It was originally reported on a blog called "Alpha rays and metal plate (α線と鉄板)" on March 18, 2012.
Email from Professor Masayoshi Yamamoto, Institute of Nature and Environmental Technology at Kanazawa University about the analysis of dust off the metal plate that had been left outside since the March 11, 2011 accident, as reported by the blog:
Here's the result of the analysis. Frankly, I am surprised that plutonium was detected.
Plutonium has been spread and deposited at low densities in wide areas because of the atmospheric testing of nuclear weapons . The ratio of plutonium isotopes, Pu238/Pu239+240, of the atmospheric testing origin is about 0.03. But this time [dusts off the metal plate in Iwaki City], this ratio is higher at 0.8, indicating the dusts contain plutonium of the Fukushima I Nuke Plant origin.
The government estimate of the ratio of these plutonium isotopes is 3. Considering the plutonium of the atmospheric testing origin is not attached to the metal plate, I wonder why the ratio is lower than the government estimate. It is possible that not all plutonium isotopes released had the ratio of 3, and there were plutonium isotopes with the lower ratio (0.8) that were released from the plant.
Further, the ratio of Pu239+240/Cs-137 is 9.0E-5, which is 200 times the government estimate of 4.3E-7, showing more plutonium being detected than the government estimate indicates. It is probably because of fractionation.
Right now, I am hoping to test for uranium, and Am-241 if possible.