Radon is a radioactive transformation product of 238U, 235U, and 232Th, and it exists in various concentrations in all soils, minerals, ground water, material buildings and air. the half-life of 222Rn (3.82-day) is long enough for it to diffuse into and build up in homes and Inhalation of radon can cause lung cancer.220Rn, or thoron from the 232Th series, and 219Rn from 235U have very short-lives (55.6s and 3.96s, respectively) and they are of minor significance compared to 222Rn in 238U series. But These sources such as thoron can be important in the minerals. Thoron,(220Rn) decays by emission of an alpha particle with energy of 6.29 MeV and 212Pb and 212Bi are short-lived thoron progeny, which decay by emitting beta particles accompanied by gamma radiation. Gamma dosimetry was performed using modified ORNL adult male phantom and MCNP code after inhalation. the comparision of the radon gamma dosimetry and thoron gamma dosimetry was performed. The results showed that the absorbed dose is the largest in the lung. Thymus and the heart are received largest absorbed dose respectively after lungs according to ICRP103 Publication. the absorbed dose from thoron is quite large in comparision to radon. Because there are more high-energy particles than the radon in the spectrum.

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