Spontaneous fission
Spontaneous fission (SF) is a form of radioactive decay in which a heavy atomic nucleus splits into two or more lighter nuclei.
In contrast to induced fission, there is no inciting particle to trigger the decay; it is a purely probabilistic process.
[1][2]: 16 Following the discovery of induced fission by Otto Hahn and Fritz Strassmann in 1938, Soviet physicists Georgy Flyorov and Konstantin Petrzhak began conducting experiments to explore the effects of incident neutron energy on uranium nuclei.
[3] Spontaneous fission arises as a result of competition between the attractive properties of the strong nuclear force and the mutual coulombic repulsion of the constituent protons.
Thus, at high mass and proton numbers, coulombic repulsion overpowers the nuclear binding forces, and the nucleus is energetically more stable as two separate fragments than as a single bound system.
[4]: 478–9 Spontaneous fission is usually a slow process, as the nucleus cannot simply jump to the lower energy (divided) state.
As nuclear mass increases, so too does the fissility parameter, eventually approaching and exceeding unity, where stability against fission is lost altogether.
[2]: 35 The semi-classical liquid-drop model provides a primarily qualitative description of the phenomenology by treating the nucleus as a classical drop of liquid to which quantum corrections can be applied, which provides a useful conceptual picture that matches in part with experimental data, but ignores much of the quantum nature of the system and fails at more rigorous predictions.
In this model, as with a classical liquid drop, a "surface tension" term is introduced which promotes the spherical shape of the nucleus.
[6]: 3 As the deformation of the nucleus increases, and particularly for large nuclei due to their stronger coulombic repulsion, the nucleus may find itself in a state where a thin 'neck' develops, forming a bridge between two clusters of nuclear matter which may exceed the ability of the surface tension to restore the undeformed shape, eventually breaking into two fragments at the "scission point".
After separation, both fragments are highly positively charged and therefore gain significant kinetic energy via their mutual repulsion as they accelerate away from each other.
Decays occurring within 10−13 s of scission are termed "prompt" and are initially dominated by a series of neutron emissions which remain the dominant decay mode until the fragment energy is reduced to the same order of magnitude as the neutron separation energy (approximately 7 MeV), when photon emission becomes competitive.
Below the neutron separation energy, gamma emission is dominant, characterised by a disordered spectrum of gamma energies with characteristic low-energy peaks corresponding to specific decays as the daughter descends the yrast line,[2]: 53–4 each decay carrying away excess angular momentum.
[2]: 53–4 Daughter products created by prompt decays are often unstable against beta-decay, and further photon and neutron emissions are also expected.
