TRAPPED!

by Dave Jacqué

In a quiet basement laboratory, in one brief moment, months of preparation for an intricate physics experiment came together in a blip of light on a computer screen — on the very first try. Surprised, physics doctoral student Li-Bang Wang literally fell out of his chair onto the floor.

“ Physics experiments rarely work on the first try,” Argonne physicist Zheng-Tian Lu (pictured at left) said, explaining the student's startled reaction. “It was amazing.”

What Wang, Lu and their colleagues in Argonne's Physics Division had done was to conduct the most precise measurement ever made of the charge radius — one aspect of the size — of the nucleus of helium-6.

The results are so precise 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 neutrons affect the structure and dynamics of nuclei and shed light on the structure of all neutron-rich systems, including neutron stars.

The helium-6 nucleus is a proving ground for nuclear physicists probing the complex, subtle atomic forces that shape the central core of every atom in the universe. Helium-6 is the simplest nucleus with a “halo”: two loosely bound neutrons in orbit around a compact core formed by two protons and two neutrons, also known as an alpha particle.

Measurements of helium-6 made in the 1980s and 1990s included some by Isao Tanahata, now a visiting scientist in Argonne's Physics Division. Physicists found that the nucleus of helium-6 is much larger than that of regular party-balloon helium (helium-4). However, they couldn't pin down the size of the helium-6 nucleus precisely enough to distinguish among various theoretical predictions.

The recent Argonne project involved a team of physicists and engineers from all four groups in the Physics Division. Led by Argonne's Ernst Rehm and Zheng-Tian Lu, the collaboration brought together the needed expertise in nuclear reaction, accelerator and atomic physics, as well as nuclear structure theory.

Wang, who received his Ph.D. early this year from the University of Illinois, believes collaborations like this illustrate a distinct advantage of working at a national laboratory versus a university, which may not have as many researchers dedicated to a single sub-discipline of physics — nuclear physics in this case.

Consequently, researchers in a national laboratory can conduct their experiments more quickly and more efficiently, he said.

Trapped

To create the helium-6, a beam of lithium-7 ions was aimed at a graphite target. Some of the lithium ions lost a proton, becoming helium-6. The helium-6 atoms were analyzed using Atom Trap Trace Analysis (ATTA), a technique developed and performed by the Physics Division's medium-energy physics group.

Lu and a team of researchers at Argonne developed Atom Trap Trace Analysis in 1999. Since then, refinements in equipment and procedures have improved the method's efficiency more than 1,000 times.

In the tabletop device, atoms of interest are selected and slowed to just 20 meters per second with lasers tuned to their resonant frequencies. Another set of lasers and magnetic fields halts the atoms and holds them in place. Cooled to near zero degrees, absolute temperature, an atom can be confined by laser light to within a cubic millimeter of space in the middle of a vacuum chamber. Wang and postdoctoral researcher Peter Mueller set up and operated the equipment to trap and detect the helium-6 nuclei.

Every atom has a characteristic resonant frequency. If you shine a laser beam with a wavelength of 389 nanometers at a helium atom, the atom will fluoresce as its electrons are excited, then release photons as they return to their normal state. This resonance is affected by the size of the nucleus, which affects the electrons' orbits. Although the frequency change due to this effect is just one megahertz, or one part in a billion, the ATTA setup can detect it.

While in the trap, the atom scatters millions of photons per second from the laser beams. A photon detector records the arrival and departure of individual atoms.

The charge radius of helium-6 was determined to be two fermis — two trillionths of a millimeter. The results of this research were published in Physical Review Letters in 2004 and were highlighted by the American Institute of Physics in its 2004 edition of Physics News.