Abstract
It is well known that there is a radioactive hot spot in south Kanto. The map is published by Japan Atomic Energy Agency. We research environmental dynamics of the hot spot. We select representative 26 points and plot the ambient dose rate. Many defects are found, which are interpolated. The graphs have vibrations even if smoothing is done. Using the least-squares method, the residence half-life is 2.3y at Toride city near the center of the hot spot. Radioactive substances in there are Cs-134 and Cs-137.To research the vibrations, we use Fourier transformations, and split the change of ambient dose rate into slow and rapid phenomena. The boundary is 48 h on Nyquist frequency. On the slow phenomenon, the vibrations are not found at all points. Thus, we calculate half-lifes and accumulated dose rates in a year for all points in Kanto.The Fourier spectrum of rapid phenomenon shows there is a wave of 24 h and the harmonics. To investigate the shape of the wave of 24h, we draw ambient dose rates at instants for 4, 12, 20 h per every day. That is to find daily waves at 3 phases.In the phase investigations, we find that ambient dose rate of 12 h is about −0.02 μSv/h smaller than those of others at Toride city. Considering the circulation of the atmosphere, the phenomenon relates with suspended particulate matter of radioactive Cs. We measure the intensity by using a technique for Lock-in amplifier, and get −0.005μSv/h. The rate is negligible in the case of external exposure. However; considering of small particles of Cs, it is rejected. We believe watchful waiting for 2 years is appropriate.
