US2561988A - Electron diffraction detector system - Google Patents

Description

'atented lui? 24,1

' ,Unirajnflisra'rss ears-,nr osare-s escasas "jaLscTnoN'nrrsnaorronnsrsoron SYSTEM Richard L. sangrar. riirseurgh. ra.; assigner 1.a Vl'estinghouse Electric Corporation, East' Pittsiburxgh, Pa., a corporation of iennsylvana.

applicati@ Juneau, 194e, sensi No. 102,292

'My invention relates t1: electron diiracti'on cameras, and in particular relates to arrangements by which the variation of intensity of the diirsction pattern with angular divergence may be registered graphically and measured.'

lr. electron diffraction cameras a ne beam of ainsi int voltage electrons is projected against a specimen of a substance making a small angle with the surface of which' it is desired, to obtain p. pattern. It is found that electrons are dif# fracted in such an arrangement' at various angles from the axis of the impinging v'oearn in suchv a Wa'y'as to Yproduce conical streams of electrons l diverging at various angles to the axis of the imnnsing stream. The intensity distribution vofthe dlffracted electrons at various angles relative to the axis of the cones is found to' be a characteristic of the material undergoing study and maybe referred to as thelelectron dirraction pattern o1 the surface of that material.

. In' accordance With the prior art, such diffrac-v tion patterns have been recorded' by positioning photographic films orplates to intersect the elec- Y tron' cones., However, this procedure is unsatisfactory in that it gives only rough measurements olf-intensity variation, and is somewhat time con vsuming because it is' necessaryto develop the photographs before the desired infomation as possible to record the' diiraction patterns accurately.

" In application Ser. o. 120,123?, led June 29,v

1949. assigned to the same assignee as the present application and entitled Electron Diraction Camera, John W. Coltman discloses an arrangement in which the system of conical electron beams forming such a diiraction pattern impinges upon a plate electrode having a small aper- 'ture through which a small section of the beam is allowed to pass to i'mpinge upon a colleci'nr electrode connected to a measuring oscilloscope. By causing; the electronheam system to lie' magnetically deflected so that this plate aperture is in eect displaced along a radius of the electron beam system', the variations in the in tnsity of the latte: are recorded on the oscilloscope;

s claims. (ci. o-49.5)

such.V angular deflection. In such case the arrangement has the lundesirable feature that, as

the aperture of xed sizeA is displaced radially outward from the center of the beam'v it soliV tends a constantly decreasing angle about the central axis oi' the electron beam and', according? ly,4 the response of thev oscilloscope represents a continually decreasing fraction of the current ovsing in the various electron cones as tl'ie' aperfture moves from'the center to the peripheral por? tions'of the beam. To put the matter another A way, it is necessary to multiplyv the current readchanges are rapid it becomes practically ini'- v While this arrangement possesses great utilf 1W; it'is usuaiLv desired to plot the number of alecm-ons deflected ai; anyv givenangle against ing, which the oscilloscope registers for any angu- Vlar deflection, by the value of the angle itself, in

order to-obtain a graph in whichl the number of electrons diflracted through' any angle' are plotted against the` angularl measurement. One princi'- pal feature of my invention is the provision afm graph relating the diffraction angle with the tot'al number of electrons diracted through the angie.

One object of my' invention is, accordingly: to providel a novel apparatus for recording. in' an electron diiraction. apparatus. the relation' bei'- tween diffraction angleand total number of elec*- trons difracted through that angle.' Y

Another object of my invention is' to' provide a novel form of electron diraction camera 4which directly registers' the relationship between' diffraction angle and the fraction of electrons de# flected through such an angle.

Still another object of my invention is to provide a novel electron diffraction camera which records on the screen of an oscilloscope acurv truly representing, as abscissa, the dini-action angle, `and as ordinate the fraction of electrons diffracted through that angle.

Still' another object of my inventionis-tesoro` vide a grapl'iicaly electron. dilraction pattern' reg'- istering device which records diffraction angle' as abscssa against electron current as ordinate, and in which the sensitivity for the ordinate scale remains the same thrcughout the abscissa range.

Other 'objects of :ny invention will become apparent upon reading the following description taken inconnection with the in which: Figure l.' is' a. diagram useful in principles or my'invention; Fig. 2 is an' illustration'. of a typical? amphi-cai curve appearing on an oscilloscope screen'ir.' the apparatus embodying myv invention; g

Fig. 3 is aschematic representationof one fom of apparatus embodying the vention;4

Fig; 4 is a. detailed illustration of one :erntet principles: ofmy .aperture plate forming 'shown inPig. 3; andne electron beam 2.

a part of the embodiment Pig. 5` ls a schematic illustration of a modied orrn or the embodiment shown in Fig. 3.

Referring in detail to Fig, i, 'a sample l of'v material 4which it is desired to obtain-an electron diraction pattern is supported by any suitable means not shown) in the path of en extremely I So-called electron guns and other arrangements for producing such line streams of electrons are welldsnown in the electronics art today and are believed to require no separate description.

The above-mentioned sample and electron beam may' be 'enclosed within a suitable container 3 which may oe vacuum tight if desired land which is believed to require no detailed description since'such arrangements are well'- known in the electronics art today. A publication entitled RCA Electron Di'raction Unit published by Radio Corporation o America, Rockefeller Center, New York city, describes an apparatus employing such an electron beam.

After being didi-acted by the surface of the sample I, it will be found that the electrons are diffracted in such a way as to produce, in effect,

gg, the cone, the aperture in the 'late 5 should be curved to correspond with the radial distance that.

' it occupies at 'any-particular instant in ils scanning Increment. W'ith a simple aperture of unchanging size and shape n; me plaie s, uns-reeel that the minimum deviation of the aperture from its idealtalne as it moves radially outward from the apex'of the V isattained ii the slot in the' plate 5 has a radius equa-l to approximatelyl vin the instrument which records the electron curr where R is the radius of the outermost electron cone which it is desired to record. The "angle between the sides of the Veshaped notch in the plate 4 should, o course, bemade as small as is consistent with obtaining the desired sensitivity rent owing through the aperture in plate 5.

Fig. 3 shows diagrammatically an'apparatus embodying theideas schematically described in connection with Fig. 1. The sample of material l a number of half cones having various apex angles, coaxial to the axis of the original elecsupported on the plate 4, it would be intersected by the conical beams of electrons 'above-men tione'd and would be iound to show successive rines representing the traces of these cones atv the plate 4. Such arrangements as I have so far described are found in some diffraction cameras of thel prior art on which my arrangement is an improvement. n

, In accordance with the principles of my invention AI provide thev plate 4 withv a V-shaped slot of angle 9, the apex of the V being positioned on the axis of the conical electron beam system. It will readily be seen. that the portions of the electron beam systcm at any radial-'distances of diierent apex angles which pass through the V-shaped slot are the same fraction 4oi the total electron current in the cone striking plate l at that radius. By collecting the electronv current passing through this aperture, I can,

accordingly, obtain a curve in which the difErac-er tion angle lsv-recorded as abscissa, and the total number of electrons diiracted at that angle is recorded as ordinate. Fig. 2 illustrates such a graph on the screen of an oscilloscope.

To obtain the best record of the electron current at any radial distance r from the axis of is held by supports (not shown) in the path of an electron beam by an electron gun system 2a of a type too well-known in the art to require detailedvdescription. Thisarrangement is en closed within a. container'S which may, desired; be made vacuum tight and-which supports the plate 4. rI "ne 'M -shaped aperture is shown in Fig. 3 at a position displaced 90"1 f rom that which is illustrated in Fig. l. The plate 5 is supported :for sliding 'movement along the lower lace of the plate 4 and both plates 4` and 5 are connected through a suitable voltage source (not shown) to the source of electronstream 2.v The plate 5 is insulatinglysupported on a collector electrode S sliding on insulated supporting rods l held by a suitable framework 8 based on the' door of the container 3. The container 3 may beat ground l bisect'or of the V-shaped opening so that the apar#l ture in the plate 5 traverses the entire from the axis I2 of the conical electrode beam system to the point of incidence of the'mos'f widely difracted electron Which it is desired to record. rI he rod 9 may be arranged to be moved thus by attachment to the' core i3 of a reciprocating motor having winding N. Beyond the core I3, the rod 9 may extend through a suitable bearing l5 and be arranged to move a contact 6 along a potential divider Il f ed with direct cur` rent from a suitable'voltage source i8. One end of the voltage divider i1 is grounded. The horizontal deflection plates of an oscilloscope i5 are connected between the movable contact i5 and the grounded end of the potential divider i7. The

Winding i4 o1' the reciprocating motor is supplied with current through a suitable circuit inter yrunter 2i which is arranged in a manner well known in the art to repeatedly energize and deenergize the winding I4 to thereby reciprocate the collector electrode S and plate iialong the radius of the conical electron beam system abo mentioned.

The vertical plates of oscilloscope I5 are respec-l tiyely connected to ground and to the collector electrode 6 through a suitable amplifier 2o. A suitable bias Spring 22 forming a part of the reciprocating motors is arranged to return the core i3 to its position after each energizelife si ure internme- While I have described afrcclprocating motor- .as employed to cause theraciial movement of .the

' .wd of the collector electrode 6, it will be obvious to those skilled in the art. that any otherjsuitahle Aerrangeinen-t. for `causing such' motion may be employed. 'I'

A moment's consideration will show that 2S the reciprocating motor moves the collector elec-V trode S radially outward along the radl Eo l circles of intersection of the conical electron beam in thel container 3, the deflectins:` voltage impressed on the horizontal' plates of oscilloscope 1S will delect the electronbeain thereof horizontally to produce abscissas corresponding w.

acanaas amplified control current for 'the oscilloscope.- In

lradial distances along the plate l and, to ahigl'ii degree of approximation, corresponding to the .apex angles o f the various conical electron beams. 'Simultaneouslx the collector electrode E' will receive through the aperture plate an electron current proportional to the total electron flow incident upon plate 4 atthe radial distance r;

and this current will produce vertical deflections 'of corresponding magnitude on' the screen of the oscilloscope i9. In order to give a; vertical de- -ilection proportional to the fraction of current in a given ring the detector aperture must move with uniform radial velocity..

There will, accordingly, be registered on the screen of the oscilloscope a graph correctly representing the variation with diffraction angle of the electron current in the conical electron beams'.

While a single circular aperture in the plate 5. having a radius of 0.83 times that corresponding division may be carried to any number. and that at. the limit oi subdtisicn an aperture having 1 continuouslycuned'sidesaillrasult.

In Fig. 5- I show'a mcgiiicaticn of m5' invece tion which avoids, the nevcisity lor providing for -mechanicalmoveinent of che collect/or electrode and also shows an arrangement for dez-lynn;- anV this arrangement. the element i, 2, 2a 4 are the same as the corresponding'elernents in Fig. -3

already described. Howerer, insteadjof using-a moving collector electrode 8. Iprovlde a atationary collector electrode comprising a fluorles-' cent screen 3l positioned at a substantial dis' tance below theplate and preferably on the erf tension of the axis of the difi'ractionelectron cones. Between the vscreen 3'! and theplate il', I provide a magnetic eld which is perpendicular to the plane bisecting the angle between-the sidesof the V-s'naped aperture in theplatel 5. 'This magnetic el'cl :hay be conveniently setup bylia pair oi windings' 32 (only one or' 'which is shown) which are suppliedv withcurrent of saw-#tooth waveform from a saw-tooh-generator 33- of a type well-known in the electronics art. The same saw-`tooth generator sup lies voltage to the horlzontal vdellectlon plates of the oscilloscope il.

The verticadeilectlon plaresof the oscilloscope tov the maximum electron cone which itis desired to record will be found satisfactory for most purposes, it is possible to provide' an aperture in plate 5 which vill 'even niore closely approximate the ideal condition by employment of the configuration shown in Fig. 4. The aperture is split up vinto three different portions. The inner portion of the aperture in plate 5 will alone be exposed during the rst one-third of the travel of the .collector electrode 6, while the collector electrode S is moved from the apex of the V for'about onethird of its total length radially outward from lthe apex of the V-shaped notch in plate 4; then,

during the next one-third of said movement, the

intermediate portion 2B vof the said aperture likewise 'collects electrons; and nally, during the -lst onethird of the'radial movement' of the collector electrode E, all three portions 25, 2S and 21 of the aperture in plate 5 will pass electrons.

The inner portion 2 5 of this aperture should preferably have a radius Rl which is (3.83 times one-third of the maximum radial movement of the collector electrode 6 from vthe axis to the outermost cone, and its length should correspond to the width of the V-shaped aperture in plate 4 corresponding to that one-third. The radius R2 of the portion 26 should be 0 .83 times twofthirds 4of the total radial travel of the collector electrode 6 and the slots 25 and 2S should together-'span .the width of the V-shaped aperture in plate 4 at the said two-thirds of the total travel, The

:radius R3 of the portions 27 of the aperture ln- I9 are connected to be energized by current proportional tobut. many times larger than that in- The amplifyingmeans comprises a photornultiplier 35 exposed to radiation .from the uorescent screen 3l and having ltsanode connected to one of the vertical plates of the oscilloscope i9 and through a resistor 3 6 to the positive terminal of a voltage source (not:

shown), The negative terminal of this'voltage source is connected through ground to the cathode of the tube 35l and to the other vertical plate of the oscilloscope l. Such a photomultiplier is believed to be too well-known to require detailed description. butexampl are described inv the ebovefentitled .application-of John W. Coltman entitled Electron Diffraction Camerali1 filed June 29, 1949,-:and in an' application Serial No.

'158,014 by Marshall and Coltman, tiled June 3l), A

1947, for an X-RayThic`eness Gage and which is now U. S. Patent 2,512,355, granted June 20, 1950. w i- The screen is supported by the electrode plate 34 which is grounded and is provided with an aperture exactly like those already described 4 as present in the plate 5 in Fig. 1.

The magnetic .ld the Winding 32 Will periodically change from a value of'zerQ to one suilcient to deect through the aperture in plate 34 the electrons which pas; through the V-shaped .aperture in plate 4 from the' electron cone of widest aperture. When the current in Winding Y32 has its zerovalue, the electrons atthe cen- 'tral axis of the conical diri-action electron beam system will pass through me narrow aperture in plate 4 corresponding to the tip of the Vshaped slot therein and will than pass throh the aperture in plate 3d to collector electrode 3 l. and cause a vertical deilection from the electron beam irom the screen of oscilloscope is; Correspondingiy,

'thedeection voltage app-lied tothe horizontal delecting plates 'n i5 will be AzeroA and the electron beam will be at one extreme ci horizontal movement. Y-

. ture in plate 4 at greater and greater radial cilstances from the apex thereof.'

The v -shaped notch in plate l insures that the current reaching collector electrodes 3l isa constant fraction of the total electron now in the conical beam sstem at the radial distance -cer- 'responding to horizontal deflections of the spot on the screen of the oscilloscope i9.

Since numerous changes may be made in the l above-described construction, and diierent em' bodiments of the invention may be made withv out departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanylng drawings shall be interpreted as Jand not in a sense. I claim as my invention:

illustrative 1. In an electron ditfraction measuring device;

means for producing a conical electron beam sys tem corresponding to the diffraction pattern of a. specimen under test. means for intersecting said electron beam system with a conducting plate having' a V-shaped slot with its apex onth'e axis of said conical beam system, means for scanning said slotlengthwise thereof by an aperture restricted in its 'dimension which is parallel to the bisector of said slot, andl means for measuring currentow through said aperture.

'2. In an electron difraction measuring device, means for producing a conical electron beam system corresponding to the diffraction pattern of a specimen under test, means for intersecting said electron beam system with a conducting plate having a V sha ped slot with its apex 'on the axis of said conical beam system, means for scanning saidslot lengthwise thereof by an aperture' reev stricted in its dimension Which'is parallel to the bisector of said slot, and means for plottingl cur- Irent flow through -said aperture'against movement of said scanning means. 3. In an electron diffraction measuring device.

' means for producing a conical electron beam system corresponding to the diifraction pattern of a specimen under test. means for intersecting said electron beam system with a. conducting plate having a Vshaped slot with its apex on theaxis of saidconical beam system, means for scanning said slot lengthwise thereof by an aperture restricted in its dimension which is parallel to the bisector ol'- said slot, and means for plotting-current flow through said aperture against move- 'ment of said scanning meansy said aperture havlng a radius which is 0.83 times the radius o the' trace on said plate of the Widest angled cone of interest in said conical beam system.

4. In an electron diilraction measuring device,

assises bis-actor 'of said slot; and means'for plotting cur'- Arens. ow through said aperture againstmovement of said scanning means, said apermre compri rg a compound curve composed of a plurality of arcshwhich are tangent to'leach other at midpoints.

5. En combination with means for prochming an electron diiraction patternfhaving the form of a conical electron beam' system. en electrode having a v-shaped aperture'with the apex of the V positioned in the axis o! Said conical beam system. meansfor producing a magnetic eld perpencular to the plane which bisects said V shaped slot, said magnetic field being loca'ied'on the side of said electrode-remote from the apex of d conical beam system, a plate electrode avisa an aperture therein which is narrow in thejdirection ofthe bisector of Said V-shaped slot and which is located in the region of the central axis of said'concal beam system, a collector elec l trode upon' which electrons passing through said.

aperture are incident after passing through-said aperture and current measuring means connected to said collector electrode. -v

6- In combination with means for producing an electron diffraction pattern having the form of a conical electron beam system. an electrode having a v-shaped aperture with the apex of the V positioned in the axis of said'conical beam system, means for producing a magnetic eld perpendicular to the plane which bisects said V- shaped slot, said magnetic field being located on the side of said electrode remote from the apex ot smd conical beam system, a. plate electrode having an aperture therein which is narrow' in the direction of .the bisector of said Vshaped slot-arid which is located in the region of the central Aaxis of said conical beam system, acollector electrode upon which electrons passing through said aperture are incident after passing tial-ouah said aperture, a source of saWQtooth voltage connected to induce said4 magnetic field. andmeans for vgraphically plotting the current from said collector electrode against the strength of said magnetic field'.

7. In combination with means for producing-an electron diffraction pattern having the form oi a conical electron beam system, an electrode hav4 ing a J-shaped aperture with the apex of the V positioned in the axis of said con-ical beam system. means orproducing a magnetic field perpendicular to the plane which bisects said Y shaped slot, said magnetic field being located on the side of said electrodes remote from the apex of said conical beam system. a plate electrode having an aperture therein which is narrow irrl v the direction of the bisector of said V-shaped slot and which is located in the region of the central of said conical beam system, la collector elec trode upon which electrons passing through said aperture are incident after passing ,through said aperture, a source of saw-tooth voltage connecte-d t9 induce said magnetic field and an oscilloscope one pair of dei'iecting plates energized from a. source of saw-tooth voltage and another pair of plates energized proportionately to the means for producing a conical electron beam system corresponding to the diil'racton pattern of a specimen under test. means for intersecting said electron beam system with a conducting plate having a v -shaped slot with its apex on the of said conical beam' system, means for scanning said slot lengthwise thereof by an aperture restricte-d in its dimension which is parallel to tte pendicular to the plane which bisects said V# I l 'shaped sot; said magnetic field being located on the side of said electrode remote from the apex of said conical beam system. a pmteelectrode having an aperture therein which is nanow in. the direction of the bisector o'said V-shaped slot and which is located-n the region of the central axis of said conical beam system, said' aperture extending in a drectitm-norznal te the piane bisecting `said V-sha'ped notch for ja distance equal l'co the width of said @auch where it is intersecped by, the'cone'of west aperture irl-said conical electwn beam.'

The olowing references are of recarq in ghe le of this paient:

STA'IES PA- A mer-mm1.. LONGINL-

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