A21 Alpha decay

In alpha decay the nucleus of an element with mass number Aj and atomic number Zj emits an alpha particle. Alpha particles are made of two protons and two neutrons, that is, a helium nucleus. The original nucleus is replaced by a new nucleus whose mass number A2 is equal to Aj — 4 and atomic number Z2 is Zj — 2, and an alpha particle.

For instance, Rn (ARn = 222, ZR„ = 86) decays into Po, meaning that the nucleus of 222Rn emits an alpha particle (AQ = 4, ZQ = 2), leaving as remainder a nucleus whose mass number is 218 (222 — 4) and atomic number (86 — 2) = 84, that is, 2j8Po.

The mass (energy) of the parent nucleus must exceed the sum of the masses (energies) of the daughter nucleus and alpha particle emitted. The condition for a-decay to occur can be expressed as follows [Mukhin, 1987]:

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