Kenneth Evans-Lutterodt

Fabrication of silicon kinoform lenses for hard x-ray focusing by electron beam lithography and deep reactive ion etching

The focusing of subnanometer wavelength x rays is limited by the ability to fabricate high-quality optics. In general, the resolution is of the order of the smallest feature of the optic, so nanometer spot sizes are extremely difficult to achieve with lenses made by traditional fabrication methods. In addition, gains in resolution for a given lithography limit are often made at a sacrifice of focusing efficiency. Kinoform lenses offer a compromise position of high resolution and efficiency. The object of this work is to describe the fabrication of kinoform lenses and to show how their unique properties could provide a path toward nanometer scale focusing. Fabrication is made easier by using higher order focusing and larger features. By combining 100keV electron beam lithography and deep reactive ion etching, the authors have fabricated cylindrical kinoform lenses in silicon. These lenses can be used in a crossed pair to produce a two-dimensional focus, but to maintain a large aperture and high resolution ...

Spatial dependence and mitigation of radiation damage by a line-focus mini-beam.

Recently, strategies to reduce primary radiation damage have been proposed which depend on focusing X-rays to dimensions smaller than the penetration depth of excited photoelectrons. For a line focus as used here the penetration depth is the maximum distance from the irradiated region along the X-ray polarization direction that the photoelectrons penetrate. Reported here are measurements of the penetration depth and distribution of photoelectron damage excited by 18.6 keV photons in a lysozyme crystal. The experimental results showed that the penetration depth of ~17.35 keV photoelectrons is 1.5 ± 0.2 µm, which is well below previous theoretical estimates of 2.8 µm. Such a small penetration depth raises challenging technical issues in mitigating damage by line-focus mini-beams. The optimum requirements to reduce damage in large crystals by a factor of 2.0-2.5 are Gaussian line-focus mini-beams with a root-mean-square width of 0.2 µm and a distance between lines of 2.0 µm. The use of higher energy X-rays (> 26 keV) would help to alleviate some of these requirements by more than doubling the penetration depth. It was found that the X-ray dose has a significant contribution from the crystals solvent, which initially contained 9.0%(w/v) NaCl. The 15.8 keV photoelectrons of the Cl atoms and their accompanying 2.8 keV local dose from the decay of the resulting excited atoms more than doubles the dose deposited in the X-ray-irradiated region because of the much greater cross-section and higher energy of the excited atom, degrading the mitigation of radiation damage from 2.5 to 2.0. Eliminating heavier atoms from the solvent and data collection far from heavy-atom absorption edges will significantly improve the mitigation of damage by line-focus mini-beams.

Local structure of human hair spatially resolved by sub-micron X-ray beam

Human hair has three main regions, the medulla, the cortex, and the cuticle. An existing model for the cortex suggests that the α-keratin- based intermediate filaments (IFs) align with the hair’s axis, but are orientationally disordered in-plane. We found that there is a new region in the cortex near the cuticle’s boundary in which the IFs are aligned with the hair’s axis, but additionally, they are orientationally ordered in-plane due to the presence of the cuticle/hair boundary. Further into the cortex, the IF arrangement becomes disordered, eventually losing all in-plane orientation. We also find that in the cuticle, a key diffraction feature is absent, indicating the presence of the β-keratin rather than that of the α-keratin phase. This is direct structural evidence that the cuticle contains β-keratin sheets. This work highlights the importance of using a sub-micron x-ray beam to unravel the structures of poorly ordered, multi-phase systems.

X-ray study of W(001) with and without hydrogen

We have carried out synchrotron x‐ray diffraction studies of the clean W(001) surface reconstruction transition as a function of temperature, and of the room‐temperature phases and phase transitions of W(001) with submonolayer coverages of hydrogen. For clean W(001) we find that below the transition temperature of ≂230 K, the (×)R45° diffraction peaks have an intrinsic width, larger than the instrumental resolution, corresponding to finite‐sized domains. Nevertheless, we are able to measure one and one‐half decades of change in diffraction linewidth, and three decades in the amplitude. The detailed dependences of the surface peak widths are consistent with the predictions of the two‐dimensional XY model with cubic anisotropy to leading order. We also find that the clean surface structure is commensurate at all temperatures measured, above and below the transition temperature. For hydrogen covered W(001) our data suggest a simple picture in which at very low coverages there is a trade‐off with increasing c...