For protection against gamma radiations, the shields used should be made of

Their attenuation is dependent upon the density of the shielding material; it can be shown that a dense shield material with a higher atomic number is a better attenuator of X-rays. Lead enjoys the advantage of being the densest of any commonly available material. Where space is at a premium and radiation protection is important, lead is often preferred. It is recognised that lead is not the densest element (e.g. tantalum, tungsten and thorium are higher on the density scale), but lead is readily available, easily fabricated and the cheapest of these materials.