Carolina Ribbing

High-energy X-ray optics with silicon saw-tooth refractive lenses.

Silicon saw-tooth refractive lenses have been in successful use for vertical focusing and collimation of high-energy X-rays (50-100 keV) at the 1-ID undulator beamline of the Advanced Photon Source. In addition to presenting an effectively parabolic thickness profile, as required for aberration-free refractive optics, these devices allow high transmission and continuous tunability in photon energy and focal length. Furthermore, the use of a single-crystal material (i.e. Si) minimizes small-angle scattering background. The focusing performance of such saw-tooth lenses, used in conjunction with the 1-ID beamlines bent double-Laue monochromator, is presented for both short ( approximately 1:0.02) and long ( approximately 1:0.6) focal-length geometries, giving line-foci in the 2 microm-25 microm width range with 81 keV X-rays. In addition, a compound focusing scheme was tested whereby the radiation intercepted by a distant short-focal-length lens is increased by having it receive a collimated beam from a nearer (upstream) lens. The collimation capabilities of Si saw-tooth lenses are also exploited to deliver enhanced throughput of a subsequently placed small-angular-acceptance high-energy-resolution post-monochromator in the 50-80 keV range. The successful use of such lenses in all these configurations establishes an important detail, that the pre-monochromator, despite being comprised of vertically reflecting bent Laue geometry crystals, can be brilliance-preserving to a very high degree.

Field emitting structures intended for a miniature x-ray source

The concept of a miniature X-ray source is presented. The source consists of a micromachined field-emitting cathode and an anode. Electrons emitted from the cathode are accelerated towards and into the anode where their deceleration produces X-rays. The energy of the X-rays is controlled by the voltage applied to the electrodes, and the dose rate by the emission current. A tentative medical application is discussed and candidate field emission structures are investigated. Spectra from different anode materials have been collected using diamond cold cathodes.

Multi-prism x-ray lens

Refractive x-ray lenses with a triangular surface profile have been used to focus a synchrotron beam to sub-μm line width. These lenses are free from spherical aberration and work in analogy with one-dimensional focusing parabolic compound refractive lenses. However, the focal length can be easily varied by changing the gap between the two jaws. Silicon lenses were fabricated by wet anisotropic etching, and epoxy replicas were molded from the silicon masters. The lenses provided intensity gains up to a factor of 32 and the smallest focal line width was 0.87 μm. The simplified geometry and associated fabrication technique open possibilities for low-Z materials such as beryllium, which should greatly enhance the performance of refractive x-ray optics.

Generalized prism-array lenses for hard x-rays

A Fresnel-like X-ray lens can be constructed by a triangular array of identical prisms whose base corresponds to the 2pi-shift length. Each column of prisms is progressively shifted from the optical axis by an arbitrary fraction of the prism height. Similarly to the multi-prism lens, quasi-parabolic profiles are formed by a superposition of straight-line segments. The resulting projected lens profile is approximately linear with a Fresnel-lens pattern superimposed on it to provide the focusing. This geometry exhibits a significantly larger effective aperture than conventional parabolic refractive lenses. Prototype lenses were fabricated by deep reactive ion etching of silicon. These one-dimensionally focusing lenses were tested at a synchrotron beamline and provided focal line-widths down to 1.4 microm FWHM and an intensity gain of 39 at a photon energy of 13.4 keV. Fabrication imperfections gave rise to unwanted interference effects resulting in several intensity maxima in the focal plane. The presented design allows the focal length to be shortened without decreasing the feature size of the lens. Furthermore, this feature size does not limit the resolution as for real Fresnel optics.

Microfabrication of saw-tooth refractive x-ray lenses in low-Z materials

Saw-tooth x-ray refractive lenses have been fabricated in silicon, epoxy and diamond. Silicon lenses were made by anisotropic wet etching of single crystalline silicon. Epoxy lenses were moulded from silicon masters. Diamond lenses were replicated by chemical vapour deposition on silicon masters and subsequent sacrificial etching of silicon. Beryllium saw-tooth test structures were embossed using a diamond master. Silicon and epoxy lenses gave sub-micron focal lines and provided gains of up to 40 when tested in a synchrotron set-up. Focal lengths ranged from 0.33 to 0.61 m for x-ray energies between 14 and 30 keV.

Diamond Membrane Based Structures for Miniature X-ray Sources

Diamond membrane based structures for a miniature X-ray source are presented. The source consists of two microstructured diamond membranes, which are transparent to the generated radiation. One membrane works as a field emitting cathode and the other is covered with an anode thin film metal. A high voltage is applied between the electrodes and electrons emitted from the cathode are accelerated towards the anode metal where they produce X-rays. 15 kV X-ray spectra were collected through both membranes. The spatial distribution of the emitted bremsstrahlung and the characteristic radiation was investigated.

A Tunable Energy Filter for Medical X-Ray Imaging

A multiprism lens (MPL) is a refractive X-ray lens, and its chromatic properties can be employed in an energy filtering setup to obtain a narrow tunable X-ray spectrum. We present the first evaluat ...

Microstructured diamond X-ray source and refractive lens

This paper treats microstructured CVD diamond in two X-ray applications, a miniature X-ray source and a refractive X-ray lens. The X-ray source consists of boron doped diamond membrane electrodes and an intermediate insulator. The cathode has a pyramidal shape, which is field-emitting and the anode is a metal film on a diamond membrane. Anode radiation emerges through both membrane electrodes. The source has not been vacuum sealed, therefore, all measurements so far have been made in a vacuum chamber. The refractive X-ray lens has saw-tooth geometry and a tunable focal length. It was made by microwave plasma assisted CVD of diamond onto anisotropically etched silicon masters. The lens has been used for one-dimensional focusing of a synchrotron beam to 1.9 μm line width.

Material‐dependent infrared transmittance of mineral wool

The specular infrared transmittance of mineral wool is reported and compared to the optical properties of the raw material, a silicate glass. A transmittance peak at 8.3 μm for the wool, not previously reported, is correlated to the reflectance minimum at the same wavelength for the bulk glass. It is concluded that the radiative behavior of this fibrous insulation cannot be described by gray models, neglecting the dispersion of the raw material. It is also demonstrated that the transmittance peak values are much higher than could be expected from the absorption of the glass. This is tentatively explained as an effect of forward scattering through very small angles, thus increasing the specular transmittance.

A low-absorption x-ray energy filter for small-scale applications

We present an experimental and theoretical evaluation of an x-ray energy filter based on the chromatic properties of a prism-array lens (PAL). It is intended for small-scale applications such as medical imaging. The PAL approximates a Fresnel lens and allows for high efficiency compared to filters based on ordinary refractive lenses, however at the cost of a lower energy resolution. Geometrical optics was found to provide a good approximation for the performance of a flawless lens, but a field-propagation model was used for quantitative predictions. The model predicted a 0.29 E/E energy resolution and an intensity gain of 6.5 for a silicon PAL at 23.5 keV. Measurements with an x-ray tube showed good agreement with the model in energy resolution and peak energy, but a blurred focal line contributed to a 29% gain reduction. We believe the blurring to be caused mainly by lens imperfections, in particular at the periphery of the lens.