US3423584A

US3423584A - Spectrometer ion source having two filaments each alternately acting as emitter and collector

Info

Publication number: US3423584A
Application number: US536921A
Authority: US
Inventors: Raymond A Erickson
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1966-03-23
Filing date: 1966-03-23
Publication date: 1969-01-21
Anticipated expiration: 1986-01-21
Also published as: GB1152014A; DE1673257A1

Description

Jan. 21, 1969 MASS SPECTROMETER ION SOURCE HAVING TWO FILAMENTS R. A. ERICKSON 3,423,584

EACH ALTERNATELY ACTING AS EMITTER AND COLLECTOR l? 1' T I I Filed March 23. 1966 Sheet l of 5 (I0 t l1 1 fi PUMP 2 :2 r9 H GAS I SOURCE v 3 J '5 AMPLIFIER S g x RECORDER (1- E] I "X SCAN L GENERATOR P 1 M 7 7 om ucal INVENTOR.

RNEY V Jan. 21, 1969 MASS SPECTROMETER ION SOURCE HAVING TWO FILAMENTS EACH ALTERNATELY ACTING AS EMITTER AND COLLECTOR Filed March 23, 1966 R. A. ERICKSON Sheet 3 of S FIG.5

2s &44 m 44 46 \I:;::/- E46 o E as 51 VOLTAGE SUPPLY 5 INVENTOR. VARIABLE RAYMOND A. ERICKSON VOLTAGE SUPPLY United States Patent 9 6 Claims Int. 01. 801d 59/44;H01j 39/34 ABSTRACT OF THE DISCLOSURE A cycloidal mass spectrometer is disclosed. The spectrometer includes an ion source. The ion source has dual electronic emitters providing alternative sources for producing an electron beam for ionizing gas within the ion source. Each of the electron emitters includes an associated collector electrode structure operated at the same potential as the electron emitter. A switching network is provided for alternatively switching into service either one of the electron emitters. The switching network includes means for operating either one of the emitters as the active source of electrons and the other emitter and its collector electrode as the electron collector electrode for the active emitter, whereby the operating conditions in the ion source are not substantially altered by failure of one of the emitters and a subsequent shift to use of the other emitter.

The present invention relates in general to ion sources for mass spectrometers and, more particularly, to an improved ion source having plural alternative sources for producing the ionizing electron beam of the ion source, whereby upon failure of one of the electron emitters the alternative emitter may be put into service to reduce down time of the spectrometer required for filament replacement.

Heretofore ion sources for cycloidal mass spectrometers have employed only one electron emitter for supplying the beam of ionizing electrons. Typically this emitter must operate in a relatively hostile environment inasmuch as the thermionic emitter is open to the gas being ionized and as a consequence the gas may readily attack the emitter making it brittle leading to its early demise. Generally the emitter is made of a material such as rheniurn which resists embrittlement but is subject to removal of metal by ion bombardment. Lifetimes of such rhenium emitters have been found in practice to vary widely from a few hours to months. Once the emitter fails the mass spectrometer must be opened to the atmosphere for filament replacement and then reassembled, baked out, and evacuated to its operating pressure range as of 10- torr. At best this filament replacement process is time consuming, requiring a down time of half a day or more.

In the present invention a dual filament is provided with associated switching networks for switching operation to a reserve one of the filaments such that operation may be quickly resumed without the necessity of replacing the burned out filament until failure of the reserve filament. Using the improved dual filament ion source of the present invention average down time due to filament failure is at least halved.

The principal object of the present invention is the provision of an improved ion source for mass spectrometers, such ion source being especially useful in cycloidal mass spectrometers.

One feature of the present invention is the provision of a reserve emitter in an ion source, whereby upon failure Patented Jan. 21, 1969 ice of the primary emitter operation may be switched to the reserve emitter and operation of the ion source resumed.

Another feature of the present invention is the same as the preceding wherein the primary and reserve emitters are located at opposite ends of the electron beam path such that one emitter assembly serves as the beam collector assembly for the other emitter, whereby placement of the emitter assemblies in the ion source is facilitated.

Another feature of the present invention is the same as any one or more of the preceding wherein the emitter assemblies include an additional electron collecting member to facilitate their dual functioning as collecting electrodes.

Another feature of the present invention is the same as any one or more of the preceding wherein the emitters include an associated switching network for shorting together the terminals of the collecting emitter assembly to assure its proper operation as a collector in the event it is open circuited as by fracture or burn out.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a fragmentary side view, partly schematic, of a cycloidal mass spectrometer system using the features of the present invention,

FIG. 2 is a partial circuit diagram for the source of potentials applied to the analyzing electrode array of FIG. 1,

FIG. 3 is a sectional view through the ion source of FIG. 1 taken along line 3-3 in the direction of the arrows,

FIG. 4 is a fragmentary sectional view of a portion of the structure of FIG. 3 taken along line 44 in the direction of the arrows, and

FIG. 5 is a circuit diagram for control of the ionizing electron beam current and switching network for the dual filaments of the ion source of FIGS. 3 and 4.

Referring now to FIG. 1 there is shown a cycloidal mass spectrometer system. More particularly, an array of generally rectangular shaped ring electrodes 1 are insulatively supported within a thin rectangular vacuum envelope 2, only partially shown, from a heavy rectangular flange, not shown, which closes off one end of the vacuum envelope.

The separate rings 1 of the electrode array are operated at slightly different electric potentials derived from a voltage source 3 via leads 4 connected at nodes 5 of a voltage divider network 6. The different potentials applied to the different rings 1 establish a region of uniform electric field E in the hollow interior of the ring electrode array. The electric field E is directed parallel to the line of development of the ring electrode array.

The electrode array is immersed in a uniform region of magnetic field H directed at right angles to the direction of the electric field E. The field H is conveniently produced by an electromagnet 7 with the vacuum envelope 2 being disposed in the narrow gap defined between a pair of pole pieces 8 of the magnet 7.

The envelope 2 is evacuated in use via pump 10 to a suitably low pressure as of 10- torr. Gas to be analyzed by the analyzer section, including the array of electrodes 1, is introduced from a source 9 into the analyzer section through the vacuum envelope 2 via an inlet tubing 11 as of stainless steel. The inlet tubing 11 feeds gas at a desired rate into an ion source 12. The ion source ionizes the gas and projects it through a slot into the crossed magnetic field H and electric field E of the analyzer.

Under the influence of the crossed electric and magnetic fields the ions are caused to execute cycloidal trajectories. However, only ions of a certain mass number, for

a given intensity of E and H, will be focused at a detector slot 13 a certain focal distance from the source and at the same electric potential. An ion detector 14 is positioned behind the slot 13 to produce an output corresponding to number of ions under analysis having the certain predetermined focused mass number, if any.

The output is fed to an amplifier 15 which amplifies the detected signal and feeds it to the Y axis of an X-Y recorder 16 wherein it is recorded as a function of a scan of the magnetic field intensity H produced by a scan generator 17. The output of the recorder 16 is a mass spectrum of the sample under analysis.

Referring now to FIGS. 3 and 4 there is shown in greater detail the ion source 12. The ion source 12 comprises a substantially closed cylindrical metallic ionizing chamber 21 as of rhodium plated stainless steel. The chamber 21 is transversely segmented into three

separate electrodes

22, 23 and 24 separated by sheets 25 of insulation as of 0.005" thick mica such that the electrodes 2224 may be operated at independent potentials in use. One end wall 26 of the chamber 21 is centrally apertured at 27 to provide a gas inlet passageway in gas communication with an insulating end segment 11' of the gas inlet pipe 11 for introducing gas to be analyzed into the ionizing chamber 21. The other end wall 28 of the ionizing chamber 21 is provided with a large diameter axial bore 29 which is closed over by a pair of knife edge plates 31. The knife edge portions of the plates 31 are slightly spaced apart to define an ion exit slit 32 as of 0.0004" wide and having a length equal to the diameter of the bore 29. The exit slit 32 is elongated in a direction parallel to the direction of the magnetic field H which is present in the ion chamber 21.

The center electrode 25 of the ion chamber 21 is apertured on opposite sides by a pair of

cylindrical bores

33 and 33 with the axes of the bores being coaxial with the direction of the magnetic field H. A first electron emitter assembly 34 is outside of the bore 33 for projecting a stream of electrons across the chamber 21 and through the oposite bore 33'. The beam is focused by the magnetic field H. The beam of electrons serves to ionize gas in its path. Ions produced are swept from the ionizing region through the ion beam exit slit 32 by potentials applied to the electrodes 22-24 to form an ion beam 35 for use in the cycloidal mass spectrometer. The three electrode ion source 12 forms the subject matter of and is claimed in copending U.S. application No. 534,857 filed Mar. 16 1966 and assigned to the same assignee as the present invention.

A second identical electron emitter assembly 36 is positioned at the end of the beam path 37 to serve as the beam collector for the first emitter assembly 34 when the first emitter 34 is in use. The emitter assemblies each include a precision machined block 38 of insulating material as of Mycalex, manufactured and marketed by Mycalex Corporation of America. The insulating block 38 has a tapped transverse bore 39 to receive a long machine screw 41 passing through a slot 42 in the edge of electrode 22. The screw includes a washer 43 and when threaded into the block 38 and tightened down serves to clamp the block 38 and its emitter assembly to the electrode 22.

The emitter assembly includes a filamentary emitter 44 as of 7 mil rhenium wire directed transversely of the axis of the ionizing chamber 21 and supported between two conductive posts 45 conveniently formed by a pair of machine screws threaded into tapped bores in the insulating block 38. An auxiliary collecting electrode 46 as of molybdenum 0.005" thick and .120" long and 0.080" wide is electrically conductively fixed at one end to one of the emitter support posts 45. The auxiliary collector electrode 46 lies behind the emiter 44 and serves to col lect electrons of the beam missed by its associated emitter wire 44 when the emitter assembly is used as a collector for its companion emitter assembly 36. Operating potentials are supplied to the

emitter assemblies

34 and 36 via posts 45 as more fully described below with regard to FIG. 5.

Operating potentials for the three

ionizing chamber electrodes

22, 23 and 24 are derived from a variable voltage power supply 47. When the ion source 12 is operated for a positive ion beam the repeller electrode 22 is operated at a positive potential as of 160200 volts derived from voltage supply 47 relative to the beam exit electrode 24 preferably operating at ground potential. The central electrode 23 is operated at a potential midway between the potentials applied to the repeller and exit

electrodes

22 and 24, respectively. The potential for the central electrode is derived from a centertapped voltage divider network 48 connected across the voltage supply 47. To operate the source 12 as a negative ion source the terminals of the voltage supply 47 are reversed and the E and H fields produced in the cycloidal analyzer section are reversed.

Referring now to FIG. 5 there is shown the electrical circuit for the ionizing electron beam including its current control and alternative switching features. One of the filaments 44 serves as the emitter and the opposed filament and auxiliary collector electrode serve as the collector. By suitable switching, to be described, the functions of the two emitter assemblies are reversed, as desired. More specifically, A.C. filament current at a suitable low voltage as of 6 volts is derived from one winding 51 of a magnetic amplifier 50 driven from a suitable source 52 via a second winding 53. The filament power is fed to a first double pole double throw switch 54, which when closed to the right, as shown in the drawing, energizes the left filament assembly 36.

The anode voltage for the emitter assembly 36 is derived from a variable voltage supply 55, variable from 5 to volts, connected between the filament 44 and the center electrode 23 serving as the anode. Under the influence of the anode voltage and as focused by the magnetic field H the electrons form a ribbon beam about 0.040 wide and 0.007" thick which passes across the ionizing chamber 21 to the opposed collector-emitter assembly 34. The opposed emitter assembly 34, at the terminal end of the beam, is connected to the terminal of a second double pole double throw switch 56 which is ganged to the first switch 54 via a mechanical linkage 57. Switch 56 has its terminals shorted together such that when closed to the right both ends of the filament 44 are shorted together and operated at the same potential as the associated auxiliary collector electrode46.

The collecting emitter assembly 34 is preferably operated at a potential more positive than the anode electrode 23 to assure complete collection of the beam on the filament 44 and its auxiliary collector 46. Accordingly, a voltage supply 58 as of +20 volts is connected between the shorting switch 56 and the anode electrode 23.

It is desirable to measure and control the electron beam current as this determines the ion beam intensity. Therefore in the circuit between the emitter shorting switch 56 and the anode electrode 23 there is provided a current meter 59 for measuring the beam current and an adjustable gain current amplifier 61, the output of which is fed to a control winding 62 on the magnetic amplifier 50. The beam current as measured by meter 59 is adjusted to a desired level by adjusting the gain of the amplifier 61. Increasing the gain of the amplifier 61 produces more current in the control winding 62 which produces increased saturation of the core of the magnetic amplifier causing the filament power to be reduced thereby decreasing the electron beam current.

Upon failure of the primary emitter assembly 36 ganged

switches

54 and 56 are merely switched thereby reversing the functions of the

emitter assemblies

36 and 34. Even if the filament 44, which is serving as the collector, has open circuited as by burning out or fracture the assembly still functions normally as a beam collector. This is because of the provision of the auxiliary collector electrode 46 and because both ends of the filament 44 are shorted together to the collector electrode 46 via switch 56.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In an ion source apparatus, means containing a region of space within which gaseous substances are ionized via bombardment with an electron beam, means for accelerating ions produced in the ionizing region through a beam defining opening to produce an ion beam, first electron emitter means for forming and projecting a beam of electrons through the ionizing region of said containing means for ionizing the gas therein, second electron emitter means, alternative to said first emitter means, for forming and projecting a beam of electrons through the ionizing region of said containing means for ionizing the gas therein, whereby upon failure of said first emitter means said second emitter means may be energized to sustain operation of the ion source, the improvement comprising, means for connecting said second emitter means in circuit with said first emitter means at a potential positive with respect to said first emitter means to cause said second emitter means to serve as a beam collector structure for said first emitter means when said first emitter means is serving as the source of electrons for the ionizing electron beam.

2. The apparatus according to claim 1 wherein at least said first emitter means includes a filamentary thermionic emitter and an associated collector electrode electrically connected to said filamentary emitter for collecting the electron beam from said second emitter means, whereby enhanced collector action of said first emitter means is facilitated.

3. The apparatus according to claim 1 wherein at least said first emitter means includes a filamentary emitter, and means forming an electrical switching network connected to said first filamentary emitter for electrically shorting together the ends thereof for operation as a collector electrode for said second emitter means when said second emitter means is put in operation.

4. The apparatus according to claim 1 in combination with a cycloidal mass spectrometer means operable upon the ion beam emerging from the ion source for separating the ions according to their mass and providing an output variable in accordance with the mass number of the separated ions.

5. The apparatus according to claim 4 wherein said containing means comprises a chamber having a pair of end walls and an intervening side wall portion encircling the ionizing region, means for operating said side and end walls at independent operating electrical potentials with said intervening encircling side wall portion operating at a potential intermediate the potentials applied to said end walls, and said emitter means being positioned to direct their electron beams across said intervening encircling wall from one side wall portion thereof to the opposed side wall portion thereof.

6. The apparatus according to claim 5 wherein said first and second emitter means are located at opposite ends of the electron beam path across said intervening side wall portion of said chamber.

References Cited UNITED STATES PATENTS 2,975,277 3/ 1961 Von Ardenne. 2,977,470 3/ 1961' Robinson 250-419 3,265,890 8/ 1966 Briggs 250-419 RALPH G. NILSON, Primary Examiner.

S. C. SHEAR, Assistant Examiner.

US. Cl. X.R.