報告書番号

V11026

タイトル（和文）

環境中に拡散した放射性物質対策に関する基礎的検討(その1) 空間線量率の測定による汚染分布状況の把握

タイトル（英文）

Monitoring of radionuclides in the environment (Part 1); A mapping of ground surface radio contamination with measurement of a directive Gamma-ray dose rate

概要 （図表や脚注は「報告書全文」に掲載しております）

水源や木材供給などの重要な機能を持つにも関わらず除染が困難な森林等においては、長期に渡って放射線量の空間的・時間的変化を把握し、風化に伴う放射性物質の移動や植生における循環を監視する必要がある。しかし、監視のための簡便・安価、かつ精度の高い手法がない。そこで、手法開発を目途として、簡易な鉛遮蔽によって特定方向からのγ線（空間線量率）を弁別して測定し、以下に示すような知見を得た｡①簡易鉛遮蔽による特定方向の空間線量率弁別；市販の鉛ブロックを用いた簡易遮蔽によって下方方向の空間線量率を弁別することが可能であった。②地形に依存するマッピングへの適用性；グラウンドなどの単純空間では、簡易遮蔽によって下方方向の空間線量率の測定精度が向上し、土壌汚染状況の詳細なマッピングが可能であった。しかし、森林などの複雑地形では、土地の凹凸や土質などの影響をより強く受けたため精度の向上効果は明らかでなかった。。

概要 （英文）

The enormous earthquake and the following accident of nuclear power plant in Fukushima at March 2011 induced contamination of huge areas of eastern Japan with radionuclides. Hereafter, decontamination of these areas are the urgent matter to be solved in Japan. However, decontamination can not be easily applied at the mountain areas mainly because economical reasons. Instead, continuous observations for long duration is expected to keep human out from the high dose rate areas and to decide the timing when the regulation can be dissolved. Japan is situated at a temperate monsoon zone and has much rain. Also, the mountain area is consisted of geographical complex, that is, steep slopes covered with many species of natural plants and/or plantation woods and their decomposition products. These could induce comparatively large weathering effects and mean a need to map the exact status of soil contamination to note the spatial/temporal changes in distribution of radionuclides. However, usual mapping procedures, such as the analysis of soil radionuclides concentration by a Ge-detector system and/or the measurement of soil surface radionuclides by a beta-ray dose detector, is exhaustive and not easily applicable for such objectives. A few option for a Gamma-ray measurement including dose-rate detectors could be seen; however, blockage of unlimited radiation is an important problem and a very large/heavy collimator has been necessary for the purpose.
Here, we used a set of lead block shield (5 cm x 10cm x 20 cm, 7-8 pices) and a portable Gamma-ray dose rate detector (PDS-100GN/ID, Mirion Tech. Inc., USA), which is comparatively lighter and freely assemblable, to test the applicability to measure radionuclide distributions in fields. Results showed that the limited Gamma-ray measurement using the portable detector could well applicable to see the exact status of radionuclide distributions in an open field. That is, the measurement could differentiate very low Gamma-ray dose rate from neighboring narrow measurement blocks (ca. 6.3 m2 x 220 blocks) in an open field before/after decontamination with soil surface scraping. Also, the values were showed higher positive co-relationship (r2=0.94) with the surface radio density. However, a geographical complex and/or a wide vertical distribution of soil radionuclides in a natural cove disappeared the improvement. Also, the measurement could differentiate only for the contamination of downward but not for upward. Further improvement or another way of measurement, such as a gamma camera technology, is necessary to see radio nuclide distributions in natural cove.

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