Gamma absorption in concrete
The absorption capacity of concrete is lower than that of lead, because its density is lighter (2.35 against 11.6) and this material does not contain heavy nuclei. The thicknesses of screens needed amount to tens of centimeters, but concrete is cheap and screens easy to manufacture. The figure shows that a 40 cm concrete screen absorbs 99.9% of ta 500 keV gamma energy. The curves are indicative because they depends on the beam characteristics.
Source K.Gerber