In 1928 Paul AM Dirac postulated that a subatomic particle existed which was equivalent in mass to an electron but carried a positive charge. Carl Anderson experimentally observed these particles, which he called positrons, in cosmic ray research using cloud chambers in 1932. Both received Nobel Prizes in physics for their contributions. The positrons observed by Anderson were produced naturally in the upper atmosphere by the conversion of high-energy cosmic radiation into an electron-positron pair. Soon after this it was shown that when positrons interact with matter they give rise to two photons which, in general, are emitted simultaneously in almost exactly opposed directions. This sequence of events touches on many of the momentous developments in physics that occurred in the first 50 years of the twentieth century: radioactivity, Einstein's special relativity (energy-mass equivalence famously described by E = mc2), quantum mechanics, de Broglie's wave-particle duality, and the laws of conservation of physical properties.

Today we produce positron-emitting radionuclides under controlled laboratory conditions in particle accelerators in the hospital setting for use in positron emission tomography (PET). In this chapter we will examine the basic physics of radioactivity and positrons and their detection as it relates to PET.

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