Patent Number: 051014220
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 illustrates in diagrammatic form a glass capillary 10 which tapers from a large end 12 to a relatively small end 14 and which has an interior bore 16 defined by the inner surface 18 of the capillary wall. The bore 16 also tapers from the large end 12 inwardly to the small end 14. The capillary may be formed into its tapered shape by conventional drawing processes, wherein a glass tube is heated and tension is applied to its opposite ends to cause the tube to stretch and draw out. As the tube is drawn, its diameter decreases substantially uniformly, producing a thin-walled capillary having a very small aperture at its smallest end. The tube may be of a fused silica; however, a higher density glass, such as lead glass, is preferred since it has better X-ray reflecting properties. Various manufacturing procedures may be used to produce the capillary, but such techniques are well known, and do not constitute a part of this invention. It has been found that a tapered, thin-walled capillary can be used to concentrate X-rays entering into the large end of the capillary if they are directed in such a way that they strike the inner surface 18 of the capillary at or below some angle less than the critical glancing angle. X-rays which enter the capillary and strike surface 18 at angles greater than the critical glancing angle will be absorbed by the wall of the capillary and will not pass through its length, while those which strike the wall surface at or below this angle will be reflected from that surface in the manner generally indicated by the dotted line 20 in FIG. 1 and will pass through the capillary, so that the capillary serves as a waveguide for these X-rays. As the reflected X-rays move from the larger end 12 toward the smaller end 14 of the capillary, they are in the form of a beam which is concentrated into a progressively smaller diameter aperture without any significant losses, so that the intensity of the beam continuously increases along the length of the capillary and the X-rays are, in effect, focused at a spot at or immediately adjacent the end 14 of the capillary. This spot is equal in diameter to the inner diameter of the end 14. Thus, the capillary reduces the diameter of the beam while increasing its intensity to provide concentrated X-ray energy at the end of the capillary. There are numerous applications for this technique of concentrating X-rays, including any use which requires high X-ray intensities at small spots. From an X-ray optical viewpoint, there is no preferred range of capillary wall thickness; this dimension is a function of the manufacturing technique used to make the capillary. It is the inner diameter of the capillary that is important. In tests of the present invention, the capillaries used had wall thicknesses of about 40 microns. As illustrated in FIG. 2, the capillary 10 preferably is coated by a plastic material 22 such as an epoxy acrylate. The acrylate layer may be applied to the outer surface of the capillary 10 after it has been drawn to the desired taper and diameter to protect the thin glass wall of the capillary and to give it strength and flexibility. The plastic coating is a thin, uniform layer which may be applied in a conventional manner. For example, after the capillary fiber has been heated and drawn to its tapered profile, it is air cooled to 50.degree. C. and passed through a cup of uncured liquid epoxy acrylate. The coated fiber is then sent through a high intensity cylindrical source of ultraviolet light that cures the acrylate coating. The coating thickness can vary from a 50 micron wall thickness at the large end of the fiber to a 225 micron wall thickness at the small end. In accordance with the present invention, both ends of the capillary are supported in the manner illustrated in FIG. 2 for the large end 12, it being understood that a similar mounting structure is used for the small end 14, as shown diagrammatically in FIG. 3. The support structure for the capillary includes, in the illustrated embodiment, an annular vertical mounting block 30 which incorporates an aperture 32 through which the end portion of the capillary extends. The aperture 32 is filled with an adhesive bonding material 34 which may be an epoxy material, and which is packed around the capillary in an uncured state. The epoxy is then cured to form an adhesive bond with the wall of the aperture 32 and with the exterior wall surface of the capillary. When cured, the bonding material 34 has sufficient strength to secure the capillary in the mounting block when tension in the axial direction is applied to it, as will be described. It has been found that if the plastic coating 22 extends completely through the aperture 32 so that the epoxy 34 contacts only the outer surface of the plastic coating 22, insufficient strength is provided to hold the capillary in tension, for when the epoxy is connected to the coating 22, the coating tends to break loose from the surface of the glass when the capillary is subjected to tension. This allows the capillary to slide longitudinally with respect to the coating and thus with respect to the mounting block, thereby releasing the tension. The angle of taper of the capillary is exaggerated in FIG. 2 for purposes of illustration, but in actual tests, wherein the degree of taper was much less than the illustrated taper, the slippage between the plastic coating 22 and the glass capillary 10 allowed the capillary to slide out of the mounting block when the blocks were moved apart to apply tension to the capillary. The epoxy did not break free from the coating, but the coating did not did not stay attached to the glass. A solution to the foregoing problem is the construction illustrated in FIG. 2, which involves stripping the plastic coating 22 away from at least a part of the end portion of the glass tube so that the coating terminates at an end 36 approximately midway through the aperture so that the bonding material 34 contacts the outer surface of the glass. The epoxy 34 bonds to the glass in the region 38 with sufficient strength to hold the capillary in the mounting block when axial tension is applied to the capillary. An additional advantage of the construction illustrated in FIG. 2 is the fact that the plastic coating 22 extends at least partially into the aperture 32 where it is in contact with, and is gripped by the bonding material 34 in the end region generally indicated at 40. This arrangement provides a cushioning effect at the point 42 where the capillary enters the bonding material 34, the plastic coating material thereby serving as a strain relief for the glass wall of the capillary to prevent breakage of the glass from the shear force applied to the capillary at point 42 by its weight. It was found that if the plastic coating 22 is stripped away from the capillary throughout the axial length of the aperture 32, so that the epoxy 34 contacts only the glass surface, a shear force is generated at the edge 42 of the epoxy which results in easy breakage of the capillary. The mounting block 30 illustrated in FIG. 2 and a matching mounting block 30' (FIG. 3) at the opposite end of the capillary are used to secure the tapered glass capillary 10 and to apply axial tension to it so that it is linear to within a a few arcseconds resolution to enable X-rays to propagate down the bore 16 of the capillary without being absorbed by the capillary wall. In order to apply the required tension, the mounting blocks 30 and 30' are mounted for longitudinal motion in the direction of the longitudinal axis 44 of the capillary on an optical rail 46 (see FIG. 3) with the capillary 10 stretched between the two mounting blocks. The mounting blocks may be located in housings 48 and 48', as by means of a suitable gimbal mount diagrammatically illustrated in FIG. 2 by inner gimbal ring 50 in which the mounting block 30 is secured, as by a mounting ring 52. The inner ring 50 is pivotally mounted by pins 54 and 56 to a gimbal outer support 58 for motion about a horizontal axis. The gimbal ring 58 may, in turn, be mounted on a rotatable base 60 for pivotal motion about a vertical axis, as illustrated in FIG. 3 in housing 48, which is broken away to better illsutrate the gimbal mounting. The gimbal mounting preferably is a motor-driven, adjustable precision mounting which permits precise alignment of the mounting block with the longitudinal axis of the capillary. A suitable gimbal mounting is the Oriel Motorized Mirror Mount, equipped with a "Motor Mike" precision drive, manufactured by Oriel Corporation, Stratford, Conn. This mounting permits alignment of the end portions 12 and 14 of the capillary with the main body portion thereof between the mounting blocks, so that the capillary remains essentially straight along its entire axial length. The housings 48 and 48' are mounted on the optical rail for relative motion parallel to the longitudinal axis 44 of the capillary which is stretched between them. Thus, for example, the housing 48 may be secured to a mounting platform 62 carried by a support base 64 fixed to the optical rail. The corresponding housing 48' is secured to a corresponding platform 62' which is also carried by a support base 64', which in this case preferably is a traveler movably mounted on the optical rail 46. The traveler is positionable by means, for example, of a threaded drive rod 68 which may be motor driven or manually operated by means of a hand wheel 70. The housing 48' is movable longitudinally along the drive rod 68 in the directions indicated by the arrow 72 so as to apply a selected amount of tension to the capillary 10. By rotation of the drive rod 68, the traveler 64' is moved longitudinally with respect to the fixed base 64 so that the capillary 10 can be pulled taut to effectively eliminate sag in the capillary and to insure that its longitudinal axis 44 is linear. The gimbal mounting for the mounting blocks 30 (and 30' for housing 48'), which permits the ends of the capillary to be aligned with the longitudinal axis between the two mounting blocks, insures a straight path completely through the capillary bore 16 for the propagation of X-rays. X-rays from a suitable X-ray source, diagrammatically illustrated at 80 in FIG. 2, are directed into the large end 12 of the capillary after it has been fastened to the mounting blocks 30 and 30', the blocks have been secured in the housings 48 and 48', and tension has been applied to the capillary by adjustment of the traveler 64'. The application of sufficient tension, together with precise adjustment of the gimbal mounting allows the axis of the capillary to be aligned within a few arcseconds precision, and through the adjustment of the gimbals and the tension of the capillary, X-ray flux through the capillary is maximized. Since, for the propagation of X-rays, the critical angles for total reflection are on the order of milliradians, and since the cladding material 22 and the glass wall of the capillary 10 absorb X-rays rather than propagating them, the requirements for maintaining a straight axial line through the capillary are very high, and are much greater, for example, than would be the requirements for propagating light through an optical fiber. In order to further maximize the propagation of X-rays through the capillary, the large end 12 of the capillary is enclosed by a gas-tight enclosure 82 including side walls 84, which may be metal, for example, and an end wall 86 formed of X-ray transparent material such as Kapton tape. Helium gas is supplied from a source 88 by way of an inlet 90 to the interior 92 of the enclosure 82 under slight pressure so that the helium flows through inlet end 12 into the interior bore 16 of the capillary. The helium flows through the capillary and exits from the aperture at the small end 14, thereby filling the capillary and displacing the air. A small flow of helium into the large opening 12 of the capillary translates into a larger flow rate exiting from the small opening 14, with the flow exerting sufficient force to clear air out of the capillary. Since helium is transparent to X-rays, a significant increase in the flux density of X-rays at the output end 14 is attained. For example, with X-rays having an energy level of 8 keV an air-filled capillary 1.6 meters long has its flux density reduced by a factor of 5 due to the absorption of X-rays in air. At lower X-ray energies, this loss is even greater. However, substituting helium for air increases the 8 keV X-ray flux by 5 times. It is noted that the helium exiting the small end of the capillary may be recaptured, if desired, but can be allowed to dissipate since the flow rate is very low. In a test of the present invention, a capillary 1.6 meters long and having a diameter of 470 micrometers at its large end 12 and 110 micrometers at its small end 14 was mounted in the manner described above. The capillary was gripped at its two ends by means of mounting blocks such as those illustrated at 30 and 30' and was pulled straight by applying longitudinal tension to the capillary. The tensile strength of the glass was sufficient to substantially eliminate sag in the capillary, and the adhesive bonding in the manner illustrated prevented shear forces from breaking the glass at its securing points. The metal/glass bond did not separate, and the metal/acrylate bond served as a strain relief. The use of a gimbal mounting at the ends of the fiber permitted precise alignment of the capillary axis, and the use of helium within the capillary bore further maximized the density of the X-rays at the small-end opening of the capillary, thereby effectively focusing the X-rays to a high intensity spot having a diameter of 110 micrometers. The provision of a plastic coating on the X-ray capillary provides another unexpected advantage. It has been found that such a coating provides a degree of flexibility to the thin glass capillary, enabling the capillary to be curved into an arc of a relatively large radius; for example, several meters, without breaking. It has been found that such a smoothly curved, large-radius arc will still propagate X-rays, although there is some absorption, and such a curvature allows an X-ray beam to be redirected. This is important, because present X-ray technology can steer a beam only over a limited angular range of about 0.3 degrees. Thus, for example, a synchrotron source generally directs its output X-rays horizontally, and present day X-ray optics do not permit such beams to be significantly redirected. However, with a carefully supported large-radius curved capillary, the output X-rays from a synchrotron could be directed vertically or in any direction in the horizontal plane. Although the present invention has been described in terms of a preferred embodiment thereof, it will be understood that variations and modifications may be made without departing from the true spirit and scope thereof. Thus, for example, variations in the mounting mechanism on the optical rail may be utilized, and the gimbal mounts diagrammatically illustrated in the drawings preferably will be precision mounts which may be adjusted for accurate alignment. Accordingly, the true spirit and scope of the invention is limited only by the following claims.