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Newly Revamped SSRL Beamlines 4-1 and 4-3 Bringing Smiles to Users

(Photo - Beamline 4-1)
Beamline 4-1 is an X-ray Absorption Spectroscopy station, outfitted with dual ion chambers and a fluorescence detector. (Photo by Lauren Schenkman.)

Two completely rebuilt beamlines at the Stanford Synchrotron Radiation Lightsource hosted their first experiments in March. Beamlines 4-1 and 4-3 are both X-ray absorption spectroscopy, or XAS, stations that cover the range of the X-ray energy spectrum most commonly used with this technique, between 2 and 35 keV. X-ray absorption spectroscopy is a bread-and-butter technique at synchrotron facilities, and the new beamlines will greatly expand access to these high-demand experiments.

"The idea is to take the simpler experiments off of other beamlines that are highly oversubscribed, and put them on here," said Joe Rogers, the engineering physicist who leads the user support team on 4-1. "We can hopefully get a lot of users through here, and collect a lot of data."

(Photo - Matt Latimer)
Beamline scientist Matt Latimer configures the 4-3 experimental hutch for a new user. (Photo by Lauren Schenkman.)

The energy range of a beamline's X-rays determines what a user can study there. "The great thing about X-rays is that you can look at specific elements, because different elements absorb different energies," said University of British Columbia chemist Pierre Kennepohl, Beamline 4-3's first user. During X-ray absorption spectroscopy, an X-ray beam is aimed at a sample and tuned through a range of energies. As the beam passes through the sample, X-rays of a given energy will eject electrons from atoms of a particular element. The resulting pattern in the X-ray beam provides information about the atoms' electronic structure. The ejected electrons act like waves, scattering off local atoms and affecting the sample's absorption of higher-energy X-rays. This gives a picture of the atoms' neighborhood—what elements live nearby, and how far away they are.

Beamline 4-3 is unique in that many of its optics are in the same vacuum atmosphere as the SSRL's storage ring, allowing greater transmission of X-rays in the 2 to 5 keV energy range. This lets users selectively zoom in on elements such as sulfur, chlorine and calcium, which star in chemical reactions that are important in biology and environmental science. Kennepohl used XAS to see how sulfur bonds to other elements when it loses or gains electrons, processes called oxidation and reduction. "We're looking at everything within a huge molecule from the perspective of just sulfur," he said.

Sulfur goes back and forth between oxidized and reduced states in the body. Kennepohl probed the structure of an amino acid with a sulfur atom that has been oxidized a step further than normal. Scientists think sulfur oxidation works like an on–off switch in biochemical reactions, and it's possible that this extra oxidation step could trigger a cascade of reactions responsible for cataract formation, Alzheimer's disease or cancer.

Beamline 4-3's first user, University of British Columbia chemist Pierre Kennepohl, was left smiling after a successful run. (Photo by Lauren Schenkman.)

"Beamlines that can do these experiments are relatively rare," Kennepohl said. "There's a huge surge of interest lately in this energy range. It was very exciting for us to run here and get really nice data."

Until now, Kennepohl and other researchers wanting to do XAS using a 2 to 5 keV beam had to join the long waiting list for Beamline 6-2, which has a wiggler-produced beam that isn't ideal for this range.

"Beamline 6-2 has a more complicated background that makes it virtually impossible to measure the extended X-ray absorption fine structure, which means you could get information about the electronic structure, but not much about distances and identities of neighboring atoms, the local structural environment," explained Beamline 4-3 engineering physicist Matt Latimer. "Beamline 4-3 doesn't have this background problem, so the whole range of experiments [in the 2 to 5 keV range] really opens up. That's the most exciting new capability."

As more and more users collect data with the new beamlines, Latimer's and Rogers' groups will gather feedback to determine what new functions and detectors they will add to the current setup.

"During this phase the users are learning what the capabilities of the beamline are," Latimer said. "The in-hutch instrumentation will continue to evolve to support the particular experiments the users want to perform."

—Lauren Schenkman
SLAC Today, April 6, 2009