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Report No.

Distribution of the $$^{134}$$Cs/$$^{137}$$Cs ratio around the Fukushima Daiichi Nuclear Power Plant using an unmanned helicopter radiation monitoring system

Nishizawa, Yukiyasu; Yoshida, Mami*; Sanada, Yukihisa   ; Torii, Tatsuo  

Many radioactive substances were released by the Fukushima Daiichi Nuclear Power Plant accident occurred on March 11, 2011 in the atmosphere. A lot of short half life nuclides which are $$^{131}$$I, $$^{132}$$Te ($$^{132}$$I) and $$^{133}$$I, etc., in addition to longer half lived nuclides such as $$^{134}$$Cs and $$^{137}$$Cs. The estimated release amount of these nuclides from the reactor 1st to 3rd unit is reported, but it's found to be quite different in the short half lived nuclides by the reactor units. Because the radioactivity ratio of $$^{134}$$Cs and $$^{137}$$Cs was slight different between the reactor units, it can be considered that the valuable source is obtained by the measurement of $$^{134}$$Cs/$$^{137}$$Cs ratio in the environment around the Fukushima Daiichi Nuclear Power Station at the present stage when the nuclides with short half lives had already decayed. We have measured high resolution $$gamma$$ ray spectrum using an unmanned helicopter equipped with LaBr$$_{3}$$(Ce) detector in a 3 km range from the power station which was near to the release source of the radioactive cesium. Because the LaBr$$_{3}$$(Ce) detector has high resolution of $$gamma$$ rays, the discrimination of many nuclides is possible. In addition, there is extremely much number of the data provided by the distribution measurement with the unmanned helicopter. Because a new map was illustrated by the analysis of the $$^{134}$$Cs/$$^{137}$$Cs ratio, we report the outline.



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