Concrete is commonly used as a radiation shield due to its availability and cost-effectiveness. However, its major limitation is its low density compared to lead, which reduces its effectiveness in attenuating gamma rays and X-rays. Lead’s higher atomic number and density provide superior shielding by absorbing more radiation per unit thickness. Therefore, to achieve the same level of protection, concrete must be much thicker and heavier than lead. This makes concrete less efficient in applications where space and weight are critical considerations.

Reference: Radiation Protection and Dosimetry, Michael G. Stabin, 2nd Edition.