The first information about the atomic nucleus was obtained at the beginning of the 20th century by studying radioactivity. For a long period of time only three kinds of nuclear decay modes (alpha, beta, and gamma) were known. They illustrate three of the fundamental interactions in nature: strong, weak, and electromagnetic. Spontaneous fission became better studied soon after its discovery in 1940 by Konstantin Petrzhak and Georgy Flyorov because of both the military and the peaceful applications of induced fission. This was discovered circa 1939 by Otto Hahn, Lise Meitner, and Fritz Strassmann.
There are many other kinds of radioactivity, e.g. cluster decay, proton emission, various beta-delayed decay modes (p, 2p, 3p, n, 2n, 3n, 4n, d, t, alpha, f), fission isomers, particle accompanied (ternary) fission, etc. The height of the potential barrier, mainly of Coulomb nature, for emission of the charged particles is much higher than the observed kinetic energy of the emitted particles. The spontaneous decay can only be explained by quantum tunneling in a similar way to the first application of the Quantum Mechanics to Nuclei given by G. Gamow for alpha decay.
Usually the theory explains an already experimentally observed phenomenon. Cluster decay is one of the rare examples of phenomena predicted before experimental discovery. Theoretical predictions were made in 1980,
four years before experimental discovery.
Four theoretical approaches were used: fragmentation theory by solving a Schrödinger equation with mass asymmetry as a variable to obtain the mass distributions of fragments; penetrability calculations similar to those used in traditional theory of alpha decay, and superasymmetric fission models, numerical (NuSAF) and analytical (ASAF). Superasymmetric fission models are based on the macroscopic-microscopic approach
using the asymmetrical two-center shell model
level energies as input data for the shell and pairing corrections. Either the liquid drop model
or the Yukawa-plus-exponential model
extended to different charge-to-mass ratios
have been used to calculate the macroscopic deformation energy.
Penetrability theory predicted eight decay modes: C, Ne, Mg, Si, Ar, and Ca from the following parent nuclei: Ra, Th, U, Pu, Cm, Cf, Fm, and No.
The first experimental report was published in 1984, when physicists at Oxford University discovered that Ra emits one C nucleus among every billion (10) decays by alpha emission.