An autoradiograph is an image created by placing a specimen in close contact with X-ray film. If radioactivity is present it will darken the image.
This image is from Professor Bin Mori of Tokyo University Graduate School of Agricultural and Life Sciences. There are more autoradiographs on his blog site. He says the pictures show that radioactive materials have been absorbed from the roots and distributed throughout the plants. (That is why the entire leaf shape can be seen.) He thinks the dark dots on the dandelion leaves are where radioactive material landed on the leaves. We don't know where the dandelions were growing.
The dark dots indicate the presence of hot particles. It is not possible to say how large they are. The size of the spots here indicates the spread of the radiation tracks emanating from the particles. It is very likely that the leaves were separated from the film by foil, as the film must be protected from light. Alpha particles from Uranium and Plutonium will not pass through the foil, so the exposure here is to beta and gamma decays. However, from the first explosions, we have predicted that Uranium and Plutonium would be found. Alpha spectrometry and mass spectrometry can confirm the composition. This is the next step, which should be carried out by the Japan authorities (and must have been done, though as far we know the results have not been published.)
