Function fitting to experimental data is one of the important procedures to derive necessary information from the data. The least-squares method has been known as a means for fitting function to normally distributed data. One of the authors (T. A.) has proposed the method for fitting to Poisson-distributed data from radioactivity measurements. We have attempted to fit functions with linear and non-linear least-squares methods and the method by T.A. and estimated the errors of the functions. We have also obtained confidences for the intervals of the errors with computer simulations and got good results.
This paper attempts to discuss the influences of global-scale radioactive fallout on wheat produced in Japan. For this purpose, a set of wheat samples was collected annually from sixteen National and Prefectural Experimental Farms during the years from 1959 to 1995. According to the graph presenting year-to-year variations in the nationwide averages of90Sr and137Cs contents (mBq/kg) in wheat kernels, the sharp peak was first marked in 1963 with 12 300 for90Sr and 43600 for137Cs. In 1966, however, the nuclide contents dropped drastically to 2360 for90Sr and 2 870 for137Cs respectively. In the years following, both nuclides continued to decrease slowly although there were minor fluctuations. It is noticeable that low-level137Cs contamination was caused in Japan's wheat in 1981, the year after the nuclear testing in China. It is also noticeable that137Cs contents in wheat kernels increased to 5960 in 1986 when the Chernobyl nuclear power plant accident happened, although it dropped back to its normal level of 40 in the year after. The following are also discussed in this paper : (1) Year to year variations in the ratio of90Sr to137Cs contents in wheat kernels and wheat flour. (2) Regional differences in90Sr and137Cs contents in wheat kernels. (3) Comparison of90Sr and137Cs contents in wheat kernels and unpolished rice. (4) Direct and indirect pathways of90Sr and137Cs that contaminate wheat. (5) Influence of the Chernobyl power plant accident. (6) Suggestions about the ways of estimating the level of137Cs contamination in case of nuclear accidents, if any.