Development Of a Simple Device To Measure The Vertical Distribution Of Radio Cesium In Soil.

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, a simple in-situ is needed to estimate the vertical distribution of radiocesium since the soil sampling method requires a lot of time and costs. Also, it is important for inhabitants to monitor the environments such as soil and groundwater for a long time. In this study, we have developed a new device that can measure vertical distribution of radiocesium concentration and conduct the long-term monitoring. The device is about 8 cm in diameter, 10 cm long, and consists of four photodiodes as detectors of radiation and five 1 cm lead plates to enhance the directionality. These sensors are connected data logger through the convert substrate.  We installed the device in an undisturbed paddy field in Iitate Village, Fukushima to evaluate the accuracy and the durability for long-term monitoring. Comparing the counting rate of sensor and radiocesium concentration measured by soil sampling method at each depth reveals that the devive is subject to interference from other depths. Thus, we definined “leakage coefficient” to compensate the counting rate of sensors. As a result, a liner relationship was obtained between the counting rate and the radiocesium concentration at each depth. In addition, it was found from in-situ testing for one month that the device would be durable enough to use in the actual field.