## 4 Results and Discussion  
### Radon Exhalation and Radioactivity Analysis  
The most representative radon growth curves to equilibrium are shown in Figure 2 for the \"Noto\" (a), \"Comiso\" (b), and \"Mendicino\" (c) natural stones, respectively.  
The fitted effective decay constant is reported in the graphs. A comparison between its values, i.e., (0.020 \(\pm\) 0.003) h\({}^{-1}\), (0.050 \(\pm\) 0.003) h\({}^{-1}\), and (0.081 \(\pm\) 0.011) h\({}^{-1}\), for the Noto, Comiso, and Mendicino natural stones, respectively, and the radon decay constant, i.e., 0.008 h\({}^{-1}\), demonstrate that the bound exhalation and the leakage cannot be neglected as their contribution is equal to about 0.012 h\({}^{-1}\), 0.042 h\({}^{-1}\), and 0.073 h\({}^{-1}\), for the Noto, Comiso, and Mendicino samples, respectively.  
Table 2 reports the average \({}^{222}\)Rn-specific exhalation rate (the mean value for the five aliquots of the investigated rocks) for all the analyzed natural stones.  
It is worth noting that, by making the requisite schematic assumptions and utilizing the findings of the radon exhalation measurements, it is feasible to calculate the impact of a specific stone, hypothesized to be employed in the confinement of a defined volume on the accumulation of radon within an indoor environment. The indoor radon concentration  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & \({}^{222}\)**Rn Exhalation Rate** \\
**(Bq h\({}^{-1}\) kg\({}^{-1}\))** \\ \hline \hline Noto stone & 0.013 \(\pm\) 0.003 \\ \hline Comiso stone & 0.040 \(\pm\) 0.006 \\ \hline \hline Mendicino stone & 0.030 \(\pm\) 0.010 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The average \({}^{222}\)Rn-specific exhalation rate for the analyzed natural stones.  
Figure 2: The most representative radon growth curves to equilibrium for the \({}^{\prime\prime}\)Noto\({}^{\prime\prime}\) (a), \({}^{\prime\prime}\)Comiso\({}^{\prime\prime}\) (b), and \({}^{\prime\prime}\)Mendicino\({}^{\prime\prime}\) (c) natural stones.  
resulting from the confining walls can be readily estimated as reported in [47], and it was found to be lower than 10 Bq m\({}^{-3}\) for all investigated natural stones. The air exchange rate of 0.2 h\({}^{-1}\) was assumed [47].  
Moreover, the \({}^{226}\)Ra, \({}^{232}\)Th, and \({}^{40}\)K mean activity concentrations (the average value across the five aliquots of the rocks under study) for the investigated samples are reported in Table 3.  
It is important to point out that, in all cases, the activity concentrations of \({}^{226}\)Ra, \({}^{232}\)Th, and \({}^{40}\)K are significantly lower than the average world values, i.e., 35 Bq kg\({}^{-1}\), 30 Bq kg\({}^{-1}\), and 400 Bq kg\({}^{-1}\), respectively [48]. According to published research, the chemical composition and mineralogical characteristics of natural stones have a significant impact on the values of the \({}^{222}\)Rn exhalation rate, as well as C\({}_{\text{Ra}}\), C\({}_{\text{Th}}\), and C\({}_{\text{K}}\)[49; 50]. For this reason, further investigations about the chemical and mineralogical composition of the investigated \"Noto\", \"Comiso\", and \"Mendicino\" stones through Micro-Raman Scattering and X-ray diffraction were carried out, as detailed in the following section.