The Filled-Out Universe

Like philatelists filling the last empty space in a series of cherished stamps, physicists have now found the last subatomic particle that is needed to make the universe neatly and electrically symmetrical. The Radiation Laboratory of the University of California announced last week that a team of physicists (Drs. Bruce Cork, Glen Lambertson, Oreste Piccioni, William Wenzel) has identified the antineutron, which differs from ordinary neutrons in the opposite direction of its magnetic field.

Like the identification nearly a year ago of the antiproton (TIME, Oct. 31), the work was done with the Berkeley Bevatron, the world's most powerful particle accelerator, and a long train of auxiliary apparatus. The Bevatron's beam of 6.2 billion-volt protons was shot into a beryllium target. Out of the target came a secondary beam of assorted atomic debris. The particles with a negative charge, separated from the rest by the Bevatron's strong magnetic field, were mostly mesons. Among them were a few antiprotons (negative protons) formed when the Bevatron's powerful projectiles smashed an atomic nucleus.

Most of these antiprotons were "annihilated" (turned into energy) when they hit an ordinary positive proton. But occasionally, when an antiproton passed close to an ordinary proton, it merely handed over its negative electric charge. The proton, its positive charge neutralized by a negative one, became an ordinary, chargeless neutron. The antiproton, having lost its negative charge and received nothing in return, also became a chargeless particle, but it did not become a normal neutron. Since its basic "anti-ness" was not changed by the loss of its charge, it became an antineutron with a reversed magnetic field. If an antineutron hits a neutron, both turn into energy.

All this action was predicted by theory. The Berkeley scientists, by carefully screening out of the beam all antiprotons and gamma rays, proved that it actually happens. The surviving mesons, neutrons and anti-neutrons were allowed to pass into a counting device which measures flashes of energy released by each entering particle. The ordinary neutrons gave small flashes. The mesons gave flashes about twice as strong. Occasional flashes 20 times as strong (2 billion volts) could be only the result of the mutual annihilation of a neutron and an antineutron.

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