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MIT researchers compare atomic masses with unprecedented accuracy

David Pritchard
David Pritchard
Photo / Donna Coveney

MIT atomic physicists have developed a technique that compares the masses of single charged atoms with unprecedented accuracy -- akin to measuring the distance between Boston and Los Angeles to within the width of a human hair.

The study, published in Science Express, reports the ratio of the masses of nitrogen and acetylene molecules with a precision below 1 part in 100 billion.

The work, led by David E. Pritchard, the Cecil and Ida Professor of Physics and a principal investigator in the MIT-Harvard Center for Ultracold Atoms, opens the door to numerous applications, including testing E=mc2 and weighing chemical bonds for weakly bound or very rare ionic species.

Pritchard, also affiliated with MIT's Research Laboratory for Electronics, and members of his group have been a leader in the field of high-precision mass spectrometry for more than 10 years. They have developed techniques to trap and detect a single charged atom, known as an ion, for more than a month at a time. They have used this method to publish the atomic masses of 13 different atoms ranging from hydrogen to cesium with an uncertainty of around 1 part in 10 billion.

Atomic mass is measured by comparing the rates at which different molecular ions orbit magnetic field lines in a magnetic trap. The precision of this widely used technique was limited by changes that occurred in the magnetic field during the minutes required to switch the two ions being compared. The MIT laboratory had its own special challenge: magnetic field variations caused by a nearby subway line. The group was forced to do all measurements between 1:30 and 5:30 a.m., when the subway and elevators in their building were shut down.

In these recent experiments, the Pritchard group for the first time put two ions in the trap at the same time. Previously, this generated problems when the two ions came too close together and generated bothersome electrostatic interactions. The researchers overcame this obstacle by placing the ions 1 mm apart in a common circular orbit. In this configuration, the ions in the trap are like a waltzing couple.

"They spin around on the dance floor, always a fixed distance from each other," said Simon Rainville, the first author of the paper and a postdoctoral fellow at Harvard. The researchers then took advantage of the coupled motion to monitor and control the trajectories of the ions in the trap.

The new technique, akin to using a weight-balanced scale like those once used for meat or produce, dramatically increases the precision with which atomic masses can be measured. And thanks to a new highly automated computer system, masses are measured in the MIT lab 24 hours a day.

The field has advanced significantly since the 19th century, when Italian chemist Amadeo Avogadro first observed that gases at the same temperature and pressure combined in definite volume ratios, and equal volumes of the gases had the same number of molecules. By weighing the volumes of gases, he could determine the ratios of their atomic masses.

In the early 20th century, Pritchard noted, mass comparisons of atomic species had a precision of around 1 part in 1,000, and when he started working in the field, the state of the art was a few parts in 100 million. Today, the precision has reached several parts in a trillion. "In a logarithmic sense, we've made nearly as much progress as in the entire previous history of mass spectrometry," he said.

In addition to Pritchard and Rainville, authors include James Thompson, a postdoctoral researcher at MIT.

This work is supported by the National Science Foundation.

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