US3476970A - Hollow cathode electron discharge device for generating spectral radiation - Google Patents

Description

' 3,476,970 13 FOR W. GILLIES AL HOLLOW CATHODE ELECTRON DISCHARGE DEVIC GENERATING SPECTRAL RADIATION Filed Sept. 12. 1966 Nov. 4. 1969 625555 +v $55.30 Box $01.90

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zz wztmzmm moEzomzuozoz n1 f9 +09 m mm United States Patent US. Cl. 313-209 1 Claim ABSTRACT OF THE DISCLOSURE A spectral light source of the hollow cathode variety capable of emission of spectral radiation characteristic of the material within the hollow cathode to enhance emis sion of the ground state lines of the radiation with respect to emission of ion lines of said radiation.

This invention relates to atomic absorption spectrometry and more particularly to electron discharge devices of the hollow cathode type which are adapted for emitting beams of radiation having a defined spectral line(s) characteristic of the cathode element(s) Generally, the constituents of an unknown chemical sample may be detected by introducing a solution of the unknown sample in the form of a mist into a suitable flame or other means to cause the solution to dissociate the compounds into its atomic constituents. Next, radiation having known spectral lines is directed through the sample vapor with the result that certain spectral lines of the beam of radiation will be absorbed by the vapor. Then the radiation can be analyzed to discover which spectral lines have been absorbed and the degree to which these lines have been absorbed by the sample of atomic vapor to determine respectively the constituents and the concentration of these constituents in the unknown sample.

Typically, beams of radiation with defined spectral lines are provided by electron discharge devices of the hollow cathode type. In such devices, the cathode element is usually shaped in the form of a hollow cylinder en closed at one end. Illustratively, an anode of an annular, wire or plate configuration may be disposed in the vicinity of the cathode element so as not to prevent the radiation emitted from the hollow portion of the cathode element from being directed to the outside of the electron discharge device. It is noted that the cathode element is generally made of a metal(s) or a material containing the metal(s), whose spectra it is required to produce. In operation, a source of potential is applied between the anode and cathode elements to cause a discharge of electrons therebetween. The discharge of electrons ionizes positively the gas contained within the envelope of this device. In turn, the positively charged gas ions are attracted to and strike the cathode element to thereby sputter atomic particles of the metal of which the cathode element is made into the region of the hollow portion of the cathode element. These sputtered atomic particles are bombarded by a portion of the electrons flowing between the cathode and anode elements and also by the excited gas atoms and ions within the envelope. Due to the bombardment of the gas ions, excited atoms and electrons, some of the electrons associated with the sputtered atoms are excited from a ground state to higher energy levels. When the excited electrons return to their ground state or from a higher energy level to a lower energy level, the energy originally absorbed through the collisions of gas ions, excited gas atoms and/or electrons is released in 3,476,970 Patented Nov. 4, 1969 "ice the form of radiation having a spectral line(s) determined by the difference in the energy levels between which the electrons have made transitions.

In the process of determining the presence of elements and/or the concentration of those elements in the unknown sample, the beam of radiation having spectral lines associated with those elements is directed through the vaporized atoms of the solution to be analyzed. If the available excitation levels of the vaporized atoms are of a value corresponding to a wavelength of interest in the beam of radiation, a portion of the beam of radiation of that Wavelength will be absorbed by the atoms of the vaporized solution thereby indicating the presence of that element corresponding to the wavelength of the absorbed radiation. It is particularly noted that the probability that a portion of the beam of radiation will be absorbed depends upon the fact that the vaporized atoms have an available electron energy level corresponding to at least one. of the wavelengths of the beam of radiation directed therethrough. In order to meet this condition, it is desirable to provide a beam of radiation of a wavelength(s) corresponding to the excitation of the atoms of the cathode element from the ground state to some higher energy level(s)generally, those corresponding to resonance radiation. It is noted that the atoms sputtered from the cathode element are readily excited from their ground state to a higher energy level due to the inherent nature of the atomic collisons with the ionized and excited gas atoms. Further, the wavelength of the radiation emitted from an ionized atom will differ from the wavelength of the radiation corresponding to the return of the atom from the resonant energy level to the ground state. More specifically, the electron transitions of an ionized sputtered atom differ from the electron transition of a non-ionized atom in the excited condition. Because the vaporized ground state atoms of the solution to be analyzed have a greater probability of absorbing the radiation emitted from sputtered non-ionized atoms which have been excited to their resonant energy level as opposed to sputtered ionized atoms, radiation from the former is to be preferred. In particular, typically more than approximately of the vaporized particles of the unknown solution are non-ionized and non-excited and at their ground state. Further, ground state atoms are most readily excited by radiation of a wavelength corresponding to the quanta of energy required to excite the electrons of the ground state atom to its resonant energy level. On the other hand, if the wavelength of the radiation does not correspond to the discrete quanta of energy required to excite the electrons to a discrete energy level, the radiation will not be absorbed and will pass essentially unaffected through the vaporized solution to be analyzed. Therefore, the quanta of energy of the radiation emitted from an ionized atom differs from that required to excite the vaporized atoms of the unknown solution and this radiation has a poor probability of being absorbed, whereas the quanta of energy of radiation emitted from an excited, non-ionized atom corresponds to that energy level required to excite the vaporized atoms in the sample, and the radiation of this wavelength has a high probability of being absorbed.

In order to increase intensity of the beam of spectral radiation and to enhance the emission of radiation corresponding to non-ionized atoms which have been excited from their ground state as compared to that radiation emitted by ionized atoms, there has been suggested an electron discharge device including an auxiliary electrode for the emission of an auxiliary discharge in the vicinity of the hollow portion of the cathode element to a second auxiliary electrode acting as an anode element for the first auxiliary electrode. The use of these devices, however, have resulted in several significant problems. First, this device utilizes four electrodes, i.e. a cathode element, an anode element, and two auxiliary electrodes for establishing an auxiliary discharge therebetween. Further, this device requires the use of extensive shields which may take the form of tubular members disposed about the first and second auxiliary electrodes to confine the auxiliary discharge to the hollow portion of the cathode element. Further, the auxiliary discharge between the first and second auxiliary electrodes utilizes a high level of current thereby typically requiring the use of a second power source and the use of a thermally emissive auxiliary electrode. Further, it may be well understood that the use of four electrodes including a thermally emissive auxiliary electrode, and the tubularly-shaped shielding members result in an electron discharge device which is expensive to manufacture.

Therefore, it is an object of this invention to provide a new and improved source of spectral radiation which overcomes the limitation of the prior art.

It is the principal object of this invention to provide a new and improved electron discharge device of the hollow cathode variety capable of emitting spectral radia tion having a high ratio of spectral radiation corresponding to the excitation of the electrons associated with non-ionized atoms (sputtered from the cathode element) between their ground state and a resonant energy level to spectral radiation emitted from ionized atoms which have been sputtered from the cathode element.

It is a further object of this invention to provide a new and improved electron discharge device of the hollow cathode variety which is capable of providing spectral radiation with a minimum of electrodes and with a simple, inexpensive construction.

It is a more particular object of this invention to provide a new and improved electron discharge device of the hollow cathode variety capable of emitting spectral radiation without the use of an auxiliary electrode.

These and other objects are accomplished in accordance with the teachings of the present invention by providing a new and improved electrical discharge device for providing a beam of spectral radiation including a cathode element and an anode element for establishing an electrical discharge therebetween whereby atomic particles are sputtered from the cathode element. More specifically, the cathode element, which is made of a material corresponding to the desired wavelength of the beam of spectral radiation, has an effective aperture or opening therein of such a diameter to reduce the ionization of the atomic particles sputtered from the cathode element to thereby provide a beam of spectral radiation emitted from atoms (sputtered from the cathode element) whose'electrons have been excited from a ground state to a resonant energy level. More specifically, the cross-sectional area of the effective openings of the cathode element should not exceed .049 square inch. Further, the electron discharge device is filled with gas at a pressure adjusted from 2 millimeters of mercury to 100 millimeters of mercury for maximizing and stabilizing the electron discharge in the hollow portion of the cathode element.

These and other advantages of the present invention will become apparent when considered in view of the following detailed description and drawings, in which:

FIGURE 1 is a diagrammatic view of a system for performing atomic absorption measurements;

FIG. 2 is a perspective view, partially broken away and partially in section, of an electron discharge device embodying the present invention; and

FIG. 3 is a graphical representation comparing the ratio of intensities of radiation of a wavelength (2,320 A.) emitted by non-ionized atomic particles sputtered from a nickel cathode element to radiation of a wavelength (2,316 A.) emitted from ionized atoms of a nickel cathode as a function of the diameter of the hollow portion 4 of the nickel cathode element as incorporated in the de vice of FIG. 2.

Referring now to the drawings and particularly to FIG. 1, there is generally shown a system for performing atomic absorption measurements including an electron device 10 of the hollow cathode variety for providing a beam of radiation having a characteristic spectral line(s). The beam of radiation emitted by the electron discharge device 10 is focused by a suitable optical assembly 96 through a means 98 for vaporizing a solution containing an unknown quantity of the material under consideration into a monochromator 100. As explained above, certain spectral lines are absorbed by the vapor of the sample from the beam of radiation as it passes through the flame means 98. More specifically, if the beam of radiation emitted from electron discharge 10 is of a specified wavelength corresponding to the energy required to excite the atoms which have been vaporized in the flame means 98 from their ground state to the resonant energy level, a portion of the beam of radiation will be absorbed by the vaporized solution of the unknown sample. On the other hand, if a beam of radiation is of a wavelength not corresponding to the quanta of energy required to excite the atoms of the vaporized material, the beam of radiation will not be significantly absorbed thereby indicating that a certain element is not present in the vaporized solution. As mentioned above, it is desirable to provide a beam of radiation of a wavelength generated by a sputtered atom which has been excited from a ground state toa resonant energy level. Further, a beam of radiation generated by ionized atoms sputtered from the cathode element has a significantly less probability of being absorbed than radiation generated by non-ionized atomic particles. Further, by adjusting the monochromator 100 to the wavelength of the spectral lines which are to be absorbed by the sample vapor, a beam of radiation of a narrow bandwidth about the wavelength to be absorbed can be obtained through an opening within the monochromator 100. The resultant beam of radiation is directed into a radiation sensitive device 101 which provides an output signal which is amplified and applied by the amplifier 102 to a meter 104 which in turn provides an indication of the intensity of the radiation directed through the opening of the monochromator 100. It is contemplated that the degree of absorption may be measured by comparing the indication displayed by the meter 104 when unknown samples are introduced into the flame means 98 and when no sample is introduced into the flame means 98.

With reference now to FIG. 2, there isillustratively shown an electron discharge device 10 in accordance with the present invention. The electron discharge-device 10 includes an envelope 11, made of a suitable insulating material such as glass and having an enlarged tubular portion 12 and a smaller tubular portion 14 which are interconnected by a transition portion 13. The tubular portion 14 is sealed off at one end by a window 16 which is made of a suitable material efliciently transmissive to radiation wavelengths produced by this device. The portion 12 is sealed off by a stemheader 18 having a tippedotf exhaust tube 20 in a manner well known in the art. There is disclosed within the envelope portion 12 a cathode element 22 which is made of or contains a selected material corresponding to the wavelength of the radiation to be emitted from electron discharge device 10. For example, if it is desired to emit spectral lines corresponding to a material such as nickel, the cathode element 22 would be made of nickel or a material containing nickel. On the other hand, the cathode element 22 may be made of several elements such as calcium, magnesium and aluminum; an electron discharge device including such a cathode element has been further described in U.S. Patent No. 3,183,393 entitled Discharge Device by James E. Patterson and assigned to the assignee of the present invention. The cathode element 22 may illustratively be of a cylindrical configuration and have a centrally extending opening or hollow portion 24 extending from an edge 23 of the cathode element 22 closely adjacent to the window 16 into the cathode element 22 for a length denoted by the letter B. The diameter of the hollow portion 24 is designated by the letter A. lllustratively, the cathode element 22 is supported by a conductive lead 26 made of a suitable electrically conductive material such as nickel. The lead 26 may be affixed to the cathode element 22 and after being attached to a tungsten portion extends through the stem-header 18 to the exterior of the envelope 11. The lead 26 not only serves to support the cathode element 22 within the envelope portion 12, but also serves to provide an electrical connection thereto.

An anode element 28, which is illustratively shown to be in the configuration of a ring, is positioned in close proximity to the cathode element 22 near its upper edge 23. The anode element 28 is made of a suitable electrically conductive material such as tantalum, nickel or tungsten and is supported, in an illustrated embodiment, within the envelope portion 12 by means of two support rods 30. At least one of the support rods is of an electrically conductive material, for example, the same material as that of the cathode lead 26. The support rods 30 are secured at one end to the anode element 28 as by welding and extend through the stemheader 18 to the exterior of the envelope 11 to thereby provide means by which a potential may be applied to the anode element 23.

As shown in FIG. 1, a source 60 of potential is connected in series with a variable resistive impedance 62 between the cathode element 22 and the anode element 28 to provide an electrical discharge therebetween. Though the element 22 has been described as a cathode, it is noted that if the source 60 supplied an AC potential that the element 22 would act as a cathode only a part of the cycle depending upon the instantaneous polarity of the source 60. The electrical discharge between the cathode element 22 and the anode element 28 through a suitable gaseousmedium capable of supporting an electrical discharge such as neon, argon or helium causes the support gas within the envelope 11 to be ionized. The positive ions of gas in turn bombard the cathode element 22 thereby sputtering atoms of the material of which the cathode element 22 is made'from the inside surface into the hollow portion 24.

In order to limit the path of the electrical discharge between the cathode element 22 and the anode element 28, shielding means are provided which include at least two insulating discs 38 and 44 which are disposed in a spaced parallel relation between the cathode element 22 and the anode element 28. More specifically, the insulating disc 38 has an aperture 40 therein which is disposed concentrically about the hollow portion 24 of the cathode element 22. Further, the insulating disc 38 is disposed so as to be in close proximity or to abut the upper edge 23 of the cathode element 22 and extends therefrom to the interior surface of the envelope 11. The second insulating disc 44 has an aperture 41 which is disposed in fitted relation about the cathode element 22 and extends therefrom to the interior periphery of the envelope 11. In addition, the insulating means includes a pair of insulating sleeves 48 disposed about the support rods 30 and extending between the stemheader 18 and the insulating disc 44. In addition, two pair of insulating rings 47 and 46 are disposed about the support rods 30, respectively, between the insulating discs 44 and 38 and between the insulating disc 38 and the anode element 28. It may be understood that the anode element 281 abuts against the insulating rings 46 and serves to hold the insulating discs 38 and 44, insulating rings 46 and 47, and the insulating sleeves 48 axially in place. For a further description of the above described shielding means, reference is made to US. Patent No. 3,264,511 entitled Glow Discharge Device by George K. Yamasaki and assigned to the assignee of this invention.

It is a desired object of this invention to enhance the emission of radiation of a wavelength derived from nonionized atoms (i.e., atoms of the sputtered cathode material having an electron which has been excited from a ground state to a resonant energy level) and to diminish the excitation of radiation of a wavelength generated by ionized ions (i.e. those atoms of the sputtered cathode material which have been excited to an extent that at least one electron has been removed from the atom). In accordance with the teachings of this invention, the generation of ions from the sputtered particles of .the cathode element and therefore the emission of radiation from the ionized particles is limited by controlling the cross-sectional area or diameter (dimension A) of the effective opening of the cathode element 22. Referring now to FIG. 3, there is shown in graphical form a plot of the ratio of the intensity of radiation emitted from non-ionized atoms to the intensity of radiation emitted from ionized atoms as a function of the diameter (i.e. dimension A) of the hollow portion 24. In particular, these measurements were taken from an electron discharge' device wherein the cathode element 22 was made of nickel and the cathode current was maintained constant at about 15 ma. A cathode element made of nickel will sputter non-ionized atoms which will emit radiation of a wavelength of 2,320 A. and ionized atoms which will emit radiation of a wavelength of 2,316 A. The radiation of a wavelength of 2,320 A. may

be called the ground state or nickel I line, and the radiation of a wavelength of 2,316 A. may be called the ion or nickel II line. It is particularly noted that less sophisticated monochromators are unable to optically resolve wavelengths of radiation that are very close to each other. As a result, a monochromator may be unable to separate radiation of a wavelength of 2,320 A. from radiation of a wavelength of 2,316 A. thereby allowing a considerable amount of useless radiation emitted by ion particles to be directed onto the light sensitive device 101.

As shown in FIG. 3, the ratio of the ground state lines of radiation to the II lines of radiation becomes increasingly larger as the diameter A of the hollow portion 24 is decreased. More specifically, as the diameter of the cylindrical hollow portion 24 is decreased below A of an inch, the intensity of the ground state lines of radiation becomes greater than the intensity of the H lines of radiation. 4

Ilustratively, the opening or hollow portion 24 has been shown in FIG. 2 to be of a substantially cylindrical configuration with a uniform circular cross-section. It may be understood that the opening 22 could be varied to be a square or other configuration and that the crosssection does not have to be uniform. However, it does appear necessary that the efiective opening, i.e. the portion of the opening 24 where the glow discharge takes place in relative proximity to the face 23, have a crosssectional area less than .049 square inch. In the particular embodiment where the opening 24 takes the form of a cylinder, the diameter (i.e. dimension A) should not exceed inch.

The diameter A of the hollow portion 24 may be decreased to that point where an electrical discharge may not be supported between the anode element 28 and the hollow portion 24 of the cathode element 22. It may be understood that as the diameter A of the hollow portion 24 is decreased it is necessary to increase the pressure of the gas within the envelope 11 and therefore the level of the voltage supplied by the potential source 60 between the cathode element 22 and the anode element 28. As the diameter A of the cylindrical hollow portion 24 is decreased to approximately of an inch, the pressure of the gas within the envelope 11 must be increased to a point where increased potential is required to establish an electron discharge between the cathode element 22 and the anode element 28. It may be understood that as the pressure of the gas within the envelope is increased, the greater the impedance is presented between the cathode element 22 and the anode element 28. Further, it is noted that though a particular embodiment has been described incorporating a cathode made of nickel, the advantages of reducing radiation emitted from ionized atoms by controlling the diameter of the cathode element will secure to cathode elements made of other materials.

The depth as taken along the dimension B of the hollow portion 24 of the cathode element 22 is determined so as to support a discharge within the hollow portion 24. More specifically, the ratio of the dimension B to the dimension A must be /2 or greater in order to insure that the discharge is concentrated within the hollow portion 24 and that the output will have the desired line intensity ratio. Thus, as a result of the above discussion, the depth B of the hollow portion 24 may assume a minimum of approximately ,3 of an inch and may be increased an any nominal value without affecting the performance of this device. In alternate embodiments, the hollow portion 24 may extend through the cathode element 22 which would then take the form of an annular member.

As explained above, the pressure of the gas within the envelope 11 must be varied as the diameter of the hollow portion 24 is changed. More specifically, the gas pressure within the envelope 11 would be adjusted from 2 millimeters of Hg to 100 millimeters of Hg corresponding, respectively, to dimensions of the hollow portion 24 of 3744: inch and & inch. Further, the current of the discharge between the anode and cathode elements may be varied to obtain a maximum ratio between the ground state radiation and the ion radiation. In one particular example, wherein the cathode element was made of nickel, a current of approximately milliamps provided a ratio of the ground state line to the II radiation line of 9:1.

It is noted that the electron discharge devices of the hollow cathode variety of the prior art have employed cathode elements having hollow portions of a diameter greater than 1 inch primarily due to their inability to confine the electrical discharge to such a narrow hollow portion. The shielding means, as described with regard to FIG. 2, allows the electrical discharge to be confined to hollow portions of inch or less in diameter to thereby efliciently generate radiation of the desired spectral wavelengths.

Since numerous changes may be made in the above-described apparatus and different" embodiments of the invention maybe made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A spectral radiation source comprising an envelope having a first and second electrode for establishing electrical discharge therebetween, said first electrode having an effective opening therein and including a solid material therein for providing a beam of characteristic spectral radiation of said solid material, the'diameter of said effective opening being not greater than inch or less than inch and the depth of said efiFective openmg being not less than $4 inch to cause the greater emission of the ground state lines of said radiation than the emission of ion lines of said radiation, said envelope containing a gaseous medium maintained at a pressure 1n the range of 2 millimeters to millimeters of mercury and shielding means for confining said electrical discharge to said opening of said first electrode, said shielding means comprising a disc member extending from said firstelectrode to the inner periphery of said envelope.

References Cited UNITED STATES PATENTS 2,433,809 12/1947 Clapp 313 X 3,089,054 5/1963 Walsh et a1 313209 X 3,183,393 5/1965 Paterson 313-185 X 3,205,388 9/1965 De Lany et a1. 313209 X 3,242,371 3/1966 Sugawara et a1. 313205 3,264,511 8/ 1966 Yamasaki.

1 OTHER REFERENCES 'Elwell, Atomic-Absorption spectrophotometry, 1962,

JAMES W. LAWRENCE, Primary Examiner RAYMOND F. HOSSFELD, Assistant Examiner U.S. Cl. X.R. 313--210, 216, 239

Disclaimer 3,476,970.Wallaee Gillies George K. Y am arsakz', and J. 6. Burger, Horseheads, N.Y. HOLLOW CATHODE ELECTRON DISCHARGE DEVICE FOR GENERATING SPECTRAL RADIATION. Patent dated Nov. 4, 1969. Disclaimer filed Sept. 12, 1972, by the assignee, Westinghouse Electric Uorpomtion. Hereby enters this disclaimer to claim 1 of said patent.

[Ofiiez'al Gazette January 16, 1973.]

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