Physicopoeia

φυσικοποιΐα
THINGS I WISH I'D KNOWN WHEN I STARTED TEACHING PHYSICS

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 ²³⁵always 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: