Physics: Tiny Tracks to Ancient Ages

When was the earth formed? What are the ancient milestones in man's development? In recent years, scientists have tackled such mysteries by means of radioactive dating. By comparing the amount of radioactive carbon 14 in a fossil with the amount contained in a living counterpart, for example, paleontologists determine when the fossil was part of a functioning organism. Using similar methods, scientists date meaningful objects as old as 3.5 billion years.

For all its startling applications, radioactive dating is inadequate for objects that are very small or contain only tiny traces of radioactive matter. Hence the significance of two recent dating techniques developed largely by Physicist Robert M. Walker of Washington University in St. Louis.

Enlarging the Evidence. Walker's most proven technique is based on the fact that most rocks and minerals contain a small impurity of uranium, which fissions (splits), leaving tiny scars or tracks inside the substance. Until recently, this phenomenon remained unobserved. Walker found that even with an electron microscope the fossil tracks were too tiny—.001 of an inch long and only ten atoms wide—to see in significant numbers.

To solve this problem, Walker and General Electric's Dr. P. Buford Price devised a method of enlarging the tracks by etching them with acid. As a result, they can now be seen under an ordinary light microscope. By counting the tracks, the age of material containing uranium can be measured. All it takes is comparison of the tracks in a sample with the amount of its uranium content, plus a complex equation.

Last year, Walker and another colleague discovered what may prove to be an even more sensitive dating method—the measurement of alpha-particle tracks. Uranium nuclei frequently emit an alpha particle. As the particle is expelled, the nucleus recoils. Walker reasoned that "recoil tracks would be there, so I looked for them." He discovered that observable recoil tracks occurred 4,000 times more than fission tracks. When acid etching and track counting are perfected for alpha particles, the method should provide a means of dating infinitesimally small objects and those too young to have accumulated measurable amounts of fission tracks.

Dating the Moon. Walker's methods have already produced important discoveries. By analyzing fission tracks, Russian scientists recently identified the 104th chemical element, named Khurchatorium, arid U.S. research scientists spotted the phenomenon of triple fission—the splitting of a nucleus into three roughly equal parts. General Electric scientists have irradiated thin strips of plastic, etched the fission tracks with acid, and produced a material of great potential medical significance—a sensitive sieve that duplicates the filtering capacity of human membranes.