Tunable laboratory x-ray sources

J. Dohnálek

Institute of Macromolecular Chemistry of the Academy of Sciences of the Czech Republic, v.v.i., Heyrovského nám. 2, 16206 Prague 6, Czech Republic

dohnalek@imc.cas.cz

The choice of wavelength of x-ray radiation is critical for many diffraction and non-diffraction experiments. The traditional laboratory x-ray sources are in large based on generation of radiation characteristic for the element of the source anode, i.e. monochromatic or close-to-monochromatic radiation. Therefore experiments relying on changing properties of samples with varied energy of the primary x-ray beam have so far been performed mostly at sources of synchrotron radiation. Synchrotron radiation of much higher flux densities, lower divergence and tunable energy produced by bending magnets or insertion devices fully satisfies requirements of many types of experiments.

Because of intrinsically weak diffraction of crystals of biological molecules higher intensity sources of radiation have always been sought. Sources of synchrotron radiation of the 3rd generation brought a substantial improvement of the methods and experiments of bio-crystallography, namely an easier implementation of multiple wavelength anomalous dispersion experiments used widely for solving the phase problem. Intensive and tunable laboratory sources of x-ray radiation would offer a long-wanted tool not only for bio-crystallographic applications.

In the last ten years several research groups have aimed at development of laboratory-size tunable x-ray sources. One direction of these efforts utilizes the phenomenon of inverse Compton scattering. Interaction of electrons accelerated to energies of tens of MeV with photons of electromagnetic radiation of micrometer wavelength leads to generation of x-ray photons. The Compact Light Source (CLS) of Lyncean Technologies, inc., Palo Alto, California has been developed with the aim of application of this phenomenon in generation of an x-ray beam suitable for various types of experiments, which could be housed within a larger laboratory. Some recent results indicate that CLS can be used both for x-ray imaging and diffraction experiments [1].

Another application of the inverse Compton scattering mechanism is applied in the newly designed type of an x-ray source - Compact X-ray Source (CXS) designed at the Massachusetts Institute of Technology/Nuclear Reactor Laboratory. In this case technology of superconductors should be applied to achieve electron acceleration with a finely controlled time structure, high repetition rate and leading to highly brilliant x-ray beam.

Another direction of these developments relies mainly on generation of Brehmsstrahlung by high-energy electrons. Large currents of electrons with energy of 25 MeV interacting with a solid target such as carbon wire generate a wide spectrum of x-ray and gamma radiation. The Photon Production Laboratory, Ltd., Shiga, Japan has designed a single-magnet "synchrotron" setup with selection of a given wavelength of radiation by a system of monochromators - "Mirrorcle". X-ray radiation is generated by two different mechanisms - Brehmsstrahlung from a micrometer target or so called "parametric" radiation produced by interaction of the intensive and energetic electron beam with a single crystal periodic lattice. The Mirrorcle design is being optimised to become an x-ray source usable for diffraction experiments [2].

While the CXS source is only in the stage of planning CLS and MIRRORCLE RAY 20 ST systems have been built and partially optimised. Both produce x-ray radiation of variable energies. Developments to provide high intensity monochromatic beams for a broad range of experiments are under way.

References

1. M. Bech, O. Bunk, C. David, R. Ruth, J. Rifkin, R. Loewen, R. Feidenhans'l, F. Pfeiffer, J. Synchrotron Rad., 16, (2009), 43.

2. H. Yamada, T. Hiraia, M. Morita, D. Hasegawa, M. Hanashima, In Developments in x-ray tomography VI, Proceedings of the Society of photo-optical instrumentation engineers (SPIE), 7078, (2008), P780.