One of the great utilities of the Safecast project is the ability to collect and analyze environmental data on an unprecedented scale. As Safecast passes the two year mark with collecting radiation data in Japan it is now possible to begin to look back over the changes in radioactivity across different parts of the country and see where this may all be heading.
A scan of two cities
While events at the Fukushima Daiichi plant have led to the common association of all of Japan with radiation we have collected enough data to show that large swaths of the country remain near background (Approximately 30-40 counts per minute). One example of this is the city of Inzai, which lies between Tokyo and Narita International Airport, and is 193km southwest of the meltdown site. As the first graph shows background levels slightly above what is considered typical, but there is no appreciable change from the middle of 2011 to the end of 2012 in terms of radioactivity.
The next city in our study, Iitate, is about 38km northwest of the Fukushima Daiichi reactor site, and shows both more significant exposure and a noticeable decay curve. What is notable about the decay curve is that a decay half-life can be estimated to reasonable precision with the best-fit exponential decay function. From this method we can estimate the decay half-life for surface radioactivity to be only one year from the two years starting in the middle of 2011. This is notable as the decay half-life for the two dominant radioisotopes, Cs-134 and Cs-137, are two and thirty years respectively. This implies that there are additional mechanisms at work in Iitate which are moving radioactive material away from the surface, such as erosion and the deposition of new topsoil, which are accelerating the decay of measurable radioactivity at the surface.