REY MATTER MADE VISIBLE WITH INVISIBLE ANTIMATTER


Positron emission tomography


Particle physicists regularly use collisions between electrons and their antiparticles, positrons, to investigate the constituents of matter and the fundamental forces at high energies. When an electron and a positron meet, they annihilate to produce energy. In high energy particle accelerators the energy can rematerialise as new particles and antiparticles. This is what happens at the new PEP-II collider (the B Factory) at the Stanford Linear Accelerator Laboratory (SLAC) in California.

At low energies, however, the electron-positron annihilations can be put to different uses, for example to reveal the workings of the brain in the technique called Positron Emission Tomography (PET).

A PET scan of the brain of BABAR physicist Adrian McKemey (courtesy of the Medical Research Council).

In PET, the positrons come from the decay of radioactive nuclei incorporated in a special fluid injected into the patient. The positrons then annihilate with electrons in nearby atoms. As the electron and positron are almost at rest when they annihilate there is not enough annihilation energy to make even the lightest particle and antiparticle (the electron and positron), so the energy emerges as two gamma-rays which shoot off in opposite directions to conserve momentum. The gamma rays can be detected and used to build up an image of the organ being investigated.