News | X-Ray | October 27, 2016

The Weird Case of the X-Ray That Came Out Blank

A ‘nonlinear’ effect that seemingly turns materials transparent is seen for the first time in X-rays at SLAC’s LCLS

SLAC, blank X-ray, Joachim Stohr

An illustration shows what happens in a typical experiment with SLAC’s LCLS X-ray laser, top, versus what happened in this study with an especially intense X-ray pulse. Normally the X-ray pulses -- which are shown coming in from the right -- scatter off electrons in a sample and produce a pattern in a detector. But when researchers cranked up the intensity of the X-ray pulses, the pulses seemed to go straight through the sample, as if it were not there, and the pattern in the detector vanished. Two recent papers describe and explain this surprising result, which is due to a ‘nonlinear’ effect where particles of X-ray light team up to cause unexpected things to happen. Image courtesy of SLAC National Accelerator Laboratory.

October 27, 2016 — Imagine getting a medical X-ray that comes out blank – as if your bones had vanished. That’s what happened when scientists cranked up the intensity of the world’s first X-ray laser, at the Department of Energy’s SLAC National Accelerator Laboratory, to get a better look at a sample they were studying: The X-rays seemed to go right through it as if it were not there.

This result was so weird that the leader of the experiment, SLAC Prof. Joachim Stöhr, devoted the next three years to developing a theory that explains why it happened. Now his team has published a paper in Physical Review Letters describing the 2012 experiment for the first time.

What they saw was a so-called nonlinear effect where more than one photon, or particle of X-ray light, enters a sample at the same time, and they team up to cause unexpected things to happen.

“In this case, the X-rays wiggled electrons in the sample and made them emit a new beam of X-rays that was identical to the one that went in,” said Stöhr, who is an investigator with the Stanford Institute for Materials and Energy Sciences at SLAC. “It continued along the same path and hit a detector. So from the outside, it looked like a single beam went straight through and the sample was completely transparent.”

This effect, called “stimulated scattering,” had never been seen in X-rays before. In fact, it took an extremely intense beam from SLAC’s Linac Coherent Light Source (LCLS), which is a billion times brighter than any X-ray source before it, to make this happen.

The observation is a milestone in the quest to understand how light interacts with matter, Stöhr said.

“What will we do with it? I think we’re just starting to learn. This is a new phenomenon and I don’t want to speculate,” he said. “But it opens the door to controlling the electrons that are closest to the core of atoms ­– boosting them into higher orbitals, and driving them back down in a very controlled manner, and doing this over and over again.”

Nonlinear optical effects are nothing new. They were discovered in the1960s with the invention of the laser – the first source of light so bright that it could send more than one photon into a sample at a time, triggering responses that seemed all out of proportion to the amount of light energy going in. Scientists use these effects to shift laser light to much higher energies and focus optical microscopes on much smaller objects than anyone had thought possible.

The 2009 opening of LCLS as a DOE Office of Science User Facility introduced another fundamentally new tool, the X-ray free-electron laser, and scientists have spent a lot of time since then figuring out exactly what it can do. For instance, a SLAC-led team recently published the first report of nonlinear effects produced by its brilliant pulses.

“The X-ray laser is really a quantum leap, the equivalent of going from a light bulb to an optical laser,” Stöhr said. “So it’s not just that you have more X-rays. The interaction of the X-rays with the sample is very different, and there are effects you could never see at other types of X-ray light sources.”

Stöhr stumbled on this latest discovery by accident. Then director of LCLS, he was working with Andreas Scherz, a SLAC staff scientist, who is now with the soon-to-open European XFEL in Hamburg, Germany, and Stanford graduate student Benny Wu to look at the fine structure of a common magnetic material used in data storage.

To enhance the contrast of their image, they tuned the LCLS beam to a wavelength that would resonate with cobalt atoms in the sample and amplify the signal in their detector. The initial results looked great. So they turned up the intensity of the laser beam in the hope of making the images even sharper.

That’s when the speckled pattern they’d been seeing in their detector went blank, as if the sample had disappeared.

“We thought maybe we had missed the sample, so we checked the alignment and tried again,” Stöhr said. “But it kept happening. We knew this was strange – that there was something here that needed to be understood.”

Stöhr is an experimentalist, not a theorist, but he was determined to find answers. He and Scherz dove deeply into the scientific literature. Meanwhile Wu finished his Ph.D. thesis, which described the experiment and its unexpected result, and went on to a job in industry. But the team held off on publishing their experimental results in a scientific journal until they could explain what happened.

“We are developing a whole new field of nonlinear X-ray science, and our study is just one building block in this field,” Stöhr said. “We are basically opening Pandora’s box, learning about all the different nonlinear effects, and eventually some of those will turn out to be more important than others.”

The study included other collaborators from SLAC and Stanford, and was funded by the DOE Office of Science.

For more information: www.slac.stanford.edu

Related Content

Agfa Brings Intelligent Radiography to RSNA 2018
News | Digital Radiography (DR) | September 17, 2018
September 17, 2018 — At the 2018 Radiological Society of North America (RSNA) annual meeting, Nov.
The DRX-Transportable System/Lite
News | X-Ray | September 12, 2018
Columbus Regional Health (Columbus, Ind.) has deployed a Carestream ...
Mount Sinai Serves as Official Medical Services Provider for 2018 U.S. Open
News | Orthopedic Imaging | September 06, 2018
For the sixth consecutive year, Mount Sinai will serve as the official medical services provider for the 2018 U.S. Open...
Scientists Advance Technique for Developing Novel Light Beams from Synchrotron Radiation
News | X-Ray | August 28, 2018
August 28, 2018 — A new study has demonstrated a method that produces novel light beams from synchrotron light source
RSNA Announces Pneumonia Detection Machine Learning Challenge
News | Artificial Intelligence | August 27, 2018
The Radiological Society of North America (RSNA) has launched its second annual machine learning challenge. The RSNA...
Hologic Acquires Digital Specimen Radiography Company Faxitron Bioptics

VisionCT 3-D breast specimen-designated computed tomography (CT) system. Image courtesy of Faxitron Bioptics.

News | Breast Imaging | July 31, 2018
Hologic Inc. announced it has completed the acquisition of Faxitron Bioptics, a privately-held leader in digital...
The Aspen XDR Straight Arm
News | Digital Radiography (DR) | July 20, 2018
The Aspen XDR Straight Arm Digital Radiography System is an advanced and complete digital X-ray solution with a compact...
Fujifilm to Host Pediatric Imaging Best Practices Symposium at AHRA 2018
News | Pediatric Imaging | July 18, 2018
Fujifilm Medical Systems U.S.A. Inc. announced that it will offer educational opportunities and exhibit its latest...
The images (before and after) depict a 50 percent dose reduction in chest X-ray without compromising  image quality.

The images (before) depict a 50 percent dose reduction in chest X-ray without compromising
image quality.

Sponsored Content | Case Study | Radiation Dose Management | July 03, 2018
There are a handful of priorities for radiologists: lowest radiation dosage, highest image quality, optimal workflow,...
Metropolitan Washington Orthopaedic Practice Upgrades DR With Agfa DX-D 300s
News | Digital Radiography (DR) | June 15, 2018
Agfa announced that it has installed two DX-D 300 digital radiography (DR) solutions at the multi-office Centers for...