The invention relates to an X-ray tube, in particular an X-ray tube with rotating anode, intended for studying the molecular structure of crystalline materials according to the X-ray diffraction method. Such an X-ray tube is usually provided with a housing, an anode and a cathode which are mounted in the housing, the cathode comprising a focusing device, an incandescent wire and a ceramic carrier, which ceramic carrier supports the focusing device and the incandescent wire in the housing.

Such X-ray tubes operate with a continuous tube power of the order of magnitude of 18 kW, only a small part of which is converted into X-radiation. This means that the temperature of the anode rises to a value of 600-1000°C at the place where the electron flow from the cathode meets the anode. Moreover, a power of 180-220 W is needed to maintain the incandescent filament at a high temperature at which sufficient electrons are released to maintain the current through the tube. The temperature of the cathode and the housing conseguently rises to such an extent that cooling of the cathode and the housing is necessary. Thereby the design of the cathode part of the X-ray tube is relatively complex, and this cathode part has large dimen¬ sions. The dimensions of the cathode part constitute a restriction for the freedom of placing the detector which is used in X-ray diffraction study.

The object of the invention is to provide an X- ray tube of the type mentioned in the preamble, in which the disadvantages of the known X-ray tubes are effectively overcome.

To this end, the X-ray tube according to the invention is characterized in that the ceramic carrier is made of a ceramic material with a thermal conductivity of at least 80 W/mK. The invention is based on the insight that by deliberately using a ceramic material with advantageous characteristics as regards thermal conductivity a compact design of the ceramic carrier is possible and separate cooling of the cathode by means of oil or another complex measure is not necessary. The heat of the cathode can be discharged by way of the ceramic carrier to the housing, which is cooled in the usual way by water or the like. Owing to the compact design of the cathode, great freedom

for placing the detector for the X-ray diffraction study is obtained.

According to an advantageous embodiment of the X- ray tube according to the invention, it is possible with a simple design of the cathode to remove and insert the incandescent filament guickly without using aids, while the accuracy of the position of the incandescent filament remains guaranteed.

The ceramic carrier is preferably made of alumin- ium nitride, the thermal conductivity of which is 140- 180 W/mK.

The invention is explained in further detail below with reference to the drawing, in which an exemplary embodiment of the X-ray tube according to the invention is illustrated very diagrammatically.

Fig. 1 shows a section of an embodiment of the X- ray tube according to the invention.

Fig. 2 shows part of the X-ray tube from Fig. 1, on a larger scale. Fig. 3 shows part of the X-ray tube from Fig. 1 in section on a larger scale, the anode being illustrated in side view.

Figs. 1 and 2 show very diagrammatically an X-ray tube which is provided with a housing 1 and an anode 2 supported rotatably in the housing. The rotatable support of the anode and the seal thereof relative to the housing

1 are not part of the subject of the invention and are therefore not illustrated. A turbo-molecular pump 4, which interacts with a pump (not shown) to take care of the vacuum in the housing 1, is also fitted in the housing 1.

A cathode 6 is supported by means of a ceramic carrier 7 in a wall 5 of the housing 1. Said cathode 6 comprises a focusing device 8 and an incandescent filament

9. The focusing device 8 is made of, for example, molybdenum and by means of the ceramic carrier 7 is placed in such a way that it is isolated relative to the housing

1, made of, for example, copper.

Figs. 2 and 3 illustrate the wall 5 of the housing 1 with the cathode 6 and the ceramic carrier 7 shown on a larger scale. As can be seen in Figs. 2 and 3, channels 10 for a cooling liquid, for example water, are formed in the housing 1, for discharging heat. During operation the X-ray tube described works continuously at a

power of, for example, 18 kW, with the result that the temperature of the anode 2 can rise to very high values, of the order of 600-l000°C. This causes the temperature of the cathode 6 also to rise to high values. In the case of the X-ray tube described the heat of the cathode, in particular of the focusing device 8 is discharged directly by way of the ceramic carrier 7 to the wall 5 of the housing 1. For this purpose, the ceramic carrier 7 is made of a material with a thermal conductivity of at least 80 W/mK. The specific resistance of the ceramic material is also at least 10 ⁹ Ω. cm. The breakdown field strength must be at least 15 kV/mm. A particularly suitable material is, for example, aluminium nitride.

The focusing device 8 has a circular periphery, and in the exemplary embodiment shown is mounted in a cylindrical chamber 11 of the ceramic carrier 7. The focusing device 8 can be fixed in said chamber 12 by, for example, soldering. In the case of the embodiment shown, a shoulder 12 is formed in the carrier 7, said shoulder forming a stop for the focusing device 8. The ceramic carrier 7 is placed in the same way in the wall 5 against a shoulder 13.

At the side facing away from the anode 2 the focusing device 8 has a mounting space 14 in which a ceramic carrier plate 15 of the incandescent filament 9 is accommodated in a close fit. The carrier plate 15 and the mounting space 14 are provided with interacting positioning means, so that the carrier plate 15 can be placed only in a predetermined position in the mounting space 14. Such positioning means can also be used to ensure a predetermined position of the carrier 7 relative to the wall 5 or of the focusing device 8 relative to the carrier 7.

The incandescent filament 9 is supported in the carrier plate 15 by two contact supports 16. Each contact support 16 has a contact element 17 which interacts with a corresponding spring contact 18 which is supported in a closing plate 19. The contacts 18 hold the carrier plate 15 in the correct position in the mounting space 14 or the focusing device 8 in the carrier 7, or the carrier 7 in the wall 5.

The use of the closing plate 19 described has the advantage that the incandescent filament 9 can be replaced

in a simple manner by removing the closing plate. The seal is achieved by a suitable rubber sealing ring 20. The closing plate 19 is held in place by spring clamps (not shown) in the absence of a vacuum in the housing 1. It will be clear that when vacuum is present in the housing 1 the closing plate 19 is retained on the wall 5 in such a way that it automatically seals. The closing plate 19, like the carrier plate 14 for the incandescent filament 9, is preferably made of the same material as the ceramic carrier 7.

The closing plate also supports a spring contact 21 for connection of the focusing device 8 to a high- voltage source (not illustrated) . Like the spring contacts 18, said spring contact 21 exerts a spring force on the focusing device 8. The supply cable for the connection of the spring contacts 18, 21 is indicated diagrammatically by 22.

It is pointed out that the X-ray tube can be designed with two windows 23 for taking X-ray radiation from a point source, and with a window 24 for taking X-ray radiation from a line source. If two line sources are desired, the construction can be modified in such a way that the inlet for the cable 22 runs parallel to the axis of the anode. This means that one point source also remains available.

It will be clear from the above that the inven¬ tion provides an X-ray tube in which using a suitable ceramic material for the carrier of the cathode means that separate cooling for the cathode is no longer necessary. This also permits a particularly compact construction, and the user can place the detector used in the X-ray diffraction study in a wide radius around the X-ray tube, as indicated diagrammatically by arrows 25 in Fig. 1.

The invention is not restricted to the exemplary embodiment described above, and can be varied in different ways within the scope of the claims.