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Alpha decay

Alpha radiation is helium nuclei, with two protons and two neutrons, emitted from an unstable nucleus. The mass number, protons plus neutrons, falls by four. The atomic number, the number of protons, falls by two, so the daughter nucleus is a different element. Paper stops alpha particles, but they do great damage if they get inside the body, because they deposit their energy over a short path. The nucleus that remains can itself be radioactive.

{}^{A}_{Z}\mathrm{X} \rightarrow {}^{A-4}_{Z-2}\mathrm{Y} + {}^{4}_{2}\mathrm{He}

alpha decay

Symbols

$A$	mass number
$Z$	atomic number

Example

Uranium-238 has atomic number 92. After one alpha decay:

mass number $238 - 4 = 234$ and atomic number $92 - 2 = 90$ , which is thorium-234.

Beta decay changes the atomic number by one and leaves the mass number unchanged. Gamma radiation changes neither.

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Part of Physics 1: Atoms, radiation and the universe.