# Design of a Low-Resolution Gamma-ray Spectrometer for Monitoring Radioactive Levels of Wastewater
## 4 Discussion  
Measuring the specific activity before draining radioactive wastewater is a resource- and time-intensive process. In this study, a method of continuously measuring radioactivity by installing a detector on a decay tank was used to reduce time and cost. For this purpose, a NaI(Tl) detector, which is commonly used in studies measuring radioactivity in seawater, groundwater, or lakes, was used to measure the count rates in a tank containing radioactive wastewater, and the _ACF_ was calculated. The specific activities in Bq\(\cdot\)m\({}^{-3}\) (\(C_{HPGe}\)) were obtained by multiplying the counts per second (\(N_{Nul}\)) measured using a NaI(Tl) detector by the _ACF_. Using the _ACF_, a radiation safety manager can observe the specific activity of wastewater stored in the wastewater tank in real time and can immediately decide whether to drain the tank or not. For this experiment, a NaI(Tl) detector with a diameter of 38 mm and a height of 38 mm was used. It is inexpensive and has low efficiency among the products sold. The _ACF_ to transfer counts obtained using a NaI(Tl) detector to the specific activity was calculated using a gamma spectroscopy system with an HPGe detector calibrated with standard reference materials. When underwater radioactivity is measured, water acts as a shield and affects the measurement radius [13]; thus, a change in _ACF_ was observed while increasing the size of the tank containing radioactive wastewater. The change in the counted value for various sizes of the tank containing radioactive wastewater was calculated using MCNP 6.2. As a result of the calculation, when the volume of the tank exceeded 38 L, the _ACF_ was fixed at 5.5389 \(\times\) 10\({}^{10}\) Bq\(\cdot\)s\(\cdot\)m\({}^{-3}\).counts\({}^{-1}\). Because the decay tank for radioactive wastewater used in the field is at least 38 L, this conversion factor is suitable for radioactive wastewater containing \({}^{18}\)F.  
The _MDA_ is the minimum detectable activity that can be separated from natural radioactivity. Because the system used in this study must prove that the specific activity of radioactive wastewater is less than the draining standard, the _MDA_ of this measurement system must be less than that of radioactive wastewater. Otherwise, this system cannot determine whether the radioactive wastewater can be drained or not. To prove this, the _MDA_ of the measurement system was calculated to be 2.09 \(\times\) 10\({}^{5}\) Bq\(\cdot\)m\({}^{-3}\) for one hour \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reading Time (h)** & _MDA_ **(Bq\(\cdot\)m\({}^{-3}\))** & **Reading Time (h)** & _MDA_ **(Bq\(\cdot\)m\({}^{-3}\))** \\ \hline
1 & 2.09 \(\times\) 10\({}^{5}\) & 13 & 5.62 \(\times\) 10\({}^{4}\) \\ \hline
2 & 1.46 \(\times\) 10\({}^{5}\) & 14 & 5.41 \(\times\) 10\({}^{4}\) \\ \hline
3 & 1.18 \(\times\) 10\({}^{5}\) & 15 & 5.23 \(\times\) 10\({}^{4}\) \\ \hline
4 & 1.02 \(\times\) 10\({}^{5}\) & 16 & 5.06 \(\times\) 10\({}^{4}\) \\ \hline
5 & 9.12 \(\times\) 10\({}^{4}\) & 17 & 4.91 \(\times\) 10\({}^{4}\) \\ \hline
6 & 8.32 \(\times\) 10\({}^{4}\) & 18 & 7.77 \(\times\) 10\({}^{4}\) \\ \hline
7 & 7.69 \(\times\) 10\({}^{4}\) & 19 & 4.64 \(\times\) 10\({}^{4}\) \\ \hline
8 & 7.19 \(\times\) 10\({}^{4}\) & 20 & 4.52 \(\times\) 10\({}^{4}\) \\ \hline
9 & 6.77 \(\times\) 10\({}^{4}\) & 21 & 4.41 \(\times\) 10\({}^{4}\) \\ \hline
10 & 6.42 \(\times\) 10\({}^{4}\) & 22 & 4.31 \(\times\) 10\({}^{4}\) \\ \hline
11 & 6.11 \(\times\) 10\({}^{4}\) & 23 & 4.21 \(\times\) 10\({}^{4}\) \\ \hline
12 & 5.85 \(\times\) 10\({}^{4}\) & 24 & 4.12 \(\times\) 10\({}^{4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Minimum detectable activity (_MDA_) for reading time. of measurement, which was smaller than the draining standard of 1 \(\times\) 10\({}^{7}\) Bq.m\({}^{-3}\) as prescribed by law in Korea, where the public is predicted to reach the dose limit owing to the draining of \({}^{18}\)F-containing wastewater into the public environment.