タンデム型パンデグラーフ加速器を用いた樹木中の微量元素分析

Abstract

タンデム型バンデグラーフ加速器から放出されるプロトンビームを用いて樹木中の微量元素を定量するため, 照射条件, 試料の処理方法などの検討を行った。この結果, 厚さ100μmのスギの切片に3MeVのエネルギーをもつプロトンをビーム直径3mm, ビーム電流20nAで, 電流積算値65μCまで照射しても, 試料からの元素の揮散や試料の破損は認められなかった。試料に導電性をもたせるため, 木粉にグラファイトを10wt％添加すると制動放射が減少して検出精度が著しく高くなった。また, 木粉を低温灰化することによってもグラファイトを添加したときと同じ効果を示した。樹木の葉部からは, K, Ca, Mn, Fe, Cu, Zn, As, Pb, Rb, およびSrなどの元素が容易に検出された。

Since tree is long-lived plant and its annual rings contain many important informations about the environmental history, it has been used as biomonitor. We have been determined the amount of trace elements along the annual rings by means of instrumental thermal neutron activation analysis (INAA). In this method the elements, Na, K, Rb, Cs, Mg, Ca, Sc, Ti, V, Mn, Fe, Co, La, Ce, Sm, Al, Sb, Cl, and Br, can be analyzed with sufficient sensitivity. Unfortunately, the elements such as Pb, Cu, As, Ni, and Cd, which are closely associated with environmental pollutions, cannot be detected, or even if they can, the sensitivity of analysis is not still satisfactory. In this study, particle induced X-ray emission (PIXE) method was applied to the wood samples, and especially bombardment conditions suitable for those samples were established. The proton beam of 3.0 Mev was obtained by a Tandem Van de Graaff accelerator. The results are as follows. The damage of the sample and the volatilization of elements from the sample were not observed up to 20 nA of beam current. Addition of 10% of graphite to wood meal decreased effectively the bremsstrahlung background, and consequently increased the sensitivity of detection. Similar effect was achieved by ashing the meal at low temperature. The elements such as K, Ca, Mn, Fe, Cu, Zn, As, Pb, Rb, and Sr could be determined quantitatively.