# Random-Specific Activity in Drinking Water and Radiological Health Risk Assessment: A Case Study
## Materials and Methods
### Liquid Scintillation Counting (LSC) Measurements  
The activity concentration of _Rn-_222 in the investigated drinking water samples was obtained through liquid scintillation counting (LSC) measurements, according to [27]. Specifically, 10 mL of each sample was inserted with a gas-tight syringe into the bottom of a 25 mL plastic vial previously filled with 10 mL of Perkin Elmer Opti-Fluor O scintillating  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Site ID**} & \multicolumn{3}{c}{**GPS Coordinates**} \\ \cline{2-3}  & **Latitude** & **Longitude** \\ \hline
1 & 38\({}^{\circ}\)21\({}^{\prime}\)03.0\({}^{\prime\prime}\) N & 16\({}^{\circ}\)10\({}^{\prime}\)35.0\({}^{\prime\prime}\) E \\
2 & 38\({}^{\circ}\)23\({}^{\prime}\)39.1\({}^{\prime\prime}\) N & 16\({}^{\circ}\)11\({}  
cocktail immiscible in water, stored and, after a rest time of 5 h, counted for 60 min together with a background [28]. The scintillator was a Perkin-Elmer Tricarb 4910 TR, with an energy range of 0-2 MeV (\(\beta\) particles) and 0-10 MeV (\(\alpha\) particles). Its minimum acceptable efficiency is 60% for H-3 (0-18.6 keV) and 95% for C-14 (0-156 keV). Its average background is 17 CPM for H-3 and 26 CPM for C-14. It operates in normal/low-activity-high-sensitivity mode, with the external Ba-133 standard to account for chemical and optical quenches and to assess the counting efficiency by using the tSIE/AEC [29].  
A picture of the detector is presented in Figure 2.  
During the radon-in-water analysis or if a water sample is taken and analyzed sometime later (rather than immediately), the sample's radon concentration will diminish, mainly due to radioactive decay and, partly, to the degassing phenomenon [30]. Then, it is essential to correct the resulting activity concentrations in order to take into account the decay from the sampling time to the analysis time. The decay correction is described by a simple exponential function with a time constant of 132.4 h, coming from the exponential law for radioactive decays:  
\[C_{Rn-222}=C_{0,Rn-222}e^{-\lambda t}, \tag{1}\]  
where \(C_{Rn-222}\) is the measured \(Rn\)-222 concentration, \(C_{0,Rn-222}\) is the initial concentration at the sampling time and \(t\) is the time elapsed since collection (hours).  
Figure 1: The map of the investigated area.  
The time elapsed from sampling to measurement is less than 48 h in all cases, in order to minimize the _Rn_-222 content due to the decay of Ra-226 in the investigated samples.  
Moreover, the uncertainty associated with the measurement of the _Rn_-222 concentration for each of the four investigated samples (per single sampling location) is given by [27]  
\[U(C_{Rn-222})=C_{Rn-222}\sqrt{\left(\frac{U_{CN}}{C_{N}}\right)^{2}+\left( \frac{U_{\epsilon}}{\epsilon}\right)^{2}+\left(\frac{U_{V}}{V}\right)^{2}}, \tag{2}\]  
where \(U_{CN}\) is the uncertainty associated with the net counts \(C_{N}\) of the detector calibration source, \(U_{\epsilon}\) is the uncertainty associated with the detection efficiency \(\epsilon\) and \(U_{V}\) is the uncertainty associated with the volume \(V\) of the analyzed sample [27].  
The quality of the LSC experimental results was certified by the Italian Accreditation Body (ACCREDIA). This implies the continued verification (with annual periodicity) of the maintenance of the LSC method's performance characteristics [31].