- All heavy nuclei are unstable to spontaneous fission into two
lighter fragments. However, for all but extremely heavy nuclei,
this process is obstructed by a potential barrier, the activation
energy, and hence the process is not significant except for some
transuranic isotopes of very large mass number.
- Nuclear surface tension tries to maintain a spherical shape, but the
coulomb energy decreases as the nucleus becomes deformed and hence
acts in the opposite "direction".
- Two fragments form which break apart and repel each other in
opposite directions due to the conservation of momentum. Due to
there being an excess of one neutron per fragment, the self-sustaining
process of a chain reaction occurs (see next
- Even though fission only occurs in about 3% of the decays, the
neutron production rate of 2.3×1012 n sec-1 g-1
of 252Cf makes it a prolific, compact source.