Although the Universe is dominated by matter, antimatter still occurs naturally in cosmic rays from space and in the radioactive decay of certain isotopes, for example fluorine-18 and potassium-40. Isotopes are variants of a particular element that differ in the number of neutrons in their nucleus (the number of protons is the same). Isotopes can be produced in a variety of ways, including within nuclear reactors and using particle accelerators. In some cases the isotopes will decay by a process known as positive beta decay where one of the protons in the nucleus is converted to a neutron, and the positive charge is emitted in the form of an antielectron (positron). This positron then annihilates with an electron, creating a pair of photons (gamma rays) travelling in opposite directions. This behaviour is useful when designing medical or engineering applications that use antimatter.
Isotopes can be produced in a variety of ways, including within nuclear reactors and using particle accelerators. In the case of accelerators, beams of protons or electrons are fired at a target material that is subsequently converted into the isotope of choice.
The medical technique of positron emission tomography (PET) is used to diagnose the presence of small cancer tumours or of abnormal brain function. It does this by tagging certain biologically interesting molecules using radioactive isotopes whose nuclei have too few neutrons. For example, a form of glucose is tagged with fluorine-18 atoms and used to map energy metabolism within the body.
A PET scanner consists of many small detectors arranged in a ring. If two detectors simultaneously identify the two gamma rays from positron-electron annihilation then the isotope must have decayed along the line joining these two detectors. Combining millions of such lines builds up a map of glucose concentration in the body, reflecting the energy uptake of different tissues.
Positron emission particle tracking (PEPT) is another technique based on detecting pairs of photons from positron-electron annihilation, following positron emission in a radioactive decay. However instead of mapping the concentration of a radioactive fluid a single radioactively-labelled object is tracked to allow the motion of granular solids and fluids to be studied. The technique has been used to understand and improve devices such as dishwashers and aeroplane engines.