Patent Number: 062333062
Section: description

FIG. 1 is a graphic representation of the intensity of the X-rays as generated by a known X-ray tube, illustrating the problem imposed by the X-ray absorption by a beryllium X-ray window. This graph has been obtained by theoretically calculating the intensity of X-rays as a function of the wavelength thereof (expressed in the reciprocal unit keV) as emitted by a nickel anode irradiated by an electron beam with an energy of 50 keV and a beam current of 60 mA. The intensity of this radiation is represented in an arbitrary measure; in this case it is expressed as a number of counting pulses per second (cps) of an arbitrary detector. This graph shows that at a relevant wavelength of the L.alpha. line of nickel of 146 nm (corresponding to an energy of 0.852 keV), an intensity of approximately 2.times.10.sup.12 cps is reached. FIG. 2 is a graphic representation of the absorption of the X-rays in a beryllium window of a known X-ray tube, illustrating the problem imposed by the X-ray absorption. For this Figure it is assumed that the X-rays must pass a beryllium window having a thickness of 100 .mu.m. The radiation is incident on this window as shown in FIG. 1. This graph shows that an intensity of approximately 2.times.10.sup.6 cps is reached for the above-mentioned wavelength of the La line of nickel of 1.46 nm (corresponding to an energy of 0.852 keV), thus implying an attenuation by a factor 10.sup.6. This attenuation is thus due to the presence of the 100 .mu.m beryllium window in the path of the X-rays. FIG. 3 shows a part of a known X-ray analysis apparatus which is of relevance to the invention and in which the X-ray source according to the invention can be used, the apparatus in this case being an X-ray spectrometer. The X-ray spectrometer includes an X-ray tube 2 for generating a beam of X-rays 10. The beam 10 irradiates a specimen 4 of a material to be examined by means of the X-ray spectrometer; the specimen is arranged in a specimen location for accommodating the specimen. The specimen 4 is arranged in a specimen holder 6 in a separate specimen space 8. X-ray fluorescent radiation which propagates in all directions is generated in the specimen as denoted by solid lines in the Figure. The fluorescent radiation irradiates an entrance slit 14 so that this entrance slit performs the function of the object 16 to be imaged for the imaging Rowland geometry to be described with reference to FIG. 2. In the Figure the width of the slit 14 is not shown to scale for the sake of clarity; in practical circumstances the width of this slit is of the order of from some tens of microns to some millimeters, depending on the relevant application. After having left the entrance slit 14, the beam of fluorescent radiation 18 is incident on an analysis crystal 28 which has a curved reflecting surface 29. The shape of the surface will be described in detail hereinafter with reference to FIG. 3. In this context it is to be noted merely that the surface 29 of the analysis crystal 28 has a cylindrical shape, i.e. the line of intersection of the crystal surface and the plane of drawing is a curved line (i.e. the line 29 in the Figure), but the line of intersection of the crystal surface and a plane perpendicular to the plane of drawing (for example, the plane perpendicular to the plane of drawing and also perpendicular to the line 29) is a straight line. In this arrangement the analysis crystal has a dual function: it selects the desired wavelength, determined by the angle of incidence, from the beam of fluorescent radiation on the basis of said Bragg relation (2d.sin'=n.lambda.), and it focuses the beam emanating from the apparent object point 16 in the image point 24. This image point is imaged on an exit slit 26 which constitutes the entrance collimator for an X-ray detector 20. Via an entrance window 22, the X-rays thus deflected are incident on the detector 20 in which they are detected, after which further signal processing is performed by means of electronic means (not shown). The analysis crystal 28 is mounted on a holder which is not shown in the Figure and is displaceable in two directions in the plane of drawing (as denoted by the arrows 30) and also rotatable about an axis 32 perpendicular to the plane of drawing. By virtue of these possibilities for displacement, the analysis crystal can be adjusted in an accurately defined orientation and position. The beam path from the X-ray tube 2 to the detector 20 extends through a hermetically sealable measuring space 24 which, in the case of X-rays of long wavelength, can be evacuated, if desired, or be filled with a gas which is suitable for such measurements. The known X-ray analysis apparatus utilizes a known X-ray tube which is provided with an exit window for the X-rays 10. When the invention is used, the X-ray window can be omitted because the function of this element is performed by the bundle of X-ray conducting capillary tubes, with the X-ray window provided thereon, which forms part of the X-ray source according to the invention. FIG. 4 is a diagrammatic representation of an X-ray source according to the invention. The X-ray source consists of an X-ray tube 7 in which an anode 40 is provided. The anode is irradiated by an electron beam 42 which forms a focal spot 56 on the anode so that X-rays 44 are generated in the anode in known manner; the X-rays can leave the X-ray tube 7 via a window opening 54. The X-ray source according to the invention is also provided with a bundle of capillary tubes 46 which conduct X-rays and one end of which is connected to the window opening 54 in a vacuum tight manner. The capillary tubes at that end of the bundle are directed towards the location 56 on the anode where the X-rays are generated. Even though FIG. 4 shows the bundle of capillary tubes as a bundle with gaps between the capillary tubes, a variety of constructions of this bundle is feasible. It is notably possible to construct an embodiment in which the capillary tubes are arranged against one another and are rigidly interconnected. The desired vacuumtightness of the bundle, required so as to enable vacuumtight connection of the bundle to the window opening 54 of the X-ray tube, can then be achieved by providing the exterior of the bundle with a layer of a synthetic material which is also connected to the inner side of the window opening 54. In FIG. 4 the vacuum sealing is diagrammatically represented by a plate-shaped support 58 in which the capillary tubes are provided in a vacuumtight manner. This plate-shaped support itself is mounted in the window opening 54 in a vacuumtight manner. An evacuated space is present in the housing 52 of the X-ray tube 7. This space is in vacuum contact with the interior of the capillary tubes, the other end 48 of which is sealed in a vacuumtight manner by means of an X-ray transparent X-ray window 50 which is made of a synthetic material or diamond of a very small thickness. This small thickness is possible because the ends of the capillary tubes of the bundle 46 act as a fine-meshed supporting grid having a periodic structure of, for example 10 .mu.m, so that a thickness of 1 .mu.m is feasible without special steps being required. At the end 48 of the bundle 46 the capillary tubes may be oriented in such a manner that the X-rays emanating therefrom are concentrated onto one location again. The specimen 10 to be examined in the apparatus can be arranged in that location. The X-ray power taken up by the bundle 46 is dependent on the space angle at which the entrance side of the bundle is perceived from the X-ray focus 56, on the transmission of the X-rays by the capillary tubes, and on the extent to which the window 50 transmits the X-rays. These parameters can all be varied within broad limits. In order to make a coarse estimate nevertheless of the improved X-ray yield according to the invention, it will be assumed that said space angle equals 0.2 staradian (corresponding to a receiving surface area of 1 cm.sup.2 at a distance of 2 cm from the anode), that said transmission is of the order of magnitude of 10% (see the cited article Proceedings of SPIE, "Polycapillary Focusing . . .", Table 2, paragraph 3.2) and that the X-ray absorption in the X-ray window is negligibly small because of the small thickness and the suitable choice of material. This means that a fraction of approximately 3% (i.e. 0.2/2.pi.) of the total amount of X-rays emitted by the anode in a space angle of 2.pi. staradians enters the capillary tubes which pass on this fraction with a transmission efficiency of 10%, so that ultimately 0.3% of the radiation generated in the anode comes to the benefit of the irradiation of the specimen. Even if all X-rays generated in the anode in the known X-ray tubes were situated within the space angle used (which is certainly not the case), the intensity at the area of the specimen would still be improved by a factor of approximately 3000 (0.3% of 10.sup.6) by carrying out the invention.