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They put sample (the grain of salt or seawater) and detector in a big lead box to get the readings - at least that is one method.
Quote from: JMLCarter on 13/04/2011 22:19:03They put sample (the grain of salt or seawater) and detector in a big lead box to get the readings - at least that is one method.I suspect the grains of salt in question were more metaphorical than physical.
All radioactive decay is very characteristic and given the right sort of equipment the type and energy of the radiation can be detected with great precision this allows precisely the sort of atom that emitted the radiation to be identified so for example individual atoms of a short lived fission product can be identified as they decay. There is no other natural source of such short lived isotopes so individual atoms getting to the UK in the air from Japan can be identified. As there are an incredible number of atoms in even a small quantity of material the test is just about a sensitive as a test could possibly be.Remember the story that if you managed to distribute a glass of water through all the water on the earth any glass of water would contain one hundred of the atoms in the original glass.