## 2 Materials and Methods
### HPGE \(\gamma\)-ray Spectrometry Measurements  
Each aliquot of the basalt aggregate for concrete was first dried, in order to completely remove the moisture and to obtain constant mass. After, it was inserted into a Marinelli hermetically sealed container of 250 mL capacity. After 40 days, the secular radioactive equilibrium between \({}^{226}\)Ra and its daughter products was reached, and then the sample was ready for gamma spectrometry measurement with a live time of 70,000 s. Spectra were analyzed with the aim to assess the activity concentration of \({}^{226}\)Ra, \({}^{232}\)Th, \({}^{40}\)K and \({}^{137}\)Cs. In detail, the \({}^{226}\)Ra activity concentration was calculated by using the 295.21 keV and 351.92 keV \({}^{214}\)Pb and 1120.29 keV \({}^{214}\)Bi gamma-ray lines, and the \({}^{232}\)Th-specific activity was determined by using the 911.21 keV and 968.97 keV \({}^{228}\)Ac \(\gamma\)-ray lines. In particular, for the \({}^{214}\)Bi radionuclide, the TCS coincidence summation correction was applied [20] by using the MEFFTRAN code [21]. Continuing, for \({}^{40}\)K, the evaluation was performed from its \(\gamma\)-line at 1460.8 keV and, finally, in order to investigate the anthropic radioactivity content, the \({}^{137}\)Cs-specific activity was evaluated through its \(\gamma\)-line at 661.66 keV.  
The experimental set-up was composed of a positive biased Ortec HPCGe detector (GEM), whose operating parameters are reported in Table 1[22].  
The detector was located inside lead wells to screen the environmental background radioactivity and, for efficiency and energy settings, a multi-peak Marinelli \(\gamma\)-source (BC-4464) of 250 mL capacity, energy range 60-1836 keV, custom made to replicate the exact designs of the specimens in a water-equivalent epoxy-resin matrix, was employed.  
The Gamma Vision (Ortec) software was used for data acquisition and analysis [22].  
The specific activity (Bq kg\({}^{-1}\) dry weight, d.w.) of the investigated radioisotopes was calculated as follows [23]:  
\[\mathrm{C}=\frac{\mathrm{N_{E}}}{\mathrm{\varepsilon_{E}}\mathrm{t}\mathrm{V _{d}}\mathrm{M}} \tag{1}\]  
where N\({}_{\mathrm{E}}\) is the net area of a peak at energy E; \(\mathrm{\varepsilon_{E}}\) and \(\mathrm{\gamma_{d}}\) are the efficiency and yield of the photopeak at energy E, respectively; M is the mass of the sample (kg); and t is the live time (s) [24]. Moreover, with the density of the basalt aggregate for concrete being higher than 1.1, the self-absorption correction on the activity concentration value was performed according to [25; 26].  
The Italian Accreditation Body (ACCREDA) certified the quality of the \(\gamma\)-ray spectrometry experimental results [27], thus ensuring continuous verification that the performance properties of the method are preserved [28].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**HPGe** GEM & \\ \hline
**Parameter** & **Value** \\ \hline Full Width at Half Maximum & 1.85 keV \\ Peak-to-Compton ratio & 64:1 \\ Relative Efficiency & 40\% (at the 1.33 MeV \({}^{60}\)Co \(\gamma\)-line) \\ Bias Voltage & 4500 V \\ Energy Range & 50 keV–2 MeV \\ \hline \hline \end{tabular}
\end{table}
Table 1: The HPGe GEM operating parameters.