Animation showing the use of a cyclotron to produce the medical radioisotope oxygen-15 (15O). A cyclotron is a type of particle accelerator. A charged particle, here a deuterium nucleus (2H), is injected at the centre of two semicircular electrodes called dees. A magnetic field (arrows) is established perpendicular to the plane of the dees. This causes the particle to move in a circular path. The voltage across the dees is reversed each time the particle is in one of the dees, constantly accelerating it towards the other. As its time in each dee is constant, the larger its path the faster it moves, and the particle spirals outwards. When it reaches the desired speed, it exits the cyclotron. To produce the 15O, the deuterium is aimed at a target of nitrogen-14 (14N). The deuterium collides with the 14N nucleus, ejecting a neutron (yellow), and forming the 15O. 15O decays quickly, emitting a positron. This feature is used in positron emission tomography (PET) scans. With 15O incorporated into biologically active molecules, the scan shows the metabolic activity of parts of the body.