US2585702A - Spectrometer - Google Patents

Description

Feb 12, 1952 R. w. THOMPSON ETAL 2,585,702

SPECTROMETER F116@ June 12, 1945 s sheetsfshefvu 1 Feb- 12, 1952 R. w. THOMPSON ETAL 2,585,702 r SPECTROMETER 3 Sheets-Sheet 2 Filed June 12, 1945 Feb- 12, 1952 R. w. THOMPSON ET AL 2.585,702

SPECTROMETER 3 Sheets-Sh-et Filed June l2, 1945 Patented Feb. 12, 1952 UNITED STATES PATENT OFFICE SPECTROMETR' v Robert W.l Thompson, Minneapolis, Minn.,4 and Wallace T. Leland, New York,.N. Y.,-,assignors to the United States of America as represented by the" United States Atomic Energy'Comm'is'- sion` Application yl`tlnef'12, 1945; Seriwl 4N0a5994066' T'Cla'ims. l

The. present invention relates to Vmass. spectrometers, andparticularly to mass spectrometers utilizing thermionic ion sources.

lIn. various massspectrometer procedures, as for example, the determination of`isotope Aabundance ratios of uranium and other heavy metals, it is necessary to employ positive ions within the spectrometer, and in the past considerable diiculty has been encountered in the provision of asatisfactory source of such ions. This difficulty vi's particularly serious in the case of uranium, and various rather complicated procedure's', some of Whi'ch'necessitate the introduction of gaseous halogen compounds of uranium into theV mass spectrometer during the analysis, have been used. These'procedures'not only are very time consuming, but in addition they can be carried out only by very highly skilled technicians. As a result, the obtaining of uranium isotope abundance ratios and similar measurementson aA production schedule basis hasfheretofore been considered substantially impossible of attainment,v despite the existing need for such determinations.

The principal object of the present invention isY to provide an improved mass spectrometer which will satisfy this need and will make possible the. determination of isotopev abundance ratios ina relatively short period of time, without necessitating unusual skill on the part of the operator, the apparatus being thereby adapted for use in connection with routine commercial manufacturing operations.

As will hereinafter appear, this object of the present invention is accomplished by the provisionof a massy spectrometer apparatus which utilizes -a ther-minnie ion source of .novel design, this arrangement contrasting withthe prior knownapparatus employingl ion-sources in which the desired positivefions were' produced by bombardment ofV suitable-.gaseous compounds by low energy electrons. The improved.A apparatus of the present invention includes Various novel and useful structural featuresV and combinations of those features which'lcontribute to 'its successful operation These various structural featuresV and other objects and. advantagesof 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: y t l 1- is afsectional view,.p `artly in elevation,

of-v anlass spectrometer embodying the. features of'our invention;-

view, partly in4 elevation,V of the thermioriicbion sourceand the associated.beam-directing' and collimatingv apparatus forming a part of the mass spectrometer illustrated more completely in Fig. l;

Fig. 3 is a sectional view, partly in elevation', vtaken on the line 3--3 of Fig. 2 of the internal construction;

Fig. 4 is.v a View, partly in plan and partly'in section, Ytaken in the direction of the arrows 4 4 of Fig. 2; I

Fig. 5 is an enlarged' fragmentary sectional View, partly in elevation, on the line 5--5 of Fig. 3; and

Fig. 6 is a circuit diagram showing the features of a control circuit for operating the spectrometer illustrated in the other gures of the drawings.

The mass spectrometer illustrated in Fig'. 1 includes a glass envelope 1 which terminates at one endgin a large female grind' 9 through which the thermionic ion's'o'ur'ce assembly I'I is inserted by means of a complementary male grind I3. The outer envelope includes a four lead, reentrant press seal I5, through which the connections fo'r the control potentials are made, and a vpumping duct l1, adapted to be connected to suitableevacuating apparatus (not shown)`. The other end of the envelopev 'l is closed by a cupshaped, metallic end closure i9', which is adapted to support an ion beam collimating and: centering structure 2| and a metallic analyzer tube 2'3 which connects therewith.y The metallic end closure I9 is sealed into the glass tube by a suitable glass to metal seal, various types of which are Well-known to the art. The analyzer tube 23 is curved, as illustrated at 25, to conform to the curvature produced in the ion beam by a magnetici-leid producing structure, indicated diagrammatically at 2"'1. A suitable ion collector 29, which includes electrical connections' 3l, by means of which the ion beam may be monitored and' its intensity measured, is provided at the end of the analyzertube 23.

The thermionic ion source Il includes a, filament 33, which in one satisfactory embodiment of the invention comprises a helical coil of 8V mil (.008in'ch)` tungsten wire', the filament having a length, prior to stretching, of' about 2.5 millimeters, and an inside diameter of 40 mils. The lament 33 when stretched hask a coil length o'f Vabout y4.0 millimeters, and it is' suspended, as by' spot-welding, upon' twolament'supports 35 of approximately" 50 mils in diameterr which pass through" a" conventional press seal 31 in a 14 millimeter' glass 'tube 39', which is joined 'to the male? grind I?.V4 by'a reentrant glass-seal 4l.

AThe satis'fa'tor'yv operation of the thermionlc ,moved along the tube 39.

ion source II of the mass spectrometer of the present invention is believed due in large measure to the provision of an electrostatic focusing system which coacts with the filament 33. This system includes a pair of opposed, focusing electrodes 43, each of which comprises a flat strip of a refractory metal, such as Nichrome, bent into the obtuse angle shape illustrated particularly in Fig. 5. The electrodes 43 are fastened together, with the base portions 44 thereof in spaced parallel relationship and with the other portions 45 thereof in outwardly aring relationship, by means of glass spacers 41, glass washers 48, metallic washers 49, and suitable rivets 50. The outwardly flaring portions 45 of the two focusing electrodes 43 define a dihedral angle of approximately 90 and are adapted to be positioned with the outer periphery of the filament 33 adjacent the apex of this angle.

The focusing electrode structure is supported independently of, and adjustably relative to, the filament 33 by means of two split clamps 5I and 53, an angle support 54, and a fiat support 55, all of which may be of Nichrome or other suitable refractory metal. The angle support 54 andthe flat support 55 are each rigidly affixed to its clamp at one end and are spot-welded to one of the outwardly flared electrodes 43 at the other end, as illustrated. The clamps 5| and 53 are riveted or otherwise fastened together about the tube 39 so as to snugly engage that tube, yet not too tightly, to prevent the clamps from being The arrangement described provides a stiff positive support for the focusing electrode structure, yet at the same time it permits adjustment of the position of the electrode structure relative to the filament 33. The two outwardly flaring electrodes 43 are insulated from each other, and electrical connections for Lthe control vpotentials are made thereto through the supports 54 and 55.

The ion beam collimating and centering structure 2I is attached to the analyzer tube 23 and kis positioned intermediate the thermionic ion source I I and the inlet end of tube 23. The structure 2I includes a pair of rectangular slit-defining plates 51 which are supported in spaced, aligning relationship with each other by means of a pair of circular supports 59 which are spaced Iapart and insulated from each other by glass are provided a pair of beam centering electrodes,

- 61, each of which comprises a right angle shaped,

metallic member, spot-welded or otherwise in .tegrally attached to a suitable supporting plate 60 and 6I and the rivets 63, as illustrated particularly in Figs. 2, 3, and 4.

In order that the filament 33 may be utilized for the thermionic production of positive, metalllic ions, it has been found necessary to either -manufaeture the filament from the particular metalA involved or to coat the filament with a suitable compound of that metal, for instance,

the oxide of the metal. In the case of uranium,

the manufacture of a lament of that metal is is as follows:

not practical for making routine measurements as a check on manufacturing operations, because of the time required and because of the inherent difficulties in working uranium. As a result, composite, oxide coated filaments have been resorted to, using a coiled filament of a metal such as tungsten, tantalum, molybdenum, or the like, for the metallic part of the composite struture.

In coating the filament with oxide, for example UzOs or U02, a binder is required, and this binder must possess certain exacting, and to some extent coniiicting, properties. Particularly, the binder should be solid at room temperatures; it should have a vapor pressure at room temperatures sufficiently high that it can be quickly pumped off in vacuum; the melting point should be low, of the order of about 60 to 90 C.; and finally, the vapor pressure at the melting point should be sufciently low that the binder does not evaporate too rapidly during the coating operation. The following organic compounds have been found to satisfy all of the above requirements:

One particularly satisfactory coating procedure A thin paste is made by stirring together powdered U02 and melted binder. The metallic lament to be coated, having previously been cleaned, is heated in air (by the passage of electrical current) to a temperature just above the melting point of the binder. The U02 paste is then applied to the filament with a one millimeter glass rod or other convenient applicator. If the filament is at the proper temperatures when this is done, the oxide paste runs around and completely fills in the space between the turns of the helix.Y The coated filament is then heated in vacuum at a suiiicient temperature to drive oif the binder and the final result is an oxide coating of uniform thickness having a reasonably smooth, sintered appearance.

In the making of abundance ratio measurements by the use of the above described type thermionic ion source, it is necessary to manufacture a new coated filament for each measurement. This resul from the fact that there is some contamination of the filament metal by the uranium oxide when the filament is heated to a suiicient temperature to effect positive ion liberation. Whether this is due to an alloying of the filament with the oxide, or whether it is due to some other reason, is not known, but the effect Ais quite evident. The cooperating grinds 9 and I3 and the adjustable support for the focusing electrodes 45 greatly facilitate the disassembling. reassembling, and adjustment of the apparatus as the different tests are made. The cooperating grinds 9 and I3 also permit convenient and accurate adjustment of the relative position of the thermionic ion source I I and the beam collimating and centering structure 2 I.

During the operation of the spectrometer, it

` has been found desirable to provide means whereby the electric potentials applied to the two focusing electrodes 45 can be varied independently Within reasonable limits. It is also desirable that assumez 'fbeienergizedfrom la 110 volt 'G0-'cycle source, a 'variael12 and 'avcenter tap filament-V 'transformer T3:which\servesthe additional function vof in- ,"su'l'ating the -lament supply fromthe control poft'entia'l.` vArrammeter 14 may Abe'includedin the "filament circuit if desired. f 1=` "Thefhigh voltage ylll-'C'. portion of the circuit V=is of-ithe voltagedivider-type. Itincludes a `2500 v olt stabilizedf'rectiler"type supply'm which is 'e'rierg'ized froml a 110 volt 60 cyclesource. and a ymainvoltage divider section'having a1.5 nieg- Vchin 'fixed' rresistor section "15, a ."5V` me'gc'il'im` section 16 tapped "at 50.000 ohm intervals, -randva-ff50g000 ohm potentiometer sectionV Tl, the `center ta-pcf which ieg-rounded as illustrated `iat' T8. il voltmeter f1! iand astabilizing con- Lfdenser' `8'0 fof about 2 microfarad capacity are 4coi-in'ecte'd acrossfthev output leads "I8 and 8|'. The i /output-'leadfhwhich is the positive lead, vconnectsto the center' tap of the filament trans- /form'er Yr'I3A -`thus vmaintainingthe coated filament,

.thermionic ion source at the desired'high-posb tive potential.

Y 'Thervoltage Isources vfor the focusing? electrodes 4'3 and the several elements ofthe beam ceiliymating-arid beam-*centering structure V'2 `l are also lconnected at, one 'end' to the positive output lead nl81'.Y `In order that lindependently adjustable potentialsmay be -supplied to the tWofocusing-electrodes'l'the'fcircuit from the output lead Bl to esclu-electrodel 43 includes -a source of D.`C. potential v83-vvhiczh may'comprise a Ibattery of about 150 volts, and a variableresistor 85 havinga total resistanceyof4 about 120,000 ohms Voltmeters 86 mayfbe connectedfasillustrated to provide visual control of the focusing electrode potentials. this arrangementthefocusing electrodes d3 can be maintained at potentials differing from each other and from the filamentby values up to the potential of the battery 83, or other auxiliary potential source.

The analyzer tube 23 and the two collimating Aslit defining electrodes 57 are-grounded, and by means offfa double pole doublethrow switch 87,

' .axed resistor 88, and a variable resistor 89,'.con-

;nected.as-illustrated, it is possible to grcundeither :.oi'fth'ev beam centering electrodes'l and tofapply a D.C. potential of the order of 0 to 250 volts above ground to the other electrode. The fixed resistance 88 may conveniently be of the order of megohms and the variable resistor 89 may be of the order of 1 megohm.

In the operation of spectrometer apparatus of this character, a high degree of vacuum appears to be required in order to make possible good resolution of the isotopic constituents of the sample under test. In the particular spectrometer described above, it was found necessary to provide a vacuum of approximately 4X 10-7 m. m. of mercury in order to obtain good resolution of the 235 and 238 uranium isotopes.

In the making of measurements with the de- .female Agrinds-9 and lf3.

scribed apparatus, the' first Vstep viiillfiofp necessity,V 'bethepreparation ;of v.a suitable ion emitting filament, v:asoby'coatingthe filament -33'vthiia -rnixture of lbinder and an f oxidefsample'of .the

material :under test. Following` this operation the focusing electrodes f 43` Will4 be adjusted to accu- 'mately-locatethe1filament33 relative thereto and :thus assure that thedesired ion beam willv be eo'btainedi. Thefiila-ment and focusing electrode assemblyl II-may then be introduced intothe Ainterior of the main body 1 of thespectrometer through the agency ofthe cooperatingvv male -and l These grinds have4 @a further i, advantage'vin Athat they.y permit` relatively close radJnstnien-t of the ,position of .thethermionic ion source 'structure-Hand the-collimating and beam-*centering `assembly 2|. The seal aty the grinds-fniay be improved-.by .the application .cte thin .ribbon .of vacuum seal-ingV Wax yalongthe meeting edges ofthe parts. The eiectricalucoiinections tothe focusing..electrodes-Micah.heattached in place by reach-ing through, theeiiacea- .-tionrlead Fluor-.maybe .attached while 4the electrodesare Voutside of the glass envelope llMe.

Y.spring lead being--zernployedso that A.the leads :will

.centract'avvhen the electrode isinplace.

'The spectrometer;.may now be evacuated,vv holes 9| in the analyzer tube 23 facilitating i rapid pumping` of rthe 4entire structure, andV during,.this :opera-tion the filament. $3rmay beheated so asto va-porize the 'binder and eifectthe formation. of

the oxide 7coating thereon.` The evacuation should be continued untildthe necessary low. pressure is obtained Within the tube. A McLeod, hot filament, `vacuum gauge, not shown, .or equivalent apparatus, may beremployedy for determining the vacuum pressure Within the spectrometer. yAt the conclusion of the pumping operation, .the 'apparatus ris `in condition to Avproceed with the `determination -of the abundance ratio or otherA tests.Y The; focusing electrode `potential controls provided snake possible easy and `rapid focusing ofi-the. thermionic ionsiliberated by thefflament intoa lbeam and 4the-'directing of that beam `ontothenearer collimating. slit andthe means .-provided for controlling the relative potential of the beam ice-riteringnelectrodes 6 1 makes possibleaccurate centering yof the ionfbeam in its ByV passage through the collimating Measurements made `with iilamentscoated with structure.

uranium dioxidepowder, accordance with the .C. .These ionseinclude U+, UQt,v and 1102+, and isotope abundance ratio measurements maybe i madezon any one.A of the liberated ions.

u Inthe foregoing there are ,described thefeattur-es .of an improved ymass spectrometer ywhich has particular Yutility .in the .determination .of abundance ratios of the isotopes of the heavier elements, especially uranium. The apparatus of thepresent invention in simple in design, and is relatively easy to operate due in part to the novel improved ion source which is included therein and in part to the combination of that ion source with the novel collimating and beam centering structure disclosed. The features of the present invention considered to be new are particularly pointed out in the appended claims.

What is claimed is:

1. A thermionic ion source for a mass spectrometer comprising a heater filament, support means for said filament, a coating on said filament which will liberate ions of the desired character when heated to a predetermined temperatial to the electrodes.

2. A thermionic ion source for a mass spectrometer comprising a heater lament, a coating on said filament which will liberate ions of the desired character when heated to a predeter- "xnined temperature, support means for said filament, a pair of fiat electrodes for focusing and directing the ions produced by said source, and means supporting said electrodes in opposed, outwardly flared relationship whereby said electrode being located adjacent to the'vertex of said angle, and said electrode support means including clampswhich engage said tube and which permit adjustment of the position of said electrodes relative to said coated filament, and means to apply a positive potential to the electrodes. "l

5. In combination in a mass spectrometer, a thermionic source of positive ions, Vion beam collimating means, said source and said means deninig a longitudinal axis, and means along said axis and between said source and said collimatng means for focusing theions produced by said source into a beam and for directing said beam into said collimating means, comprising arpair define the sides of a dihedral angle with said i' filament located adjacent to the vertex thereof, said electrode support means including means for insulating said electrodes from said coated filalment and means to adjust the position of said electrodes relative to said coated lament, and Vmeans to' apply a positive potential to the electrodes.

V trodes define the sides of a dihedral angle with said lament located adjacent to the vertex thereof, said electrode support means insulating v said electrodes from said coated lament and Vfrom each other, and including means to adjust the position of saidelectrodes relative to said coated filament connected to the electrodes, and means to apply an independently adjustable positive potential to eachbf the electrodes.

4. A thermionicrion source for a mass spectrometer comprising a glass tube having a press sealin one end thereof, a pair of spaced apart filament supports contained in said press seal, a helical `filament suspended on said lament supports, a coating on said filament which will liberate ions of the desired character when heated v to a predetermined temperature, a pair of flat electrodes for focusing and directing the ions produced by said coated lament, means supporting said electrodes on said glass tube in outwardly flared relationship, so as to dene the sides of a dihedral angle, said coated filament of at electrodes, means supporting said electrodes in opposed, outwardly flared relationship adjacent vto the source of positive ions, saidsupportmeans insulating said electrodes from said ion source and including means attached to the electrodes to adjust the position of saidxelectrodes relative to the ion source, and means Vvto apply a positive potential tothe electrodes.

6. A thermionic source of positive ions comprising in combination: a cylindrical insulating support member, two rigid filament supports extending from within the cylindrical support member and secured thereto at one end, a coated filament connected between ther filament supports, two flared electrodes mounted on the lament supports adjacent to the filament and insulated therefrom, said electrodes defining the sides of a dihedral angle, at least'one movable circular clamp mounted upon the cylindrical support member, an adjusting element connected between each electrode and a clamp, and means to apply a positive potential to each of the electrodes.`

7; A device for generating a beam of ions comprising, an ionfsource, and means to focus the ion source into a beam, said means comprising a pair of electrodes, and means supporting said electrodes in opposed, outwardly flared relationship adjacent to the ion source, said support means including means attached to the electrodes to adjust the position of lsaid electrodes relative to the ion source and means to apply a positive potential to the electrodes.

ROBERT W. THOMPSON.' WALLACE T. LELAND.

REFERENCES' CITED The following references are of recordin the le. of this patent:

UNITED STATES PATENTS

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