Patent Number: 055704085
Section: summary

FIELD OF THE INVENTION This invention relates broadly to the field of x-rays. More particularly this invention relates to the field of x-ray optics. This invention provides a device and a method for improvement in the capability of capillary x-ray optic/x-ray source systems to produce high intensity, small diameter x-ray beams. BACKGROUND OF THE ART When samples are analyzed by various x-ray techniques, such as x-ray diffraction, it is desirable that the dimensions of the x-ray beam hitting the sample be on the order of the sample size, or of the order of the spot on the sample to be examined. This criteria on beam size is important because it maximizes spacial resolution, while minimizing background noise produced by unwanted photons. In many cases, for example in the case of x-ray diffraction of protein crystals, sample sizes are very small, and conventional x-ray diffraction equipment does not function efficiently. When traditional laboratory x-ray sources are used to analyze such small samples, beams of appropriate size are typically obtained by collimation methods. This includes such things as passing the x-ray beam through pin holes cut into x-ray absorbing materials such as lead. Because low beam divergence is also desirable, these pin holes must be placed a significant distance away from the source. This means that the solid angle of collection from the source is quite small. This in turn results in a very low intensity beam reaching the sample. One significant disadvantage of a low intensity beam is that measurement times can be extremely long. For some samples this is merely an inconvenience. However, for samples like protein crystals which have relatively short life times, this extended period of analysis can render the analysis technique useless. In all cases, extended measurement times lead to a decrease in the signal-to-noise ratio. Also, it is important for commercial analysis operations to maximize the sample through-put by minimizing analysis time. Shorter analysis times can thus lead to substantial financial rewards. It is known in the art that to obtain more x-rays from a source, a larger spot size on the anode is required. Thus, conventional wisdom dictates that in order to decrease power transmitted to a sample, either with or without an optic, a more powerful source with a larger spot size should be used. A general rule that is followed is that the source spot size should be the size of the sample being analyzed. It is known to the art that single hollow glass capillaries can form x-ray beams of very small dimensions see for example P. B. Hirsch and J. N. Keller, Proc. Phys. Soc. 64 369 (1951). Tapering these single capillaries to further limit output spot size is also known to the art see E. A. Stern et. al.Appl. Opt. 27 5135 (1988). However, both these devices only capture x rays from a very small portion of the source. Thus, their use also leads to x-ray beams of less intensity than is desired. Yet another disadvantage of the tapered devices is that the minimum x-ray spot size is located right at the tip of the device. This places strict limitations on the positioning of a sample. In addition, these single tapered capillaries can only form a small spot with considerable divergence. Often times for diffraction experiments, a parallel beam is desirable. Also known to the art are multi-fiber polycapillary x-ray optics. These devices form a particular class of a more general type of x-ray and neutron optics known as Kumakhov optics. See for example U.S. Pat. No. 5,192,869 to Kumakhov. Disclosed in this patent are optics with multiple fibers which are designed to produce high flux quasi-parallel beams. Although these optics can capture a large solid angle of x-rays from diverging sources, their potential for capturing from a small spot source or for forming small dimension output beams is limited by the relatively large outer diameter of the individual polycapillary fibers. The outer diameter of the fibers is on the order of 0.5 millimeters. Because of the fiber outer diameter these multi-fiber optics have a minimum input focal length roughly 150 millimeters. The critical angle for total external reflection at 8 keV for glass is four milliradians. Effective transmission after many reflections is obtained only if the photons are approximately one-half the critical angle. So using 0.5 mm diameter fibers, geometry shows that with a source as small as 100 .mu.m, the source-optic distance should be at least 150 mm for the outer channels to transmit effectively. Because of this relatively long input focal distance to capture a large angular range of x-rays from the source the input diameter needs to be relatively large which in turn constrains the minimum diameter and maximum intensity (photons/unit area) of the output beam. The minimum beam diameter for a multi-fiber polycapillary optic with a 0.15 radian capture angle which forms a quasi-parallel beam is on the order of 30 millimeters. These optics are thus not appropriate to produce the intense small diameter x-ray beams needed for small sample diffraction experiments such as protein crystallography. For focusing optics, because of the fiber diameter, the minimum focused spot sized has a diameter on the order of 0.5 millimeters. OBJECT OF THE INVENTION Thus it is the object of the subject invention to provide a solution to the long felt need in the art for laboratory based, small dimension, high intensity x-ray beams. It is another object of this invention to allow the analysis sample to be placed at a position removed from the output end of the device. It is yet another object of this invention to provide a small, intense x-ray beam which is highly collimated with a minimum of divergence. Yet another object of this invention is to produce small, high intensity, focused x-ray spots. Another object of this invention is to provide these benefits in a relatively compact, and cost effective system. BRIEF SUMMARY OF THE INVENTION The subject invention accomplishes these objects with a carefully engineered x-ray source/capillary optic system comprising: 1) A monolithic multiple-channel capillary optic with scaled down input and output diameters minimized with respect to photon energy, source diameter, and channel diameter; and, PA1 2) an x-ray source with a spot size designed to maximize optic output intensity for a desired output beam diameter. The specially designed optic is positioned within 60 mm or less relative to the x-ray source. Monolithic optics are an essentially integral one-piece structure in which fiber channels are closely packed and self-aligning along their entire length. At the input end of the optic the channels are oriented to aim substantially at the x-ray source. The output end of the optic can be shaped to form either a converging, or a quasi-parallel beam, depending on the intended use of the invention. The smaller source, although less powerful, provides an increase in the areal density of x-rays. The monolithic optic enables the efficient capture of the small spot x rays, because each individual channel can be aligned more efficiently with the source spot. Surprisingly, it has been discovered that a small spot, lower power source, when combined with a monolithic capillary optic's superior x-ray collection abilities, can lead to a higher intensity of x-rays at the output of the optic when compared with the use of a large spot, higher power source with or without an optic. The basic idea behind the invention then, is to continue to capture the x-rays from the source, and to squeeze these photons into a smaller output space in order to produce the desired high intensity, small diameter beam. This requires significant reengineering of existing optic designs, and modification of the x-ray source used. The first modification is that the input diameter of the optic must be decreased from what is currently known. A critical point to the invention is that in order to keep the same amount of photons entering the input end of the optic, the optic must be moved closer to the x-ray source to maintain the same capture solid angle. Characteristic input focal lengths of the subject invention are less than half of the roughly 150 millimeters required for the best multi-fiber polycapillary optics. Moving closer and using smaller input diameters all aimed at a common point, means the optic will "see" a smaller portion of the source. Thus, another key element of the subject invention is to decrease the source spot size in order to increase the power density and therefor the x-ray production from the area of the source from the which the optic captures photons. This is done in spite of the fact that the total number of x-rays emerging from the source is decreased. This invention provides for more efficient use of existing x-ray power.