It is probably a stupid question easy to answer by any of you experienced people. But I have some problem in understanding how exactly is the specification of the atomic percentages (ao) meant inside the or section. Of course, there is a very nice Wiki as use manual, and I really don’t want to complain. But the examples therein show no clear pattern of this specification. For water, it is used ao=“1” for oxygen, and ao=“2” for hydrogen. Ok, this is clear - although IMHO this are also no percentages as those would sum up to 1. 
The other example show now many real numbers, also not summing to 1. Is there any strikter rule or a consistency pattern for which I can check for correctness?
Thank you very much for any kind of hint in this respect. I have not found any other post specific on this problem. So, I’m sorry if I’m the only blind guy
The atom percentages are all relative and are renormalized based on the specified density of the material. So using ao=“1” for oxygen and ao=“2” for hydrogen is the same as using ao=“5” and ao=“10” The only situation in which the values are taken as absolute, rather than relative, values is if you were to specify in which case the ao values are taken to be in units of atom/b-cm. In that case, the sum of the ao values gives the total atom/b-cm of the material. Does this make sense now?
Best regards,
Paul
There may be a very helpful PDF available online: Handbook of Nuclear Engineering, by Cacuci.
It is one of the most extensive nuclear texts out there is actually a few thousand pages in length. It doesn’t contain any exercises or problems but is nonetheless a ‘must have’.
Shawn W.