This is a classic article from 快猫短视频鈥檚 archive, republished as part of our 50th anniversary celebrations
AT THE last count, the periodic table contained 111 chemical elements. Its shadowy antimatter counterpart contains just one, now that physicists from Germany, Italy and Switzerland have for the first time created antihydrogen, the simplest antiatom.
鈥淭his is the first step in the antiperiodic system,鈥� says the team鈥檚 spokesman, Walter Oerlert of the Institute of Nuclear Physics at the National Research Centre in J眉lich, Germany. 鈥淚t鈥檚 really the proof that there is an antiworld.鈥�
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Although antiatoms are entirely new, physicists have been studying their constituent parts for years. Like conventional matter, antimatter is built from subatomic building blocks. Antiprotons, the negatively charged antimatter shadows of protons, can be made by smashing protons into one another at immense energies. And positrons, the positively charged counterparts of electrons, are emitted naturally during the radioactive decay of some unstable atomic nuclei. Antimatter is annihilated whenever it meets normal matter, releasing large amounts of energy.
Oerlert and his colleagues have now combined single antiprotons and positrons to make antiatoms of antihydrogen. They used a machine called the Low Energy Antiproton Ring (LEAR) at CERN, the European Centre for Particle Physics in Geneva. For a total of 15 hours in September and October last year, the physicists fired a jet of xenon atoms across LEAR鈥檚 antiproton beam. Collisions between antiprotons and protons in the xenon nuclei generated pairs of electrons and positrons. These positrons then combined with further antiprotons in the beam to make antihydrogen.
LEAR is shaped like a running track, with straight sections joined by bends. The researchers created antihydrogen in one of the straights, and placed detectors made of silicon at the beginning of the next bend. Because antiatoms have no charge, the antihydrogen was not pulled around the bend by LEAR鈥檚 powerful magnets, and instead careered into the detectors at nine-tenths the speed of light.
Inside the detectors, the antiatoms were stripped down once more into their component antiprotons and positrons, which left traces in the silicon. Each positron was also quickly annihilated by an electron, giving off two photons of gamma radiation as it disappeared. These photons shot out of the silicon in opposite directions, and were detected in crystals of sodium iodide held around the silicon.
This article was originally published in 快猫短视频 on 6 January 1996
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