Cluster Decay

Cluster decay (also named heavy particle radioactivity or heavy ion radioactivity) is a type of nuclear decay in which a parent atomic nucleus with A nucleons and Z protons emits a cluster of Ne neutrons and Ze protons heavier than an alpha particle but lighter than a typical binary fission fragment (although ternary fission into three fragments produces products which overlap cluster decay). A chemical transformation of the parent nucleus leads to a different element, the daughter, with a mass number Ad = A - Ae and atomic number Zd = Z - Ze where Ae = Ne + Ze. For example:

88Ra → 14
6C + 209

This type of rare decay mode was observed in radioisotopes that decay predominantly by alpha emission, and it occurs only in a small percentage of the decays for all such isotopes.

The branching ratio with respect to alpha decay

is rather small (see the Table below). Ta and Tc are the half-lives of the parent nucleus relative to alpha decay and cluster radioactivity, respectively.

Cluster decay, like alpha decay, is a quantum tunneling process: in order to be emitted, the cluster must penetrate a potential barrier. This is a different process than the more random nuclear disintigration that precedes light fragment emission in ternary fission, which may be a result of a nuclear reaction, but can also be a type of spontaneous radioactive decay in certain nuclides, demonstrating that input energy is not necessarily needed for fission, which remains a fundamentally different process mechanistically.

Theoretically any nucleus with Z > 40 for which the released energy (Q value) is a positive quantity, can be a cluster-emitter. In practice, observations are severely restricted to limitations imposed by currently available experimental techniques which require a sufficiently short half-life, Tc < 1032 s, and a sufficiently large branching ratio B > 10 −17.

In the absence of any energy loss for fragment deformation and excitation, as in cold fission phenomena or in alpha decay, the total kinetic energy is equal to the Q-value and is divided between the particles in inverse proportion with their masses, as required by conservation of linear momentum

where Ad is the mass number of the daughter, Ad = A – Ae.

Cluster decay exists in an intermediate position between alpha decay (in which a nucleus spits out a He4 nucleus), and spontaneous fission, in which a heavy nucleus splits into two (or more) large fragments and an assorted number of neutrons. Spontaneous fission ends up with a probabilistic distribution of daughter products, which sets it apart from cluster decay. In cluster decay for a given radioisotope, the emitted particle is a light nucleus and the decay method always emits this same particle. For heavier emitted clusters there is otherwise practically no qualitative difference between cluster decay and spontaneous cold fission.

Read more about Cluster Decay:  History, Theory, Experiments, Fine Structure

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