DE4440448C2 - X-ray source with increased radiation yield - Google Patents

Description

The invention relates to an X-ray source, which has the same electrical power as conventional tubes in a certain spectral range an increased radiation power provides or vice versa, with the same radiation power of a much lower electrical Performance requires.

The efficiency of X-ray tubes when converting electrical energy into radiation energy is due to the physical principle of action, it is very low and is usually <1%. In the case of fine focus tubes in particular, the overall efficiency is up to immediate use the radiation deteriorated considerably due to the size of the solid angle used.

In addition to structural analysis, fine focus tubes have been used in particular in total reflection X-ray fluorescence analysis (DE-OS 29 11 596; "Instr. Developments in TRXRF", X-Ray Spectrometry, Vol. 20, pp. 23 ff, (1991)). If the usual dimensions of such measuring arrangements are taken into account, it can be estimated that less than 10 ^-4 of the X-rays emitted by the anode reach the sample.

For many applications, therefore, X-ray tubes with a power <500 W and so are connected complex high-voltage generators and cooling systems necessary. This situation prevents the improvement of many X-ray measuring systems.

For the deflection, focusing and filtering of X-rays, total or Bragg reflection-based systems are known ("A simple model for...", X-Ray Spectrometry, 22, pp. 216 ff (1993); DE-OS 39 40 251; DE-OS 38 42 561; EP 0244 504).

The systems are used as independent assemblies according to the intended effect arranged in the free beam path.

In the simplest case, waveguides of this type consist of two opposite polished ones Quartz glass plates or from stacks of such plane-parallel mirror surfaces (DE 38 04 798). Other systems are based on glass capillaries with also highly coated surfaces. Of the With simple arrangements, the gain in irradiance already reaches values between 3 and 7 (DE-Z .: "Experimentelle Technik der Physik", Volume XII, 1964, Issue 5, pp. 320 ... 326).

In principle, all surfaces of solids are suitable for total reflection if they have one have sufficient quality (surface roughness) and the angle of incidence of the radiation is one does not exceed a certain critical angle.

This angle, at which there is a transition from reflection to scattering and absorption, is for Radiation of higher energy is smaller than for low-energy radiation. For quartz applies e.g. B .:

E _cut = 32.3 / ϕ _crit

E _cut is the maximum radiation energy (keV) that is still reflected at the angle ϕ _crit (mrad). Higher energy radiation is scattered or absorbed.

By restricting the limit angle possible in the waveguide, a spectral angle can be obtained Realize edge filter function.

The main advantage of total reflection is its high efficiency, which is around 95%. They are disadvantageous very small angle in the range of 2 mrad.

If multilayer systems made of heavy and light elements or the network planes of crystals are used for the surface of the waveguide, the radiation transport through the waveguide is realized at larger angles via the Bragg condition:
nλ = 2d sin θ.

Here λ is the wavelength of the radiation, d is the lattice spacing of the crystal lattice and θ the angle of incidence of the radiation.

Under these conditions, bandpass filter functions of the waveguide can be implemented relatively easily. The poorer reflection coefficient is disadvantageous.

Defined opening angles of the waveguide also have a collector effect and enable improved focusing of the X-ray beam.

The aim of the invention is a substantial increase in the ratio of electrical here Input power and the radiation power available at the exit window of the tube Overall efficiency of an X-ray tube, as well as a targeted influence on the Beam parameters of the X-rays emerging from the tube, such as beam divergence or Energy spectrum.

For this purpose, according to the invention, a conventional X-ray tube consisting of a cathode, a Acceleration path for electrons and an anode, where the X-rays are generated, modified in such a way that the photons produced in the X-ray tube are not only transferred as usual be emitted a window transparent for X-rays, but that immediately on Focal spot of the electrons on the anode is a waveguide arrangement that corresponds to the Condition of total reflection is designed, is arranged, the resulting X-rays couples with maximum efficiency, possibly with regard to the forwarding Divergence and / or spectrum influenced and then the coupling of the radiation is realized.

The essence of the invention is to be explained on the basis of exemplary embodiments.

Fig. 1 The principle solution.

Fig. 2 design of the X-ray source according to the invention with waveguide adjustment.

Fig. 3 coupling out radiation by means of a geometrically structured waveguide to reinforce the edge filter effect and limit the exit angle.

Fig. 4 radiation coupling with combined total and Bragg reflection for additional energy and wavelength selection.

In Fig. 1, a beam emitted from the cathode 1 electron beam 2 generated on the anode 3, the primary X-rays 4. Immediately at the focus on the anode 3 , the resulting x-ray radiation 4 is coupled into a waveguide arrangement, which forwards it to the exit window of the x-ray tube 6 and in this way is able to change / influence it in accordance with the principles of wave optics.

Fig. 2 shows the structural design of a waveguide X-ray tube with the possibility of adjusting the relative position of the focal spot on the anode and the inlet opening of the waveguide. The principle of generating X-rays corresponds to that which is generally used in fine focus tubes. An electron beam 2 emitted by a cathode 1 generates the primary X-ray radiation 4 on the anode 3 . The waveguide here consists of two quartz glass plates 5 . In conjunction with an adjustment mechanism and a bellows 9, the waveguide can be adjusted along a plane 8 parallel to the anode. In this example, the electron beam creates a line focus that is parallel to the entrance opening of the waveguide. X-ray radiation, which enters the opening of the waveguide and strikes its surface at an angle smaller than the critical angle of total reflection, is transported through the waveguide to the exit window 6 of the tube.

A shift of the waveguide parallel to the anode surface changes the proportion of coupled radiation, its spectrum and the beam profile at the exit window of the tube. If the focus is in the extension of the longitudinal axis of the waveguide before its opening, then the radiation at the exit of the x-ray tube consists of one from the waveguide by reflection transported portion and the portion that came directly from the focus to the tube window. If you move the waveguide parallel to the surface of the anode, you will only get it reflected radiation to the exit window of the tube.

A change in the relative position of focus and waveguide is of course also a Electron beam deflection possible.

The end face of the waveguide is advantageously covered by a metal coating Anode potential. This prevents disruptive space charges caused by backscattering or Primary electrons.

Various variants can be used for the waveguide. So instead of two reflective surfaces also stack systems of such surfaces are used. Same thing applies to capillaries or bundles of capillaries.

Fig. 3 shows a specific waveguide 5, wherein the structuring of the surface 10 enhances the edge filter effect and at the same time limits the opening angle of the exiting radiation 7. The structuring of the surface of the waveguide consists of stripe-shaped areas that are suitable for total reflection, separated from areas that cannot reflect the X-rays due to their geometrical position or surface condition. The reflective strips of the waveguide are arranged with respect to one another in such a way that the X-rays are mutually reflected from strips to strips.

Only X-rays that are reflected in such a way that they are just reflected on the next one reflective area of the other side of the waveguide, is to the exit of the tube transported.  

This makes the angular range at which the radiation is reflected from the surface strong limited. Radiation with higher energy, which is only reflected when it is under one This waveguide does not make a particularly shallow angle on the surface transported. This enhances the edge filter effect.

Such waveguides can advantageously be produced by etching or by vapor deposition non-reflective stripes on a polished quartz plate.

Fig. 4 shows the same waveguide using a surface 11 which enables an energetic bandpass filter by Bragg reflection. Due to the total reflection condition, the radiation emerging from the waveguide has only a very low divergence. If a multilayer system of light and heavy elements (e.g. W / Si or W / C layers) or the network planes of a crystal (LiF; graphite...) Are used for the last reflecting surface, the X-rays become corresponding at larger angles of incidence reflected the Bragg condition. In contrast to the total reflection condition, the effect here is an energetic bandpass filter.

Claims (5)

1. X-ray radiation source with increased radiation yield on the basis of the electrons accelerated in a vacuum from a cathode to an anode with an X-ray optical system arranged inside the X-ray tube, characterized in that the X-ray optical system is based on total reflection and consists of at least two opposite surfaces or crystal planes consisting of waveguides ( 5 ), which couples as large a portion of the radiation ( 4 ) directly to the anode as possible, changes it spectrally and directs it to the exit window ( 6 ) of the X-ray tube. 2. X-ray source according to claim 1, characterized in that in one of plates existing waveguide through mechanical devices the plate spacing at the entrance and Exit opening and the relative position of the X-ray focus and the entrance window of the waveguide are changeable. 3. X-ray source according to claim 1 and 2, characterized in that the adjustment with Piezoceramic components are used. 4. X-ray source according to claim 1 to 3, characterized in that instead of one Waveguide made of reflective plates are used one or more capillaries attached to their Realize a total reflection inside. 5. X-ray source according to claim 1 to 4, characterized in that by a Structuring the surface of the waveguide so that it is strip-shaped reflective and non-reflective areas of the surface perpendicular to the direction of propagation detach the x-ray beam together, preventing particularly small angles of reflection and thus improves the edge filter effect of the waveguide for higher energies and the divergence of the emerging X-rays is reduced.