A live geiger counter that is set up in Western Tokyo. That upward tail is pretty troubling.
http://park18.wakwak.com/~weather/geiger_index.html
What is the bottom graph representing?
EDIT: Looks like data from a little over a year ago- The date range in the screenshot is from Dec 4th to 5th 2010.
Other text: "100 CPM is equivalent to exposure of 1 micro-sievert per hour". Or, approximately one tenth your rate of exposure when flying commercial.