TEMPORAL FEATURE OF GLOBAL FALLOUT NP-237 DEPOSITION IN PADDY FIELD THROUGH THE MEASUREMENT OF LOW-LEVEL NP-237 BY HIGH-RESOLUTION ICP-MS

The global fallout Np-237 in paddy field soils collected periodically between 1959 and 1989 have been studied with emphasis on the measureme nt of low-level Np-237 by high resolution ICP-MS. The Np-237 contents of samples ranged from 0.39 to 4.55 mBq/kg for three locations. The Np -237/Pu-239, Pu-240 activity ratio was evaluated to be (2.0 +/- 0.5) X 10(-3) for the first series of weapons test, during 1952-1958, and th e somewhat higher value of (3.6 +/- 0.7) X 10(-3) for the second serie s of weapons tests, during 1961-1962. Behavior of Np-237 in paddy fiel d soil was investigated by using the integrated deposits of Np-237 as well as Pu-239, Pu-240 and Cs-137 with a simple compartment model. The apparent residence time for Np-237 in surface soils within the depth studied was found to be shorter than those for Pu-239, Pu-240 and Cs-1 37, implying greater mobility of Np-237 than of Pu-239, Pu-240 or Cs-1 37.