第4章患者救済に奔走した活動記録〈論文・研究発表〉FUKUSHIMA いのちの最前線499T. KOBAYASHIactivities outside are not so long, and they can be careful not to stay too long near those hotspots.DISCUSSION From the early stage of the increase of gamma-ray background caused by the crisis in Fukushima Daiichi nuclear power plant, the author distributed the data as Figure 1 to the relevant people within the campus almost every day, to confirm that there was no further accident in the nuclear power plant. Frequency of the data distribution became roughly once a month nowadays. The audience of this information asked the author several questions. "How could we translate the unit cps in Figure 1 to more familiar unit of μSv/h?" This was a serious but quite difficult problem. The author had no high precision dosimeters for environmental level. Only a pocket dosimeter (Panasonic, ZP-145) was placed in the author's office from the 18th of March, and comparing cps of NaI with this tiny dosimeter, a conversion factor of 9.4×10-4(μSv/h)/cps was temporarily obtained. The author intends to get more precise dosimeter. "The reason of the increase of long half-life is the influence of supplies from nuclear reactor?" Concerning this question, there is an interesting report by Meteorologica1 Research Institute4). This report says that the fallout of Cs-137 away from the Chernobyl nuclear disaster faded environmentally with the half-life of 25 days. Thus, the present author guessed that the newly supplied radioisotopes from Fukushima Daiichi Nuclear Power Station might be the origin of the half-life longer than 25 days. Nowadays, however, environmentally accumulated long HL radioisotopes such as Cs-137 might be the main origin of the apparent half-life longer than 100 days. "ls the expression with half-life really appropriate for the change of cps?" The answer is not so clear either. However, radioisotopes from nuclear power plant are divided into two groups, i.e., with short half-life one (I-131) and with long half-life ones (Cs-134, Cs-137 and Sr-90). Therefore, the idea of the sum of two groups of RI with different half-life may be one of the simplest models, at the present time of three months or 100 days after the accident.CONCLUSIONS Three groups of radiation survey through Tohoku Region Pacific Coast Earthquake and the subsequent Fukushima Daiichi nuclear disaster were reported. First, at the head of Associate Dean, School of Medicine, the group of interested people assembled from the faculty members of "Life Sciences and Social Medicine" and "Human and Natural Sciences" began radiation surveillance immediately after the earthquake, and gave precious information and confirmation of a sense of security for the staff of Fukushima Medical University. Second, serial measurements of natural radiation revealed various responses from the nuclear power plant accidents. For the gamma radiation data, non-linear least squares fit indicated short and long half-life decrease of the radiation. Shorter half-life is clearly recognized as the contribution of iodine 131, while longer half-life is attributed to the radiation from cesium-134, cesium 137 and strontium 90. Third, the team of radiation dosimetry under the direct control of Dean, School of Medicine, started late April and continues the surveillance and will continue for all the people in Fukushima Medical University.REFERENCES1. Kobayashi T. Indoor-atmospheric radon-related ra­dioactivity affected by a change of ventilation strategy. Fukushima Journal of Medical Science, 52 : 59-64, 2006.2. Mathematical Systems, INC. http://www.msi.co.jp/splus/3. The R Project for statistical Computing. http://www.r-project.org/, http://www.okada.jp.org/RWiki/4. "Artificial Radionuclides in the Environment" series artides. http://www.mri-jma.go.jp/Dep/ge/2007Artifi_Radio_report/Chapter5.htm