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==Key Stage 4==
===Meaning===
[[File:HalfLifeGraph1.png|right|300px|thumb|A [[graph]] showing the [[fraction]] of [[Unstable Isotope|unstable isotope]] left compared to the '''half life'''.]]
A '''half life''' is the time it takes for half of the [[Unstable Isotope|unstable isotopes]] in a [[radioactive]] sample to [[Radioactive Decay|decay]].
: The '''half life''' of an [[isotope]] depends on how [[Stable Isotope|stable]] the [[isotope]] is.
: The '''half life''' of an [[isotope]] can be determined by measuring the [[emission]] of [[Ionising Radiation|ionising radiation]]. The time it takes for the [[Count Rate|count rate]] of [[Ionising Radiation|ionising radiation]] to halve, is the same as its '''half life'''.
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|[[File:UnstableIsotopeDecay.png|center|600px]]
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| style="height:20px; width:200px; text-align:left;" |This [[diagram]] shows the number of [[Fermium]]-252 [[atom]]s halves each day, so the '''half life''' is 1 day.
|}
{| class="wikitable"