GB2133208A

GB2133208A - X-ray sources

Info

Publication number: GB2133208A
Application number: GB08232985A
Authority: GB
Inventors: Quentin Charles Herd
Original assignee: Kratos Ltd
Current assignee: Kratos Ltd
Priority date: 1982-11-18
Filing date: 1982-11-18
Publication date: 1984-07-18
Also published as: GB2133208B

Abstract

An X-ray source for an X-ray photo-electron spectrometer has a multi-facetted anode 1 and preferably two filaments 5, 5' for emitting electrons to bombard a selected face of the anode. The anode is rotatable to bring any selected anode face opposite a selected filament. The filaments can be energised selectively to enable at least two anode faces to be selected individually or together without movement of the anode. The anode shown has four facets 14a, b, c and of different materials, eg. Ti, Mg, Zr and Al on a Cu base, with a centrally located electron trap in the form of a recess 16 with Be- or C- coated walls. A shield 4 for focussing the electron beam onto a selected facet has four baffle electrodes 15- 18. A double sealed vacuum lock permitting adjustment and replacement of the anode is also disclosed. <IMAGE>

Description

SPECIFICATION Improvements relating to X-ray sources This invention relates to X-ray sources, in particular for an X-ray photoelectron spectrometer. However the invention is also applicable to other analytical equipment using Xray sources and to sources used for medical purposes.

There are a number of ways of producing Xrays e.g. electron bombardment of a suitable target, X-ray fluorescence by illuminating a suitable target with suitable higher energy X-rays, and synchrotron radiation resulting from the acceleration of high energy electrons. For most analytical work the electron bombardment method or the fluorescence method are the most generallv used. This application refers specifically to the electron bombardment method.

It is known to provide an X-ray photoelectron spectrometer with an X-ray source capable of giving one or two types of X-radiation. However there is a need for an X-ray source having greater versatility.

According to the present invention there is provided an X-ray source assembly having an anode formed with multiple faces, at least one filament which can be energised to cause electrons to be emitted for bombarding a face of the anode, and means for rotating the anode relative to said filament to enable a selected face of the anode to be located in a position in which it can receive electrons from said filament.

The invention will now be particularly described with reference to the accompanying drawings in which: Figure 1 is a diagrammatic section through a known type of dual anode source; Figure 2 is a diagrammatic section through a X-ray source assembly according to the present invention as applied to an X-ray photoelectron spectrometer; Figures 3a and 3b are diagrammatic side and end elevations respectively of a multifacetted anode structure according to the invention; Figure 4 is a diagrammatic view of a mechanism for rotating or indexing the anode round in the apparatus of Figure 1 so that different anode faces can be selected, and Figure 5 is a perspective view of the complete X-ray source assembly.

Hitherto, X-ray photoelectron spectrometers have had X-ray sources capable of giving one or two types of X-radiation. Such a 'dual-anode' Xray source is availableon most commerciai X-ray photoelectron spectrometers and its general form is shown diagrammatically in Figure 1. The anode a has a chisel-shaped end portion each face of which has a different anode material on it. In Figure 1 the anode materials are shown as A1 and A2. To excite the target material A1, an appropriate filament f1 is switched on and electrons are accelerated onto A, by the application of suitable electrical potentials to the filament f,, the anode a and to a screen electrode c. X-radiation produced at A, passes through an X-ray window w to reach the specimen s.To obtain X-radiation from material A2, filament f1 is switched off and a second filament f2 is switched on. Filaments f1 and f2 are connected by leads b to a suitable electric supply. The whole structure is shielded from the rest of the system by means of a metallic cover h.

in order to render such a system more versatile the present invention provides multiple anodes and means for bringing each anode into operation selectively.

Figure 2 shows one embodiemnt of an X-ray source assembly according to the invention as it might be fitted to an X-ray photoelectron spectrometer. The electron spectrometer 9, a specimen 7 and the excitation part of the X-ray source are all subjected to high vacuum conditions. In the source, the anode 1 has a multifacetted end portion, as best seen in Figure 3b, with different anode materials deposited on each face. The method of deposition of the anode material can be evaporation, ion-plating, ion sputtering, plasma spraying or any other deposition technology applicable to the anode material. The basic anode substrate is conveniently copper or some other high heatconductivity material from which the anode can be made. The X-ray source structure is housed in a body 2 which prevents stray electrons leaving the source region.The body 2 contains an X-ray window 3 through which rays emitted from the anode pass before striking the sample 7.

The method of excitation to produce X-rays is by electron bombardment. Filament 5 or 5' is heated to emit electrons which are then attracted to the selected face of the anode by means of a suitably applied electrical potential. A shield electrode 4 surrounds the anode to enable adjustments to be made to the electron trajectories by varying the potential applied to the electrode and thereby altering the position at which electrons first strike the anode face.

A vacuum flange 6 is provided vvhereby the Xray source is fixed to the vacuum chamber (not shown in figure 2) of the spectrometer. The actual anode structure passes through a double sealed lock 10, the space between the seals of the lock 10 being evacuated by a pumping line 11. As seen in Figure 4, a valve 1 Oa seals off the lock.

When open the valve permits the passage of the anode body into and out of its operative position.

When the valve 1 0a is closed, the system can be maintained under vacuum without the presence of the anode. Alternatively the valve may be omitted if a quick withdrawal of the anode is not required. The anode structure is cooled by a suitable cooling medium pumped round the system via cooling ducts 12.

Changing the selected anode face is performed by rotating the anode, which can rotate relatively easily in the double sealed lock 10, by means of a drive mechanism 1 3 to be described below.

Figures 3a and 3b show the excitation region of the X-ray source in more detail The anode 1 is circular in cross-section except for the end portion 14 which has a tapered and facetted shape. Each face can take a different anode material and the number of faces can be made dependent on the size of the anode and the degree of localisation of the exciting electron beam. As an example, the illustrated anode has four faces bearing four different anode materials. The faces 1 4a, b, c, d, could, by way of example, be coated with titanium (Ti), magnesium (Mg), zirconium (Zr) and aluminium (Al) respectively. Such anodes would be useful in X-ray photoelectron spectroscopy as they provide high (Ti) and low (Zr) energy lines together with two narrow lines (Mg and Al) thus providing means of observing different escape depths and differing photoelectric cross-sections.

The anode tip has a recess 1 6 which acts as an electron 'sink' minimising the chances of electrons hitting an unwanted face. The number of electrons reaching this sink will be low. Hence any X-ray background will be low. The geometry of the trap can be made such that X-rays generated in the trap will tend to be shielded from the specimen. Coating the inside of the trap with a material which gives a low energy Ka line e.g.

benyllium or carbon will similarly reduce background.

The two filaments 5, 5', which are selectively heated to emit electrons, enable the operator to change the X-ray source by switching on different filaments without rotating the anode. Another mode of operation is to excite two anodes simultaneously by simultaneous heating of both electrodes. Another possibility is to excite, for example, the Mg anode only with a selected filament, study the photoelectric emission from the sample, and then rotate the Mg face round to be opposite the other filament which is then excited, the first selected filament being switched off. In this latter case radiation from the Mg face will bombard the sample at a different angle of 'attack' due to the fact that the Mg face has moved to the opposite side of the axis of the anode and the centre of the face is therefore at a different angle to the portion of the target under investigation.

The shield 4 aids focussing of the electron beam. Further trimming of the electron beam so as to ensure that it hits one face only and does not produce unwanted radiation, due to 'crossover' to adjacent faces, is provided by four baffle electrodes 1 5 to 18.

The source is generally run with the anode at a high positive electrical potential and the filament at an 'earth' or near-earth potential. This prevents backscattered electrons from bombarding the Xray window of the X-ray source. This window is typically very thin aluminium or beryllium metal and provides some 'filtering' of the unwanted continuous X-radiation; it also prevents stray electrons from reaching the sample.

A mechanism for rotating or indexing the anode is shown in Figure 4. The hollow anode tube 1 with its facetted end 14 passes through the mounting flange 6 which is fitted with the vacuum lock 10. The lock uses at least two spring loaded seals 22, 23 and the space 24 between them is pumped out via the manifold 26. Outside the lock, the anode has a gear wheel 29 keyed to it. The end of the anode terminates in the cooling pipes 12. The gear wheel 29 keyed to the tubular anode 1 is driven via a second gear wheel 30 which in turn is driven by a third gear wheel 31 from a drive motor 33. Gear wheels 30 and 31 are mounted for rotation on a bearing plate 35 which is attached to the vacuum lock.

As the anode will generally be at high voltage (1-15kV) the lock and gear drive mechanism will also be at this voltage. Consequently an insulating coupling shaft 32 runs between gear wheel 31 and the drive motor 33. A cover 34 is provided for safety. The motor is fitted with microswitches so that the anode can be indexed by the correct amount.

With the high voltage switched off and the cover removed it is possible to remove the used anode 1 via the vacuum lock 10 and valve 1 Oa and replace it with a new or different anode in a short period of time (of the order of minutes).

Alternatively the anode can be withdrawn into a controlled environment (e.g. a nitrogen-filled glove-box) and coating materials placed on the anode. The anode can then be inserted in the system and emission X-ray spectroscopy carried out by using a suitable sample as a 'converter'.

The drive motor 33 can be easily interfaced to a computer system so that different anode faces can be selected automatically when the instrument is running under computer control.

Claims

1. An X-ray source assembly having an anode formed with multiple faces, at least one filament which can be energised to cause electrons to be emitted for bombarding a face of the anode, and means for rotating the anode relative to said filament to enable a selected face of the anode to be located in a position in which it can receive electrons from said filament.

2. An X-ray source assembly according to claim 1 having at least two said filaments and means for selectively energising the filaments.

3. An X-ray source assembly according to claim 1 or claim 2 having a fast insertion vacuum lock through which the anode can be withdrawn to enable a replacement anode to be inserted.

4. An X-ray source assembly according to any preceding claim having a shield electrode interposed in the line-of-sight path of electrons travelling from the filament or filaments to the anode.

5. An X-ray source assembly according to any preceding claim having electron baffle means for reducing the emission of unwanted radiation from anode faces adjacent said baffle means.

6. An X-ray source assembly according to any preceding claim wherein the anode has an electron trap located substantially symmetrically with respect to said multiple faces to reduce the flow of electrons to faces other than the face or faces for the time being intended to be exposed to electron bombardment.

7. An X-ray source assembly substantially as herein described with reference to Figures 2, 3a and 3b of the accompanying drawings.

8. An X-ray photoelectron spectrometer having an X-ray source assembly in accordance with any one of the preceding claims.