In earth and environmental sciences, radioactive isotopes, atom variants that decay over time, play a major role in age determination. A radioactive isotope of the inert gas argon 39, for example, is used to determine the age of water or ice. Such isotopes are extremely rare, however -- only a single 39 Ar isotope occurs in a thousand trillion argon atoms. Hence researchers' attempts to isolate and detect such atoms remain the proverbial search for the needle in a haystack. Physicists have now succeeded in rendering usable an experimental method developed in basic research for ground water dating using 39 Ar....