A note on estimating the wall thickness of PET-CT rooms in order to provide radiation protection

The present work examines the possibility of combining new structures to an existing brick wall in order to meet the radiation protection standards, 50 mSv y-1 to staff, for 511 keV gamma rays.

We derive an expression for the attenuation provided by a composite wall, taking into account photon build-up factors, for a point source of gamma rays. The attenuation coefficients of brick and cement were first calculated by determining their respective densities and using the corresponding mass attenuation coefficients at 511 keV, as given by the NIST tables. From these calculated values we estimated the necessary thickness of an extra reinforcement, composed of brick, mortar, ceramic, and lead, keeping in mind the dose limits.

Experimentally, we found the densities of brick and cement to be 1.52 and 1.80 g per cc, respectively, giving their attenuation coefficients as 0.1322 cm-1 and 0.1566 cm-1, respectively, at 511 keV. Hence, for brick alone, the necessary wall thickness has to be 35 cm while for the brick and mortar at our disposal, a 30-cm wall suffices giving the average dose to the staff to be within the safe limits.

To achieve the necessary attenuation, several options exist; however, the cheapest options are with brick, cement, and their suitable combinations, the thickness being decided by the composition of the proposed shielding wall. Neglect of photon build-up overestimates the attenuation and thus underestimates the amount of leakage.