Compact Laser Generates Research-Quality X-rays

A novel method generates research-quality x-rays using a "tabletop" laser, making such light sources more practical for advanced research.

High-quality x-rays — needed for applications ranging from analyzing the structure of matter to advanced medical imaging — are now generated using gigantic and costly synchrotron sources. Most are available at a select few locations around the world, and some equal the size of a college campus.

Nathan Powers, a member of the research team that developed a laser-driven x-ray device, shows the accelerator used to generate synchrotron x-rays. Courtesy of Greg Nathan, University of Nebraska-Lincoln.

A team at the Extreme Light Laboratory at the University of Nebraska-Lincoln developed a new way to generate synchrotron x-rays by using a compact but powerful laser.

Because the new device could be small enough to fit in a hospital or on a truck, and also because synchrotron x-rays result in lower doses of radiation as well as high-quality images, it could lead to more widespread applications for advanced x-ray technology, UNL scientists said. Those might include detecting nuclear materials concealed within a shielded container for homeland security, earlier discovery of cancerous tumors, or the ability to study chemical reactions that are too fast to observe with conventional x-rays.

In traditional synchrotrons, electrons are accelerated to extremely high energy and then made to change direction periodically, causing them to emit energy at the x-ray wavelength. At the European Synchrotron Radiation Facility in Grenoble, France, the electrons circle in a huge storage ring at near the speed of light. Magnets are used to change the direction of the electrons, producing x-rays.

In an alternative approach, the UNL team replaced both the electron accelerator and the magnets with laser light. They first focused their laser beam onto a gas jet, creating a beam of relativistic electrons. They then focused another laser beam onto the accelerated electron beam. This rapidly vibrated the electrons, which in turn caused them to emit a bright burst of synchrotron x-rays — a process referred to as Compton scattering. Remarkably, the light's photon energy was increased during this process by a millionfold, yet the combined length of the accelerator and synchrotron was less than the size of a dime.

Physics professor Donald Umstadter and doctoral student Nathan Powers in the Diocles Extreme Light Laboratory. Courtesy of Greg Nathan, University of Nebraska-Lincoln.

"The x-rays that were previously generated with compact lasers lacked several of the distinguishing characteristics of synchrotron light, such as a relatively pure and tunable color spectrum," said physics professor and Extreme Light Laboratory Director Donald Umstadter, who led the project. "Instead, those x-rays resembled the 'white light' emitted by the sun."

The new laser-driven device produces x-rays over a much larger range of photon energies, extending to the energy of nuclear gamma rays, they said. Even fewer conventional synchrotron x-ray sources can produce such high photon energy. Key to the breakthrough was finding a way to collide the two microthin beams — the scattering laser beam and the laser-accelerated electron beam.

"Our aim and timing needed to be as good as that of two sharpshooters attempting to collide their bullets in midair," Umstadter said. "Colliding our 'bullets' might have even been harder, since they travel at nearly the speed of light."

The work appears in Nature Photonics (doi: 10.1038/nphoton.2013.314)

For more information, visit: www.unl.edu/diocles