Unexplained energy in nuclear process

[LHarriger]

I was talking to a guy the other day who said that back in 1954 he worked for the military on nuclear reactors (or something along those lines). He said that back then there were nuclear processes where all of the energy could not be accounted for. This missing or extra energy was very small and they used the symbol $\zeta$ to represent it. Since then he has not been involved in physics and still doesn't know what this energy term was due to. I told him it was probably due to not including neutrinos in the processes but he said they took those into account. Does anyone know what this energy is from?

 

[LHarriger]

Sorry, that's the wrong symbol. It wa xi, $\xi$ that they used.