X-ray microscopy, shadow imaging, Kumakhov optics, polycapillary cone, spatial resolution

Essentially new approach to the x-ray microscopy, based on application cone-shaped polycapillary structure of Kumakhov optics is described. The given approach allows on the basis of usual middle-focus source to receive images of shadow objects with the resolution about 1 um. In addition it is possible to receive images of self-luminous objects with the same resolution. The description of experimental installation and results of researching of shadow and self-luminous objects is submitted. First results of the image computer processing with the purpose of elimination polycapillary grid are also presented.

In order to receive an images with the micron resolution an existing x-ray shadow imaging microscopes require reception expensive x-ray sources, usually on a basis synchrotron. But work on synchrotron stations is extremely expensive and not readily available. The use of microfocus x-ray tubes is also often done, however its brightness is substantially smaller. To increase of x-ray tube brightness one use increase of electronic beam density by means of electromagnetic focusing systems. Such approach requires cooling both the focusing system and a target that results in complication of design and rise the price of a microscope.

Use of polycapillary Kumakhov optics could be way out of this situation. Polycapillary optics affords improvement of resolution and contrast of images. In a foreign works on use of polycapillary optics authors accentuate the ability of the contrast increase at the expense of scatter component rejection in medical images. Probably, it is accounted for the technological level of manufacture of polycapillary systems with the submicron channels is not sufficient, that necessary for reception of images with the micron resolution. It is necessary to note that modern commercial x-ray visualizing systems have the resolution at a level 5-6 μm.

In the home works both directions of polycapillary optics use are considered – increase the contrast of images with not high resolution for medical and technical diagnostics, and receiving micron resolution for x-ray microscopy. There is also a possibility of use the imaging properties of Kumakhov polycapillary optics in x-ray astronomy. From the first experimental works on polycapillary X-ray microscopy technological level of polycapillary structures manufacture has considerably grown, that allows to increase essentially characteristics of schemes of microscopy on the basis of Kumakhov optics. Thus, works under the application of modern Kumakhov optics to X-ray microscopy are expedient. In the given work research of use the high-resolution polycapillary system in x-ray microscopy is made.

For development of scheme and technical decisions the prototype of microscope was created (Fig. 1). We choose the scheme with vertical optical axis. The microscope consists of two paths: x-ray one and auxiliary – optical. The optical path is intended for a choice of the zone of research on an object. It incorporates an optical microscope, such as MIKMED-1, connected with the TV-camera. After arrangement of object in the center of the optical path field it is transferred to the field of a x-ray path.

Fig.1 X-ray imaging microscope with polycapillary cone

The x-ray path contains a generator on the basis of x-ray tube BS-11.For decrease of the tube focal spot size up to 30 um the electromagnetic focusing is applied. The tube is placed in the case of a generator. The case rests on spherical plate, which moves in a vertical direction by the screw. Thus change of distance up to the object and visualizer in an axial direction is provided. Movement of a focal spot in a cross direction is made by change of angle of inclination concerning an axis of the x-ray path at its turn on spherical plate. Two micrometric screws established transversely carry out turn.

For arrangement of micro object in a field of a x-ray path three-coordinate positioner is used. Moving along a direction between optical and x-ray paths is made by two-level positioner. The motion is applied to moving on distances up to 100 mm from an optical path in x-ray with the stepping motor with step 1.25 μm. The three-coordinate motion is applied for exact positioning in a field of a x-ray path with engines of a direct current with accuracy 0.1 um. With the help of this motion the object is established in an axial direction closely to an entrance plane of the polycapillary optical system intended for increase of the shadow image of object in a x-ray range.

The system contains the polycapillary cone placed in the holder on an optical axis on a focal length from a source. In the way alignment a generator micrometric screws achieve overlapping of a focal spot with a point of focus of a cone. X-ray radiation of a focal spot, passing through object, creates the shadow image on an input of a cone. This image is divided by capillaries on an input of a cone on the separate elements corresponding to the sizes of channels. On an output of a cone the image consisting of increased elements is formed. The increased X-ray image acts on an entrance surface of a visual analyzer where it will be transformed in visible. For preservation of the high resolution the backlash between target section of a cone and an entrance plane gets out minimal and does not surpass 0.5 mm.

In a microscope two types of visual analyzers are used. In the elementary variant the principle of direct detecting of the x-ray image on the CCD-matrix of the TV-camera is realized. At a target end face of a cone the reception surface of the CCD-matrix camera Sony SK 1043 is established. Dimension of a matrix 720×576 elements in the size 6.5 μm. The received TV-signal is entered in a computer with the help of a standard capture card. Processing of the image consists in accumulation of the separate frame. Division of the saved up frame into the image of an output of a cone is made for reception of the image of object without object.

In the other type of a visual analyzer the method of transformation of the x-ray image in visible on scintillation screen is used. The oxysulfide gadolinium screen with terbium is put on a optical transparent substrate. The optical image from the screen with the help of a micro objective is projected on photocathode electron-optical image intensifier. After amplification of brightness the image optics of carry goes con the CCD-matrix of the TV-chamber. The further way of processing of the image is made under the scheme described above. X-ray images of an output of a cone without object and the image with the object, divided on thee image without object are submitted (Fig. 2).

Fig. 2 X-ray images of a cone output A) without object and B) with the object

The described units of a microscope: the optical microscope, a x-ray generator with a power unit, three-coordinate positioner of a sample, a visual analyzer are placed on the basis consisting of two plates, connected by racks, as shown in Fig. 1.

The principle of the microscope action based on projection a x-ray image of shadow or self-luminous object on a polycapillary cone input with the large relation entrance d_k and exit D_k diameters (M_k=D_k/d_k=5-15 of times). The image is broken up into elements by a grid of entrance apertures of polycapillary structure, that is the raster is formed. Each capillary locks in an element of the image and moves it with increase to the detector(a film, a x-ray visual analyzer). Due to this the first step of increase – in a x-ray range is provided. The increase in similar systems is determined by the relation: M₀=D₀/d₀, where d₀-diameter of a capillary on a system input; D₀ – diameter of a capillary on output, and M₀=M_k=M. Functions of a cone consist in formation and increase in the image, increase of the resolution, increase in contrast. Length L and increase M of a polycapillary cone are connected to a focal length f (distance between x-ray tube focus and cone input) by the following expression: f = L/(M-1). For example, for M=10 and L=45 mm, f=5 mm. The entrance area of a cone determines the field of vision.

Unlike the projection scheme of increase on the basis of a point source, where the resolution depends on its size, in the scheme with polycapillary optics it is determined basically by the entrance sizes of capillary. Polycapillary structures in the optical scheme of projection microscope allow to receive x-ray images with the resolution at a level of 2-3 entrance diameters of the channel.

Thus, a technological level of manufacture of polycapillary optics at the Institute for Roentgen optics (IRO), allowing to create X-ray optical systems with the submicron channels, theoretically enables to achieve the submicron resolution of x-ray images even on the basis of a usual low-power X-ray tube without a high degree of focusing of an electronic beam. It is necessary to note, that capillary structures of Kumakhovoptics having a discrete structure, it is well combined with modern raster digital visualizing systems on the basis of CCD-matrixes.

By present time the technological level at the IRO allows to receive polycapillary cones with entrance diameter of the channel less then 50 nm and relation of entrance and target diameters about 100. However transmission cones with extremely small diameters are very low, that demands very long term of exposure.

The optical scheme of experiment consists of a source, object, a polycapillary cone, and visual analyzer of x-ray images (Figs. 1, 3). As source transmission type with the copper anode and electromagnetic focusing microfocus tube BS-11 serves in a mode 2.5 W, 40 keV. Electromagnetic focusing is carried out for increase in density of a flux of radiation within the limits of an input of a cone. The effective size of a tube focal spot is about 200 um. Thus the exposition takes 10 sec.

Fig. 3 Experimental scheme of shadow object microscopy

After transformation of the x-ray image from a cone output on the scintillation screen the light image is exposed to the further increase a micro objective in a light path, to amplification of brightness in electron-optical image intensifier and is registered by the CCD-camera as the digital image. The total resolution of the visualizing device resulted from a luminophor plane ~5 μm. Video signal from the CCD-camera is captured by a standard card of a personal computer. The program with pixel accumulation, averaging and division makes processing of the image. The further processing is made by computer methods before reception on the monitor of a computer of the final increased image.

X-ray shadow images of test-object, representing pixel by pixel division of the image of an cone output with object and without (Fig. 4), representing tungsten plate in height 2 um on a silicon pedestal in height 2.5 um, towering above a thick silicon plate are received.

Fig. 4 Images of the test object A) in the electron microscope; B) in the X-ray shadow microscope with high-resolution polycapillary Kumakhov optics

A silicon pedestal on a direction of distribution of x-ray radiation has much smaller thickness (~10 um), than the silicon plate on which it is (~0.5 mm). Therefore on the X-ray image the pedestal is created much with smaller contrast, than a plate. Thickness of tungsten on a direction of a course of beams makes about 10 um that is enough for reception of good contrast. Apparently, details of the micron sizes that corresponds to theoretical estimations of resolution R=3d₀=0.9 um are distinctly resolved.

Also, on the image the grid formed by polycapillary walls is visible (see Fig. 4). The technique of elimination of the given grid is developed by software (see Fig. 7).

Images of two gold wires by diameter 10 μm with a backlash between them in 3 μm have been received. However from reasoning resulted in the given work follows, that the real resolution of that system makes 1.5-2 um. In our work images of test-object with the micron resolution are really received.

In x-ray microscopy on the basis of polycapillary Kumakhov optics there is an opportunity to use advantage of the given optics, such as broadbandness. Broadbandness allows to use, for example, K-jump in spectra of absorption of various elements for increase of contrast as against other types of the (optics traditionally used in x-ray microscopy, distinguished monochromaticity. Also, it is necessary to note that besides use of hard X-ray radiation, distribution of the given technique to a soft range that modern Kumakhov optics allows is possible.

The problem of reception of images self-luminous objects in X-ray range is still actual, that is determined by perfection as traditional X-ray sources, such as X-ray tube, and rather new, for example, laser-plasma sources. For determination of the X-ray tube focal spot sizes by means of reception of its image the cylindrical X-ray-optical system of Kumakhov optics was used. However, for measurement of the microfocus tube spot sizes, accuracy of the given technique frequently has not enough, because the size of an error brought by polycapillary structure, is comparable to the size of a determined anodic spot. Besides at the micron sizes of a focal spot, the sizes of the received image of an anodic spot on an output of a column can be lower than a limit of the resolution of detecting system: modern commercial x-ray visualizing systems have the resolution at a level 5-6 um, and the sizes of the microfocus tube focus spots are some microns.

In the given situation use of opportunities of Kumakhov optics is expedient at work with X-ray images. By application of increasing high-resolution imaging optical element of the given optics it is possible to take off restrictions of the detector on resolution.

The principle of a technique will consist in the following. To the anode on some distance, smaller focal, the capillary structure, in the form of the truncated cone is established on which output detecting system is brought (Fig. 5).

Fig. 5 Experimental scheme of self-luminous object microscopy

The focal length is determined by a point of a geometrical convergence the truncated cone generator. As detector for an express estimation the x-ray visual analyzer is the most expedient to use systems. Preliminary calibration of system allows determining the sizes of the received image of a focal spot.

Functions of a cone in the given technique will consist in formation and increase in the image. Diameter of a capillary of polycapillary system on an input, i.e. before a focal spot, should be much less than an an anode spot. On an output of a lens it is necessary for diameter of a capillary to correspond to the resolution of a visual analyzer. In such systems the capillary optics gives the information on object to the detector with accuracy corresponding 2-3 entrance diameters of a capillary. Thus, for the resolution 1 um it is necessary, that diameter of a capillary was no more than～0.5 μm. Naturally, there should be a rigid control of diameter of a capillary over an input and an output that the system transferred the information without distortions. Resolution of the given technique raises both due to reduction of the entrance size of the channel, and for the account conicity, i.e. increases in the image.

The given approach allows increasing accuracy of determination of the spot sizes with use of modern optical systems up to 1 micron. Lack of the given method in comparison with a method on the basis of a column is the big difficulty alignment a cone concerning a tube. Also can bring the contribution of distortion in connection with an insufficient regularity of capillary structure that arises owing to technological difficulties of manufacturing with reduction of the size of the channel. The cone-shaped polycapillary structure was applied to realization of a technique(cone Kumakhov half lens), the polycapillary element is attached to the special holder capable of moving in three orthogonal directions. It allowed carrying out exact cone alignment concerning focal spot of x-ray tube. Besides the x-ray visual analyzer was used.

Examples of x-ray images of a focal spot of a microfocus X-ray tube with the resolution about I micron, received on the basis of the described technique, are resulted in Fig. 6.

Fig. 6 Image of microfocus tube focal spot obtained in X-ray microscope with polycapillary cone

As a result of the analysis of this image determine the necessary sizes of an anode spot of a tube. On a luminescence of a spot are visible shadows, formed by polycapillary walls X-ray optical structures. But they do not bring significant distortions in the image for the purposes of definition of the sizes of a focal spot.

Works on realization of computer processing the x-ray images received with use of imaging polycapillary Kumakhov optics are conducted, allowing to eliminate on the image the characteristic defect connected to display of polycapillary walls on the final image of object (Fig. 7).

Fig. 7 The work example of a image reconstruction technique for a polycapillary microscope: A – removed polycapillary walls; B – final partial the image

Displacement of object be relative X-ray optical input (an inout of a polycapillary cone) on distances more then thickness polycapillary walls, with the purpose of providing for the analysis closed earlier sites by polycapillary walls, allows to receive the information on intensity of a luminescence of the given sites. Thus, there is an opportunity of integrity restoration of the x-ray image in all a field of a review of a microscope. It is carried out by carry of the information on intensity of X-ray radiation (pixels of a graphic file) to the basic image from other (displaced) images. Restoration and reconstruction of the image of object is carried out on the basis of reception and processing of a series of images of object with various displacement concerning an input of polycapillary structure. In result we have partial image with eliminated polycapillary walls.

In Fig. 7 the example of elimination polycapillary walls on the image of a metal grid is resulted. In Fig. 7A the image of a metal grid with removed by program way polycapillary walls is shown. In Fig.7B the result of application of a technique of elimination is shown. It is visible, that polycapillary walls it is considerably eliminated. Full elimination of a grid needs the further improvement of a technique.

The given processing is actual both for imaging and for scanning microscopy on basis Kumakhov optics as the problem of display polycapillary walls on a modern technological level of manufacture Kumakhov optics is actual for all listed cases.

We are received a x-ray shadow images with micron resolution on the basis of scintillation visualizing device with 5 μm spatial resolution and usual 4 W x-ray tube with 200 um focal spot. Such approach permits to realize, for example, a precision technique of determination an effective focal spot size for various x-ray tubes. The technique of elimination of a polycapillary grid on the images received with the help of Kumakhov optics is realized.

These results testify to an opportunity of the further improvement of quality of the images received by means of Kumakhov optics by optimization of the optical scheme, improvement of characteristics of separate components, first of all-a polycapillary cone, and due to program processing of final images.