München 2004 – wissenschaftliches Programm

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A: Atomphysik

A II: HV II

A II.1: Hauptvortrag

Dienstag, 23. März 2004, 11:30–12:00, HS 133

High Resolution Imaging of Microscopic, non-Translation Periodic Objects using Coherent X-rays — •Edgar Weckert — HASYLAB at DESY, 22603 Hamburg

Imaging in the nano or micro world is one of the most essential diagnostics tool needed for any science dealing with objects in that size regime. For samples smaller than 1 µm, with some limitations, electron microscopy is in general the probe of choice if also the interior of these objects should be investigated at very high resolution. X-ray micro tomography is able to investigate much larger samples down to a spatial resolution slightly better than 1 µm. Further developments in that field are expected but from a theoretical point of view this method will be limited to about 10-20 nm resolution at best due to the need of X-ray optics, which is limited in numerical aperture. A further increase in resolution will only be possible by imaging techniques not using any optics on the detection side, called lensless imaging. Prerequisite to this technique is the availability of coherent X-ray radiation which is already now available at very small fluxes from the highest brilliant third-generation synchrotron radiation sources and which will be the domain of the X-ray free electron laser. In contrary to crystals non-translation periodic objects give rise to a continuous diffraction pattern if illuminated with coherent X-rays. From such a diffraction pattern it is quite straight forward to reconstruct the scattering density if sampled higher than the Nyquist frequency. This technique is at present under development. The achievable resolution will be limited by the total dose tolerated by the sample. For such objects, if they can be produced in an identical configuration and in large numbers, the short pulses of the XFEL open another window for imaging. Since most radiation damage processes have little chances to manifest themselves during the 100 fs pulse duration of a XFEL each object can be exposed to a significantly higher dose during a single focussed pulse. It is planned to reconstruct the total continuous 3D diffraction pattern of such systems from the individual diffraction images of a large number of identical objects exposed at different orientations. Present theoretical studies try to explore the lower limit in sample size (single cell, virus particle, …) and upper limit in resolution that can be achieved under such conditions.