As reported in November’s issue of the optics journal Nature Photonics, a research team has developed a novel method to generate research-quality synchrotron X-rays using a ‘tabletop’ laser, at the University of Nebraska-Lincoln’s Extreme Light Laboratory in the USA. It is hoped that the new technology will lead to applications that will benefit both society and the scientific field.
Although the high quality of synchrotron X-rays make them ideal for research, most traditional devices are gigantic and costly, available only at a few sites around the world. Shrinking components of advanced laser-based technology will increase the feasibility of producing high-quality X-rays in medical and university research laboratories, which in turn could lead to new applications for the X-rays.
Possible applications will include the detection of cancerous tumours at earlier stages, the discovery of nuclear materials concealed within shielded containers or scientists studying extremely fast reactions that occur too rapidly for observation with conventional X-rays.
In traditional synchrotron machines, electrons are accelerated to extremely high energy and then made to change direction periodically, causing them to emit energy at X-ray wavelength. At the European Synchrotron Radiation Facility in France, the electrons circle near the speed of light in a storage ring which is 844m in circumference. Magnets are used to change the direction of the electrons and produce X-rays.
Pursuing 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, and then focused another laser beam onto the accelerated electron beam. This led to a rapid vibration of the electrons, which 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 million-fold, yet the combined length of the accelerator and synchrotron was less than 18mm.
‘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 colour spectrum,’ said Umstadter. ‘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. Few conventional synchrotron X-ray sources are capable of producing such high photon energy. The key to the breakthrough was finding a way to collide the two micro-thin 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 bullets in mid-air,’ explained Professor Donald Umstadter, director of the Extreme Light Laboratory, who led the research project. ‘Colliding our "bullets" might have even been harder, since they travel at nearly the speed of light.’