RADIOISOTOPES

An apparatus for on-site measurement of radiocaesium in pond sediment was developed by assembling commercially available parts. It gives vertical radiocaesium distribution in sediment from γ-ray count data measured by an array of PIN photodiode sensors. Successful conversion from γ-ray count rates to radiocaesium concentration in Bq/kg-wet was attained by an inversion code based on the maximum-entropy method. On-site tests at three ponds in Fukushima prefecture revealed that the apparatus gives vertical distributions close to those of core sampling method by 10 min measurement. It works about 5 h by a small mobile battery. The total cost for the apparatus is about 100,000 Japanese Yen which may not be unacceptable to individuals. The present method measures vertical distribution directly and is superior to other methods which assume empirical profile such as exponential distribution. Experimental error and application to other target such as soil or containers were also discussed.