## 2 Related Works  
### Framework  
Uranium-238 (\({}^{238}\)U) is a naturally occurring radioactive element found in soils and rocks in different concentrations. The decay of \({}^{238}\)U occurs through a series of shorter-lived radionuclides that can eventually produce Radium-226 (\({}^{226}\)Ra), which has a half-life of 1620 years, and which decays by alpha-particle emission directly to Radon-222 (\({}^{222}\)Rn), which has a half-life of 3.82 days. Other radon isotopes also can be formed naturally in these decay sequences. One of these isotopes, Radon-220 (\({}^{220}\)Rn), is formed by the decay of Radium-224 (\({}^{224}\)Ra), in the decay sequence of Thorium-232 (\({}^{232}\)Th), and has a half-life of 56 s, while Radon-219 (\({}^{219}\)Rn) has a half-life of 4 s, and is formed from the decay of Radium-223 (\({}^{223}\)Ra), in the decay sequence of Uranium-235 (\({}^{235}\)U) [39; 40].  
Despite having been discovered in 1900, it was only in the 50s of the 20 th century that awareness of its potential as being hazardous to humans was realized, through the perception and recognition of the high levels of exposure of uranium miners [41]. This noble gas, formerly called emanation, was discovered a few years after radium, but because they are elements with such short half-life times, it was only recently possible to proceed with the characterization of radon compounds, even though there are definite indicators for the existence of such compounds. Since radon isotopes are all gases, they are normally released into the atmosphere and dispersed by air currents. As radon is relatively short lived, its applications are also very limited. For example, for medical applications, radon can be used as a radiation source. In this case, radon is placed inside small sealed glass tubes, and after its decay, the whole series of Po, Bi, and Pb-isotopes are formed and whose penetrating radiation is useful for therapy [42]. \({}^{222}\)Rn has a half-life long enough to be used as a marker in hydrogeological studies, while the remaining isotopes have a half-life too short to be used as tracers in environmental investigations [39].  
The occurrence of radon is directly related to the geological substrate, since all rocks contain some uranium, although some rock types only have very low contents [43; 44]. Natural diagenetic processes cause the mechanical and chemical breakdown of rocks, releasing uranium compounds, which also become part of the soil. In other words, from this perspective, it is expected that the uranium content of a soil to be approximately the same as the rock that gave rise to it. However, some rock types have higher values for uranium concentration, such as light-colored volcanic rocks, granites, dark shales, phosphate sedimentary rocks, and all the metamorphic rocks derived from those mentioned above. This higher concentration of uranium will contribute, through its natural decay, to the occurrence of higher radon concentrations as well [45]. In a gaseous state, radon has much greater mobility than uranium or radium, which are fixed to solid matter, in rocks and soils, while radon can simply migrate to the pores and cracks of rocks and soils, and thus exiting outside. When this release takes place in the outdoor environment, the gas is immediately dispersed, not accumulating in a specific location, and therefore does not cause any problem. On the other hand, if this release occurs in a confined space, such as a building, the accumulation of radon indoor can reach levels that can become worrisome, especially if it is a space with human occupation [46].