FUKUSHIMA Lives on the Line

232maximum value of 9.3 times as usual values. This sudden increase was attributed to the hydrogen explosion at the nuclear power plant. Thereafter, the counts seemed to attenuate exponentially. This early exponential attenuation was expressed roughly 3 days of HL that was attributed to the existence of I-131 (HL : 8.06 days). This early environmental HL of 3 days was reasonably shorter than the physical HL of I-131.After one month has passed, semi-log plot of the cps versus time did not fit to a single line and the apparent HL became longer and longer, indicating the appearance of the second long HL. Thus, the author tried a model equation of CPS=a2-t/Ta+b2-t/Tb, where Ta is the short HL and Tb is the longer HL. Nonlinear least squares regression, using a command, nls, of S-PLUS2) or R3), obtained the value of Ta=3.63±0.02 days and Tb=181±5 days. This second longer HL increased longer and longer afterwards and reached 181 days after about five months Fig. 2. Change of atmospheric radon concentration after the magnitude 9.0 earthquake in a room of the Radioisotope Center. CPRD means continuous passive radon detector (Pylon, AB-5).Fig. 1. Serial gamma radiation measurements in air with sodium iodide (NaI) scintillation counter before and after the magnitude 9.0 earthquake, tsunami, and subsequent nuclear power plant crisis. Results were stored every 4 hours as the average cps from the accumulated counts. Measured place was Fukushima Medical University Department of Natural Sciences (Physics) professor's office. Radiation surveillance is a routine activity of this department.(144 days after the peak value was observed). This long HL might be contributions of newly supplied radioisotopes from nuclear reactor or environmentally accumulated long HL isotopes such as Cs-134, Cs-137 and Sr-90. The value of b/(a+b) was 0.13 that means long HL radioisotopes in FMU were 13% at the first attack of the explosion of the reactor to Fukushima.Atmospheric radonFigure 2 shows the change of atmospheric radon in one room of the Radioisotope Center. This Center is normally ventilated extremely because of the prevention of non-sealed radioisotopes. This extreme ventilation stopped when the earthquake broke out and RI users were immediately prohibited to enter. The stop of ventilation naturally raised atmospheric radon concentration as indicated in the Figure 2.Usual radon level was quite low and less than the lower detection limit of the device (5 Bq/m3), while after the ventilation stopped, it raised as high as 250 Bq/m3 that exceeds intervention level of the U.S.A.(150 Bq/m3) or Europe (200 Bq/m3). The origin of the elevated radon might be the thick concrete wall of the room that was devised to handle sealed radiation sources. Although the level became high, there was no problem of radiation protection because people were inhibited to enter during the accident.Radon level at other places also became slightly high after the earthquake, and again the ventilation stopped for about 10 days. However, the level did not exceed usually observed maximum level since these rooms have not so thick concrete as RI center's room.Second cosmic raysSecond cosmic rays showed a little decrease and growth through the earthquake. However, these changes were explained with the contrary change of the atmospheric pressure ; second cosmic rays decrease when the atmospheric pressure increases (thicker air disturbs cosmic rays to reach the ground). Atmospheric pressure at the time was later checked with the data from automated meteorological data acquisition system (AMeDAS) of Japan Meteorological Agency.3. The team of radiation dosimetry under the direct control of Dean, School of MedicineThe results of surveillance by the team of radiation dosimetry under the direct control of Dean, School of Medicine, are informed to FMU staff twice a month nowadays. Indoor levels are now no problem, while outdoor values are a little high, especially on some 'hotspots.' However, times for students' club activities outside are not so long, and they can be careful not to Radiation Measurements at the Campus of FMU