# Design of a Low-Resolution Gamma-ray Spectrometer for Monitoring Radioactive Levels of Wastewater
## 3 Results
### Determination of \({}^{18}\)F ACF in Water  
In this study, the count rates of wastewater stored in a decay tank were measured using a NaI(Tl) scintillation detector and then converted into specific activity. To this end, a radioactive wastewater sample containing \({}^{18}\)F radioisotopes was prepared. Because our experimental data were used to determine whether radioactive wastewater could be discharged safely, the specific activity of test water containing \({}^{18}\)F should be less than \(1\times 10^{7}\) Bq\(\cdot\)m\({}^{-3}\), which is the discharge standard for radioactive wastewater provided by law.  
The count rates measured for the sample including \({}^{18}\)F and the background count rates are shown in Figure 2a,b, respectively. Figure 2a confirms that the 511 keV gamma rays are emitted by \({}^{18}\)F. Because the background count rate was only approximately 5.2% around the 511 keV peak, that of the sample including \({}^{18}\)F, the graph subtracting the background count rate from Figure 2a was almost the same as in Figure 2a; therefore, it was not separately indicated in the graph.  
To estimate the _ACF_ that can change the count rates (counts\(\cdot\)s\({}^{-1}\)) of the \({}^{18}\)F sample measured with a NaI(Tl) detector to the specific activity (Bq\(\cdot\)m\({}^{-3}\)), 500 mL of the wastewater sample used in the above experiment was selected. It was measured using a gamma spectroscopy system for one hour. The results are shown in Figure 3. The gamma spectroscopy system used in this measurement was calibrated using a certified reference material (Table 2); thus, the background radioactivity was automatically removed.  
The count rates of the full energy peak at 511 keV, obtained by subtracting the background counts from the counts of the wastewater sample measured using the NaI(Tl) detector, was 0.9217 counts\(\cdot\)s\({}^{-1}\), and the specific activity of the same sample measured using the gamma spectroscopy system was 1.15031 \(\times\) 10\({}^{6}\) Bq\(\cdot\)m\({}^{-3}\). Thus, the _ACF_ value calculated using Equation (2) was 4.6415 \(\times\) 10\({}^{10}\) Bq\(\cdot\)s\(\cdot\)m\({}^{-3}\)-counts\({}^{-1}\).  
To determine the change in _ACF_ for the variation in tank size, a calculation was implemented by a Monte Carlo simulation using MCNP 6.2 with the same geometry and physical conditions as in the above experiment. Figure 4a was obtained by subtracting the background count rates shown in Figure 2b from the count rates of the wastewater sample measured using the NaI(Tl) detector shown in Figure 2a.  
In Figure 4a, a small peak appears at approximately 440 keV. This peak might be attributed to the difference between the Monte Carlo geometry in our calculation and the actual geometry of the material used in our experiment. This is because there was no peak around 440 keV when measuring the same sample using the gamma spectroscopy system, as shown in Figure 3.  
Figure 3: Count rates of \({}^{18}\)F source obtained by experiment using HPGe detector.  
Figure 2: Count rates of \({}^{18}\)F source including background (a) and background (b) obtained by experiment using NaI(Tl) detector.  
As shown in Table 3, the _ACF_ calculated in computer simulations continued to increase and was fixed at 5.5389 \(\times\) 10\({}^{10}\) Bq\(\cdot\)s\(\cdot\)m\({}^{-3}\cdot\)counts\({}^{-1}\) when the volume exceeded 38 L. It was concluded from those that the larger the water tank, the larger the value measured using the NaI(Tl) detector, and it is fixed when the volume exceeds 38 L. Because the decay tank for radioactive wastewater used in the field was at least 38 L, the _ACF_ of 5.5389 \(\times\) 10\({}^{10}\) Bq\(\cdot\)s\(\cdot\)m\({}^{-3}\cdot\)counts\({}^{-1}\) was suitable as an _ACF_ for wastewater containing \({}^{18}\)F.