I am trying to find information on the relative merits of using a gas, liquid, solid or even a plasma as a radiation shield.
Ice, water and water vapour.
Iron, molten Iron, Iron vapour
Water is a little peculiar as the solid form, ice is less dense than the liquid form water. Water vapour is over 1000 times less dense than the liquid.
If a shield of water vapour 1KM thick is as good a protection as 1 meter of ice then the water vapour is a better shield per unit mass.
Earths shield is atmosphere. I saw somewhere that the top 70 Km provides almost all the shield, a link to that information might be helpful.
Please no theory just links or actual experimental results.
Answers can be partial, a specific material, in specific form under specified condition for a specific form of radiation at a minimum, but the more information in each answer is better.