DE102018206514A1 - Method and device for controlling a focal spot position - Google Patents

Abstract

The invention relates to a device (1) for controlling a position of a focal spot (50) on a target (30) in an x-ray tube (10, comprising:
at least one charge collection element (60) for collecting a portion of secondary electrons generated in the focal spot (50), the at least one charge collection element (60) being spaced from the target (30) in the x-ray tube (10) and being electrically isolated,
wherein the at least one charge collecting element (60) is arranged so that in each case a detection space angle (8) spanned by free lines of sight (101, 102; 101 ', 102', 101 ", 102") of the secondary electrons is smaller than the solid angle of the hemisphere is above the target surface (31) at a target focal spot position (55), with free line of sight (101, 102, 101 ', 102', 101 ", 102") being the straight lines along which one of the secondary electrons passes unhindered from the target (30). to which at least one charge collecting element (60) can move; and
a measuring device (270) for measuring the charge collected per time interval and
an evaluation device (280) connected to the measuring device (270), which device is suitable for converting a change in the charge detected per time interval into a change in position and for providing a position signal (290). Furthermore, the invention relates to a method which detects changes in the focal spot position (52).

Description

The invention relates to a method and an apparatus for controlling a position of a focal spot in a surveillance area about a target focal spot position on a target in an x-ray tube, in particular in an x-ray tube of an industrial computed tomography measuring device.

With the measurement technology of industrial computed tomography, a precise knowledge of the various geometric parameters of the measuring device is of crucial importance. This also includes the knowledge of the position of the focal spot of the X-ray source, which is also called source spot or English spot. An inaccurate knowledge of the position of the origin of the X-ray radiation, d. h the focal spot position, is directly in the measurement errors of a test object, which is measured by means of an industrial computed tomography measuring device.

Even if the focal spot position is known at the beginning of each measurement and is preferably the same for all measurements, there is still a problem that even during a measurement of a test object, the focal spot position can drift or wander on the target of the X-ray source. In particular, when a new thermal equilibrium must be established during the measurement, such fluctuations or changes in the focal spot position occur in an X-ray source.

From the prior art, various ways are known to consider or compensate for focal spot drift. In one variant, projections are recorded at very large angular steps at the beginning of a scan of a test object with the X-ray radiation. If, for example, angular steps of 45 ° are used, then eight projections must be recorded at the resulting eight reference angles. These projections of the test object then serve as reference projections. If one of the reference angles is reached during the fine scanning of the test object, an offset between the current projection and the previously acquired reference projection is determined via an image processing method, and this offset is used for the translational correction. This is not a complete correction of a focal position change because a perspective effect of changing the focal spot position is not taken into account.

Another variant provides that the measurement of a test object is interrupted after a predetermined number of measuring steps and in each case instead of the test object or additionally a reference object is introduced into the measuring range. Based on this reference object, the focal spot position is then determined. In such a case, the new geometry can be assigned directly to the projections and taken into account in the reconstruction. However, a measurement of a test object must be interrupted regularly. If continuous measurements are made, the individual sequences of the measurement must be performed overlapping. In addition, it is very disadvantageous, especially for very small objects that are measured with a high magnification, not only to rotate the object by small angular steps, but also repeatedly to drive out of the beam path. In each case there is the risk that an object movement takes place in the holder, which is in the order of magnitude of the detected resolution.

Another possibility is to constantly keep a known high-precision reference object in the picture in addition to the test object actually to be measured. The disadvantage of this is that this valuable space is lost in a height axis and graduated for different magnifications, a variety of reference objects is needed.

The invention is thus based on the object to provide an improved method and an improved device, which control a change in the focal spot position. In the narrower sense, the term controlling is understood to mean the detection and monitoring of the focal spot position, for example the detection of a change in the focal spot position. In a broader sense, the controlling may also include quantitatively determining the position change and / or additionally including controlled tracking of the focal spot position in an output focal spot position or a target focal spot position.

The invention is achieved by a device having the features of patent claim 1 and a method having the features of patent claim 9. Advantageous embodiments will be apparent from the dependent claims.

The invention is based on the idea of detecting a change in a position of the focal spot on the basis of secondary electrons which are generated in the focal spot during operation of the X-ray tube in addition to the X-ray radiation and emitted in a field-free hemisphere over a target surface of a target. If one uses a charge collecting element, which is spaced from the target surface in the X-ray tube is arranged and due to its geometric shape and orientation and optionally of diaphragms and other objects between the Target surface and the charge collecting element can collect only secondary electrons emitted in a detection space angle at the focal spot, which does not cover the entire half space above the target surface, it can be concluded from changes in the charge collected with the charge collecting element per time interval on a change in the position of the focal spot , Only secondary electrons, which are emitted along a free line of sight between the focal spot position and a position on the charge collecting element, reach the charge collecting element. A free line of sight is a line along which an undisturbed view from a position in the focal spot to a position on the charge collecting element is possible, or in other words along which secondary electrons can pass in a straight line undisturbed by material obstacles from the focal spot to the charge collecting element. The charge collection element is arranged such that, as the focal spot position changes, the detection space angle subtended by the free line of sight between the current focal spot position and the charge collection element changes. From the change of a detected charge amount per time interval can thus be concluded that there is a change in the visible area of the charge collecting element for the secondary electrons in the focal spot.

The term "over" the target surface is to be understood in the sense of in the direction of radiation of the primary electrons in front of the target surface, regardless of an orientation of the target surface in space.

• mindestens ein Ladungssammelelement zum Sammeln eines Teils von im Brennfleck erzeugten und in die Hemisphäre über einer Targetoberfläche des Targets emittierten Sekundärelektronen, wobei das mindestens eine Ladungssammelelement beabstandet von dem Target in der Röntgenröhre angeordnet ist und gegenüber anderen Elementen der Röntgenröhre elektrisch isoliert ist,
• wobei das mindestens eine Ladungssammelelement relativ zu der Targetoberfläche des Targets so angeordnet ist, dass jeweils ein Erfassungsraumwinkel, der von freien Sichtlinien der Sekundärelektronen aufgespannt wird, kleiner als der Raumwinkel der Hemisphäre über der Targetoberfläche in der Sollbrennfleckposition ist, wobei freie Sichtlinien jene geraden Linien sind, die von einer der Entstehungspositionen der Sekundärelektronen im Brennfleck zu einer der Positionen auf dem mindestens einen Ladungssammelelement geradlinig verlaufen und entlang derer sich eines der Sekundärelektronen ungehindert von dem Target zu dem mindestens einen Ladungssammelelement bewegen kann; und
• eine Messeinrichtung, die mit dem mindestens einen Ladungssammelelement verbunden ist, zum Messen der pro Zeitintervall auf dem mindestens einen Ladungssammelelement gesammelten Ladung von Sekundärelektronen und
• eine mit der Messeinrichtung verbundene Auswerteeinrichtung, welche geeignet ist, eine Änderung der pro Zeitintervall erfassten Ladung in eine Positionsveränderung umzuwandeln und ein Positionssignal bereitzustellen.

In particular, an apparatus for controlling a position of a focal spot on a target in an x-ray tube in a surveillance area about a target focal spot position is proposed, which comprises:

• at least one charge collection element for collecting a portion of secondary electrons generated in the focal spot and emitted into the hemisphere above a target surface of the target, the at least one charge collection element being spaced from the target in the x-ray tube and electrically isolated from other elements of the x-ray tube,
• wherein the at least one charge collection element is disposed relative to the target surface of the target such that each detection space angle subtended by free lines of view of the secondary electrons is less than the solid angle of the hemisphere above the target surface in the target focal spot position, with clear line of sight being those straight lines which are rectilinear from one of the formation positions of the secondary electrons in the focal spot to one of the positions on the at least one charge collecting element and along which one of the secondary electrons is free to move from the target to the at least one charge collecting element; and
• a measuring device, which is connected to the at least one charge collecting element, for measuring the charge of secondary electrons collected on the at least one charge collecting element per time interval
• an evaluation device connected to the measuring device, which device is suitable for converting a change in the charge detected per time interval into a change in position and for providing a position signal.

• Sammeln von Sekundärelektronen auf mindestens einem Ladungssammelelement, die einen Anteil der im Brennfleck auf dem Target erzeugten und in eine Hemisphäre über einer Targetoberfläche des Targets emittierten Sekundärelektronen bilden, wobei das mindestens eine Ladungssammelelement relativ zu der Targetoberfläche des Targets so angeordnet ist, dass jeweils ein Erfassungsraumwinkel, der von freien Sichtlinien der Sekundärelektronen aufgespannt wird, kleiner als der Raumwinkel der Hemisphäre über der Targetoberfläche in der Sollbrennfleckposition ist, wobei freie Sichtlinien jene geraden Linien sind, die von einer der Entstehungspositionen der Sekundärelektronen im Brennfleck zu einer der Positionen auf dem mindestens einen Ladungssammelelement geradlinig verlaufen und entlang derer sich eines der Sekundärelektronen ungehindert von dem Target zu dem mindestens einen Ladungssammelelement bewegen kann;
• iteratives Messen der pro Zeitintervall gesammelten Ladung auf dem mindestens einen Ladungssammelelement;
• Ermitteln, ob eine Änderung bei der pro Zeitintervall gesammelten Ladungsmenge aufgetreten ist, und falls dieses der Fall ist, Ermitteln einer Positionsabweichung der Brennfleckposition, die mit einer Änderung des Erfassungsraumwinkels des mindestens einen Ladungssammelelements korrespondiert, wobei die Änderung des Erfassungsraumwinkels wiederum mit der ermittelten Änderung der pro Zeitintervall erfassten Ladung korrespondiert, und
• Ausgeben eines Positionssignals, welches eine Positionsinformation umfasst.

Furthermore, a method is proposed for controlling a focal spot position on a target in an x-ray tube in a monitoring area around a target focal spot position, comprising the steps:

• Collecting secondary electrons on at least one charge collecting element that form a portion of the secondary electrons generated in the focal spot on the target and emitted into a hemisphere over a target surface of the target, wherein the at least one charge collection element is disposed relative to the target surface of the target such that each detection space angle , which is spanned by free lines of sight of the secondary electrons, is smaller than the solid angle of the hemisphere over the target surface in the target focal spot position, where free line of sight are those straight lines that are from one of the secondary electron formation positions in the focal spot to one of the positions on the at least one charge collection element straight and along which one of the secondary electrons can move freely from the target to the at least one charge collecting element;
• iteratively measuring the charge accumulated per time interval on the at least one charge collection element;
• Determining whether a change has occurred in the amount of charge accumulated per time interval, and if so, determining a positional deviation of the focal spot position corresponding to a change in the detection space angle of the at least one charge accumulation element, wherein the change in the detection space angle again coincides with the determined change corresponds to the charge detected per time interval, and
• Outputting a position signal comprising position information.

Advantage of the invention is that used directly in the focal spot secondary electrons are used to determine the position of the focal spot and / or a change in the focal spot position. In particular, in X-ray sources in which the primary electrons are directed onto the target of the X-ray tube at a shallow angle and the generated X-ray also exits the target surface at a shallow angle, a space perpendicular to the target surface can be used to detect the generated secondary electrons. It has been shown that changes which correspond to one tenth of the diameter of the focal spot can already be detected. This applies at least for focal spot diameter in the range of 10 microns to 1 mm.

It is exploited that when hitting high-energy electrons on the target in the focal spot only a portion of the electrons is slightly reflected, but a significantly larger number of low-energy secondary electrons are emitted in the focal spot. These are emitted in the focal spot in all spatial directions in the hemisphere above the target surface. It has been found that the total amount of charge emitted from the target into the hemisphere above the target surface is approximately 50% of the charge applied to the target by the primary electrons. However, only a fraction of this is collected by the charge collecting element, since the detection space angle of the charge collecting element is smaller, generally much smaller than the solid angle of the hemisphere over the target surface.

Preferably, in one embodiment, the at least one charge collection element is spaced from the target such that the detection space angle subtended by the free lines of sight extending from the focal spot to a position on the at least one charge collection element and along which they emit a rectilinear propagation at the focal spot Secondary electrons is possible to a position on the at least one charge collecting element, is dependent on the focal spot position along an excellent or predetermined direction on the target surface. This ensures that not only a displacement of the focal spot position can be detected from its original position, but also a qualitative and possibly quantified statement along at least one excellent direction is possible. This also opens up the possibility of actively returning the focal spot position to the original focal spot position along this excellent direction via electron-optical elements acting on the primary electrons.

For example, in those embodiments in which the positional deviation can be quantified at least along an excellent spatial direction, the output position information may indicate, for example, the determined direction of the positional change or, optionally, the positional deviation or zero. Here, the positional deviation relative to a previously detected focal spot position may be indicated, and a zero correspondingly outputted as positional information, if no relative change in the focal spot position has occurred. Alternatively, a deviation from the target focal spot position may also be indicated as a positional deviation, the zero being output when the current focal spot position coincides with the target focal spot position. Other embodiments only indicate the direction of the deviation when it has exceeded a tolerance threshold.

In a particularly preferred device it is provided that a detection surface is formed, which has a proximal boundary edge and a distal boundary edge, wherein the proximal boundary edge and the distal boundary edge are oriented parallel to each other and to the target surface and by aperture edges and / or edges of other elements X-ray tube and / or edges of the at least one charge collecting element and limit the detection space angle of the free line of sight from the focal spot at the target focal spot position to the at least one charge collecting element, wherein the proximal boundary edge has a smaller distance from the target surface than the distal boundary edge, wherein the proximal boundary edge and distal boundary edge oriented perpendicular to the excellent direction on the target surface. The detection surface is narrowed by the proximal boundary edge and the distal boundary edge perpendicular to the excellent direction such that an opening angle range of the free line of sight is limited in planes perpendicular to the proximal boundary edge and the distal boundary edge, so that positional change of the focal spot along the designated direction on the boundary Target surface, which is oriented perpendicular to the boundary edges, is well detected. Perpendicular to the excellent direction, the detection space angle is thus restricted by two edges parallel to each other and to the target surface in the focal spot. Due to the fact that the boundary edges, ie the proximal boundary edge and the distal boundary edge, have different distances to the target surface, the detection area is not oriented parallel to the target surface in the focal spot, but angled relative to the target surface.

The proximal boundary edge and thus also the distal boundary edge are generally rectilinear.

Any clear line of sight of secondary electrons detected on the charge collection element will pass through the detection surface or end on the detection surface if an area of the charge collection element coincides with the detection surface. The detection surface is thus always formed between the charge collecting element and the target surface or coincides with the charge collecting element. The design is thus dependent on an arrangement of the charge collecting element above the target surface and, if appropriate, on the diaphragm edges or edges of other objects arranged between the charge collecting element and the target surface between the charge collecting element and the target surface. The orientation of the newly formed detection surface in space and thus relative to the target surface is determined by the proximal boundary edge and the spaced apart and parallel oriented distal boundary edge. These also limit the detection area perpendicular to the excellent (predetermined) direction.

In some embodiments, it is therefore provided that the position signal indicates a positional deviation of the focal spot position in a direction of the target surface that is oriented perpendicular to the boundary edges of the detection surface.

Particularly large changes in the detected charge per time interval arise when the detection surface includes an obtuse angle with a line of sight from a desired focal spot position of the focal spot to the proximal boundary edge, the line of sight being oriented perpendicular to the proximal boundary edge.

The flatter the angle between the line of sight to the proximal boundary edge and the detection surface, the smaller the detection space angle, which is spanned by free lines of sight of the secondary electrons between the focal spot and the charge collecting element. The surface of the charge collecting element visible to the secondary electrons thus becomes smaller, the flatter the angle between the line of sight to the proximal boundary edge and the detection surface becomes. Conversely, the steeper the angle between the line of sight to the proximal boundary edge and the detection surface becomes larger the detection space angle. Movement along the excellent direction (which is also referred to as a predetermined direction) on the target thus results in a reduction of the detection space angle and thus a smaller amount of charge collected on the charge collection element per time interval and a movement in the opposite direction to one Increasing the amount of charge collected on the charge collector per time interval.

In a preferred embodiment it is provided that the detection surface forms an angle with a surface normal of the target surface α which is preferably selected from the angular range between 1 ° and 10 °, more preferably from the angular range between 3 ° and 7 °. The angle α Here, the apex angle between the surface normal and the detection surface. The smaller the vertex angle α is, the more sensitive is the detection with respect to change in position of the focal spot along the excellent direction.

Particularly preferably, the at least one charge collecting element is arranged vertically above the desired focal spot position. This is equivalent to having the surface normal in the desired focal spot position intersect the charge collection element and the surface normal represents a clear line of sight. Here, on the one hand, in particular, there is sufficient space available in an X-ray tube as well as an efficient detection at favorable angles for a well-resolved position detection possible.

However, an embodiment is also possible in which the at least one charge collecting element is not disposed and formed perpendicular to the target focal spot position and there is no free line of sight along the surface normal of the target surface in the target focal spot position.

Preferably, the charge collecting element is electrically conductive. Preferably, the charge collecting element is formed as a metal layer or sheet metal. This makes it possible to collect a large amount of charge on the charge collecting element.

Preferably, the at least one charge collecting element is designed as an electrically conductive plate. In these embodiments, both the proximal boundary edge and the distal boundary edge may be formed by edges of the conductive plate. However, it is also possible that the proximal boundary edge or the distal boundary edge or both boundary edges, ie, the proximal boundary edge and the distal boundary edge, by electrical conductive diaphragm edges or edges of other elements of the x-ray tube are formed.

When a plate is used as the charge collecting element, the detection area may be provided on at least a part of the plate when the charge collecting element has both the proximal boundary edge and the distal boundary edge. Such an embodiment thus represents a simple embodiment of the device. A distance to the target must be so great that a significant heating of the charge collecting element due to heat radiation emitted by the focal spot on the target surface is avoided.

In another embodiment, it is provided that the detection surface is not physically formed and the proximal boundary edge is an edge of a diaphragm or another element of an X-ray tube measuring head of the X-ray tube. Such an embodiment may be advantageous for shielding heat radiation emitted from the focal spot at the target surface from the charge collecting element and for increasing the distance between the focal spot and the charge collecting element.

In order to achieve the least possible interference with movements perpendicular to the excellent direction on the target surface, in one embodiment it is provided that the at least one charge collecting element has a greater extent parallel to the direction of the proximal and distal boundary edges than perpendicular to the target surface on the target surface Projected projections of the proximal boundary edge and the distal boundary edge are away from each other. Movements transverse to the direction of excellence thus influence the angles along this transverse direction, under which the charge collecting element is seen by the generated secondary electrons, not at all or only minimally and at least significantly less than a change in position parallel to the excellent direction.

Embodiments in which the detection surface and the charge collecting element have a greater extent parallel to the boundary edges of the detection surface than the projections of the boundary edges generated perpendicular to the target surface on the target surface are therefore advantageous.

In order to determine the charge accumulated on the at least one charge collecting element, in one embodiment of the device it is provided that the at least one charge collecting element is connected to the measuring device via a capacitor, and the measuring device is designed to maintain a potential across the capacitor, and the charge quantity , which must feed the measuring device into the capacitor to compensate for the charge change caused by incident secondary electrons, is used as a measurement signal for measuring the change in charge per time interval.

In an alternative embodiment, the voltage drop across a resistor during discharge of the at least one charge collecting element is used as the measurement signal.

In yet another embodiment of the invention, it is provided that the current caused by the impinging charges is measured as a measured signal for the collected charge.

All embodiments have in common that a reliable determination is possible even with small amounts of charge that are collected.

High dynamics in the charge or current measurement enables a measurement of small changes on a high fundamental signal and thus a high position resolution of the focal spot position. A resolution between the minimum detected current or the minimum detected charge and a maximum detected current or a maximum detected charge, which comprises at least ^210, ie 1024 digitization steps, is preferred.

A particularly compact design is obtained when the charge collecting element is designed as a closing element or part of a closing element of a recess in an X-ray tube measuring head. In such a design, the X-ray tube measuring head is preferably made of a solid metallic block, in which two recesses are incorporated, for example, two holes whose directions of extension or bore axes intersect and meet at a shallow angle. At this point of intersection the target is arranged. One of the two recesses (holes) is used to irradiate the primary electrons, the other of the two recesses serves as an exit channel for the generated X-rays and is completed with a possible X-ray transparent exit window.

In a particularly preferred embodiment, in such an X-ray tube measuring head, a third recess, again introduced for example in the form of a hole in the metal block, which is preferably oriented perpendicular to the target surface and up to the target surface, preferably up to the Target fuel spot position extends on the target surface. The at least one charge collecting element can thus be arranged in this third recess.

In a particularly preferred embodiment, the charge collecting element is designed as a closing element or part of the closing element of this third recess of the X-ray tube measuring head. Optionally, in addition to the outside of the X-ray tube measuring head a protective device as Berührschutz necessary.

In order to be able to take into account changes in the charges detected by the at least one charge collecting element per time interval, which are not due to changes in the focal spot position but are attributable to changes in the secondary electron emission as a whole at the target, it is provided in one embodiment that the evaluation device is formed, to detect a normalization signal that is correlated with a total number of generated secondary electrons, and to consider the normalization signal in determining a change in the charge detected per time interval. In this way, a very reliable position determination and control of the focal spot position can also be carried out in those operating states in which, for example, the primary electron current is not constant or a change in the focal spot size occurs due to a change in the focusing of the primary electron beam. Here, the power density and thus the temperature of the target change, which in turn affects the emission of secondary electrons.

In order to also be able to detect changes in the focal spot position in another direction on the target surface, which is oriented transversely, preferably perpendicular, to this excellent direction, it is provided in one embodiment that at least one further charge collecting element is arranged above the target surface in an electrically insulated manner in the x-ray tube and a further detection surface is formed, which has a further proximal boundary edge and a further distal boundary edge, which are oriented perpendicular to a further excellent direction, wherein the further proximal boundary edge and the further distal boundary edge are oriented parallel to each other and to the target surface and are formed by further diaphragm edges or further edges of other elements of the x-ray tube or edges of the at least one further charge collecting element and a further detection space angle of further free Sichtlini The further proximal boundary edge has a smaller distance from the target surface than the further distal boundary edge, wherein the further charge collecting element with the measuring device or another measuring device for measuring the time interval on the other Charge collecting element accumulated charge of secondary electrons is connected and connected to the measuring device or the other measuring device evaluation is adapted to convert a change in the detected charge per time interval on the further charge collecting element in a position change of the focal spot along the other excellent direction on the target surface and another position signal to provide the further excellent direction oriented transversely to the excellent direction.

The further boundary edges are oriented transversely, preferably perpendicular, to the boundary edges in such an embodiment.

In order to obtain an independence of the charge determinations for the at least one charge collecting element and the at least one further charge collecting element, the at least one charge collecting element and the at least one further charge collecting element are preferably arranged such that the amount of free line of sight from the desired focal spot position to the at least one charge collecting element also at Changing the position of the focal spot position by the target focal spot position is disjoint from the amount of further free line of sight from the target focal spot position to the at least one further charge collecting element.

• 1 eine schematische Darstellung einer ersten Ausführungsform der erfindungsgemäßen Vorrichtung;
• 2A eine schematische Darstellung der Geometrie in einer Schnittebene senkrecht zur flächigen Ausdehnung eines Ladungssammelelements einer Vorrichtung gemäß der ersten Ausführungsform, bei der eine Nachweisebene durch Kanten des Ladungssammelelements eingegrenzt ist;
• 2B eine schematische Darstellung der Geometrie in einer Schnittebene senkrecht zur flächigen Ausdehnung eines Ladungssammelelements einer Vorrichtung gemäß einer zweiten Ausführungsform der Erfindung, bei der die Nachweisebene durch eine Kante des Ladungssammelelements und eine Blendenkante eingegrenzt ist;
• 2C eine schematische Darstellung der Geometrie in einer Schnittebene senkrecht zur flächigen Ausdehnung eines Ladungssammelelements einer Vorrichtung gemäß einer dritten Ausführungsform der Erfindung, bei der die Nachweisebene durch zwei Blendenkante eingegrenzt ist;
• 3 einen Grafen, welcher einen funktionellen Zusammenhang eines Versatz-Signal-Verhältnis, im Folgenden auch als OSR - Offset Signal Ratio bezeichnet, von einem Scheitelwinkel α zwischen einer Oberflächennormale der Targetoberfläche und einer Nachweisfläche angibt;
• 4 eine grafische Darstellung eines funktionalen Zusammenhangs des Versatz-Signal-Verhältnisses, OSR, von einer Bewegung der Brennfleckposition für unterschiedliche Brennfleckdurchmesser;
• 5 eine schematische Darstellung einer vierten Ausführungsform der Erfindung mit einem Röntgenröhrenmesskopf, bei dem das Ladungssammelelement als Abschlusselement einer Funktionsöffnung ausgebildet ist; und
• 6 eine schematische Darstellung einer fünften Ausführungsform der Erfindung, bei der das Ladungssammelelement als Bestandteil des Röntgenfensters ausgebildet ist.

The invention will be explained in more detail with reference to a drawing. Hereby show:

• 1 a schematic representation of a first embodiment of the device according to the invention;
• 2A a schematic representation of the geometry in a sectional plane perpendicular to the planar extension of a charge collecting element of a device according to the first embodiment, in which a detection plane is bounded by edges of the charge collecting element;
• 2 B a schematic representation of the geometry in a sectional plane perpendicular to the planar extension of a charge collecting element of a device according to a second embodiment of the invention, in which the detection plane is bounded by an edge of the charge collecting element and a diaphragm edge;
• 2C a schematic representation of the geometry in a sectional plane perpendicular to the planar extension of a charge collecting element of a device according to a third embodiment of the invention, in which the detection plane is limited by two diaphragm edge;
• 3 a graph, which has a functional relationship of an offset signal ratio, hereinafter also referred to as OSR - offset signal ratio, from a vertex angle α between a surface normal of the target surface and a detection surface;
• 4 a graphical representation of a functional relationship of the offset signal ratio, OSR, of a movement of the focal spot position for different focal spot diameter;
• 5 a schematic representation of a fourth embodiment of the invention with an x-ray tube measuring head, in which the charge collecting element is designed as a closing element of a functional opening; and
• 6 a schematic representation of a fifth embodiment of the invention, in which the charge collecting element is formed as part of the X-ray window.

In 1 a section of an X-ray tube measuring head 11 an x-ray tube 10 shown schematically. A primary electron beam 20 meets a target 30 , At the place of impact 21 the primary electrons of the primary electron beam 20 becomes x-ray radiation 40 generated, for example, for the measurement of a test object (not shown) in a computer tomograph. Part of the energy of the primary electrons leads to a heating of the target. Therefore, the place of generation of the X-ray becomes 40 focal spot 50 called. The position where X-ray generation takes place becomes the focal spot position 52 or also called source position.

Part of the target 30 incident primary electrons is reflected at the target. In addition, low-energy secondary electrons in the focal spot 50 in all directions of the hemisphere above the target surface 31 emitted. These spread out in a straight line. The angular distribution depends on the energy of the high-energy primary electrons.

To determine the focal spot position based on the emitted secondary electrons 52 to be able to determine is in the x-ray tube 10 spaced apart from the target 30 a charge collection element 60 arranged. The charge collection element 60 is only under a limited detection space angle Θ for secondary electrons that are in a desired focal spot position 55 be emitted. When changing the focal spot position 52 on the target surface 31 occurs a change in the detection space angle Θ below which the secondary electrons from the changed focal spot position 52 ' the charge collecting element 60 see.

To the largest possible change in the detection space angle Θ at a position change of the focal spot position 52 to get is the charge collection element 60 opposite the target surface 31 angled arranged.

In the illustrated embodiment, the charge collection element is 60 as a metallic plate 61 educated. The metallic plate 61 is almost perpendicular to the target surface 31 oriented. The charge collection element 60 and thus the plate 61 have a proximal edge 62 and a distal edge 63 on. The proximal edge 62 has a smaller distance 72 to the target surface 31 as the distal edge 63 on, cf. 2A ,

The proximal edge 62 and the distal edge 63 each extend perpendicular to the plane of the drawing. This is also the second extension direction of the plate 61 or the charge collecting element 60 , As well as the target surface 31 has an extension perpendicular to the plane of the drawing, are the proximal edge 62 and the distal edge 63 to each other and to the target surface 31 oriented in parallel.

Only those secondary electrons that are in a straight line from the focal spot 50 to the charge collecting element 60 be on the charge collecting element 60 collected. A straight line along which a secondary electron emanates from its emission site in the focal spot 50 on the target 30 to a position on the charge collecting element 60 without being obstructed by an obstacle, is called a free line of sight. For a target fuel spot position 55 is a free line of sight 101 drawn from the target focal spot position 55 to the proximal edge 62 of the charge collecting element 60 runs. In addition, there is a clear line of sight 102 drawn from the target focal spot position 55 to the distal edge 63 of the charge collecting element 60 runs. These are the two free lines of sight 101, 102, which in the drawing plane are the angle range θ for the directions that make up secondary electrons at the target focal spot position 55 are emitted, the charge collecting element 60 to reach. Therefore, the proximal edge 62 and the distal edge 63 also as boundary edges 65 or as a proximal boundary edge 66 and accordingly as the distal boundary edge 67 designated. Just Line of sight, whose angle is relative to the target surface 31 in the angle range θ fall through the line of sight 101 . 102 is limited, thus leading to the charge collecting element and provide free line of sight for secondary electrons. On the charge collecting element 60 Thus, only a portion of the secondary electrons collected in the hemisphere above the target surface 31 be emitted. The angle range θ is the viewing angle, below the secondary electrons, which is at the target focal spot position 55 are emitted, the charge collecting element 60 see.

The focal spot position changes 52 along an excellent direction 80 to a changed focal spot position 52 ' , so also results in a changed angle range θ ' that of free line of sight, 101 ' . 102 ' spanned by the changed focal spot position 52 ' to the charge collecting element 60 ie to the proximal boundary edge 66 or the distal boundary edge 67 run. The changed angle range θ ' , under the secondary electrons, which at the changed focal spot position 52 ' are emitted, the charge collecting element 60 see, is smaller than the angle range θ , under the secondary electrons, the charge collecting element 60 see that at the target spot position 55 be emitted. This reduction in the size of the angular range θ when changing the focal spot position 52 on the target surface 31 along the excellent direction 80 perpendicular to the proximal boundary edge 66 and the distal boundary edge 67 also results in that per time interval the amount of charge on the charge collecting element 60 is collected is reduced. From the reduction of the charge detected per time interval can thus on the change of the angular range θ and to be deduced therefrom on the change in position of the focal spot position.

The focal spot position changes 52 however, from the desired focal spot position 55 opposite the excellent direction 80 into another changed focal spot position 52 " , so the angular range increases θ to a changed, enlarged angle range θ " that of the free line of sight 101 ' . 102 ' to the proximal boundary edge 66 and the distal boundary edge 67 is limited.

From this other changed focal spot position 52 " become more secondary electrons on the charge collecting element 60 collected. An increase in the charge detected per time interval corresponds to the increased angular range θ " , This can then also another change in position of the focal spot position 52 which of the excellent direction 80 is opposite, be determined.

Parallel to the excellent direction 80 perpendicular to the boundary edges 65 Thus, a positional deviation of the focal spot position can be determined on the target based on the measured collected charge.

It is noted at this point that the detection space angle Θ a solid angle is and θ indicates the associated angle range in the drawing plane. In this and the following figures, therefore, for reasons of simplicity, only the angular range θ or the changed angle range θ ' or changed angle range θ " drawn in the plane of the drawing. The change in this angular range is determinative of a change in the charge detected per time interval when the focal spot is in its position along the excellent direction 80 relocated.

It turns out that not the actual orientation of a charge collecting surface, ie the charge collecting element 60 , but only the position of the boundary edges 65 ie the proximal boundary edge 66 and the distal boundary edge 67 , is important to each other. Based on 2A to 2C this should be clarified. The same technical features are designated in all figures with identical reference numerals.

In 2A is schematically the situation analogous to that in 1 shown embodiment shown. That as a plate 61 trained charge collecting element 60 has a proximal edge 62 and a distal edge 63 to which the free line of sight 101 . 102 run the angle range θ limit, among the secondary electrons that at the target focal spot position 55 are emitted, the charge collecting element 60 see. Thus, the proximal edge represents 62 of the charge collecting element 60 the proximal boundary edge 66 and the distal edge 63 of the charge collecting element 60 the distal boundary edge 67 for a so-called detection area 90 which is the area through which the free line of sight passes, that of the focal spot 50 to a position on the charge collecting element 60 run. For the sake of simplicity, only the line of sight 101 . 102 shown the angle range θ in the drawing plane. In the drawing plane, parallel to the target surface 31 also runs the excellent direction 80 along which focal spot position changes are detectable. In the embodiment of the 2A falls the detection area 90 with the surface of the charge collecting element 60 together.

A change in the focal spot position perpendicular to the plane of the drawing, ie parallel to the boundary edges 65 , but does not lead, or only minimally, to a change in the Detection space angle, under which the secondary electrons, the charge collecting element 60 see. It is particularly preferred that a distance of projections 165 , a projection 166 the proximal boundary edge 66 and a projection 167 the distal boundary edge 67 , on the target surface 31 a smaller distance 168 to each other than the charge collecting element 60 perpendicular to the plane of the drawing, ie parallel to the boundary edges 66 . 67 , is extensive.

In 2A is also the focal spot diameter 51 located.

In 2 B an embodiment is shown in which the proximal boundary edge 66 through an edge 112 a panel 110 is trained. The distal boundary edge 67 is an outer edge in this embodiment 64 of the charge collecting element 60 , Also in this embodiment, there is a detection surface 90 whose size is identical to the one after 2A is. Only the size and location of this detection area 90 however, determines how much charge on the charge collection element 60 is collected per time interval. In this embodiment, the charge collecting element extends 60 parallel to the target surface 31 over the proximal boundary edge 66 out.

In 2C Finally, an embodiment is shown in which, in addition to the proximal boundary edge 66 passing through the edge 112 the aperture 110 is formed, including the distal boundary edge 67 through an edge 122 another aperture 120 is trained. A size and location of the detection area 90 is identical to the detection surfaces also in this embodiment 90 the other embodiments according to 2A and 2 B , Also in this embodiment, the charge collecting element extends 60 parallel to the target surface 31 over the proximal boundary edge 66 and the distal boundary edge 67 out.

It is again noted at this point that a change in the detection space angle Θ by the free lines of sight or their directions, among the secondary electrons from the focal spot to the charge collecting element 60 can be spanned, essentially by changing the angle range θ in the plane perpendicular to the boundary edges 65 , ie here in the drawing plane, is determined.

In the illustrated embodiments of the 2A to 2C is the charge collector 60 vertically above the desired focal spot position 55 , This means that a surface normal 35 the target surface the charge collecting element 60 cuts. The surface normal 35 and the detection area 90 close a vertex angle α one that is chosen as low as possible. Preferably, the apex angle is α in an angular range between 1 ° and 10 °, more preferably between 3 ° and 7 °.

The line of sight 101 to the proximal boundary edge 66 closes with the detection area 90 a shallow angle β one.

A sensible minimum vertex angle α depends on the one hand on the maximum expected size of the focal spot, ie the maximum expected focal spot diameter, a manufacturing accuracy and a distance 72 the proximal boundary edge 66 from the target surface 31 from.

oder anders ausgedrückt $$\text{OSR}=\text{L}/\Delta \text{L}\text{.}$$

In 3 is graphically a signal-to-signal ratio or signal-to-noise ratio, hereafter referred to as OSR, in a graph 200 against apex angle α between the surface normal 35 and the detection area 90 applied. The signal-to-signal ratio OSR is defined as the ratio of the focal spot position 52 , For example, the target focal spot position 55 , Detect charge L (x) per time interval divided by the difference ΔL of the charge per unit time detected by a change in position Δx of the focal spot position 52 is caused, ie $$\text{OSR}=\text{L}\left(\text{x}\right)/\left(\text{(L}\left(\text{x}\right)-\text{L}\left(\text{x}+\Delta \text{x}\right)\right)$$

or in other words $$\text{OSR}=\text{L}/\Delta \text{L}\text{,}$$

Deviations are considered where ΔL> 0. For ΔL → 0, OSR → 0, d. H. for minimum deviations of the detected amount of charge per time ΔL caused by focal position changes, the signal-to-signal ratio OSR tends to zero.

It should be detected as small changes of a large signal. The smaller the offset signal ratio OSR is, the more accurately deviations of the focal spot position can be detected.

On the abscissa are the different vertex angles α and plotted on the ordinate, the signal-to-signal ratio OSR. From the illustrated functional connection of the count 200 it turns out that the vertex angle α should be chosen as small as possible. Nevertheless, the apex angle should be α be chosen so that secondary electrons emitted from the focal spot, also in the embodiment according to 2A not on the back of the charge collecting element 60 reach. This restriction does not apply if on the back of the cargo collection element 60 is a second charge collection element for which the collected charge is evaluated separately per time interval. However, this requires twice the cost of charge measurement and evaluation to the focal spot position along the excellent (predetermined) direction 80 on the target 30 to be able to determine.

In 4 For each of three different focal spot diameters, a functional relationship of the signal-to-signal-to-noise ratio OSR to changes in the focal spot position is schematically plotted. Shown are a functional context in a count 151 for a focal spot diameter of 1 mm, a functional relationship in a graph 152 for a focal spot diameter of 100 μm and a functional relationship in a graph 153 for a focal spot diameter of 10 μm. From the counts 151 to 153 It can be seen that even smaller deviations can be detected for smaller focal spot diameter than for larger focal spot diameter. Again, the smaller the signal-to-signal ratio OSR, the easier and more accurate a focal position detection is possible.

A measurement of the charge on the charge collecting element 60 can be deposited in different ways by means of a measuring device 270 be executed. On the one hand, it is possible to measure the voltage across a large resistor, via which the charge is derived from the charge collecting element. Alternatively, one can directly amplify the current signal which arises during the discharge. Yet another embodiment provides that charge collection element such as a capacitor 260 to hold on a voltage which is discharged by the deposited charge. This embodiment is in 1 indicated.

The charge measurement can be done by means of the measuring device 270 over longer time intervals, since a change of the focal spot position also usually takes place only over longer periods. Therefore, even small changes in the amount of charge collected or deposited per time interval can be reliably detected. Preferably, the deposited charge or charge current necessary to obtain a given potential is sampled and integrated at short intervals. If a charge change per time interval, so to speak, a change in the secondary electron current, by means of an evaluation 280 determined, it may change the position of the focal spot position 52 be determined. In a simple embodiment is used as a position signal 290 just output that the focal spot position has changed. In a preferred embodiment, in addition, the direction of the position change as a position signal 290 specified. In yet another improved embodiment, the distance, or the quantitative size of the change in position is additionally indicated. The position signal 290 can be from a downstream controller 300 used to electron optical devices 310 to control which the primary electron beam 20 affect and so the focal spot 50 back to the desired focal spot position 55 relocate.

In order to be able to take into account fluctuations in the secondary electron emission, which are not caused by a change in the focal spot position, the evaluation device 280 a signal input 281 on, over which a normalization signal 315 can be detected. The normalization signal 315 is for example a current measuring device 320 provided, one on the target 30 current and measures the normalization signal 315 generated.

In 5 is schematically an embodiment of an X-ray tube measuring head 11 an x-ray tube 10 partially shown in a sectional view. The X-ray tube probe 11 is from a metal block 12 manufactured. In these are a first recess 13 and a second recess 14 , for example both in the form of a bore, whose axes are at a shallow angle to a surface of a target 30 meet that yet another functional recess 18 fills. Through the first recess 13 hits the primary electron beam 20 in the focal spot 50 on the target 30 , The x-ray radiation 40 occurs over the second recess 14 through an x-ray window 15 from the X-ray tube probe 11 out, that the second recess 14 and is used to measure a test object 45 used.

Vertical or nearly vertical above the target surface 31 of the target 30 is another recess 16 , For example, in the form of a hole or milling, formed. An outer surface 19 of the X-ray tube probe 11 is opposite to an axis 25 the recess 16 and opposite the target surface 31 bent. Preferably, the outer surface 19 in the place where this from the recess 16 is penetrated, compared to the surface normal 35 of the target 30 a vertex angle α on that for a cargo-collecting element 60 suitable is. A degree 26 the recess 16 is here as charge collecting element 60 trained and opposite the metal block 12 isolated attached to this and closes the recess 16 , Is a suitable inclination of the outer surface 19 the X-ray tube probe 11 in the region of the recess 16 opposite the surface normal 35 the target surface 31 not producible, so can the detection area 90 , which is crucial for the charge measurement, via an optional aperture 110 or two optional irises 110 . 120 in the region of the recess 16 be determined.

In a further embodiment, in 6 is shown, the charge collecting element as part of the X-ray window 15 educated.

In addition to the measurement and control of the focal spot position 52 along an excellent spatial direction 80 which is in each case in the illustrated plane of the drawing in the embodiments shown so far in the figures, it is also possible to additionally use a further charge collecting element, which is also angled relative to the target surface, in a spatial direction transverse to this excellent direction 80 , in particular perpendicular to the excellent direction 80 , Position change of the focal spot position 52 to investigate. Analogously, in such an embodiment, there is another detection surface which is defined by a further proximal boundary edge and a further distal boundary edge with respect to an angular region φ is limited in a plane perpendicular to the plane of the previous figures. The arrangement of the further charge collecting element also perpendicular to the desired focal spot position is not possible. The arrangement of the further boundary edges perpendicular to the boundary edges 65 However, if possible, should be done so that the free line of sight from the focal spot position 52 to the charge collecting element 60 passing through the detection area 90 run and the other free lines of sight to the further charge collecting element, which extend through the further detection surface, each disjoint to each other even given predetermined deviations of the focal spot position of the target focal spot position.

The invention is characterized in that in particular movements of the focal spot in the direction of enlargement, i. the direction along which movements on the target surface cause a change in magnification in a CT scan, and if necessary corrected. As an excellent direction, this enlargement direction is selected.

Movements of the focal spot in a direction perpendicular to the direction of enlargement lead to "blurring" of the CT image, which has the same effect on all imaged edges of a test object and thus causes a loss of resolution. This can be additionally improved if the displacement of the focal spot on the target transversely to the enlargement direction is monitored via a second charge collecting element.

LIST OF REFERENCE NUMBERS

1

contraption

10

X-ray tube

11

X-ray tube probe

12

metal block

13

first recess

14

second recess

15

X window

16

another recess

18

Funktionsausnehmung

19

outer surface

20

primary electron beam

21

impingement

25

axis

26

graduation

30

target

31

target surface

35

surface normal

40

X-rays

45

UUT

50

focal spot

51

Focal spot diameter

52

Focal spot position

52 '

changed focal spot position

52 "

other changed focal spot position

55

Target focal spot position

60

Charge collecting element

61

plate

62

proximal edge

63

distal edge

64

outer edge

65

boundary edges

66

proximal boundary edge

67

distal boundary edge

72

distance

80

excellent direction

90

detection area

101, 102

free sightlines

101 ', 102'

(changed) free sightlines

101 ", 102"

(other changed) free line of sight

110

cover

112

edge

120

cover

122

edge

151

Count (1mm)

152

Graf (100 μm)

153

Graf (10 μm)

165

projections

166

Projections of the proximal boundary edge

167

Projection of the distal boundary edge

168

distance

200

earl

260

capacitor

270

measuring device

280

evaluation

281

signal input

290

position signal

300

control device

310

Electron-optical device

315

normalization signal

320

Current measurement device

α

Angle between detection surface and a surface normal of the target surface

β

Angle between detection surface, and a free line of sight to the proximal boundary edge

Θ

Detection solid angle

θ

of the free lines of sight in a plane spanned angle range

Claims (9)

Apparatus (1) for controlling a position of a focal spot (50) on a target (30) in an x-ray tube (10) in a surveillance area about a target focal spot position (55), comprising: at least one charge collection element (60) for collecting a portion of secondary electrons generated in the focal spot (50) and emitted into the hemisphere above a target surface of the target (30), the at least one charge collection element (60) being spaced from the target (30) in the x-ray tube (10) is arranged and is electrically insulated from other elements of the x-ray tube (10), wherein the at least one charge collection element (60) is arranged relative to the target surface (31) of the target (30) so that in each case a detection space angle (8) that is independent of visual lines (101, 102; 101 ', 102', 101 ", 102"). ) of the secondary electrons is less than the solid angle of the hemisphere over the target surface (31) in the target focal spot position (55), wherein free line of sight (101, 102, 101 ', 102', 101 ", 102") are those straight lines which are straight from one of the secondary electron formation positions in the focal spot (50) to one of the positions on the at least one charge collecting element (60); along which one of the secondary electrons is free to move from the target (30) to the at least one charge collection element (60); and a measuring device (270), which is connected to the at least one charge collecting element (60), for measuring the charge of secondary electrons collected per time interval on the at least one charge collecting element (60) an evaluation device (280) connected to the measuring device (270), which device is suitable for converting a change in the charge detected per time interval into a change in position and for providing a position signal (290). Device (1) according to Claim 1 characterized in that a detection surface (90) is formed having a proximal boundary edge (66) and a distal boundary edge (67), the proximal boundary edge (66) and the distal boundary edge (67) being parallel to each other and to the target surface (31) are oriented and formed by diaphragm edges (112, 122) and / or edges of other elements of the x-ray tube (10) and / or edges (62, 63) of the at least one charge collecting element (60) and the detection space angle (Θ) of the free Limiting line of sight (101, 102) from the focal spot (50) at the target focal spot position to the at least one charge collection element (60), the proximal boundary edge (66) being closer to the target surface (31) than the distal boundary edge (67) proximal boundary edge (66) and the distal boundary edge (67) are oriented perpendicular to the salient direction (80) on the target surface (31). Device (1) according to Claim 2 , characterized in that the detection surface (90) with a free line of sight of a Desired focal spot position (55) of the focal spot (50) to the proximal boundary edge (66) oriented perpendicular to the proximal boundary edge (66) includes an obtuse angle (β). Device (1) according to one of Claims 2 or 3 characterized in that the detection surface (90) includes an angle (α) with a surface normal of the target surface (31), which is preferably selected from the angular range between 1 ° and 10 °, more preferably from the angular range between 3 ° and 7 ° , Device (1) according to one of the preceding claims, characterized in that the at least one charge collecting element (60) is formed as an electrically conductive plate (61). Device (1) according to one of the preceding claims, characterized in that the charge collecting element (60) as a closure element (26) or part of a closure element (26) of a recess (16) in an X-ray tube measuring head (11) is formed. Device (1) according to one of the preceding claims, characterized in that the evaluation device (280) is adapted to detect a normalization signal (315), which is correlated with a total number of generated secondary electrons, and the normalization signal (315) in the determination of a change to consider the charge detected per time interval. Device (1) according to one of the preceding claims, characterized in that at least one further charge collecting element is disposed above the target surface (31) electrically isolated in the x-ray tube (10) and a further detection surface is formed, which has a further proximal boundary edge and a further distal Having boundary edge, which are oriented perpendicular to another excellent direction, wherein the further proximal boundary edge and the further distal boundary edge in each case parallel to each other and to the target surface (31) are oriented and by further diaphragm edges or other edges of other elements of the X-ray tube (10) or Edges of the at least one further charge collecting element are formed and limit a further detection space angle of further free lines of sight from the focal spot (50) at the desired focal spot position to the at least one further charge collecting element, wherein the further proximal Boundary edge has a smaller distance from the target surface (31) than the further distal boundary edge, wherein the further charge collecting element is connected to the measuring device (270) or another measuring device for measuring the charge accumulated per time interval on the further charge collecting element charge of secondary electrons and with the measuring device (270) or the further measuring device connected evaluation device (280) is adapted to convert a change in the detected charge per time interval on the further charge collecting element in a position change of the focal spot (50) along the other excellent direction on the target surface (31) and another position signal to provide the further excellent direction oriented transversely to the excellent direction. A method of controlling a focal spot position (52) on a target (30) in an x-ray tube (10) in a surveillance area about a target focal spot position (55), comprising the steps of: Collecting secondary electrons on at least one charge collection element (60) which form a portion of the secondary electrons generated in the focal spot (50) on the target (30) and emitted into a hemisphere over a target surface (31) of the target (30), the at least one Charge accumulation element (60) relative to the target surface (31) of the target (30) is arranged so that in each case a detection space angle (8) spanned by free lines of sight (101,102; 101 ', 102', 101 ", 102") of the secondary electrons is less than the solid angle of the hemisphere over the target surface (31) at the target focal spot position (55), with free line of sight (101, 102, 101 ', 102', 101 ", 102") being straight lines from one of the origin positions of the secondary electrons in the focal spot (50) to one of the positions on the at least one charge collecting element (60) are rectilinear and along which one of the secondary electrons freely from the target (30 ) can move to the at least one charge collecting element (60), iteratively measuring the charge accumulated per time interval on the at least one charge collection element (60); Determining whether a change in the amount of charge accumulated per time interval has occurred, and if so, determining a positional deviation of the focal spot position (52) corresponding to a change in the detection space angle (Θ) of the at least one charge accumulation element (60); Changing the detection space angle (Θ) in turn corresponds to the determined change in the charge detected per time interval, and Outputting a position signal (290) comprising position information.

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