In order for momentum to be conserved, the thorium nucleus and the helium nucleus must have ``equal and opposite" momenta. The relationship between momentum and total energy is . The kinetic energy is defined as . Therefore, the relationship between momentum and kinetic energy is or
The derivative of with respect to is
which is always less than . Therefore, the helium nucleus will have more kinetic energy because it has smaller mass. Therefore, answer (E) is correct.
Please send questions or comments to X@gmail.com where X = physgre.