X-ray tube with a reduced working distance

Abstract: In an X-ray tube for an X-ray analysis apparatus an optimized minimum working distance is achieved, together with an adequately high radiation yield, by integrated optimization of the radiation end of the tube, the position and the mounting of the exit window (6), and the electron-optical configuration (8,10) in the tube. This results in an X-ray tube having a conical end (16), an angle of cone of approximately 45° and the use of the cone as such in the electron-optical system of the tube. Exact determination of the anode (14) position relative to a reference face outside the tube enables exact positioning of each individual tube in an analysis apparatus.

Description: The invention relates to an X-ray tube, comprising a cathode for providing a beam of electrons, an anode for providing X-rays upon impingement of the beam of electrons thereon, and an electron-optical system which serves to direct the electron beam to be emitted by the cathode onto the anode, which cathode, anode and electron-optical system are accommodated in a cylindrical envelope having a radiation exit window at one axial end for passing the X-rays, the axial window end of the tube being constructed so as to be conical with an angle of substantially 45°, the exit window being arranged at the narrow end of the axial window end. An X-ray tube of this kind is known from EP 439 852. The X-ray tube described therein comprises a conical axial end which supports a window, a cathode which is mounted adjacent an anode pipe, and an electron-optical system which includes a deflection electrode, mounted between the cathode and the anode, an aperture in a cathode housing, and the anode itself. Using the electron-optical system, an electron beam to be emitted by the cathode is directed onto the anode surface so that the electrons for generating X-rays are incident at an angle of at least approximately 45° thereon. Such an X-ray tube satisfies the demand for an X-ray source having a comparatively high radiation power in order to realise a comparatively short working distance for irradiation of an object or specimen in an X-ray analysis apparatus. For various applications, the working distance associated with the output power is still found to be excessive, so that the relevant measurements cannot be optimally performed. It is an object of the invention to comply with said demand to an even greater extent; to achieve this, the X-ray tube of the kind set forth in accordance with the invention is characterized in that the exit window is surface mounted on an axial sealing plate of a minimum, uniform material thickness, a conical portion of the axial window end surrounding the exit window forms part of the electron-optical system as an electron-optical electrode, thereby the use of an additional electrode in the cathode-anode space of the tube being dispensed with, a cooling duct which does not project beyond the 45° cone is provided around the envelope, the electron-optical elements being chosen so that operation is possible with a minimum distance between the anode and the exit window. Because relevant parts of an X-ray tube in accordance with the invention are integrally conceived to minimize the anode-object distance in an analysis apparatus, a substantial gain is thus achieved. Because the window plate, customarily made of beryllium, is surface mounted on an end sealing plate without special mounting rims or being seated into a recess, the entire sealing plate can have a uniform thickness as claimed, said thickness being chosen to achieve adequate vacuum-tightness of the tube, for example an iron- nickel or copper-nickel plate having a thickness of only approximately 1 mm. Notably because of its coefficient of expansion, copper-nickel is particularly suitable for use in conjunction with a beryllium window. Because the conical tube wall portion itself forms part of the electron-optical system, a substantial reduction of the transverse dimension of the tube can be achieved in comparison with the arrangement of the deflection electrode between the anode pipe and the cone, thus offering a substantial gain when mounted in an analysis apparatus. A cooling duct is arranged around the tube so that no part thereof projects substantially from the cone. It should be remarked that DE 27 49 856 discloses an X-ray tube comprising a cathode, an anode and an electron-optical system which serves to direct an electron beam to be emitted by the cathode onto the anode and which is accommodated in a cylindrical envelope having a radiation exit window at one axial end, the axial window end of the tube being constructed so as to be conical. Moreover, the conical portion of the axial window end forms part of the electron-optical system. It is, however not known from this document that the axial window end of the tube is constructed so as to be conical with an angle of substantially 45°, neither is it known from this document that the exit window is surface mounted on an axial sealing plate of a minimum, uniform material thickness, and that a cooling duct which does not project beyond the 45° cone is provided around the envelope. An X-ray tube, comprising a cathode, an anode and an electron-optical system which serves to direct an electron beam to be emitted by the cathode onto the anode is also known from an article in NUCLEAR INSTRUMENTS AND METHODS, Vol.126, No.1, 1975, NL, pages 99-101. This article however does not disclose that the axial window end of the tube is constructed so as to be conical with an angle of substantially 45°, that the exit window is surface mounted on an axial sealing plate of a minimum, uniform material thickness, that a conical portion of the axial window end forms part of the electron-optical system, and that a cooling duct which does not project beyond the 45° cone is provided around the envelope. In a further embodiment, there is provided on the envelope an abutment face parallel to the anode surface for exact axial determination of the distance from and centring with respect to the anode surface. In a preferred embodiment, the anode is arranged at an extremity of an anode pipe which extends axially in the X-ray tube, in which the cathode extends around the anode pipe in an annular fashion and in which an end of a sleeve-shaped electrode (8) of the electron-optical system extending coaxially with the anode pipe which faces the exit window extends at the most as far as the anode surface. Some preferred embodiments in accordance with the invention will be described in detail hereinafter with reference to the drawing. Therein: Fig. 1 shows an X-ray tube in accordance with the invention; Fig. 2 shows relevant parts of a simultaneous spectrometer in which an X-ray tube is mounted as a radiation source, and Fig. 3 shows relevant parts of a sequential spectrometer comprising an X-ray source in the form of such an X-ray tube. An X-ray tube 1 as shown in Fig. 1 comprises, arranged within an envelope 2 with a connector socket 4 and a window 6, an electron emitter 10 which is accommodated in a cathode sleeve 8 and which consists of, for example a filament. Electrons emitted by the emitter are directed onto an anode 14. The electron paths are determined by the geometry of the cathode sleeve, the cathode, the anode and in this case also by the shape of a conical portion 16 of the tube envelope. The geometry of the cone 16 of the tube is chosen so as to achieve a minimum working distance between the anode 14 and an object to be irradiated. The other electron-optical elements are also chosen so that operation is possible with a minimum distance between the anode 14 and the window 6; this is why the cone 16 as such acts as an electron-optical electrode and the use of an additional electrode in the cathode-anode space of the tube is dispensed with. The window 6 is mounted on the cone so as to have a minimum structural length. This is achieved by mounting the window directly on the edge of minimum thickness instead of providing a recess in a window edge of the cone to support the window; mounting on the inner side or on the outer side of the tube is also possible. The working distance is thus realised by the internal geometry of the sleeve, by the external geometry of a radiation end thereof, as well as by integrated cooperation of these two factors. Because of the small working distance, for suitable radiation reproducibility it is desirable that the distance between the anode and the specimen surface is exactly defined and known. To this end, the position of the anode in the tube is determined relative to a flange 20 outside the tube. A face 22 then serves as a reference face for mounting the tube in an X-ray analysis apparatus. Fig. 2 shows the X-ray tube 1 mounted in a simultaneous spectrometer, comprising a specimen table 30, a mounting plate 32, surface portions 34 of which can act, for example as reference faces, a housing 36 for a number of measuring channels, two channels 42 and 44 which are symmetrically situated relative to an object or specimen 40 being indicated. From a point of view of radiation efficiency it is important to minimize the distance between the window 6 of the tube and the specimen 40. As appears from the Figure, the thickness of the tube and the shape of the cone 16 are of major importance in this respect. Optimization thereof in combination with said optimization of the tube itself, subject to the secondary condition imposed by mounting, offers a substantial gain in respect of radiation efficiency which can be translated into tube service life, speed of measurement, resolution etc . Fig. 3 shows the mounting of the X-ray tube in a sequential spectrometer in which the feasible mounting distance between the tube 1 and the specimen 40 is limited by the space for an entrance collimator 50, preferably consisting of several portions which can be exchanged in respect of position and, therefore, occupying a comparatively large amount of space, whereto the geometry of the tube must be adapted again. Optimization of the working distance again imposes a preferred shape for the cone 16, leading to a similar shape due to the geometry which is comparable with the position in the simultaneous spectrometer. The sequential spectrometer also comprises a crystal turret 52 and a detector system 54 which is in this case provided, by way of example, with a first detection collimator 56, a gas ionization detector 58, a second detection collimator 60 and a scintillation detector 62. Both positions result in a conical shape with an angle of cone of substantially 45°.

Claims: An X-ray tube, comprising a cathode (8,10) for providing a beam of electrons (12), an anode (14) for providing X-rays upon impingement of the beam of electrons thereon, and an electron-optical system (8,10,14,16) which serves to direct the electron beam to be emitted by the cathode onto the anode, which cathode, anode and electron-optical system are accommodated in a cylindrical envelope (2) having a radiation exit window (6) at one axial end for passing the X-rays, the axial window end of the tube being constructed so as to be conical with an angle of substantially 45°, the exit window (6) being arranged at the narrow end of the axial window end, characterized in that the exit window is surface mounted on an axial sealing plate of a minimum, uniform material thickness, a conical portion of the axial window end surrounding the exit window forms part of the electron-optical system as an electron-optical electrode, thereby the use of an additional electrode in the cathode-anode space of the tube being dispensed with, a cooling duct which does not project beyond the 45° cone is provided around the envelope, the electron-optical elements being chosen so that operation is possible with a minimum distance between the anode (14) and the exit window (6). An X-ray tube as claimed in Claim 1 in which on the envelope (2) there is provided an abutment face (22) parallel to the plane anode surface (14) for exact axial determination of the distance from and centring with respect to the anode surface. An X-ray tube as claimed in Claim 1 or 2 in which the anode (14) is arranged at an extremity of an anode pipe which extends axially in the X-ray tube, in which the cathode extends around the anode pipe in an annular fashion and in which an end of a sleeve-shaped electrode (8) of the electron-optical system extending coaxially with the anode pipe which faces the exit window extends at the most as far as the anode surface. An X-ray analysis apparatus comprising an X-ray tube as claimed in any one of the preceding Claims.

Inventors: D ACHARD VAN ENSCHUT J F M, VALKONET LOURENS, D'ACHARD VAN ENSCHUT,J.F.M., VALKONET, LOURENS, D'ACHARD VAN ENSCHUT,J.F.M.,C/O INT.OCTROOIBUREAU, Valkonet, Lourens, c/o INT.OCTROOIBUREAU
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1993
Main classification: H01J35
IPC codes: H05G1, H01J35