Quasi-parallel Beam Research Articles

A new kind of collimator made of photonic crystal with negative refraction

A new kind of collimator is presented. This collimator is made of a photonic crystal slab with a half-circle removed. A point source is placed in the hole. A numerical simulation based on the finite-difference time-domain method is carried out to verify the light propagation in the collimator. The results show that with negative refraction the collimator can produce a narrow intense quasi-parallel light beam and other small side beams. Compared with ordinary collimators, this collimator can make light beams narrower, and it is easy to integrate optically. It is hoped that this collimator will play a role in future photonic microdevices.

Beam collimation with polycapillary x‐ray optics for high contrast high resolution monochromatic imaging

Monochromatic imaging can provide better contrast and resolution than conventional broadband radiography. In broadband systems, low energy photons do not contribute to the image, but are merely absorbed, while high energy photons produce scattering that degrades the image. By tuning to the optimal energy, one can eliminate undesirable lower and higher energies. Monochromatization is achieved by diffraction from a single crystal. A crystal oriented to diffract at a particular energy, in this case the characteristic line energy, diffracts only those photons within a narrow range of angles. The resultant beam from a divergent source is nearly parallel, but not very intense. To increase the intensity, collimation was performed with polycapillary x-ray optics, which can collect radiation from a divergent source and redirect it into a quasi parallel beam. Contrast and resolution measurements were performed with diffracting crystals with both high and low angular acceptance. Testing was first done at 8 keV with an intense copper rotating anode x-ray source, then 17.5 keV measurements were made with a low power molybdenum source. At 8 keV, subject contrast was a factor of five higher than for the polychromatic case. At 17.5 keV, monochromatic contrast was two times greater than the conventional polychromatic contrast. The subject contrasts measured at both energies were in good agreement with theory. An additional factor of two increase in contrast, for a total gain of four, is expected at 17.5 keV from the removal of scatter. Scatter might be simply removed using an air gap, which does not degrade resolution with a parallel beam.

High-intensity X-ray microbeams obtained using a cylindrical polycapillary structure

High-intensity quasi-parallel X-ray microbeams with a radiation flux density on the order of 1010 photon/(s mm2) and a divergence of several milliradians, which is close to the parameters of synchrotron radiation, were obtained using a microfocus source based on a transmission-type X-ray tube. The divergent X-ray radiation was converted into a quasi-parallel beam using a cylindrical structure of Kumakhov polycapillary optics with a micron channel diameter.

Using a quasi-parallel X-ray beam of ultrashort wavelength for high-pressure virus crystallography: implications for standard macromolecular crystallography

Data acquisition from crystals of an icosahedral virus, cowpea mosaic virus (CPMV), was carried out to 2.8 A resolution under an elevated hydrostatic pressure of 330 MPa. This was the first example of a complex macromolecular assembly to be studied by high-pressure crystallography. The data were obtained from the ESRF ID30 beamline using a quasi-plane wave of ultrashort wavelength with a diamond anvil cell and an imaging-plate detector. The results of the high-pressure data analysis are given and are compared with those obtained under standard conditions, showing that the experimental procedures implemented are very efficient in terms of diffraction information collected per unit volume of crystal. These results suggest that the use of a quasi-parallel synchrotron radiation beam of ultrashort wavelength should also be considered for conventional macromolecular crystallography data collection.

First results from a macromolecular crystallography system with a polycapillary collimating optic and a microfocus X-ray generator

The design and performance of a high-flux X-ray crystallography system, optimized for diffraction measurements from small macromolecular crystals, is described. This system combines a microfocus X-ray generator [40 µm full width at half-maximum (FWHM) spot size at a power level of 40 W] and a short focal length (F= 2.6 mm) polycapillary collimating optic, and produces a small-diameter quasi-parallel X-ray beam. Measurements of the X-ray flux, divergence and spectral purity of the resulting X-ray beam are presented. The X-ray flux through an aperture of 250 µm diameter produced by the microfocus system is 16 times higher than that from a 3.15 kW rotating-anode generator equipped with a pyrolytic graphite monochromator. Diffraction data from lysozyme test crystals collected with the microfocus X-ray system are of high quality and can be reduced with standard crystallographic software, yielding an overall merging factorRsym≤ 6%. Significant additional improvements in flux are possible, and plans for achieving these goals are discussed.

Parallel-beam X-ray diffractometry using X-ray guide tubes

A parallel-beam X-ray diffraction geometry using X-ray guide tubes is proposed to eliminate preferred-orientation effects in powder X-ray diffraction (XRD) patterns and for new applications of XRD. A bundle of X-ray guide tubes (polycapillaries) is used to provide an intense quasi-parallel (approximately 0.2° divergence) and large-diameter (approximately 20 mm) beam of X-rays needed for parallel-beam diffractometry. Mica and silicon particles were agitated inside a cylindrical chamber by a steady flow of N2gas so that they were randomly oriented. The quasi-parallel incident X-ray beam passed through the cloud of floating particles. The diffracted X-rays were detected using a standard 2θ diffractometer. The integrated intensities observed agree well with those calculated from the known model of the crystal structure. This result demonstrates that this type of diffractometry is capable of avoiding preferred-orientation effects and of collecting XRD data for moving powder samples.

A new approach for in-laboratory XAFS equipment.

A so-called linear spectrometer with bent-crystal has been usually used for an in-laboratory EXAFS apparatus, because of its high efficiency of utilizing X-rays from an ordinary X-ray tube and its better energy resolution. In a linear spectrometer, the monochromator crystals have to be aligned very accurately along a Rowland circle, so the goniometer movement becomes very complicated. The mechanism of the monochromator can be very much simplified when parallel X-rays are used. We propose to use polycapillary optics to obtain a quasi-parallel X-ray beam from a conventional X-ray source and measure EXAFS spectra by a simple system utilizing an ordinary powder diffractometer.

Neutron absorption measurements using converging beams – increased reaction rate

Focusing lenses using neutron reflection optics may considerably increase the neutron beam current density over a small area. The technique enables neutron absorption measurements to be taken with higher sensitivity, greater spatial resolution and lower background. Experimental concerns associated with the beam converging onto the sample are considered and compared to those for a quasi-parallel beam. In particular, we show the connection between the interaction rates for the parallel beam and the isotropic field.

High-resolution x-ray diffraction measurements of residual stresses in polycrystalline thin films deposited on single crystalline substrates

A nondestructive method for stress determination in polycrystalline/amorphous thin films deposited on the nearly perfect crystals was developed. High-resolution x-ray diffraction rocking curves are taken from the substrate using a monochromatized and quasiparallel incident x-ray beam. Changes in the rocking curve shape, caused by stress-induced bending of the substrate, are compared with the newly derived analytical expression of the diffraction spectrum, and as a result the radius of curvature and hence the biaxial stress is obtained. For the (004) Si reflection and the Cu Kα1 radiation the radii of curvature within the interval, 3 m<R<500 m, can be resolved. The method was successfully applied to measure stresses in heat-treated Ni90Ti10 alloy films deposited on a Si (001) substrate. The stress behavior (as a function of the annealing temperature) was found to reflect the sequence of the formation of silicides and enabled the evaluation of their thermal expansion coefficients.

Characterization of Concave-Curved Spectrometers for 2D X-Ray Optics

A procedure for calculating X-ray intensity proffles analytically for various X-ray diffraction geometries has been developed, which takes into account the misalignment, the solid angle factor, the effects of convergence and/or divergence of the diffracted X-rays. The approach is applicable to X-ray optics with either a point source or a quasi-parallel beam. Moreover, using this procedure allows one to calculate the magnified image from a plasma source, the intensity profiles of topographs of bent crystals, and the spectral resolution of varions focusing geometries. Several examples are presented to demonstrate the applications of this procedure. Using non-dispersive and dispersive double-crystal spectrometers, rocking curves were measured for singly and doubly bent crystals. The agreement was satisfactory with the X-ray dynamic theory of bent crystals. Furtlermore, we have also extended the study of X-ray optics to include the crystal anisotropic effects. The anisotropic elasticity theory is applied to bend crystals for calculating the dichracting region on the crystal surface. The anticlastic curvature effects are analytically demonstrated with respect to the crystals' diffracting area. PACS numbers: 07.85.+n, 29.30.Kv

Production of quasi-parallel X-ray beams using microchannel plate “X-ray lenses”

We describe the soft X-ray (0.28 and 1.74 keV) images produced by a spherically-slumped microchannel plate (MCP) optic with a nominal radius of curvature, R, of 1.4 m. With a pointlike X-ray source situated at the focus, an axial distance R2 from the MCP on the concave side, this structure produces, in the manner of a lens, a quasi-parallel output beam whose energy-dependent intensity profile corresponds closely to the predictions of (i) a simple 1D analytical model and (ii) a 2D Monte Carlo ray-trace code. With the source on the convex side of the slumped plate, we observe X-ray focusing which is again consistent with our models. Analysis of the X-ray image structure observed in focusing mode yields an upper limit to the rms surface roughness of the lead glass channel walls of 50 Å.

Monochromatic spatial resolution in the soft X-ray wavelength range

The author suggests a new method which make use of glass capillary arrays to diagnose isoelectronic laser plasma ions. This new set up includes a fiber-like array, located close to the soft-X-ray source, to generate a quasi parallel polychromatic beam which is then diffracted by a grating. This quasi parallel beam is then diffracted by a plane grating; use of a holographic grating with a nonuniform groove distribution and focusing properties increases the spatial resolution as well as the luminosity of the set up. Rough approximation shows that diffracted images may be obtained in a single shot.

Developments in the Design of Efficient X-ray Optical Elements Using Large Arrays of Glass Capillaries

We have characterized an optical element designed to convert divergent x rays into a quasi-parallel beam. For Cu K radiation, it transmits four times the x-ray power of a slit-based collimation system of comparable energy resolution.

The kumakhov lens: Prospects for x-ray microanalysis

A new optical system based on multiple reflection of X-rays and neutrons along specially shaped solid surfaces first proposed by Kumakhov has undergone intense and rapid development in the USSR and more recently in a joint development effort between the Institute for Roentgen Optical Systems (IROS) in Moscow, Russia, and the Center for X-Ray Optics (CXO) in Albany, NY. The most used X-ray Optical System (XOS) utilizes thin hollow microcapillary tubes arranged appropriately to capture X-rays from a divergent source to produce a quasiparallel beam or a focussed beam as shown schematically in Figure 1. Alternatively, a parallel beam of X-rays (or neutrons) can be deflected or focused. It is also possible to use such systems for energy filtering or energy selection.The first such systems were constructed from single hollow capillaries and showed that X-rays could indeed be captured and focussed as shown in Figure 2. These "first generation" Kumakhov optics are most efficient for relatively low energy X-rays (0.5-2 keV).

Focusing and collimation of X rays using microchannel plates: An experimental investigation

Focusing and collimation of 8 keV x rays has been demonstrated using microchannel plate (MCP) blanks with cylindrical channels. The focusing effect arises from total external reflection of x rays at the interior surfaces of the channels of a MCP and has been described previously by the authors. Point to point focusing was observed with flat and curved MCPs, and collimation from a point source to a quasi-parallel beam was observed with a curved MCP. Intensity profiles at the image plane and at other planes behind a flat MCP were obtained for a 20 × 40 μm 2 x-ray source, and agree well with theoretical predictions. The flux in a 40-μm-diameter collector in the image plane was compared with the flux in the same plane, but without a MCP. The relative gain in flux increases linearly with the source to MCP distance, as predicted. A maximum relative gain in flux of 18 ± 1 was observed for a source to detector distance of 50 cm.

Focusing and Collimation of X Rays Using Microchannel Plates: An Experimental Investigation

Focusing and collimation of 8 keV x rays has been demonstrated using microchannel plate (MCP) blanks with cylindrical channels. The focusing effect arises from total external reflection of x rays at the interior surfaces of the channels of a MCP and has been described previously by the authors. Point to point focusing was observed with flat and curved MCPs, and collimation from a point source to a quasi-parallel beam was observed with a curved MCP. Intensity profiles at the image plane and at other planes behind a flat MCP were obtained for a 20 × 40 μm2 x-ray source, and agree well with theoretical predictions. The flux in a 40-μm-diameter collector in the image plane was compared with the flux in the same plane, but without a MCP. The relative gain in flux increases linearly with the source to MCP distance, as predicted. A maximum relative gain in flux of 18 ± 1 was observed for a source to detector distance of 50 cm.

X-ray performance of the LAMAR protoflight mirror

A grazing incidence x-ray mirror in the Kirkpatrick-Baez geometry was constructed and tested. Its geometric aperture measures 20 by 30 cm and its length is 100 cm. The focal length is 3.4 m measured from the front to the focal plane. It is the first of eight mirrors to be built for the LAMAR experiment of the Shuttle High Energy Astrophysics Laboratory. Its angular resolution was measured at 1.5 and 6.4 keV in a quasiparallel x-ray beam at the Marshall Space Flight Center. The half-power width (HPW) of the resolution function projected along a horizontal axis is 31 sec of arc at both energies and in visible light. With the addition of small isolation pads the mirror is able to withstand the vibration and acceleration levels of a Space Shuttle launch. The resolution remains under 35-sec of arc HPW for changes in temperature of 9.5 degrees C and when a modest temperature gradient is imposed on the mirror.

An energy-dispersive spectrometer for the rapid measurement of X-ray absorption spectra using synchrotron radiation

The design and evaluation of an energy-dispersive spectrometer to measure X-ray absorption spectra rapidly using a synchrotron-radiation source is presented. The method employs a cylindrically bent triangular crystal to focus and disperse a quasi-parallel polychromatic X-ray beam onto the sample. The beam passing through the sample then diverges towards an X-ray detector where beam position can be correlated to energy. Both concentrated and dilute samples were measured on X-ray film and with an electronic linear photodiode array detector and the data analysed to determine the resolution obtained and the data quality. This method is shown to provide an efficient way to obtain high-quality EXAFS and absorption-edge data and should permit kinetic studies to be performed on small samples with good counting statistics. The method should find application in the fields of biophysics, chemistry and materials science.

The development of an energy dispersive EXAFS camera for the rapid measurement of EXAFS spectra

A conceptually different approach to obtaining EXAFS data is being developed at SSRL which should increase the efficiency of data collection and minimise the effects of beam instabilities. Furthermore, a high-speed system of this type should permit time-resolved studied to be performed on chemical and biophysical systems. Polychromatic radiation is used to record a complete EXAFS spectrum simultaneously in a dispersive mode rather than the normal point-by-point measurement. We utilise a cylindrically bent triangular Si (111) crystal (capable of providing energy ranges of greater than 1000 eV) to disperse the quasi-parallel polychromatic beam. This energy dispersed beam converges to a focus at the sample position; the beam transmitted through the sample then diverges towards an X-ray detector. The position of the beam, incident on the detector, can be correlated to energy. An energy dispersive EXAFS camera has been designed and built and has been used to obtain EXAFS spectra in a few seconds from a number of metal foils. The resolution of EXAFS features was found to be better than 5 eV. A detailed description of the camera and data obtained with this technique will be presented.

A Fast X-Ray Absorption Spectrometer for Use with Synchrotron Radiation

A quasi-parallel and polychromatic beam of synchrotron radiation is focused and dispersed by a curved crystal, so that the energy of each ray of the focused beam varies as a function of convergence angle through the focus. The specimen is placed at the focus. By measuring the X-ray intensity distribution across the beam behind the focus, in the presence and absence of the specimen, the absorption spectra of Cu and Ni metal foils were obtained. Using an X-ray film as the detector, a spectrum from a Cu foil was obtained in 0.1 seconds when the SPEAR storage ring at Stanford was operated at 3.1 GeV and 80 mA. The energy resolution is approximately 2.0 eV and the energy range of the spectrum is approximately 1 keV.