Monte Carlo calculations of effect of chest wall thickness on the assessment of lung burden of 241Am

Full text: Inhalation intakes of actinides such as 239Pu and 241Am by radiation workers are commonly detected and assessed by in-vivo lung monitoring technique using sensitive, low background external phoswich detector. Assessment of lung burdens of actinides which emit low energy photons is rendered difficult due to severe attenuation of these photons in the chest wall thickness which may vary significantly in humans. A computer program based on Monte Carlo photon transport technique, has been developed to simulate the response of a 20 cm dia. phoswich detector to lung burdens of actinides (241Am) in MIRD mathematical phantom. The computer program can be broadly divided into four routines: (1) Source routine to generate photons from 241Am source distributed in both the lungs of the MIRD phantom which are traced through the trunk of the phantom. (2) The path length routine determines the distance to next interaction in heterogeneous media of MIRD phantom using Coleman technique. (3) The interaction routine decides the type of interaction by random sampling from the possible interaction processes, namely Photoelectric and Compton scattering. (4) The scoring routine is used to score the photons that are incident on the detector face, in respective energy bins and the effect of the detector on this incident spectrum is simulated using detector variables, i) intrinsic efficiency, ii) iodine K-X-ray escape probabilities and iii) Gaussian resolution function of the detector, to obtain the pulse height spectrum. The computer program was utilized to study the effect of different chest wall thicknesses on computed detection efficiency of the detector placed right above the lungs over the chest. The detection efficiencies are found to decrease by ∼ 50 % as the overlay thickness increases by 3.5 cm over the basic phantom. The results will be useful for assessment of lung burdens of 241Am in radiation workers of different body builds