http://www.ctbto.org/press-centre/highlights/2011/fukushima-related-measurements-by-the-ctbto/fukushima-related-measurements-by-the-ctbto-page-1/Comprehensive Nuclear-Test-Ban Treaty Organization
http://www.ctbto.org/------------- SNIP
To date, more than 35 radionuclide stations that are part of the International Monitoring System (IMS) have provided information on the spread of radioactive particles and noble gases from the Fukushima accident. The IMS is a global network that will comprise 337 facilities when complete. Sixty-three of the 80 planned IMS radionuclide stations are already operational and able to detect airborne radioactivity.
Initial findings
The first analysis results of the monitoring data became available a few days after the accident. A clear picture quickly emerged. Initial detections of radioactive materials were made on 12 March at the Takasaki monitoring station in Japan just 250 km away from the troubled power plant. The dispersion of the radioactive isotopes could then be followed to eastern Russia on 14 March and to the west coast of the United States two days later.
Spreading across the entire globe
Nine days after the accident, the radioactive cloud had crossed Northern America. Three days later when a station in Iceland picked up radioactive materials, it was clear that the cloud had reached Europe. By day 15, traces from the accident in Fukushima were detectable all across the northern hemisphere. For the first four weeks, the radioactive materials remained confined to the northern hemisphere, with the equator initially acting as a dividing line between the northern and southern air masses. As of 13 April, radioactivity had spread to the southern hemisphere of the Asia-Pacific region and had been detected at stations located for example in Australia, Fiji, Malaysia and Papua New Guinea.
The CTBTO’s monitoring system, custom-tailored to detecting nuclear explosions, can detect a range of radioactive isotopes, among them Iodine-131 and Caesium-137. Looking at the ratios between the various radioactive isotopes – in particular Caesium-137 – enables the source of the emission to be identified. In the case of the current readings, findings clearly indicate radionuclide releases from a damaged nuclear power plant, which is consistent with the recent accident at Fukushima in Japan. As of 13 April, the average level of radioactivity picked up by the stations worldwide continued to decline, which is also due to the relatively short half-lives of Iodine-131 (8 days) and Xenon-133 (5.2 days). MORE AT LINK