High-brightness X-rays from synchrotron light sources like the Advanced Photon Source (APS) at the U.S. Department of Energy’s (DOE) Argonne National Laboratory are an invaluable tool to study the structure and function of materials, from proteins that cause disease to metals that store electricity in batteries. In many cases, a smaller, brighter beam can open up a new world of scientific discovery.
That’s why teams of scientists and engineers at the APS, a DOE Office of Science User Facility, are planning for the APS Upgrade, which will deliver X-rays that are 100 times brighter than those offered by today’s APS.
“It’s really exciting to be thinking about new equipment that will allow transformational science taking full advantage of the revolutionary properties of the APS Upgrade, …” — Stefan Vogt, Associate Division Director for Beamline Operations in the X-ray Science division
Long-lead procurements for technical components for the APS Upgrade are now underway, and Argonne is also focusing on the scientific and engineering challenges that arise from the capabilities the new machine will provide, in the form of X-ray beams 100 billion times brighter than a typical X-ray machine from a doctor’s office.
To address these challenges and opportunities, last year Argonne commissioned the “Velociprobe,” a new scanning tool to explore the limits of fast, high-resolution X-ray microscopy. The instrument, which will be used at the APS before the Upgrade is completed, was built under the Laboratory Directed Research and Development program.
“The APS beams will be so much brighter that, if we were to try to use the scanning tools that are out there today, we’d have to attenuate or lessen the brightness of the beam, which defeats the point,” said the Velociprobe designer, mechanical engineer Curt Preissner of the Argonne X-ray Science division.
How it works
The Velociprobe instrument is designed with five core elements: large granite blocks, a movable lens, a sample of interest, a laser system and a detector.
The lens focuses incoming X-rays on a sample, which can be biological or nonbiological. The beam will scatter as it passes through the sample, creating a diffraction pattern that will then be captured by the detector. All the while, optical lasers will transmit data about the relative position of the lens and the sample. The granite blocks provide thermal and mechanical stability, to increase the accuracy of the measurements.
Then, researchers will use sophisticated software algorithms to analyze the diffraction data and positional information to reconstruct the 2-D structure of the sample, overcoming resolution limitations traditionally imposed by imaging optics.
With components that provide better stability, speed and more precise measurements, the Velociprobe is designed to achieve a higher spatial resolution at high speed than similar instruments. With current conventional methods, spatial resolution, which describes how well one can distinguish two distinct points that are relatively close together, is limited by the size of the incoming X-ray beam on the sample.
“This means that if you have a 1-micron beam, the best resolution you can get is 1 micron. So if you have a sample with features smaller than 1 micron, you won’t be able to resolve them,” said Junjing Deng, an APS physicist involved in the project. “The Velociprobe will not have this limitation.”
The Velociprobe is capable of scanning samples faster than other hard X-ray scanning tools because the lens it uses is designed to move back and forth rapidly, causing the X-ray beam to “walk” across specific regions of the sample. The process, known as fly-scanning, is faster than step-scanning, the traditional approach used with other scanning instruments. In the step-scanning method, the microscope works in a move-settle-measure sequence at each scan point, so a lot of time is wasted in the motor motion and settling.
“Because the lens has a lower mass than the sample, moving the lens rather than the sample lets you move much more quickly, which is what helps to enhance the speed of scanning,” said Chris Roehrig, an electronic engineer in the X-ray Science division who leads the project. “And while you’re moving the beam all the way across the sample in one motion, you have electronics that are continually acquiring data in the detector.”
With traditional methods, the time it takes to scan large sample areas or scan many different angles of a sample can be hours or even days. But the Velociprobe can scan an area much faster than other hard X-ray instruments. The initial goal of the project was to scan a 1-micrometer by 1-micrometer area in less than 10 seconds. The Velociprobe performance far exceeded the goal, scanning a 4-micrometer by 4-micrometer area in 2.1 seconds, which was 75 times faster than the goal.
The Velociprobe will have multidisciplinary applications, from biology to chemistry to energy storage materials.
Later this year, researchers plan to use the Velociprobe to examine the insides of computer microchips, to look for differences between design specifications and actual production pieces. This information could potentially be used to detect long-term reliability problems, or possible unexpected circuitry.
“In general, these chips are relatively large and thick and, with today’s capabilities, could take weeks or even months to scan, depending on the size,” Roehrig said. “We’re trying to speed things up quite a bit with these projects by looking ahead to the APS Upgrade and developing instrumentation that can allow us to take advantage of that Upgrade to get scanning speeds way down to reasonable levels.”
In fact, the team is now rebuilding the Velociprobe to further improve its speed for scanning “large” samples such as computer chips. And going forward, the ideas and ingenuity behind the Velociprobe will be used to directly feed in to develop even better scanning devices and complementary tools, said Stefan Vogt, principal investigator on the project and Associate Division Director for Beamline Operations in the X-ray Science division.
“It’s really exciting to be thinking about new equipment that will allow transformational science taking full advantage of the revolutionary properties of the APS Upgrade, and the Velociprobe is one of the first excellent examples of this new equipment,” Vogt said. “Of course, there is the added advantage that even though this was designed with the Upgrade in mind, we will be able to use it now, before the Upgrade, to drive scientific results.”