土壌を採取し,深さ毎に放射性セシウム濃度を測定するには多大な労力と時間がかかる.そこで,本研 究では現場で放射性セシウム濃度の鉛直分布を測定する装置を開発した.放射線を検出する 4 つのフォ トダイオードセンサを鉛板で挟み,深度別で指向性を高めた.しかし,指向性は十分ではないため,点 線源実験により測定範囲外からの影響度合いを示す漏れ係数を独自に定義し,補正行列を提案した.福 島県飯舘村の不撹乱水田において本測定器を試験したところ,土壌放射能の鉛直分布がほぼ正確に測定 できることが確認できた. キーワード:放射性セシウム,鉛直放射能分布,フォトダイオード,漏れ係数行列,除染
土壌の物理性, 125: 29-33.
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.
ASA, CSSA and SSSA International Annual Meetings, Nov. 3-6 @Tampa, Florida
On March 11, 2011, great earthquake hit in northeast Japan that caused Fukushima Daiichi Nuclear Power Plant accident. Although radiocaesium (134Cs, 137Cs) was fixed strongly to the soil from surface to 3〜5 cm in depth, agricultural fields have been disturbed by wild boar and weed after three years from the accident. For the effective decontamination of the agricultural fields, it is needed to measure the vertical distribution of radiocaesium concentration in the soil in a short time. In order to measure the vertical distribution of radiocaesium concentration in Fukushima soil, we have developed an in-situ device “DOJYO-kun” which is 8 cm in diameter and 10 cm length with four Geiger-Mullar tubes (LND 712). The device was installed for three minutes in a hole after soil core (5 cm diameter, 16 cm long) was taken an undisturbed paddy field in Iitate Village, Fukushima before and after decontaminating by soil paddling method. Then, the applicability of the device was evaluated by comparing cpm (count per minute) of the device and analyzed radiocaesium concentration of the soil core by NaI-scintillator in each depth. As a result, the device could detect the decrease of radiocaesium concentration of soil after decontaminating in the agricultural fields. However, some significant differences were found in the deep layer from the surface between the device and NaI-scintillator. These differences are probably attributed to insufficient evaluations of radiation’s scattering and attenuation, disturbance of the top soil when we set the device. However, the device is promising to monitor trends and decontamination effects in a short time in the agricultural field in Fukushima. More precise calibrations are needed to measure radiocaesium concentration at each depth accurately.
ASA, CSSA and SSSA International Annual Meetings, Nov. 2-5 @Long Beach, Tampa, California
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 method is needed to estimate the vertical distribution of radioceasium 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 radiocaesium 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 to a 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 radioceasium concentration measured by soil sampling method at each depth reveals that the device is subject to interference from other depths. Thus, we defined “leakage coefficient” to compensate the counting rate of sensor. As a result, a liner relationship was obtained between the counting rate and the radiocaesium concentration at each depth. In addition, it was found from in-site testing for one month that the device would be durable enough to use in the actual field.
ASA, CSSA and SSSA International Annual Meetings, Nov. 3-6 @Tampa, Florida
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 consists of four Geiger-Muller tubes 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. With a field test of the device in an undisturbed rice field in Iitate Village, Fukushima, we confirmed that the vertical distribution of soil radioactivity was properly measured with the device developed.
20th World Congress of Soil Science, Jun. 8-13 @ Jeju, Korea
2014年10月16日＠明治大学