Map by Prof. Hayakawa from Gunma University, graphics by Ms.
I live in Abiko City in the north east of Chiba prefecture, only a stone’s throw from the border with Ibaraki prefecture. A few weeks ago, a concerned mother of our area’s day care facility showed me her Geiger counter, which showed elevated values between 0.6µSv/h and over 1µSv/h, when the officially published value was less than 0.1µSv/h. I asked around some friends who had competent contacts in regards to measuring radiation, but nobody could offer an explanation other than device error or presence of a local radiating source, such as a transmission antenna.
Shortly later Pieter of Safecast shared with me a blog entry he came across, written by a Professor Takeda of Chubu University. Using measurement data shared between universities and other data, Takeda had established, that an area commencing at Misato City close to Tokyo up to Kashiwa City close to me had been substantially contaminated at the end of March. This random deposition of radioactive fallout more than 200km away from the Fukushima plant turned the area into a so-called hotspot. Around us everything was lower, but inside the spot the contamination was many times higher. Prof. Hayakawa at Gunma University also produced a map, which was used in Takeda’s article, but my city was not in it.
On the same day of our discovery, Pieter and Stig arrived by car with the bGeigie installed. We decided to safecast the claimed area by criss-crossing through it. Indeed we found that the area highlighted by Professor Takeda and others was a hot spot, but we also found that it extended even beyond it. The Safecast Map can be seen here.
We continued by taking some detailed measurements of various types of ground since we realised straight away that the radiation was mainly not in the air, but it rose from the ground up. Measurements showed that all surfaces exposed to rain had substantially higher readings.
Looking at SPEEDI data and other sources, some initial conclusions were as follows:
- the fallout was deposited by rain on or shortly after the 21 Mar 2011; below is a graph compiled with data published by the University of Tokyo with measurements of their Kashiwa Campus. The increase on the 21st is clearly visible.
- rain must have been light and falling straight down; measurements confirm that recessed areas are much less contaminated compared to areas more exposed to the elements
- the majority of radiation is now emanating from the ground, especially from asphalt and concrete surfaces; measurements taken from higher up (e.g. 4th floor) are slightly elevated but less than half from those measured at 1m (~0.25uSv/h vs 0.55uSv/h)
- almost all horizontal man-made surfaces exhibit radiation at almost one order of magnitude higher when measured at close distance contact compared to 1m height; the strongest are asphalt and concrete surfaces (>1400 CPM), closely followed by wood (e.g. exposed benches, ~1000 CPM). On the opposite side, sand, gravel and soil including children sandpits show very little increase on the surface, just over half of a measurement performed on asphalt and concrete with a height of 1m (~0.35uSv/h vs 0.55uSv/h).
The exercise of using quality equipment and consistent methodology clearly helped in confirming the nature and extent of the contamination reaching an area far away from the Fukushima plant. In this case the discovery was by several academics and publicly pointed out Professor Takeda, but the validation by Safecast was invaluable for me and other residents in the area.