Differences of natural radioactivity and radon emanation fraction among constituent minerals of rock or soil

岩石又は土壌の鉱物成分中の自然放射能とラドン散逸係数の相違

迫田 晃弘*   ; 西山 祐一*; 花元 克巳*; 石森 有  ; 山本 祐紀*; 片岡 隆浩*; 川辺 睦*; 山岡 聖典*

Sakoda, Akihiro*; Nishiyama, Yuichi*; Hanamoto, Katsumi*; Ishimori, Yuu; Yamamoto, Yuki*; Kataoka, Takahiro*; Kawabe, Atsushi*; Yamaoka, Kiyonori*

花崗岩質の物質を構成する主要な鉱物間の放射性特性の相違を調査した。鉱物中の自然放射能(UとRa)とラドン散逸係数は、線スペクトロメトリーにより測定した。土壌鉱物のラドン散逸係数(27-43%)は、岩石鉱物のそれら(0.6-4.6%)より高かった。さらに、散逸係数は岩石の場合も土壌の場合も鉱物の種類によって大いに異なった。これらの結果について、表面積と鉱物粒子中のラジウム分布の相違の観点で議論した。岩石と土壌サンプルの石英について、予想外に散逸係数が大きかった。バルクのサンプルから発生するトータルのラドン散逸量へのそれぞれの組成鉱物の寄与を見積もった。結果はラドン散逸係数だけでなく、Ra放射能の強さと鉱物含有量に依存することがわかった。

We examined differences in the radioactive characteristics among the main minerals forming granite materials. Using a non-toxic high-density agent, minerals were separated from rock (granite-gneiss) and soil (weathered granite) samples. The natural radioactivity (U and Ra) and radon emanation fraction of the minerals were then studied by -ray spectrometry. The radon emanation fractions (27-43%) of the minerals from the soil were much higher than those (0.6-4.6%) of the rock minerals. Additionally, the emanation fractions differed greatly among the minerals separated from both the bulk rock and soil. These results were discussed in terms of the differences of surface area and radium distribution in the mineral grains. It was noticeable that a higher emanation fraction than expected for quartz was commonly observed in the rock and soil samples. We then estimated the contribution of each constituent mineral to the total radon exhalation from the bulk samples. The result depended not only on the radon emanation fraction, but also on the Ra activity and the mineral content. Furthermore, using the obtained data, we also discussed the effect of grain size on radon emanation and why this has been reported to vary markedly in previous studies.