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Half matter, half antimatter, positronium atoms teeter on the brink of annihilation. Now there's a way to make these unstable atoms survive much longer, a key step towards making a powerful gamma-ray laser.

All the elements in the periodic table consist of atoms with a nucleus of positively charged protons, orbited by the same number of negatively charged electrons. Positronium, symbol Ps, is different. It consists of an electron and a positron orbiting each other (see diagram). A positron is the electron's antimatter counterpart. Though positively charged like the proton, it has just 0.0005 times its mass. Positronium "atoms" survive less than a millionth of a second before the electron and positron annihilate in a burst of gamma rays.

In principle, positronium could be used to make a gamma ray laser. It would produce a highly energetic beam of extremely short wavelength that could probe tiny structures including the atomic nucleus - the wavelength of visible light is much too long to be of any use for this.

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