US2622204A - Mass spectrograph - Google Patents

Description

Dec. 16, 1952 A. E. SHAW ETAL MASS SPECTROGRAPH Filed 0ct.'3l, 1946 4 Sheets-Sheet l INVENT0R5 Zl/ZZfFld fall BY yfie rz S/zaw Dec. 16, 1952 A. E. SHAW ETAL MASS SPECTROGRAPH 4 Sheets-Sheet 2 Filed Oct. 31, 1946 FaZZ BY 229376617"? 2:. 57%aa/ 2 via QM /J w i \fi \2 HHI H .rl mm QM RN QN Dec. 16, 1952 A. E. SHAW ETAL 2,622,204

MASS SPECTROGRAPH Filed Oct. 31, 1946 4 SheetsSheet 3 Dec. 16, 1952 A. a SHAW ETAL MASS SPECTROGRAPH 4 Sheets-Sheet 4 Filed Oct. 31, 1946 Patented Dec. 16, 1952 MASS SPECTROGRAPH Albert E. Shaw and Wilfrid Rall, Chicago, 111., assignors to the United States of America as represented by the United States Atomic Energy Commission Application October 31, 1946, Serial No. 706,880

7 Claims. (01. 250-413) thorium and other heavy metals in relatively short periods of time without necessitating unusual skill on the part of the operator.

A more specific object of the invention is to provide a unitary sub-assembly for collimating and electrostatically deflecting the ion beam. This assembly is operatively coupled to a removable ion source and 'a magnetic deflecting unit in such a manner that the combination of the collimating slits and the electrostatic condenser will produce parallelism of the ion beam upon entering the magnetic deflecting unit of-- the mass spectrograph.

A further object of the invention is to provide a unitary collimating and deflecting sub-assembly which is easily removable as a complete unit which can be accurately adjusted for parallelism 1 and alignment external to the spectrographic vacuum system.

A further object of the invention is to provide a photographic plate positioner unit for adjusting and operating the photographic plate in or- 'der to secure a plurality of successive exposures without removal of the photographic plate from the vacuum system.

A further and more specific object of the invention is to provide a removable ion source which will accommodate the rapid insertion of the sample to be analyzed, simple adjustment of a sample electrode, and the entire removal of the ion source from the mass spectrographic system.

The improved apparatus of the present invention includes various novel and useful structural features and combinations of those features which contribute to its successful operation. These various structural features and other objects and advantages of the invention will be made more apparent by reference to the accompanying drawings and the following description of a preferred embodiment thereof. In the drawings: v 1

Fig. 1 represents a plan view, partly in section, showing a mass spectrograph embodying the invention;

Fig. 2 is an enlarged fragmentary longitudinal cross sectional view of the removable ion source;

Fig. 3 is an enlarged fragmentary longitudinal cross sectional view, partly in section, of the unitary collimating and electrostatic deflecting assembly;

Fig. 4 is a transverse cross sectional view of the collimating condenser slit taken along the line 44 of Fig. 3;

Fig. 5 is a transverse cross sectional view of the anterior portion of the magnetic deflecting unit taken along the line 5-5 of Fig. 3;

Fig. 6 is an enlarged fragmentary plan view, partly in section, of the photographic plate positioner unit attached to the magnetic deflecting unit, as shown generally in Fig. 1; and

Fig. '7 is a vertical cross-sectional view, partly in elevation, of the photographic plate positioner unit, taken along the line of Fig. 6, of a mass spectrograph,

In general, the mass spectrograph of the present invention, as shown in Fig. 1, comprises an ion source I2, wherein ions are produced by a high frequency spark discharge developed between an outer electrode and a central electrode containing aspecimen of the material to be analyzed. The discharge occurring between these electrodes produces ions which'subsequently are accelerated to the cathode region where they are collimated. The ion source I2 is a selfcontained vacuum tight unit which is readily removable from the remainder of the mass spectrographic system so that a new specimen, to be analyzed, can be easily inserted into the mass spectrograph .in order to facilitate rapid determinations.

The ion source I2 is attached to a collimating and electrostatic deflecting unit, generally illustrated by numeral I4, by a vacuum tight connection. The accelerated ions developed by the ion source I 2 enter the collimating and electrostatic deflecting unit l4 and are then collimated and deflected by the associated components of said unit I4, so that the ion beam will be substantially parallel upon entering a magnetic deflecting unit, shown generally by numeral IS.

The associated ion beam is then deflected magnetically so as to impinge upon a recording means positioned adjacent to the magnetic deflecting unit I5 and hereinafter described in detail.

Referring to Fig. 2 the ion source I2 comprises .a central electrode 20 which may be a solid :specimen of "the sample -'to be analyzed or may nular face plate 28.

aperture 24 in an annular ring 2! and extends outwardly through a vacuum tight seal assembly, generally indicated by numeral 36 and hereinafter described in detail; The central electrode 20 is mounted within an ion source housing generally indicated by numeral [3 which consists of the vacuum tight assembly 36, a flexible bellows 34, the annular rin 21, an insulating cylindrical member 38, for example, a, heavy walled glass.

tube, and the annular face plate 28 wherein the insulating member 38 ismounted in conventional manneiby a plurality of insulating rod member's- 39 which are'attached to the platen and the ring 21. The rods 39 being preferably spaced approximately 120 around the longitudinal axis 7 The rod 23 is attached to a clamp nut 25' which is adjusted by a drive screw 21A so that an inward-outward travel is produced on the rod 23. The rod 23 extends'through the vacuum seal assembly 36, which comprises an annular cap-plate '49 and an annular disc 4!, v'vhich'threadsinto said plate 40. The disc 6| compresses upon an inner annular ring 44' which in turn compresses an annular resilient gasket 45, which conforms to the outer periphery of the rod 23, so as toafford a vacuum tight seal. The cap plate 43 is soldered to a flexible bellows 34, which in turn, is similarly attached to said annular ring 27 thereby making flexible connection therebetween. The flexible bellows 34 and the rod 23 are adjusted by means of an adjusting'screw dfi of which there are three and 120 apartwhich threads through said cap plate '40. Springs 41 which are three in number and 120 apart and 60 f-r'omsaid screws '46 are 'extended between said cap'plat'e 40' and said annular ring 2T to'afio'rd 'tensiodtherebetwe'en. An annular roove lfi' is" machined in the posterior face of theannular ring 21 wherein an annular resilient g'asket'dil'is embedded; and similarly a correspondin groove'5l is machined in the" anterior surface of the annular face plate 28 wherein-"an annular resilient gasket 53 is embedded so that the insulating tube 38 may be inserted and compressed'by drawing up on" the insulating menibers'39 by adjusting the take up screws 42. The aperture 26' is greater in diameter than the' outer diameter of the rod'23 wh'i'cnextends' through said aperture 26"s'o that the annular faceplate 28 is electrically insulated from the central electrode 20. The 'outer'electrode generallydesignated 30 comprises an annular anode 'cap 3| composed of material, such as stainless steel and having a central axial aperture 32. The cap3l is'held by means of friction to an outer telescopic shell 35- which slides upon an inner telescopic shell 35A which is attached centrally" to said an- At the posterior portion of the outer telescopic 'shell 35" and attached thereto, is' a disc insert 31A having a central opening therethrough'. Attached to said disc 31A are clamps 33, which in turn, hold a metallic insert 31 formed of material having'a high melting point, such as tantalum or tungsten, in close proximity" with the central electrode 20. The

shell 35 and the structure supported thereby constitutezan outer electrode 30' which is insulated 4 from the central electrode 20 so that a discharge occurring between the electrodes produces ions which subsequently are accelerated to the cathode region.

As will be noted in Figs. 2 and 3, the plate 28 is connected to a ring 58 by one or more electrically insulating rods 50; Thus, by disconnecting the rods 50 from the plate 28, the entire ion source may be removed as a unit from the device. A tubular insulating member 55 of any suitable material, such as glass, is recessed in the plates 28 and 58 and is afforded vacuum tight connections 'ther'ewith as by annular gaskets 55 and BI respectively. The collimating and electrostatic deflecting unit [4 comprises a collimating imageslit indicated by numeral 64 and a collimating condenser slit 10, as well as an electrostatic condenser I9, and a parallel collimating slit 90 which are mounted in said sequence so as "to comprise a sub-assembly defined as aunitary electrically conducting material, such as stainless steel, said cap 63 being mounted on an outer shell or tube'68." The shell68 is slidably mounted on an inner shell 69 to accommodate adjustment of the'spacing between the slit 64 and the electrode 33. It'may be noted, that the shells-38 and 69- and the members 63 and 61 function as a cathode generally designated-60. Theinner telescopic shell 69" is an integral part of a tube 1!, having a flange 12' connected tothe annular ring 58 so as to'aliord a rigid support for the unitary collimating and electrostatic deflecting assembly. The condenser-slit m is shown slightly enlarged in Fig. 4' and is machined centrally in'a metallic plate 13, for example, brass, and: said plate 13 is 'mounted' on the tube TI. Attached rigidly to the inner surface of the tube H by means of a plurality ofscrews 74' is an electricallyinsulatin -mounting l5. 7

The condenser generally designated by numeral 19 comprises: spaced-j plates 83' and 32; formed of any suitable material such as brass, said plates being preferablyin the formof segments 6f concentric circles to define at path 8| therebetween in the form 'of' a segment of a circle. Thus the plate is disposed radially outwardly of the plate- 82 from the'common center of av curvature thereof. The path 8! preferably has a mean radius of 15.0 cm., a subtended angle of 31 50, and a width ofapproximately 0.25 inch..- The plates 89 and 82 areattached'to the insulating mounting 15 by a plurality of mounting screws.

The collimating slit 90* is disposed in an annular disc 9| which is positioned at the posterior end of said mounting. I5 and arfixed thereto. Po-

sitioned slightly beyond and posteriorlyto' said for 'the condenser 19 which is provided with spring clips Sis-engaging the'contacts 91 and making electrically conducting connections to the-condenser "plates illland82. Each of the contacts '91 is supported from the -tube 93' by an electrically insulated vacuum seal 95 having a vacuum-tight connection to the tube 93. A similar contact I02 engages a spring slip IOI attached to an insulated lug I on the disc 9| to afford an electrical connection to the collector 94.

It may be noted, that the structure comprising the image slit 64, the condenser slit II, the condenser 19 and the parallel slit 90 constitute a unit or sub-assembly which can be simply removed from the mass spectrograph. This affords an efficient method to adjust and establish the correct interrelationship and alignment between the slits 64, I0 and 90 and the condenser I9 by optically projecting a light beam through the image slit 64 and visually correcting the internal alignment of these component members by the alignment of the exit light beam.

The condenser housing 93 is attached to the magnetic housing H by means of a plurality cap screws I I 0, preferably four in number, and a gasket 83 being provided to afford a vacuum-tight seal.

As shown in Figure 5, the housing I'I comprises upper and lower magnetic pole pieces I I2 and I I3 and spacers I I4 of non-magnetic material interposed therebetween, the spacers II4 being preferably about 0.125 inch thick. Thus, the pole pieces 2 and I I3 define a magnetic gap aperture or path II5 therebetween wherein the ions are deflected by a magnetic field.

At the posterior end of said magnetic deflecting unit I5 is a recording means in the form of a photographic plate positioner unit I20 for recording the ion beam which is deflected and impinges upon a photographic plate I2l which is mounted adjacent to said aperture I I5 in an evacuated channel I30, as shown in Figs. 6 and 7. The photographic plate I2I which has a photographic emulsion thereon is cradled in a nonmagnetic photographic plate holder I22. Operatively connected to said holder I22 is a nonmagnetic rack I24, which engages a non-mag netic pinion I26 so that the rotation of the pinion I26 will afford an upward and downward motion to the photographic plate I2I so that a plurality of exposures on a single plate may be obtained.

The pinion I26 is connected to a rod I28 which extends through an opening I32 in a non-magnetic side wall I 34 integrally formed with the associated spacer H4 and through a central aperture in a face plate I36 which is attached to the side wall I34. An annular ring disc I38 having an opening accommodating the rod I28 is threaded into the outer portion of the said face plate I36 and presses upon an annular ring I40 which, in turn, cups an annular resilient gasket I 42; for example, rubber with sufficient compression so that the resilient gasket I42 will bear upon the outer periphery of the rod I28 to form a vacuum tight rotary seal. A rotatable knob I39 which is aflixed to said rod I28 by means of a set screw MI is operable to rotate the pinion I26. The rotation of the knob I39 may be calibrated for a plurality of successive predeterminedpositions by means of indentations I43, spaced on the inner surface of the knob I39, said indentations I43 being adapted to engage a conventional ball and spring assembly I45.

The photographic plate I2l may be removed from the evacuated system by opening a conventional gate lock I46 (Figs. 1 and 6) which bears upon an annular resilient gasket I48, for example rubber so as to afford a vacuum seal for said channel I30. A vacuum line I50 is provided for evacuating the device to any desired value.

In the operation of the mass spectograph, ions are produced by a discharge between the outer electrode and the central electrode 20 and said ions are subsequently accelerated to the cathode 60 where they are collimated upon entering the image slit B4.

The ion source unit is so designed that the outer electrode 30 and the central electrode 20 are electrically insulated from each other by the insulating member 38 and 39. This feature makes it unnecessary to provide the conventional tungsten seals through the walls of a vacuum envelope, and in addition, provides the mass spectrograph system with an ion source unit I2 which can be easily assembled and disassembled from the balance of the mass spectrograph in order that new samples to be analyzed may be inserted rapidly and efliciently.

The accelerated ion beam then is directed toward the collimating condenser slit I0 where said beam is collimated and then enters the electrostatic deflecting field which is developed across the condenser plates 80 and 82. The ion beam is electrostatically deflected and directed toward the parallel collimating slit '90 which tends to produce parallelism 0n the ion beam prior to em tering the magnetic deflecting unit I5.

The ion beam is then deflected by the homogeneous magnetic field developed between said pole pieces H2 and H3, wherein said magnetic field, in this particular instance, is developed by energizing said magnetic pole pieces H2 and H3 by the use of a conventional electromagnet (not shown). If desired, the magnetic pole pieces I I2 and [I3 may be constructed of materials which are considered as permanent magnets, such as a cobalt, nickel, and aluminum alloy (Alnico V), thereby eliminating the associated electromagnet.

In operation, it has been found that the unitary collimating-electrostatic deflecting sub-assembly has provided a means for adjusting the critical inter-relationship of the electrostatic condenser I9 and the collimating slit system; and the alignment of these components, external to the vacuum system by optical methods, affords a rapid method for adjusting the alignment and also affords a simple method for removal of the assembly in order to clean the surfaces from contaminants produced by past exposures.

It should also be noted that we have provided a mass spectrograph that is simple in design and construction, and has a substantially high resolving power for the determination of the isotope abundance ratios of solid materials having heavy atomic Weights, such as uranium and thorium.

It will be apparent that modifications will be readily suggested to others skilled in the art as the result of the teachings of our invention, hence, it should be understood that our invention is not limited by specific construction hereinbefore described, which is an exemplary embodiment, and should be restricted only insofar asset forth in the following claims.

What is claimed is:

1. A unitary sub-assembly for a mass spectrograph comprising a supporting element, spaced entrant and exit collimating slits mounted on the supporting element, an intermediate condenser collimating slit mounted on the supporting element, and spaced condenser plates mounted on the supporting element and insulated therefrom defining a path between the plates, said path extending between said condenser and said slits, and contact springs having electrically conducting connections to the remote sides of saidf condenser plates and: adapted to engageaassociated: support contacts.

2. A unitary collimating and electrostatic deflecting assembly comprising ashell with spaced aligned slits at opposite ends thereof, an insulating support member rigidly attached to said shell, an element comprising another slit said slit also being supported by said member, and a condenser comprising spaced condenser plates mounted on said member between said shell and said other slit, said plates defining an arcuate deflecting ion path between said other slit and adjacent to the first mentioned slit.

3. A device of the class described, comprising a housing having an orifice therein wherein a unitary collimating and electrostatic deflecting assembly is mounted, said assembly comprising a shell with spaced aligned slits at opposite ends thereof, an insulating support member rigidly attached to said shell, an element comprising another slit Supported by said member, and a condenser comprising spaced condenser plates mounted on said member between said shell and said other slit, said plates defining an arcuate deflecting ion path between said other slit and adjacent to the first mentioned slit, and mounting means attached to said housing andengageable and disengageable with the co'llima-ting and deflectingassembly to afford support'therefor, whereby the assembly may be removed from the housing as a unit.

4. A device of the class described comprising a housing having an orificetherein, and unitary colli-mating and electrostatic deflecting assembly sealed within the orifice, said assembly comprising a shell with spaced aligned slits at opposite ends thereof, an insulating support member rigid- 1y attached to said shell, an element comprising another. slit supported by said member and a condenser comprising spaced condenser plates mounted on said member between said shell and said other slit, said plates defining an arcuate defleeting ion path between said other slit and the adjacent of the first mentioned slit, and disengageable mounting means on the. housing adjacent to the orifice for sealing the assembly within the orifice.

5. A sub-assembly for a. massspectrometer comprising, in combination, a source ofzionsof diverse energies, a housing having an orifice therein, and a unitary collimating and electrostatic deflecting assembly, the ion source being attached'to the assembly, said assembly including a shell with spaced aligned slits at opposite ends thereof, a support member rigidly afiixed to the shell, an element having another slit mounted upon the support member, and a condenser comprising spaced condenser plates mounted on the support member between theshell and the other slit, said plates defining an arcuate deflecting ion path between the other slit and the contiguous slit in the shell, and means for removablymounting said assembly within the orifice in the housing, said means including means to secure the assembly to the housing.

6. A sub-assembly for a massspectrometer comprising the elements of claim 5 in combination with contact springs mounted on the remote sides of the condenser plates, and support contacts mounted on the housing in snug engagement with said springs.

7. A mass spectrometer comprising the elements of claim 5 in combination with means to record the ion currents attached to the housing adjacent to the unitary collimating and deflecting assembly.

' ALBERT E. SHAW.

WILFRID RALL.

REFERENCES CITED The following references are of recordin the file of this patent:

UNITED STATES. PATENTS Number Name Date 1,809,115 Goddard June 9, 1931 2,323,148 McLaughlin. June 29, 1943 2,341,551 Hoover Feb. 15, 1944 2,425,833 Runge Aug. 19, 1947 OTHER REFERENCES Bleakney American Physics Teacher, Feb. 1936, vol. 4, pages 12-18.

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