Purpose: Shielding of radioactive isotopes presents a unique challenge due to the complex decay spectra involved. The workflows and half-lives for different isotopes necessitate accurate transmission factors in order to safely shield a facility while minimizing cost. Prior works have estimated radionuclide transmission factors, but have used decay spectra simplifications resulting in data of limited utility. In this work, a novel modification of GEANT4 decay and annihilation physics was used to facilitate the determination of true broad beam transmission factors for any radionuclide.
Methods: The Monte Carlo toolkit Geant4 v10.5 was used to simulate semi-infinite slab geometries. Radioactive decay and electromagnetic physics was modified to collimate radionuclide decay products, allowing for the production of an infinitesimal pencil beam of stable decay products. This pencil beam is transported through a semi-infinite barrier slab, and scored by a semi-infinite air slab implemented as a true air-kerma scorer. Simulations were performed for the following isotopes: 11C, 13N, 15O, 18F, 60Co, 64Cu, 68Ga, 82Rb, 99mTc,124I, 137Cs, 169Yb, 170Tm, 192Ir, and 198Au. The barrier materials considered were baryte concrete, concrete, gypsum wallboard (drywall), iron, lead, leaded glass, and stainless steel. Barrier thicknesses of 1cm up to 50cm were consideredconsidered, depending on the material.
Results: Preliminary results for the 511 keV emitters demonstrate excellent agreement between the simulations and the results provided in the AAPM Task Group 108 for 511keV photons, for all positron emitting isotopes considered above except 68Ga and I124, which have non-negligible decay photon energies in addition to the 511keV considered in TG-108.
Conclusion: This investigation illustrates the shortcomings of approximating decay schema when considering radionuclides. Additionally, this work demonstrates a method for accurate, efficient determination of broad beam transmission factors for any variety of radionuclides without approximations to either decay spectra or compromise to the broad beam geometry.