The decay series of uranium and the type of radiation and range of energy of decay products are shown in Table 2. The important daughter product of uranium series is radon and its progenies. Radon is a naturally occurring radioactive gas. This was discovered by F.E. Dorn in 1900. It is found everywhere as part of our environment (i.e., in soil, water, and air). The ubiquitous radioactive gas is formed by radioactive decay of radium (226Ra), which is the daughter product of uranium decay series. The half-life of radon is 3.82 days; it decays by emission of alpha particle to form radon decay products or progeny, which are divided into short-lived and long-lived progeny. These are the significant contributor of natural radiation. On the basis of the epidemiological studies, it has been established that the enhanced levels of indoor radon in dwellings can cause health hazards and may lead to serious diseases like lung cancer in human beings .
Thorium decay series
The decay series of thorium and types of radiation with range of energies of decay products are as shown in Table 3. The important daughter products in this series are thoron and its progenies. The thoron progeny has relatively long half-life than that of radon progeny; therefore thoron progeny would give a significant dose to the lungs [11, 12, 13]. The decay of thorium 232Th leads to the subsequent formation of thoron (220Rn), its half-life 55 seconds. It is more abundant than 238U, but the short half-life of 220Rn allows only a fraction to escape into the atmosphere. The 222Rn is the one of the most significant isotope and it contributes significant dose to publics as ionizing radiation.