Inside the "brain" of a BaBar physicist

30 September 1998

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Particle physicists regularly use collisions between electrons and their antiparticles, positrons, to investigate matter and fundamental forces at high energies. When electron and positron meet, they annihilate, turning into energy which at high energies can rematerialise as new particles and antiparticles. This is what happens at machines such as the Large Electron Positron Collider (LEP) at CERN, and the new PEP-II collider 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). 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.

(This image is of the brain of one of the physicists working with high-energy electron-positron collisions at the BaBar detector at PEP-II.)

Credit: Adrian McKemey (Brunel University)/MRC

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