Fluorine-18 (${}^{18}\mathrm{F}$)

Fluorine-18 is a radioactive isotope of fluorine with 9 protons and 9 neutrons. It decays to oxygen-18, mainly by emitting a Positron and also by electron capture. Its approximately 110-minute half-life makes its radioactivity change appreciably over an ordinary laboratory session. It is used in positron emission tomography.^[ref]

The half-life can be measured by following the falling radioactivity, rather than by watching one nucleus wait to decay. A 2010 experiment used ionization chambers, germanium detectors and coincidence counting with fast scintillators, following decay for up to nine half-lives. The combined result was $1.82871(18)\mathrm{h}$. Comparing different detector systems tests whether the inferred decay rate depends on one measurement technique.^[ref]

For a closed sample governed only by radioactive decay, the surviving parent fraction is $N(t)/N(0)=2^{-t/T_{1/2}}$. Taking 110 minutes as an illustrative rounded half-life, 220 minutes leaves one quarter of the original fluorine-18 nuclei. Four half-lives leave $1/16$, not zero. This calculation concerns nuclear survival; movement of a labelled molecule into or out of an observed region is an additional process.