Ultrasensitive Analysis Technique Detects Single Atoms In Large Samples

Called Atom Trap Trace Analysis, or ATTA for short, the technology holds promise for advancing the state of the art in many fields, from solar-neutrino research to groundwater studies and environmental monitoring. This method was first announced in Science magazine on Nov. 5th, 1999.

The Argonne team -- including Kevin Bailey, Chun-Yen Chen, Xu Du, Yimin Li, Zheng-Tian Lu, Tom O'Connor, all of Argonne’s physics division, and Linda Young of Argonne’s chemistry division -- has been able to count 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.

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 its depth. Trace analysis of krypton-81, which lives 40 times longer than carbon-14, can be used to date samples up to one million years old.

Until this development, ultrasensitive techniques capable of detecting trace isotopes at the parts-per-trillion level have fallen into two categories. “Low-level counting,” pioneered by Nobel Laureate Willard Libby at the University of Chicago, essentially detects radiation from a sample as radioactive isotopes decay. This method requires lots of shielding from cosmic rays and other environmental activities that generate “noise” in the detectors, and in the case of long-lived isotopes, a lot of patience while the atoms decay.

In “accelerator mass spectrometry,” or AMS, atoms from a sample are sent through a particle accelerator, stripped of their electrons, and have their mass measured in a sensitive detector. Pioneering work on radio-krypton dating of groundwater with AMS was recently conducted by an international team including Argonne physicists Philippe Collon and Richard Pardo, both of Argonne’s physics division. However, this approach is difficult in practice because it requires access to large accelerator facilities.

In contrast, the ATTA technique uses a table-top laser to slow, trap and count atoms of interest. Atoms absorb light particles (photons), which excite their electrons to a higher energy level. 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. For example, 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 directions, and slow the atoms from a pace of a jet plane to a mere crawl within an arm's length.

In the ATTA device, a trickle of krypton gas enters a source and is directed into a one-meter-long tube. There the krypton atoms run into a laser beam shining at them head-on.

There is about a one part-per million difference in the “resonant frequencies” of the various krypton isotopes. “We can tune our laser to the resonant frequency of krypton-81,” said 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. 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.

Many 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 lead-205 to shed light on the “solar neutrino problem,” Lu said. Neutrinos are ghostly subatomic particles produced by thermonuclear reactions in the sun’s core. They only rarely interact with matter and most pass unhindered through the earth. Physicists using huge underground neutrino detectors are finding fewer particles than expected of the type predicted by current theories of how the sun produces energy.

Neutrinos produced by the sun will convert a small number of atoms in rocks to lead-205, an extremely long-lived isotope. Ancient, deeply buried rocks are shielded from to cosmic rays that would produce spurious results.

By measuring the amount of lead-205 in these ancient rocks, scientists may be able to get an accurate measure of the sun’s past activity. If more neutrinos were produced in the distant past, it may signal that the sun’s core is currently in a relatively dormant phase, accounting for the dearth of neutrinos.

Another potential application 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 just 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 minuscule 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 of krypton-85, some other fission-produced isotopes of strontium and cesium can be detected with ATTA at an unprecedented sensitivity.

The nation’s first national laboratory, Argonne National Laboratory supports basic and applied scientific research across a wide spectrum of disciplines, ranging from high-energy physics to climatology and biotechnology. Since 1990, Argonne has worked with more than 600 companies and numerous federal agencies and other organizations to help advance America's scientific leadership and prepare the nation for the future. Argonne is operated by the University of Chicago as part of the U.S. Department of Energy's national laboratory system.