EP1102302B1 - Monochromatic x-ray source - Google Patents

Description

The invention relates to an X-ray source for generating a largely monochromatic fluorescent X-ray radiation having a primary and a secondary target.

An x-ray source of this kind is known from US Pat. No. 3,867,637 and comprises in an x-ray tube essentially a (primary) target, which is opposite to a cathode and in which x-rays are generated by the incidence of an electron beam. The target rests on a substrate, which can be made of a light metal such as aluminum or beryllium, for example, and serves to mechanically hold the target and to ensure a vacuum-tight closure of the x-ray tube. The substrate is substantially transmissive to the X-rays emanating from the target and selected to be thick enough to absorb any incident electrons. On the other side of the substrate, a fluorescent material (secondary target) is applied, which may be, for example, cerium oxide, so that by the incident from the primary target X-rays, a material-dependent monochromatic fluorescence X-radiation is excited.

A problem with these known X-ray sources is that it is relatively difficult to couple a large proportion of the X-radiation generated in the primary target into the secondary target. This has the consequence that the intensity of the excited monochromatic fluorescence X-ray radiation is correspondingly low or can be increased by changing the targets only at the expense of the spectral purity.

The invention is therefore an object of the invention to provide an X-ray source of the type mentioned, can be generated with the substantially monochromatic fluorescence X-ray radiation with a higher radiation intensity at the same time high spectral purity.

This object is achieved with an X-ray source of the type mentioned, which is characterized in that the primary target is a liquid metal or a liquid metal alloy, the / between a first, permeable to an electron beam and a second, permeable to X-ray window to which the secondary target adjoins, is guided in such a way that electrons striking the primary target through the first window produce x-rays which, upon reaching the secondary target, substantially have a maximum energy corresponding to an absorption edge of the secondary target , so that in the secondary target a largely monochromatic fluorescence X-radiation is excited.

The (at least in the operating state of the X-ray source) liquid metal or the metal alloy fulfills not only the function of the primary target, but at the same time effective removal of heat from the target and cools the windows, in particular at the first window by the incident electron beam a relatively strong heat development occurs. The cooling has the consequence that the electron radiation and thus the thermal power density can be significantly increased, so that also increases the radiation intensity of the monochromatic fluorescence X-rays accordingly.

The dependent claims have advantageous developments of the invention to the content. The execution of the window according to claim 2 has the advantage that they are on the one hand particularly stable, so that they can withstand the flow pressure of the flowing liquid metal even at relatively low thickness and on the other hand elicit the electron or X-ray only a very small amount of energy.

The embodiment according to claim 3 has the advantage that a particularly effective dissipation of heat from the windows is achieved.

Finally, the embodiment according to claims 4 and 5 makes it possible to considerably improve the spectral purity of the X-ray radiation coupled out of the secondary target.

• Fig. 1 eine schematische Darstellung der Ausführungsform;
• Fig. 2 eine schematische Darstellung eines Teils der Röntgenstrahlenquelle;
• Fig. 3 einen schematischen Querschnitt durch eine erste Targetanordnung;
• Fig. 4 einen schematischen Querschnitt durch eine zweite Targetanordnung;
• Fig. 5 eine graphische Darstellung der spektralen Verläufe der Röntgenstrahlung bei verschiedenen Auslesewinkeln; und
• Fig. 6 eine graphische Darstellung der spektralen Reinheit einer Röntgenlinie in Abhängigkeit von dem Auslesewinkel.

Further details, features and advantages of the invention will become apparent from the following description of a preferred embodiment with reference to the newspaper. It shows:

• Fig. 1 is a schematic representation of the embodiment;
• Fig. 2 is a schematic representation of a part of the X-ray source;
• 3 shows a schematic cross section through a first target arrangement;
• 4 shows a schematic cross section through a second target arrangement;
• 5 shows a graphic representation of the spectral characteristics of the X-radiation at different read-out angles; and
• 6 shows a graphic representation of the spectral purity of an x-ray line as a function of the read-out angle.

In Figure 1, an electrically preferably grounded tube piston 1 is shown, which is closed vacuum-tight by a first window 2. In the vacuum space of the tube piston is a cathode 3, which emits an electron beam 4 in the operating state, which passes through the first window 2 on a primary target 10 in the form of a liquid metal, so that by interaction with the electrons, an X-ray radiation. The liquid metal (or liquid metal alloy) is in a system 5. This system includes conduits 50 through which the liquid metal is driven by a pump 52 having a portion 51 opposite the first window 2 and a heat exchanger 53 the heat generated in the liquid metal can be dissipated by means of a cooling circuit.

At the opposite side of the first window 2, the section 51 has a second window 6 through which the X-radiation excited in the liquid metal (primary target) enters a secondary target 11 to excite monochromatic fluorescent X-radiation there. This radiation is finally masked out via a device 8 adjacent to the secondary target.

The purpose of the first window 2 is to vacuum-tightly close both the tube piston 1 and the section 51 through which the liquid metal flows. The first In addition, the window should be made of a material which is as transparent as possible to the electron beam, so that the energy loss of the electrons as they pass through the window and thus also the resulting heat is as low as possible. Finally, the window should also have the highest possible thermal conductivity.

Diamond has proven to be a particularly suitable material, which offers sufficient mechanical stability even at a window thickness of 1 μm. The energy loss that electrons with an energy of e.g. 150 keV in such a window is less than 1%, so that the heat flow in the window caused by the electrons is lower than 500 W when the liquid metal is heated by the electrons of 50 kW. Further advantages of diamond include its high thermal conductivity and the fact that it can be heated up to 1500 ° C in an oxygen-free environment without irreversible changes.

The pump 52 preferably operates according to the magnetohydrodynamic principle, so that it has no mechanically moving parts. An example of such a pump is described in U.S. Patent 4,953,191.

FIG. 2 shows the region of the section 51 of the system 5 with the first window 2, which comprises a silicon carrier 22 with a thickness of, for example, 300 μm, and a diamond layer 23 with a thickness of, for example, 100 μm, wherein in the region of the passage of the electron beam an opening 21 is made in the silicon carrier. The production of such a window is described, for example, in EP-A-0 957 506 [PHD 98-044] .

The first window 2 opposite second window 6 of the section 51 is preferably constructed in the same manner as the first window. It is important here that it has a good permeability to the X-rays excited in the liquid metal. Diamond has also proved to be advantageous for this because it not only has a high thermal conductivity, but absorbs the X-rays generated in the target in only a very small extent, since on the one hand be very thin due to its strength can and on the other hand has a low atomic number.

Finally, the secondary target 11 with the masking device 8 is attached to the second window 6, which will be explained in more detail with reference to FIG.

In order to increase the efficiency of the heat removal by the liquid metal, located in the region of the windows 2, 6 of the section 51, a cross-sectional constriction 54, with which an accelerated and turbulent flowing flow is generated in this area. The cross-sectional constriction, for example, is asymmetric as shown and has a cross-sectional wing-like profile, wherein the free passage area for the liquid metal may be about 100 microns versus a diameter of the conduit 50 of about 10 mm. Furthermore, the cross-sectional constriction 54 and the second window 6 are preferably made of the same material and form an element fulfilling both functions.

For the primary target metals or metal alloys can be used which have a high atomic number and are liquid at the lowest possible temperature, preferably room temperature. Examples are mercury, a metal alloy of 62.5% Ga, 21.5% In and 16% Sn or a Mecallregierung of 43% Bi, 21.7% Pb, 18.3% In, 8% Sn, 5% Cd and 4% Hg (all figures in weight percent). The secondary target may be, for example, tantalum.

Particularly in the case of the target arrangements shown in FIGS. 3 and 4, non-liquid metals (for example gold) or metal alloys may also be used for the first target.

FIG. 3 shows a schematic cross section through a first target arrangement in the form of a layer structure. The electron beam E hits through the first window 2 on the primary target 10, which serves as a converter and in which the X-rays are excited. These enter through the second window 6 in the secondary target 11 and produce there the largely monochromatic fluorescence X-ray Rfl.

wobei in Figur 3 diese Dicke als Radius R₁ um den Eintrittspunkt des Elektronenstrahls E in das primäre Target dargestellt ist.The operating principle is based on the following considerations: Let it be assumed that the incident electron beam has the energy E ₀ , while the energy of a (material-dependent) absorption edge K of the secondary target E _k . As the electrons diffuse through the primary target 10, they produce x-rays (ie, essentially bremsstrahlung with a relatively broad frequency spectrum) in a known manner and thereby lose energy. The thickness R _{1 of} the primary target, that is, the path length of the electrons through the primary target, is chosen to approximate the following condition: $${\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="imgf0001.png"\; state="\{\{state\}\}">R</figure-callout>}}_1=({\mathrm{e}}_0-{\mathrm{e}}_{\mathrm{k}})\square \mathrm{X}/\square \mathrm{e}$$

wherein in Figure 3, this thickness is shown as the radius R ₁ to the entry point of the electron beam E in the primary target.

In this equation, □ E / □ X means the mean energy loss of electrons per unit of path length over the energy interval E _{0 -Ek} . The electrons, which have passed through the primary target or the path length R ₁ , now only have the energy E _k and thus can not excite bremsstrahlung in the secondary target 11 with an energy that is greater than E _k . Since this energy corresponds to an absorption edge of the secondary target, there rather takes place an absorption of the corresponding X-rays and an excitation of higher energy states, by their return to the ground state the characteristic radiation (monochromatic X-ray line, fluorescence X-radiation) is generated.

If the path length through the primary target is significantly shorter than the value R ₁ calculated with the above equation, then the intensity of the generated X-radiation is correspondingly lower. If the path length is significantly larger, although a much higher proportion of the electrons is converted into X-radiation, it is also absorbed in the primary target before it can reach the secondary target. In Thus, the intensity of the monochromatic X-ray radiation is very low in these two cases.

wobei µ den linearen Dämpfungskoeffizienten für Röntgenstrahlen in dem sekundären Target darstellt. Die Photonen-Energie, bei der µ berechnet wird, sollte näherungsweise (E₀ - E_k) / 2 betragen.The thickness of the secondary target, which is represented in FIG. 3 by the radius R ₂ around the point of entry of the electron beam into the primary target, is chosen such that the intensity of the fluorescence X-radiation is as large as possible. A maximum value is reached if the following condition is met: $${\mathrm{R}}_2-{\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="imgf0001.png"\; state="\{\{state\}\}">R</figure-callout>}}_1=1/\mathrm{\mu }$$

where μ represents the linear attenuation coefficient for X-rays in the secondary target. The photon energy at which μ is calculated should be approximately (E ₀ - E _k ) / 2.

The monochromatic fluorescence X-radiation produced in the region of the secondary target calculated according to the above equation should be read out at an angle at which as far as possible no disturbing influence of Bremsstrahlung from the primary target with the path length R ₁ occurs. An optimal suppression of this Bremsstrahlung is observed when the fluorescent material itself serves as a radiation filter for this radiation. This is the case when the X-ray Rfl is read at a relatively small angle to the plane of the primary target. Such a direction is shown in FIG.

In order to further improve the spectral purity and to further minimize the bremsstrahlung spectrum present in the fluorescence X-ray spectrum, an increased filtering effect can be achieved with the second target arrangement shown in FIG.

Here, too, the electron beam strikes through the first window 2 onto the primary target 10, which may be a liquid or solid metal or a metal alloy. The generated X-radiation enters the secondary target 11 through the second window 6.

The excited monochromatic fluorescence X-ray Rfl is masked out via the device 8.

This device 8 consists of a material substantially opaque to the X-radiation with a high atomic number. Due to the funnel-shaped opening in the material, which narrows in the direction of the secondary target and whose major axis is at an angle of between about 65 ° and 90 ° to the direction of the incident electron beam, only such radiation from the secondary target is hidden has traveled certain path length.

The design of the optimal path length depends on the intended use of the X-ray source and is always a compromise between maximum intensity of the monochromatic X-ray and its spectral purity, that is, the filtering effect of the secondary target.

These relationships are graphically illustrated in FIGS. 5 and 6, in both figures for a target assembly of a 5 μm primary target of gold, a diamond window of 195 μm thickness and a secondary target of tantalum with a thickness of 150 μm in which an electron beam E with an energy of 150 keV is incident on the primary target.

FIG. 5 shows the course of the energy spectra of the monochromatic fluorescence X-ray radiation read at different angles, curve (1) in reflection for an Z angle of 90 to 180 degrees, curve (2) in transmission for an Z angle from 0 to 90 degrees and curve (3) in transmission for an Z angle of 65 to 90 degrees. The Z-angle extends as shown in Figures 5 and 6 between the direction of incidence of the electron beam and the readout direction.

Curve (1) shows the usual course in known X-ray tubes, which indeed show two distinct frequency lines, but also have a considerable Bremsstrahlung spectrum above and below these lines. In contrast, curve (2) indicates significantly reduced Bremsstrahlung spectrum and frequency lines with only slightly reduced intensity, while curve (3) is characterized by an extraordinarily high spectral purity at a significantly reduced intensity of the two frequency lines. Curve (2) in particular, however, represents a compromise between high spectral purity and only a slightly reduced intensity of monochromatic X-rays, which has not been achieved to that extent with the state of the art for many applications.

FIG. 6 shows the purity of the spectral monochromatic X-ray radiation (K □ line) in percent at every 5 degree intervals as a function of the Z angle. These measurements yielded a clear maximum at an Z angle of 82.5 degrees.

Claims (5)

1. An X-ray source for generating substantially monochromatic fluorescent X-rays with a primary and a secondary target, characterized in that the primary target (10) is a liquid metal or a liquid metal alloy which is conducted between a first window (2) being transparent to an electron beam, and a second window (6), being transparent to X-rays and adjoined by the secondary target (11), in such a manner that electrons which are incident on the primary target via the first window produce X-rays which exhibit, upon reaching the secondary target, essentially a maximum energy which corresponds to an absorption edge of the secondary target, so that substantially monochromatic fluorescent X-rays are excited in the secondary target.
2. An X-ray source as claimed in claim 1, characterized in that at least one of the two windows (2; 6) is a diamond window.
3. An X-ray source as claimed in claim 1, characterized in that the liquid metal or the liquid metal alloy is conducted between the first and the second window (2; 6) in a turbulent flow.
4. An X-ray source as claimed in claim 1, characterized in that it includes a device (8) for forming a monochromatic X-ray beam which has traveled a mean path length through the secondary target (11) in such a manner that an as large as possible part of the Bremsstrahlung from the primary target (10) is absorbed by the secondary target.
5. An X-ray source as claimed in claim 4, characterized in that the device (8) is formed by an X-ray shield on a free surface of the secondary target (11) which has a funnel-shaped opening which is constricted in the direction of the secondary target and whose main axis encloses an angle between approximately 65° and 90° relative to the direction of the incident electron beam (E).

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