EP0567183A1

EP0567183A1 - X-ray tube using M-line of radiation spectrum

Info

Publication number: EP0567183A1
Application number: EP93201081A
Authority: EP
Inventors: Johannes Jacobus De Koning
Original assignee: Koninklijke Philips Electronics NV; Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1992-04-21
Filing date: 1993-04-14
Publication date: 1993-10-27

Abstract

The anode of an X-ray tube is provided with an anode target material in which effectively usable M-radiation can also be generated, notably with a heavy element such as Th, Pa, or U. Thus, a radiation spectrum to be emitted by the tube is suitable for analysis of light elements, notably by means of the M-radiation, as well as heavier elements, using radiation from the comparatively energy-rich continous spectrum and the spectrally attractively situated L-radiation of the anode material.

Description

The invention relates to an X-ray tube, comprising a cathode and an anode for generating X-rays. The invention also relates to an X-ray analysis apparatus comprising an X-ray tube in accordance with the invention.
An X-ray tube of this kind is known from US 4,205,251. An X-ray tube described therein aims to generate X-rays in an effective spectral range for X-ray analysis of elements. A tube described therein is particularly suitable for generating copper K-α radiation having a characteristic wavelength of approximately 0.15 nm.
For analysis of different elements, notably elements having a low atomic number, the copper K-α wavelength is not very effective. In order to improve this situation, solutions have been proposed such as, for example the use of scandium as the anode material as described in US 4,583,243 for analysis of notably light elements, or a double-layer anode as described in US 4,622,688 for generating a radiation spectrum which can be controlled by the anode voltage and which can be used for analysis of light as well as heavier elements. For light elements, the anode thereof comprises a layer of scandium and, underneath the comparatively thin scandium layer, it comprises a layer of tungsten or molybdenum for analysis of heavier elements by means of an increased anode voltage.
It is an object of the invention to provide an X-ray tube offering a comparatively high radiation yield over a comparatively wide spectral range, without it being necessary to vary the anode voltage. To achieve this, an X-ray tube of the kind set forth in accordance with the invention is characterized in that in order to generate an X-ray spectrum with an effective wavelength range for X-ray analysis, the anode contains a material in which effectively usable M-radiation can also be generated.
Because in an X-ray tube in accordance with the invention use can be made of comparatively long-wave M-radiation as well as more short-wave characteristic radiation and radiation from the continuous spectrum, the tube is suitable for analysis of light elements, to be excited notably by the M-radiation, as well as heavier elements, to be excited notably by means of, for example L-radiation and effective radiation from the continuous spectrum. Thus, the X-ray tube very effectively utilizes the M-radiation, experienced as a nuissance thus far and situated in an attractive wavelength range for the analysis of light elements if the anode material is suitably chosen.
In a preferred embodiment, the anode contains an element from the group of heavy elements thorium (90), protactinium (91) and uranium (92) or mixtures thereof. M-radiation of these elements has a wavelength of approximately 0.4 nm and is sufficiently energetic for fast analysis of elements having an atomic number of between, for example 4 (beryllium) and 18 (argon). The continuous spectrum to be generated in such anode materials with the L-radiation of approximately 0.1 nm is sufficiently energetic for analysis of heavy elements as high as the anode material. An additional advantage consists in that an exit window of beryllium with a customary thickness of, for example 150 µm exhibits a comparatively high transmission for the M-radiation to be generated in these tubes. The anode material is notably low-radioactive U-238.
In a preferred embodiment, the anode material is deposited on a support, for example in the form of an anode body in a side-window tube or an end-window tube, as a comparatively thin layer of, for example from 0.2 to 2.0 µm thickness. In a target transmission tube a thin layer of anode material is deposited on an exit window, preferably by sputtering or a CVD technique. Because of the necessary transmission for the X-rays to be generated therein, the layer then has a thickness of at the most, for example approximately 0.1 µm. The layer of anode material can alternatively be provided on an anode body as a foil. The thickness of the layer is then again determined by the deposition technique and the temperature conditions and may also be substantially greater than approximately 0.1 µm.
An X-ray tube in accordance with the invention is particularly suitable for use in, for example an X-ray spectrometer because therein a wavelength range generated by a single, customary high voltage can be used for excitation of a wide range of elements.
Some embodiments in accordance with the invention will be described in detail hereinafter with reference to the drawing. Therein:

Fig. 1 shows an end-window X-ray tube in accordance with the invention,
Fig. 2 shows an X-ray analysis apparatus comprising such a tube,
Fig. 3 shows diagrammatically a radiation spectrum to be generated by such a tube.

An X-ray tube 1 as shown in Fig. 1 comprises, accommodated in an envelope 2 with a connector 4 and an exit window 6, an electron emitter 10, for example in the form of a filament which is accommodated in a cathode sleeve 8. Electrons emitted by the emitter 10 are directed towards an anode 14 along paths 12. The electron paths are determined by the geometry of the cathode sleeve, of the cathode itself, and of the anode and by the shape of a conical portion 16 of the tube envelope. On an anode body 18 of the anode, being made of, for example copper, there is provided an anode target plate 19 of, for example uranium 238. In the target an X-ray beam 20 is generated in a customary manner by the electrons incident along the paths 12, which beam emanates from the tube via the exit window 6 which consists of, for example a plate of beryllium having a thickness of 150 µm.
Fig. 2 shows an X-ray tube 1 mounted in an X-ray analysis apparatus 20, which is in this case formed as a simultaneous spectrometer comprising a specimen table 30, a mounting plate 32, and a housing 36 for a number of measuring channels. The Figure shows two measuring channels 42 and 44 which are symmetrically arranged relative to an object or specimen 40 to be examined. Each measuring channel comprises an entrance slit 46, an analysis crystal 48, and a detector 50. Each of the measuring channels is oriented relative to the specimen in conformity with a respective element assigned thereto. Via the exit window 6, the specimen is irradiated by an X-ray beam 52 which, as a result of the use of an anode material in accordance with the invention, covers a wide effective wavelength range and is hence suitable for simultaneous excitation of a wide range of elements.
A curve 50 in Fig. 3 represents the continuous X-ray emission spectrum of uranium for an anode voltage of 60 kV, with, as a relative measure, the number of X-ray quanta (N) to be emitted as a function of the wavelength in nm. For 60 kV the continuous spectrum commences at the short wavelength side at approximately 0.2 nm and a maximum is reached at approximately 0.3 nm.
A first favourable effect consists in that this maximum, and hence the entire continuous spectrum, reaches a level which increases in proportion to the atomic number of the anode material. Single lines in the continuous spectrum denote the line spectrum with N-lines at approximately 0.88 nm, M-lines at approximately 0.39 nm and L-lines at approximately 0.09 nm. In the case of 60 kV, K-lines in uranium of approximately 0.01 nm are not excited.
A line 52 diagrammatically represents the absorption (Ab) of a beryllium window having a thickness of 150 µm. Because the absorption reaches a value of 50% only at approximately 0.45 nm, the M-radiation can still adequately pass such a window so that it can be effectively used for analysis. A radiation spectrum emanating from the tube is denoted by a dashed line 54; because of the attractive combination of emission and absorption, it is extremely effective for analysis of a wide range of elements.

Claims

An X-ray tube, comprising a cathode and an anode for generating X-rays, characterized in that in order to generate an X-ray spectrum with an effective wavelength range for X-ray analysis, the cathode contains a material in which effectively usable M-radiation can also be generated.
An X-ray tube as claimed in Claim 1, characterized in that the anode material contains an element from the group of heavy elements thorium, protactinium and uranium.
An X-ray tube as claimed in Claim 1 or 2, characterized in that the anode material is low-radioactive uranium 238.
An X-ray tube as claimed in Claim 1, 2 or 3, characterized in that the anode material is deposited on an anode body as a comparatively thin layer.
An X-ray tube as claimed in Claim 4, characterized in that the anode material is deposited on a support by sputtering, vapour-deposition or a CVD technique.
An X-ray tube as claimed in Claim 5, characterized in that the anode material is deposited on a radiation exit window of a target transmission X-ray tube.
An X-ray tube as claimed in any one of the preceding Claims, characterized in that a voltage of up to approximately 60 kV can be applied between the anode and the cathode.
An X-ray spectrometer comprising an X-ray tube as claimed in any one of the preceding Claims.