Patent Number: 055704085
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to FIG. 1, the basic elements of a typical x-ray source are shown. Filament 10, is heated, by applying a voltage, to a temperature such that electrons 12, are thermally emitted. These emitted electrons are accelerated by an electric potential difference to anode 14, which is covered with target material 16, where they strike within a given surface area of the anode which is called the spot size 18. X-rays 20, are emitted from the anode as a result of the collision between the accelerated electrons and the atoms of the target. In order to control the spot size, electromagnetic focusing means 22, is positioned between electron emitting filament 10, and anode 14, so that the electron beam passes within its area of influence. X ray sources with spot sizes of 2 microns or less are available commercially. However, as the electron spot size decreases, so does the production of x rays. FIG. 2 shows how x ray power (production of x rays), and the power density (power/spot area) of a source varies with spot diameter. Noting that the linear vertical scale on the right of the graph is used for the total power, it can be seen from the lower tail 24, of total power curve 26, that power decreases nearly linearly with spot diameter for very small spot sizes. Turning our attention now to the power density curve 28, and noting that the vertical scale on the left of the graph, which applies to this curve is logarithmic, it can be seen that there is an inverse relationship between the power density and the spot diameter. The reason for this is that the total power varies linearly with spot diameter, while the area varies as the inverse of the square of the spot diameter. Thus it can be seen that even though total x-ray production is decreased, the power density increases with decreasing spot size. Monolithic capillary optics allow unprecedented possibilities for efficient use of the increased power density of small spot x-ray sources. The combination of the smaller spot source, and properly engineered monolithic capillary optic of the subject invention can thus lead to a substantial increase in intensity of small diameter output x-ray beams. Specific design parameters vary depending on the energy of x-rays used. Two types of systems are particularly pointed out. First, a system in which a very intense small diameter quasi-parallel beam is formed and second a system in which a very small, intense converging x-ray spot is formed. In all cases, systems of the type defined by the subject invention can be easily differentiated from other prior art systems based on a much reduced source to optic distance. FIG. 3 shows an x-ray source 30, and multi-fiber polycapillary optic 32. In order for the polycapillary fiber 33 to efficiently capture radiation from source 30, the collection angle 34 of the capillary must be less than the critical angle for total external reflection. This angle is dependent on the x-ray energy. For a typical example of an approximately 8 keV optic with polycapillary outer diameters of around 0.5 millimeters, simple geometric considerations lead to the conclusion that the optic must be placed at least 150 millimeters away from the source. The subject invention is defined by optics which are placed no more than half that distance from the source. The first embodiment of the subject invention is shown in FIG. 4. The system 40, for producing a high intensity, small diameter x-ray beam comprises two main components; a small spot x-ray source 42, and a monolithic capillary optic 44. The two components are separated by a distance f, known as the focal distance, measured along optical axis 46. The optic 44 comprises a plurality of hollow glass capillaries 48 which are fused together and plastically shaped into configurations which allow efficient capture of divergent x radiation 43 emerging from x-ray source 42. In this example the captured x-ray beam is shaped by the optic into a quasi-parallel beam 50. The output beam is not completely parallel because of divergence due to the finite critical angle of total external reflection. The channel openings 52 located at the optic input end 54 are roughly pointing at the x-ray source. The ability of each individual channel to essentially point at the source is of critical importance to the subject invention for several reasons: 1) It allows the input diameter of the optic to be sufficiently decreased, which in turn leads to the possibility of smaller optic output diameters; 2) it enables efficient capture of x-rays even when the source spot is decreased; 3) it makes efficient x-ray capture possible for short optic to source focal lengths. The diameters of the individual channel openings 52 at the input end of the optic 54, are smaller than the channel diameters at the output end of the optic 56. The class of optics used in the subject invention are monolithic. This means that the walls of the channels themselves 70, form the support structure which holds the optic together. For this case, the maximum capture angle is given by 2.psi., where .psi. is the maximum bend angle of a curved capillary. In a preferred embodiment the x-ray source 42 has a spot size of roughly 30 microns and is located approximately 1.0 millimeter from the input end 54 of capillary optic 44. The collection angle .psi. for this optic is around 0.2 radians. The optic produces an output beam 50 with a diameter of essentially 1.0 millimeter. The overall length of the optic is approximately 8.0 Millimeters. The increase in intensity is expected to be more than roughly 2 orders of magnitude brighter than currently available laboratory sources. FIG. 5 shows a second embodiment of the subject invention. Again the source/optic system 80, comprises small spot x-ray source 82, and monolithic capillary optic 84. The optic has channels formed by individual glass capillaries 89 which have been fused together. The channel openings 86 at the input end 88 are positioned to capture radiation from divergent source 82. In this particular embodiment, however, the optic output end 90 is shaped to form a very small spot converging beam. For this case, because the radiation is turned through twice the angle of the quasi-parallel output optic, so the maximum capture angle is just .psi., the maximum bend angle. A preferred embodiment of this system, designed for approximately 8 keV x-rays, can be specified as follows. Again referring to FIG. 5, the x-ray source 82, has an anode spot size of around 100 micrometers. The converging optic 84, is placed essentially 27 millimeters in front of the source. The acceptance angle of the optic 85 is roughly 0.13 radians, and the optic has an output focal length 87 of nearly 2 millimeters. The overall length of the optic is about 165 millimeters. The optic input diameter 88 is approximately 7 millimeters, with input channel diameters of essentially 14 micrometers. The output diameter 90 is roughly 0.6 millimeters. The maximum channel diameter is around 10 micrometers. This invention has been specified in part by specific embodiments. It is to be understood that it will be apparent to those skilled in the art that various modifications, substitutions, additions and the like can be made without departing from the spirit of the invention, the scope of which is defined by the claims which follow and their equivalents.