Cathode element for a microfocus x-ray tube

A cathode element for a microfocus x-ray tube includes a heatable filament formed of a wire for thermionic emission of electrons for generating an electron beam. The filament, in a source area of the electron beam, has an elongate extension in two directions perpendicular to the electron beam.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is filed herewith for the U.S. National Stage under 35 U.S.C. §371 and claims priority to PCT application PCT/EP2010/002223, with an international filing date of Apr. 9, 2010. The contents of this application are incorporated in their entirety herein.

Not applicable.

TECHNICAL FIELD

The present invention relates to a cathode element for a microfocus x-ray tube including a heatable filament formed of a wire for thermionic emission of electrons for generating an electron beam.

BACKGROUND OF THE INVENTION

In microfocus x-ray tubes, hairpin filaments are used where the wire is bent to a pointed tip to emit a fine electron beam in order to obtain a focal spot size in the pm range. However, due to increasingly higher tube currents and higher filament temperatures associated therewith, hairpin filaments have only a relatively short lifetime, and therefore the cathode needs to be replaced at regular intervals after a limited number of operating hours. Significant additional maintenance efforts and corresponding downtimes are thus caused, which constitutes an obstacle to the use of microfocus x-ray tubes in industrial manufacture.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a cathode element for a microfocus x-ray tube having an extended lifetime. The present invention achieves this object as described in the specification, drawings and claims presented herein.

Due to the elongate extension of the filament in two directions perpendicular to the electron beam, in the source area of the electron beam, the effective electron emitting surface can significantly be increased so that, compared to the essentially point-shaped extension of the electron emitting tip of a hairpin filament, a significantly lower filament temperature suffices for emitting the same electron current. Elongate extension of the filament means that the extension is significantly larger, in particular at least 50% larger than the thickness of the wire, and preferably at least twice as large, and further preferably at least three times as large. The lower filament temperature results in a significant extension of the lifetime of the filament and thus of the cathode element. A multiplied filament lifetime, increased by an order of magnitude and more, can be achieved with the present invention. Surprisingly, it has turned out that in spite of the increased electron emitting surface a focal spot size of less than 10 μm, and preferably 7 μm or less, can still be obtained. Owing to the present invention, high-resolution microfocus x-ray inspection systems can therefore be used in industrial manufacture.

In the source area of the electron beam the filament comprises a plurality of wire portions which are arranged next to each other. Thus, the present invention can be realized in a simple way by one single wire. In one embodiment which is particularly simple to manufacture, the wire portions are formed by a plurality of wire loops so that the electron emitting area of the filament has the shape of a wire coil.

The wire portions are arranged so as to be spaced from each other. Thus, the wire flanks, i.e. the side surfaces of the wire between the wire portions, can further contribute to the electron emitting surface, with the result that the inventive effect can be enhanced.

At least three wire portions may be utilized to obtain a significant increase of the electron emitting surface. Up to ten wire portions may be utilized, and preferably a maximum of six electron emitting wire portions may be utilized, so as to be able to obtain a microfocus, i.e. a focal spot of the electron beam not exceeding 10 μm. An uneven number of wire portions is advantageous, since the beam profile of the electron beam improvise due to the presence one wire portion located exactly in or substantially near the center. Therefore, embodiments may utilize three, five or seven wire portions accordingly.

The cathode element is designed as a replaceable unit for being inserted into an adapted mounting of a microfocus x-ray tube. In this manner, depending on the application, an inventive cathode element or a conventional cathode element comprising a hairpin filament can be inserted into the adapted mounting of an inventive microfocus x-ray tube.

An inventive microfocus x-ray tube may also include a condenser lens to align the electron beam approximately parallel when using an inventive cathode element. That way, in particular when using a downstream conventional focusing lens, the specified nominal values of the tube can be obtained independent of the type of the inserted cathode element. When using a cathode element comprising a hairpin filament the condenser lens is conveniently switched off. There is no need for adapting the focusing lens to the inventive cathode element.

The present invention will be described in more detail on the basis of preferred embodiments as follows and with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

The micro-computer tomography system shown inFIG. 1includes an x-ray system10which is adapted to record a set of x-ray projections of a sample13. For this purpose, the x-ray system10includes a microfocus x-ray tube11which emits x-radiation14originating from a focal spot or focus16of the x-ray tube11, an imaging x-ray detector12and a sample holder20which preferably is adapted to rotate the sample13around a vertical axis. The x-ray detector12is preferably an area detector, in particular a flat panel detector; however, a line detector is also possible. A set of x-ray projections of the sample13is obtained for example by successively rotating the sample holder20around a pre-defined small angular step at a time and recording an x-ray projection at each angle of rotation. The x-ray system10is not limited to a rotation of the sample holder20around a vertical axis. Alternatively, for example the x-ray tube11and the x-ray detector12can be rotated around the fixed sample13.

The x-ray projections are read out from the x-ray detector12and are transmitted to a computer device41in which reconstructed three-dimensional volume data of the sample13are calculated from the recorded set of x-ray projections using a generally known reconstruction algorithm and are displayed for example on a monitor42. As shown inFIG. 1, the computer device41can also be adapted to control the x-ray source11, the sample holder20and the x-ray detector12; alternatively, a separate control device may be provided.

The microfocus x-ray tube11in particular includes a cathode element15, a Wehnelt cylinder21, an anode19, a focusing lens22which preferably is designed as an electromagnetic lens, and an electron beam target23. Moreover, a further electromagnetic lens25may be provided which preferably is designed as a condenser lens for aligning the electron beam24approximately parallel; however, the condenser lens25is not compulsory. The microfocus x-ray tube11conveniently further includes a not-shown deflector unit for beam position adjustment.

The cathode element15includes a filament17which is made of a suitable wire27, in particular of tungsten, and is mounted on an insulating socket34which for example is made of a ceramic material. The filament wire27preferably has a strength in the range of 100 μm to 300 μm, for example approximately 200 μm. A heating voltage is applied to the ends of the filament17for thermionic emission of electrons from the filament wire27. An accelerating voltage generated by a not-shown high-voltage generator is applied between the filament17and the anode19to accelerate the electrons extracted from the wire towards the anode19and to generate an electron beam24. The maximum accelerating voltage preferably is at least 100 kV, preferably at least 200 kV.

The generated electron beam is focused on the target23by the focusing lens22in order to generate x-radiation14. The target23preferably is arranged in a reflecting arrangement (direct beam target). The massive target23can absorb a comparatively high power so that the x-ray tube11is advantageously adapted to generate a maximum tube current of at least 1 mA and/or a maximum tube power of at least 100 W. Thus, the x-ray tube11is suited for the inspection of relatively thick samples like for example casted parts.

The present invention is not limited to a direct beam target. The inventive filament17in particular may also be used in an x-ray tube11comprising a transmission target. In view of this, the maximum tube current preferably is at least 0.5 mA and/or the maximum tube power is at least 50 W.

In order to obtain a detail resolution in the x-ray image of well below 10 μm, which is desired in micro-computer tomography, it is necessary that the size of the electron beam focal spot16on the target23is below 10 μm. For this purpose, the electron beam24first is focused using a Wehnelt cylinder or grid21lying on a suitable negative potential relative to the filament17, in order to create a sharp cross-over point26. Cathode17, Wehnelt cylinder21and anode19thus form a triode. Behind the anode19the electron beam is further focused on the focal spot16of the target using a focusing lens22. In general, the electron optics of the tube11, here consisting of Wehnelt cylinder21, focusing lens22and, if required, condenser lens25, is adapted to create a focal spot16having an average diameter of 10 μm or less.

In a preferred embodiment according toFIG. 3andFIG. 4, the electron emitting area28of the filament17is formed by a plurality of loops29which may be arranged essentially parallel to each other. The filament17in this embodiment is a single-coiled filament. Preferably, there are at least three loops29. In the embodiment ofFIG. 3andFIG. 4, three loops29are shown which may be an optimum number. Furthermore, there preferably are not more than ten loops29, further preferably not more than seven loops29, in order to limit the extension of the electron emitting area in respect of the requested resolution of detail in the x-ray image.

The surface of the filament facing the target23, which forms the main source of the electron beam24, is formed by a plurality of wire portions30, as is shown inFIG. 4. The wire portions30preferably are aligned essentially parallel and as a result show an overall planar extension of the surface of the filament17facing the target23, with a first elongate extension l1perpendicular to the electron beam and a second elongate extension l2perpendicular to the electron beam and perpendicular to the extension l1(seeFIG. 4). Elongate extension means that l1and l2are significantly larger than the thickness d of the wire, in particular at least 50% larger, preferably at least twice as large, further preferably at least three times as large, in the present embodiment approximately four times as large. In comparison to the “point-shaped” surface of the tip of a hairpin filament having an extension of approximately d2, an electron emitting surface of the filament17which is extended by up to a factor of three and more is provided by the present invention. Consequently, for generating the same tube current the heating temperature of the filament17can be reduced significantly and thus its lifetime can be extended by a factor of ten and more. The extensions l1and l2preferably are about the same size, i.e. they differ from each other for example by not more than 50% in relation to the larger one of the two extensions. The filament17preferably is free of tips or kinks with a bending radius in the range of the wire diameter d.

The loops29and thus the electron emitting wire portions30preferably are spaced from each other, as can be seen inFIG. 4. The distance preferably is smaller than or equal to the thickness d of the filament wire27and preferably is in the range of 0.1 d to d, in the present case for example 0.5 d or approximately 100 μm. The spaced arrangement of the wire portions30provides the advantage that the flanks or the side surfaces of the wire portions30further contribute to the electron emitting surface forming the source of the electron beam. Hereby, the effective electron emitting surface can be further increased without additional effort.

The wire portions30may also be formed by other means than wire loops29. In a not-shown embodiment for example each wire portion30can be formed by a separate single filament. In the embodiment shown inFIG. 5, for example five wire portions30are formed by a serpentine filament. The embodiment according toFIG. 6shows clearly that an overall planar extension of the surface of the filament wire27facing the target23can also be realized without straight wire portions30.

The x-ray tube11has an open design, i.e. the tube11comprises means for venting and in the vented state can be opened to take out a cathode element15and insert a new cathode element15in particular when a filament has reached or passed a predetermined operating time. The housing34of the x-ray tube11for this purpose consists of two housing halves35,36which can be separated from each other at a flange37. The cathode element15designed as a replaceable unit preferably includes the Wehnelt cylinder21, in order that the centering of the filament17relative to the front end opening31for the electron beam24can already be carried out by the manufacturer and does not have to be carried out by the user of the x-ray tube11.

After the insertion of a new cathode element15the x-ray tube11is sealed to be vacuum-tight by connecting the two housing halves35,36and is evacuated to the operating vacuum using a vacuum pump33permanently mounted on the x-ray tube11.

In a preferred embodiment, in particular if a higher detail resolution of the x-ray images is desired, the x-ray tube11is adapted to optionally being used with a hairpin filament17. For this purpose, a cathode element15comprising a hairpin filament can simply be inserted into the mounting32; the x-ray tube11in this high-resolution operating state is shown inFIG. 7. There is no need for a further change in the design of the x-ray tube11, besides the replacement of the cathode element15, or of the not-shown high-voltage generator. To render this possible, essential parameters of the filament17to be used having the essentially planar extension, like wire length and diameter, dimensions like for example loop diameter as well as distances are optimally chosen. When operating the x-ray tube11with a hairpin filament the condenser lens25preferably is switched off. Thus, the x-ray tube11is operated in a conventional manner with the focusing lens22. The condenser lens25preferably is switched off automatically as a result of inserting a cathode element comprising a hairpin filament.

The embodiment shown inFIG. 1relates to a micro-computer tomography system10. However, the x-ray tube11is also suited for a two-dimensional radiographic testing system without CT reconstruction.