The most important source of natural radioactivity is Radon-222 For this reason, Radon dosimetry has attracted quite a lot of attention which resulted in a vast number of research activities carried out in scientific communities by scientists. A great deal of work has been devoted to determine the damage caused by alpha radiation emitted from radon and its progeny Radon-222 daughters such as 214Bi and 214Pb deposit in human lungs and emit gamma quanta followed by the beta particles which cause smaller damage in comparison with alpha particles. In our country an exact estimate has not been done on the amount of exposures from radon in residential houses, bringing need to more attention on taking necessary efforts in this area. In this study, using the equations of Bateman and the FORTRAN code, the decay curve of radon daughters and the energy spectrum of the gamma radiation are calculated for five hours and twenty days respectively and doses from gamma rays emitted by radon progeny in the lungs have calculated in all main organs using MCNP code for adult male.

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