two PRLs on x-ray phasing

Two new PRLs are dealing with x-ray phasing.

The first paper is de Jonge et al., “Quantitative Phase Imaging with a Scanning Transmission X-Ray Microscope” Phys. Rev. Lett. 100, 163902 (2008). Typically the differential phase contrast measurements become non-trivial for thick specimens, when the adsorption and phase-wrapping effects become significant. This paper resolves this problems when differential phase contrast measurements are done in scanning transmission x-ray microscopy mode (STXM), since the solution is overconstrained, allowing to arrive at unique phase and adsorption values.

The second paper is Johnson et al., “Coherent Diffractive Imaging Using Phase Front Modifications” Phys. Rev. Lett. 100, 155503 (2008).

Since phase is lost during the measurements, it is impossible to simply fourier-transform the coherent x-ray diffraction pattern to obtain a real-space image of an object with nanoscale resolution. There are numerous numerical approaches of phase-retrieval based on oversampling the diffraction pattern. This paper presents an alternative approach of introducing a phase plate, and deconvolving the set of phases resulting from the sample by scanning the phase object around, making the contribution from the phase plate known, and providing information on un-altered phases that would be observed if no phase plate was present. This technique is similar to ptychography, as it provides additional constraints that help arriving at unique solution in a rapidly convergent manner, except it scans the known phase plate, rather than the object being imaged.

Dark-field X-ray Phase Contrast Imaging

This week’s featured paper is the paper by Franz Pfeiffer and colleagues at Paul Scherrer Institute in Switzerland:

Hard-X-ray dark-field imaging using a grating interferometer, Nature Materials 7, 134 – 137 (2008) .

This Nature Materials paper is related to the previous papers by the same group: Pfeiffer et al., Phase retrieval and differential phase-contrast imaging with low-brilliance X-ray sources, Nature Physics 2, 258 – 261 (2006)

as well as Pfeiffer et al., Shearing Interferometer for Quantifying the Coherence of Hard X-Ray Beams, Phys. Rev. Lett. 94, 164801 (2005).

The use of shearing inteferometer, which to x-rays look like series of micron-sized “combs” or diffraction gratings, allows imaging of milimeter-sized objects using the differential phase contrast, rather than adsorption, as a contrast mechanism. These are the techniques that can be adopted using rather primitive “highly incoherent” in-house x-ray sources – such as x-ray tubes and rotating anodes, and therefore do not require a trip to a synchrotron.

Xraying Nanoparticles in Ball-Lightning and Fuel Injection jets

Observations of ball-lightning – long-lived (2 to 50 seconds) bright fireballs size of baseball to beach ball – have been observed for centuries. Previous theoretical work ascribes the longevity of the ball lightnings to the slow oxidation process of silicon, forming nanoparticle networks. Now, Mitchell et al. Phys. Rev. Lett. 100, 065001 (2008) have created artificial ball-lightning using localized microwaves and have studied them in-situ with synchrotron radiation (using small-angle x-ray scattering), proving that the fireballs do indeed contain nanoparticles with sizes of the order 50 nm.

Small Angle X-ray Scattering, or SAXS, has been used to all kinds of samples – liquid, vapor or solid, but this may be the first time this technique was applied to plasma.

Meanwhile, Wang et al. (Nature Physics, advanced publication) performed an ultra-fast time resolved study of morphology of optically dense jets from fuel injector nozzle. The achieved microsecond temportal resolution is due to application of time-resolved full-field phase contrast imaging. Unlike typical radiography (such as used at the dentist’s office) that is sensitive to the mass density (adsorption) of material through intensity measurements, phase contrast measurements rely on phase changes.

For more details on phase contrast imaging see Wilkins et al., Nature 384, 335 – 338 (28 November 1996)

Techniques using visible light scattering are suffering from problems due to multiple scattering from various interfaces of jet droplets – ironically, these interfaces are precisely what serves as a contrast mechanism in the x-ray phase contrast imaging technique used by Wang et. al in this study.