EP0349388A1 - X-ray tube with self-limitation of the electron flux by saturation - Google Patents

X-ray tube with self-limitation of the electron flux by saturation Download PDF

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Publication number
EP0349388A1
EP0349388A1 EP89401762A EP89401762A EP0349388A1 EP 0349388 A1 EP0349388 A1 EP 0349388A1 EP 89401762 A EP89401762 A EP 89401762A EP 89401762 A EP89401762 A EP 89401762A EP 0349388 A1 EP0349388 A1 EP 0349388A1
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EP
European Patent Office
Prior art keywords
cathode
tube according
anode
tube
focusing
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Granted
Application number
EP89401762A
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German (de)
French (fr)
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EP0349388B1 (en
Inventor
Guillaume Fournier
François Caire
Gilles Lemestreallan
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General Electric CGR SA
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General Electric CGR SA
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
    • H01J35/06—Cathodes
    • H01J35/064—Details of the emitter, e.g. material or structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
    • H01J35/06—Cathodes
    • H01J35/066—Details of electron optical components, e.g. cathode cups
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/14—Arrangements for concentrating, focusing, or directing the cathode ray
    • H01J35/147—Spot size control

Definitions

  • the present invention relates to an X-ray tube with self-limitation of the electronic flow by saturation, usable in particular in the medical field.
  • the main characteristics of these tubes are their resistance to the drift of their emission characteristics as a function of their temperature as well as the homogeneity of the illumination X produced by all the points of their focus.
  • the invention aims to improve such tubes by avoiding their possible destruction under the effect of excessive heating of their anode.
  • X-rays are produced by electron bombardment, in a vacuum enclosure, of a target made from a material with a high atomic number.
  • the electrons necessary for the bombardment of this target are released by thermoelectronic effect, generally in a helical tungsten filament, from a cathode placed precisely within a room of concentration.
  • the concentration piece plays a focusing role at the same time as a Wehnelt role.
  • the target is formed by the anode of the tube.
  • the initial velocities of the electrons at the level of the emitter are very dispersed. Their trajectories therefore have a disordered structure and the focusing system is responsible for rectifying them.
  • the focusing system is generally not efficient enough. Consequently, instead of the impact on the target of the bombardment electrons, a fairly complicated tangle of trajectories is obtained. This gives the fireplace x-ray thermal energy profile rather unfavorable with good image quality.
  • the modification of the load of the tube is not without effects on the energy distribution of the hearth.
  • the modification of this load of the tube one can reach energy densities in certain places of the anode which are beyond the acceptable thermal densities for this anode. In this case the anode can be destroyed.
  • the phenomena of expansion and compression of the useful surfaces of the thermal focus are essentially linked to the importance of the space charge conveyed by the electrons before striking the target. Still it is necessary to link the importance of this charge of space to the high pull-out voltage of the electrons of the cathode.
  • the object of the present invention is to remedy this drawback by proposing a flat emitting device which also offers mechanical rigidity which makes it possible to overcome the problems of corrugated sheet mentioned above.
  • the solution of the problems of limitation of the thermal density along the hearth as a function of the load of the tube can then be provided by the installation of such a planar cathode in a focusing part known as on. It was discovered that there was then a self-regulation of the characteristics of this home. One can then in particular ensure that the quotient of the electronic flow by the surface of the hearth is maintained within limits bearable from the thermal point of view by the target.
  • the advantage of the solution thus presented is that it applies over a wide range of high voltage between the anode and the cathode so that the same tube can be used for several applications.
  • the subject of the invention is therefore an X-ray tube provided with a cathode and an anode, opposite the cathode, for emitting X-radiation, characterized in that - the cathode is a flat cathode, - placed at the base of a walking focusing device.
  • FIG. 1 schematically shows an X-ray tube according to the invention.
  • This X-ray tube comprises, in a vacuum enclosure not shown, a cathode 1 located opposite an anode 2.
  • the anode receives electronic radiation 3 on its hearth 4 and re-emits X-radiation 5 in particular in the direction of a use window 6.
  • the use window is part of the envelope of the tube.
  • the cathode has the particularity of opposing a planar face 7 opposite the anode 2. It also has the particularity of being inserted into a focusing optic 8 known as on.
  • This walking focusing optic is to create a distribution of the electric field between the anode and the cathode such that the radiation 3 of the electrons is of the convergent type.
  • the focusing device 8 can also be a single step, it has been found here more advantageous to make it double step.
  • the focusing part 8 has a prismatic shape, FIG. 1 of which represents the right section plane.
  • the part 8 comprises the two steps, respectively 9 and 10 distributed symmetrically in 9 ′ and 10 ′ on either side of the cathode 1.
  • Each step has a step 91 or 101 and a riser 92 or 102. (respectively 91 ′ 92 ′ 101 ′ 102 ′).
  • the plane 7 of the cathode 1 is distant from the anode 2 by a distance of approximately 7.5 mm.
  • the tops 91 and 91 ′ of steps 9 and 9 ′ are spaced about 6.5 mm from the anode.
  • the tops 101 and 101 ′ are spaced about 6 mm from the plane of the anode 2.
  • the width of the cathode 1, measured in the plane of cross section of the focal prismatic part 8, is equal to 2 mm.
  • the width of a housing 11 where this cathode is placed inside the focal piece 8 is 2.2 mm.
  • the distance between risers 92 and 92 ′ is approximately 3.65 mm while the distance between risers 101 and 102 ′ is approximately 4.65 mm.
  • the device has a symmetrical appearance with respect to a plane passing through the axis 12 of the radiation, perpendicular to the plane of the figure.
  • the assembly can be circular, the axis 12 serving as an axis of revolution for the cathode as well as for the focusing part.
  • the anode 2 is a rotating anode and even that it has an inclined face on the axis 12. In this case the distances indicated are rather the distances measured on this axis 12 between the plane 7 of the cathode and the trace of the axis 12 on the anode 2.
  • the thermal flux FT is then self-limited, for a given high operating voltage, as a function of the load of the tube D.
  • the diagram in FIG. 2 presents three curves respectively 20 to 22 configured by high voltages respectively of 20 KV, 40 KV, or 50 KV, displaying in a range of use located between 150 milliamperes and 350 milliamperes, a limited pace.
  • the FT heat flux is expressed in KW per mm2. In the example shown, it is always less than 50 KW per mm2, even for the highest operating high voltage.
  • the increase in the dose rate causes the convergence point 19 to move towards the anode 2.
  • this cross-type radiation l spacing 17 18 of the lateral rays of the X-ray beam before the point of convergence 19 causes the dimension 16 of the focal point to shrink.
  • this narrowing which could be disastrous, is in fact self-limited by a phenomenon of saturation of the emission of the electrons torn from the upper face 7 of the cathode 1.
  • the space charge which naturally tends to increase with the charge of the tube (there are more electrons) increases to such an extent that it constitutes in certain conditions a screen for the emission of the following electrons. In a way, this charge of space acts like a grid.
  • this phenomenon could be used as a self-regulation, provided that a particular focusing optic is chosen.
  • This focusing optic is that described above: it includes the steps with the given dimensions. The phenomenon still occurs if we deviate from these values. This phenomenon has the advantage of occurring regardless of the high operating voltage of the tube. Understandably, this saturation phenomenon causes a saturated heat flux over the hearth, the value of which depends on this high voltage. Indeed, if the high voltage is low, the electrons are relatively less accelerated, the charge of saturation space is felt more quickly: the congestion of saturation is caused all the more easily as the electrons go slower. It is also interesting to note that the curves 20 to 22 showing the different effects on the heat flow of this saturation phenomenon are, when approaching saturation, substantially vertical.
  • the cathode 1 has the appearance of a beam shown in perspective in FIG. 3.
  • This beam is prismatic, hollow, and has substantially the appearance of a house.
  • the base of the house constitutes the emissive face 7 of the cathode, the walls of the house such as the wall 23 have windows such as 24.
  • the advantage of manufacturing a hollow beam lies in the reduction of the quantity of metal to heat. As this quantity is lower, the thermal inertia of the cathode is less, the starting of the tube can be faster. Furthermore, the consumption of the cathode heating supply can be reduced, which is an advantage when we know the insulation problems which must be faced with the heating circuits of such cathodes.
  • heating filament 25 for example of the same type as the heating filament used in the state of technology as a transmitter.
  • This filament 25 is itself negatively polarized (several thousand volts) relative to the cathode 1.
  • the beam cathode is made of tungsten.
  • the ceiling 26 and the interior of the walls are provided from this of a fiber mat 27 to concentrate the heating on the emissive part of the cathode.
  • the fibers are ceramic fibers which allow good insulation of the internal walls of the house. The electrons emitted by the heating filament then bombard the rear of the cathode according to a drawing represented by the electric field curves 28. This bombardment is limited to the front wall. Furthermore, this front wall has a concave profile.
  • this concave profile is even so concave that the wings respectively 29 and 30 of this cathode have internal faces, respectively 31 and 32, closer to the filament 25 than is the internal face of the cathode at the place 33 from its middle.
  • the wings which are both thicker and which would be harder to heat are however more heated so that the active face of the beam is brought at all points to a substantially constant temperature so as to emit with a substantially constant flow the expected electron radiation.
  • the beam according to the invention now has the advantage that its emissive face 7 no longer distorts under the effects of heating, it nevertheless undergoes expansions which should be guided without upsetting them.
  • the cathode is fixed by a single tab 34 constituting in a way the chimney of the house.
  • the method of attachment is preferably obtained by blocking this tab 34 between two screws 35 and 36 which come to grip it between them respectively.
  • This mounting at a fixing point has the advantage of leaving the cathode all the desired degrees of freedom. It is in particular preferable to a method of fixing with two points which would have the drawback that the reactions between these two points would inevitably have repercussions on the flatness of the emissive surface 7.
  • the walls of this cathode are held in the focal room 8 by pins of ceramics such as 37 and 38 which come to rest on both sides on it. This makes it possible to avoid any phenomenon of bending or vibration harmful to an exact positioning of the transmitter in the focusing part.
  • the pins allow the transmitter to thermally expand along its greatest length while keeping it laterally in its reference position.
  • the electrical supply of the cathode can be obtained by passing the high voltage through the screws 35 or 36.
  • the focal piece 8 can be electrically decoupled from the beam.

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  • X-Ray Techniques (AREA)

Abstract

The problems are resolved of temperature holding of an anode by making a flat cathode (1) engraved in a device (8) for running focusing. It is shown that, depending on the shape of this device, the heat flow at the anode is limited by a saturation value lower than a thermal holding limit for this tube. In order to improve the thermal holding of the cathode, a hollow beam-shaped cathode is made. This ensures it a rigidity inherent in its beam-shape without however enclosing it with the disadvantages of too large a thermal inertia. <IMAGE>

Description

La présente invention a pour objet un tube à rayons X à auto-limitation du flux électronique par saturation, utilisable notamment dans le domaine médical. Les principales caractéristiques de ces tubes sont leur résistance à la dérive de leurs caractéristiques d'émission en fonction de leur température ainsi que l'homogénéité de l'illumination X produite par tous les points de leur foyer. L'invention vise à perfectionner de tels tubes en évitant leur éventuelle destruction sous l'effet d'un échauffement trop important de leur anode.The present invention relates to an X-ray tube with self-limitation of the electronic flow by saturation, usable in particular in the medical field. The main characteristics of these tubes are their resistance to the drift of their emission characteristics as a function of their temperature as well as the homogeneity of the illumination X produced by all the points of their focus. The invention aims to improve such tubes by avoiding their possible destruction under the effect of excessive heating of their anode.

D'une façon générale des rayons X sont produits par le bombardement électronique, dans une enceinte à vide, d'une cible élaborée dans un matériau à haut numéro atomique. Les électrons nécessaires au bombardement de cette cible sont libérés par effet thermoélectronique, généralement dans un filament hélicoïdal de tungstène, d'une cathode placée avec précision au sein d'une pièce de concentration. La pièce de concentration joue un rôle focalisateur en même temps qu'un rôle de Wehnelt. La cible est constituée par l'anode du tube. Dans ce type de configuration très classique, les vitesses initiales des électrons au niveau de l'émetteur sont très dispersées. Leurs trajectoires présentent donc une structure désordonnée et le système de focalisation est chargé de les rectifier. Mais le système de focalisation n'est généralement pas suffisamment performant. En conséquence au lieu de l'impact sur la cible des électrons de bombardement, on obtient un enchevêtrement assez compliqué des trajectoires. Ceci confère au foyer thermique des rayons X un profil énergétique assez peu favorable avec une bonne qualité d'image.In general, X-rays are produced by electron bombardment, in a vacuum enclosure, of a target made from a material with a high atomic number. The electrons necessary for the bombardment of this target are released by thermoelectronic effect, generally in a helical tungsten filament, from a cathode placed precisely within a room of concentration. The concentration piece plays a focusing role at the same time as a Wehnelt role. The target is formed by the anode of the tube. In this very conventional type of configuration, the initial velocities of the electrons at the level of the emitter are very dispersed. Their trajectories therefore have a disordered structure and the focusing system is responsible for rectifying them. However, the focusing system is generally not efficient enough. Consequently, instead of the impact on the target of the bombardment electrons, a fairly complicated tangle of trajectories is obtained. This gives the fireplace x-ray thermal energy profile rather unfavorable with good image quality.

Dans des développements récents, par exemple dans ceux décrits dans la demande de brevet européen n° 85 106753.8 déposée le 31 mai 1985 on fait référence à une cathode qui n'est plus constituée par un filament mais constituée par une portion d'un ruban présentant, à l'émission des électrons, une surface plane en face de l'anode. L'intérêt d'utiliser un émetteur d'électrons plan a déjà été présenté antérieurement à cette demande. Il consiste à maintenir une certaine cohésion des charges électroniques au cours de leur trajectoire vers la cible. En effet, l'expérience a montré qu'on obtient dans ce cas une répartition de potentiel électrostatique favorable à une meilleure focalisation des charges électriques. Le foyer X ainsi obtenu présente alors un profil énergétique pratiquement homogène, ce qui est bénéfique à la qualité de l'image. La littérature scientifique relate certaines expérimentations basées sur ce principe général. On y fait toujours usage d'émetteur élaboré sous la forme de ruban de tungstène. Mais ces rubans présentent des problèmes de tenue thermomécanique. C'est d'ailleurs pour résoudre de tels problèmes que la demande de brevet européen ci-dessus évoquée a été déposée. En particulier, malgré tous les soins portés au laminage des rubans, des phénomènes de contraintes différentielles se produisent dans ceux-ci. Ces rubans prennent alors du fait des échauffements et des refroidissements successifs dans le tube une allure dite en tôle ondulée. Les avantages de disposer d'un émetteur plan sont alors perdus.In recent developments, for example in those described in European patent application No. 85 106753.8 filed May 31, 1985, reference is made to a cathode which is no longer formed by a filament but formed by a portion of a ribbon having , on emission of electrons, a flat surface opposite the anode. The advantage of using a planar electron emitter has already been presented prior to this request. It consists in maintaining a certain cohesion of the electronic charges during their trajectory towards the target. In fact, experience has shown that an electrostatic potential distribution is obtained in this case favorable to better focusing of the electric charges. The focal point X thus obtained then has a practically homogeneous energy profile, which is beneficial to the quality of the image. The scientific literature relates certain experiments based on this general principle. There is always use of an emitter developed in the form of a tungsten ribbon. However, these tapes present thermomechanical behavior problems. It is moreover to resolve such problems that the European patent application mentioned above was filed. In particular, despite all the care taken in rolling the ribbons, phenomena of differential stresses occur in them. These ribbons then take on, due to the successive heating and cooling in the tube, a so-called corrugated sheet appearance. The advantages of having a plan transmitter are then lost.

En plus de ces défauts, les émetteurs plans ou même les émetteurs filaments présentent l'inconvénient que l'allure du profil énergétique du foyer varie d'une manière non maîtrisée avec la charge du tube. La charge du tube correspond au débit de rayonnement X. Ce débit est lié à l'importance de l'effet thermoélectronique dans la cathode, à la température de cette cathode. Or de plus en plus d'appareils de radiologie sont munis de circuits de régulation pour réguler la charge du tube. Cette régulation tient compte du coefficient d'absorption radiologique d'un patient donné à examiner de façon à ce que le rayonnement qui traverse ce patient soit minimum. Cette régulation agit bien entendu sur le circuit de chauffage de la cathode. La technique de régulation tendant à faire agir cette régulation sur la haute tension entre anode et cathode a été abandonnée car cette technique conduit à modifier pendant l'examen la dureté du rayonnement X utilisé.In addition to these faults, flat emitters or even filament emitters have the disadvantage that the shape of the energy profile of the hearth varies from one uncontrolled manner with the load of the tube. The charge of the tube corresponds to the X-ray flow rate. This flow rate is linked to the importance of the thermoelectronic effect in the cathode, at the temperature of this cathode. However, more and more radiology devices are provided with regulation circuits to regulate the load of the tube. This regulation takes into account the radiological absorption coefficient of a given patient to be examined so that the radiation passing through this patient is minimum. This regulation of course acts on the heating circuit of the cathode. The regulation technique tending to make this regulation act on the high voltage between anode and cathode has been abandoned because this technique leads to modifying during the examination the hardness of the X-ray used.

Mais la modification de la charge du tube n'est pas sans effets sur la distribution énergétique du foyer. En particulier dans certaines situations, compte tenu de la modification de cette charge du tube, on peut atteindre des densités énergétiques en certains endroits de l'anode qui se situent au delà des densités thermiques acceptables pour cette anode. Dans ce cas l'anode peut être détruite. Les phénomènes de dilatation et de compression des surfaces utiles du foyer thermique sont essentiellement liés à l'importance de la charge d'espace véhiculée par les électrons avant d'aller frapper la cible. Encore faut-il lier l'importance de cette charge d'espace à la haute tension d'arrachement des électrons de la cathode.But the modification of the load of the tube is not without effects on the energy distribution of the hearth. In particular in certain situations, taking into account the modification of this load of the tube, one can reach energy densities in certain places of the anode which are beyond the acceptable thermal densities for this anode. In this case the anode can be destroyed. The phenomena of expansion and compression of the useful surfaces of the thermal focus are essentially linked to the importance of the space charge conveyed by the electrons before striking the target. Still it is necessary to link the importance of this charge of space to the high pull-out voltage of the electrons of the cathode.

Il pourrait être envisageable de modifier la fonction de la pièce de focalisation en fonction de la charge d'espace de manière à limiter par exemple les effets destructeurs d'une augmentation brutale trop importante de la densité thermique du foyer. Indépendamment de la complexité d'un tel asservissement, dans l'état actuel non envisageable, il faudrait en plus que cet asservissement puisse anticiper avec rapidité les dérives thermiques et la densité thermique du foyer. Cette solution n'est actuellement pas possible.It could be conceivable to modify the function of the focusing part as a function of the space charge so as to limit, for example, the destructive effects of a sudden excessive increase in the thermal density of the hearth. Regardless of the complexity of such a control, in the present unthinkable state, it would also be necessary for this control to be able to anticipate quickly the thermal drifts and the thermal density of the hearth. This solution is currently not possible.

En conséquence, dans l'état actuel de la technique, la régulation apportée sur la charge du tube retentit automatiquement en une variation de l'illumination X, et donc sur la qualité des images résultantes. En définitive, le caractère hétéroclite des effets combinés de la charge d'espace et de la haute tension ( de la charge du tube), ne permet pas de disposer de tubes dont certaines caractéristiques au moins d'émission seraient maîtrisées quelle que soit la charge.Consequently, in the current state of the art, the regulation brought about on the load of the tube automatically sounds in a variation of the illumination X, and therefore on the quality of the resulting images. Ultimately, the heterogeneous nature of the combined effects of space charge and high voltage (of tube charge), does not allow the availability of tubes whose at least certain emission characteristics would be controlled regardless of the charge .

La présente invention a pour objet de remédier à cet inconvénient en proposant un dispositif émetteur plan offrant par ailleurs une rigidité méchanique permettant de s'affranchir des problèmes de tôle ondulée évoqués ci-dessus. La solution des problèmes de limitation de la densité thermique le long du foyer en fonction de la charge du tube peut alors être apportée par l'installation d'une telle cathode plane dans une pièce de focalisation dite à marche. On a découvert en effet qu'il y avait alors une autorégulation des caractéristiques de ce foyer. On peut alors en particulier assurer que le quotient du débit électronique par la surface du foyer est maintenu dans des limites supportables du point de vue thermique par la cible. L'intérêt de la solution ainsi présentée est qu'elle s'applique sur une large gamme de haute tension entre l'anode et la cathode de telle façon qu'un même tube peut servir à plusieurs applications.The object of the present invention is to remedy this drawback by proposing a flat emitting device which also offers mechanical rigidity which makes it possible to overcome the problems of corrugated sheet mentioned above. The solution of the problems of limitation of the thermal density along the hearth as a function of the load of the tube can then be provided by the installation of such a planar cathode in a focusing part known as on. It was discovered that there was then a self-regulation of the characteristics of this home. One can then in particular ensure that the quotient of the electronic flow by the surface of the hearth is maintained within limits bearable from the thermal point of view by the target. The advantage of the solution thus presented is that it applies over a wide range of high voltage between the anode and the cathode so that the same tube can be used for several applications.

L'invention a donc pour objet un tube radiogène muni d'une cathode et d'une anode, en regard de la cathode, pour émettre un rayonnement X, caractérisé en ce que
- la cathode est une cathode plane,
- placée à la base d'un dispositif de focalisation à marche.The subject of the invention is therefore an X-ray tube provided with a cathode and an anode, opposite the cathode, for emitting X-radiation, characterized in that
- the cathode is a flat cathode,
- placed at the base of a walking focusing device.

L'invention sera mieux comprise à la lecture de la description qui suit et à l'examen des figures qui l'accompagnent. Celles-ci ne sont données qu'à titre indicatif et nullement limitatif de l'invention. Les figures montrent :

  • - figure 1 : une coupe schématique d'un tube radiogène selon l'invention;
  • - figure 2 : un diagramme énergétique pour le tube de la figure 1 ;
  • - figure 3 : Une vue en perspective d'un exemple d'une cathode rigide utilisée dans l'invention,
  • - figure 4 : une vue en coupe de la cathode de la figure 3.
The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These are given for information only and in no way limit the invention. The figures show:
  • - Figure 1: a schematic section of an X-ray tube according to the invention;
  • - Figure 2: an energy diagram for the tube of Figure 1;
  • - Figure 3: A perspective view of an example of a rigid cathode used in the invention,
  • - Figure 4: a sectional view of the cathode of Figure 3.

La figure 1 montre schématiquement un tube radiogène selon l'invention. Ce tube radiogène comporte, dans une enceinte à vide non représentée, une cathode 1 située en vis à vis d'une anode 2. L'anode reçoit un rayonnement électronique 3 sur son foyer 4 et réémet un rayonnement X 5 notamment en direction d'une fenêtre d'utilisation 6. La fenêtre d'utilisation fait partie de l'enveloppe du tube. Selon l'invention la cathode présente la particularité d'opposer une face plane 7 en vis à vis de l'anode 2. Elle présente en outre la particularité d'être insérée dans une optique de focalisation 8 dite à marche. Cette optique de focalisation à marche a pour objet de créer une répartition du champ électrique entre l'anode et la cathode telle que le rayonnement 3 des électrons soit du type convergent. On distingue deux types de rayonnement convergent. Dans un premier type, représenté sur la figure 1, le point de convergence des électrons est situé devant le plan de l'anode : il est réel. Dans ce cas, le rayonnement est dit croisé. Dans un deuxième type de rayonnement, dit direct, le point de convergence des électrons se situe derrière l'anode 2 : il est virtuel.Figure 1 schematically shows an X-ray tube according to the invention. This X-ray tube comprises, in a vacuum enclosure not shown, a cathode 1 located opposite an anode 2. The anode receives electronic radiation 3 on its hearth 4 and re-emits X-radiation 5 in particular in the direction of a use window 6. The use window is part of the envelope of the tube. According to the invention, the cathode has the particularity of opposing a planar face 7 opposite the anode 2. It also has the particularity of being inserted into a focusing optic 8 known as on. The purpose of this walking focusing optic is to create a distribution of the electric field between the anode and the cathode such that the radiation 3 of the electrons is of the convergent type. There are two types of radiation converge. In a first type, represented in FIG. 1, the point of convergence of the electrons is located in front of the plane of the anode: it is real. In this case, the radiation is said to be crossed. In a second type of radiation, called direct, the point of convergence of the electrons is located behind the anode 2: it is virtual.

Bien que le dispositif de focalisation 8 puisse être également à simple marche, on a trouvé ici plus avantageux de le réaliser à double marche. La pièce de focalisation 8 a une forme prismatique dont la figure 1 représente le plan de section droit. La pièce 8 comporte les deux marches, respectivement 9 et 10 réparties symétriquement en 9′ et 10′ de part et d'autre de la cathode 1. Chaque marche comporte un dessus de marche 91 ou 101 et une contremarche 92 ou 102. (respectivement 91′ 92′ 101′ 102′). Dans un exemple préféré de réalisation le plan 7 de la cathode 1 est distant de l'anode 2 d'une distance d'environ 7.5 mm. Les dessus 91 et 91′ des marches 9 et 9′ sont distants de l'anode d'environ 6,5 mm. Les dessus, 101 et 101′ sont distants eux d'environ 6 mm du plan de l'anode 2. La largeur de la cathode 1, mesurée dans le plan de section droite de la pièce prismatique focale 8, vaut 2 mm. La largeur d'un logement 11 où est placée cette cathode à l'intérieur de la pièce focale 8 vaut 2.2 mm. La distance qui sépare les contremarches 92 et 92′ est d'environ 3,65 mm tandis que la distance qui sépare les contremarches 101 et 102′ est d'environ 4,65 mm. On peut considérer que les contremarches sont ainsi accolées à des cylindres parallelépipèdiques (pris au sens théorique du terme) de largeur respectives 4 mm et 5mm. De préférence le dispositif a une allure symétrique par rapport à un plan passant par l'axe 12 du rayonnement, perpendiculairement au plan de la figure. En variante cependant, plutôt que d'être prismatique, l'ensemble peut être circulaire, l'axe 12 servant d'axe de révolution à la cathode ainsi qu'à la pièce de focalisation. Il est possible que l'anode 2 soit une anode tournante et même qu'elle présente une face inclinée sur l'axe 12. Dans ce cas les distances indiquées sont plutôt les distances mesurées sur cet axe 12 entre le plan 7 de la cathode et la trace de l'axe 12 sur l'anode 2.Although the focusing device 8 can also be a single step, it has been found here more advantageous to make it double step. The focusing part 8 has a prismatic shape, FIG. 1 of which represents the right section plane. The part 8 comprises the two steps, respectively 9 and 10 distributed symmetrically in 9 ′ and 10 ′ on either side of the cathode 1. Each step has a step 91 or 101 and a riser 92 or 102. (respectively 91 ′ 92 ′ 101 ′ 102 ′). In a preferred embodiment, the plane 7 of the cathode 1 is distant from the anode 2 by a distance of approximately 7.5 mm. The tops 91 and 91 ′ of steps 9 and 9 ′ are spaced about 6.5 mm from the anode. The tops 101 and 101 ′ are spaced about 6 mm from the plane of the anode 2. The width of the cathode 1, measured in the plane of cross section of the focal prismatic part 8, is equal to 2 mm. The width of a housing 11 where this cathode is placed inside the focal piece 8 is 2.2 mm. The distance between risers 92 and 92 ′ is approximately 3.65 mm while the distance between risers 101 and 102 ′ is approximately 4.65 mm. We can consider that the risers are thus joined to parallelepiped cylinders (taken in the theoretical sense of the term) of width 4 mm and 5 mm respectively. Preferably, the device has a symmetrical appearance with respect to a plane passing through the axis 12 of the radiation, perpendicular to the plane of the figure. In a variant, however, rather than being prismatic, the assembly can be circular, the axis 12 serving as an axis of revolution for the cathode as well as for the focusing part. It is possible that the anode 2 is a rotating anode and even that it has an inclined face on the axis 12. In this case the distances indicated are rather the distances measured on this axis 12 between the plane 7 of the cathode and the trace of the axis 12 on the anode 2.

Les dimensions données ci-dessus présentent l'avantage que le flux thermique FT est alors autolimitée, pour une haute tension d'utilisation donnée, en fonction de la charge du tube D. En effet, le diagramme de la figure 2 présente trois courbes respectivement 20 à 22 paramétrées par des hautes tensions respectivement de 20 KV, 40 KV, ou 50 KV, affichant dans une plage d'utilisation située entre 150 milliampères et 350 milliampères, une allure bornée. Le flux thermique FT est exprimé en KW par mm². Dans l'exemple indiqué il est toujours inférieur à 50 KW par mm², même pour la haute tension d'utilisation la plus forte.The dimensions given above have the advantage that the thermal flux FT is then self-limited, for a given high operating voltage, as a function of the load of the tube D. In fact, the diagram in FIG. 2 presents three curves respectively 20 to 22 configured by high voltages respectively of 20 KV, 40 KV, or 50 KV, displaying in a range of use located between 150 milliamperes and 350 milliamperes, a limited pace. The FT heat flux is expressed in KW per mm². In the example shown, it is always less than 50 KW per mm², even for the highest operating high voltage.

Dans l'invention, où le rayonnement 3 est convergent et converge en un point de convergence 19, l'augmentation du débit de dose provoque le déplacement en direction de l'anode 2 du point de convergence 19. Dans ce rayonnement de type croisé l'écartement 17 18 des rayons latéraux du faisceau de rayonnement X avant le point de convergence 19 provoque le rétrécissement de la dimension 16, du foyer. On a découvert dans l'invention que ce rétrécissement qui pouvait être désastreux, est en fait autolimité par un phénomène de saturation de l'émission des électrons arrachés de la face supérieure 7 de la cathode 1. En effet, du fait de la concentration, la charge d'espace, qui a naturellement tendance à augmenter avec la charge du tube (il y a plus d'électrons) augmente à un point tel qu'elle constitue dans certaines conditions un écran pour l'émission des électrons suivants. En quelque sorte cette charge d'espace agit comme une grille. On a en particulier découvert dans l'invention que ce phénomène pouvait être utilisé comme une auto-régulation, à condition de choisir une optique de focalisation particulière.In the invention, where the radiation 3 is convergent and converges at a point of convergence 19, the increase in the dose rate causes the convergence point 19 to move towards the anode 2. In this cross-type radiation l spacing 17 18 of the lateral rays of the X-ray beam before the point of convergence 19 causes the dimension 16 of the focal point to shrink. It has been discovered in the invention that this narrowing which could be disastrous, is in fact self-limited by a phenomenon of saturation of the emission of the electrons torn from the upper face 7 of the cathode 1. In fact, due to the concentration, the space charge, which naturally tends to increase with the charge of the tube (there are more electrons) increases to such an extent that it constitutes in certain conditions a screen for the emission of the following electrons. In a way, this charge of space acts like a grid. In particular, it was discovered in the invention that this phenomenon could be used as a self-regulation, provided that a particular focusing optic is chosen.

Cette optique de focalisation est celle décrite ci dessus : elle comporte les marches avec les dimensions données. Le phénomène se produit encore si on s'écarte de ces valeurs. Ce phénomène présente l'avantage de se produire quelle que soit la haute tension d'utilisation du tube. D'une manière compréhensible, ce phénomène de saturation provoque un flux thermique à saturation sur le foyer dont la valeur dépend de cette haute tension. En effet, si la haute tension est faible, les électrons sont relativement moins accélérés, la charge d'espace de saturation se fait plus rapidement sentir : l'embouteillage de saturation se provoque d'autant plus facilement que les électrons vont moins vite. Il est par ailleurs intéressant de remarquer que les courbes 20 à 22 montrant les différents effets sur le flux thermique de ce phénomène de saturation sont, à l'approche de la saturation, sensiblement verticales. Ceci signifie, que dans ce cas les dimensions du foyer du tube sont sensiblement constantes, et que donc les images vont être acquises selon un même protocole, quelle que soit par ailleurs la charge imposée au tube par son système de régulation. L'avantage procuré par l'invention est représenté par le fait qu'au moment de la saturation le débit ne peut plus augmenter, mais surtout le flux thermique ne le peut plus non plus. En choisissant correctement les matériaux d'anode et de cathode ou les conditions d'utilisation des tubes de telle façon que le point de saturation ne soit pas situé hors des tolérances de fonctionnement on obtient alors ainsi le résultat recherché.This focusing optic is that described above: it includes the steps with the given dimensions. The phenomenon still occurs if we deviate from these values. This phenomenon has the advantage of occurring regardless of the high operating voltage of the tube. Understandably, this saturation phenomenon causes a saturated heat flux over the hearth, the value of which depends on this high voltage. Indeed, if the high voltage is low, the electrons are relatively less accelerated, the charge of saturation space is felt more quickly: the congestion of saturation is caused all the more easily as the electrons go slower. It is also interesting to note that the curves 20 to 22 showing the different effects on the heat flow of this saturation phenomenon are, when approaching saturation, substantially vertical. This means that in this case the dimensions of the focal point of the tube are substantially constant, and that therefore the images will be acquired according to the same protocol, whatever the load imposed on the tube by its regulation system. The advantage provided by the invention is represented by the fact that at the time of saturation the flow can no longer increase, but above all the heat flow cannot either. By correctly choosing the anode and cathode materials or the conditions of use of the tubes so that the saturation point is not located outside of the operating tolerances, the desired result is thus obtained.

Dans un exemple préféré, la cathode 1 a l'allure d'une poutre représentée en perspective sur la figure 3. Cette poutre est prismatique, creuse, et a sensiblement l'allure d'une maison. La base de la maison constitue la face 7 émissive de la cathode, les murs de la maison tels que le mur 23 possèdent des fenêtres telles que 24. L'intérêt de fabriquer une poutre creuse se situe dans la réduction de la quantité de métal à chauffer. Comme cette quantité est plus faible, l'inertie thermique de la cathode est moins grande, le démarrage du tube peut être plus rapide. Par ailleurs la consommation de l'alimentation de chauffage de la cathode peut être réduite ce qui est un avantage quand on sait les problèmes d'isolement auxquels doivent être confrontés les circuits de chauffage de telles cathodes.In a preferred example, the cathode 1 has the appearance of a beam shown in perspective in FIG. 3. This beam is prismatic, hollow, and has substantially the appearance of a house. The base of the house constitutes the emissive face 7 of the cathode, the walls of the house such as the wall 23 have windows such as 24. The advantage of manufacturing a hollow beam lies in the reduction of the quantity of metal to heat. As this quantity is lower, the thermal inertia of the cathode is less, the starting of the tube can be faster. Furthermore, the consumption of the cathode heating supply can be reduced, which is an advantage when we know the insulation problems which must be faced with the heating circuits of such cathodes.

Bien qu'on puisse envisager un chauffage direct de cette cathode en faisant passer un courant électrique directement au travers de celle-ci, on préfère utiliser un filament de chauffage 25 par exemple du même type que le filament de chauffage utilisé dans l'état de la technique comme émetteur. Ce filament 25 est lui même polarisé négativment (plusiers milliers de volts) par rapport à la cathode 1.Although it is possible to envisage direct heating of this cathode by passing an electric current directly through it, it is preferred to use a heating filament 25 for example of the same type as the heating filament used in the state of technology as a transmitter. This filament 25 is itself negatively polarized (several thousand volts) relative to the cathode 1.

Dans un exemple préféré la cathode en poutre est réalisée en tungstène. Afin de limiter également la quantité d'énergie thermique à fournir pour chauffer la cathode on munit le plafond 26 et l'intérieur des murs de celle-ci d'un matelas 27 de fibres pour concentrer le chauffage sur la partie émissive de la cathode. Dans un exemple les fibres sont des fibres de céramique qui permettent un bon isolement des parois internes de la maison. Les électrons émis par le filament chauffant bombardent alors l'arrière de la cathode selon un dessin représenté par les courbes de champ électrique 28. Ce bombardement est limité à la paroi avant. Par ailleurs cette paroi avant présente un profil concave. Dans un exemple préféré ce profil concave est même tellement concave que des ailes respectivement 29 et 30 de cette cathode présentent des faces intérieures, respectivement 31 et 32, plus proches du filament 25 que ne l'est la face intérieure de la cathode à l 'endroit 33 de son milieu. De cette manière les ailes qui sont à la fois plus épaisses et qui seraient plus dures à chauffer sont cependant plus chauffées de manière à ce que la face active de la poutre soit portée en tous points à une température sensiblement constante de manière à émettre avec un débit sensiblement constant le rayonnement d'électrons attendu.In a preferred example, the beam cathode is made of tungsten. In order to also limit the amount of thermal energy to be supplied to heat the cathode, the ceiling 26 and the interior of the walls are provided from this of a fiber mat 27 to concentrate the heating on the emissive part of the cathode. In one example, the fibers are ceramic fibers which allow good insulation of the internal walls of the house. The electrons emitted by the heating filament then bombard the rear of the cathode according to a drawing represented by the electric field curves 28. This bombardment is limited to the front wall. Furthermore, this front wall has a concave profile. In a preferred example, this concave profile is even so concave that the wings respectively 29 and 30 of this cathode have internal faces, respectively 31 and 32, closer to the filament 25 than is the internal face of the cathode at the place 33 from its middle. In this way the wings which are both thicker and which would be harder to heat are however more heated so that the active face of the beam is brought at all points to a substantially constant temperature so as to emit with a substantially constant flow the expected electron radiation.

Bien que la poutre selon l'invention présente maintenant l'intérêt qu sa face émissive 7 ne se distorde plus sous les effets des échauffements, elle subit cependant des dilatations qu'il convient de guider sans les contrarier. Dans ce but la cathode est fixée par une patte unique 34 constituant en quelque sorte la cheminée de la maison. La mode de fixation est de préférence obtenu par blocage de cette patte 34 entre deux vis 35 et 36 qui viennent l'enserrer entre elles respectivement. Ce montage à un point de fixation présente l'avantage de laisser à la cathode tous les degrés de liberté voulus. Il est en particulier préférable à un mode de fixation avec deux points qui présenterait l'inconvénient que les réactions entre ces deux points se répercuteraient immanquablement sur la planéïté de la surface émissive 7. Pour guider les déplacements de la cathode avec la temperature, les murs de cette cathode sont maintenus dans la pièce focale 8 par des pions de céramique tels que 37 et 38 qui viennent s'appuyer de part et d'autre sur elle. Ceci permet d'éviter tout phénomène de flexion ou de vibration néfaste à un exact positionnement de l'émetteur dans la pièce de focalisation. Les pions permettent à l'émetteur de se dilater thermiquement suivant sa plus grande longueur tout en le maintenant latéralement dans sa position de référence. En pratique, l'alimentation électrique de la cathode peut être obtenu en faisant passer la haute tension par les vis 35 ou 36. La pièce focale 8 peut être découplée électriquement de la poutre.Although the beam according to the invention now has the advantage that its emissive face 7 no longer distorts under the effects of heating, it nevertheless undergoes expansions which should be guided without upsetting them. For this purpose the cathode is fixed by a single tab 34 constituting in a way the chimney of the house. The method of attachment is preferably obtained by blocking this tab 34 between two screws 35 and 36 which come to grip it between them respectively. This mounting at a fixing point has the advantage of leaving the cathode all the desired degrees of freedom. It is in particular preferable to a method of fixing with two points which would have the drawback that the reactions between these two points would inevitably have repercussions on the flatness of the emissive surface 7. To guide the movements of the cathode with temperature, the walls of this cathode are held in the focal room 8 by pins of ceramics such as 37 and 38 which come to rest on both sides on it. This makes it possible to avoid any phenomenon of bending or vibration harmful to an exact positioning of the transmitter in the focusing part. The pins allow the transmitter to thermally expand along its greatest length while keeping it laterally in its reference position. In practice, the electrical supply of the cathode can be obtained by passing the high voltage through the screws 35 or 36. The focal piece 8 can be electrically decoupled from the beam.

Claims (12)

1 - Tube radiogène à limitation de flux thermique comportant une cathode (1) et une anode (2), pour émettre (4) un rayonnement X (3), situé en regard de la cathode, caractérisé en ce que
- la cathode est une cathode plane (7)
- placée à la base d'un dispositif (8) de focalisation à marche.1 - X-ray tube with thermal flow limitation comprising a cathode (1) and an anode (2), for emitting (4) X-ray radiation (3), located opposite the cathode, characterized in that
- the cathode is a flat cathode (7)
- placed at the base of a walking focusing device (8). 2 - Tube selon la revendication 1 caractérisé en ce que la forme du dispositif de focalisation est agencée pour que le rayonnement X soit croisé (19).2 - Tube according to claim 1 characterized in that the shape of the focusing device is arranged so that the X-ray is crossed (19). 3 - Tube selon la revendication 1 ou la revendication 2 caractérisé en ce que le dispositif de focalisation est à double marche (9,10).3 - Tube according to claim 1 or claim 2 characterized in that the focusing device is double step (9,10). 4 - Tube selon la revendication 3 caractérisé en ce que
- le plan de la cathode est éloigné d'environ 7.5 mm de l'anode,
- le dispositif de focalisation comporte
- un plan profond commun avec le plan de la cathode, et limité par un cylindre d'environ 3.65 mm de largeur,
- un plan intermédiaire (91, 91′) situé à environ 6.5 mm de l'anode, et limité par un cylindre d'environ 4.65 mm de largeur,
- et un plan supérieur (101,101′) situé à environ 6 mm de l'anode.4 - Tube according to claim 3 characterized in that
- the plane of the cathode is approximately 7.5 mm away from the anode,
- the focusing device comprises
- a common deep plane with the cathode plane, and limited by a cylinder about 3.65 mm wide,
- an intermediate plane (91, 91 ′) located about 6.5 mm from the anode, and limited by a cylinder about 4.65 mm wide,
- and an upper plane (101,101 ′) located about 6 mm from the anode. 5 - Tube radiogène selon l'une quelconque des revendications 1 à 4 caractérisé en ce que cette cathode comporte une poutre.5 - X-ray tube according to any one of claims 1 to 4 characterized in that this cathode comprises a beam. 6 - Tube selon la revendication 5 caractérisé en ce que la poutre est creuse (24).6 - Tube according to claim 5 characterized in that that the beam is hollow (24). 7 - Tube selon la revendication 6 caractérisé en ce que la cathode est chauffée par un dispositif (25) de chauffage indirect.7 - Tube according to claim 6 characterized in that the cathode is heated by a device (25) for indirect heating. 8 - Tube selon la revendication 7 caractérisé en ce que le dispositif de chauffage comporte un matelas (27) de fibres pour concentrer le chauffage sur la partie émissive de la cathode.8 - Tube according to claim 7 characterized in that the heating device comprises a mattress (27) of fibers to concentrate the heating on the emissive part of the cathode. 9 - Tube selon la revendication 7 ou la revendication 8 caractérisé en ce qu'une face interne de la cathode, opposée à la face plane, a une forme concave avec des ailes (29,30) plus proches du dispositif de chauffage qu'une partie centrale interne de cette forme concave.9 - Tube according to claim 7 or claim 8 characterized in that an internal face of the cathode, opposite the flat face, has a concave shape with wings (29,30) closer to the heating device than internal central part of this concave shape. 10 - Tube selon l'une quelconque des revendications 5 à 9, caractérisé en ce qu'au moins une des parois (23) de la poutre comporte un évidement (24).10 - Tube according to any one of claims 5 to 9, characterized in that at least one of the walls (23) of the beam has a recess (24). 11 - Tube selon l'une quelconque des revendications 5 à 10 caractérisé en ce que la poutre est fixée au tube par un seul point (34) de fixation.11 - Tube according to any one of claims 5 to 10 characterized in that the beam is fixed to the tube by a single point (34) of fixing. 12 - Tube selon l'une quelconque des revendications 5 à 11 caractérisé en ce que la poutre est guidée par des pions (37,38) de céramique fixés de part et d'autre d'elle sur un dispositif, (8) de focalisation.12 - Tube according to any one of claims 5 to 11 characterized in that the beam is guided by pins (37,38) of ceramic fixed on either side of it on a device, (8) for focusing .
EP89401762A 1988-07-01 1989-06-22 X-ray tube with self-limitation of the electron flux by saturation Expired - Lifetime EP0349388B1 (en)

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FR8808961A FR2633773B1 (en) 1988-07-01 1988-07-01 RADIOGENIC TUBE WITH SELF-LIMITING ELECTRONIC FLOW
FR8808961 1988-07-01

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FR2633773A1 (en) 1990-01-05
ES2026735T3 (en) 1992-05-01
JPH0254848A (en) 1990-02-23
JP2840615B2 (en) 1998-12-24
EP0349388B1 (en) 1991-11-13
DE68900433D1 (en) 1991-12-19
US5033072A (en) 1991-07-16
FR2633773B1 (en) 1991-02-08

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