Novel source of X-rays for imaging purposes

Photo: Thorsten Naeser standing next to the lasersystem

The ATLAS Lasersystem based in the Laboratory for Extreme Photonics of the Ludwig-Maximilians University Munich, serves as a light source for the new brilliant X-ray radiation; © LMU Munich/Thorsten Naeser

Physicists at LMU Munich and the Max Planck Institute for Quantum Optics have validated a novel laser-driven means of generating bright and highly energetic X-ray beams. The method opens up new ways of imaging the fine structure of matter.

For over a century, medical imaging has made use of X-rays produced in a specialized type of vacuum tube. The major disadvantage of this method lies in the poor quality of the emitted radiation. The source emits radiation from a large spot into all directions and over a broad energy range.

These features are responsible for the relatively modest resolution attainable with this mode of imaging. X-rays generated in synchrotrons provide much higher resolution, but their dimensions and cost preclude their routine use in clinical settings.

However, an alternative approach is now available, for two laser pulses can generate X-rays of similar quality to synchrotron radiation in devices with a far smaller footprint: One pulse accelerates electrons to very high energy and the other forces them into an undulating motion. Under these conditions, electrons emit X-radiation that is both highly energetic ('hard') and highly intense, and is therefore ideal for probing the microscopic structure of matter.

Now, physicists based at the Laboratory for Attosecond Physics (LAP) at LMU Munich and the Max Planck Institute for Quantum Optics (MPQ) have developed such a laser-driven X-ray source for the first time. With the aid of two laser pulses, the researchers have generated ultrashort bursts of X-rays with defined wavelengths tailored for different applications.

The new source can image structures of varying composition with a resolution of less than 10 micrometers. This breakthrough opens up a range of promising perspectives in materials science, biology and – in particular – medicine.

One of the great advantages of the new system in comparison with conventional X-ray sources is that the wavelength of the emitted light can be tuned over a wide range. This ability to alter the wavelength allows radiologists to analyze different types of tissue, for instance. By fine-tuning the incident beam, one can gain the maximum information about the sample one wishes to characterize.

Not only is the laser-driven radiation tunable and extremely bright, it is produced in pulsed form. Each 25-fs laser pulse gives rise to X-ray flashes of a few fs duration. This makes it useful for applications such as time-resolved spectroscopy, which is used to investigate ultrafast processes at the level of atoms and electrons.

The intensity of the pulses for example the number of photons per pulse generated by the new source is not yet high enough for this task, but the researchers hope to overcome this obstacle with the aid of the facilities at the new Centre for Advanced Laser Applications (CALA), now being built on the Garching Campus.

The new optically generated radiation can also be combined with phase-contrast X-ray tomography, an imaging procedure that is being refined by Prof. Franz Pfeiffer of the Technical University of Munich (TUM). This technique extracts information from the light that is scattered (rather than that absorbed) by an object.

"Using this method, we can already image structures as small as 10 micrometers in diameter in opaque materials," Stefan Karsch explains. "With our new X-ray source, we will be able to obtain even more detailed information from living tissues and other materials," he adds.; Source: LMU Munich

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