報告書番号

O14002

タイトル（和文）

半導体型積算線量計による環境放射能の測定(その1) ―従来測定法との比較による可能性評価―

タイトル（英文）

Measurement of environmental radioactivity using a cumulative gamma radiation dosimeter (1); A capability study

概要 （図表や脚注は「報告書全文」に掲載しております）

森林管理の観点から、森林内における放射性物質の移動や循環の把握、および動態予測を目途として、コストや労力低減に優れ、より広範囲を長期にわたり観測可能な新たな簡易測定手法の開発するため、半導体型積算線量計(D-シャトル）によって、環境中での特定対象からの放射線量測定が可能か、定量的比較・評価を行った。その結果、以下の点を明らかにした。１．0.08 μSv h-1のバックグラウンド（BG）放射線量率の室内でサクラ切断枝からの放射線量を測定した結果、樹体の遮蔽効果を仮定した補正を行うことにより、有意な検出力（BGの3σ以上）、GMサーベイメータと同程度の水準で採取地区間の有意な差異が確認できた。また、134Cs+137Cs濃度測定値とも相関（r=0.59）が認められた。２．0.24 μSv h-1のBG放射線量率の自然環境下で、スギ立木からの放射線量を測定した結果、樹体の遮蔽効果を仮定した補正によりBGとの差分の有意性が認められた。一方、樹体内汚染分布等の影響を受け、放射性Cs濃度との相関は低下した（r=0.28-0.39）。以上より、積算線量計による環境中での樹体等からの放射能弁別・汚染度推定は可能であるが、樹体の遮蔽効果等を踏まえた適切な補正が必要であることが明らかとなった。

概要 （英文）

The accident of Fukushima Dai ichi Nuclear Power Plant (FDNPP) on 11 March 2011 dispersed radionuclides to large areas of northern Japan. Although the decontamination programs are completed in many residential areas, forest areas still have received less attention. Then, concerns for potentially contaminated forest products, such as mushrooms, timber, and litter based composts are closed up with the progress of rehabilitation of the distressed areas. Possible discharges of radionuclides into drainage basins and drifts from decomposed particles of plant debris in flowing water are also a large concern. These mean that more attentive on site survey is expected for each subject; however, due to the nature of the forest environment, the current method for the surveying is labor intensive, expensive, inapplicable, or potentially ineffective for such long term purpose, and thus, needs for alternative method become greater.
Here, we present the concept and results of trial measurements of an alternative method using a cumulative gamma radiation dosimeter (D shuttle, Chiyoda Technol Corp., Japan) and a lead shield. The trials were conducted at our laboratory in Abiko, Chiba, where is one of the comparatively higher radionuclides deposited area from the Fukushima accident. The targets of the trial measurements were cut boughs of a flowering cherry (Prunus x yedoensis cv. Somei Yoshino) and living trunks of a cedar (Cryptomeria japonica). Measurements using NaI and GM survey meters were carried out as a comparison. The D shuttle demonstrated a good detection ability of the targeted radiation from the environment and a certain level of correlation (Pearson's r=0.28 to 0.59) between cumulative doses and the activity concentration of radiocesium, although such attributes were dependent on the measurement condition (e.g., air dose rate and shield). Even though further investigations (e.g., specific shielding effects of trees) are required to establish a specific correlation between the cumulative dose and the activity concentration, the D-shuttle could be counted as a useful tool to see a long-term changes of radionuclide distributions under the natural condition.

報告者

キーワード