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X-ray and gamma radiation are best absorbed by atoms with heavy nuclei; the heavier the nucleus, the better the absorption. In some special applications, depleted uranium or thorium[4] are used, but lead is much more common; several centimeters are often required. Barium sulfate is used in some applications too. However, when cost is important, almost any material can be used, but it must be far thicker. Most nuclear reactors use thick concrete shields to create a bioshield with a thin water cooled layer of lead on the inside to protect the porous concrete from the coolant inside. The concrete is also made with heavy aggregates, such as Baryte, to aid in the shielding properties of the concrete. Gamma rays are better absorbed by materials with high atomic numbers and high density, although neither effect is important compared to the total mass per area in the path of the gamma ray.
