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==Key Stage 4==
===Meaning===
A '''half life''' is the time it takes for half of the [[Unstable Isotope|unstable isotopes]] in a [[radioactive]] sample to [[Radioactive Decay|decay]].
===About Half Life===
: Every [[isotope]] has a unique '''half life'''.
: 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 '''half life'''.
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