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Lab Fano detector works to resolve mysteries of plutonium

Jim Tobin and colleagues are trying to solve a mystery that has plagued scientists for more than 65 years.

What is the electronic structure of plutonium (Pu), and how does electron correlation in Pu work?

Over the last 60 years, the mystery of Pu's electronic structure has been unraveled slowly. In a series of experiments and linked theoretical modeling, the range of possible solutions for plutonium's electronic structure has been reduced dramatically.

So far scientists have discovered three factors about the electronic structure: it contains only five electrons, it has a large spin orbit coupling and the 5f electron states are predominantly localized.

The next step is to figure out the electron correlation (the non-centro-symmetric interactions between two or more electrons). That’s where Tobin and his team, made up of Brandon Chung, Sung-Woo Yu and Tom Felter of the Materials Science and Technology Division, come in. "We needed to come up with a new technique, but it’s really, really hard," Tobin said. "We need to look at the spins but we need it to be dynamic. So how do you do that?"

Create your own custom instrument: the Fano spectrometer.

Not only did his team design and build it, but it has been tested on cerium and platinum. Using the Fano Effect (named after Ugo Fano, a 20th century leader in theoretical physics), chirally polarized X-rays are aimed at the sample. In the case of cerium, the data showed that the spins were dynamically coupled and reversed, proving the essence of the Kondo shielding model.

According to the Kondo shielding model, electrical resistance will diverge as the temperature approaches absolute zero. The team next plans to study the actinides and ultimately Pu.

In the past, Tobin’s team used spin spectroscopy to study electron correlation in non-magnetic materials (cerium) at the Advanced Photon Source at Argonne National Laboratory and the Advanced Light Source at Lawrence Berkeley. The team decided to extend the approach to other non-magnetic materials, including the actinides. But studying the correlation in Pu at off-site facilities proved to be difficult.

So they built the Fano spectrometer at the Lab, where Pu is more accessible than at other facilities.

While chemically toxic and highly radioactive, Pu may be the most scientifically interesting element in the periodic table. Its properties include the face-centered-cubic phase is the least dense; Pu expands when it solidifies from a melt; and it basically lies at the boundary between the light and heavy actinide elements.

Just in case the Fano experiment doesn’t resolve the Pu dilemma, the Fano spectrometer has another unique capability: BIS, which is short for Bremstrahlung Isochromat Spectroscopy. While the Fano experiment is directed at the occupied electronic strucure, the BIS experiment is aimed at determining the conduction electronic structure of Pu and other actinides.

There have been many experiments to determine the occupied electronic structure of Pu, but there is essentially no experimental data on the unoccupied (conduction band, above the Fermi energy) electronic structure of Pu. The Fermi energy is a concept in quantum mechanics usually referring to the energy of the highest occupied quantum state. The team determined that the BIS experiment is the best way to determine the unoccupied structure of the actinides. They were driven by an analysis of earlier experiments, where the same data exists for thorium and uranium. The BIS experiment is essentially the time reversal of photoelectron spectroscopy. While photoelectron spectroscopy sends a photon in and an electron out, the BIS sends an electron in and photon out. By analyzing the photons on the way out, the conduction band can be seen, Tobin said.

"There are a lot of theories to solve the electron correlation question, but in our two experiments, Fano and BIS, we’ll be able to differentiate the models based upon how the spins are dynamically coupled and the nature of the unoccupied states," he said.

April 4, 2008

Contact

Anne M. Stark
[email protected]