US2619598A - Electron diffraction detecting system - Google Patents

Description

Nov. 25, 1952 J. w. COLTMAN EIAL 2,619,598

ELECTRON DIFFRACTION DETECTING SYSTEM Filed June 29, 1949 2 F' $-l i m :1 Sawtooth l: H Generator I I I 26 a Fig 4. 5

Fig.7.

2- I l I k A l E E 2 4.3+

34 INVENTORS JohnW. Coltmon and RichardL.Longini.

Patented Nov. 25, 1952 UNITED 1 mum? or burgh, Pa -.assignors ;to Westinghouse -Electric-' corporation, Ea's't'Pittsburgh, Pa.-,-a-cor-poration-offPenn-sylvania" Application'dune 29;;1949, swarm. 1 0211-22 19o1'aim's: (o 2511-49 15) Our invention relates-= ton electron diffractioncameras, 1 and in particular relatestearrangements. by which the -variation of intensitjof the diffraction pattern withangle-of diffraction may be registered graphicallyandmeasured; 5

In. electron diffr-action. cameras-la vfine beam of constant voltage electrons is projectedJonto'v a I specimen of a substance" fdriwhicli it' is. desired? to obtainanpatternh Iti'is' found that'lelectroiis arediffr'actedfin. suchanarrangement at various 10 angles from tli'efaxis" of the impinging tbeaiii in 1 such a way as to produce conical streamsor ae trons diverging at-variou's angles to tl'ie'hXis of the impinging stream; The intl'is'iti disti'i'bfitfori of the diffracted electrons-atvarious angles'i'ela five to the axis'of'tlie c'o'nes"is'foundt6 be'a characteristic of tl'ie' material undergoing'fstu'dy and may berefr'red toa's theelectronidiffraction pattern of that. material. I

In accordance with thepr'ionartfsuch diff fa'c tion patternshave been'recorded'by positioning" photographic films or" plates" to" beinters'ectefd by the electron" cones." However'fthis"pro'cedure 'i unsatisfactory in that it 'givesonlv-rouglf-meas f ur'em'entsof"intensitwvariatiorr, and is'somewha time-consuming because=it--is necessary to develop the---photographs'"before the desired 'infoi-mat-ion as-to the diffraction pattern is obtained T e-- procedure becomes especially unsatisfactoryasis sometimes the case the ph y'sioal cha'r'a'ct istics of the samp1e-being studied are: 'cl'ia'n'gwith'tiin'ea It iSObVlOlIS th'a-t in' anyzcas'ei I the changes are rapid it becomes practically im possible to record thei difiraction patterns adcuratelyiv In application Serial No. 10 ,123 filed substan tiallvcontemporaneously awith thisona and-" as signed' to the sameassign and er ititled l-11160 tron Difiraction Camerafl-John W. coltmamdia closesairarrangement -in wh-ioh the system 40 conical electron beams forming-such a difi raetion pattern impingesiupona plate: electrode haV-ingi-a small aperture'throughwhich 'asmall -sect-ion p a thebeam isallOWd to' pass: toi imping c upomia 1-1 collectorielectrode connected to' a measuring oscilloscope; I By magnetically" deflecting the ele'c tron" beam, this platei aperture is :Lin's: efiect :'dis:- placed alonga Iradi'us-v ofcthe'electron beam system; 11 and var-iations in'l'th'ei iHtGIISitY Of 'thQ-Z' latter are ir.

foatl'ehgth-is varied to inake possible a ree'or 2Q. ing of the --current distribution in the-- difi'racted-='=- beam.

One feature of our-inventio-n-- isthe provision of an arrangement whereby an oscilloscope records a graph relatingthe diffraction an le with the total :number of electrons -difiracted through-+ the angle, k 0

One object of our invention is ,--accordingly, "to provide a novel apparatus for recordingfin anelectron diffraction -apparatus,-; the- -relation b tween" difiractionangleandtotal number of elec'--' trons diffracted throl ghthat-angle.' I

Another-objectof our-invention is to mar ne a novel forin -of fe1ectron difiractioii caiiiefa 'whicndirectlyr registers the relatidnship b'tweii diffraction angle and the total huihb? of alestrons I deflected'throu'ghsuch an amine;

Stillanother Objectof'ourinvefitiofi'i' V videa novel electron "difiractioh cani'era whilf records on the screen of anoscillosopea truly. representingthe" difirac'tiorif ang f abscissa and as ordinate tot'al'"n1irnber"o'f" electrons diffractd'thfdufii a the-angle."

Still "another"object'ofour 'i'nv'ention'is to pre vide a graphical electron diffraction pattern registering device which records diffraction angle as abscissa against to'tal electron current as ordinate, and in which the sensitivity for the ordinatescale "'remains the same"throughouttlfe .abscissa range;

Other objects ofou'i' invhti'ofi will :b'ecome 'apa parent 'uponreading the fol'lowin zdescriptiontakeriin connection -with-"the" drawings;- in'dvhi'ch a Figi'ir -l -is' a' 'diagram u'seful inexpl'aihing'dihe principle {of 'on r invention;

Fig; 2 is --scheiiiaticshowingiofian lemeiitafi form of device embodyin'gxthe principles otour invention:

FigSl' 3; 4 and- 5 are 'scheniat'i'c represent'atibns of alternative 1 forms or one portion -ofthe device-- shownin Fig;2';'

Figi-"fi is 'a schematio illustratior'r*of an alterna tiv form of thee'mbodime'nt shown Fig-L 2mm Fig. 7 is an illustrationpf a mddified for'm of a portioriof-the embodiment shown'in Fig. 6'.

Referrifig 'ir'fdetair to Figure a sample l of I material which' it is desired 'to obtain ari'electron diffraction pattern is i supp rted by .any suitmeans (not shown) :in the path "of *arr extrenil andother arrangements fo'i' pro'ducirig such fin streamsof electrons are Wen -known inthe elec' troni'cs' art today and are: believed to -reciuire' ne separate description.

rimpin ing ava 'gl'ancin'ga'angl' onsfhgw sample I, it will be found that the electrons are diffracted in such a way as to produce, in efiect, a number of half-cones having various apex angles, coaxial to the axis of the original electron beam 2. If a fluorescent plate 4 were supported in the path of the conical beams, it would be intersected by the conical beams and would be found to show successive semicircles representing the traces of these cones at the plate 4. Such arrangements as I have so far described are found in certain diffraction cameras of the prior art on which my arrangement is an improvement.

The above-mentioned sample and electron beam may be enclosed within a suitable container which may be vacuum-tight if desired and which is believed to require no detailed description since such arrangements are well known in the electronics art today. A publication en titled RCA Electron Diffraction Unit published by Radio Corporation of America, Rockefeller Center, New York city, describes an apparatus employing such an electron beam.

Our present invention employs the general arrangement as illustrated in Fig. 2 in which an electrical field is impressed by what we may term a focusing electrode 5 on the paths of the electrons in the above-mentioned conical beam and acts, in effect, as an electron lens which by imparting curved paths to these electrons brings all the electrons in a cone of a given aperture angle to a focus, as indicated by the dotted lines in Fig. 2. The conical beams nearer the central axis are brought to a focus at points just below the focusing electrode 5 while the foci of conical beams of larger aperture angle are positioned at greater and greater distances from electrode 5. The actual distance f from the focusing electrode to the focus at which electrons constituting a cone of any given aperture angle converge is dependent upon the magnitude of the voltage impressed on the electrode 5. Thus mathematical analysis shows that the distance f above-mentioned in cm. is given approximately by the formula where A is the radius in cm. of the electrode 5,

13 its potential in volts relative to container 3, V the energy in electron-volts of an electron when it emerges from the sample and R is the perpendicular distance in cm. from the center of electrode 5 to the path the electron would have followed had focusing electrode 5 not been used.

While we have described a simple electron lens system, it will be evident to those skilled in the art that other electron lenses may be used provided they are axially symmetric and have a marked dependence of focal distance on the distance R; i. e. have high spherical aberration.

It will be evident from these considerations that by locating a catcher-electrode l2 on the central axis of the conical beam system, the electrons in some one cone will be caused to focus upon it; and that by varying the potential of the focusing electrode 5 (e. g. by a periodic voltage generator ll), one of the conical beams after another may be focused on the catcher-electrode [2. By causing current collected by the catcherelectrode l2 to deflect the luminous spot on the screen of an oscilloscope M in one coordinate, and by deflecting the luminous spot along another coordinate by means of the periodic voltage derived from source H, a graph representing the variation of electron current in the cone with aperture angle thereof may be traced on the screen of the oscilloscope l4.

In conformance with the foregoing, a focusing electrode 5, which may be of extremely small diameter, is supported by an insulator 6 at a point on the extension of the path of the electron beam 2, and is maintained at a suitable positive potential relative to the container 3, by a source of direct-current voltage I, in series with the movable tap B on a potential divider 9. The potential divider 9 is supplied with voltage from a saw-tooth generator I l of a type too well known in the art to require separate description. It will be evident that the combined effect of the voltage drop across the potential divider 9 and the voltage source is to impress a periodically varying (i. e., a saw-tooth) wave voltage on the electrode 5, relative to the container 3. Some distance from the extension of the electron beam 2 is po sitioned the catcher-electrode 12 which is supported by an insulating bushing l3 from the wall of the container 3. The electrode I2 is connected to impress on one of the deflecting plates of the oscilloscope [4 a voltage which is propor-' tional to a number of electrons brought to a focus on electrode [2 at any instant by the focusing electrode 5. The electrode l2 should preferably be of relatively small surface area. The opposite deflecting plate to that just referred to in oscilloscope i4 is connected to the envelope 3.

The other set of deflecting plates in oscilloscope M is impressed with a saw-tooth voltage derived from the saw-tooth generator H.

The cathode of the electron gun from which the electron beam is derived is connected through a direct-current voltage source l5 sufficient to accelerate the electrons in the beam 2 to the desired velocity for bombarding the sample I. The envelope 3 is preferably grounded.

With the arrangement above described, a moments consideration will show that at any instant the electrons in some particular one of the conical beams will impinge on the catcher-electrode [2, the electrons in the other conical beams passing the electrode 5 to strike the walls of the container 3 and be returned to the electron gun through the voltage source 15. However, the electrons of the particular conical beam incident upon the catcher-electrode l2 will flow back to the electron gun through a resistor 16 and will thereby deflect the luminous spot on the oscilloscope H to a distance proportional to the current flow focured on catcher-electrode l2 at that instant.

Correspondingly, the position of the luminous spot on the screen of oscilloscope [4 will also be determined by the instantaneous value, at the moment, at the voltage impressed on the horizontal deflection plates of the oscilloscope l4.

As the voltage of saw-tooth generator ll changes, the conical beams having wider and wider apertures will be focused on the catcherelectrode 32, while at the same time, the luminous spot on screen M is deflected further and further along the axis of abscissas. The luminous spot will, accordingly, trace on the screen of oscilloscope [4 a graph representing the variation during the periodic cycle of saw-tooth generator ll, of the current flow in conical beams of different aperture angle.

One of the practical problems in connection with the arrangement thus described, is to impart the desired potential to the electrode 5 withoutdistorti-ng the electricgradient of the" electrical field whichiocuses-the electron beams.

One 'way ot-doing this-is-- to-make-the supporting member- 5 for--theelectrode 5 of some high resistancematerial (e: g.-, carbon),- which will permit-' current "fiowtherein to produce apotential-gradient in-the regionit occupies whichis a-replica of' the potential gradient which Woulde'xist there werethe support--- 5 and -thelead wi're which itenclcses absent from-thesys tem, The wall thickness of the member 8 may betapered -in' such awayas'to produce the desiredwariation of =potentialgradient frorrr point 'In- -'Fig: 3; weshow an' alternative expedient foravoiding "th'e' distortion" of the electric fieldin the neighborhoodof the leadfwhich-suppliedvoltage s to-they electrode 5:" This arrangement iS ;S QnfleWhat* similar inappearance to" the condenser bushings -now' known in the electrical art and comprises an insulating tube 2| closely surroundingtheinvleadtothe electrode 5 and provided-with; a conductive 'coatingon its exterior; a secondinsulatingtube 22- of -somewhatshorter length surrounds the first insulating tube Hand is likewise provided with a conductive coatingfonits exterior; a third insulating tube- 23 j'likewise providedywi-th aconductive coating andshorter than the tube 22 surrounds the latter,v

and afourth insulating tube 24 provided with a conductive-coating connected to the container W2113 surroundsthe tube 23. The various cone-- ducting coatings above-mentioned are respectively connected to suitable tap pointson apotential 'divider- 25' which is connected between the container -3-'and the-external terminus ofthe in-lead toelectrode 5. By varying the "position of "these; tap points, the potential gradient in the-neighborhood of the insulating tubes inside ductors passing throughnthecenters of therings- 26* to variable taps onapotential-divider similar tothat'already described in'Fig. 3. By varying the;p osition of these taps, the potential gradient in-the space surrounding the rings 26- may be madefltoapproximatethat which would exist wre-thein-lad to-theelectrode 5' absent from' the system.

Fig.5 shows-stilanother arrangement for imparting the desired potential to the electrode 5 andcomprises-a support 'for the latter positioned along-the extensionof'theelectrode beam 2 in the-direction awayfrom'the sample I. pointsome distance from the-electrode 5, this support is turned at =right angles and passes through-thewall ofzcontainer 3 to the terminal of--the-voltage=source 29 which imparts to the electrode 5 its desired potential. The in-lead to" the electrode 5 is-provided with a coating or sheath 21- of-insulating material.

InaFig. -6, weshowa modified form of the arrangement in-Fig. 3 which is designed to make it possible to employ a-smaller voltageto energize the electrode 5.- IIr-the Fig. 6 modification, the wall orthe containenis interrupted at a point At a.

somewhat belowthe sample I by an insulating wall portion 3| and the container wall below the-- insulators--3t 1 comprises a-= conductive wall portion 32 through which thein-lead and sup Insulatingly supported by a member 33 on this annular plate is a-wallportion 34 which insulatingly supportsat its lower--end a fluorescent screen 35 which-is given apositive potential; relative to wall portion 34-bya voltage source 35A. The wall portion 34 is preferably grounded.- The wall portion 3-i's likewise grounded, whilethe wall portion 32 is kept at a negative potential-- relative to the wall portion- 3 by a voltage source 35. The electron gun producingthe electron stream 2 is-provided with-a; voltage somewhat more negativethanthat of wall portion '32 by means of the adjustable voltage source 43 As a result of this arrangementit will be seen that the electrons in the stream 2'arefirst-ac celerated to the voltage, corresponding to that 'ofthe sources 36 and. 43 in series, and thereupon.

impinge upon the; sample I'. Passing throughthe latter, they produce a system of conical electron beams representing the diffraction pattern"- of the substance I and then after passing by the insulator 3i, enter the field" within thecontainen 32 and are largely deprived of their kineticenergyby virtue of'the negative potential on'- container-32. They, therefore, approach the electron lensconstituted by the electrode 5 'witha comparatively low velocity correspondingto the voltage of source'43 and-as aresult'; th'epoten tialto which electrode 5 must be elevated in order to readily focus the electrons'ontheaper ture-irithe annular plate above-mentioned, is reduced to a readily usable value. To give an example, the voltage source 36 may be of 45 kilovolt's and the potential of the electrode 5 may be of 5 kilovolts. The above-mentioned potential is impressed on the electrode 5 by the voltage source 37 in series with the potential divider 35" whichis supplied with'saW-tooth waves by a sawtooth generator 39.

The saw-tooth-generator 39 supplies the hori-' zontal'deflection plates of the-oscilloscope; while the vertical plates of that. oscilloscope. are respectively connected to ground and to be energized in correspondence with" the current reaching the catcher-electrode 35.

Those electrons which" are not focused on the aperture'in the annular plate above-mentioned, pass to the cathode-of thersource of electron beams 2, while those electrons which are focused on said aperture-strike catcher-electrodes- 35 and pass through source 35A to energize the vertical deflection plates of'the oscilloscope 4|.

For the sake of simplicity, the vertical plates of the oscillscope M in Fig. 2 have been shown as driven from a voltage derived directly. from the electron current impinging on collector la in Fig.: 2; In practice, there is seldom enough current to accomplish this, and it may be necessary to insert amplifiers in this circuit'such as are well known in the art. Alternatively, it may be desirable to use a detector of the fluorescent screen-photomultiplier type such as is described in the application to be filed contemporaneously with this one, entitled Electron Diffraction Camera John W. ColtmarnSerial No; 102,123 filed June 29, 1949. Fig. 6 has been arranged to show such a type of detection. In using such a detector, it is advantageous-if the electrons being detected-have a high energy. and therefore 7' in Fig. 6 means have been shown by which the original kinetic energy of the electrons may be restored after the selection process is completed. This means comprises the use of wall portion 34, insulatedly supported from wall portion 32 by means of insulator 33, and held at ground potential, and this may be assisted by a positive potential impressed on screen 35 by voltage source 35A. Electrons leaving enclosure 32 are accelerated on entering the region of strong electric field between wall enclosure 34, which is at ground potential and enclosure 32 which is at high negative potential supplied by source 36. The high speed electrons then may enter the enclosure containing screen 35 for detection by that fluorescent screen and photomultiplier 35A. Such a photomultiplier and energizing circuits therefor are too well known to require detailed description, but reference may be made to F. H. B. Marshall and J. W. C-oltman Patent 2,512,355, X-ray Thickness Gauge, which issued June 20, 1950. Suitable configuration of the aperture in the base of enclosure 32 and of the chamber 34, together with potential fixing devices typified by adjustable voltage source 35A, may be chosen in accordance with well known principles of electron optics to shape the electron beam in a manner convenient for detection.

Fig. 7 shows still another way of supporting the electrode in the container of the type shown in Fig. 6 and comprises an in-lead wire which passes from the voltage source 31 through an insulated bushing in the wall of the container 3 and then passes along the extension of the electron beam 2 downward to support the electrode 5. The in-lead thus supporting electrode 5 is cased in insulating sleeve 44. Otherwise, the connections of Fig. '7 are similar to those of Fig. 6.

It will be recognized that the electrode 5 in Fig. 2 may likewise be supported in position by an arrangement similar to that just described in connection with Fig. '7, and that the electrode 5 in Fig. 6 may be supported by the expedients shown in Figs. 3, 4 and 5. While the diameter of the sphere constituting electrode 5 may be given various values, it will, in many cases, be

found sufficient if it is no larger than the diameter of the in-lead to it, and this electrode may consist of simply an exposed portion of such an in-lead.

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

We claim as our invention:

1. An electron diffraction recording device comprising means for bombarding a sample material with a concentrated electron beam to produce a conical electron beam system, and .an electron lens system having a high spherical aberration for converging the electrons in each of the various cones simultaneously to a diiierent point on the extended axis of said conical beam system.

2. An electron diffraction recording device comprising means for bombarding a sample material with a concentrated electron beam to produce a conical electron beam system, 'an electron lens system for focusing the electrons in various cones at different points on the extended axis of said conical beams, and a catcher-electrode of restricted size positioned at a point on said axis,

3. An electron diffraction recording device comprising means for bombarding asample material with a concentrated electron beam' to produce a conical electron beam system, an electron lens system for focusing the electrons in various cones at different points on the extended axis of said conical beam system, a catcher-electrode of restricted size positioned at a. point on said axis, and means for varying the focal length of said electron lens.

4. An electron difiraction recording device comprising means for bombarding a sample material with a concentrated electron beam to produce a conical electron beam system, an electron lens system for focusing the electrons in various cones-at different points on the extended axis of said conical beam system, a catcher-electrode of restricted size positioned at a point on said axis,

and means for periodic-ally varying the focal length of said electron lens system.

5. An electron diffraction recording device comprising means for bombarding a sample material with a concentrated electron beam to produce a conical eiectron beam system, an electron lens system for focusing the electrons in various cones at different points on the extended axis of said conical beam system, a catcher-electrode of restricted size positioned at a point on said axis, means for periodically varying the focal length of said electron lens system, and means for plotting the current flowing from said catcher-electrode against said variations in focal length of the electron lens system.

6. In combination with means for producing a conical electron beam system constituting the diffraction pattern of a sample material, an electrode positioned on the axis of said conical beam system, means for impressing a periodically varying voltage on said electrode relative to said sample, a catcher-electrode positioned on said axis more remote from said sample than said electrode, and current-responsive means energized by the output of said catcher-electrode.

7. In combination with means for producing a conical electron beam system constituting the diffraction pattern of a sample material, an electrode positioned on the axis of said conical beam system, means for impressing a periodically varying voltage on said electrode relative to said sample, a catcher-electrode positioned on said axis more remote from said sample than said electrode, current-responsive means energized by the output of said catcher-electrode, said ourrent-responsive means recording one coordinate of a graph, and the other coordinate of said graph being responsive to said periodic voltage.

8. In combination with means for producing a conical electron beam system constituting the diffraction pattern of a sample material, an elec-- trode positioned on the axis of said conical beam system, means for impressing a periodically varying voltage on said electrode relative to said sample, a catcher-electrode positioned on said axis more remote from said sample than said electrode, current-responsive means energized by the output of said catcher-electrode, said ourrent-responsive means comprising one pair of deflecting plates of an oscilloscope, the other pair of deflecting plates of said oscilloscope being energized by said periodic voltage.

9. The arrangement described in claim 6, in which said periodic voltage has a saw-tooth wave form.

10. The arrangement described in claim 7 in which said periodic voltage has a saw-tooth Wave for 11. The arrangement described in claim 8 in. which said periodic voltage has a saa tooth wave form.

12. In an apparatus for recording electron diffraction patterns of sampie materials, means for bombarding said material with a concentrated electron beam to produce a conical electron diffraction pattern, said means being positioned in one portion of a container having a first conducting Wall portion, said container also having a second conducting Wall portion insulated from said first conducting wall portion and surrounding the axis or said conical beam system, means for supporting a focusing electrode within said second conducting Wall portion and for imparting to it a predetermined potential relative thereto, an aperture in said second conducting Wall portion positioned on the axis of said conical beam system, a third conducting wall portion surrounding the extension of said axis through said aperture, and a catcher-electrode positioned on said axis beyond said third conducting Wall portion.

13. The arrangement set forth in claim 12 in which means are provided for impressing a sawtooth voltage upon said focusing electrode, relative to said second conducting Wall section.

14. The arrangement set forth in claim 12 in which means are provided for maintaining said second Wall section at a potential close to that of the cathode from which said electron beam originates, While said first and third conducting wall sections are maintained at a substantial positive potential, relative to said cathode.

15. The arrangement described in claim 12 in which said focusing electrode is supported from said second wall portion by means for regulating the potential gradient.

16. The arrangement described in claim 14 in which said catcher-electrode comprises a screen emitting light in response to electron impact.

17. The arrangement according to claim 16 in which a photomultiplier irradiated by said screen energizes an indicating device.

18. The arrangement according to claim 17 in which the indicating device is a graph recorder having its amplitude-coord'mate energized by the output of said photomultiplier, and its time-coordinate energized by a periodic-voltage source which also impresses voltage on said focusing electrode.

19. The arrangement according to claim 18 in which the indicating device is an oscilloscope.

JOHN W. COLTMAN. RICHARD L. LONGINI.

REFERENQES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,971,277 Rupp Aug. 21, 1934 2,257,774 Von Ardenne Oct. 7, 1941 2,452,919 G-arbor Nov. 2, 1948 2,457,092 Simard et a1 Dec. 21, 1948

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