Monitoring Fallout Decay


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.

Radioactivity in Inzai, Japan

Radioactivity in Inzai, Japan. 18 months worth of data.

Safecast data from Iitate, Japan.

Radioactivity in Iitate, Japan. 24 months worth of data.

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.

The start of a larger analysis

Below is a series of radioactivity versus time graphs for eight locations across Japan over the past two years.  All of the data was taken from the Safecast public database with selection areas consisting of squares 2km on a side selected around a given GPS coordinate.

latitude: 37.7608337N
longitude: 140.4747282E
Distance to Fukushima Daiichi reactor: 62km NW
Start time: 5/5/2011  10:05:08 AM
Estimated half-life: 1.0 years
Stop time: 5/1/2013  9:32:00 AM
Estimated half-life: 1.0 years

Data link

latitude: 37.4472222N
Distance to Fukushima Daiichi reactor: 4km NW
Start time: 9/24/2011  1:32:22 AM
Stop time: 5/1/2013  7:35:37 AM
Estimated half-life: 1.8 years

Data Link

latitude: 37.6791903N
longitude: 140.7352474E
Distance to Fukushima Daiichi reactor: 39km NW
Start time: 5/6/2011  4:41:58 PM
Stop time: 4/29/2013  8:03:56 AM
Estimated half-life: 1.1 years

Data Link

latitude: 35.8323485N
longitude: 140.1458154E
Distance to Fukushima Daiichi reactor: 193km NW
Start time: 6/26/2011  5:05:39 PM
Stop time: 11/17/2012  11:04:28 PM
Estimated half-life: NA

Data Link

latitude: 37.5131313N
longitude: 140.3938904E
Distance to Fukushima Daiichi reactor: 57km W
Start time = 5/25/2011  3:21:34 PM
Stop time = 5/1/2013  9:14:07 AM
Estimated half-life: 1.4 years

Data Link

latitude: 37.4945855N
longitude: 141.0006538E
Distance to Fukushima Daiichi reactor: 9km N
Start time: 9/16/2011  11:56:10 PM
Stop time: 5/1/13 7:44
Estimated half-life: NA

Data Link

latitude: 37.2826405N
longitude: 140.9934863E
Distance to Fukushima Daiichi reactor: 16km S
Start time: 9/24/2011  1:12:37 AM
Stop time: 4/26/13 5:12
Estimated half-life: 2.7 years

Data Link

latitude: 37.4044283
longitude: 140.9833544
Distance to Fukushima Daiichi reactor: 5km SW
Start time: 9/24/2011  1:28:22 AM
Stop time: 1/1/2013  8:40:52 AM
Estimated half-life: 0.6 years

Data Link

Where to go from here?

All of the above decay curves were generated using uniform squares of four square kilometers in area.  While this decreases spatial resolution it is useful for gathering significant statistics for a given location.  Further analysis could compare the relationship between the area of a study and the characteristics of the decay curve.  Similarly, this first look doesn’t consider the geography of the location.  It would stand to reason that the topography, soil, and climate of a given location should have a noticeable impact on the rate of change in surface radioactivity.  Generally, as the number of readings increases over time, the significance of the analysis from any location should improve.