X-ray tube comprising an exit window

Abstract: An X-ray tube includes an electron-optical system which directs an electron beam (20)which emanates, for example from a line-shaped or annular emissive element (14), onto an anode target face (10) in a collimated fashion so that a large part of reflected electrons is incident on the target face again, thus contributing to the generating of X-rays. The construction of the electron-optical system and the high degree of reincidence of scattered electrons allows for a minimum distance between the anode target face and the exit window. A filament wire (14) of the cathode may be constructed so as to achieve an optimum uniform operating temperature over a comparatively large part of its length.

Description: The invention relates to an X-ray tube, comprising a cathode, an anode and an electron-optical device which are accommodated in an envelope comprising a radiation exit window, in which the electron-optical device is adapted to generate a field which collimates an electron beam emanating from the cathode and which directs this beam onto an anode target face, via a trajectory including an inversion point. An X-ray tube of this kind is known from "Patent Abstracts of Japan", vol.10, no.87, (E-393) [2144] April 5, 1986 & JP-A-60 232 650. In the said Japanese Patent document a cathode structure is placed in the neighbourhood of the anode target face and providing an electron-optical device which is adapted to generate a potential field which directs the electron beam emanating from the cathode structure onto the anode target face. To achieve this, the X-ray tube of the kind set forth in accordance with the invention, as defined in Claim 1, is characterized in that the electron-optical device comprises an annular auxiliary electrode surrounding the anode like a ring which is situated adjacent and near the anode target face. The use of the X-ray tube, according to Claims 1-5, in an X-ray analysis apparatus is defined in Claim 6. Inter alia because of this electrode geometry, a high efficiency can be achieved in combination with a comparatively high degree of freedom as regards the distance between the anode and the window. It is remarked that also in US-P-4 017 757 there is disclosed an X-ray tube with a cathode structure in the neighbourhood of the anode target face and with an electron-optical device which directs the electron beam onto the anode target face. According to one embodiment of the invention the anode target face is situated at a minimum distance from the exit window, which distance is determined by the required breakdown strength of the electrostatic potential difference between the anode and the exit window. The filament wire of the cathode may be circular, in which case the anode member also has a circular cross-section. The cathode filament wire as well as the anode member may alternatively have a non-circular shape, for example the shape of a possibly rounded square. According to another embodiment of the invention the cathode comprises an emissive element in the form of a straight line segment, notably for the formation of a line focus on an adjacently arranged anode target face. For the formation of an elongate, or more generally speaking a non-circular focus, the cathode filament wire, and preferably also the anode member, may be constructed so as to be substantially elliptical, the elipticity being, for example 4. In order to increase the service life of the filament wire, the X-ray tube according to the invention may be provided with an emissive element in the form of a filament wire provided with supports which are clamped in a cooled portion of the cathode housing in an electrically insulated manner and which are located lengthwise along the filament at mutually equal distances so that a comparatively large part of the filament wire can be adjusted to a uniform temperature in the vicinity of an emission temperature. According to the invention the life-time of the cathode filament can still further be increased by heating the filament in an atmosphere from which applicable material is deposited on hot spots of the wire as W from WF6. Thereto the X-ray tube according to the invention comprises a cathode filament wire and is further characterized in that the uniformity of temperature of the wire is increased by depositing additive material such as W from a gas atmosphere on parts of the wire being at the highest temperature during activating. Some preferred embodiments in accordance with the invention will be described in detail hereinafter with reference to the drawing. The sole Figure of the drawing shows an X-ray tube which comprises an envelope 2 in which an exit window 4 is arranged at a first end, a socket 6 being provided at a second end. An exit window consists of, for example a beryllium plate having a thickness of, for example 125 µm. An anode member 8 is centrally arranged in the envelope 2. At its end which faces the exit window the anode member supports an anode target face 10, the anode member supporting a high-voltage connector 11 at an opposite end. Adjacent the cylindrical anode member having a round, a rectangular, an elliptical or other cross-section, there is arranged a cathode 12. In the present embodiment the cathode comprises an annular filament wire 14 which is arranged in a cathode housing 16 having an annular aperture 18, which cathode housing may also accommodate a control electrode. An electron beam 20 to be emitted by the filament wire emanates from the cathode housing via the beam aperture 18. The filament wire is connected, by way of conductors 22 and 24, to a cathode current supply source 30, via wall passages 26 and 28. The electron beam emitted by the emitter is radially collimated by the aperture 18 which acts as an electron lens. Further collimation and alignment of the beam can be achieved by means of an auxiliary electrode 32 and possibly a collar 38 in the form of a thickened or restricted portion of the anode member. Additional collimation can be imparted to the beam by way of the shape and potential of the auxiliary electrode and the shape of the collar 38, and the beam can thus be directed as a ring onto the anode target face at a desired angle. The ring is preferably chosen so that electrons which are reflected upon first incidence are for the better part incident again on the anode target face. The efficiency is thus enhanced and undesirable heating and other disturbances by reflected electrons are avoided. Fig. 1a shows an example of a filament wire 14 which serves as an electron emitter for an X-ray tube as shown in Fig. 1. Using supports 34 which are clamped in a cooled portion 36 of the cathode housing in an insulated manner, for example via pins 35, the filament is retained in a fixed position. The conductors 22 and 24 can also serve as positioning supports. To this end, these conductors may be constructed so as to be comparatively rigid on the one hand, and on the other hand they may be provided with heat shields. As a result, a substantial local cooling of the filament wire occurs along said conductors. The shape, composition and notably position, measured along the filament wire, are determined in accordance with the invention so that comparatively large segments of the filament wire, extending between supports, exhibit an optimum uniform temperature in the vicinity of the desired emission temperature during operation, notably temperature differences between the various segments being minimized. As a result, the service life of the X-ray tube can be substantially reduced for otherwise the same circumstances. In known annular filament wire cathodes burning of the filament wires more readily occurs because this process is strongly positive in a sense that a comparatively hot location assumes a higher resistance and therefore becomes hotter again, so that it evaporates more quickly and hence assumes a higher temperature again, etc. In the described embodiment the filament wire 14 has a circular shape, but it may also have another shape, for example it may be shaped as a straight line segment.

Claims: An X-ray tube, comprising a cathode (12), an electron-optical device and an anode (8) having an anode target face (10) which, are accommodated in an envelope comprising a radiation exit window, in which the electron-optical device is adapted to generate a field which collimates an electron beam emanating from the cathode and which directs this beam onto the anode target face, via a trajectory including an inversion point, characterized in that the electron-optical device comprises an annular auxiliary electrode (32) surrounding the anode (8) like a ring which is situated adjacent the anode target face such that the electron beam (20) is passing between the ring shaped annular auxiliary electrode (32) and the anode (8) and such that the angle between the anode target face (10) and the trajectory of the electron beam (20) which is incident on the anode target face (10) is at least 45 degrees. An X-ray tube as claimed in Claim 1, characterized in that the anode target face is situated at a minimum distance from the exit window, which distance is determined by the required breakdown strength of the electrostatic potential difference between the anode and the exit window. An X-ray tube as claimed in Claim 1 or 2, characterized in that the cathode comprises an emissive element in the form of a straight line segment. An X-ray tube as claimed in any one of the preceding Claims, characterized in that the emissive element is a filament wire provided with supports (34) which are clamped in a cooled portion (36) of the cathode housing in an electrically insulated manner and which are located lengthwise along the filament at mutually equal distances so that a comparatively large part of the filament wire can be adjusted to a uniform temperature in the vicinity of an emission temperature. An X-ray tube comprising a cathode filament wire as claimed in Claim 4 characterized in that the uniformity of temperature of the wire is increased by depositing additive material such as W from a gas atmosphere on parts of the wire being at the highest temperature during activating. An X-ray analysis apparatus, comprising an X-ray tube as claimed in any one of the preceding Claims in order to minimize a distance between an anode target face and a specimen.

Inventors: D ACHARD VAN ENSCHUT JOHANNES, JENNESKENS THEODORUS JAN JEANN, SWEMMERS ANTONIUS HENDRIKUS MA, D'ACHARD VAN ENSCHUT, JOHANNES FRANCISCUS MARTINUS, JENNESKENS, THEODORUS JAN JEANNETTE MARIA, SWEMMERS, ANTONIUS HENDRIKUS MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: H01J35
IPC codes: H01J35