High Intensity Atomic Spectral Lamp With Interchangeable Cathode

Abstract

A high intensity atomic spectral lamp having a first set of electrodes including a primary cathode consisting of or comprising a selected element adapted to produce a primary electric discharge which gives rise to an atomic vapor of said element by cathodic sputtering from the primary cathode; a second set of electrodes adapted to produce a secondary electric discharge which passes through the said atomic vapor, to excite the atoms in the vapor to emit radiation characteristic of said element; a window to allow the passage of said radiation out of the lamp, said window being placed in front of the operative surface of the primary cathode; means to pass a stream of inert gas through the lamp close to and across the operative surface of the primary cathode and between the primary cathode and the window in such manner as to sweep away from the vicinity of the primary cathode surface atomic species formed by said primary discharge. The preferred form of the lamp includes demountable cathode support means to permit ready removal and replacement of the primary cathode. This invention is concerned with an improved form of high intensity atomic spectral lamp. The principal type of atomic spectral lamp at present in use is the hollow cathode discharge lamp, in which the cathode is shaped in the form of a hollow cylinder and is made of a material which consists wholly or partly of the elements of which the atomic spectrum is to be obtained. This type of lamp suffers from the principal disadvantage that the electrical discharge between the anode and cathode serves to generate or produce an atomic vapour by sputtering from the cathode, and also to supply the excitation, which is necessary for the production of atomic spectra, to at least some of the atoms in the vapour. These two functions of the discharge cannot be separately controlled, and a variation in one parameter of the discharge, e.g. current or pressure, will affect both functions. The amount of atomic vapour produced must be limited to relatively small quantities if the widths of the spectral lines are not to be increased by self-absorption and resonance broadening. Thus the discharge current that can be used, and therefore the degree of excitation that can be imparted to the atomic vapour, are similarly limited. Consequently the intensities of the spectra emitted by such discharge lamps are limited if sharp lines are required. Our prior Australian Pat. Nos. 260,726 and 289,307 described forms of high intensity atomic spectral lamp in which the atomic vapour of an element was generated by a first electrical discharge giving rise to cathodic sputtering and this vapour was then excited by a means of a second independent discharge to thereby increase the radiation emitted by the lamp, in comparison with ordinary cathode sputtering lamps, without incurring significant line broadening. A further modification of this type of lamp is described in our Australian Pat. No. 295,985, the lamp described therein producing atomic vapour by thermal means rather than by cathodic sputtering. Such prior art high intensity lamps have found significant applications in the field of atomic adsorption spectroscopy although they still share with ordinary hollow cathode lamps the disadvantage that one lamp is required for each element to be determined. Most prior art lamps, whether of the normal or high intensity type generally conform to one basic configuration in which the lamp envelope is of elongated tubular shape, with a flat optical window at one end, the hollow cathode being situated at or near the other end of the tube and facing the window. In the high intensity type of lamp the second set of electrodes which provide the exciting discharge are usually arranged to be diametrically opposed across the long axis of the lamp and close to the mouth of the hollow cathode. Other arrangements are, of course, possible but the principal criterion of any such lamp is that the window must be fairly remote from the hollow cathode in order to minimise the amount of cathode material which is deposited on the window as a result of cathodic sputtering. Even so, after a long period of operation, windows of most lamps, particularly those used for producing the spectra of fairly volatile elements become coated with a sputtered film of the element and lose their usefulness. In lamps of the sealed off type, which are the most common, another problem is that clean-up of the filler gas inevitably occurs thereby limiting the useful lifetime of the lamp. Another difficulty which is encountered with our earlier high intensity lamps, e.g. of the type described in Australian Pat. Nos. 260,728 and 289,307, is that there is substantial interaction between the primary discharge, which produces the atomic vapour, and the secondary (exciting) discharge. It is found, in fact, that if the secondary discharge current exceeds about 80 to 100 mA the light output of the lamp actually falls off, due to effective lowering of the voltage across the primary discharge electrodes. While the prior art high intensity lamps have been used with some success for atomic fluorescence spectroscopy, they are by no means ideal for this purpose. Because of the configuration of the lamps as described above, they essentially have a small numerical aperture. This coupled with the fact that they cannot be run at their maximum light output reduces the useful intensity of radiation which they provide to the point where it may not be sufficient to enable satisfactory use of atomic fluorescence techniques. The principal objects of the present invention can therefore be stated as the provision of a high intensity atomic spectral lamp having at least one and preferably all of the following features: Barnes Engineering Company of Stamford, Conn., U.S.A., have produced a demountable, hollow cathode, atomic spectral lamp which includes a gas flow-through system but the design of the lamp is essentially conventional, having an elongate tubular body with a window at one end and the hollow cathode at the other. In the Barnes lamp, the flow direction of the gas is from the cathode towards the window, which inevitably will lead to deposition of the sputtered cathode material on to the window. The use of gas flow through systems is also described in our Australian Pat. No. 414,987, and patent application No. 59106/73 which are concerned with a type of apparatus (now known as a "sputtering chamber") in which an atomic vapour is produced by a sputtering discharge from a replaceable cathode comprising a solid sample in a chamber through which a constant flow of gas is passing. This arrangement allows contaminants, introduced by the opening of the chamber to exchange cathodes, to be removed from the chamber and also avoids "clean-up" problems. Particularly in the system described in our patent application No. 59106/73, the cathode has a comparatively large area and this necessitates the use of a rather complicated "arrester" to control both the discharge characteristics and the gas flow which is directed essentially from and perpendicularly away from the face of the cathode. Gas pressure and flow-rate are also critical in this system and must be carefully controlled. It should be noted that in the sputtering chamber the cathode is the sample under test and the object is to ensure the production of an atomic vapour from the sample which is properly representative in composition of the sample. A sputtering chamber is not designed or intended to produce emitted radiation. A high intensity spectral lamp on the other hand is essentially a spectral source, for which it is often desirable that the cathode be as pure as possible, for example in atomic fluorescence work. This therefore implies the use of a cathode of relatively small area. The present invention is concerned with a high intensity lamp of an entirely new configuration which utilizes both the flow-through principle and a demountable cathode system and, which, by virtue of its design, maximises the advantages to be obtained from these features. According to the present invention there is provided a high intensity atomic spectral lamp having a first set of electrodes including a primary cathode consisting of or comprising a selected element adapted to produce a primary electric discharge which gives rise to an atomic vapour of said element by cathodic sputtering from the primary cathode; a second set of electrodes adapted to produce a secondary electric discharge which passes through the said atomic vapour, to excite the atoms in the vapour to emit radiation characteristic of said element; a window to allow the passage of said radiation out of the lamp, said window being placed in front of the operative surface of the primary cathode; means to pass a stream of inert gas through the lamp close to and across the operative surface of the primary cathode and between the primary cathode and the window in such manner as to sweep away from the vicinity of the primary cathode surface atomic species formed by said primary discharge. Preferably the window is substantially parallel to the operative surface of the primary cathode and the gas stream also passes substantially parallel to that surface. It is further preferred that the lamp includes demountable cathode support means to permit ready removal and replacement of the primary cathode. A principal feature of the lamp of the invention is thus the use of a gas flow to sweep atomic vapour sputtered from the primary cathode away from the area between the cathode and the window in a direction substantially parallel to the window. This arrangement permits the window to be relatively close to the front surface of the primary cathode and thus subtend a relatively large angle at the cathode surface. This enables the provision of high numerical aperture with a window of relatively small diameter, while still minimizing deposition of sputtered material on the window. In one preferred embodiment the high intensity lamp of the present invention comprises an elongate, generally tubular body portion with the primary cathode mounted in the wall of the body portion; the secondary cathode and common anode being mounted within the body portion and arranged to direct the second discharge across and through a region in front of the primary cathode; and gas inlet and outlet means arranged to pass a stream of gas through said region in a generally axial direction with respect to the body portion and in the direction of the secondary discharge. More specifically, the preferred lamp of the invention comprises: The body portion is preferably of reduced diameter in at least part of those portions of the body which lie between the cathode compartment and the secondary cathode and anode respectively. This arrangement helps to increase the current density of the secondary discharge and therefore the degree of excitation of the atomic vapour. Preferably the optical window is of substantially larger diameter than the said aperture or the primary cathode, thereby to provide a high numerical aperture. Preferably also, the primary cathode has a substantially flat front face and the arrangement of the cathode compartment and supporting means is such that in use the flat face of cathode is substantially flush with the inside wall of the body portion adjacent the cathode compartment. The use of the flat-faced cathode permits higher operating voltages for the primary discharge (up to about 550V) as compared with a hollow cathode (about 200V). This further contributes to minimising interaction between the discharges. It is further preferred that the secondary cathode is of the electrically heated (filament) type and/or is coated with a thermionically emissive material. The preferred coating material is lanthanum hexaboride which is stable to repeated exposure to air. To facilitate replacement of the secondary cathode, when necessary, it is preferred that the secondary cathode is supported on supporting means which can be removed from and replaced in the body portion. 1. Low or minimal interaction between the primary and secondary discharges. 2. Provision for interchangeable cathodes. 3. The capability of working at optimum filler gas pressure for maximum light output, without compromising service life. 4. High numerical aperture. a generally tubular body portion having gas inlet and outlet means to allow a flow of gas through the body portion in a generally axial direction; a cathode compartment located in the wall of the body portion; primary cathode means located in said compartment and comprising a primary cathode and demountable primary cathode support means adapted to allow the first cathode to be readily removed from and replaced in said compartment; window means attached to the wall of the body portion, said window means comprising a chamber in open communication with an aperture in the wall of the body portion diametrically opposite the cathode compartment and an optical window at the end of the chamber remote from the aperture; a secondary cathode located within the body portion and spaced away from the cathode compartment; a common anode located within the body portion and spaced away from the cathode compartment in the opposite direction to the secondary cathode.

Patent Drawings (1 sheets)

Sheet 1