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Dose response of a natural-polymer-based gel dosimeter; Effects of composition on white turbidity increase after irradiations

天然高分子ゲル線量計の線量応答; 照射後の白濁化に対する組成の影響

山下 真一; 廣木 章博; 長澤 尚胤; 村上 健*; 田口 光正

Yamashita, Shinichi; Hiroki, Akihiro; Nagasawa, Naotsugu; Murakami, Takeshi*; Taguchi, Mitsumasa

既に提案されている高分子ゲル線量計には、材料の毒性が高い、線量評価に高価で大掛かりな装置が必要、熱安定性が必ずしも高くないといった弱点があった。これらを克服するため、毒性の低い2-ヒドロキシエチルメタクリレート(HEMA)やポリエチレングリコールジメタクリレート(9G)を放射線検出用の溶質に用い、透明性の高いセルロース誘導体のヒドロキシプロピルセルロースを電子線照射で橋架けしてゲル母材に用い、照射により白濁化が視認できる新たな高分子ゲル線量計を開発した。白濁は1-2Gyの$$^{60}$$Co $${gamma}$$線照射で確認でき、組成にも影響を受けた。具体的には、9Gが4%で主成分の場合には1Gy前後の低線量で白濁化が確認できるものの10Gy程度で白濁化が飽和し、HEMAが4%で主成分の場合には低線量での白濁化がやや薄いものの10Gyを超えても白濁が濃くなっていった。白濁化の度合いを定量的に評価するために濁度測定並びに吸光測定も行った。さらに、$$^{60}$$Co $${gamma}$$線照射と比べると感度が1/3-1/7程度と小さいものの、がん治療に用いられる135MeV/uのCイオンビームの照射においても白濁が生じることが確認できた。

A new polymer gel dosimeter was developed to overcome weakness of earlier polymer gel dosimeters. Hydroxyethyl 2-methacrylate (HEMA) and polyethylene glycol ester dimethacrylate (9G) were less toxic constituents, which polymerize with irradiations. One of cellulose derivatives, hydroxypropyl cellulose (HPC), was crosslinked by irradiations of electron beam and used as a gel matrix, which doesn't soften with increasing temperature. Hydrogel of HPC is very transparent, so light scattering and absorption of products of radiation-induced polymerization are easily observable. As a result of irradiation, white turbidity was observed inside the gel at around 1-2 Gy. Increasing behavior of white turbidity with irradiation depended on composition of constituent monomers. White turbidity appears at rather lower dose and its increase saturates at around 10 Gy of $${gamma}$$-irradiation when 9G is predominant. On the other hand, it becomes stronger even beyond 10 Gy when HEMA is predominant. In order to quantify the degree of white turbidity, haze measurement and UV-vis absorption spectrometry were conducted. In addition, white turbidity evolution was also confirmed after irradiations of therapeutic C ion beam although degree of white turbidity was less clear than that observed after $$^{60}$$ Co $${gamma}$$-irradiation.

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