The atomic mass of fluorine-18 is 18.00 atomic mass units. Scientists create fluorine-18 using a oxygen in a particle accelerator. It does not exist in nature and has a half-life of approximately 20 minutes, degrading to form oxygen-18 by emitting a positron.
The medical field uses fluorine-18 in performing positron-emission-tomography scans of the human body. PET scans give doctors a three-dimensional image of structures in the body. One of its applications is in determining if a tumor has metastasized. The relatively short half-life of fluorine-18 minimizes the radiation dosage required for the patient, but it increases the need to recalibrate the machine to account for the radioactive decay.