To determine the recently accumulated state of the radiocesium137Csin surface sediment in the brackish-water Lake Obuchi, surrounded by facilities reprocessing spent nuclear fuel in Rokkasho Village, we investigated concentration of137Csin the sediment and speciation of137Csin the sediment according to standard procedures of sequential extraction. Concentrations of137Csin the sediment, the silty types (8.0 -13.8Bq kg-1dry) were higher than the sandy type (0.2Bq kg-1dry) . Concentrations of137Csin the sediment tended to be higher in the smaller particle sizes. Moreover, we observed a high correlation coefficient between137Csconcentration in the sediment and amount of organic matter. We found that most137Csin the sediment were the speciations (contained within the crystal structures of minerals etc.) that non-eluted under a natural condition; however, the137Csbonded to organic matter in the sediment were only about 5 % of the sum total of each extraction. This suggests that organic matter acts as a carrier in137Csaccumulation on the lake bottom. Also, 137Csin the sediment can more readily exist in a stable state when bonded to minerals, possessing a crystal structure, rather than to organic matter.
Gamma-ray spectrometry with germanium detectors has been frequently applied for monitoring radioactive materials in sites of nuclear facilities. When cascade gamma-rays simultaneously interact with the detector, spurious peaks, so-called coincidence-sum peaks, and the full energy peaks with reduced intensity are observed in an apparent pulse height spectrum. Although there are some reports on the correction of the coincidence-sum effect, most of them cannot be easily applied for routine measurements. In order to make the correction simple, the method with the experimental formula of peak-to-total ratio given by the authors is presented. The intensities of full energy peaks corrected by the present method show good agreement with those by the method using total detection efficiencies.