiation interact with matter results in the actual emission of more radiation from the matter involved. Therefore, if your shielding is too think, it may stop all the radiation entering the shield from one side, but new radiation created in the interior of the shielding can exit through the other side and may itself be a hazard. So the amount of shielding must be carefully calculated to block as much as reasonable of the external radiation without creating too much additional radiation inside the shield itself.