# Physicopoeia

###### φυσικοποιΐα

# Fission Decay Products

Many syllabuses require pupils to know the basics of nuclear fission, usually of Uranium-235, including the idea of a chain reaction. Textbooks and exams often show the fission products of ²³⁵U as being Krypton and Barium: in fact, to show how much copying goes on between textbooks, it's often as specific as ⁹⁰Kr and ¹⁴⁴Ba. This is fine, but it does sometimes lead pupils (and teachers) to the conclusion that fission of ²³⁵U *always* produces those two daughter nuclei. Given how catastrophically fission happens, it's perhaps not surprising that, in fact, a huge number of fission reactions are possible with the same starting conditions and that Kr and Ba are definitely *not *the only possibilities. In fact, the two products are likely to be asymmetric in mass terms, with mass numbers around 90 and 140, as this graph shows.

At pre-16 level, I simply mention that Kr and Ba are *possible *but definitely not the *only *products of ²³⁵U fission.

For older pupils who can calculate Binding Energies, I wrote this sheet of practice calculations, taking them through a number of possible fission reactions. It also reinforces the understanding that many daughter products are possible: