Alpha Decay

The atomic nucleus is composed of protons and neutrons, yet 2 of the protons and 2 of the neutrons can group to form an α particle inside the nucleus, that is, to form a ⁴He nucleus. The escape of an α particle is known as α decay.

α particles feel an attractive force from the combined proton and neutrons, however, the protons will also exert a repulsive force due to Coulomb interaction. Additionally, the α particle angular momentum will push it towards the surface of the nucleus.

In α decay, the original nucleus with Z proton, N neutrons, and A total of nucleons will lose 2 protons, 2 neutrons and a total of 4 nucleons:

    α decay: (Z,N,A) -> (Z-2,N-2,A-4)

The energy available for the decay is known as α- decay Q-value. For transitions between nuclear ground states it is calculated as:

    α- decay Q-value=Mass(Z,N) - Mass(Z-2,N-2) - Mass(α)

Numerical values can be obtained from the web application QCalc.

α decay typically occurs for heavy nuclei, Z larger than 50. Nuclei that exhibit α decay are indicated with a   yellow   background in the chart of nuclei:

α decay half-lives can span the whole range of known values, from very short to very long. The larger the enegy available for the decay, the shorter the half-life. Additionally, nuclear levels with similar values of spin and parity to the initial level are preferentially populated. The larger the difference in angular momentum the larger the half-life, yet this dependence is not as strong as it is for β and proton decays.

A quantity often used is the "Hindrance Factor", which is the ratio between the alpha-decay partial experimental half-life and a calculated half-life, for instance that obtained using Preston's model :

    HF = T_1/2 (experimental)/ T_1/2(calculated)

The relation between Hindrance Factors values and T_1/2 has been studied in detailed by Akovali.