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Abstract
Monthly plutonium and thorium depositions at Tsukuba (28m asl) and Mt. Haruna (1370m
asl) were measured during 2006 and 2007 (Jan 2006-Dec 2007 at Tsukuba, Nov 2006-Dec
2007 at Mt. Haruna). The monthly (239,240)Pu depositions ranged from 0.044 to 2.67mBq
m(-2) at Tsukuba and from 0.05 to 0.9mBq m(-2) at Mt. Haruna during the measurement
periods. Monthly (239,240)Pu deposition did not differ markedly between the two sites
except in April 2007. Seasonal pattern of monthly (239,240)Pu depositions at both
sites showed high in spring and low in summer, and typical of seasonal variations
in northeastern Asia. Thorium deposition at Tsukuba was higher than that at Mt. Haruna
except in May and June 2007. (230)Th/(232)Th activity ratios were used to partition
deposition samples into locally and remotely derived fractions. The results revealed
that a major proportion of total (239,240)Pu and Th deposits are derived from remote
sources, especially in spring.