Temporal variation and provenance of thorium deposition observed at Tsukuba, Japan

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• 作者：Katsumi Hirose^a ; ¹ ; ^{hirose45037@mail2.accsnet.ne.jp} ; Yoshikazu Kikawada^a ; Yasuhito Igarashi^b
• 关键词：232Th ; Deposition ; 230Th/232Th ratio ; Seasonal change ; Long-term variability
• 刊名：Journal of Environmental Radioactivity
• 出版年：2012
• 期刊代码：45_0265931x
• 类别：env
• 出版时间：June, 2012
• 卷：108
• 期：Complete
• 页码：24-28
• 文件大小：733 K

摘要

Temporal variations of monthly thorium (Th) deposition observed at Tsukuba, Japan during the period 1990-2007, comparing with plutonium deposition, was studied. The monthly ²³²Th deposition as did ^239,240Pu, varied according to season and inter-annually. In particular, ²³²Th deposition increased significantly in spring coinciding with Asian dust (Kosa) events. The ²³⁰Th/²³²Th activity ratios vary according to sources and can therefore be used to differentiate between locally derived and remotely derived ²³²Th. The ²³⁰Th/²³²Th activity ratios in deposition samples showed large variability with high ratios occurring in early spring. These high ²³⁰Th/²³²Th ratios in deposition can be attributed to local dust storms, especially in early spring, that cause resuspension of soils from cultivated fields which are characterized by high ²³⁰Th/²³²Th activity ratios. The results reveal that both locally and remotely derived ²³²Th deposition showed seasonal variations with maxima in spring, although the remotely derived fraction is dominant rather than the locally derived one. The ²³²Th deposition maxima later in spring is attributable to the remotely derived fraction, corresponding to the Kosa events. Annual ²³²Th deposition exhibited an increasing trend, suggesting the presence of sources other than soil dust such as fly ash from increasing coal burning.