The concept of antimatter was so revolutionary that it had to wait for experimental confirmation before being accepted.

Antimatter is now used every day in medicine to analyze our brain with the Positron Emission Tomography technique (PET). It allows us to measure the metabolic functions and the biochemical reactions in the brain; it has many applications in neurosciences, oncology and cardiology.
For the PET, positrons come from the decay of radioactive nuclei, incorporated in a special fluid injected into a patient. The emitted positrons  annihilate with the electrons of the atoms in the brain and give rise to two gamma rays emitted in opposite directions. These are observed using special detectors which surround the head of the patient; they find the point where the annihilation took place: PET allows us to reconstruct the exact locations where the radioactive medicine was distributed.

Cosmic ray particles come from outer space, from all directions. They are mainly composed  of high energy protons and helium nuclei, and of some heavier nuclei. When they reach the top of the earth's atmosphere they interact with the nitrogen and oxygen  nuclei of the air, producing new particles and antiparticles, transforming their kinetic energy into the mass of the created particles. Cosmic rays are thus a source of some antiparticles and in 1932 Carl Anderson detected the first antiparticle: the positron.

Fig. 2: A cosmic ray interacts in the high atmosphere yielding a cascade of new particles and antiparticles. (Credit: INFN-Notizie N.6 April 2001 "Dai raggi cosmici all'universo")

Fig. 3: Interaction of a cosmic ray particle in the atmosphere. Click on the picture to see the animation. (If you don't see the animation, you need to install Flash Player [here]). (Credit: Davide Centomo, Dep. of Physics University of Bologna)