Application Of a Simple Device To Measure The Vertical Distribution Of Radiocesium Concentration In Soil, Fukushima.

On March 11, 2011, a great earthquake hit in northeast Japan that caused Fukushima Daiichi Nuclear Power Plant accident. Immediately after the accident, ¹³¹I, ¹³⁴Cs and ¹³⁷Cs were detected in the fallout. Among them, ¹³⁴Cs and ¹³⁷Cs have been fixed so strongly on clay minerals in soil that topsoil in agricultural field has been contaminated. For the decontamination of agricultural field, we need to estimate an appropriate soil thickness for decontamination which is normally determined by soil sampling method. However, since the soil sampling method requires a lot of time and costs, a simple in-situ method is needed. In this study, therefore, we have developed a new device that can measure vertical distribution of radiocesium concentration. The device consistes of four photodiodes as detectors of radiation and five lead plates to enhance the directionality. We installed the device in an undisturbed paddy field in Iitate Village, Fukushima and compared counting rate in each depth and the vertical distribution of radiocesium concentration in the sampled soil. By defining leakage coefficient to remove influences out of range of measurement, we calculated counting rate of sensors. As a result, a liner relationship was obtained between the corrected counting rate and the radiocesium concentration. The result means that our device can be promising to measure the radioactive cesium concentration in each depth of soil in in-situ fields.