EP0314552B1 - X-ray radiation generating set with full protection against radiation leakage - Google Patents

Description

The invention relates to an X-ray emitting device of the type in which an X-ray tube contained in a sheath is supplied with high voltage via at least one electrical connection device mounted on the sheath. The invention particularly relates to means for producing shielding for protection against leakage X-rays.

In diagnostic X-ray installations, X-rays are generally produced using an X-ray tube. According to a current arrangement, on the one hand, the X-ray tube is contained in a metal sheath filled with an electrically insulating oil, the sheath being intended in particular to ensure the protection of users against electric shocks and X-rays; the sheath equipped with an X-ray tube being called an X-ray assembly. On the other hand, the supply elements of the X-ray tube, and in particular the elements of a high-voltage generator producing the high supply voltage of the X-ray tube, are grouped in another enclosure located at a distance from the X-ray assembly. . Depending on the type of high-voltage power supply, monopolar or symmetrical, one or more cables with high electrical insulation carry the positive and negative polarities of the high voltage into the sheath containing the X-ray tube, via connection devices. electric. Such an electrical connection device is formed in two parts, one of which, called receptacle, constitutes a female part permanently fixed to the sheath; the other part constitutes a male part or end piece intended to be fitted into the receptacle and which is integral with the cable.

Given the very high values that the high supply voltage of an X-ray tube can reach, 150 KV for example, the receptacle and the end piece are made of electrically insulating materials, and have shapes and dimensions imposed by international standards. In order to protect users, the sheath is brought to ground or earth potential, as well as an outer casing of the cable (s); one of the manufacturers' constant concerns being that all the external surfaces of the sheath, of the connection device, and of the cables are metallic, and that there is electrical continuity between all of these members.

The electrical contact (s) which are made by an electrical connection device, are made in the bottom of the receptacle between contact members secured to the end piece and, second contact members secured to the receptacle and which pass through the bottom wall of the latter and thus open inside the sheath. The receptacle plunges deep enough inside the sheath, so that the end of the second contact element opens out inside the sheath at a sufficient distance from the internal wall of the latter, to avoid electrical breakdown between the metallic sheath which is grounded, and this contact element which is at the potential of the high voltage. Assuming that the polarity transported is the negative polarity, this contact is connected by a connecting wire to the cathode of the X-ray tube, and the anode of the latter is brought to positive polarity by another connection, inside the sheath, between the anode and another contact member of another electrical connection device.

However, these precautions alone are not sufficient to avoid the risk of electrical breakdowns inside the sheath, and the correct electrical insulation in the latter is only perfectly achieved if the sheath is filled with a fluid or electrically insulating oil. Under these conditions, it is practically impossible, despite the presence of the insulating oil, to place a metallic element inside the sheath near the receptacle, and particularly if this metallic element tends to decrease the length of the vanishing lines, that is to say to decrease the length of electrical insulator between the contact member and the inner wall of the sheath. This results in an important problem which relates to the protection of users from leakage X-rays.

Indeed, in an X-ray tube, X-ray radiation is produced under the effect of the bombardment of an anode by an electron beam. At the point of impact of the electron beam, a focal point is formed, which constitutes the source of the X-rays. The X-rays are emitted in all directions, and only a fraction of this X-rays, called the useful beam, is used to produce images of an object. The useful beam leaves the sheath through an exit window which absorbs very little X-ray radiation. To absorb the X-rays which are emitted in the sheath in directions different from those of the useful X-ray, the interior walls of the sheath are coated with '' a layer of lead which constitutes a shielding against X-ray radiation. Generally, the layer of lead has a thickness of approximately 3mm, which makes it possible to absorb most of the X-ray produced by the X-ray tube when it is operating under the maximum expected high voltage supply voltage, that is to say usually of the order of 150 KV.

Given the problem of electrical insulation mentioned above, the lead layer is stopped around the holes in which the electrical connection receptacles are inserted, so that these holes are not shielded and allow the emitted X-ray sheath to exit in their direction. These parasitic X-rays or leakage X-rays which exit from the sheath outside the exit window, can reach energies close to those of the useful beam, and represent a significant danger for users.

A known solution to this problem consists in placing the orifices, intended to receive the receptacles for electrical connectors, outside of areas accessible to emitted X-rays. directly from the fireplace. This solution is interesting, but it only makes it possible to limit the amount of leakage X-rays, in particular because by secondary radiation effects, X-rays can have in the sheath all directions.

Another solution to X-ray leakage in the region of the electrical connectors is proposed in French patent 542 253. Two circular metal plates covered with a thick layer 3 of anti-X metal, such as lead, are supported by the holder. bulb in front of the connectors. Again, the solution is not satisfactory because it does not completely eliminate the leakage X-rays.

The invention relates to an X-ray assembly of the type described above, the arrangement of which makes it possible to suppress all leakage radiation, in particular at the level of the electrical connection device (s), without requiring any particular positions of these electrical connection devices, and while retaining the required electrical insulation qualities.

According to the invention, an X-ray assembly comprising an X-ray tube placed in a protective sheath filled with an electrically insulating fluid and pierced at least by an orifice in which is mounted an electrical connection device with a cable with high electrical insulation used to supply the tube with high voltage, said sheath comprising a first shield against X-radiation on its internal walls and a second shield disposed at the orifice absorbing the X-rays emitted in the sheath in the direction of said orifice , and characterized in that the second shield is constituted by a bell-shaped screen fixed to the sheath around the orifice and made of an electrically insulating material charged with a material with high atomic number, intended to suppress leakage X-rays at the level of electrical connection devices.

The invention would be better understood with the aid of the description which follows, given by way of nonlimiting example, and with the aid of the appended figure which shows, schematically, in a sectional view, an X-ray unit according to the invention.

The figure shows an X-ray unit 1, the representation of which is limited to the means useful for understanding the invention. The radiogenic assembly 1 comprises a metal sheath 2, made of aluminum for example. The sheath 2 contains an X-ray tube 3 and an insulating oil 4 in which the X-ray tube is immersed 3. The X-ray tube 3 is of a conventional type comprising in particular a casing 5 in which are contained a cathode 6 and an anode 7. In the nonlimiting example described, the anode 7 is a rotating anode whose rotation about an axis of rotation 8 is obtained using a rotor 9 and a stator 10 which is outside the casing 5; the anode 7 is secured to the rotor 9 by a metal support axis 11, and the rotor 9 is carried by a metallic support shaft 12 which is also fixed to a first end 13 of the casing 5. The cathode 6 is carried in screw with respect to the anode 7 by a support 14, which is fixed to a second end 15 of the envelope 5. The cathode 6 is electrically connected in a conventional manner (not shown) to watertight electrical bushings 16,17 which is provided the second end 15 of the envelope 7. The cathode 6 is of a traditional type comprising one or more filaments (not shown), one end of which is connected to the cathode 6 itself, so that for both connecting the cathode 6 at high voltage and supply the cathode filament (s) with heating voltage, just connect cathode 6 to the negative polarity of high voltage and add a filament connection to supply. Also, assuming that the cathode has a single filament, two watertight electrical bushings 16, 17 are enough, the first bushing 16 being connected both to the cathode 6 itself and to one end of the filament and the second bushing 17 being connected to the other end of the filament.

The high supply voltage of the X-ray tube 3 is produced by a high voltage generator (not shown) which is located outside the sheath 2. Generally, the high voltage generator is placed inside an enclosure ( not shown) also containing means for supplying a first and a second low voltage used respectively for heating the filament contained in the cathode 6 and for supplying the stator 10; the negative polarity of the high voltage being connected, in this enclosure, to one of the poles of the first low voltage.

In the nonlimiting example described, the high voltage supply to the X-ray tube 3 is of the symmetrical type, that is to say that the negative and positive polarities of the high voltage are respectively negative and positive with respect to the sheath potential 2 which is connected to earth or ground.

The negative polarity -HT of the high voltage as well as the first low voltage used to heat the filament contained in the cathode 6, are transported to the interior of the sheath 2 using a first cable with high insulation electrical 20, via a first electrical connection device 21. On the other hand, the positive + HT polarity of the high voltage as well as the second low voltage used to supply the stator 10 are transported into the sheath 2 , by a second cable with high electrical insulation 22, by means of a second electrical connection device 23.

The electrical connection devices 21, 23 are of the same type, and each comprise an end piece 25, 26 and a receptacle 27, 28 made of an electrically insulating material. The first end-piece 25 which extends the first cable 20, is engaged in the first receptacle 27, and the second end-piece 26 which extends the second cable 22 is engaged in the second receptacle 28. The sheath 2 has a first and a second orifices 29, 30 in which the first and second receptacles 27,28 are inserted respectively, which are thus immersed in the sheath 2. In the nonlimiting example described where two connections are sufficient to supply the cathode 6, two wires (not shown) contained in the first cable 20 are each connected to a male contact 31, 32; these male contacts protrude from the end 33 of the first end piece 25 and are respectively engaged in a first and a second female contact 35, 36 belonging to the first receptacle 27. The female contacts 35, 36 are mounted in a bottom wall 37 of the receptacle 27, and pass through this bottom wall in a sealed manner. The first female contact 35, which carries the negative polarity -HT, is connected in the sheath 2 to the first electrical bushing 16 by a first link 37, and the second female contact 36 is connected to the second electrical bushing 17 by a second link 38; the connections of cathode 6 being thus made.

The orifices 29.30 are formed by the interior of protrusions in the form of a tube 40 in which the receptacles 27, 28 and the end pieces 25, 27 are fixed in a conventional manner, so as to maintain the tightness of the sheath 2; the elements used for these fasteners are well known, and they are not shown for clarity of the description.

The second electrical connector 23 is constituted in the same way as the first electrical connector 21: the second receptacle 28 comprises a third and a fourth female contacts 41, 42 which open into the sheath 2; a third and a fourth male contacts 43,44 belonging to the second endpiece 26, are engaged respectively in the third and fourth female contacts 41,42. The third female contact 41 carries the positive + HT polarity of the high voltage, and this third female contact 41 is connected to the support shaft 13 by a third link 48, so that the anode 7 is connected to the positive polarity + HT via the support axis 11, the rotor 9 and the support shaft 12. In the nonlimiting example of the description, the stator 10 is brought to the positive polarity of the high voltage + HT, c that is to say that one of these inputs 46 is connected to the third female contact 41 which not only transports the positive + HT polarity of the high voltage, but also transports one of the polarities of the second low voltage intended for the 'stator 10 supply; the second polarity of the second low voltage being introduced into the sheath 2, by the fourth female contact 42 which is connected to a second input 47 of the stator 10, by a fourth connection 49.

In operation, the cathode 6 produces an electron beam 51 which bombards the anode 7 and generates on the latter a focal point 52 representing the source of X-radiation which, from the focal point 52, is emitted in all directions. . A fraction of this X-ray radiation constitutes a useful beam 53 which leaves the sheath 2 through a window 54 which has little absorption of X-ray radiation.

Inside the sheath 2, X-rays (not represented are emitted in all directions, not only from the focal point 52 which constitutes the source of the primary X-ray, but also from any points inside the sheath 2 where objects can be struck by secondary electrons, or, by X-rays of the primary radiation which generate a scattered X-ray which, itself, can undergo multiple diffusions. To avoid a leakage of these X-rays through the sheath 2, it is known to coat the interior walls of the latter with a shield consisting of a layer of lead 56. To avoid any electrical breakdown between the ground to which the sheath 2, and the negative and positive polarities -HT, + HT of the high voltage, the lead layer 56 is stopped around the orifices 29.30; so that an X-ray, called leakage X-ray, can escape from the sheath 2 through these orifices 29.30.

According to a characteristic of the invention, the first X-ray shielding, constituted by the lead layer 56, is completed by a screen or second shielding which is both electrically insulating and absorbing X-ray radiation, and which is arranged at the level orifices 29.30 in order to suppress leakage X-rays at the level of the latter.

In a first version of the invention, this screen or second shield is constituted by the receptacles 27, 28 themselves. To this end, the receptacles 27, 28 are made from an electrically insulating material, such as for example an epoxy resin, which is charged with a material with a high atomic number, so as to constitute a material which is both electrically insulating. and absorbing X-rays.

These high atomic number materials can be electrically insulating materials such as, for example, oxides of tungsten, uranium, thorium or even lead such as minium.

But these materials with a high atomic number can also be electrically conductive materials such as in particular bismuth, tungsten, uranium, thorium, lead. In fact, tests have shown that an epoxy resin charged, for example, up to 50% in lead or tungsten, on the one hand retains largely sufficient electrical insulating properties with respect to the values of alternating or continuous voltages ( 150 KV) which are necessary for the operation of a conventional X-ray tube. It has also been observed that the epoxy resin thus loaded with a material with a high atomic number, and having a thickness of the order of 7 to 8 mm, absorbs X-ray radiation in substantially the same way as the layer of lead having a thickness of the order of 3mm.

This solution to the shielding of the orifices 29.30 is particularly advantageous, in that it does not compromise the electrical insulation inside the enclosure 2, and does not require any additional work during the mounting of the receptacles 27, 28 in the sheath 2. Of course, additional work is required to load the chosen base material, epoxy resin for example, with a powder of a material with a high atomic number. The method used to obtain the material which is both electrically insulating and absorbent is substantially the same as that which is used to reinforce a resin, by light metals or by glass fibers. By way of non-limiting example, it may be mentioned that in the case of an epoxy resin loaded up to 50% of a tungsten or lead powder, the particle size of this powder is of the order of 10 to 100 microns.

The receptacles 27, 28 can be obtained by molding for example, so as to have an appropriate thickness e , that is to say about 7 mm, particularly as regards the bottom wall 37; it should be noted that such a thickness e corresponds substantially to the thickness of the partitions of the receptacles produced according to the prior art.

The ends 25, 26 can also be made of an electrically insulating and absorbing material of X-ray in order to constitute the second shield, or also so as to add their mass to that of the receptacles 27, 28, this which reduces the thickness e of the latter. But the end pieces 25, 26 are integral with the cables 20, 21 which can be replaced, so it is preferable to give the receptacles 27, 28 directly the appropriate thickness e to suppress leakage radiation.

According to another version of the invention, it is also possible to cover the receptacles 27, 28, inside the sheath 2, by a screen 60, 61 in the shape of a bell and made of an electrically insulating material charged by a high atomic number material as explained above; these bell-shaped screens 60, 61 being fixed to the sheath 2, and having small openings 62 intended to allow the passage of the insulating oil 4 and electrical connections 37, 38 to the X-ray tube 3.

According to yet another version of the invention, illustrated by the second screen 61 which covers and surrounds the second receptacle 28, inside the sheath 2, the screens 60, 61 can be produced in a sandwich structure in which, a layer 70 of a material with a high atomic number such as lead for example, is coated between two layers 71, 72 of a conventional electrically insulating material.

Claims (7)

1. Radiation producing assembly comprising an x-ray tube (3) arranged in a protective sheath (2) filled with an electrically insulating fluid and having at least one orifice (29, 30) therethrough, in which orifice there is mounted an electrical connection device (21, 23) having a cable (20, 22) with high voltage electrical insulation for supplying the tube with high voltage, said sheath comprising a first shield (56) against x-rays on its inside wall surfaces and a second shield arranged in alignment with the orifice (29, 30) absorbing x-rays emitted in the sheath towards said orifice, characterized in that the second shield is constituted by a bell-shaped screen (60, 61) secured to the sheath (2) around the orifice (29, 30) and manufactured of an electrically insulating material filled with a material having a high atomic number intended to absorb stray x-rays adjacent to the electrical connection device (21, 23).
2. Radiation producing assembly in accordance with the preamble of claim 1, whose electrical connection device (21, 23) is constituted by a receptacle (27, 28) mounted in the orifice (29, 30) of the sheath (2) and by a plug (25, 26) integral with the cable (20) carrying high voltage and fitted in the receptacle (27, 28), characterized in that the second shield is constituted by the receptacle (27, 28) made of the electrically insulating material, filled with a material having a high atomic number and having a predetermined thickness (e) in order to absorb stray x-rays.
3. Radiation producing assembly in accordance with claim 1 or claim 2, whose electrical connection device (21, 23) is constituted by a receptacle (27, 28) mounted in the orifice (29, 30) of the sheath (2) and by a plug (25, 26) integral with the cable (20) and fitted in the receptacle (27, 28), characterized in that the second shield is constituted by the plug (25, 26) made of the electrically insulating material, filled with a material having a high atomic number.
4. Radiation producing assembly in accordance with claim 1, characterized in that the second shield is constituted by the bell-shaped screen (60, 61), manufactured of the electrically insulating material filled with a material having a high atomic number, encircling the orifice (29, 30) and secured in the interior of the sheath (2) and comprising small openings (62) more particularly intended to permit the passage of the insulating fluid (4).
5. Radiation producing assembly in accordance with claim 1, characterized in that the second shield is constituted by the screen (61) surrounding the receptacle (28) and secured to the interior of the sheath, embodied in the form of a sandwich structure in which a layer (70) of a material having a high atomic number is located between two covering layers (71, 72) of an electrically insulating material.
6. Radiation producing assembly in accordance with any one of the claims 1 through 5, characterized in that the material with a high atomic number is an electrically conducting material.
7. Radiation producing assembly in accordance with any one of the claims 1 through 5, characterized in that the material with a high atomic number is electrically insulating.

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