Laser Used for Sensitive Trace Analysis

Michael D. Wheeler

Researchers at Argonne National Laboratory have demonstrated an ultrasensitive trace analysis technique based on magneto-optical trapping that can measure the isotopic ratio of atmospheric krypton gas. Atom trap trace analysis, described in the Nov. 5, 1999, issue of Science, has potential in archaeological dating, environmental monitoring and solar neutrino research.

In experiments, Zheng-Tian Lu and his colleagues inserted a sample of krypton gas into a 1-m-long tube, where the krypton atoms ran head-on into a beam of laser light generated by an 899 Ti:sapphire ring laser pumped by a Sabre argon-ion laser from Coherent Inc. of Santa Clara, Calif. At the end of the tube lies the trap, where six laser beams -- one from each horizontal direction and from above and below -- hold the atoms in place. Atoms can be kept in the center of a trap for many seconds -- an eternity in a science that measures time in billionths of a second.

While in the trap, a krypton atom scatters tens of millions of photons per second from the laser beams and appears as a bright dot. An avalanche photodiode records the arrival and departure of individual atoms.

Free of contamination

What makes this method unique is its potential advantages over the other techniques that are capable of detecting trace isotopes at the parts-per-trillion level: low-level counting and accelerator mass spectrometry. Unlike these other forms of trace analysis, atom trap trace analysis does not require pure samples or isolation from environmental noise.

Potential applications abound for the atom trap trace technique.

Low-level counting -- known to many in the form of carbon-14 dating -- detects radiation from a sample as radioactive isotopes decay, but this demands shielding from cosmic rays and other phenomena that generate noise in the detectors. In accelerator mass spectrometry, atoms from a sample are sent through a particle accelerator and stripped of their electrons, and a sensitive detector measures their mass. However, the large accelerator facilities required make this process expensive and impractical for many applications.

"In the game of ultrasensitive trace analysis, contamination is the killer," said Lu. "Accelerator mass spectrometry, which uses a mass analyzer, suffers from isobar contamination caused by atoms or molecules that have the same mass number. Low-level counting is prone to all kinds of internal or external radioactive background."

According to Lu, his team's work could open up a range of applications, including monitoring bone-loss rates in the diagnosis and assessment of osteoporosis, and monitoring nuclear reactor safety and environmental contamination. In the near term, the researchers will continue to focus on improving the technique using krypton, but they could look to other tracers in the future.