Move over carbon-14 dating–Argonne develops an ultrasensitive trace analysis technique

A new ultrasensitive trace analysis technique — able to detect single atoms in a large sample — has been developed by Argonne researchers.

Called Atom Trap Trace Analysis, or ATTA for short, the technology holds promise for advancing the state of the art in many fields, from groundwater studies to environmental monitoring.

One of the first applications for ATTA may be dating ancient Greenland ice cores. While carbon-14 dating has been used to date ice up to about 50,000 years old, ages of older samples have to be inferred from circumstantial evidence, such as their depth. Trace analysis of krypton-81, which lasts 40 times longer than carbon-14, can be used to date samples up to 1 million years old.

An Argonne team has counted individual atoms of krypton-85 and krypton-81 isotopes in a sample of natural krypton gas. Krypton-85 atoms have one more, and krypton-81 three fewer, neutrons than the common krypton-84 atoms. The two isotopes are extremely rare: in a natural sample of the gas, every krypton-81 atom is mixed in with a trillion krypton-84 atoms that are chemically similar.

Until this development, ultrasensitive techniques capable of detecting trace isotopes at the parts-per-trillion level required either a particle accelerator or detectors that had to be shielded from cosmic rays and other "noise."

In contrast, the ATTA technique uses a table-top laser to slow, trap and count atoms of interest. In the ATTA device, a trickle of krypton gas enters a source and is directed into a 1-meter-long tube. There the krypton atoms run into a laser beam shining at them head-on, tuned to their "resonant frequency."

Atoms can be compared to springs, with the positively charged nucleus at one end and the negatively charged electrons at the other. If the oscillating electric field of photons in a laser beam matches the frequency of this "spring," electrons will be excited to higher energy levels, just like well-timed pushes make a schoolyard swing go higher.

The atoms then emit photons as the electrons drop back to their usual places. Following Newton’s third law, the atom gets a kick when a photon is absorbed or emitted — a krypton atom changes its velocity by about 6 millimeters per second with each kick. This absorption-emission process occurs millions of times per second, so with a modest laser beam, researchers can push atoms in any chosen direction, and slow the atoms from a pace of a jet plane to a mere crawl within an arm’s length.

There is about a one part-per million difference in the "resonant frequencies" of the various isotopes of krypton. "We can tune our laser to the resonant frequency of krypton-81," said Argonne physicist Zheng-Tian Lu. "When we do that, we only trap krypton-81."

At the end of the tube lies the trap, where six laser beams — one from each horizontal direction, plus 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 usually measures times in billionths of a second.

While in the trap, a krypton atom scatters millions of photons per second from the laser beams, and appears as a bright dot, visible to the naked eye. A photon detector records the arrival and departure of individual atoms. In the current experiment, one krypton-81 atom is detected every 15 minutes or so.

POTENTIAL APPLICATIONS
Although the Argonne team has only worked with krypton atoms so far, ATTA should apply to some other isotopes for applications in a range of scientific disciplines.

One possibility is using calcium-41 as a medical tracer to monitor bone loss from osteoporosis. With the powerful detection abilities of ATTA, a patient can ingest a small, non-harmful amount of the isotope, which would be taken up into the bones. Over the following years, ATTA could be used to detect the miniscule amounts of calcium-41 in urine samples, which signals the loss of calcium from bones. This method would give medical doctors a powerful new tool to diagnose and test treatments for the condition.

ATTA may also find use as an extremely sensitive leak sensor and environmental monitor at U.S. Department of Energy "legacy waste" cleanup projects. Besides krypton-85, extremely small concentrations of fission-produced strontium and cesium can be detected with ATTA.

For more information on the ATTA technique, see the Web site at: www-mep.phy.anl.gov:80/atta/main.htm.

For more information please contact Dave Jacqué