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Cluster decay is a type of nuclear decay in which a radioactive atom emits a cluster of neutrons and protons heavier than an alpha particle. This type of decay happens only in nuclides which decay predominatly by alpha decay, and occurs only a small percentage of the time in all cases. Cluster decay is limited to heavy atoms which have enough nuclear energy to expel a portion of its nucleus.

Cluster decay has an intermediate position between alpha decay (in which a nucleus spits out a ⁴He nucleus) and spontaneous fission in which a heavy nucleus splits into two (or more) large fragments and a variable number of neutrons. Spontaneous fission ends up with a probabilistic distribution of daughter products, which sets it apart from cluster decay. In cluster decay the emitted particle is a light nucleus and the decay method always emits the same particle. More than 20 nuclei have been found which occasionally decay by emitting clusters.^[1]

Tritons and deuterons are also known as radioactive decay products. Helium-6 occasionally decays via deuteron emission and Helium-8 decays a small part of the time with a triton emission. It is possible that other exotic isotopes decay in these methods as helium is studied in particle accelerators to a great degree.

Cluster decay was discovered in 1984 when researchers at Oxford University detected that ²²³Ra emits one ¹⁴C nucleus for every billion (10⁹) alpha decays.^[2]

The known cluster emissions are as follows:

Isotope	Particle emission	Branching ratio	Reference
114Ba	12C	~3.0×10−3	[1]
221Fr	14C	8.14×10−13	[1]
221Ra	14C	1×10−12	[1]
222Ra	14C	3.07×10−10	[1]
223Ra	14C	8.5×10−10	[1]
224Ra	14C	6.1×10−10	[1]
225Ac	14C	6×10−12	[1]
226Ra	14C	2.9×10−11	[1]
228Th	20O Ne	1×10−13  ?	[1]  ?
230Th	24Ne	5.6×10−13	[1]
231Pa	23F 24Ne	9.97×10−15 1.34×10−11	[1]
232U	24Ne 28Mg	2×10−12 1.18×10−13	[1]
233U	24Ne 25Ne 28Mg	7×10−13   1.3×10−15
234U	28Mg 24Ne 26Ne	1×10−13 9×10−14
235U	24Ne 25Ne 28Mg 29Mg	8×10−12   1.8×10−12
236U	24Ne 26Ne 28Mg 30Mg	9×10−12   2×10−13
236Pu	28Mg	2×10−14
237Np	30Mg	1.8×10−14
238Pu	32Si 28Mg 30Mg	1.38×10−16 5.62×10−17
240Pu	34Si	6×10−15
241Am	34Si	2.6×10−13
242Cm	34Si	1×10−16

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National Nuclear Data Center

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