Most Precise Measurement Ever Made Of Helium-6 Charge Radius

Borromean refers to the symbol of the medieval princes of Borromeo. Their symbol was a trio of rings that were intertwined in such a way that removing any of the rings caused the entire structure to fall apart. Physicists use the term to describe the behavior of some atomic nuclei, including helium-6, because if any one constituent is removed, the rest of the nucleus disintegrates.

The new measurements are so precise that they can be used to determine the accuracy of predictions made by a variety of nuclear structure theories. The data also offer new insight into how adding neutrons affects the structure and dynamics of nuclei and shed light on the structure of all neutron-rich systems, including neutron stars.

The goal of Argonne's Physics Division and this research, said Director Don Geesaman, “is to understand the properties of the nucleus at the center of every atom and how these properties affect the origin of matter and the operation of the universe.”

The helium-6 nucleus is a proving ground for nuclear physicists probing the complex, subtle forces that shape the central core of every atom in the universe because:

* Helium-6 is the simplest nucleus with a “halo” — two loosely bound neutrons in an orbit around a compact core formed by two protons and two neutrons, also known as an alpha particle.

* It is a sufficiently small system that accurate calculations, with techniques pioneered by Argonne theorists, can be compared to the experimental results, yet sufficiently large that new features of the forces come into play.

* The isotope lasts about one second — that's almost forever on the timescale of nuclear forces.

Measurements of helium-6 were made in the 1980s and 1990s, some by Isao Tanahata, now a visiting scientist in Argonne's Physics Division. From those measurements, physicists learned that the isotope's nucleus is much larger than that of helium-4 — regular party-balloon helium, which doesn't have the extra pair of neutrons in orbit around the alpha particle — although they couldn't pin down the size of the helium-6 nucleus precisely enough to distinguish among various theoretical predictions.

To experimentally determine the correct prediction required the combined resources of the Physics Division in another Borromean effort. The research was funded by the U.S. Department of Energy's Office of Nuclear Physics. Led by Ernst Rehm and Zheng-Tian Lu, the collaboration brought together the Physics Division's expertise in nuclear structure theory, nuclear reaction, accelerator and atomic physics.

“We do the world's best calculations of nuclear structures starting from the basic forces between neutrons and protons,” Geesaman said, “and with our accelerator, experimental equipment and creative researchers, we are able to make critical tests of the predictions of these calculations. Helium-6 is one example.”

The accelerator is the Argonne Tandem-Linac Accelerator System. ATLAS provides high-precision heavy-ion beams of all elements from hydrogen to uranium at energies as high at 17 million electron volts per nucleon, which is about 15 percent of the speed of light. Physicists from across the world use this DOE national collaborative research facility to probe the structure of the atomic nucleus by studying the gamma rays and particles emitted when ion beams smash into targets.

Trapped

The first step in conducting these high-precision measurements began with producing and extracting helium-6. The ATLAS group working with the heavy-ion group to optimize the helium-6 production, accelerated a beam of lithium-7 ions into a graphite (carbon) target. Some of the lithium ions lost a proton, becoming helium-6. From there, the helium-6 atoms were directed to an atom trap trace analysis facility in the ATLAS facility and probed with lasers. This atom-trapping technique was developed and performed by the medium-energy physics group.

The helium-6 atoms were trapped with a combination of magnetic fields and laser beams. Cooled to nearly absolute zero, an atom can be confined with laser light to within a cubic millimeter of space in the middle of a vacuum chamber. When illuminated by laser beams tuned to its resonant frequency, a helium atom absorbs and re-radiates the light at a rate that depends on the state of the atom's electrons. The nucleus' size has an effect on electron orbits, causing a very small shift in frequency — about one part per billion.

In one brief moment, several years of preparation came together in a blip of light on a computer screen — on the very first attempt. Surprised, Li-Bang Wang, a Ph.D. student from the University of Illinois at Urbana-Champaign, literally fell out of his chair onto the floor.

“Physics experiments rarely work on the first try,” Lu said. “It was amazing.”

The charge radius was determined to be two fermis — two-trillionths of a millimeter. The results of this research were published in Physical Review Letters on Oct. 1, 2004. The measurement will be a key benchmark for all future few-body nuclear structure theories.

The Physics Division's nuclear theory group is using this data to predict the size of the helium-6 nucleus using two different methods. These measurements require hundreds of hours using the JAZZ supercomputer in Argonne's Mathematics and Computer Science Division.

“Determining the charge radius of helium-6 is a small but important step toward reaching the goal of finding the way to describe the force that binds nuclei together,” Lu said.

“In the last century, we learned to understand atomic structure very well,” Lu said. “We can calculate atoms to exceedingly high precision using quantum mechanics and electromagnetic theory. But our understanding of nuclear forces is still growing. And I hope that in this century, we'll be able to solve this part of the mystery.”

Next steps

The next step for the collaboration is to tackle helium-8, which is the most neutron-rich matter that can be made on Earth.