Novel method for determining of <scp> ¹³⁷ Cs </scp> in seawater using liquid scintillation counter

Abstract Robust and efficient monitoring of radiocesium in the marine environment has become increasingly important, especially following the Fukushima Daiichi Nuclear Power Plant accident. A liquid scintillation counter (LSC) has been used for 137 Cs analysis—which was identified as one of the hazardous radionuclides; however, a major limitation of this approach is that the target radionuclide must be purified from the sample. Therefore, we present a novel application of LSC coupled with ammonium molybdophosphate (AMP)–Poly Acry Nitrile (PAN) resin for 137 Cs determination in seawater. The method involves preconcentration and purification of 137 Cs from large seawater volumes (40 L) using a Cs‐specific ion exchanger, followed by selective removal of interfering potassium (K) and rubidium (Rb) ions through optimized rinsing procedures. Potassium and Rb have radioisotopes, 40 K and 87 Rb, respectively, which are the main interferences for quantifying 137 Cs by LSC. Critical mass thresholds of K (&lt; 0.01 mg) and Rb (&lt; 0.001 mg) were established to avoid spectral overlap in LSC. Compared with the high‐purity germanium (HPGe) detector, the LSC method yielded ~ 20‐fold higher counting efficiency and enabled quantification of 137 Cs with a markedly shorter measurement time. The minimum detectable activity was 0.22 mBq L −1 , which is comparable to that of the HPGe method (0.2–0.6 mBq L −1 ) at the same counting time of 80,000 s. For the same seawater samples, this method showed consistent results with the typical HPGe technique. These findings suggest that LSC combined with AMP–PAN resin is a robust and high‐throughput alternative for monitoring of 137 Cs in seawater.