DE102010030713B4 - X-ray source for generating X-rays with a hollow body target and a method for generating X-radiation in a hollow body target - Google Patents

Abstract

X-ray source for generating X-radiation by impingement of electron beams on a target, wherein in the beam path of the electron beam (5) a hollow body target (15) for advantageous use of the backscattered electrons (8) is arranged, whose entire inner surface is acted upon by the electron beam (5) , the X-ray source is operated with acceleration voltages from 100 kV and utilizes the preferred direction of the X-ray radiation.

Description

The invention describes an X-ray source for generating X-rays with a hollow body target and a method for generating X-radiation in a hollow body target and is applicable in various fields of X-ray technology, for example in medicine, the technology for analysis, irradiation and baggage and container inspection. While z. B. in medical and technical irradiation technology and in the technical Rückstrahltechnik a high total beam power or in the radiometry is a highly constant beam current in the foreground, require other X-ray techniques such. As radiography, radioscopy, CT or the use of X-ray optics in addition a well-defined and specific high-loadable focal spot. The focal spot describes the location and size of the point of impact of the electrons on the target of the anode and is the place of generation of the X-rays. In medicine and engineering, the focal spot is described and measured using various standards or procedures. As an example, the technical standard DIN EN 12543-1 to 5 should be mentioned.

State of the art

In the vacuum vessel of an X-ray source, the electrons emerging from the cathode are accelerated by means of an electric field to the target of the anode, where they trigger X-rays. These propagate in all directions, with the technically usable X-rays leaving the vacuum space through a specially arranged beam window.

In principle, two arrangements of the target for the electron beam are possible: The reflection target designates an arrangement in which the electron beam and the used X-ray radiation lie on the same side of the target. In a so-called transmission target, the electron beam from one - namely the vacuum side - hits the target, while the usable X-ray beam on the other - the air side - emerges from the target and thus also from the X-ray source. The target is thus simultaneously the X-ray exit window.

The operation of the X-ray source presents a variety of problems, four of which are briefly described below.

Electron backscatter

Firstly, it is known to the person skilled in the art that with reflection targets and transmission targets-depending on the target material, the target angle and the acceleration voltage-a significant proportion of the electrons accelerated onto the target leave the target without generating radiation as so-called backscattered electrons. The proportion of backscattered electrons increases, the shallower the electrons strike the target surface.

These backscattered electrons not only degrade the X-ray generation (efficiency), but also lower the stability of the high voltage insulators in the X-ray source.

In order to prevent or at least reduce this, various means are known to the person skilled in the art, such as the conventional voluminous electron trap around the target and the formation of electrostatic fields by electrodes or insulators for deflecting the backscatter electrons to areas in which they cause little or no interference can.

In the EP 1 618 585 B1 For example, an X-ray tube with an array of multiple channels having a target at about its center will be described. Although this construction, which is intended to provide a clear separation of the different emission channels, also makes it possible to capture the electrons backscattered by the target, they do not contribute to increasing the conversion of electrons into X-rays since the material used for channel separation (copper ) has a much lower degree of conversion for the conversion of electron into X-rays. In this case, too, the return electrons would generate radiation at a considerable distance from the actual focal spot and thus unnecessarily increase the focal spot.

Also in the WO 2009/081 312 A1 describes a particularly large and efficient collector for trapping backscattered electrons in X-ray tubes as well as the storage and removal of heat energy carried by these electrons. However, the construction is not aligned with any use of these electrons.

The focal spot

Secondly, when considering the imaging properties of an X-ray source, one has to choose such a small focal spot on the one hand that it is capable of producing the finest density and element distributions in the examined material, which are necessary for quality testing. To make shadow projection or CT visible. However, this is also required for the lateral resolution in the fluorescence analysis and the precision of diffractive reflections.

On the other hand, the focal spot must be so large that the flow of generated X-rays necessary for rapid testing and thus also the flow of generating electron beams is ensured, without evaporating or even melting the heated target in the focal spot too quickly.

The electron energy actually absorbed here is only converted into X-ray energy to a very small extent, while the excess amount is converted into heat.

For the reasons given above, instead of an ideal point, one must start with a small (approximately circular or rectangular) beam surface on the flat target as the focal point and design the target such that a high electrical power per unit area of the effective and usable focal spot - here brilliance called - is achieved.

The target is therefore constructed as a multi-layer system in which the conversion material, ie the material in which the electron is efficiently converted into X-ray energy, is applied to a carrier material with better thermal conductivity. The two layers can also consist of several different materials to improve the respective properties.

The prior art is the use of a tungsten, or tungsten rhenium target, unless for other reasons such. As the X-ray analysis (eg diffraction, fluorescence) or the use of low-energy radiation (eg mammography) another target material is needed. These materials are characterized by both a high atomic number, which is advantageous for the conversion of electron into X-ray energy, as well as by a relatively good thermal conductivity, high melting temperature and low vapor pressure.

As is known, in rotating anode tubes, rotary tubes and tubes with oscillating targets, the electron beam is offered a constantly renewing target surface, the focal spot remaining in space. This advantage is paid for with a greater technical effort.

There are also known for decades X-ray tubes with so-called cone targets of conversion material, in which an electron hollow beam is directed to the massive cone tip, which generates a radiating zone in the form of a hollow cone-like ring around the massive cone apex. Here, the goal of the development was to create an X-ray source which allows the weld on pipe joints or the carcass of vehicle tires to be radiated vertically from the inside to the outside with a single exposure. The radiating zone forming on the cone target is three-dimensional and leads in the projection perpendicular to the electron beam (and the cone axis) to a dash or crescent-shaped focal spot.

After US Pat. No. 7,050,540 B2 For example, a tapered solid pin of conversion material is irradiated perpendicular to its axis with electrons. Both of these latter cases are reflection targets in which a massive spatial structure is exposed to accelerated electrons from the outside and no precautions are taken against the backscattered electrons.

preferred direction

Third, it is known to the person skilled in the art that the X-ray radiation is in a so-called preferred direction (FIG. US 4,229,651 A ; US 4,821,304A ; US 5 029 195 A ; US 5 128 977 A ; US 5,206,895 A , A. Ganguly, NJ Pelc, 'On the Angular Deposition of Bremsstrahlung X-Ray Emission', Proc. f SPIE, Vol. 6913, 69134P-1 (2008)) spreads. The higher the acceleration voltage, the shallower the angle of incidence of the electrons to the target surface and the decrease angle to the target surface of the X-ray should be to exploit the highest possible intensity. It should also be noted that this results in the circumstance advantageous for the defectoscopy that the X-ray spectrum hardens. This means that the intensities are amplified, especially at the higher energies of the spectrum. In the construction of X-ray sources of this latter circumstance could not be implemented so far, since the negative effect of the backscattered electrons at small angles of incidence against the target surface with the positive effect of the preferred direction almost compensated.

However, it is possible to compensate for the loss of the current of the backscattered electrons by means of a so-called compensation current and to load the focal spot again higher. However, since this compensation current, like the actual tube current, comes from one and the same cathode, the power of the cathode must be increased and thus its service life must be shortened.

However, the energy balance is still burdened by the unused backscattered electrons.

By ray target

Fourth, the requirements of the target construction are much more difficult to meet in the conventional transmission targets than in the case of reflection targets, since here with the construction several functions and thus several properties, such as high Ordnungszahl, low radiation attenuation, good heat conduction, high mechanical strength and sufficient vacuum capability must be combined.

Thus, the previously used geometry of a thermally loaded in their middle thin disc for the removal of heat from its center to the edge is extremely unfavorable. The after DE10 2008 014 897 A1 Although the proposed use of pyrolytic graphite as a window membrane with conversion layer applied centrally improves the heat dissipation in comparison to previously customary window materials, the larger conversion layer from the target to the cooling medium with the same focal spot as the holkörper target is not achieved.

The US 4 521 903 A describes an X-ray source with a hollow body target, wherein the X-radiation exits from the same opening of the hollow body target into which the electron beam enters. Thus, it is not possible with this X-ray source to use the X-ray preferred direction, as it occurs at higher energies in the direction of the primary electrons.

The DE 195 44 203 A1 describes an x-ray tube having an anode with a passageway through which the electrons are directed to a conventional target.

With the US 2004/0 165 699 A1 a miniature X-ray tube is disclosed, which is designed with an acceleration voltage up to 70 kV maximum. This acceleration voltage is not sufficient to achieve the effect of the X-ray preferred direction.

US 2004/0 120 463 A1 discloses an X-ray source for generating X-ray radiation by impinging electron beams on a target, wherein a hollow body target, from which a coated surface is irradiated, is arranged in the beam path of the electron beam.

The DE 20 04 359 A finally describes an X-ray generator for X-ray diffraction and X-ray microscopy. An X-ray source with a hollow body target is used, the X-ray source operating at voltages around 30 kV.

task

The invention has for its object to provide an X-ray source with such a target, which avoids the aforementioned disadvantages, the energy of the backscatter electrons effectively used, ensures a high specific load capacity, exploits the preferred direction and can be reliably operated with high energy efficiency.

This object is achieved by the features in the main claim 1 and 8. Advantageous embodiments of the invention are contained in the subclaims. The present invention surprisingly improves the prior art of x-ray sources.

Disclosure of the invention

The hollow body target according to the invention consists of a hollow body of conversion material which is open at least on one side. The hollow body target is acted upon by this open end with a quasi-parallel or more or less divergent electron beam or a hollow electron beam on its inner side and is constructed as a reflection or as a transmission target. Due to the shape as a hollow body creates a larger spanned surface of conversion material with the same focal spot size after DIN EN 12543-1 to 5 , The focal spot thus has a higher brilliance than the prior art. The use of the preferred direction and the backscattered electrons increases the efficiency.

It is advantageous to have a very homogeneous electron beam. The electrons can come from all kinds of sources, such as. As cold, hot and hot cathodes, linear or circular accelerators or others. The guidance of the electron beam may be effected by the acceleration field and / or by additional electric fields or magnetic fields.

The hollow body target is cooled on its outside in a manner known to those skilled in the art.

This makes the entire X-ray system in both mobile and stationary design more efficient and energy-saving than conventional X-ray systems. The focal spot size can be a few mm, but can also be reduced arbitrarily down to the nano range.

The focal spot is stable in its shape and position to a reference point on the tube shell and thereby more efficient and more resilient and thus more brilliant than the focal spot of otherwise comparable X-ray sources.

The present invention relates to a completely novel design of the target in X-ray sources with the four following main aspects.

A first aspect of the present invention deals with the advantageous use of the backscattered electrons.

A large part of these backscatter electrons reemerges from the conversion layer after only one or only a few collisions with the conversion material and has therefore lost relatively little of their original acceleration energy. In the narrowness of the hollow body target, the backscattered electrons have little chance of leaving the hollow body target, but instead re-encounter another location on the interior wall of the hollow body target, and have a renewed chance of producing X-radiation rather than just useless heat. The efficiency of X-ray generation is thus significantly improved. This also avoids the introduction of a compensation current mentioned above, increases the efficiency of the X-ray source and meets the first aspect of the invention.

Since only a few backscattered electrons emerge from the hollow body target and an upstream target-oriented electronic catcher basket, the effective focal spot is no longer enlarged by its eventual reoccurrence with conversion material and thus generation of intensified background radiation around the focal spot. As a result, the backscattered electrons which otherwise stray in the vacuum space are reduced so much that high-voltage flashovers caused by them are markedly reduced.

The second aspect of the invention relates to the thermal focal spot of the hollow body target and its thermal capacity.

In X-ray technology is a measure of the quality of a focal spot its brilliance, so the thermal capacity based on the effective focal spot size.

The thermal load is given by the energy of the radiation / heat generating electrons multiplied by their number per unit of time.

In contrast to the hitherto known solid plane or cone-shaped reflection targets which are acted upon from the outside by electrons or the equally planar transmission targets, the surface generating X-rays forms three-dimensionally in the hollow body target in the hollow body of the hollow body target spanned by a conversion layer.

The size of the effective, so-called optical focal spot is described by the projection of an X-ray emitting structure, so the spatially distributed and acted upon by accelerated electrons conversion material from a point, so to speak as a virtual surface and is z. B. after DIN EN 12543 - 1 to 5 determined.

In order to achieve high brilliance, it is important to accommodate the largest possible area of conversion material in the "optical shadow" or in the projection of this optical focal spot. This task is solved by the hollow body target in a particularly effective manner.

The radiating surfaces generated in the hollow body target are the surfaces heated by the electron beam, for the cooling of which not only a base area according to the usual flat focal spot, but also its sidewalls contribute significantly. Based on the previously customary focal spots obtained according to the invention in the three-dimensional conversion surface converted an enlarged conversion area. However, improved cooling allows increased heat input by the incident electrons. This achieves an increased specific electron power, an increased overall beam power, and finally the desired shortening of the recording time, based on the effective focal spot.

A third aspect of the invention is based on the fact that the preferential direction of the generated X-radiation always has an amplified and - with the acceleration voltage - - growing component in the direction of the accelerated electrons. For a hollow body target, the acceptance angle of the X-radiation is also at a relatively small angle to the direction of the accelerated electrons. As a result, the advantage of the preferred direction can be utilized with the structure of the hollow body target.

A fourth aspect of the invention is based on the geometry of the hollow body target which causes a flat to grazing incidence of the accelerated electrons at the point of impact on the cylindrical or conical conversion layer.

Due to this oblique incidence and penetration of the accelerated electrons on and through the conversion layer, this can be carried out even thinner than the range of the incident electrons. As a result, a better heat-conducting layer can be joined closer to the conversion layer and better cooling can be achieved with full absorption of the electrons. Compared to a conventional thinly coated transmission target, the conversion layer can be reduced to about 1/3 to 1/10.

This very small thickness of the conversion layer of the hollow body target also allows the generated X-ray radiation, the hollow body target with only extremely low attenuation in the conversion material - called self-absorption - to leave this and become usable.

• - Die Reduzierung der nutzlosen und schädlichen Rückstreuelektronen;
• - die Nutzung eines großen Teils der eingefangenen Rückstreuelektronen zur Strahlenerzeugung;
• - die Vergrößerung der Konversionsschicht und der Wärmeübergangsfläche für die Kühlung des Targets;
• - die Nutzung der Vorzugsrichtung;
• - eine hohe Absorption der einfallenden, beschleunigten Elektronen bei dünnen Konversionsschichten;
• - Dadurch kann eine besser wärmeleitende Schicht dichter an der Konversionsschicht gefügt werden und bei voller Elektronenabsorption eine bessere Kühlung erreicht werden;
• - die Verringerung der Eigenabsorption der erzeugten Röntgenstrahlung

The advantages can be summarized as follows:

• - The reduction of useless and harmful backscattered electrons;
• - The use of a large part of the trapped backscattered electron beam generation;
• - The increase of the conversion layer and the heat transfer surface for the cooling of the target;
• - the use of the preferred direction;
• a high absorption of the incident, accelerated electrons in thin conversion layers;
• - This allows a better heat-conducting layer are joined closer to the conversion layer and at full absorption of the electrons better cooling can be achieved;
• - The reduction of the self-absorption of the generated X-radiation

• 1 Stand der Technik der Röntgenstrahlerzeugung, dargestellt an einer anodengeerdeten Seitenfenster-Röntgenröhre mit ebenem Reflektionstarget
• 2a - 2k Räumliche Prinzipdarstellungen verschiedener Hohlkörpertargets
• 3a - 3b Ausführungsbeispiele für erfindungsgemäße Hohlkörpertargets
• 4 Zur Messung des sich auf einem Hohlkörpertarget ergebenden Brennflecks nach Norm am Beispiel eines Hohlpyramiden-Targets.
• 5 Darstellung einer Strahlgeometrie für eine Röntgenröhre mit Doppelfokus
• 6 Prinzip-Darstellung einer anodengeerdeten Endfenster-Röntgenröhre mit Hohlkörpertarget
• 7 Prinzip-Darstellung einer anodengeerdeten Seitenfenster-Röntgenröhre mit Hohlkörpertarget
• 8 Prinzip-Darstellung der Anordnung eines Hohlkörpertargets in einer bipolaren Röntgenröhre
• 9 Prinzip-Darstellung der Anreihung mehrerer Elektronenemitter-Hohlkörpertarget-Paare in einem gemeinsamen Vakuumgefäß
• 10 Prinzip-Darstellung der Austrittsrichtung des nutzbaren Röntgenstrahls entgegen der Richtung des Elektronenstrahls

The invention will be explained in more detail with reference to embodiments shown at least partially in the figures. Show it:

• 1 Prior art X-ray generation shown on an anode-grounded side window X-ray tube with a flat reflection target
• 2a - 2k Spatial schematic representations of various hollow body targets
• 3a - 3b Exemplary embodiments of hollow body targets according to the invention
• 4 For measuring the resulting on a hollow body target focal spot according to standard on the example of a hollow pyramid target.
• 5 Representation of a beam geometry for an x-ray tube with double focus
• 6 Schematic representation of an anode-grounded end-window X-ray tube with hollow body target
• 7 Schematic representation of an anode-grounded side window x-ray tube with hollow body target
• 8th Schematic representation of the arrangement of a hollow body target in a bipolar X-ray tube
• 9 Schematic representation of the arrangement of several electron emitter hollow body target pairs in a common vacuum vessel
• 10 Principle representation of the exit direction of the usable X-ray beam opposite to the direction of the electron beam

In 1 the state of the art is shown using the example of an x-ray tube: in a vacuum vessel 1 is the cathode 2 at negative potential to the grounded anode 3 isolated. The one in the cathode 2 built-in electron emitter 4 For example, a heated tungsten filament provides an electron beam 5 in the high voltage electric field (not shown) between cathode 2 and anode 3 into a small thermal focal spot on the massive and flat target 7 focused and accelerated. A noteworthy share 8th the on the target surface 7 incident accelerated electrons are backscattered here and generate useless heat unless it results in unwanted enlargement of the focal spot or disturbance of the high voltage strength of the tube. Also in the target 7 absorbed portions of accelerated electrons 5 generate predominantly heat while the small remainder generate X-rays 11 leads through the X-ray exit window window 9 the vacuum space 10 can be left and used or absorbed inside the tube. The useless heat in the anode 3 converted energy of the electrons is through the cooling channels 14 , which are traversed by water, for example, dissipated.

In contrast, the present invention is based on the use of anodes with hollow body targets 15 as reflection or transmission targets as in the 2 a - 2 k, for example.

To an electron beam 5 on the inner walls of a hollow body target 15 To send, the radiation must be inclined at a small angle against the walls, as in 2 a, 2 b and 2 c is reproduced. It is obvious that with this geometry not all the available conversion surface can be hit, but at most a fraction ≤ ½.

Around the entire inner surface of the hollow body target 15 with accelerated electrons 5 to irradiate, for example, a divergent electron beam 5 that uses the entire inner surface of a hollow body target 15 evenly applied, as z. B. on a square ( 2 d) and a circular cross section ( 2 e) is demonstrated.

On the other hand, if you form the hollow body target 15 for example, so that it is - in the direction of the electron beam 5 seen - rejuvenated and a hollow pyramid ( 2 f) or a hollow cone ( 2 g), the hollow body target can 15 also with a weakly divergent, quasi-parallel electron beam 5 be charged.

It is also possible to use the hollow body target 15 so that it forms - in the direction of the electron beam 5 seen - not rejuvenated but expanded and again a hollow cone or a hollow pyramid 15 forms. The target tip, which now faces the electron source, has an opening through which the electron beam, which is highly divergent, passes through 5 radiates in order to then fan out and to meet the sloping outward surfaces inside.

Other forms of hollow body targets 15 , such as B. with elliptical or star-shaped cross-section are possible.

The construction according to the invention of the hollow body target 15 causes the primary electron beam 5 not only X-rays and heat generated on the impacted surfaces, but also backscattered electrons have the chance again, on a wall of the Holkörpertarget 15 interact. The person skilled in the art becomes the shape of the hollow body target 15 so that a uniform distribution of primarily incident electrons and backscattered electrons on the hollow body walls is formed. The atomic number of the target, the acceleration voltage, the angle of incidence of the electrons, the acceptance angle of the X-radiation are optimization parameters. Comparing the achievable with accelerated electrons acted surfaces in the cases of 2 a to 2 h, one obviously obtains a larger spanned conversion surface than with the usual flat targets.

The purely geometric enlargement of the heat transfer surface from a flat target to a hollow body target 15 z. B. after 2 d is on the order of four.

In a simple version ( 2 i and 2 j) may be the hollow body target 15 but also by only one or even two erected walls of conversion material in the vicinity of the focal spot, for example by omitting one or two walls of a hollow body target 15 to 2 B , are formed without increasing the focal spot according to DIN EN 12543-5. These walls of conversion material can capture scattered electrons from the focal spot and thus contribute to the generation of X-rays. The efficiency is not as high as with a hollow body target 15 to 2 a to 2 However, the simplified arrangements can still increase the efficiency effectively. Is waived on the use of the preferred direction can in this case as in 2 k is the electron beam 5 also much steeper, z. B. in an incidence angle of 70 ° -90 °.

Such a hollow body target 15 , can preferably be constructed within the vacuum vessel of the X-ray source so that the axis of the acceleration voltage dependent cone of the preferred direction coincides with the window normal or at least within the beam exit angle of the usable X-rays 11 is and therefore the preferred direction is available.

Will the hollow body target 15 for example, by a recess after 3 a as a reflection target in a massive block of target material or highly absorbent cooling material 16 formed, so is a thin, highly collimated pencil beam from a cylindrical or square hollow body target 15 emerge and be usable.

In 3 b is the formation of the hollow body target 15 For example, shown as a transmission target.

Therefore, according to the invention, the hollow body target 15 constructed with a low-attenuation of the generated X-ray, thin conversion layer and is the generated X-ray radiation through the surrounding cooling material 16 also weakly weakened, is also the use of the entire radiation (in the direction of the accelerated electrons) over the solid angle 2 π (hemisphere) also allows.

Characterized in that the irradiated for generating the X-ray electron beam 15 on the inner surface of a hollow body target 15 will be conducted at a same, after DIN EN 12543-1 to 5 measured focal spot the aspects 1 to 4 reached.

To dissipate the higher load capacity of a hollow body target that is possible according to the invention 15 resulting higher heat loss on the hollow body target 15 In a preferred embodiment, the thin hollow body target 15 on its entire outer surface with a heat sink 16 connected or in a heat sink 16 embedded ( 3 b).

But in the hollow body target 15 X-rays generated in all directions as low as possible, is in the design of this heat sink 16 According to the invention, the physical fact that the penetrating power of X-rays is extremely higher than that is used that of electron beams of the same energy. The X-ray radiation generated in a thin conversion layer can leave this layer directly to its outside as well as only to its inside, and then easily penetrate even an approximately opposite layer and finally, with a suitable material choice and dimensioning, the surrounding heat sink 16 only a little weakened to leave the exterior.

In 3 b this geometry is explained in more detail.

On the thin, for example, hollow cone-like wall of the hollow body target 15 , which is in the good heat-conducting and low-X-ray absorbing cooling material 16 embedded, for example, meets the quasi-parallel electron beam 5 and generates x-rays here 11 and backscattered electrons 8th the latter having a renewed chance of generating X-rays.

The corresponding applies to a design of the hollow body target 15 as a hollow cylinder ( 2 e), as a hollow pyramid ( 2 f), as a hollow cuboid ( 2 d) or everted cones ( 2 H).

The heat sink 16 In turn, according to the invention for continuous operation preferably made of good heat-conducting and preferably pulsed operation of good heat-storing material and can in principle be used in solid, liquid or gaseous state.

So z. For example, at lower total loads forced air, otherwise a liquid such as water or oil are used, all weaken the X-ray radiation generated relatively low.

At higher power losses, the heat sink 16 used as heat spreader as well as ultimately as a heat exchanger in a known manner to air, water, insulating gas, oil or other medium. Here it is ensured that the X-ray useful radiation of this is not or only slightly affected.

Furthermore come as a particularly good heat conducting / storing and good radiolucent cooling material metals, non-metals and ceramics low atomic number, such. As aluminum, beryllium, AlN and carbon, the latter in all modifications, especially as diamond and HOPG used. All these elements / compounds have a higher thermal conductivity than tungsten. It can also cool materials with higher atomic number such. As copper can be used, since they can be used simultaneously for beam filtering and beam hardening.

So it has z. For example, in the measurement of automotive components by X-ray shadow projection, it has been found helpful to use relatively hard X-rays of about 80 kV filtered through 2 mm aluminum to improve the image sharpness of edges that are still contaminated with lubricating oils from production.

Finally, also the pure conversion material in the hollow body target 15 serve for monochromatization.

Another advantage of the construction according to the invention is the possibility of the highly heat-conductive cooling material 16 low atomic number itself as a radiation exit window 9 , as in 3 a, and thus also to be used as a vacuum termination, which also survives stray accelerated electrons. In 3 b is shown as a heat sink 16 and beam exit window 9 can be made of a material and also of one piece. The usual separate X-ray exit window can be omitted in this case.

As an additional precaution against leakage of scattered electrons from the hollow body target 15 can either ( 3 a) a target-oriented electron trap 12 of diameter or width D be used from weakly converting material or ( 3 b) a target near aperture 17 with diameter D of highly radiation absorbing material in front of the entrance of the hollow body target 15 be set.

However, these two measures are used simultaneously, from the focused and accelerated electron beam 5 about ausscherende rays 20 either 3 a) not to encounter conversion material or ( 3 b) these electron beams 20 Although generate X-rays, but not in the direction of the X-ray Nutzstrahlen 11 to let escape.

Despite their three-dimensional shape are with hollow body targets 15 but also extremely small focal spots thereby achievable that z. B. made of conversion material or coated tubes in the nano-area hollow body targets 15 manufactured and used in accordance with this invention.

Based on 4 the three-dimensional formation of the X-ray generating surface is explained using the example of a hollow pyramid target. Looking at the radiating surfaces of a hollow body target 15 from the perspective 6 the measuring device, the optically effective, two-dimensional shadow area of the focal spot, the optical focal spot appears with its length I and its width w.

The edge length of the opening of the hollow body target 15 or its diameter D determines the optical focal spot in w and I when viewed axially.

To achieve a high image sharpness as small as possible optical focal spot is desired. If one uses a pyramidal hollow body target 15 with the entrance opening D = 1mm and the pyramid height 5 mm results in a range for the technically usable beam angle of about +/- 12 °, within which the optical focal spot size w ≤ 1 mm or I ≤ 1 mm remains.

With a pyramid height of only 2.5 mm, this range expands to about +/- 23 ° for the usable beam angle.

This is another decisive advantage over the hitherto conventional X-ray sources with reflection target, in which even at relatively low angles to the central beam, an enlargement of the optical focal spot takes place.

Part of today's standard X-ray tubes for radiographic testing is equipped with two focal spots: a large focal spot with high power for overview images of lower resolution and a small focal spot with low power for detail images of increased resolution. A disadvantage of these tubes is the fact that when switching from one to the other focal spot the image "jumps", because the two focal spots generating electron emitter are constructed laterally side by side in the cathode.

This disadvantage is in an inventive X-ray source with hollow body target, z. B. with a hollow cone or Hohlpyramiden target after 5 thereby avoiding that the electron beam to be accelerated 24 and 25 for the two focal spots of two concentric circular and stripe-shaped electron emitters 22 and 23 come. The electron beam 24 of the inner circular emitter produces the small focal spot 26 in the tip of the hollow body target 15 while the outer, larger emitter stripe is the electron beam 25 for the larger focal spot 27 on the inside of the wide end of the hollow body target 15 sets. Both focal spots are therefore also concentric and the "jumping" of the image is avoided.

Will the hollow body target 15 irradiated from a cathode, which is the electron beam 5 may vary in diameter, so the power depending on the irradiated area in the hollow body target 15 be adjusted. This allows continuous optimization of the focal spot size and test speed in dynamic test processes. The cave body target 15 can be further split to accommodate more than two focal spot sizes, or build the conversion layer of materials of different atomic number next to each other.

In this 5 For example, the cooling of the hollow cone or hollow pyramid target 15 over the cooling material 16 and the the cooling channels 14 shown.

The 6 shows the installation of a hollow body target 15 with heat sink 16 to 5 into the anode 3 an X-ray source, for example for a quasi-parallel electron beam 5 , The cathode assembly 2 is on the insulator 19 with high voltage socket 21 constructed in isolation and emits the electron beam 5 from the emitter 4 , The good heat-conducting cooling material 16 Here, for example, the cooling channels 14 on and the anode 3 is with a recess 13 provided to the in the hollow body target 15 generated x-ray radiation 11 from the material of the anode 3 leave completely undisturbed. At lower power losses, it also makes sense, instead of the cooling channels 14 the recess 13 with a cooling gas, eg. As air, to rinse.

The thin hollow body target 15 in good thermal contact fitting cooling material 16 In these cases, it must be formed into a high-vacuum-sealed hollow cone / hollow pyramid, which withstands the external air pressure and only weakens the generated radiation.

On the other hand, this is for the anode 3 provided volume so large that here also other cooling devices such as heat pipe and / or Peltier element could be accommodated.

An X-ray tube of this geometry is referred to as Endfensterröhre.

A corresponding structure as a side window tube is in 7 reproduced, where also the cooling over the cooling material 16 and the cooling channels 14 he follows.

Furthermore, in 8th the basic structure of a bipolar X-ray tube with hollow body target 15 demonstrated. Here are cathodes 2 and anode 3 each in its own part of the vacuum envelope 1 passing through a pipe socket 18 connected to each other, which the electron beam 5 from the emitter 4 to the hollow body target 15 in the anode 3 pass through. The in the hollow body target 15 generated x-ray radiation 11 enters through the recess 13 and through the X-ray window 9 out of the tube.

The high load capacity of the hollow body target 15 makes X-ray source with such a target for applications both in pulse and in continuous operation excellent.

This is especially true for CT applications. In addition to the use of such X-ray sources as end or side window tubes or as bipolar tubes, there is also a highly integrated design with a larger number of electron emitter hollow body target pairs 4 and 15 in a common vacuum envelope 1 like this in 9 is shown schematically. While the hollow body targets 15 all are on a common positive high voltage potential, are the individually isolated electron emitter 4 individually occupied with negative potential and can also be controlled individually. These pairs may be constructed in rows or circles or other suitable arrangements.

The Z. B. necessary for CT mechanical approach of individual angular positions can then be omitted, if for each of these angular positions in each case an electron emitter hollow body target pair 4 and 15 is provided.

But only two electron emitter hollow body target pairs 4 and 15 At about eye relief and built in a common vacuum housing are used advantageously for recording stereo X-ray images.

In the above, the focal spot design was treated for an imaging geometry with Goetze's optimized small focal spots. For X-ray sources for pure radiation technique, z. B. for X-ray therapy or sterilization, germ inhibition, polymerization u. Ä. No small focal spot with high specific load capacity is required. Nevertheless, the use of hollow body targets 15 singly, in pairs, lines, circles or areal grids already because of the use of the backscatter electrons of great advantage. Also, the possibility of the individual hollow body targets 15 grouping together into groups and thus creating the desired radiation reliefs in a flexible way offers a new basis for an economical irradiation technique.

In a design after 10 becomes the usable x-ray beam 11 contrary to the direction of the incident accelerated electrons 25 decreased. In this case, the preferred direction can not be exploited. Since the preferred direction forms appreciably only at higher acceleration voltages (approximately from 100 kV), this arrangement and use of the hollow body target 15 rather suitable for soft radiation, as z. B is needed for the analysis. The X-ray useful beam is opposite to the inlet opening of the hollow body target 15 taken on the side of the electron emitter. The hollow body target 15 acts as a reflection target, only with increased efficiency and increased brilliance.

The invention is not limited to the embodiments shown here. Rather, it is possible to realize by variation and combination of said means and features further embodiments, without departing from the scope of the invention.

LIST OF REFERENCE NUMBERS

1

vacuum envelope

2

Cathode, cathode assembly

3

Anode, anode assembly

4

electron emitter

5

Electron beam of accelerated electrons

6

Viewing angle of the measuring device

7

target

8th

Backscatter electrons, scattered electrons

9

X-ray emission window

10

vacuum

11

Usable X-rays

12

Target close electronic catch basket

13

Recess in the anode material

14

Channel for cooling water

15

Hollow target

16

Cooling material, heat sink

17

Target near aperture

18

pipe extension

19

insulator

20

Badly focused accelerated electrons

21

High voltage socket

22

Small electron emitter

23

Larger electron emitter

24

Small electron beam

25

Larger electron beam

D

Diameter or width of the opening of the hollow body target

Claims (11)

X-ray source for generating X-radiation by impingement of electron beams on a target, wherein in the beam path of the electron beam (5) a hollow body target (15) for advantageous use of the backscattered electrons (8) is arranged, whose entire inner surface is acted upon by the electron beam (5) , the X-ray source is operated with acceleration voltages from 100 kV and utilizes the preferred direction of the X-ray radiation. X-ray source after Claim 1 characterized in that the hollow body target (15) is a hollow body of rectangular or square or circular or elliptical or star-shaped cross-section, and / or the hollow body target (15) is a hollow body of L- or U- or V-shaped cross-section, and / or the hollow body target (15) is hollow-pyramidal hollow cone-shaped or hollow-spherical. X-ray source after Claim 1 , characterized in that the hollow body target (15) has an inlet opening for accelerated electrons of 1 nm to 5 mm, and / or the hollow body target (15) is optimized according to the Goetze principle, and / or the length of the hollow body target (15) 1 to 20 times its diameter. X-ray source after Claim 1 , characterized in that the hollow body target (15) is formed of conversion material, and / or the hollow body target (15) is formed three-dimensionally and as a conversion material coated surface of a concave recess in a thermally conductive material, and / or the hollow body target (15) of a solid, liquid, or gaseous conversion material is formed. X-ray source after Claim 1 or 4 , characterized in that the hollow body target (15) consists of conversion material of the elements Be, B, C, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, La, Ce, Pr, Nd, Gd, Ta, W, Re, Os, Ir, Pt, Au, Hg, Th, U or their compound is trained. X-ray source after Claim 1 , characterized in that the hollow body target (15) is connected to a heat sink (16) or embedded in a heat sink (16) and / or a part of the cooling material (16) itself forms the radiation exit window (9) and thus realizes the vacuum seal. X-ray source according to one of the preceding claims, characterized in that the X-ray source is designed as a bipolar X-ray tube. Method for producing a focal spot of the X-ray radiation of an X-ray source, wherein the electron beam (5) irradiated to generate the X-ray radiation is directed onto the entire inner surface of a hollow body target (15), the reflected electrons are used to improve the efficiency and the preferential direction of the generated X-ray radiation is amplified and having an acceleration voltage from 100 kV growing component in the direction of the accelerated electrons. Method according to Claim 8 , characterized in that the hollow body target (15) is acted upon by its open end with a quasi-parallel or divergent electron beam or a hollow electron beam on its inner side. Method according to Claim 8 , characterized in that the electrons are emitters, such as cold, hot and hot cathodes, linear or circular accelerators. Method according to Claim 8 , characterized in that the electron beam (5) at a very shallow angle in the range of 2 to 30 degrees on the inner surface of the hollow body target (15) is passed.

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