In 1984, Rose and Jones at Oxford University announced the discovery of a new rare type of radioactive decay in the nuclide Ra223. Their article entitled “A new kind of natural radioactivity” was published in Nature. The possibility that such a decay process, intermediate between alpha decay and spontaneous fission, may exist was postulated by A. Sandulescu, D. N. Poenaru, and W. Greiner a few years earlier in 1980. Rose and Jones showed that part of the Ra223 parent nuclide decays directly to Pb209 by the emission of a 30 MeV C14 ion i.e.
Ra223 → C14 + Pb209 + 31.87 MeV
Observations also have been made of carbon-14 from radium-222, radium-224, and radium-226, as well as neon-24 from thorium-230, protactinium-231, and uranium-232. Such heavy-ion radioactivity, like alpha decay and spontaneous fission, involves quantum-mechanical tunnelling through the potential-energy barrier. Shell effects play a major role in this phenomenon and in all cases observed to date the heavy partner of carbon-14 or neon-24 is close to doubly magic lead-208.
The ratio of carbon-14 decay to alpha decay is about 5.5 × 10−10. This low value explains why the spontaneous decay mode had not been observed earlier. Since the probability of cluster emission is expected to be greatest when the daughter nuclide configuration is close to that of a full shell, attempts have been made to observe the phenomenon with parent nuclides near Z = 88 (Z = 82 corresponds to a magic proton line). Hence the search has concentrated on the elements francium and actinium with potential daughter of thallium and bismuth, e.g.
Fr221 → Tl207 , and Ac225 → Bi211
Oxygen cluster emission was discovered by Hussonois et al. in the decay of thorium, i.e.
Th226 → O18 + Pb208
Similarly, Si34 cluster should result from the decay of Am241 and Pu240.
H. J. Rose, G. A. Jones: Nature 307, 245 (1984) Full paper
M. Hussonis et al.: Phys. Rev. C 43, 2599 (1991)
J. Sandulescu: Phys. G: Nucl. Part. Phys. 15, 529 (1989)