Results of observation of atmospheric radioactivity by Japan Meteorological Agency

Atmospheric and sea water radioactivities in Japan observed mostly in 1975 are reported. The observation was made by the network of some twenty stations including ships. The measurement of the collected samples was made at the respective observation sites except the ships. The samples collected by the ships were measured at nearby land stations. The analysis for the detection of some compositions was made at the analysis center in the Geochemical Laboratory of Meteorological Research Institute of Japan Meteorological Agency. First, the gross beta radioactivity in precipitation was measured every 24 hours when the 24 hour precipitation quantity was equal to 5 mm or more, excepting the case of nuclear explosion in the north hemisphere. In case of the explosion, the period was several days and the quantity limit was 1 mm or more. Secondly, the gross beta radioactivity in airborne dust was measured by the method that the dust deposited on the filter paper was heated to ash. Thirdly, the gross beta radioactivity in sea water collected by coprecipitation method by ships was measured with Geiger-Muller counter. Fourthly, the depositions of Sr-89, Sr-90 and Cs-137 in the mixture of precipitation and settled dust collected outdoors were measured. Fifthly, natural background radioactivity in the atmosphere in terms of gamma radiation dose rate was observed. Sixthly, the vertical distribution of radioactivity in free atmosphere in terms of gamma radioactivity was observed, using a radio sonde. The results of the observation were given in attached tables. (Iwakiri, K.)