I have found some details of the radioactivity in the sea water close to the Japanese plant, these results are from an examination of water with a germanium detector. Now first the results, in the following bar chart the left hand blue bar is the measured value while the right hand purple bar is the limit for that radioisotope.
It is clear that for cesium and iodine that the sea water contains far more radioactivity than the limit. You might ask “what is a germanium” detector. Firstly it is not a geranium detector, it does not have a special ability to detect those nice delicate flowers.
A germanium detector is a crystal of germanium, it is common to use a very pure germanium crystal. The crystal is a semiconductor which has a very low back ground current due to its high purity and the fact that it is normal to cool them to a very low temperature.
What happens is that when a gamma photon is absorbed by the crystal it causes the formation of charge carriers (free electrons and holes) these are able to flow through the crystal under the influence of an electric field (A bias voltage is applied to the crystal), the greater the energy of the photon the more charge carriers it forms and therefore the more charge which passes.
What happens is that the electronics attached to the germanium crystal will measure how much charge passes, as a result it is possible to measure how much energy was in the gamma photon.
The Cobalt-58 is an activation product which is formed from Ni-58 by a np reaction, the nickel will be present in the form of stainless steel in the reactor. SS316L is the classic stainless steel for making nuclear reactor parts and also bits of reprocessing plants. SS316 is mostly iron, nickel and chromium. I imagine that a SS316 part which has been neutron irradiated will also contain nickel-63 (long lived beta emitter), iron-55 (electron capture), iron-59, cobalt-57, cobalt-56 and chromium-51.
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