I have searched high and mighty on google and it is hard to find a comparison chart of different materials and their effectiveness to stop radiation from fallout. I am assuming the fallout source is of a standard thermonuclear weapon of 2 megatons at a distance of 10 miles.

So let us say I would like to compare lead to 8 inches of concrete or 3 feet of earth how much thickness of lead would it take to stop the same amount of gamma rays emanating from the fallout particles suspended in the air at their maximum emission time.

A trained physicist may have come across this question.

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    $\begingroup$ Water is SUPER cheap, and very easy and practical to use. Your best bet, based on the data from Bob Jacobsen, appears to be a shelter that is covered by a relatively thin layer of water. $\endgroup$ – David White Apr 15 '18 at 2:22
  • $\begingroup$ ....I did not know water was such good shielding...thank you $\endgroup$ – Sedumjoy Apr 15 '18 at 3:44
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    $\begingroup$ BTW, using heavy atoms like lead for gamma ray shielding has a problem with very high energy gammas: secondary radiation. The heavy nuclei permit a gamma photon to decay into an electron and a positron, the positron quickly annihilates with an electron to produce 2 or 3 gammas. Of course, the new gammas are lower energy than the original, but they're still annoying. $\endgroup$ – PM 2Ring Apr 16 '18 at 0:48

Lead and concrete affect radiation in different ways: There's not a simple ratio between their effectiveness. For example, I've attached below a table of thicknesses for gamma ray "half value layers": the amount of material needed to reduce gamma's from a particular source by 50%. Note that lead is from 4 to 10 times better (thinner) than concrete. (The table is from a nice presentation of how to do basic shielding calculations)

But generally, about a factor of 5-6 over concrete would be about right. So your 8 inches of concrete would be the equivalent of about 1.5 or 2 inches of lead.

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