= Alpha decay
{title2=$\alpha$}
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<Alpha decay> is <radioactive decay> emitting an <alpha particle>, a bound group of two <protons> and two <neutrons>. The daughter <atomic nucleus> therefore has <atomic number> reduced by two and <mass number> reduced by four.
A released <alpha particle> must escape the <Coulomb barrier> outside the short-range nuclear attraction. <Quantum tunnelling> explains how emission can occur even when the alpha particle's measured <kinetic energy> is below the barrier's classical height. The strong sensitivity of tunnelling to <energy> helps explain widely differing decay lifetimes.
For a specified transition from a parent initially at rest, the alpha particle and daughter share the available <Q value> through two-body recoil. Different daughter <nuclear energy levels> can produce distinct alpha lines. Their line <energies> and populations provide information beyond simply counting the sample's total <radioactive activity>.
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