Determination of atmospheric radiocesium by analyzing filter-tapes used in automated suspended particulate matter monitors

We have been determinig atmospheric radioactive cesium (¹³⁴Cs and¹³⁷Cs) concentrations in the early period after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011 using suspended particulate matter (SPM) collected hourly on a filter tape at many automated SPM monitoring stations located widely in eastern Japan. The hourly atmospheric¹³⁴Cs and¹³⁷Cs concentrations just after the FDNPP accident in wide areas of eastern Japan were first revealed in our study. The determined¹³⁴Cs and¹³⁷Cs concentrations are expected to play an important role in reducing uncertainties of estimations of the internal radiation dose rates from inhalation and time-series radionuclides release rates from FDNPP, and an evaluation of the atmospheric transport models. There are four steps for the determination of radioactive cesium; i.e., identification of the date and time of SPM samples collected on filter-tapes, preparation of samples for gamma-ray measurement, measurement of gamma-rays, and calculation of concentrations and screening of their values. In this article, we introduced the technical methods to conduct the quality assurance through these four steps, based on our experience for about 7 years.