X-ray tube, has anode partially comprising surface coatings provided outside stopping area of focal spot, where surface coatings are made of material with nuclear charge number less than nuclear charge number of material of anode

The invention relates to an x-ray tube with a vacuum housing, in which an anode is arranged, which generates useful x-ray radiation in a location area of ​​the focal spot when electrons generated in an electron source strike exits the vacuum housing through an X-ray exit window.

X-ray radiation is usually generated in X-ray tubes by bombarding an anode with electrons. The electrons are in turn released from an electron source (cathode with a thermionic emitter or a field emitter) and accelerated to the desired primary energy via a high voltage that is present between the electron source and the anode. When the electrons strike the material of the anode in the focal spot, the interaction of the electrons with the atomic nuclei of the anode material partially converts the kinetic energy of the electrons into X-rays. The yield of the generated X-rays, ie the number of X-ray quanta over the entire energy range has an almost linear dependence on the atomic number (atomic number) Z of the anode material used (for example tungsten, Z = 74) f.

For correct functioning, the entire arrangement must be housed in a vacuum housing (vacuum envelope). Usually, the vacuum housing consists of metal and / or a vacuum-tight insulator such as. B. glass or ceramic. Depending on the configuration of the X-ray tube, the vacuum housing is connected to the anode (vacuum housing and anode at the same potential, single-pole structure) or the vacuum housing is insulated from the anode and cathode (e.g. anode is at a higher potential than the vacuum housing located at near-earth potential, two-pole structure).

The X-ray radiation intended for use (X-ray useful radiation) should be able to leave the X-ray emitter in which the vacuum housing is arranged as far as possible without losses. For this purpose, an X-ray exit window made of an X-ray transparent material is incorporated into the vacuum housing. Because this X-ray exit window as part of the vacuum housing also meets certain requirements When it comes to the mechanical stability, a connection technology that is appropriate for the intended purpose and the necessary vacuum tightness, the choice of material often has to be a compromise regarding the optimal fulfillment of all the properties mentioned. While in older X-ray tubes the vacuum housing or at least a large part of it is made of glass, in modern X-ray tubes the vacuum housing is often made of metal and there is only an X-ray exit window made of an X-ray transparent material in the exit area of ​​the X-ray radiation from the X-ray tube. With the “Straton” rotary lobe X-ray tube from Siemens, it is known to realize the X-ray exit window by means of a smaller wall thickness than the vacuum housing made of steel. The useful x-ray radiation can thus emerge from the x-ray tube largely unfiltered.

The technically planned and constructively realized The location of the focal spot, i.e. the location of the anode where the primary beam of the electrons generated in the cathode impinges, can either be stationary (standing / fixed anodes) or form a focal path (rotating anodes in rotating anode X-ray tubes or rotary piston X-ray tubes).

The focal spot or focal path in turn emits a large number of electrons. On the one hand, these are secondary electrons, which are additionally removed from the anode material by excitation processes, and, on the other hand, they are also electrons of the primary beam, which leave the anode again after elastic scattering or after inelastic scattering or excitation processes. The latter electrons are referred to below as backscattered electrons.

In particular, the backscattered electrons at least partially still have a comparatively high energy (on average approx. 80% of the energy of the incident electrons). If the backscattered electrons on neighboring Parts of the vacuum housing, on the exit window or on the anode itself (this time also outside the actual focal spot or outside the actual focal path), they generate a more or less strong X-ray radiation due to their high energy depending on the material at the secondary point of impact and cause the material to heat up. Particularly in the case of high-performance X-ray tubes with vacuum housings made of a stable metal, the secondary points of impact are sources of a non-negligible X-ray radiation, which is referred to as extra-focal radiation.

In addition, the secondary point of incidence is again a source of backscatter and secondary electrons. The backscatter rate, i.e. the ratio of the number of re-emitted to incident electrons, varies with the atomic number Z of the material in a range from 0.2 at Z = 10 to 0.5 at Z = 50 (at an angle of incidence of the electrons of 40 ° to the surface normal). Particularly in the case of high-performance X-ray tubes, there is considerable backscattering at the secondary point of impact.

The heating that occurs due to the backscattered electrons in the vacuum housing, in the X-ray exit window and in the connection points between the vacuum housing and the X-ray exit window are often problematic.

This problem lies, for example, in the <de-docref CY="US" DNUM="7260181" KI="B2">US 7,260,181 B2</de-docref> underlying. The X-ray tube disclosed therein comprises a vacuum housing in which an X-ray exit window is installed in the vicinity of the anode surface, through which the X-ray radiation emitted by the anode can pass. In addition to the vacuum housing and the transparent X-ray exit window, a coating with a material with a high atomic number is applied in this area, primarily with an atomic number Z ≥ 35. This material has a comparatively high backscatter coefficient and causes electrons that have been backscattered from the anode and would hit the vacuum housing in the region of the window, in turn, backscattered, so that the thermal load on the vacuum housing and the X-ray exit window is reduced. However, the thermal protection of the vacuum housing is inevitably opposed by further heating of the anode, since some of the electrons backscattered by the layer strike the anode. In addition, more undesirable extrafocal radiation is generated by the layer, on the one hand by the impact of the backscattered electrons on the layer with a comparatively high atomic number, and on the other hand by the new impingement of multiply backscattered electrons on the anode.

An estimate of the penetration depth of the electrons into a material can be found in the publication of <nplcit><text>Kanaya K, Okayama S., "Penetration and energy-loss theory of electrons in solid targets", J. Phys. THERE ppl. Phys., Vol. 5, 1972, pp. 43-58</text></nplcit>, can be taken, for example, from the drawing shown on page 47 or from the reference number <b>10</b> formula on page 46.

It has been shown that the penetration depth D can be estimated very easily and easily instead of using the formula mentioned in this publication (penetration depth there with “R”) if the maximum occurring energy (approx. 140 keV) is taken into account. Then the depth of penetration is approximately D = 280 μm · Z^–0,165G / cm³· Ρ^–1

In the <de-docref CY="US" DNUM="2009/0279669" KI="A1">US 2009/0279669 A1</de-docref> describes an x-ray tube which comprises a vacuum housing with an x-ray exit window. A cathode and an anode are arranged in the vacuum housing. The generated X-ray radiation exits through the X-ray exit window in the vacuum housing. The vacuum housing has a coating in the area of ​​the X-ray exit window a material with a nuclear charge number Z ≥ 35 up to Z ≥ 70. For this X-ray tube, the explanations for<de-docref CY="US" DNUM="7260181" KI="B2">US 7,260,181 B2</de-docref> also.

The extra focal radiation generated by the backscattered electrons at the secondary impingement points, if it is not masked out by suitable countermeasures, leads to a considerable impairment of the image quality that can be achieved with the X-ray tube.

Subsequent masking of the extrafocal radiation, however, requires additional, not inconsiderable effort and can often not be carried out, depending on the area of ​​application of the X-ray tube. This applies in particular to applications that require a large illumination field and can therefore only be operated with a wide collimation, or to systems with a variable focus position, such as are used in high-resolution computed tomography.

In addition to the problem that El Electrons contribute to extrafocal radiation, the backscattering of the electrons also has a positive effect. The backscattering leads to a not insignificant energy flow away from the anode, which is proportional to the rate of the backscattering or secondary electrons and their electron energy. Since the entire energy of the primary beam is therefore not converted in the area where the focal spot is located, the heating of the focal spot or the focal path is less, so that the anode is exposed to less thermal stress.

Depending on the further path of the backscattered electrons, however, they can still contribute to the heating of the anode, for example by the backscattered electrons hitting the anode again at another point or being scattered back from a secondary point of impact and hitting the anode.

The problem of anode heating is generally increased by an increase in heat storage Ability of the anode, countered by direct anode cooling as possible and by using anode materials and connection techniques that allow the highest possible operating temperature of the anode structure. Here too there is a need to keep the heating of the anode as low as possible.

Due to the high temperature in the focal spot (up to approx.2,600 ° C) and the high kinetic energy of the electrons hitting the anode (typically approx. 70 keV up to a maximum of 140 keV), positively charged ions (cations ) from the material of the anode. The cations emerging from the anode are accelerated towards the cathode, which has a negative potential, and hit it. When the cations hit the cathode, contamination and immediate mechanical damage can occur. The contaminants can also be found in field emitters, for example made of carbon nano tubes due to their geometric shape and their filigree structure (approx. 10 nm diameter with a few μm length) lead to further damage. Even minor damage to the cathode leads to a deterioration in the emission properties and thus to a deterioration in the X-ray intensity. Greater damage inevitably leads to failure of the x-ray tube.

From the <de-docref CY="US" DNUM="2008/0112538" KI="A1">US 2008/0112538 A1</de-docref> an X-ray tube with a backscattered electron capture device is known. The backscattered electron trapping device has an electron absorption layer made of a material with a relatively low density and a relatively low atomic number of Z &lt;50. The backscatter electron capture device is intended to reduce the probability of a second scatter of backscatter electrons.

In the <de-docref CY="US" DNUM="2010/0046716" KI="A1">US 2010/0046716 A1</de-docref> is an X-ray tube with a vacuum housing in which a backscattered electron blocking device is arranged. The backscattered electron blocking device acts in the area of ​​the useful x-ray radiation on the backscattered electrons in such a way that they do not reach the x-ray exit window. For this purpose, the backscatter electron blocking device comprises a backscatter electron capture device and optionally a backscatter electron deflection unit and a backscatter electron deflection unit.

In the <de-docref CY="US" DNUM="4352196" KI="A1">US 4,352,196 A1</de-docref> described a rotating anode X-ray tube which has a shield in the area of ​​the rotating anode, which is coated with a material having a low atomic number.

Furthermore, in the <de-docref CY="DE" DNUM="19914825" KI="A1">DE 199 14 825 A1</de-docref> discloses, in an X-ray tube, to provide the inner surfaces of the vacuum housing with a coating made of a material having a low atomic number, at least in sections.

From the <de-docref CY="WO" DNUM="2008/090518" KI="A1">WO 2008/090518 A1</de-docref>&gt; an X-ray tube with a vacuum housing is known, in which a cathode and an anode are arranged. The X-ray radiation generated in the X-ray tube emerges through an X-ray exit window. A heel effect compensation filter, which is made of the same material as the anode body, is arranged on the outside of the vacuum housing in front of the X-ray exit window. The heel effect compensation filter consists e.g. B. from the material tungsten or a tungsten alloy, the material being applied by means of thin-film coating technology and therefore having a very small layer thickness.

Finally is out of <de-docref CY="US" DNUM="5199059" KI="A1">US 5,199,059 A1</de-docref> a rotating anode for an X-ray tube is known, in which the anode surface is provided with an oxide layer, for example aluminum oxide, titanium oxide. This coating of the anode surface improves the heat radiation capability.

Task of v The present invention is to provide an x-ray tube that provides improved image quality.

The object is achieved by an X-ray tube according to claim 1. Advantageous embodiments of the X-ray tube according to the invention are the subject of further claims.

The x-ray tube according to claim 1 comprises a vacuum housing, in which an anode is arranged which, when electrons generated in an electron source strike, generates useful x-ray radiation in a stay area of ​​the focal spot, which emerges from the vacuum housing through an x-ray exit window, wherein the anode outside of the location of the focal spot has at least partially a surface coating made of a material with an atomic number less than the atomic number of the material of the anode.

The location of the focal spot is the area of ​​the anode on which the primary electrons are used to generate the desired desired X-ray radiation. This area consists for example of tungsten or tungsten alloys. The location of the focal spot can be a stationary focal spot (standing / fixed anode) or an annular focal path (rotating anode in a rotating anode X-ray tube or in a rotary piston X-ray tube).

According to the invention, the material of the surface coating consists of a material with an atomic number that is smaller than the atomic number of the anode material.

By using such a material, only a few backscattered electrons are created. Accordingly, little extrafocal radiation arises in the material of the surface coating of the anode. Due to the low extra-focal radiation, the useful x-ray radiation generated in the area where the focal spot is located is correspondingly “contaminated” by the undesired extra-focal radiation. The proportion of the X-ray useful radiation in the through the X-ray radiation emerging from the X-ray exit window is accordingly correspondingly higher, as a result of which the image quality is significantly improved.

With molybdenum (Z = 42) as the anode material, the material used for the surface coating has an atomic number (atomic number) Z ≤ 42.

Materials with a nuclear charge number Z 16. 16 are preferably used. In the case of coating materials composed of several elements (e.g. alloys), the atomic number of the elements involved, which is average over the number of atoms, is decisive.

The use of a material with such a low atomic number reduces the generation of backscattered electrons and thus the generation of extra focal radiation, which further improves the image quality.

According to the invention, the anode has a coating on at least one surface. According to one embodiment, the top of the anode is with a surface Chen coating is provided. Alternatively or additionally, the front of the anode and / or the underside are provided with a surface coating. The top of the anode is the side on which the focal spot is located. All sides of the anode, that is to say the upper side and the front side and the lower side, preferably have a surface coating.

The optimal layer thickness for the surface coating depends on the kinetic energy E.₀ the incident electrons and the atomic number (atomic number) Z of the material or layer system used.

The maximum penetration depth L_Max of electrons with the kinetic energy E₀ here is: <de-figure num="1"><img file="DE102010040407A1_D0001.tif" /></de-figure>where A denotes the atomic weight, Z the charge number and ρ the density of the solid.

In a preferred embodiment of the X-ray tube applies to the Di cke D of the surface coating and the density ρ and the mean atomic number Z of the material of the surface coating of the anode the condition <de-math align="left">D&gt; 280 μmZ^–0,165G / cm³· Ρ^–1.</de-math>

If the incident electrons have a kinetic energy E₀ = 140 keV, then on the basis of simulation calculations according to formula (1) approx. 90% of the kinetic energy E₀ of the incident electrons must be deposited in the surface coating.

Since the density of the material of the surface coating and the atomic number (atomic number) for each material are fixed, the condition mentioned is a determination of the minimum thickness of the surface coating. The average atomic number here is calculated from the sum of the relative frequency of occurrence of the atomic number of the components of the material of the surface coating.

According to a further advantageous embodiment of the x-ray The surface coating of the tube has a thickness that is greater than the maximum penetration depth L._Max Electrons in the material of the surface coating.

In the context of the invention, the surface coating can consist of a multiplicity of materials or material connections or material combinations.

For example, the surface coating on the anode can consist of a carbon (diamond, graphite) or of carbon-containing compounds. Examples include tetrahedral coordinated amorphous carbon (ta-C), nitrogen-containing amorphous carbon (CN_x), DLC compounds (Diamond Like Carbon compounds), such as amorphous hydrogen-containing carbon layers (aC: H), metal-containing DLC ​​layers (Me-DLC, e.g. Me-C: H) or modified DLC layers (X -DLC, e.g. aC: H: X).

Also compounds such as titanium carbide (TiC), titanium carbo-nitride (TiCN), titanium aluminum um-carbo-nitride (TiAlCN), silicon nitride (SiC), tantalum carbide (TaC), hafnium carbide (HfC), chromium carbide (Cr_xC._y) and zirconium carbide (ZrC) are particularly suitable as materials for surface coatings of the anode.

In an advantageous embodiment, the material of the surface coating consists of an oxide, e.g. B. silicon oxide (Si_xO_y, especially SiO₂), Aluminum oxide (Al_xO_y, especially Al₂O₃), Titanium oxide (Ti_xO_y, especially TiO₂, TiO_x) or oxide compounds with chrome (Cr₂O₃), Zirconium (ZrO₂), Yttrium (Y₂O₃) or magnesium (MgO).

In a further preferred embodiment there is the material of the surface coating from a boride, for example from boron nitride (BN, TiBN), chromium boride (CrB₂), Titanium boride (TiB₂) or boron carbide (B_xC._y, especially B₄C)

Alternatively, the material of the surface coating consists of a nitride, e.g. B. aluminum nitride (AlN), chromium nitride (CrN), titanium nitride (TiN), silicon nitride (Si₃N₄) or titanium aluminum nitride (TiAlN)

According to a further alternative, the material of the surface coating consists of a silicide, e.g. B. molybdenum silicide (MoSi₂) or tungsten silicide (WSi₂).

Sulfides, for example molybdenum sulfide (MoS₂), Tungsten sulfide (WS₂) or tantalum sulfide (TaS₂) can also be used advantageously as the material of the surface coating.

Finally, there are also metals (e.g. Be, Ti, Fe, V) or alloys n, e.g. B. TiAl, NiAl or TZM (mixed crystal hardened and particle-reinforced Mo-based alloy) suitable for a surface coating of the anode.

For the surface coating, multilayer layers can also be produced from the aforementioned materials.

Depending on the material of the surface coating and the layer thickness on the anode, different coating techniques can be used. These are, for example, sputtering, plasma coating, physical vapor deposition or chemical vapor deposition. A combination of at least two of the aforementioned coating techniques is also possible within the scope of the invention, in particular for multilayer layers.

Two schematically illustrated exemplary embodiments of the invention are explained in more detail below on the basis of the drawing, but without being restricted thereto. Show it:

<figref idref="S18">1</figref> an anode of a first embodiment of the X-ray tube according to the invention u nd

<figref idref="S18">2</figref> an anode of a second embodiment of the X-ray tube according to the invention.

In <figref idref="S18">1</figref> is an anode <b>1</b> a first embodiment of the X-ray tube according to the invention and in <figref idref="S18">2</figref> is an anode <b>2</b> a second embodiment of the X-ray tube according to the invention each shown in longitudinal section. The material of the anode<b>1</b> exists in the configurations according to <figref idref="S18">1</figref> and <figref idref="S18">2</figref> Made of molybdenum (Mo, Z = 42) and is designed as a rotating anode that rotates around an axis of rotation <b>3</b> is rotatable. The same applies to the anode<b>2</b>.

The rotating anodes <b>1</b> and <b>2</b> are each arranged in a vacuum housing of an X-ray tube, not shown, and generate electrons generated in an electron source in a focal path <b>4</b> (Residual area of ​​the focal spot) X-ray radiation which emits through an X-ray exit window emerges from the vacuum housing. With the rotating anodes according to<figref idref="S18">1</figref> and <figref idref="S18">2</figref> is the area of ​​the focal track <b>4</b> made of tungsten (W, Z = 74) or a tungsten alloy.

In the <figref idref="S18">1</figref> shown rotating anode <b>1</b> has a concave top <b>11</b>on the focal track <b>4</b> runs, and one parallel to the axis of rotation <b>3</b> trending front <b>12</b> as well as a concave bottom <b>13</b>.

The top <b>11</b> is in the area outside the focal path <b>4</b> with a surface coating <b>51</b> Mistake. The face<b>12</b> and the bottom <b>13</b> each have a surface coating <b>52</b> or. <b>53</b> on. The surface coatings<b>51</b>, <b>52</b> and <b>53</b> According to the invention consist of a material with an atomic number (atomic number) less than the atomic number of the material (molybdenum) of the Drehano de <b>1</b>.

In the <figref idref="S18">2</figref> shown rotating anode <b>2</b> has a top <b>21</b>on the focal track <b>4</b> runs, and one parallel to the axis of rotation <b>3</b> trending front <b>22</b> as well as a flat bottom <b>23</b>.

The top <b>21</b> points in the area of ​​the focal path <b>4</b> a bevel <b>21a</b> so that an anode angle of α ≈ 5 ° -20 ° results. The top<b>21</b>, including the bevel <b>21a</b>, is in the area outside the focal path <b>4</b> with a surface coating <b>61</b> Mistake. The face<b>22</b> and the bottom <b>23</b> each have a surface coating <b>62</b> or. <b>63</b> on. The surface coatings<b>61</b>, <b>62</b> and <b>63</b> According to the invention consist of a material with a nuclear charge number (atomic number) smaller than the nuclear charge Number of materials (molybdenum) of the rotating anode <b>2</b>.

As materials for an at least partial surface coating of the anode <b>1</b> or 2 are, for example, silicon oxide (Si_xO_y, especially SiO₂) or aluminum oxide (Al_xO_y, especially Al₂O₃) particularly advantageous. These materials are characterized by high temperature resistance, high mechanical strength, and there are already established standard processes for producing layers from such materials.

Alternatively, the material for the surface coating of the anode <b>1</b> or. <b>2</b> for example also a carbide (e.g. boron carbide, B₄C), or a nitride (e.g. silicon nitride, Si₃N₄) include.

QUOTES INCLUDE IN THE DESCRIPTION

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Patent literature cited

<ul><li>US 7260181 B2 <b>[0010, 0013]</b></li><li>US 2009/0279669 A1 <b>[0013]</b></li><li>US 2008/0112538 A1 <b>[0020]</b></li><li>US 2010/0046716 A1 <b>[0021]</b></li><li>US 4352196 A1 <b>[0022]</b></li><li>DE 19914825 A1 <b>[0023]</b></li><li>WO 2008/090518 A1 <b>[0024]</b></li><li>US 5199059 A1 <b>[0025]</b></li></ul>

Non-patent literature cited

<ul><li>Kanaya K, Okayama S., "Penetration and energy-loss theory of electrons in solid targets", J. Phys. D: Appl. Phys., Vol. 5, 1972, pp. 43-58<b>[0011]</b></li></ul>