Detection of uranium fission tracks in minerals using tandem accelerator and its application to paleo-geothermal history analysis

1. 在古地热区进行了野外调查和采样。调查地区有：(1)后福岛古地热田，(2)大分县山谷-翁太地区热液蚀变带，(3)大分县Kunisaki-Bajyo金矿；(4)鹿儿岛县Nansastu和Hokusatsu金矿；(5)长崎县Hasami金矿和佐贺县Arita瓷粘土矿床；(6)长崎县云仙地热田；(7)福冈县星野金矿；(8)大分县Taio金矿。xrd研究明确了各研究区热液蚀变分带。测定了岩石样品的锆石和磷灰石的裂变径迹年龄、黑云母和角闪石的K-Ar年龄以及石英的热释光年龄。这些地区的寄主岩或蚀变岩的年龄为中新世中期至更新世晚期。将锆石和磷灰石装入铁铁盘，在九州大学的串联加速器上进行重离子辐照。束流能量为80 ~ 95 MeV，离子流通过Au箔的卢瑟福散射控制。辐照后对样品进行化学蚀刻，成功探测到铀与重离子轨道相交的约束裂变径迹。镍便于生成重离子。硅半导体探测器用于控制离子束的影响。

1. Field survey and sampling were performed in the paleo-geothermal area. Investigated areas are (1) paleo-geothermal field in Goto-Fukue island, (2) hydrothermal alteration zone in Yamaguni-Onta area in Oita Pref., (3) Kunisaki-Bajyo gold field in Oita Pref. (4) Nansastu and Hokusatsu gold fields in Kagoshima Pref. (5) Hasami gold field in Nagasaki Pref.and Arita porcelain clay deposit in Saga Pref. (6) Unzen geothermal field in Nagasaki Pref. (7) Hoshino gold field in Fukuoka Pref. (8) Taio gold field in Oita Pref.XRD study clarified hydrothermal alteration zoning in each study field. Then fission track age of zircon and apatite, K-Ar age of biotite and hornblende and thermoluminescence age of quartz were determined for rock samples. Host rocKs or altered rocks in such areas indicated ages of middle Miocene to late Pleistocene.2. Zircon and apatite were mounted in tefron disk and irradiated with heavy ion at a tandem accelerator in Kyushu University. Beam energy was 80-95 MeV,and ion flux was controlled with Rutherford scattering with Au foil. After irradiation, sample was chemically etched, and then confined fission tracks of uranium intersecting heavy ion tracks were detected successfully. Ni is convenient to generate heavy ions. Silicon semi-conductor detector is useful to control fluence of ion beam.