US3182190A

US3182190A - Magnetic field free ion source with adjustable electron gun

Info

Publication number: US3182190A
Application number: US213823A
Authority: US
Inventors: Jr Arnys C Lilly; Emmett B Shutes; Theodore J Weismann
Original assignee: Gulf Research and Development Co
Current assignee: Gulf Research and Development Co
Priority date: 1962-07-31
Filing date: 1962-07-31
Publication date: 1965-05-04
Anticipated expiration: 1982-05-04

Description

May 4, 1965 A. c. LILLY, JR., ETAL MAGNETIC FIELD FREE ION SOURCE WITH ADJUSTABLE ELECTRON GUN Filed July 31, 1962 7 Sheets-Sheet INVENTORS ARA/Y5 C. L/LLY, JQ. EMA/Err B. SHUTES BY THEODORE J I'VE/SHAW ATI'OkA/EV w" May 4",1965 A. c. LILLY, JR., ETAL 3,132,190

MAGNETIC FIELD FREE ION SOURCE WITH ADJUSTABLE ELECTRON GUN Filed July 31, 1962 7 Sheets-Sheet 2 INVENTORJ 2%,; "5% By rye-owes 1/. WasMw/v 47" r ORA E Y May 4, 1965 A. c. LILLY, JR., ETAL 3,182,190

MAGNETIC FIELD FREE ION SOURCE WITH ADJUSTABLE ELECTRON GUN Filed July 31, 1962 7 Sheets-Sheet 5 INVBNTORS ARM s c: LILLY. JR. EMA/E77 a. .sflurss 71/500025 J. M/SMA/V/V- W nrroglvsy Filed July 31, 1962 May 4, 1965 A. c. LILLY, JR.. ETAL 8 MAGNETIC FIELD FREE ION SOURCE WITH ADJUSTABLE ELECTRON GUN 7 Sheets$heet 4 www w. 3 1-3-33- "5 IQ N F En)(; Q 2e W- a g I R (Q Q '1' N 3 N $1 m R "B 0 miz I Q & Q

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\ EMA/E77 a. .sHZ/rEs BY 71150004 5 .1 wE/smN/v I I k ITF'OR/VEY y ,1965 A. c. LILLY, JR., EM 1 3,182,190

MAGNETIC FIELD FREE ION SOURCE WITH ADJUSTABLE ELECTRON GUN Filed July 31, 1962 7 Sheets-Sheet 5 .47 7' OIP/VE Y May 4, 1965 v A. c. LILLY, JR., ETAL MAGNETIC FIELD FREE ION SOURCE WITH ADJUSTABLE ELECTRON GUN Filed July 31, 1962 7 Sheets-Sheet INVENTORS C. L ILL Y JQ.

BY THEODORE J WE/SMA/VN ATTOR/VEV y 1965 A. c. LILLY, JR., ETAL 3,182,190

MAGNETIC FIELD FREE ION SOURCE WITH ADJUSTABLE ELECTRON GUN Filed July 31, 1962 '7 Sheets-Sheet 7 i i t u g ATTORNEY United States Patent 0 M 3,182,190 GNETIU FEEED FREE HUN SUURCE Wl'H-l ADJUSTABLE ELECTRUN GUN Arnys C. Lilly, Jrn, Penn Hills Township, Allegheny County, Emmett E.Shutes, Wilhins Township, Aliegheny County, and Theodore J. Weismann, Ross Township, Allegheny County, Pa, assignors to Gulf Research & Development Company, Pittsburgh, Pa., 2 corporation of Delaware Filed July 31, 1962, Ser. No. 213,823 5 Claims. (Cl. 25-4l.9)

This invention relates to a source of positive ions and in particular relates to a highly efiicient source of substan tially monoenerget-ic positive ions such as may be employed in a mass spectrometer, calutron, or the like.

In the operation of high-vacuum devices that require operations to be performed on material in the form of positive ions, it is necessaryto provide some means for ionizing the material to belstudied or analyzed. Thus for example, in a calutron, which is a means for separating an element into its isotopic constituents, it is necessary to ionize the material and the isotopic ions are subsequently separated by magnetic and/or electric fields. Similarly in a mass spectrometer the material being studied must be ionized and supplied to the analyzing system of the mass spectrometer in order to study the mass/ charge ratio of the various ions formed by the material being analyzed. An inherent diificulty arises in that the sample material must be in a vapor or gaseous state when it enters the apparatus just prior to ionization, but upon entering the apparatus the sample encounters a high vacuum. The material will then quicldy disperse, and it is evident that the ionization process must take place within the limited space and time interval between a when the material enters the apparatus and its dispersion due to the vacuum encountered. ize materials for devices such as mass spectrometers and calutrons by means of an are or by impact with thermally emitted electrons. eters the electron stream is produced by thermionic emission from a filament and subsequently accelerated by an electric field.

In ionizing a vapor in a mass spectrometer by means of an electron beam it is essential that the ions be withdrawn from the ion chamber substantially immediately after the ions are produced. The vapor pressure in the ion chamber must necessarily be sufficiently high to prowide appreciable density of the material to be ionized, and unless the ions formed are immediately extracted from the ion chamber, the ions may become neutralized by collisions particularly with the chamber Walls. Furthermore it is desirable to ionize as much as possible of a the vapor leaving the ion chamber since any un-ionized vapor will find its way into the deflecting system which is rnore desirably maintained at a lower pressure in order to avoid dispersion of the ion beam by collisions with un-ionized gases.

It is the purpose of this invention to provide a highly eflic'ient ion source that produces ions of substantially uniform energy.

it is another object of this invention to provide an ion source that produces an ion beam of optimum intensity by ionization of material in the vapor state by means of an electron stream.

It is a further object of this invention to provide an ion source in which ionization of the material occurs substantially immediately prior to exit of the ionized material from the ion chamber.

It is a further object of this invention to provide an ion source in which the ionizing electron stream is geometrically adjustable to provide maximum ionizing eifect with More commonly in mass spectrom- It is customary to ion- I insane Patented May 4, 1965 minimum escape of un-ionized material into the apparatus to which the ion source is connected.

In an ion source in which ionization is effected by an electron stream there is conventionally provided a thermionic filament together with a potential gradient for accelerating the electrons emitted from the filament. In conventional devices of this type a broad diffuse stream of electrons is employed and the energy spread of these electrons may be substantial. This results in the production of ions over a diffuse region of the ion chamber, with the result that many of the ions are lost by neutralization prior to leaving the ion chamber and those useful ions that are extracted from the ion chamber usually have substantially diiicring energies. In the subsequent analysis of the ions so produced the energy spread results in a dhfused focus thereby materially reducing the resolution of the apparatus. By employing the present invention the ions are produced within a small well-defined region immediately before exit from the ion chamber whereby the energy spread of the ions extracted from the ion chamber is substantially reduced. A high degree of efficiency together with a high degree of resolution is thereby attained. In this invention the ionizing electron stream is formed into a penciliform beam of high electron density and means are provided for applying the penciliform electron stream to the region of the ion chamber proximately inside the ion chamber exit opening. Provision is made so that this condition can be achieved under various potentials that may be applied to the respective elements of the apparatus. i

These and other objects of this invention are obtained by the apparatus described in this specification of which the drawings form a part and in which i FIGURE 1 is a top view (partly in section) ofthe ion source of this invention;

FIGURE 2 is an enlarged top View or the electron gun employed in the ion source of this invention;

FIGURE 3 is a front elevation of the electron gun portion of FIGURE 2; 7

FIGURE 4 is a partial rear elevation of the electron gun as indicated by the numerals T.V-IV of FIGURE 2;

FIGURE 5 is a partial longitudinal section of the electron gun as indicated by the numerals VV of FIG URE 2;

FIGURE 6 is a transverse section through the electron gun taken at the plane Vii-VI of FIGURE 3;

FIGURE 7 is a transverse section through the electron gun taken at the plane VII-VII of FIGURE 3;

FIGURE 8 is an end view of the electron gun as indicated by the numerals VIII-VIII of FIGURE 3;

FIGURE 9 is a front elevation of the ion chamber and ion-accelerating portion of this invention;

FIGURE 10 is a partial rear elevation of the ion chamher as indicated by the numerals XX of FIGURE 1;

FIGURE ll is an end View of the ion source as indicated by the numerals XI-XI of FIGURE 9;

FIGURE 12 is a section through the ion source taken at the plane XII-XII of FIGURE 9;

FIGURE 13 is an end view of the ion source as indicated by the numerals XIIIXIH of FIGURE 9;

FlGURE 14 is a longitudinal section through the ion source taken at the plane X1VXlV of FIGURE 9; and

FIGURE 15 is a diagram showing an example of electrical potentials that may be applied to the several elements of the apparatus.

In the ion source of this invention there is provided an ion chamber to which the sample material to be analyzed is introduced in vapor form. The ion chamber is provided with an exit opening from which ions are extracted by means of an appropriately applied electric field, and the extracted ions are subsequently accelerated and focused into a penciliform ion beam. The ion chamber is also provided with two openings whose axis is transverse to that of the ion exit opening, and into one of these openings a stream of accelerated electrons is projected by an electron gun, the electrons being collected by a trap at the diametrically opposite opening. The electron gun that supplies the ionizing electron stream comprises a thermionic filament and adjustable electron accelerating and focusing means. The entire electron-gun structure is pivoted on an axis substantially perpendicular to the plane defined by the axis of the electron beam and the axis of the ion beam, whereby it becomes possible to change the geometrical configuration between the electron beam and the ion exit opening thereby to provide mechanical adjustment to compensate for undetermined transverse deflections of the electron beam. These transverse deflections of the electron beam result from changes in the potentials on the electron accelerating system and from changes in the ion-extracting field that is present in the ion chamber. By means of this invention it is possible to adjust the geometrical position of the electron beam to obtain optimum ionizing effect under various conditions of electron energy as may be required to ionize various materials, and under various ion extracting potentials applied to the ion chamber as may be required to extract ions of various masses from the ion chamber. The invention thus attains control of the energy characteristics and intensity of the ion beam by means of the aiming of the electron gun.

Referring first to FIGURE 1, there is shown a top view of the ion source of this invention with the outer case removed. It is to be understood that the entire apparatus herein described is part of an evacuated system and therefore requires a conventional gas-tight outer envelope which may also serve as a housing. It has been found convenient to make this housing from a single block of a well-known vacuum-tight metal and by way of example the alloy Inconel has been found satisfactory for this purpose. The block of material forming the housing is conveniently attached and sealed to other parts of the apparatus by means of appropriate flanges which may be appropriately gasketed and bolted together or by means of other conventional devices such as glass-to-metal seals or the like. Many electrical leads must be brought out from the ion source of this invention and these leads are conveniently brought out through conventional multiterminal glass-to-metal seals mounted on plates which are bolted to flanges on the envelope as is entirely conventional. Inasmuch as the housing and envelope and electrical-lead seals do not form part of this invention they are not shown in detail in the several figures and further description of such conventional features will not be made. The inner wall of the housing is merely indicated diagrammatically in FIGURE 1 by the outline 1.

The invention as shown in FIGURE 1 is comprised of two housing portions indicated respectively by the brackets 2 and 3, the portions 2 and 3 being substantially at right angles to each other. The portion 2 houses an electron gun 4 which projects a stream of electrons into the ion chamber 6 located substantially at the head of an ion-accelerating system 5 that is housed in the portion 3. Ions formed in the ion chamber 6 are extracted and subsequently accelerated to be projected in the direction of arrow 7. The two structures 4 and 5 inside the housing portions 2 and 3 are mechanically independent, and their relative configuration and alignment are determined by the geometry of their respective mountings on the inside of the housing 1. The electron gun 4 inside the portion 2 is supported from the end 9 of the housing, and the ion-forming and ion-accelerating system 5 inside the portion 3 is supported from the end 8 of the housing. Each of the units 4 and 5 are electrically insulated from ground, i.e. from the housing 1.

The unit 5 is mounted by means of pillars 17 which may be two in number, supported on a base plate 10. The base plate 10 is provided with small holes 11 preferably three or more in number and small balls 12 of insulating material such as glass are clamped by means of clamping plate 13 in such manner that the balls 12 rest in the holes 11, the balls 12 being seated in recesses 15 in the housing end 8 and in holes 14 in a clamping plate 13 so as to align the balls and the holes 11 in the plate 10. Clamping screws 16, preferably two or more in number, engage tapped holes in the housing end 8, draw the plate 13 toward the housing end 8 thereby clamping the base plate It) firmly in position between the balls 12. Clearance slots 18 in the base plate 10 are sufliciently large that the clamping screws 15 do not contact the plate 10. In this manner the base plate 10 is firmly held in position while being at the same time insulated from the housing so that the desired electrical potentials may be applied to the unit 5. The clamping plate 13 has a large central opening 21 and is further out out to clear the pillars 17 as best seen in FIGURE 11. A small hole 19 may be drilled from the edge of the plate 10 to the holes 11 in order to release gas trapped inside the hole 11 between the glass balls 12, as is customary in structures for use in a vacuum. In similar manner all blind holes, channels, or the like encountered in the structure of this invention are provided with holes or slots to allow escape of trapped gas as is customarily done in structures to be used in vacuum.

Referring again to FIGURE 1, the electron gun portion 4 of the apparatus is supported on base plate 21 which is clamped by means of insulating balls 22 held down by clamping ring 24 drawn to the housing end 9 by means of screws 25. Columns 26 are supported on base plate 21 and serve to support the electron gun structure 4 to be described. Clamping ring 24 has a central opening 27 through which the posts 26 extend and the base plate 21 has clearance holes for the clamping screws whereby the structure mounted on the base plate 21 is electrically insulated from the housing 1. The recesses into which the

balls

12 and 22 fit in the housing ends 7 and 8 are so dimensioned that the electron gun structure 4 supported on rods 26 is in a predetermined position with respect to the structure 5 supported on pillars 17 as will be described.

The pillars 17 are fastened to the base plate 10 by means of a socket 28 that is fastened to the plate 10 as for example by welding. The socket 28 has an internal bore that is substantially the same diameter as the outside diameter of the pillar 17. The socket 28 is split longitudinally so that the pillar 17 may easily slide into the socket 28. A generally U-shaped clamp 29 (best seen in FIGURE 11) slides over the split socket 28. By tightening a screw (not shown) in the U-shaped clamp 29 the latter will contract the socket 2S and clamp tightly around the pillar 17. This type of mounting provides that the; longitudinal position of the assembly mounted on pillars 17 may be adjusted in order to bring the assembly mounted thereon into the proper alignment with the assembly mounted on columns 26. Columns 26 are supported on the base plate 21 in similar manner by means of split socket 28 welded to the base plate 21 and carrying U-shaped clamp 29 which when tightened contracts the socket 28 to clamp tightly on the column 26.

Again referring to FIGURE 1, the columns 26 serve to support the electron gun unit 4 in such manner that the electron gun 4 may pivot on an axis perpendicular to the plane of FIGURE 1. The mechanical pivot axis is located close to the ion chamber 6 and coincides with the axis of two pivot pins as will be more clearly described later. The columns 26 support a ring 32 with a diametral cross bar 33 more clearly seen in FIGURES 3 and 6. Columns 26 and auxiliary posts 50 in turn support a ring 34 on which is mounted a yoke 35. Yoke 35 carries two pivot pins on which the entire electron gun assembly 4 may rotate for purposes of adjustment.

FIGURE 2 is an enlarged top view of the electron gun assembly 4 and shows the structure of the electron gun in more detail. In FIGURE 2 the

elements

21, 22, 24, 25, 2'7, 28, and 29 are the same as those shown in FIGURE 1 and these serve to support the rods 26. In FIGURE 2 the ring 32 serves to support the rods 59 and the cross bar 33 serves to support a terminal assembly as to be described later. In FIGURE 2 the rod 26 nearest the observer has been removed and the rod 26 shown is that behind the assembly. The yoke is fastened to the ring 34 carried on the rods 26 and by means or" screws 3'7 as is best seen at the right-hand side of FIGURE 2. The yoke 35 extends close to the ion chamber 6 and terminates with two arms 38 of a fork best seen in FIG URES 3 and 5, the arms 38 each being drilled with a conical center hole 39. The electron gun 4 itself carries pivot pins 45 (see FIGURES 3 and 5) as will become evident. In FIGURES 3 and 5 pivot pins td are shown engaging center holes 39 in the arms 33 which are part of the yoke 35 shown in FIGURES 1 and 2.

Referring now to FIGURES 2 and 3 the terminal unit 36 is mounted on a T-shaped fixture 23 fastened to the cross bar 33 by means of screws 43. The support 23 has a central opening through which a rod d4 extends without contact. The rod 44 is threaded at each end for nuts as shown. Recesses are provided in the fixture 23 for small balls 41 made of insulating material. A number of terminal plates 42 shown as twelve in number in FIGURES 3 and 6, are clamped between the balls 41 by means of the nuts at each end of rod 44. Each of the terminal plates 42 is drilled with three small holes in each of which a ball 41 seats so that by clamping the nuts at the ends of rod 44 all the plates 42 are held rigidly together supported on the member 23 but each plate 42 is electrically insulated from the other. This structure is similar to that shown in Robinson Patent 2,581,446. In numerous parts of this invention a structure similar to that taught by the Robinson patent is employed, and this will hereafter he referred to simply as a glass-ball structure.

Referring now to the right-hand portion of the electron gun 4 of FIGURES 2 and 3, a fixture 51 best seen in FIGURE 3, comprises a ring 52 with two extensions 53 each of which carries a pivot pin 40. Theextensions 53 are split as seen in FIGURE 2 and may be tightened by means of clamping screw 89 to clamp the pivot pins 40. The pivot pins may be adjusted to fit snugly into the center holes 3? in the arms 38 without lost motion. The side of the ring 52 opposite to the extension 53 is provided with a pair of

arms

55 and 56 that are drilled with parallel holes into which two

rods

57 and 58 fit. The

arms

55 and 56 are drilled and tapped for set screws 59 and 60 at right angles to the holes in which the

rods

57 and 58 fit. The set screws 59 and 60 have conical points as shown in FIGURE 3. Each of the

rods

57 and 58 is provided with a V-shaped groove 61 and 62 and the set screws 59 and till respectively engage these grooves. In this manner the

rods

57 and 58 are held in a fixed longitudinal position with respect to the fixture 51, in other words the

rods

57 and 58 are thus supported on the pivot pins 4% in a parallel configuration by the fixture 51. The

rods

57 and 58 each are provided with a sleeve 63 having an extension 64 that is drilled with two small holes 65 to accommodate the balls 66. The ring 52 which forms part of fixture 51 is also provided with small holes 54 for the balls 67. Between the ring 52 and the extension 64 there are clamped by means of a glass-ball structure a number of electrode elements of the electron gun to be described in more detail later. The manner in which the electrode elements are clamped is similar to that shown in aforementioned Robinson Patent 2,581,446. The

rods

57 and 58 carry at their left-hand ends a filament-mounting block 69 best seen in FIGURE 2. The filament mounting block 69 serves to tie together the left-hand ends of the

rods

57 and 58. The ends of the

rods

57 and 58 are threaded and have nuts which draw up the

elements

69 and 63 to clamp the electrode elements of the electron i gun by means of the glass-ball structure (including balls 66 and 67) to the fixture 51.

Longitudinal sections of parts of the assembly shown in FIGURE 2 are shown in FIGURES 4 and 5, these sections being taken over the regions indicated by the corresponding numerals IV and V. Looking first at FIGURE 5, the nuts 76) tighten the filament mounting block 6 9 and the sleeves 63 with their extensions 64 to clamp the electrical elements of the electron gun against the fixture 51 by means of the glass-ball structure. The block 69 carries by means of a glass-ball structure two filament terminal blocks 71 and 72 that form terminals for a thermionic filament 34, the glass-ball structure being clamped by screw 82. The filament 84 is a conventional tungsten ribbon of dimensions .062 inch by .032 inch. ends of the filament are clamped against the respective terminal blocks 71 and 72 by means of clamping blocks '73 that are drawn to the respective terminal blocks 71 and 72 by means of screws (not shown). The two

terminal blocks

71 and 72 are thus supported insulated from each other and serve to support the filament 84 and conduct the filament current thereto.

FIGURE 4 shows a longitudinal section. between the arrows IV of FIGURE 2. FIGURE 4 is reversed (right to left) as compared with FIGURE 5 since the assembly is viewed in FIGURE 4 from the opposite side asindicated by the directions of the arrows IV and V. In FIGURE 4 the sleeve 63 and block 69 are shown clamped by means of the nuts 70. The block 6% has an extension 75, shown also in FIGURE 2, and an adjusting arm 76 seen both in FIGURES 2 and 4 is screwed to the end'of the arm '75 by means of screw 77 which is threaded into the extension of the filament mounting block 69.

Referring now to FIGURE 1, there is sealed to the inner wall of the housing 1 the flange of a metal bellows 74. The inner end of the bellows is fastened as by soldering to a plate that has a socket 68 accommodating a glass ball 78. Inside the bellows the member 68 has a partially covered slot 79 that is engaged by a mating enlargement on the end'of an adjusting screw 80 threaded in the wall of the housing 1. The space inside the bellows will be at atmospheric pressure due to leakage at the threads of screw 86, but the enlargement on the end of screw holds the bellows against this pressure. By adjustment of screw 89 the glass ball 78 may be moved and this in turn pushes the extension arm 76. In order to decrease the angle between the axis of electron gun 4 and the axis of the supporting

columns

26 and 50, the adjusting screw 8t may be turned inward to allow extension of the bellows 74 whereby the glass ball 78-pushes on the arm '76 thereby to change the alignment of the electron gun 4. Inasmuch as the bellows is completely sealed against the wall of the housing 1 and there is no communication between the inside and outside of the bellows 74, this structure permits adjustment of the axis of the electron gun 4 inside the vacuum chamber without leakage.

Returning now again to FIGURE 5, there is shown the electrostatic electron focusing system comprising the electrode elements clamped by glass balls 66 to 67 between the ring 52 of fixture 55 and the extension arms 64 of sleeves 63. The

electrical elements

99, 91, 92, 93, and 9-4- are held by means of the glass-ball structure as shown, there being four balls 66 and four balls 67 as well as four in each intervening set of balls. Element 98 comprises a plate with an aperture that is covered by a grid 96. Element 91 is an annulus having an aperture 97 somewhat larger than aperture S S. Element 92 is a plate with [an aperture 98 that is covered by a grid 99. By means of electrical potentials applied to the elements 9%, 91, and 92 with respect to the filament 84, electrons are drawn oil? the thermionic filament 84, and accelerated toward the grid 96. The electrons emitted from filament 84 are accelerated as'they traverse the openings of grid 96, aperture $7, and the openings of grid 99. A certain de gree of geometrical and electrostatic focusing of the elec- The trons is attained through this electrode system which also accelerates the electrons substantially uniformly. Further geometric focusing of the electron stream is effected by the

elements

93 and 94 each of which comprises a quadrupole lens. The elements of the

quadrupole lenses

93 and 94 are cylinders (best seen in FIGURE 8) whose taxes are parallel to the longitudinal axis of FIGURE 5, the four cylinders of each lens being of equal diameter and equidistant from the longitudinal axis of FIGURE 5. FIGURE 8 is a view looking into the end of the electron gun as indicated by the arrows VIII in FIGURE 3, and shows an end view of each of the four cylinders 94(a, b, c, d) of the quadrupole lens 94. The four similar cylinders 93((1, b, c, d) forming quadrupole lens 93 are directly behind the cylinders 94(a, b, c, d) appearing in FIGURE 8. The respective cylinders are supported on arms 88 (best seen in FIGURE 8) that are staggered and are clamped in the glass-ball support as shown in FIG- URES and 8, the arms 88 being indicated by dotted lines in FIGURE 3. The respective arms 88 carrying the cylinders 93(a, b, c, d) and 94(a, b, c, d) are further stabilized by means of balls 83 which appear in FIG- URES 3 and 5. In FIGURE 8 the central aperture 98 of electrode 92 is on the axis of the

quadrupole lenses

93 and 94, and the figure also shows the wires forming the grid 99. The quadrupole lens cylinders 93(a, b, c, d) and 94(11, b, c, d) are spaced and dimensioned in known manner as described in an article by C. F. Giese in Rev. Sci. Inst. Vol 30, No. 4, April 1959, pp. 260 and 261 entitled Strong Focusing Ion Source for Mass Spectrometers. As will become apparent from the electrical potentials applied to the respective cylinders of the

lenses

93 and 94, the first of these lenses effects divergence of the electron beam in one axial plane and convergence in the axial plane 90 thereto, and the second lens effects convergence and divergence respectively in these same two planes. The resulting effect is to produce a penciliform electron beam of small solid angle.

The electrical potentials on

elements

90, 91, 92, and on the respective cylinders of the

quadrupole lenses

93 and 94 are designed in known manner so that the pencil of electrons emerging from the electron gun (toward the right in FIGURE 5) will have a vertical dimension in the plane of FIGURE 5 limited by the vertical height of aperture 98, and will have a dimension perpendicular to the plane of FIGURE 5 that converges with distance and reaches a focus at a point a short distance to the right of FIGURE 5. The focal point of the electron stream emerging from the electron gun 4 is located substantially on the axis of the cylindrical ion chamber 6 as seen in FIG- URE 1, as will become more evident later.

The respective

electrical elements

90, 91, 92, and the eight cylinders 93(11, b, c, d) and 94(11, b, c, d) that make up the

quadrupole lenses

93 and 94 are connected by means of wires 49, only two of which are shown in FIGURE 2, to the plates 46 of a terminal unit indicated generally at 48. The plates 46 are supported by a glassball structure by means of rod 47 each end of which is threaded for a clamping nut. Each of the plates 46 of the terminal unit 48 has an extension forming a lug to which the wires 49 are attached as by soldering or welding. The plates 46 are insulated from each other by glass balls and the assembly 48 is supported on an exten sion 81 of the filament mounting block 69 as is best seen in FIGURE 7. Two of the plates 46 are also connected to the filament terminal blocks 71 and 72 previously mentioned. It is apparent that the terminal unit 48 moves with the entire electron-gun structure 4 when the aiming of the electron gun is changed by turning the adjusting screw 80. Flexible electrical connections from the terminal unit 48 to the fixed terminal unit 36 are provided by means of springs 45 best seen in FIGURE 2. The springs 45 serve to electrically connect the plates 46 to corresponding plates 42 of the fixed terminal assembly (indicated generally by 36 in FIGURES 2 and 3) previously described. Each spring 45 comprises a helical compression spring into each end of which a wire hook extends to catch the far end of the spring, the other end of the hook engaging the hole in the terminal lug. This construction of the springs 45 insures that the spring does not become slack and thereby avoids loose electrical connections. The springs 45 and their wire hooks are made of stainless steel spring wire. In order to avoid mechanical interference of adjacent springs, the plates 46 have staggered lugs best seen in FIGURE 2, the lugs alternating in position in order to separate the springs in the plane of FIGURE 2 as shown. Terminal plates 42 have lugs for convenient permanent wiring to incoming leads (not shown). I

It is seen that the electron gun 4 may pivot about the pivot pins 40 by adjustment of the screw and that this provides for aiming of the electron gun to adjust the direction with which the electron stream is projected into the ion chamber 6. The auxiliary post 50 may be cut out slightly as shown in FIGURES 1 and 2 at 126 in order to avoid mechanical interference with the electron gun terminal strip 48 when the electron gun is aimed nearly perpendicular to the axis of the ion gun 5. As seen in FIGURE 1, the pivot axis 40 of the electron gun is perpendicular to the plane of the figure, i.e. is perpendicular to the plane defined by the axis of the electron beam and the axis of the ion beam. The pivot axis 40 of the electron gun 4 is located as close as is mechanically feasible to the ion chamber 6 as indicated at the right-hand side of FIGURE 3. As will become evident later, certain of the ion gun electrodes are cut out in order to avoid mechanical interference with the movable electron gun structure.

Referring now to the ion-accelerating system generally indicated by 5 in FIGURE 1, it has already been explained how this system is mounted on base plate 10 and supported on the end 8 of the housing. FIGURE 9 shows in more detail a side elevation of the ion-accelerating system 5. The ion chamber (previously indicated generally by 6 in FIGURE 1) comprises an open-ended cylinder 106 (FIGURE 9) fastened with its axis perpendicular to supporting arms 135 (whose outline is best seen in FIGURE 12). The arms 135 as well as a number of

square electrode plates

133 and 137 to 145 and two

quadrupole lenses

103 and 109 are all held in a glass-ball structure indicated generally by 101 in FIGURE 9 and which is supported by pillars 17 previously mentioned. As shown in FIGURE 9, the right-hand portion 19 of each pillar is reduced in diameter forming shoulder 115, and the outer (right-hand) end of the pillar is drilled and tapped for a screw 116. A sleeve 114 slips onto the reduced portion 19 of each pillar, the sleeves 114 being provided with arms that are drilled for glass balls 102. The glass-ball structure 101 is clamped between the arms 125 and a ring 112 that is drawn down by means of screws 116. The ring 112 is provided with receptacles 113 that slide over the pillar ends 19 to facilitate alignment during assembly.

Starting from the left-hand side of the structure 101 as shown in FIGURES 9 and 14, the first member 133 comprises a square plate supported near its four corners by the balls 102 of the glass-ball structure. Provision is made for introducing the gas to be analyzed into the ion chamber through an aperture at the center of the plate 133 as best seen in FIGURE 14. A nipple 131 is fastened as by swaging in the aperture of the plate 133. The nipple 131 is provided with a small perforation that forms a leak through which the gas flows from a gas-inlet fixture that makes a tight fit and clamps to the nipple 131. Appropriate tubing not shown is connected at 132 to an appropriate means for introducing gas to be analyzed through the outer housing of the apparatus.

Again referring to FIGURE 14, the function of

members

134 and 136 of the structure 101 will be explained later. Member 135 mechanically supports the ion chamber cylinder 106 so as to have close clearance at 104 between the edge of the cylinder 1116 and the plate 133. Member 137 is a plate which has a central aperture 103 best seen in FIGURE 12. A grid 105 is. located across the aperture 103 of plate 137. The cylinder 1% has close clearance at 107 betweenit-sedge and the plate 137. The clearances 104 and 107' are of the order of .010 inch. Each of the plates 138 to 144. has a central. aperturethat is larger in diameter than the aperture 103 as seen in FIGURE 14-. The member 145 of the structure 101 has a central aperture 121 that is provided with a grid 122 as shown. Each of the plates 133 and137 to 145 is provided with a connecting lug 124 (seeFIGURE 1) to which a lead wire is connected.

To the right of the, electrode 145 (see FIGURE 9) there are two quadrupole lenses, 1138 and 109. The elements of each of the respective quadrupole lenses 1158 and 1119 are four cylinders whose axes are parallel to the longitudinal axis of FIGURE 9. The four cylinders 108(a, b, c, d) and 109(a, b, c, d) of each-lens are of equal diameter and are equidistant fromthe longitudinal axis of FIGURE 9. The cylinders 111802, b, c, d) and 109(42, b, c, d) are mounted on arms 111 that are staggered and are clamped in the glass-ball structure 161. The arms 111 carrying the cylinders are further stabilized by means of glass balls 110. FIGURE 13 is an end view looking into the assembly of FIGURE 1 in a direction opposite to that of arrow 7, and shows the ends of cylinders 109(a, b, c, d) and the arms 111 on which these cylinders are mounted.

The four similar cylinders 108(11, b, c, d) forming the quadrupole lens 108 are located directly behind the cylinders 109(a, b, c, a) appearing in FIGURE 13. The cylinders 108(a, b, c, d) and 169(0, b, c, d) of the

quadrupole lenses

108 and 109 have electrical potentials applied to them so that the first lens effects divergence of the ion beam in one axial plane and convergence in the axial plane 90 thereto, and the secondlens effects convergence and divergence respectively in these same two planes, thereby producing a penciliform ion beam of small solid angle. The aperture 121 in the plate 145' together with grid 122 is seen at the center of FIGURE 13.

The cylinder 1% forming the ion chamber also has two diametrically

opposed openings

117 and 118 best seen in FIGURE 12 which is a section through the ion chamber as. indicated by the arrows XII in FIGURE 9.. The opening 117 faces toward the left. of ion chamber 6 as seen in FIGURE 1 and permits the electron beam emerging from the electron gun 4 to enter the ion chamber 6. The opposite opening 113 in the ion chamber leads to two

electrodes

119 and 121 that form an electron trap. The electrode 119 has an opening covered by a grid as shown in FIGURE 12' and the electrode 1241 is the plate of the electron trap. Electrons from the electron gun assembly 4 traverse the ion chamber 6 from the opening 117 to the opening 118 and are eventually caught on the plate 121 The

electrodes

119 and 121 are respectively supported on the

plates

134 and 136 of the assembly 1431- of FIGURE 9. FIGURE 14 is a longitudinal section through the ion chamber assembly and shows the manner of supporting elements 119 and 1211 as well as other elements ofthe ion gun previously mentioned. The plates 133 to 149 have a portion cut out as seen at 128 in FIGURES 11 and 14, this cutout being necessary to provide mechanical clearance for the electron gun structure which closely appreaches the opening 117 of the ion chamber.

In order to withdraw positive ions formed by electron bombardment of gas in the ion chamber a negative potential must be placed on the electrode 137, that is the electrode 137 must be negative with respect to the cylinder 106. The variousplates 138, to 145 in the assembly 1611 to the right of the electrode 137 are successively more negative and serve to accelerate the positive ions extracted from the opening 1113 to a desired velocity with which they emerge from the opening 121 and enter the

quad rupole lenses

108 and 109. The negative potential on electrode 137 results in an electrical field distribution inside the ion chamber cylinder 1% such that electrons entering the opening 117 will not traverse the ion chamber in a straight line, but the electron beam will be bent into a roughly parabolic trajectory that is convex with respect to the electrode 137 since the electrons are repelled by the negative potential on electrodes 137. In order to compensate for the curved trajectory of the electrons inside the ion chamber 6, the entire electron gun 4 may be tilted by adjustment of the screw (FIGURE 1) so that the electron beam that enters the ion gun 6 has a generally forward component of velocity to counteract the repulsion of electrode 137, i.e., is aimed toward the right in FIG- URE 14. The electron trajectory is indicated in FIGURE 14 by the dashed line 123'. It is apparent that each time the potential on electrode 137 is changed, the electric field distribution inside the cylinder s will change and in order to bring the electron trajectory substantially grazing the exit opening 163 of the ion chamber 6 it is necessary to change the angle with which the electron trajectory 123 enters the opening 117. In this invention adjustment of the electron gun aiming angle is made by means of the screw 81} (FIGURE 1) whose adjustment controls the aiming of the electron gun as previously described.

FIGURE 15 indicates the electrical potentials employed on the various electrical elements of the ion source of this invention when operated with a mass spectrometer. The values given are by way of example only and operation of this invention is not limited thereto. The potentials are obtained from a conventional power supply (not shown) and may include a voltage divider (not shown) as is customary. The various potentials are given with respect to the potential of the outer housing 1 which is at ground potential. In FIGURE 15 the various electrical elements are indicated by the same numerals as in the FIGURES 1 to 14. In a typical application the electron-acceleration voltage V may be between 25 and volts, and the ion-acceleration voltage V may be between 500 and 5,000 volts. Power supply to the filament 84 is conventional and may for example be about two volts for the filament described. The high-voltage power supply indicated by 127 in FIGURE 15 is a conventional rectified and filtered electronic power supply of sufficient current capacity to supply the various voltage dividers required to obtain the desired voltages.

Inasmuch as the entire apparatus is maintained in a high vacuum, all parts are made of materials that can be outgassed by heating or other well-known procedures. All metal parts are made of non-magnetic material preferably of the alloy Inconel or of stainless steel type 303. Connecting wires which must flex such as filament leads are preferably made of gold as is customary in apparatus of this type. The glass balls employed in the electrode supporting glass-ball structures are preferably not made of glass, but are preferably made of synthetic sapphire.

The apparatus of this invention attains a high degree of control over the energy characteristics and intensity of the ion beam. For example, when operating. the ion source with a mass spectrometer for analyzing ions of a particular mass range a particular ion extraction voltage will be employed on plate 137. Optimum results as to minimum energy spread of the extracted ions together with maximum intensity of ion current is then obtained with the electron gun aimed at aparticular angle. Whenever the mass range of interest is changed, the required voltages will change and the optimum results will be found to obtain at a different electron-gun aiming angle. The apparatus of this invention by providing aiming adjustment of the electron gun permits attainment of optimum ion-beam characteristics over substantially all ranges of ion masses. Furthermore the apparatus of this invention by employing a sharply focused electron beam effects ionization in a very restricted region of the ion chamber and this materially reduces the energy spread of the accelerated ion beam so that by the combined action of the sharply focused electron beam and adjustment of the electron beam aiming results in an ion beam having very little energy spread. This highly desirable result permits making very precise measurement at high resolution in the subsequent electromagnetic analysis of the ion beam produced.

What we claim as our invention is:

1. An ion source for a mass spectrometer or the like comprising an electron gun adapted to produce a geometrically and electrostatically focused electron beam,

an ion chamber located in the path of said electron beam,

means adapted to introduce sample material into said ion chamber,

an opening in said ion chamber permitting entrance of the electron beam from said electron gun,

means producing a segmented electric field adapted to withdraw ions from said ion chamber and accelerate said ions,

an electrostatic double quadrupole lens adapted to geometrically focus said accelerated ions into a focused ion beam, and

means adapted to rotate said electron gun about an axis substantially perpendicular to the plane containing the axis of said electron beam and the axis of said ion beam.

2. An ion source for a mass spectrometer or the like comprising an electron gun adapted to produce a geometrically and electrostatically focused electron beam,

an ion chamber located in the path of said electron beam,

means adapted to introduce sample material into said ion chamber,

a divergent electrostatic quadrupole lens and a convergent electrostatic quadrupole lens coupled thereto adapted to geometrically focus said accelerated ions into a focused ion beam, and

3. An ion source for a mass spectrometer or the like comprising an electron gun having a thermionic filament,

means producing a first segmented electric field adapted to extract from said filament electrons and uniformly accelerate the same,

a first electrostatic double quadrupole lens adapted to geometrically focus said accelerated electrons into a focused beam,

an ion chamber located in the path of said electron beam,

means adapted to introduce sample material into said ion chamber,

means producing a second segmented electric field adapted to withdraw ions from said ion chamber and uniformly accelerate said ions.

a second electrostatic double quadrupole lens adapted to geometrically focus said accelerated ions into a focused ion beam, and

4. An ion source for a mass spectrometer or the like comprising an ion chamber including means adapted to introduce thereinto sample material to be ionized,

an electron gun providing a geometrically and electrostatically focused beam of electrons of substantially uniform energy sutficient to ioniz/e the sample material and directed into said ion chamber,

an electrode forming a boundary of said ion chamber and maintained at a potential to extract ions from said ion chamber,

a plurality of apertured electrodes maintained at potentials to uniformly accelerate said ions,

an electrostatic double quadrupole lens system adapted to geometrically focus said accelerated ions into a focused ion beam,

said electron gun supported with respect to said ion chamber so that the focal point of said electron beam lies within said ion chamber proximate the region of ion extraction,

said electron gun being rotatably adjustable about an axis substantially perpendicular to the plane containing the axis of said electron beam and the axis of said ion beam.

5. An ion source for a mass spectrometer or the like comprising an electron gun comprising a source of electrons,

a first series of apertured electrodes maintained at potentials to uniformly aceclerate said electrons, and

an ion chamber located in the path of said electron beam,

means adapted to introduce sample material into said ion chamber,

a first opening in said ion chamber permitting entrance of said focused electron beam into said ion chamber,

a second opening in said ion chamber in the path of said electron beam,

an electrode outside said ion chamber opposite said second opening maintained at a potential to trap electrons emerging from said second opening,

an apertured electrode forming one boundary of said ion chamber maintained at a potential to withdraw ions from said ion chamber,

a second series of apertured electrodes maintained at potentials to uniformly accelerate said ions,

a second electrostatic quadrupole lens adapted to geometrically focus said accelerated ions into a focused beam, and

mounting means for said electron gun adapted to rotate said electron gun about an axis substantially perpendicular to the plane containing the axis of said electron beam and the axis of said ion beam.

References Cited by the Examiner UNITED STATES PATENTS 2,643,341 6/52 Leland 2504l.9 2,706,788 4/55 Wiley 2504l.9 2,911,531 11/59 Rickard et al. 2504l.9 2,939,952 6/60 Paul et al. 2504l.9 3,084,249 4/63 Enge 2504l.9

RALPH G. NILSON, Primary Examiner.

Claims

1. AN ION SOURCE FOR A MASS SPECTROMETER OR THE LIKE COMPRISING AN ELECTRON GUN ADAPTED TO PRODUCE A GEOMETRICALLY AND ELECTROSTATICALLY FOCUSED ELECTRON BEAM, AN ION CHAMBER LOCATED IN THE PATH OF SAID ELECTRON BEAMS, MEANS ADAPTED TO INTRODUCE SAMPLE MATERIAL INTO SAID ION CHAMBER, AN OPENING IN SAID ION CHAMBER PERMITTING ENTRANCE OF THE ELECTRON BEAM FROM SAID ELECTRON GUN, MEANS PRODUCING A SEGMENTED ELECTRIC FIELD ADAPTED TO WITHDRAW IONS FROM SAID ION CHAMBER AND ECCELERATE SAID IONS, AN ELECTROSTATIC DOUBLE QUADRUPOLE LENS ADAPTED TO GEOMETRICALLY FOCUS SAID ACCELERATED IONS INTO A FOCUSED ION BEAM, AND MEANS ADAPTED TO ROTATE SAID ELECTRON GUN ABOUT AN AXIS SUBSTANTIALLY PERPENDICULAR TO THE PLANE CONTAINING THE AXIS OF SAID ELECTRON BEAM AND THE AXIS OF SAID ION BEAM.