C. K. Gary

Two-dimensional x-ray focusing from compound lenses made of plastic

We have measured the intensity profile and transmission of x rays focused by a series of either spherical or parabolic lenses fabricated using Mylar® (C5H4O2) or Kapton® (polyimide). The use of plastics can extend the range of operation of compound refractive lenses, improving transmission and aperture size and reducing focal length. The number of unit lenses range from 193 to 600 for each compound refractive lens. Two-dimensional focusing was obtained for photon energies 8–14 keV with imaging distances of less than 1 m. For example, full-width-half-maximum linewidths down to 16 μm at a distance of only 47 cm from the lens were achieved at 9 keV. The effective apertures of the refractive lenses were measured between 250 and 364 μm with peak transmissions between 10% and 33%.

Observation of bright monochromatic x rays generated by relativistic electrons passing through a multilayer mirror

We have observed the emission of 15 keV x rays produced by 500 MeV electrons passing through a x-ray multilayer mirror. The mirror consisted of 300 pairs of W and B4C layers with layer spacing of 12.36 A and supported by a 100 μm Si substrate. The x rays were emitted at the Bragg angle θγ=33.15 mrad with respect to the mirror surface and at the angle θD=66.3 mrad with respect to the electron-beam direction. The spatial distribution and the spectral angular dependence of the x rays were measured and shown to be larger than the parametric x rays emitted from the Si substrate. The value of the differential photon efficiency was estimated to be about 0.22 photons/electron/str.

CYLINDRICAL COMPOUND REFRACTIVE X-RAY LENSES USING PLASTIC SUBSTRATES

We have measured the intensity profile of x rays focused by compound refractive lenses (CRLs) fabricated using acrylic (Lucite) and polyethylene plastics. A linear array of closely spaced holes acts as multiple cylindrical lenses. The important parameters for this type of focusing are the focal length and absorption, and, for wavelengths shorter than 3 A, low atomic number plastics are suitable. We have experimentally demonstrated that we can achieve one-dimensional focusing for photon energies between 9 and 19.5 keV with focal lengths between 20 and 100 cm. For example, using 12 keV x rays we have achieved focal full width at half maximum linewidths down to 21 μm at a distance of only 20 cm from the CRL. The x-ray source was a synchrotron emitter whose source size in the vertical dimension was 445 μm.

Characteristics of the thick, compound refractive lens

A compound refractive lens (CRL), consisting of a series of N closely spaced lens elements each of which contributes a small fraction of the total focusing, can be used to focus x rays or neutrons. The thickness of a CRL can be comparable to its focal length, whereupon a thick-lens analysis must be performed. In contrast with the conventional optical lens, where the ray inside the lens follows a straight line, the ray inside the CRL is continually changing direction because of the multiple refracting surfaces. Thus the matrix representation for the thick CRL is quite different from that for the thick optical lens. Principal planes can be defined such that the thick-lens matrix can be converted to that of a thin lens. For a thick lens the focal length is greater than for a thin lens with the same lens curvature, but this lengthening effect is less for the CRL than for the conventional optical lens.

The Effect of Unit Lens Alignment and Surface Roughness on X-ray Compound Lens Performance

The required alignment tolerances and surface roughness for unit lens elements in a compound refractive lens (CRL) for x rays are discussed. Contrary to what one might expect and what has been stated in the patent literature, alignment tolerances are large and for typical parameter values the effect of misalignment is minor. For a parabolic lens the focusing properties of the CRL are unaltered by misalignment and there is a small increase in absorption. For a lens with spherical aberration, there is a slight change in focal length, a minor translation of the image, and a small increase in absorption. This article also shows that lens gain is not appreciably reduced if the phase shift that is introduced by the roughness is limited to ±π/4 or if the transverse period of the roughness exceeds a specified value. The CRL can benefit from a managed misalignment of the elements to reduce the phase error introduced by surface imperfections of the lens.

Noninvasive digital energy subtraction angiography with a channeling‐radiation x‐ray source

Channeling radiation could provide a viable source for digital energy subtraction angiography (DESA). A signal to noise ratio (SNR) of 6.2 for a resolution of 0.5 mm x 0.5 mm could be achieved using a 6-mA 100-ms 20-MeV electron-beam pulse and a diamond channeling crystal as the x-ray source. This article investigates the choice of a DESA contrast agent and the parameters of a channeling-radiation x-ray source to develop a channeling-radiation DESA imaging system. The production of dual-energy peaks, the maximum available x-ray flux, the advantages of an area exposure, the necessity of a mosaic Bragg-crystal filter to reduce patient dose, the optimal energy separation of the peaks for a quasi-monochromatic x-ray source, and the reduction of the signal from bone are discussed, leading to estimated SNRs and image resolution for a channeling-radiation imaging system. The computer analysis developed to calculate the image quality is also discussed.

Microscope using an x-ray tube and a bubble compound refractive lens

We present x-ray images of grid meshes and biological material obtained using an unfiltered x-ray tube and a compound refractive lens composed of microbubbles embedded in epoxy inside a glass capillary. Images obtained using this apparatus are compared with those using a synchrotron source and the same lens. We find that the field of view is larger than that obtained using the synchrotron source, whereas the contrast and resolution are reduced. Geometrical distortion around the edges of the field of view is also reduced. The experiments demonstrate the usefulness of the apparatus in a modest laboratory setting.

X-ray focusing with compound lenses made from beryllium.

We have measured the intensity profile and transmission of x rays focused by a series of biconcave spherical unit lenses fabricated from beryllium. The use of beryllium extends the range of operation of compound refractive lenses, improving transmission, aperture size, and gain. The compound refractive lens was composed of 160 biconcave unit lenses, each with a radius of curvature of 1.9 mm. Two-dimensional focusing with a gain of 1.5 was obtained at 6.5 keV with a focal length of 93 cm. The effective aperture of the compound refractive lens was measured as 600 mum , with 9% peak transmission.

Biological imaging with a neutron microscope

A simple microscope employing a compound refractive lens (CRL) composed of 100 biconcave lenses was used to image a biological sample with a 9.4× magnification using 10A cold neutrons. The microscope’s resolution, 0.5mm, was primarily determined by the neutron detector 5.0mm pixel size. Unlike previous work the CRL’s field of view was large (44mm) and increased as the distance between the exit of neutron-waveguide and the specimen decreased. Short source-to-specimen distances allowed the 1.2-cm-diam CRL to view a biological sample with this field of view.

Optimization of the channeling radiation source crystal to produce intense quasimonochromatic x rays

Channeling radiation is a source of intense, tunable, quasimonochromatic x rays produced by electrons traveling along a direction of symmetry through a crystal. We analyze the effect of the channeling source crystal, through both its composition and lattice structure, on the number of photons emitted per electron, the linewidths of the radiation, and the maximum sustainable currents to identify the crystal which will yield the most photons in a bandwidth of less than 10% full width at half maximum (FWHM). Although high atomic number (Z) crystals produce a greater number of photons per electron, given their poor thermal properties these crystals cannot sustain as high average currents as lower Z crystals. The linewidths of the channeling radiation emitted from high Z crystals are as large as 50% FWHM, and electrons are dechanneled more quickly in these crystals, limiting their use as a quasimonochromatic x‐ray source. Given its exceptional thermal conductivity and narrow linewidths, diamond is the optimal ...