Number 232 (Story #1), July 6, 1995 by Phillip F. Schewe and Ben Stein ONE STEP CLOSER TO PRODUCING ANTI-ATOMS , Gerald Gabrielse and his colleagues at Harvard have devised a method for trapping and cooling positrons, the antimatter version of electrons, in the proper environment needed for producing anti-hydrogen, an atom consisting of an antiproton orbited by a positron. Over the past decade, Gabrielse (617-495-4381) and his colleagues have honed techniques for trapping and cooling antiprotons in a tabletop device connected to an antiproton source at CERN in Geneva. Combining antiprotons and positrons to make the world's first anti-atom, a major pursuit in physics, will not be a trivial feat. The trick is to get them to slow down enough so that the positron is gently ensnared by the antiproton. To do this, you need to cool each type of antiparticle and store them in separate traps. The traps must exist in an environment as free as possible from background gas atoms, so that the antiparticles are not knocked out of the traps. The researchers have now stored approximately 35,000 positrons in an ultra-high vacuum environment having a pressure estimated to be better than 5x10**-17 torr, corresponding to fewer than 100 background gas atoms per cubic centimeter. Released from the radioactive decay of a sodium-22 source, 3 million positrons per second strike a tungsten crystal, which acts as a "moderator" to slow down the particles. Some of these positrons then enter a Penning trap, a device which uses a combination of electric and magnetic fields to prevent sluggish positrons from escaping. The circulating particles induce a current in an electrical circuit, which further reduces the positrons' energy. At a temperature of 4.2 K, the trapped positrons are an ultracold plasma, a frigid collection of charged particles. Incidentally, this cold bath of positrons can be used to significantly cool down highly charged ions, such as the recently produced bare uranium ion, perhaps the most highly reactive form of matter in nature. Based on studies of how electrons and protons combine to form hydrogen atoms, Gabrielse believes that having 10-100 times the present number of trapped positrons is desirable for successful anti-hydrogen production. The necessary increase may come through the use of a more efficient crystal moderator and a more intense radioactive source. The researchers plan to refine their device at Harvard and test it at CERN as early as next summer. In future attempts to produce anti-hydrogen, the researchers would probably combine the positron and anti-proton traps into a single device. (L.H. Haarsma et al., 31 July 95, Phys Rev. Lett. Reporters: For a preprint of the article and some accompanying graphics, please contact AIP Public Information at physnews@aip.org or 301-209-3091).