X-ray target

Abstract: The x-ray target assembly (22) comprises an x-ray target member (24) having an opening (27) disposed therethrough and a support member (28) having: a mounting surface (30) disposed transversely to the axis (32) of rotation, a lower surface (29) of the target member (24) being disposed on the mounting surface (30); and, a threaded stem (53) extending from said mounting surface (30) through the opening (27). The support member (28) also comprises a nut (54) receiving the threaded stem (53) and engaging an upper surface (26) of the target member (24). Brazing material (36) is disposed at the threaded interface between the nut (54) and the threaded stem (53) of the support member (28).

Description: This invention relates to a process for mounting a rotatable X-ray target member to a support member as described in the first part of the indepentent claim. As is known in the art, x-ray tubes have a wide variety of applications and constructions. In one type of x-ray tube, a high-energy electron beam is emitted by a heated-filament cathode and focused onto an x-ray emissive region of an anode. The x-ray emissive region comprises a material, such as a tungsten-rhenium alloy, which emits x-rays in response to the incidence of such focused electron beam thereon. Hence, the anode is commonly referred to as the x-ray target. The incidence of the high-energy electron beam on the x-ray target generates a large amount of heat in the target which is deleterious to the target structure and hence to the operation of the x-ray tube. Hence, in conventional x-ray tubes of this type, the target anode comprises a member, such as a disk, which is mounted on a supporting shaft and rotated by a motor. The disk and supporting shaft are typically made of a high-melting-temperature metal alloy, such as titanium-zirconium-molybdenum (TZM). The electron beam is focused onto a focal track, an annular portion of a surface of the disk comprising the tungsten-rhenium x-ray emissive material. The motor rotates the target disk at high speeds, for example 10,000 RPM, thereby rotating portions of the focal track into and out of the path of the focused electron beam. The electron beam therefore is incident on only a portion of the x-ray emissive focal track at a time, allowing the remainder of the focal track to cool during the time taken to rotate such portions back into the path of the focused electron beam. In conventional x-ray tubes, the supporting shaft comprises a flanged mounting member with a threaded stem projecting therefrom. The stem fits through an opening disposed in the center of the target disk, such disk engaging a surface of the flanged mounted member. The TZM x-ray target disk is secured to the TZM supporting shaft by clamping the disk against the flanged mounting member with a TZM nut threaded onto the stem. The target disk is then dynamically balanced on the shaft, for example by shifting weights on the underside of the disk or removing small amounts of material from the underside of the disk. While the above-described securing arrangement has performed satisfactory in some applications, in other applications the target disk has been found to slip on the supporting shaft due to the high speed rotation of the disk, thereby causing the target disk to become unbalanced on the shaft. Consequently, the unbalanced target disk vibrates during rotation, such vibration often becoming so pronounced as to destroy the x-ray tube. In another conventional x-ray tube, the stem of the supporting shaft is unthreaded and is made from material other than TZM, such as niobium (also referred to as columbium). During manufacture, the stem and target disk are heated, thereby diffusion bonding the target to the stem and hence to the supporting shaft. While such mounting method functions satisfactorily in some applications, a stem made from material such as niobium is weaker than a stem fabricated from a refractory metal alloy, such as TZM, especially in the high temperatures commonly encountered during x-ray tube operation. Hence, at such high temperatures the diffusion bond between the stem and target disk is relatively weak, often resulting in the above-described slippage of the target on the supporting shaft, with accompanying target imbalance and vibration. Additionally, it is noted that the stress put on the diffusion bond between the stem and the target disk is tension or compression stress, which is the most destructive type of mechanical stress for a diffusion bond. Further, through extended use the size of the opening in the target disk through which the niobium stem fits expands slightly. Such expansion puts additional tension stress on the diffusion bond between the niobium stem and the target disk. NL-A-8501406 (corresponding to US-A-4574388) discloses a process for bonding a rotatable x-ray target member to a support member which can be rotated by a motor, the process comprising the steps of placing the x-ray target member on a mounting surface of the support member and securing the target member to the support member using brazing material disposed on a surface of the support member. In this prior arrangement, the stem and the target member bore have smooth cylindrical surfaces and an integral collar also with smooth cylindrical surfaces is provided therebetween, all of which are secured by way of diffusion bonding which is accomplished in a conventional manner. The present invention improves on this arrangement according to the characterising clause of claim 1. The brazing material may comprise alloys of palladium, nickel or cobalt. The target member and support member may comprise a refractory metal or alloy thereof, such as titanium-zirconium-molybdenum (TZM). An example of the invention will now be described with reference to the accompanying drawings in which: FIG. 1 is a partially sectioned view and schematic diagram of an x-ray apparatus incorporating the x-ray target assembly of the present invention fabricated by the process of the present invention; and FIG. 2 is an enlarged, cross-sectional view of the x-ray target assembly of FIG. 1. Referring now to FIG. 1, there is shown therein an x-ray generating apparatus 10 including an x-ray tube 12 electrically connected as shown to an adjustable filament supply unit 14, and adjustable bias unit 16, and an adjustable high voltage supply unit 18. X-ray tube 12 comprises cathode assembly 20 and rotatable x-ray target assembly 22 of the present invention, the details of which will be described in detail hereinafter. Suffice it here to say that rotatable x-ray target assembly 22 comprises a target member 24, here including anode electrode 26, and a support member 28 having a mounting surface 30 disposed transversely to an axis of rotation 32 of x-ray target assembly 22, target member 24 being disposed on mounting surface 30. Target member 24 is secured on support member 28 using brazing material. In EP-B-229697 from which this application is divided, the brazing material is disposed between target member 24 and mounting surface 30, a first portion of brazing material 36 being diffused into target member 24 and a second portion of brazing material 36 being diffused into support member 28, such diffusion bonding process being performed at temperatures below the melting temperature of the brazing material, thereby preventing vapor deposition of the brazing material on the inner surface of x-ray tube envelope 38. Such process is described in detail hereinafter. With such arrangement, target member 24 is substantially prevented from slipping on support member 28 during rotation of x-ray target assembly 22, thereby preventing target member 24 from becoming imbalanced on support member 28, and thus substantially eliminating vibration of target member 24 during the operation of x-ray generating apparatus 10. X-ray tube 12 comprises a generally tubular envelope 38 which may be made of dielectric material, such as lead-free glass, for example. One end of envelope 38 is provided with a reentrant portion 40 which is peripherally sealed to one end of a metal collar 42. The other end of collar 42 is hermetically attached, in a well-known manner, to one end of a conventional anode rotor 44 which is made of electrically conductive material, such as copper, for example. A shaft 46 of rotor 44 extends externally of the envelope 38 and provides terminal means for electrically connecting the rotor 44 to a positive terminal of adjustable high voltage supply unit 18. Within the envelope 38, support member 28 comprises an electrically conductive stem portion 48 made of refractory material, such as an alloy of titanium-zirconium-molybdenum (TZM), for example, such stem 48 extending longitudinally from the internal end of rotor 44 and being in electrical communication therewith. Additionally, support member 28 includes nut 54, here also made from a refractory metal such as TZM. Support member 28 also comprises flange portion 50, here made of TZM and being integrally formed on the opposite end of stem 48 from rotor 44. Flange portion 50 terminates in mounting surface 30. Fixedly attached to support member 28 against flange mounting surface 30 of stem 48 is target member 24, comprising anode disk 26. Anode disk 26 is disposed perpendicularly to rotational axis 32 and is rotated by rotor 44 in a well-known manner. The inner end of anode disk 26 has a frusto-conical configuration for providing a sloped annular target surface or focal track 52 adjacent the outer periphery thereof. Focal track 52 is made of a material, such as tungsten, for example, or a tungsten-rhenium alloy, which readily emits x-rays when bombarded by high energy electrons incident thereon from filamentary cathode assembly 20 along path 21. However, other portions of anode disk 26 are here made of suitable electrically conductive, refractory material, such as TZM, for example, for purposes to be discussed. TZM nut 54 engages a threaded terminal portion (see FIG. 2) of TZM stem 48 and rests against the upper surface of TZM anode disk 26. Although anode disk 26 is rotatable, a portion of focal track 52 is continuously positioned in spaced, opposing relationship with cathode assembly 20 and is sloped toward a radially aligned x-ray transparent window 56 in envelope 38. Cathode assembly 20 is fixedly supported on a suitably angled end portion of a hollow arm 58 which has an opposing end portion hermetically attached to one end of an axially disposed support cylinder 60. The other end of support cylinder 60 is circumferentially sealed to a reentrant portion 62 of envelope 38, out of which hermetically extend electrical terminal lead members 64, 66 and 68, respectively. Terminal lead members 64, 68 are electrically connected to respective output terminals of bias unit 16 and filament supply unit 14. Another output terminal of bias unit 16 is connected electrically to terminal lead member 66 in common with an output terminal of filament supply unit 14 and the negative output terminal of high voltage unit 18. Within envelope 38, terminal lead members 64, 66 and 68 extend through the hollow arm 58 and into the cathode assembly 20. Leads 66, 68 supply an AC signal, riding on the high negative voltage output of high voltage suply unit 18 (for example -60 kV-DC with respect to ground) across the ends of cathode filament 70 in cathode assembly 20. Filament 70 is heated by such AC signal and responds by emitting electrons therefrom. High voltage supply unit 18 also supplies a high positive voltage with respect to ground (for example, +60 kV-DC) to shaft 46. Such positive potential is coupled to anode disk 26 by electrically conductive shaft 46, rotor 44 and stem 48. Thus, a high potential difference (for example, 120 kV-DC exists between anode disk 26 and filamentary cathode 70, drawing the electrons emitted by filament 70 toward anode disk 26. Filament 70 is insulatingly mounted in an electrically conductive body 72 of cathode assembly 20. Lead 64 couples a suitable DC voltage, for example negative with respect to the -60 kV-DC signal applied on lead 66, to "bias" body 72 in a well-known manner in order to focus the electrons emitted by filament 70 in an electron beam along path 21, such beam being incident on focal track 52 of anode disk 26, which emits x-rays through window 56 in response to such electron bombardment. During operation, rotor 44, which is the rotor component of an induction motor (not shown), rotates target member 24 at a predetermined speed, for example 10,000 RPM, about rotational axis 32. Thus, the focused electron beam strikes continually different portions of annular focal track 52 as such focal track revolves about axis 32, thereby allowing regions of focal track 52 which have been bombarded by the high-energy electron beam time to cool before the electron beam is again incident on such regions. Without such rotation, the electron beam would quickly burn through tungsten-rhenium focal track 52, resulting in cessation of x-ray emission from tube 12. Due to the high rotational speed attained by target assembly 22, such assembly must be precisely balanced to prevent vibration which, in addition to varying the regions on focal track 52 on which the electron beam is incident and thereby destabilizing x-ray emission from tube 12, will destroy x-ray tube 12 if the vibration is severe. Thus, target member 24 is secured onto stem 48 of support member 28, and small amounts of material are removed, such as by grinding, until assembly 22 is balanced when rotating. However, if target member 24 later "slips", that is, changes position on support member 28, target assembly 22 will be imbalanced and will vibrate during rotation. Such often occurs in prior art x-ray tubes in which the target member is secured onto the stem using only a nut, such as nut 54. FIG. 2 is a cross-sectional view of target member 24 and support member 28, enlarged for clarity. Support member 28, here made from TZM, comprises stem 48 having flange portion 50 formed thereon. Flange 50 includes a generally circular mounting surface 30 on which target member 24 is supported, as will be described. Neck portion 51 of support member stem 48 extends beyond flange 50 and terminates in threaded end portion 53. Thus, support member 28 is adapted to receive and support target member 24, here comprising anode disk 26 made from TZM and having a central bore 27 disposed therein. Neck portion 51 is selected to have a diameter slightly smaller than that of central bore 27. Anode disk 26 fits over stem end portion 53 and snugly around neck 51, the bottom surface 29 thereof resting on mounting surface 30. Anode disk 26 is set onto support member 28 and is centered thereon about rotational axis 32 by the tight fit between neck 51 and central bore 27. Threaded end portion 53 of support member 28 is adapted to receive nut 54, here made from TZM. Brazing material is disposed on the threads of nut 54 or end portion 53, and hence at the interface between TZM nut 54 and TZM end portion 53, taking care to ensure that the brazing material is covered by such TZM members and not -left exposed (thereby avoiding metallic deposition of such brazing material on the sides of x-ray tube 38). Brazing material 36 comprises relatively low melting temperature metal, such as an alloy of palladium-cobalt, Pd-Co, (melting temperature, approximately 1238°C) or palladium-nickel, Pd-Ni, (1230°C melting temperature). It is noted that the melting temperature of TZM is relatively high, approximately 2600°C. Nut 54 is tightened on end portion 53 and engages the upper surface of anode disk 26, exerting downward pressure on anode disk 26 against flange 50 mounting surface 30. Such mechanical force compresses the relatively soft braze material between its relatively hard defining surfaces. After TZM nut 54 has been securely tightened as above described, target member 24 is heated. Here, such target heating is accomplished by applying electrical power, in the form of an electron beam, to focal track 52 of anode disk 26. Alternatively, anode disk 26 may be heated inductively by exposing disk 26 to a high-frequency electromagnetic field. Preferably, target assembly 22-is not rotated during application of electrical power to anode disk 26, thereby minimizing any chance of target member 24 slipping on support member 28. Therefore, the electron beam applied to focal track 52 has relatively low power in order to avoid damaging focal track 52. The heat generated in anode disk 26 dissipates through disk 26 by thermal radiation and by conduction down stem 48 of support member 28. However, flange 50 and nut 54 are not heated to the same extent as anode disk 26, since the former experience a rise in temperature only due to conduction from disk 26, while heat is generated directly within anode disk 26 due to the electron beam or the inductive heating applied thereto. Thus, target member 26 undergoes mechanical expansion in response to such heat to a greater degree than do either cooler stem flange 50 or nut 54. Thus, flange 50 and nut 54 restrict the amount of mechanical expansion of target member 24, creating very high mechanical forces at the interfaces between which the brazing material is disposed. Thus, a very strong mechanical bond is formed substantially preventing target member 24 from slipping on support member 28 during the high rotation rates of target assembly 22 experienced during the operation of x-ray apparatus 10. Reference is made to the specification of EP-B-229697 for a discussion of the diffusion of brazing material into the surrounding components which keeps vapour pressures low and hence avoids deposition of brazing materials on the surfaces of the x-ray tube 12. Brazing materials other than Pd-Co or Pd-Ni may be used. Also, suitable materials other than TZM may be used as the material for support member 28, nut 54 and target member 24. Other shapes for target member 24 may also be employed.

Claims: A process for mounting a rotatable x-ray target member (24) to a support member (28) which can be rotated by a motor, the process comprising the steps of placing the x-ray target member on a mounting surface (30) of the support member and securing the target member to the support member using brazing material, characterised in that the support member is provided with a threaded end portion (53), the brazing material being disposed at an interface between such threaded end portion and a nut (54) adapted to receive such threaded end portion, the process further comprising the steps of fastening the nut on the threaded end portion and tightening the nut against the x-ray target member (24) to secure the x-ray target member against the mounting surface (30), the heating of the x-ray target member causing the brazing material to diffuse into the nut and the threaded end portion.

Inventors: KOLLER THOMAS J, TAVOLETTI STEVEN, KOLLER,THOMAS J., TAVOLETTI, STEVEN
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1987
Main classification: H01J35
IPC codes: H01J35, B23K20, B23K15, C23C26