Determination of age specific 131I S-factor values for thyroid using anthropomorphic phantom in geant4 simulations

Using anthropomorphic phantom in Geant4, determination of β- and γ-absorbed fractions and energy absorbed per event due to 131I activity in thyroid of individuals of various age groups and geometrical models, have been carried out. In the case of 131I β-particles, the values of the absorbed fraction increased from 0.88 to 0.97 with fetus age. The maximum difference in absorbed energy per decay for soft tissue and water is 7.2% for γ-rays and 0.4% for β-particles. The new mathematical MIRD embedded in Geant4 (MEG) and two-lobe ellipsoidal models developed in this work have 4.3% and 2.9% lower value of S-factor as compared with the ORNL data.