Debris-free laser-plasma source for x-ray microscopy

Abstract

X-ray microscopy allows high-resolution imaging of biological samples in their natural aqueous environment without staining, sectioning or fixation. In particular attention has been focused to the “water window”, i.e., the wavelength region between the oxygen K-edge at λ = 2.3 nm and the carbon K-edge at λ = 4.4 nm, where different absorption coefficients for water and proteins create a natural contrast. Current, operational x-ray microscopes normally utilise zone-plate optics in combination with synchrotron sources. However, these sources suffer from drawbacks such as high cost, limited accessibility for biological researchers and relatively long exposure times. We have developed a compact high-brightness x-ray source based on a laser-produced plasma from a liquid droplet target [1]. A 10 Hz, 70 mJ, 120 ps laser is focused onto ~15 μm droplets generated from a capillary nozzle. By choosing different target liquids spectrally tailored emission can be obtained. For x-ray microscopy we use ammonium hydroxide droplets which generates nitrogen line-emission in the water window [2]. The spectrum over this region is shown in Fig. 1. The emission has been filtered through 600 nm of titanium in order to achieve quasimonochromacy. By using only N VI radiation at λ = 2.88 nm chromatic aberrations in zone plates are efficiently eliminated. Furthermore, λ/Δλ has been estimated to >440, which makes high-resolution imaging possible. Depending on the temporal laser parameters we obtain plasmas from a diameter of ~10 μm with unfiltered single-line flux of 3x1011 photons/(sr pulse) up to a diameter of ~35 μm with 3×1012 photons/(sr pulse) [3]. A major advantage using this new droplet target is the elimination of debris from the laser-plasma. Measurements have proven that the debris emission from the plasma based on ammonium hydroxide is more than 5 orders of magnitude less than from any other conventional low-debris target [4].