Ionising radiation
The radiation emitted from unstable nuclei is called ionising radiation because as it passes through matter it can dislodge outer electrons from atoms causing them to become ions.
Types of radioactive decay
An unstable nucleus can decay by emitting an alpha particle, a beta particle, or a gamma ray.
Alpha particle
An unstable nucleus can emit a ‘package’ of two protons and two neutrons, called an alpha particle, to become more stable.
Alpha radiation is made up of a stream of alpha particles emitted from unstable nuclei.
An alpha particle is also a Helium-4 nucleus.
It is written as \(_{2}^{4}\textrm{He}\) and is also sometimes written as \(_{2}^{4}\alpha\).
Alpha decay causes the mass number of the nucleus to decrease by four and the atomic number of the nucleus to decrease by two.
Alpha decay
\(_{Z}^{A}\textrm{X}\rightarrow_{Z-2}^{A-4}\textrm{Y}+_{2}^{4}\textrm{He}\) (or\(_{2}^{4}\textrm{α}\))
Note that the mass numbers and atomic numbers are equal (i.e. they balance) on both sides of the equation.
Example
Alpha decay of Uranium-238
\(_{92}^{238}\textrm{U}\rightarrow_{90}^{234}\textrm{Th}+_{2}^{4}\textrm{He}\)
Beta particle
An unstable nucleus can emit a fast-moving electron called a beta (β) particle, to become more stable.
Beta radiation is made up of a stream of beta particles emitted from unstable nuclei.
Beta radiation is normally emitted from unstable nuclei in which the number of neutrons is much larger than the number of protons.
A beta particle has a relative mass that can be considered to be zero, so its mass number is zero, and as the beta particle is an electron, it can be written as \(_{-1}^{~0}\textrm{e}\). Sometimes, it is also written as \(_{-1}^{~0}\beta\).
Electrons are not normally found in the nucleus but, in an unstable nucleus, a neutron can split into a positive proton and a negative electron.
The proton remains inside the nucleus, but the electron is ejected at high speed.
This is called beta decay.
Beta decay causes the atomic number of the nucleus to increase by one (because there is an extra proton) but the mass number remains the same (because the total number of protons and neutrons remains unchanged).
Beta decay
\(_{Z}^{A}\textrm{X}\rightarrow_{Z+1}^{A}\textrm{Y}+_{-1}^{0}\textrm{e}\) or (\(_{-1}^{~0}\beta\))
Beta decay of carbon-14
\(_{6}^{14}\textrm{C}\rightarrow_{7}^{14}\textrm{N}+_{-1}^{0}\textrm{e}\)