CHESS has integrated a new operating mode that empowers researchers to capture processes on the nanosecond to microsecond timescale using 100 picosecond pulses of X-rays. At this scale, researchers can investigate swift processes in their experiments, such as crack formations racing across a material or proteins folding and unfolding – minute actions occurring in the blink of an eye.
In standard experiments investigating millisecond-scale or slower phenomena, X-rays from the storage ring can be thought of as a steady beam of light. But ‘under the hood,’ positrons race around the storage ring in discrete bunches. For years, CHESS has operated with relatively small positron bunches separated by 14 nanoseconds or less.
Now, in the newly-commissioned “timing mode”, more positrons are packed into a smaller number of bunches, generating rapid, intense bursts of x-rays that can be isolated by cutting-edge modern detector technology.
These quick bursts of light open up a world of possibilities for conducting time-resolved experiments with unparalleled precision and accuracy, and empowering researchers to investigate ultrafast phenomena such as chemical reactions, material phase transitions, and biological processes, with exquisite timing resolution.
Kate Shanks, a staff scientist at FAST – the CHESS beamline where the bursts of X-rays are used – compares the timing mode technique to photographing a moving object. “The optical analogy would be photographing a bouncing ball with a long exposure using continuous light versus taking snapshots with short bursts of light,” says Shanks. “This process of timing mode accesses a different level of temporal information.”
Developing alongside CHESS users
One research group has taken advantage of this technique at CHESS while also helping to shape the process along the way, culminating in a six-plus year collaboration with CHESS scientists, accelerator scientists, and the Cornell detector group.
The research group, led by Todd Hufnagel at the Department of Materials Science and Engineering at Johns Hopkins, has been crucial to bringing this technique to CHESS, organizing workshops, and discussing potential experiments that leverage CHESS’s advancements since CHESS-U.
“Many of the phenomena we are interested in happen very quickly,” says Hufnagel. “For example, the leading edge of a crack propagating in a brittle material can be moving at a speed of 1,000 meters per second or more.”
Hufnagel explains that capturing these fast phenomena is difficult, due in part to the small field of view in a typical dynamic x-ray imaging experiment. “A crack tip crosses a one-millimeter field of view in about one microsecond,” he says. “We need to be able to take several images in that time to watch the crack as it advances. Otherwise, the exposure would result in a blurry image.”
Shanks adds that CHESS is primed to be a premier light source for timing mode experiments, citing the close relationship with the detector group at Cornell.
“CHESS not only provides the rapid bunches of X-rays, but we are also developing the detectors that are needed for these experiments,” says Kate Shanks. “When the beamlines are equipped with the right kind of detectors researchers can do dynamic timing experiments that capture the ‘bouncing ball’, taking a crisp photo every 280 nanoseconds.”
Kate Shanks explains that timing mode allows researchers to readily use the existing cutting-edge detectors and is excited about the future of detector development at CHESS and Cornell. “The CHESS/Cornell detector team is one of a few groups in the world that do this type of work,” says Shanks.
Leveraging Cornell’s expertise
The first hybrid X-ray Pixel Array Detector (PAD) used for synchrotron science experiments was fabricated by the CHESS/Cornell detector team and tested at CHESS in 1999. It was used to study the dynamics of fuel injectors and shock waves. In the ensuing years the Cornell detector group has continued to develop increasingly capable PADs, including the Cornell-SLAC PAD, which became the workhorse imaging detector at the world’s first hard X-ray free electron laser at SLAC. Today, PADs are the routine X-ray imaging devices at synchrotron radiation facilities around the world.
The work continues at Cornell. The Cornell detector group, led by Julia Thom-Levy and Sol Gruner, were recently granted an award to develop a new detector for single-bunch experiments at the Advanced Photon Source, APS. “The new timing mode at CHESS is allowing us to test early prototypes of these new detectors right here at Cornell” says Julia Thom-Levy, Professor of Physics.
Even with today’s current state-of-the-art detectors, Shanks explains, it is still difficult to get enough signal from the short pulses of X-rays. “Even with a single-bunch short snapshot, it is difficult to get enough signal of those images. When you can’t get enough charge inside the single bunch, you lose intensity of the images.”
This is exactly what the new 9x1 timing mode was designed to overcome – by operating the CESR storage ring with nine “large” bunches, each of which produces an intense pulse of X-rays at the experimental stations.
"Timing mode keeps the same amount of charge within CESR and places it into single bunches” says Jim Shanks, CESR accelerator scientist. “This creates five times as many photons in each pulse while eliminating the ghosting [blurriness] effect of a longer exposure.”
Jim Shanks credits this new technique to the forward thinking at CHESS, particularly during the CHESS-U upgrade, when new capabilities - such as timing mode - were still being explored. “This collaboration with the Hufnagel group is what finally made it happen,” says Shanks.
“It is essential to have support from the users for these types of experiments, and to develop these techniques. This first demonstration experiment, from a user that understands the nuances of the whole setup, can now go anywhere. It is a team approach from the accelerator side to the staff scientists, detector group, and the CHESS users.
“This is the first time a user group has been able to do a timing experiment here at CHESS. I am hoping that by groups such as Hufnagel's submitting additional requests for timing mode capabilities at CHESS, we will have satisfied customers, and that we have something to offer these types of users.”