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Why do we hear so much about isotopes such as Cs-137, I-131 and Te-132

OK I imagine that the news has a lot on the subject of iodine, cesium and tellurium. Some of my readers will know that the isotopes which I mentioned in the title are fission products from uranium.

But some of you will ask why are they a bit on the heavy side, uranium-235 has a mass of 235 amu, if we assume that a neutron makes it split and that 3 neutrons are given out by the fission process then we should have 234 amu of fission products. I imagine that some of you have a GCSE in maths (in actual fact primary school maths should do the trick) and will understand that half of 234 is equal to 100 + 15 + 2 = 100 + 17 = 117.

Some of you might have noticed that isotopes with masses of about 117 seem to be absent from the news, for such a mass I would expect that the stable isotope would be somewhere around about tin in the periodic table.

Now I will have to admit that some isotopes are of special interest because of their half lives, chemistry or biology. But it is a case that the yield of isotopes right in the middle of the weight range is very poor.

What happens to a uranium-235 atom is that it absorbs a neutron and forms an excited state of uranium-236. This excited state of uranium-236 can either lose its energy through the emission of gamma rays or it can split into two parts.

When the energy of the excited state is low then it is normal for the uranium to split into two very unequal sized fragments. This is why we have two peaks in the fission yield against mass graph.

In the case of a very high energy excited state which undergoes fission it forms two equal sized lumps. When uranium is bombarded with fast neutrons then the yield for these isotopes which lie in the middle of this bathtub shaped graph increase because the energy of the excited state which undergoes nuclear fission is now higher. For a super heavy isotope which undergoes spontaneous fission I would always expect a far higher yield of these isotopes which are in the valley than I would in a normal power reactor.


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