Concrete and Lead Shielding Requirements for PET Facilities

Abstract

We investigate shielding thickness requirements in concrete and lead walls for positron emission tomography (PET) facilities. F\textsuperscript{18}, the most commonly used PET radiotracer, emits two back-to-back 511 keV photons, necessitating effective shielding to protect hospital staff while patients are in treatment rooms and during their hospital stay. Photon transmission measurements were conducted through a standard Israeli B30 concrete wall (3 m high, 20 cm thick) using photons from an F\textsuperscript{18} source. A narrow-beam transmission coefficient of \( T = (3.0 \pm 1.0)\% \) was recorded with the source positioned 0.05 m from the wall, and the detector at distances of 0.05--3.0 m on the opposite side. When the source is 3 meters from the wall, photons within a 0.64 m radius circular disk (wide-beam) strike the wall. This produces a dose ``buildup'' effect, whereby photons striking within the disk reach the dosimeter after Compton scattering within the wall. The wide-beam transmission coefficient was measured as \( T = (8.8 \pm 1.8)\% \), corresponding to a buildup factor \( B = 4.0 \pm 0.8 \), consistent with Monte Carlo (MC) simulations \( B = 3.9 \pm 0.6 \), validating the simulation reliability for this specific setup. Note however that reliance solely on thick concrete walls presents construction challenges due to weight, space, and cost inefficiency. Since ordinary concrete effectively interacts only through Compton scattering, while lead walls interact via the photoelectric effect, lead is preferable for shielding. A 3.3 cm lead wall results in \( < 0.5\% \) transmission, and lead sheets can be supported by a 5 cm gypsum wall, optimizing shielding design. However, considering the environmental toxicity of lead, note that recent studies advocate the use of tungsten carbide.