ATOM TRAP TRACE ANALYSIS, the search for tiny isotope fractions among atoms using a magneto-optic trap, may soon be preferable to accelerator mass spectrometry (in which atoms are heated, accelerated, and sent through a strong magnet, which sorts the atoms by mass) for certain radio-dating purposes. To demonstrate this idea, physicists at Argonne (Zheng-Tian Lu, 630-252-0583, lu@anl.gov) have detected traces of krypton-85 (with an abundance of only 10^-11) and krypton-81 (abundance of 10^-13) in an atom trap with an efficiency of 1 part in 10⁷; accelerator mass spectrometry, which requires an accelerator, currently has a counting efficiency of a part in 10⁵. Keeping track of Kr-85 atoms is important since they are produced chiefly in nuclear-fuel reprocessing plants, and (arising mostly since the 1950s) are used as a tracer of air and ocean currents. Kr-81, in contrast, is made in cosmic-ray showers in the upper atmosphere and (with a half life 40 times longer than C-14's) is preferable to carbon-dating for calibrating the antiquity of million-year-old samples of ice and ground water. (Chen et al., Science, 5 November 1999.)