報告書番号

V11027

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2)木本性植物近傍の空間線量率と植物体の放射性セシウム濃度

タイトル（英文）

Monitoring of radionuclides in the environment (Part 2); A measurement of gamma-ray dose rate and radiocesium concentrations around/in 20 woody plant species at 5 months after the accident of Fukushim nuclear power plant

概要 （図表や脚注は「報告書全文」に掲載しております）

2011年8月中旬に、千葉県我孫子市に生育する20種30本の木本性植物について、樹体周辺（樹体から約50cm）の空間線量率を様々な高さにおいて調べるとともに、樹種、測定高別に、樹体各部（当年葉、旧年葉、枝）の放射性Cs濃度を調べた。その結果、常緑針葉樹以外の樹体近傍では、地表からの放射線が樹体に遮られることによって減衰される割合が高く、地上1mの線量率が地上高12m程度でおよそ半減した。一方、ヒマラヤスギやヒノキなどの常緑針葉樹の樹体近傍では、測定位置の地上高(m)によらず空間線量率が地上高1mでの測定値と同程度に高いことが明らかとなった。これらの違いは、植物種による葉の放射能濃度や当年葉／旧年葉の放射能濃度の差異と一致した。このため、主に放射性物質の降下があった2011年3月中旬に葉が展開していたか、またその葉が測定を行った8月中旬に残存していたかが関係している可能性が高いと考えられた。その他、常緑広葉樹における葉や各樹種における枝の表面性状が、それぞれの放射能濃度に関係している可能性や、根から吸収・転流される放射性物質、およびウェザーリングの関与など、種々の可能性が考えられた。このため、森林における放射性物質対策に資するためには、継続した観測に加え、さらに詳細な検討が必要と考えられた。

概要 （英文）

The huge earthquake and the following accident of nuclear power plant in Fukushima at 11 March 2011 induced contamination of large areas of eastern Japan with radionuclides. Then after, decontamination/surveillance is the indispensable matter in these areas. A considerable amount of radionuclides would be deposited on trees and affect the surrounding gamma-ray dose rate. Although many forest observations in euro-countries after the accident of Chernobyl, Fukushima differs from that in the climate, geochemical properties and the related vegetation.
Here, we measured radionuclide depositions on 20 woody plant species grown in our laboratory at Abiko (about 210km SSW from the NPP) using a portable gamma-ray detector (PDS-100GN/ID, Mirion Tech. Inc., USA) at middle of August, 2011. The measurement was basically conducted for four layers of each trees, which was differentiated by their height; top-foliar layer, middle-foliar layer, bottom-foliar layer (measured at 0.5 m apart from targeted trees), and ground layer (1 m above the ground). Absolute height from the ground level for each was also measured using a laser-beam range finder (Laser 550A, Nikon, Japan). In addition, leaves (pre-/post-accident-developed) and twigs were sampled and precisely analyzed the radiospecies and concentrations by a Ge semiconductor gamma-ray detector (GEM20-70, Ortec, USA). Results showed that the gamma-ray dose rate around trees was basically decreased with their height; however, the tendency was not significant in evergreen coniferous species, such as Japanese cedar, Himalayan cedar, and Japanese cypress. The précised analysis confirmed the phenomenon in evergreen coniferous species that pre-accident-developed leaves were still major in these species at 5 months after the accident, and this made higher the radio cesium (134Cs+137Cs) concentration in these species. However, there was no difference in radio cesium concentration in twig between evergreen coniferous species and the others. It is noteworthy that higher radio cesium concentration was observed in leaves developed in top-foliar layers in evergreen coniferous species, whereas no tendency or the reversed tendency was observed in the other species. These results suggest that major radio dose rate around trees commonly comes from radio nuclides on the ground; however, it also comes from tree crowns around evergreen coniferous species.

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