Patent Number: 050162675
Section: summary

This invention is concerned generally with x-ray and neutron beam instrumentation. In a first aspect, the invention relates to the focusing and collimation of x-rays or neutrons and provides both a method of focusing or collimating x-rays or neutrons and an x-ray or neutron instrument. In a second aspect the invention provides a condensing-collimating monochromator. BACKGROUND OF THE INVENTION X-ray mirrors of various types have long been used in some x-ray scattering instruments to provide a means of focusing x-rays and improving flux and intensity, relative to pin-hole optics, by increasing the angular acceptance of the system with respect to the x-ray source. These methods for enhancing intensity have not found widespread application in x-ray scattering instruments because they lack spatial compactness, and flexibility in use, and are awkward to align. In the case of x-ray optical systems, simultaneous high-resolution in wavelength, angular collimation and spatial extent are usually achievable only at the expense of considerable loss in flux and intensity. An early proposal for an x-ray collimator consisted of two glass plates facing each other at a small angle. This principle was extended in a conical x-ray guide tube proposed by Nozaki and Nakazawa [J. Appl. Cryst. (1986) 19,453]. In a recent paper, Yamaguchi et al [Rev. Sci. Instrum. 58(1), Jan. 1987, 43], there has been proposed a two dimensional imaging x-ray spectrometer utilizing a channel plate or capillary plate as a collimator. It is apparent that Yamaguchi et al are treating the channel plate as a large aperture device acting solely as a set of Soller slits consisting of an array of channels surrounded by opaque walls. SUMMARY OF THE INVENTION It is an object of the invention, in its first aspect, to provide for focusing and collimation of x-ray beams as an aid to achieving both optimum angular resolution and optimum intensity in x-ray optical systems. It is believed that the solutions disclosed herein are also useful in the field of neutron scattering and in other instruments. The invention accordingly provides, in its first aspect, an x-ray or neutron instrument incorporating x-ray or neutron lens means disposed in a path for x-rays or neutrons in the instrument, the lens means comprising multiple elongate open-ended channels arranged across the path to receive and pass segments of an x-ray or neutron beam occupying said path, which channels have side walls reflective to x-rays or neutrons of said beam incident at a grazing angle less than the critical grazing angle for total external reflection of the x-rays or neutrons, whereby to cause substantial focusing or collimation of the thus reflected x-rays or neutrons. The invention also provides a method of focusing, collimating and/or concentrating an x-ray or neutron beam, comprising directing the beam into the open ends of multiple elongate open-ended channels which have side walls reflective to said x-rays or neutrons incident at a grazing angle less than the critical grazing angle for total external reflection of the x-rays or neutrons, at least a portion of said beam being incident at a grazing angle less than said critical grazing angle so that the beam is at least in part focused or collimated. The instrument will typically though not necessarily include a source of x-rays and may have one or more slit assemblies, a monochromator, a sample goniometer stage and/or adjustable x-ray detector. Advantageously, the inclinations of the side walls are uniform in each channel but progressively change from channel to channel with respect to the optical axis of said path whereby to enhance focusing or collimation of said incident beam. Preferably, the outer side wall of each channel itself varies in inclination along the length of the channel to further enhance said focusing and collimation. The device is preferable such that these inclinations can be adjusted, at least finely, on installation of the device in the instrument. As employed herein, the terms "focus" and "collimate" are not strictly confined to beams convergent to a focus or substantially parallel, but respectively include at least a reduction or increase in the angle of convergence or divergence of at least a part of the x-ray beam in question. The term "lens" embraces beam concentration devices generally. The term "channel", as employed in the art, does not specifically indicate an open-sided duct but also embraces wholly enclosed passages, bores and capillaries. The channels are preferably hollow capillaries or other bores and may comprise collectively a micro-capillary or micro-channel plate. For example, the latter may be formed of multiple hollow optical fibres or multiple optical fibres from which the core has been etched out. In general, the interior of the channels can be air and should be of a higher refractive index for x-rays than the surrounds. This requirement is met by hollow air filled ducts or channels in a suitable glass. An alternative micro-capillary device may comprise a thin film, for example of methyl methacrylate, through which multiple elongate holes have been burned, for example by means of electron beam lithography. The film thickness, and therefore the lengths of the holes, may be of the order several micron while the width of the holes may be around 100 angstrom. A quite different embodiment of the device may consist of a stack of thin, highly polished x-ray reflective metal sheets held apart by suitable spacers. This embodiment would be very suitable for use with line sources. For optimum efficiency with only one reflection in each channel, the channels should have a diameter to length ratio d/t approximately equal to said critical angle, .gamma..sub.c. In general, d/t is preferably in the range one to two times .gamma..sub.c. It will be appreciated that not all rays will necessarily intercept channel walls and that a substantial portion of the x-ray beam will typically be absorbed in the channel walls or pass undeviated through the focusing device. In an advantageous application of the invention, the x-ray lens device comprises a micro-capillary plate which is curved so that the angular tilts of the reflecting side walls in the channels vary parabolically with distance perpendicular to the optical axis. By parabolic bending in one or two dimensions, appropriate focusing and collimating effects may be simultaneously produced in the two dimensions-and may well be different in the two dimensions. Preferably, the side walls of the channels are good reflectors of x-rays and have a large value for the critical grazing angle .gamma..sub.c for total external reflection of x-rays. The side walls may be treated to enhance these properties, for example by coating them in gold. A larger .gamma..sub.c may be produced by applying a suitable thin-filmed coating on the side walls of the channels with a denser material such as gold or lead (for example by reduction of a lead glass micro-channel plate in a hydrogen atmosphere, or by vapour deposition). Micro-channel plates suitable for application of the invention may consist of an array of nearly parallel hollow optical fibres or optical fibres from which the core has been etched or otherwise removed. Channels may be typically of diameter in the 1-100 micron range and may have typical length to diameter ratios in the range 40-500. The channel or capillary matrix may be fabricated from lead glass. BACKGROUND OF THE INVENTION Turning to the second aspect of the invention, the highest resolution small angle x-ray scattering systems developed to date have been those based on the Bonse-Hart diffractometer which utilizes two parallel grooved channel-cut perfect-crystals, one for the collimator-monchromator and the second for the collimator-analyser. These systems are capable of both extremely high angular resolution of the order of one second of arc and high intensity, since the two collimator monochromators operate in a non-dispersive mode. The principal disadvantage of systems of the Bonse-Hart type is that the intensity at each scattering angle is collected separately and so the collection of a complete set of data will be quite time consuming, especially if two dimensional scattering data is required. This disadvantage becomes even more significant if the sample or diffraction conditions are changing with time. A further disadvantage is the quite wide beam required to achieve high intensities, rendering the system rather inefficient for narrow samples or for scanning large samples. The data collection times can be greatly improved, however, by employing the recently developed position-sensitive detectors of, for example, the micro-channel plate, diode array or charge-coupled device type, in which each detection pixel is of a width as small as 1 micron. Conventional channel-cut perfect crystal monochromators are not capable of spatially condensing the x-ray beam to this extent and indeed, as just mentioned, a quite wide beam is often unavoidable. Thus it is not possible to realize the full potential of position-sensitive detectors with Bonse-Hart type x-ray diffraction systems. Improved beam condensation is also desirable where imaging techniques are used, such as with photographic film or imaging plates. Kikuta and Kohra (J. Phys. Soc. Japan 29 (1970) 1322) have described an arrangement for reducing the angular spread of an x-ray beam by employing successive asymmetric Bragg diffractions at perfect-crystal faces. This was effective for the purpose but gave rise to a corresponding increase in the spatial width of the beam. SUMMARY OF THE INVENTION It is an object of the invention, in its second aspect, to provide an improved condensing-collimating monochromator which exhibits an enhanced beam condensing property when compared with prior channel-cut crystal monochromators. The invention accordingly provides, in its second aspect, a condensing-collimating channel-cut monochromator comprising a channel in a perfect-crystal or near perfect-crystal body, which channel is formed with lateral surfaces which multiply reflect, by Bragg diffraction, an incident beam which has been collimated at least to some extent, wherein said lateral surfaces are at a finite angle to each other whereby to monochromatize and spatially condense said beam as it is multiply reflected, without substantial loss of reflectivity or transmitted power. By "substantial loss" is meant a reduction by more than one order of magnitude. This aspect of the invention effectively entails the employment of successive asymmetric Bragg deffractions at perfect-crystal faces to spatially condense an incident beam, in contrast to the spatial broadening described in the Kikuta et al article. It is very surprising that condensation can be achieved similtaneously with collimation, monochromatisation and high reflectivity, the latter resulting in good intensity and flux. The result is a very versatile general purpose instrument. The lateral surfaces may provide a significant increase in intensity of the exit beam relative to that of the partially collimated incident beam when measured over the given band-pass and angle of acceptance of the monochromator. The lateral surfaces of the channel may also further collimate the incident beam by virtue of the effect of partial overlap of the reflectivity curves for each surface. The beam may comprise, for example, an x-ray beam or a beam of neutrons. It is also found that the respective asymmetry angles for said lateral surfaces (i.e. the angle between the respective surfaces and a selected Bragg plane), should be jointly selected to optimize the bandwidth, angular collimation, integrated reflectivity and spatial condensation characteristics of the exit beam. Optimum selection of asymmetry angle has been disclosed in relation to parallel multiply reflecting surfaces but the present inventor has appreciated that the optimum conditions where some spatial condensation of the beam is desired will be found to apply where the two asymmetry angles are not equal in magnitude and opposite in sign (i.e. parallel sided channel). In an especially advantageous embodiment of the invention, the first and second aspects described above are combined into a single instrument, in which collimated x-rays or neutrons from the lens means are directed to the monochromator.