# Design of a Low-Resolution Gamma-ray Spectrometer for Monitoring Radioactive Levels of Wastewater
## 4 Discussion
Thus, it is possible to use this system to determine whether the specific activity of radioactive wastewater containing \({}^{18}\)F can be measured for one hour. However, the following limitations exist when applying the results of this study to the field. First, \({}^{18}\)F is a nuclide that emits 511 keV gamma rays and has sufficient energy to pass through the aluminum outer wall of the NaI(Tl) detector. Therefore, radioisotopes emitting low-energy gamma rays that do not pass through the outer wall of the detector cannot be analyzed. This is a common problem with general radiation counters, and this type of radioisotope should be used with a specialized instrument, such as a liquid scintillation counter. Another limitation is that the _ACF_ is determined by the measurement system, and the _MDA_ is determined by the background radioactivity of the measurement environment. In other words, the _ACF_ should be determined for each measurement system, and the _MDA_ should be determined by measuring the background level after installation. For a lower _MDA_, the shielding of the measurement system can be considered. Finally, the cause of the peak around 440 keV in Figure 4 could be the difference between the MCNP geometry in the calculation and the real geometry of the material used in the experiment. This is because there was no peak at approximately 440 keV when the same sample was measured using a gamma spectroscopy system with HPGe. Despite these limitations, \({}^{18}\)F and \({}^{13}\)I, the most commonly used misealed radioisotopes in hospitals, emit radiation with sufficient energy to be measured with this system--\({}^{18}\)F used for cancer diagnosis and \({}^{13}\)I used for thyroid cancer treatment emit 511 and 971 keV gamma rays, respectively. In particular, many hospitals use only \({}^{18}\)F and \({}^{13}\)I; therefore, the possibility of applying this system is high. The results of this study indicate that specific activity measurements for the draining of radioactive wastewater into the public environment can be conducted using a relatively inexpensive NaI(Tl) detector without relying on an expensive HPGe detector or a gamma counter. When measuring radioactivity using a NaI(Tl) scintillation detector with a diameter of 38 mm and height of 38 mm, which is the smallest commercially available detector, the _MDA_ was found to be smaller than the value that is the draining standard for radioactive wastewater provided by law. This system can be improved by using a larger scintillation detector or installing a shield on the detector to reduce background radioactivity, which is planned for further research. In this system, a detector is fixed on the top of a decay tank for continuous measurement; therefore, the measurement time can be extended to several hours or days; in such cases, the _MDA_ decreases. These properties are not applicable to radioactive wastewater containing \({}^{18}\)F, which has a short half-life. However, it can be used for wastewater with a long half-life. In addition, this system is very similar to the underwater, in situ spectrometer system for measuring the radioactivity contained in rivers or seas, and it has potential applications for measuring radioactivity in ballast tanks or live fish transport containers because it measures the radioactivity of stored water. We expect that the proposed monitoring system can be used to monitor the radioactivity of the water contained in a ship's ballast tanks [27] and live fish containers. The proposed system can help identify radioactivity because many people are concerned about the marine pollution possibility by radioactive materials after the Fukushima nuclear disaster. Investigation, T.K. ; methodology, T.K. and S.K. ; simulation and experiment, S.K. ; writing--original draft, S.K. ; writing--review and editing, T.K. and H.Y. All authors have read and agreed to the published version of the manuscript.