Cathode for an X-ray tube and a tube including such a cathode

The invention relates to X-ray tubes, and more particularly to cathodes for such tubes. The invention lies in the cathode, which includes an electron-emitting filament (26), being made in the form of a main body (10) of insulating material having metal electrodes (16 and 19) deposited thereon which are insulated from one another by the main body. The filament (26) and the electrodes (16 to 19) are connected to conductors (27, 21 to 23) which pass through the main body.

The invention relates to X-ray tubes, and more particularly to a cathode 
for an X-ray tube. 
An X-ray tube comprises a vacuum enclosure containing a cathode constituted 
by a heated filament which emits electrons, and a concentrator device 
behind the filament to focus the emitted electrons towards an anode which 
is raised to a positive potential relative to the cathode. The point of 
impact of the beam of electrons on the anode constitutes a source of 
X-radiation in the form of a beam. 
Development in X-ray imaging systems has lead to the use of X-ray tubes 
which are more and more complex in order to obtain the functions desired. 
It is therefore desirable to control the geometry of the electron beam 
which strikes the anode in order to obtain a point of impact of accurate 
dimensions and having an energy distribution which is as uniform as 
possible. 
In some applications, it is necessary to obtain beams of X-rays at 
different energies and/or different angles of incidence; to this end, the 
cathode may have a plurality of electron-emitting filaments from which 
both the dimensions and the directions of electron flux must be controlled 
in order to obtain accurate points of impact of the electron beam on the 
anode. 
In other applications it is necessary to modify the electron flow rate and 
possibly to switch it off completely by means of an electrode called the 
grid. 
In order to provide the functions listed above, cathodes are geometrically 
complicated in shape and may comprise a plurality of metal electrodes 
which are mechanically separated and which are electrically insulated from 
one another. 
In French patent number 2 538 948 filed Dec. 3, 1982 and entitled "Scanning 
X-Ray Tube", and also in French patent application number 89 03888 filed 
Mar. 24, 1989 and entitled "Scanning X-Ray Tube with Deflection Plates", 
the present Applicant describes the cathodes of X-ray tubes constituted by 
multiple electrodes which are difficult to manufacture and assemble and 
which are expensive. 
An object of the present invention is thus to provide an X-ray tube cathode 
having multiple electrodes or otherwise, which is simple to manufacture 
and which is of reduced cost price. 
The present invention provides an X-ray tube cathode including at least one 
electron-emitting filament, and characterized in that it comprises a main 
body made of an insulating material, metal electrodes which are disposed 
at determined locations of said main body and which are insulated from one 
another by virtue of the insulating material of the main body, and 
electrical conductors passing through said main body in order to feed said 
filament and to apply bias potentials to said metal electrodes. 
The main body is made of a ceramic such as alumina, and the electrodes are 
made of molybedum, maganese, or tungsten, or an alloy thereof.

In prior art cathodes, and in particular in those described in the 
above-mentioned French patent documents, there is always a metal 
supporting part which acts as the electrode that is taken to a negative 
potential in order to repel the electrons emitted by a heated filament, 
and that also serves to support said filament and other electrodes. These 
other electrodes are also made of metal and they need to be electrically 
isolated from said metal supporting part and from one another. 
The invention provides a cathode in which the main body is made of an 
insulating material and on which the various electrodes are formed, with 
the insulation between various electrodes being obtained by the insulating 
material of the main body. 
More precisely, the main body 10 is constituted by a block of alumina, for 
example, which is appropriately machined in order to obtain the shape 
shown in FIGS. 1 and 2, i.e. a circularly symmetrical cylinder having a 
diametricallyextending slot portion with stepped sides, one side having 
steps referenced 12 and 13 and the other side having steps referenced 14 
and 15. The electrodes are obtained by metal deposits which are disposed 
at specified locations on the surface of the main body, in particular on 
the vertical faces and on the horizontal faces of the steps 12, 13, 14, 
and 15. For example, the entire area of the horizontal face of each of 
steps 12 and 14 is covered with a respective metal deposit referenced 16 
or 18. In addition, the entire surface of the vertical face of each of 
steps 13 and 15 is covered with a metal deposit referenced 17 or 19 
depending on the step in question. 
These electrode-forming metal deposits 16 to 19 are biased to appropriate 
potentials by means of electrical conductors 20 to 23 passing through the 
main body 10 and emerging through the horizontal faces of the steps. For 
steps 12 and 14, the conductors 20 and 22 abut directly against the metal 
deposits 16 and 18 to which they are connected. For the steps 13 and 15, 
the conductors 21 and 23 are connected to the metal deposits 17 and 19 via 
metal tabs 24 and 25 which are disposed on the horizontal faces of the 
steps 13 and 15 and which are electrically connected to the metal deposits 
17 and 19. 
The, or each, cathode filament (reference 26) is disposed in conventional 
manner in the diametrically-extending slot so as to project beyond the 
level of metal deposits 16 and 18. The filament(s) is/are fed with 
electricity via conductors (referenced 27) passing through the main body 
10. 
The metal deposits 16 to 19 and the metal tabs 24 and 25 can be obtained in 
various different ways, in particular by thin film deposition on the 
alumina substrate of the main body, with said substrate being 
appropriately doped in order to allow the thin films to take hold. The 
doping substances may be metals such as molybdenum and manganese which are 
deposited as layers of liquid in the locations to be occupied by the 
electrodes, after which the assembly is heated to dope the surface layer 
of the body 10. 
The material of the metal deposits must adhere to the alumina of the main 
body 10 and must withstand high temperatures. Suitable materials include, 
for example, molybdenum, manganese, tungsten, and alloys of these 
materials together or with other metals. The deposit may be obtained by 
vacuum evaporation, by subliminating the metals, by ion bombardment, or by 
plasma torch. 
It is mentioned above that the main body 10 is made of alumina. The purity 
of the alumina needs to be about 95% to 97%, which is a commonly-available 
quality. The alumina could be replaced by a different ceramic. 
The geometrical accuracy with which the electrodes are positioned in an 
X-ray tube cathode of the present invention is determined by the accuracy 
with which the main body is machined, and this can be both very high and 
reproducible. Compared with prior art cathodes, this avoids assembly 
operations in which accuracy depends on the dexterity of humans, and for 
which reproducibility is not constant over time nor from one person to 
another. The manufacturing cost is also lower than for prior art cathodes. 
By using a one-piece main body 10, the effects of differential expansion 
between various parts are avoided, which effects are particularly 
troublesome with the separator electrodes of prior art cathodes. This 
means that deformations are very small. 
Finally, since the main body 10 is insulating, the electrical conductors 27 
for feeding the filament 26 and the conductors 21 to 23 for biasing the 
electrodes 16 to 19 pass through the said body without there being any 
covering around the conductors 21 to 23 and 27, thereby simplifying 
cathode manufacture and lowering cost price.