Answer to Question #8043 Submitted to "Ask the Experts"

Typically concrete that is used for shielding has a density of about 2.35 g/cm³ (147 lb/ft³). Iron or aggregates containing iron such as magnetite (Fe₃O₄) or hematite (Fe₂O₃) are added to concrete to increase its density. The effective atomic number of the concrete also increases and the combined effect is an increase in attenuation. The density of iron is 7.85 g/cm³. The photon cross-section of iron in g/cm² is greater than that of concrete except for a narrow band around 1 MeV. So for all practical purposes density scaling of the concrete tenth-value layers for the primary barrier of a therapy linear accelerator is acceptable and is conservative for high-energy bremsstrahlung (> 5 MV). It is not conservative for the lower energies. Thus the form of the iron that is added to the concrete does not impact the attenuation of high-energy bremsstrahlung as long as it is mixed uniformly with the concrete. Therefore the question that you should really be asking the concrete contractor is which form of iron provides better uniformity of mix. The smaller the iron pieces, the easier it is to achieve a homogeneous distribution.

The mixing and pouring of high-density concrete is an art and should only be undertaken by those who are experienced in the process. For example, if mixing and pouring procedures are not properly controlled, the high-density aggregates could sink to the bottom. There are several vendors who specialize in providing high-density concrete products.

Nisy Elizabeth Ipe, PhD, CHP