The Production and Separation of Magnesium Ions Citation Evvard, John Cooper (1943) The Production and Separation of Magnesium Ions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/E6R3-Y720. https://resolver.caltech.edu/CaltechTHESIS:03092018-104307758 Abstract No abstract. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: (Physics) Degree Grantor: California Institute of Technology Division: Physics, Mathematics and Astronomy Major Option: Physics Thesis Availability: Public (worldwide access) Research Advisor(s): Smythe, William Ralph Thesis Committee: Unknown, Unknown Defense Date: 1 January 1943 Record Number: CaltechTHESIS:03092018-104307758 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:03092018-104307758 DOI: 10.7907/E6R3-Y720 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 10766 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 12 Mar 2018 15:56 Last Modified: 10 Nov 2023 17:49 Thesis Files Preview PDF - Final Version See Usage Policy. 18MB Repository Staff Only: item control page

OF EAGI:ESIUE IONS r.rhesis by Jorm c. Evvard In pax·tial fulfilllnGnt of' the requirGments fol" the degree of Doctor of Philosophy Californla Institute of Technology Pasadena, California 19LJ:3 Page .......... I Introduction • • II The Px•incipal of Operation of the Ion Sou1~ce A. ......... £11.e ll :tndelstein IJ:odif:tcatlon . . • • Original Form. • 1 III IV 1 1 5 5 Construction Details ... cC" .• 8 . 10 A. The Sou:r•ce of Magnesiurn Vapor . B. The Container for the l,:agnesium. c. Construction of the Copper Plate • Source I ..• • B. 1'he Wiring Diagram • • . . . . . . . c. Experimental Results • . . . . . . . A. D. Further Construction Detail s . The Problem of Resol u tion • • • • • 12 lB 20 22 v Sour c e II . • . . ............. 32 VI Theore t lce.l He solu tion • • • • • • • • • • 42 VII 11'ield Penetrati on . • • • • • • • • • • • • 48 VIII Bibliogr·aphy • • ............. 53 l Since the definite proof 0f ~~e existsnce of t., of s e pl:~-:> at :1 .-.: n of th0 isot ope s , in 1 9U2 , t he mo-cnoo.s ·1 vs_r l ou. s mf ss uni ts of a giv en ele~ment e r e bec01~i in g rn.ore nnd more importGnt . ~he use of isotopic tr a cers in chemis try , the need far 9 ure Sb~plss for nucle a r snin data , etc _ may b e mentioned as some of the l'.l01'0 COEIUlO rl USGS ret~ son , n1l~.ny o f pure isotopic s amples . ingenious sep ~:. rat:ton !/or this schemes , sone good , some bad , have be en des c ribed in the litere t ure . ~ A mo re criticcl analysis of t he ad v ante_ges and disHdv:;.;ntt-~g es of the B.L~<I.:0 1938 . t.~10se t :t1nt ·l- "(' '-., O 1 YI ,:::, 1 " J...i '-...,, t .-L u_ br~st of t he se :l s '.Chese can be c le.ased. in tHo groups : ( 1) depend upon t l1e rates of :mig:eetions of a ·I-V 0·1·r1 $ i ' q l""•L \:..: .... C !.. i o:) ) f...J \ those t hB t depend upon the accelerations of ionized atoms . the first group are : 1. Separation through diffusion . (~e~tz) Separation through f r a c tional di s ti l ~a tion . :2 . Sept::.r e.tion through the r mal d.itfusion . V • t~ ( Cl csius Lic1i::Gl) 2 4. 5. Separation by means of a centr i f uge . Separati on by gr n.vits.ti ::m . ( So:u GtiI:~e s c el led pres sure dif fu sion) . t ha t fall in t J:1e second cl a sslfic G.tion a r e : ~£'hose 1. 2. 3. Separation by mass -r ay analysis . Separation by e l ectro lysis. Separation by chemical and photo - chemical act:l on . Of the v arious me thods l isted &bove , by f& r the mo st c omp l e te sep~)r a tion so fa.I· s.ch i Gv ecJ. h£, s be en tJcc.:;rr1.p lis hed by m.eans of' the rnass spectrometer . ';fhil ::; practically l 007b si::>pe 1'" s.tlon ha.s been achi eved ;;:ith ltE.1 us e, t he size of the samp l e t ~1t:·. t CDn b s coll e ct ed :i.s nec e ssaril y limit ed by tho str·eng t h tmd. durabilit y of the ion sourc e . of t ht.;> :i on sour c e g'J:.. eater ~o make the strengt h it 1s des i re.b l <.: to u se nn extended rat::J.i:: l'" t h an £. po int SO Lll,. CG • i.'. n <~X.tend.ed sour c e inc rea se s the ion c urrent to the c oll e ct or because: (1) there i s a l a r ger ion emit ~ing sur - face , and ( 2 ) the mut u e.l s ~~H:i ce c 1rn.:r~ge x~e ;;u_lsi on per unit ion current is l ess in en exte ~ ded ion bea:m t ~.:1f;_n in a cor:fin ed ion bea1r1 . :u or t 11:is i~eas on Dr . 1 ·.i:i. ll iarr~ :d . ,Smythe de s :i.grrnd and constru ct ed a ma ss s pectr oli:eter* a t the talif ·i~· rrype I . :Note n ot Ht i on of L;rnn H. Rumbaugh , Ph . D. the sis, 1 932 . 3 of the same mass and charge from a. line source to a point collector after passing through the magnetic field. fi1is was successfully used b·,y Dr. Smythe dm:·:Lng 1924 and 1925, and la te1~ by other men at the h1boratory. An imp:r'oved 1l 1ype II instru:ment vms design.ed a.nd constructed by D1"'. Smythe and L:y-nn H. Rumbab.[~h between 1929 Hnd 1932. From 1932 to 1934 Dr. George Steward West designed and built ion sources for the Type II lens. Kunsman catalyst var:i.ety. These were of the Dr • ..Arthur Hemmendinger took over the work of Dr. West in 1934 and put the whole instrument a.nd ion sou:r'ce in satisfacto1~y operation. \'Ji th it he was able to collect samples containing several milligrams of potassimn and rubidi ,;m 2.nd to settle successfully the long disputed controversy concerning the radio-active iso- tope of' potassiu:m. I111e si tue.tion after the comple- tion of the vwrk b-y lJ:t). He~rJJt1endj_nge1l -..va s thE.1 t the application of the Type II spectrometer was limited by tho lack of new ion sources. }ior this reason, Dr. Smythe and the author decided to build a.n ion source which depended upon electron bombardmcmt to ionize the atoms. It was hoped. that this source cou1d b·a a_q};:>lied to c'ertain vaporizable meta.ls. /i ~: Incorporated in this source was the :Finklestein iEproveme.nt to increase greatly ion current. This thesis deals with the problems and theil• solutions which were met in the production of a high intensity posi tive ion source of magnes:tum ions. vc.· THE PRINCIPAL 0] 1 OPER.A~L1ION 01? 'fi-fo ION SOURCE .A hot filament A (Figure ·1) :Ls used to produce electrons. These electrons are accelerated toward the plate B .,lvh:ich is at a positive potential vv.I.th respect to the filament. rhe gas to be ionized is 1 injected into the cham.be1') C thr)ough t:he inlet tubes D. OccBsionally an electron will give up its energy to a neutral atom., thereby producing an ion. l his 1 1 positive ion is then accelerated toward the negative .filament wh:i.ch is wo-und in such a manner that most of the ions will pass rigb.t through it int o the region of an accelerator plate G. with such a source: It is seen that (1) an electron is lost as far as the so urce is concerned after it has crossed the region C once without producing an ion. a v~~·ry (2) Only few of those electrons vvhich are e:rni ttod will produce ions. (3) To get apprecialbe ionization rnany electr :J ns must be emitted by the fil81nent. The Finkelstein Bodification A schem&.tic diagram of the Pinkelstein modi- fication is shown in J.1 igure 2. 1 ~-,. e thG.t t:m electron was cap tured afte1') region C. have seen above c1~ o ssing the However, if we werB to put in an electrode F1 set at; the sa.rn:e :potent:tal as the file.ment A, the 5ources c5c/Jemal 1c 6 _ _ _ G_ _ _ ~ gas 1nlef --->- ~ ~ D C D - - _(3._ - _gas inlet - ~ A 3as inlet ~ IB D C D _ _ _F _ _ _ ga_s 1n/ef 7 electron would come to a. halt just before striking F and tb.en accelerate back toward, the region C. lilurthex·more, if a :"nagnetic fleld is placed parallel to the axis A-C, an electron which tends to diverge toward the plate B will be kept close to the axis and hence will be bounced back and forth between A and F until it finally ionizes an atom 01'1 is captured. It is seen that with this &rrangement of the source, there is a saddle point in the r•egion C. If a positive ion is formed above the saddle point, the ion will go up toward tho filament .J~. If it is formed belo'itv th0 saddle point, l t vdll go down towai'1d B 1 • However, only the ions which are going a given direction can be utilized. Hence, it is seen that we have two alternatives. il 1ype I: the soUTce can be arranged so that the ions utilized will go toward the fl lament, and 1l1ype II: ~1rranged the sou:tlce can b e so that the ions go away from the filament. Both of' these a1"rf;:.ngements have been constructed and Esed successfully to produce ions. !J:'he re111£-dnde1" of this pap ex· 1.viJ.l dea.1 with thedete.ils of construc- tion and operation of each. 8 THE 801:.THCE OF l\'.LAGNESI UM V.APOH 'I111e source of the magnesium vapor vms a cylin- drical pipe of pure magnesimn~~ about ten inches long, 5/Bu O.D. arm. , 5/l::H o I. D. rl 'l1i s ~t;)ipe ( Pig;..:i.re 3) was heated by a furnace constructed by winding #24 nic h ro:me wii,.e between tvw concentric stupakoff tu.bes; .12ou r~ · b., .196n . O.D., and .2?8 11 \J. L., respectively. This furnace was then inserted into the magnesiurn Pi::·p e. l wo such fu1'lnaces in series had a total 1 1 resistance of' 10 o1:Lms and p1•oduced arnple evapo1,.2.tion when they were run at frOEi. three to five amperes. 11he hot magneslum. alJ.oys readily with :many of the connnon metals below tb.eir natural melting tempera.tures. Purthermore, the resulting alloy decomposes readily in H vacuum. rf i1e problern was to find a material t h at would be non-rnagnetic a.nd yet wo1.1ld he.v E: little tendency to alloy with :m e.gnesimn. l he 1 1 fir·st tubul£ir ·container to be tried was ms.de of br")ass • ..After several hours of operation, this tube 11ad fa.irly l a rge holes in the walls. Copp er was then tried but found to be only sl:lgl1tly better. . .~~9..§.S .,.,._( 3/4 n ~~ Purchased S t ainless steel c. D., U.S. lB-8, permes.bili ty--1. 0 0 3) from Dovv Metal Co. 9 rurna.ce P /dte .T r1gure 4 10 were:'.) then t1.,led and fo und to be setisfa c t ory. ~I.111e ste e l t abes ·w ere then screwed to a milled copporl block. :hvo s u ch blocks screwed together fcr·med the electron plate. Sta:lnless steel is undoubtedly the best material to use for the plate, but copper is applicable in that it is a good conductor, does not contam.irio.te the system extremely and has a thG:cmal expe.nslon that· is not too different from that of stainless steel. A piece of coppe1., bar 12u by l/2H by 11/16tt was first milled to an approximately circular c u t by means of e_ 3/4n end mill set at a very slight angle (Figure 4.) The stainless steel tube was then fastened to this by a single screw in the center and co ~:;p er plugs on the ends of the t u bing. 'f i1ese plugs were made to fit freely intq the stainless steel tubes and then to ~lange out to a 3/4rt d:l.a.meter. 11he l~n.,ger section was screwed securely to the copper, wh1.le the stainless steel tube fi ttGd loosely ovGr th~ smaller section. This allowed for the difference in therm.al expansions of cop p ~)r and stldnless steel while t.b. :·~ single screw in · the center kept the tube from rotating. Having been scr·ewed toget:b.er, holes 11 were drilled. with a //18 drlll a.bout 3/8 u apart. These were the gas inlets D of Figures l and 2. 'l1vw s u.ch sectlons screwed together formed tho Chnmber C ( 1?i gur e 5 • ) 12 SOURCE I Figure 5 shows a scale drawing of Source I. The steel plate J was screwed to the copper plates B. rfi1e bouncer electrode F was made of iron and ran the length of the smfrce. It was insulated from the copp er plates by means of small pieces of glass K cut from thin spectroscopic plates. The magnetic pole pieces, N and S, furnished the field which kept the electrons on the axis. _4.,~~, ifi1e tantalum ftlmnert s, were supported to the iron pole pieces every 2i- inches. If fewer supports were used, the interaction of the alternating heating current with the magnetic field caused the filaments to vibrate badly. Oxide coated filaments were tried successfully in a previous trial so Ln·ce of smaJ. ler dimensions. Hom~ver, it was thought that due to the long length of the p1•esent source, these would prove 1:msatisfacto1"\y~H(. Calculations showed that some mechanism is ~~ Strip tantalum filaments .00;3" x .085n x lOff pu1')clw sed from the F'&.nsteel Cor ::; oration vrnre us ed. 1I1!1ese had extremely constant cross:. sections ( a.s is defdrable for long life and un:lform emission). ~H~ ~thermal expansion of the filaments plus ·excessive vibration would probably destroy the oxide coating. If was found empirically th2t filaments or electrodes arranged perpendicQlar to, rather than parallel to, the source tended to destroy the resolution. 13 Source 1 14 necessary to compensate for t h e the r m.al expansion of the file. rnents. ~L1he mechanism which p roved satlsfact- ox·y consisted o f an 1/f3 ° t h ic k whe ul mounted on a. b e aring. 1jhe filament and a s p x•ing act LLated lever were fastened to the peripher7 of this d isc, l igur e 6. 11 to the large currents t h ect t he filaments carri ed (in excess of 20 amps apiece ) it was neces !:Jcry to grou.nd the wheels to the polE~ p iece by means of an auxiliary wire rather· than to depend up on the bearing s u rfe.. ce for cont e.ct. 'H1.e sou1'1ce block was used as one of the filament leads~~. .J:he other end of the two 1 para llel filaments were sppt~w_ eld~d to .a l/8H t h ick nickel pl a te, cut to the width of ·the spacing between the filaments. This could be insulated from t h e sup- porting pl a te by a small soapstone insul_e_tor or b y e_n arx·a ngement similar to that · shown in Figu:rle Be. '11he nickel pl a te, e, wai held solidly to th~ a~ p p orting p late by the screw., a, which was insulated by t he st upakoff t ubing, b, arnl, . mica washers, d. n·iis method ga.ve no trouble. 11he fi.rst . a:-ux·iliary filament s up p o1.. ts to b'e tried w er~ made of sb,'e.et mica. · 11hese proved to be unset.' , '- i ; .5 f a ctory becaus·e , lipparently, there vv-as a che:rr:. ica.l ~ctJ.._or~_E.etween % t h e hot fil aments and the mic e. , The two filaraents we1,e operated in p a rallel. 15 r1 la.ment Take-up 6gure 6 hgure 7 r1 lament Supports 16 I I I I I J L - - - --:.J.:"":;....,.,,_ _....i rtgure 8a figure 8b rigure Be 17 1' igur·e 7. 1 'J.!he second supports tx·i ed were na de of molybdenum sheets • 002n thick, li:igure Ba. These were. held in place by mica lined slots in the iron at the position of the mi ca supports se en in Figure 7. While they served their function of support :tng the filaments, they destroyed the resolution (to be explained lat(~ r. ) The next support that was tried, Figure 8b, consisted of a tungsten wire bent to sh&p e and set in a screw head drilled lengthwise. ir he wire was clarnped by small steel pins wedged into the hole in tl)e screw. 'These s up_~)orts proved to be satis- factory both from the standpoint of resolution and support. Occasionally, the steel pins would loosen up; this s1.::.ggested that as an improvement the t ung- sten coul d be welded to nic kel screws. filament supports that vve1~e The final used, J.;'igure 8c, b.si.d molybdenum strips riveted t o the screvv head, a. 18 A schematic arrangement of' the w11~ing diagram is shown in Figure 9. T·he plate B was connected to the positive terminals of a 6000 volt and a 150 volt source*. The negative side of the 6000 volt line was connected to the accelerator plate G and to the case of the spectrograph. The negative terminBl of the 150 volt generator was connected to the source block to vY-hich one end of the fila.ment vvas attached. 11he filament current we..s furnished by an :l.nsula.tion transform0r controlled by means of a variable resistance ln the p1,.irne.ry~Ht-. A similar arrangement furn- ished cc1_rrent to the magnesiu_m heaters·. Al1 connect- ions cou..16. be altered at will as test conditions r eql:_ired. It is interesting to note that when the el<::ctronbouncing electrode F' was left floating, :lt e,ppPoxims.tel-:· assurned the ss.me potential as th<:-; filament. v~-i10·n it reached the.t velue, no more electrons cot:ld be captured by it. ~hus, the potenti&l of the bouncer electrode vms 2.utomatica.llv self be.lancing. ·-. ~The voltages were furnished by D.C., hl.G. sets. _ _ _..........._.. , , •1 ~~ t..' ... _ The current in the secondary was of the order of 20 amps pe::-:1~ filament virhile t1u:d:; of' the :J:Pimary was b.alf as gi,eat. 19 f2 ~ ~ '-...... ~ ~ ............... ~ ~ 0) ~::::, ~ 20 Preliminary trials ~ei · o1~ the sake of convenience, let us e.dopt the following definitions: Filament f'ilament s • C '~lrrent--current used to heot the .B:lnission cur·1·ent--electror:. plus posi. tivG ion cur:r·ent ..to or f1"om t11e fi1e:ments. Pl~1te voltHge--potentie.l diffe:rence bet~veen the plate, E, and filaments, A. Accelerator vol~sge--potential difference between the µlutes, B, and the accelerator grid, G. Bouncing field--~egnetic field required to keep the electrons on bhe axis. rieater current--cur~ent carried by the heaters of the :nwgne s ium. l.;'or the purpose of mE::as t.J.:r•ing tT-1e positive ion current :produced, the pla.te H~ (Pigr:.1•e 9) was installeC. .sn6. set ::' t; c potentiel cd' (!.) vo1ts E..bov 2 g:c·ou.i.:~c.t. this potential was adequate to prevent secondary emission. A positive ion curr~nt of 1.2 mils. wes measured under the following conditions: Electrode r.r G A B }1 Potentie.l 45 volts 0 650 700 650 ll File.mEmt current 3b e:ri11JS • lfoL- ter current 3 ari19s. Em:'.Lssion cln'·,:ent 100 mils. Vacuum 2x10""5 :r.un. The second test yielded a 9ositlve current of 1.4 m:i.ls un<i.er the f'ol1ovling conditions: 21 Electr·ode Potent:Lal 45 volts G 0 " .Li D 975 1000 F 975 ·1 ·1 Ir ii ~· l lrnn ent cm->rent 3:3 amps . Iiee.ter c c.r:eent 3 am113 • :iii.mission current 100 mils . V~lcu:un ~~x10 -fi mm. H If It is interestin g to note ths_·.~ th<0 :positive ion current was an eztre~ely criticel function of the _plate V (?.~tage. Appar·ently , if the l&.ttor wes to o hi gh, the electrons d.id net rern.ain in tho ionizing re g ion long enough to produce ions. In e~:,;ch of the above cases, the pl Dt e voltage was set to give maximum positive ion ctr :-·r·ent . 11hese results •.vere go9d enough to warr·ant n trial f.;tt separs.ting magnesium isotopes in the rncrns spect1~ometor. •:ch:ts attempt brought on thH problem of resol u tion. 22 fhe me ss spectrometer collecting slit width corres ponding to one m2.ss unit is app J:-o.ximL tely W = 20/IvI whe re Wi s in centimeters. 1'h u s, if e~ 2 .nmi . slit is used to·· collect mass 2·1, the slit intcgr L·tes the cu~e rent over • 2,.1 of a ms.ss unit. £:hl s is small enough so that defects in 1')esolution 1 can not be attributed to the slit width. J:l' or the s ake of comparison, :B igu1'les 10 to lf:S 1 inclu~ive, have all been normalized. The follo ~ in g t[-1b le gives s. com::) e.rison of operating conditions~H~. P:i.g. Accelerator Vacuum at voltage Estimated Bouncer vacuum at field source collector (mass 24) 10 11 12 13 14 15 3430 3430 3440 3470 3470 3470 10 - 5111111. io-5mm. lo- 5:mm. 3x10-5rr. m. 2x10-5mm. 1 • 2xl o- v-.ann. i:: good good on poor fair off off off off good on? good Pigure 10 shows the resol u tion curve of the :source with the bouncing field at approxi~ately The resolution is obviou sly not good . 400 gausses. Figure 11 shows· the observ :::, d curve under similar conditions 7 '" See Rumbaugh, Ph.D. thesis for resolution. t }:10 theory of ·~H~ Iihe collecting slit width was 2 mm. 24 25 r1gure 10 26 t 23 24 25 Mdss-- 26 25 these t :;vc cu.x·ves, it ·vv ould appGa.r· that use of' t he 1 m.agnetic bouncing field tended to destroy the res olution. this mi ght be CBJ :-;. sed by the scattering eff'ects d:.le to increased spe.ce cho.1"'ge . 'fi'le rrrn.gne tic fi e lC:~ :me.de 8 d :L fference of abo u t 3 to 1 in the E:S?_~L}..~.§'.£ ions t hough less effect ln collected ions. The resol ution is very sens i tive to ·i on velac i ties parallel t c t :::1e li':.:; r th:i.. s res.son, it so t.~ rce. Nr:::.s thought t het the lack of resolution E:i g ht bo caused by the Ll lterna t ing electric field ~roduced by the IR drop along the filaments.~~ Pigure 12 shows a C\.:. rve traced while heating the fils.rnents with direct cu:r•1~ent. Its shape :Ls so singular t tw.t doubtf u l imp1~over~1ent resulted from this test. Dr. Smyth e sLLggested that the sharp edges of ·the n:wlybdenum fihm1ent s-Llpports (Figures ? 2nd Ba) might be causing the scattering. ~o test this, the tung~ten vvire supports (Fig ure Bb) were installed. Pig·u r GS 13, 14 and 15 certainly demonstrate that this wns the chief' factor in the poor resol l:~ tion. Compa1--is~m of J:ii i garr:1 s 1 3 and l '-± shows the effect of' va.cuum c one.- I' igu.re 15 shows a c1n-ave l ti.ans on the resol u_tion. simila.1· to J?igu.re 1 4 except - · -· ·• ..... ,r ...•••"'T"'"' ... -~ . ........, _ th~:- t the vacuum was • ~~ 'lihis fiel d. amounted to abo t.J_t 2 vol ts per centimeter peak t;o peak. 26 1 23 24 25 MdSS - - - - Figure 12 26 t Z3 24 25 Mass-~ r1gure 13 26 28 l 23 24 25 MdSS - - r;gure 14 26 29 1 23 Z4 2S Ma.sc5-~ r1gure 15 26 30 better end the magnetic field was on . Unf ortunately, th:i.s curve is subj e ct to some dou bt since it 'Nas dis covered l~ter that a protective neon bulb in par a l lel with t h (3 bouncer magnet hHd s.n intormittent s.ho1')t in its cifn. . c u it • 1J:lhe experimental results l ed to the f o11ovdng c onclusions: 1. All condu.cting or insulating st1')ips vvhlch are in the ion b eam should be eliminated, if possibl e . fi1ose wh:i ch c annot be elimirn.:i. tGd sho1J.l d r•un p arall e l to the line s ource so as to .a ffe ct the focus r nther 2. Filaments parallel to the source c an be ho&ted with 60 cycle AG without se:-ei ou sly affectin g the resolt:i. tion . 3. 11he bet cer the vac uum, t he better wil l be the i ... eso l ution . LJ: . Ap9ar ent ly, the magn;;:· ti c bouncing fie l d aff ects the r eso l ution to some extent but prob a bly c o ul d be used in the c ol lection of' t he main com- ponent. 5. Eeso l ution of th~"3 ion current is im~9roved by ru.ri. .ning the source at l ower tem_;.) ers:tu.x·es higher• v oltages (Figu re 29 .) f.~nc~ 31 Varying the plate vol t~tge ha~ f1')0111 25v. to 125v. little effect on the resolution but greatly affected the intensity of the ion current to the collector. The maxim.urr1 coll.ecto~r current usuo.lly occurred when the plate voltage was about L_W volts bu'l:i this VHlue varied from test tot est. 32 The Type I source yielded good resolution. The ion current amounted to 1.5 mils at a pressure of lo-5 rnrn Hg. Using only H. single acceleri:::.ting slit, the cur1"'ent to the collecto1'} wa.s ;24 microamps. How- ever, it was f'ounct that aft er about 1000 volts were applied hetvveen the plate, B, and the accele1')ator• electrode, G, the ion current ceased to increase with the inc:t.,easing voltage. two ways: This could be explairi..e·d in ( 1) the 1000 volts pr oduced a fieJd· h:lgh 4 enough to draw out all the ions formed. This possi~ bility was discounted by increasing tho filament heating current~ Neither the electron current nor the positive ion current increased in the ratio expected. ( 2) The field dv.e to the accelerator vol te.ge was suppressing the electron emission.* The 11ype II source (Figure 16) had the filament mounted far down between the plates, B, so the.t it was elect1"lically shielded from this latte:e effect. (For approximate penetra tion of this field see Figure 31.) -..,..·-- ··~"'"" ~t· '.I1he In addition, the field produced by the high field from the accelerator grid to the filament tended to cancel the field due to the plate voltage, thereby retarding the electron emission. 33 5ource 2 w • r1gure 16 34 v oltage aut omatically boun ced the electrons b a c k t o ward t he filamE:mt. It is to be n.:YGE-)d that t :l'.lOSG posi - ti v e i ons which \Vere for med above e. c e1.,ta in sadd l e po i n t were t he only one s utilized. 'l'hi.s meant that t he ions came from a c oo ler region and hence, on the basis of th.e ii;~axw e llian t he r lr1al d i stribution, one exp ected· t he r e s ol uti on to be better than in the ':Pype I sov_1.,ce. r ·:rpe II source h&:.d the disadvantage 1 that to work efficiently, the bo ~ncer magnetic field had t o b e: i u sed . Pigure 16 shovrn the arr a n ,__, g:e:ment of t he 1rype II sou.roe tried.. Various vertical pos:i. t:i.ons of the filam.ent were ti.,ied. Copper ·was s ub stituted for .iron on t he p l a t e , S , while . S was mov e d to the poRition of G. Vario ~ s slit vddths between the two pieces, S, were tried . }'igure 17 shows a rep1.,esent e.tj_ ve curv e taken vvlth Source II. 'l1he maximum Cl.u.,r·ent to the c ollect or (aided by a. foc u sing wire) was 40 micro a.mp s. the l~esolution wa.s n ot good. .dovrnv .~: r, In e.dcati on , the l ife of the sou.r.ce was abo ut four to five hours because t he n1agnesium va p o1~ in t h e ionizati on region c on- d ensed on the c ol d iron fi l ament s1~ por t b l o c k , N, pi ling u.p beautiful laye rs of magnesium cr};rst als until it fint-:::i. lly short ed a gain st the h o t p l a~t e , B. 1~:J.is SS 23 24 25 Mass-~ Figure 11 26 36 acted 1:1.ke a powerful JYmnp keeping the ionization region at a low· pressu.re, and hence, decreasing the :~fo ion cu.rr·ent. counteract t his, the magnesium was heated to a higher· temperature. 'l1his cre a ted quite a magnesium gas circulation which tended to destroy the resolution. Type II source had the further disadvantage t hat the use.ful iont; were produced in a region of low electron density, that is, farther from the filament. In view of the short life of the source, s_nd the poo1"l i~es o lution as shown by the representative ex.per:lmental curve, the research on Type I I source was abandoned. A now sou rce I embody ing 'the constructional advantages of the 1_Pype I I source was . built with the assistance of T. H. Pi. rhe gdral plan of the source was similar to that shown 1 in Pigur·e 5 except that copper filament supports were substituted for the pole pieces, S, the latter being placed above the cop r) er. In addition, the width be- tvv-een ·the' filament s was decrea.secL to retard the magnesimn losses. The various .part o f this source shown in Figures 18 to 26, · inclusive. arr·anged in the order of assembly. m~e 'I1hese are rhe author re- 1 grets tha t he did not have time to test fully this new source. C'est la gu erre! 37 i·· 6gure /CJ rigure 5ource 13/ock 19 .· Mdgnes1um Rods 38 rigure ~I lie of ers rlgure 22 I !7Sld lid f 1017 r;gure 23 !Jldment Sup;:;orls l· I L Jupports figure 24 flldme11f r'!Jure 25 rl!a.ments Installed 41 Assembled 5ource ,Figure 26 42 The ~axwe ll - Bo ltzmann thermal distribution limits the resolving power of' a perfect source . Hu:mbaugh1~ hf:.d shovm tht::tt the numbe1" of ions em(0rging from. the mngnetic field at an angle oc. is ( F1igure - ..£. - CA e dN.c 4k ~?,?) : (L\VJvJZ .X.T s n.l 1 cie doe (1) wher>e f., = chc.rge on the ion k = Boltzmann c onstant r = absol ute temperature V = voltage of the center of the r:e ak of the given mass LlV = actual voltage minu s V . 1 Eowevex• , h is method of summing this equation wa.s :Ln erx·or and hen ce , will be corrected here • .As s ume the source enJ.i ts C ions per unit length. Let X re}n·esent the position t:~.long the source as shown . 1-ie~JI"esent ·Let r peths . the radius of' c u r vature of t:h.e ion It is clear from the geometry that = f2r sin /3 + r = rf2 sin 1.3 + 1) dX = 1'"12 cos 1.3 d/3. (3) But c( = /3 + 1r/ 4 . (4) X Hence dX = r(c os ~ + sin~) d~. ( 2) ( b) J?o rt he seke of' convenience , let us def':l:ne r S --I-Ir (Av)Z ~ v T ( ()) ~---· - ;~ .F h . D. 'l 1hesi s , Califor~nin Institu_t e of 11ecrmology . 43 5pecfromefer ~ r~ ~ \ \ \ \ \ I \ / ) • \ / -- / ~---4-r ~--· x -~ -&-----~ Sovrce r1gure 2 7 44 Subs ti tu ting ( !5) ·and. ( 6) into ( 1) and integr·ating from ot = 11/4 .to o<. = 11"'/2, we have j/rcoso( + s117oe) e -rs,;: N = cAr (?) 4 But jl' COJrX. e .,. -FV 1 = it 1 e du I -;z- and J: 1" S//JoC e -rs1n"r.c dq:_ ( 9) 4- ~ . I · '"== CAr N = L/Cl -FJYle F~u .... u~ • • {10) The fix· st term in the ~-~·~ _<?}~et of ( 10.) hB.s a rapidly conv erging series and hence is easily SlJ.JJJmed. ;J:he second term of the bracket is the 1r obr:~bility This was evaluated integral. value s of P with c o:Pr•es p ond ing ,c._ s 11 . ., c: co1·1··· son ' _r .~J ari· . by tables. v~:;. l ues e- • 7ff1:i1 i· s 11· ste c:;·. • A few of I Eire given. Prom this , .L.. it :i.s seen that Rtunbaugh' s equa tion, N = N0 e-,. . .ti .A'r .t .( 11) is approximc tely true. The value of 8 1 is in this C D.Se . 78 . p 0 .146 . 584 1. 31 LJ 2 . 336 3 . 65 5 . 26 7.1 6 9 . 35 1 • . 625 e- •'I d.ti 1 . 291 . 6 32 I 8C)0 Jv '-' • 3hr4 Du .1 63 . 35D .1 6~~ . 062 . 019 . 005 . 001+ or:::..p . 017 • OOtl . oe:1• t.J V 45 Figure 28 sho ~s a plot of I as a function of F. Prom this, Pigure 29 was comrnt.t ed under t h e fellowing ass t:;.m-o tlons: Isotope Ji abundance Voltage of peak 24 25 26 .T74 4000 .115 .111 J~bs. 'remp. 20009 K Since, ' ( 12) curve 29 is approximately equiv&lent to a source ope1'la.ted at 3fi00 vol ts and a temper·ature of 1750° K. Pigure 29 is to be compared. w:t th the experimental curve E" igure 15. 1 Combining the app1'loximate equations (11) and (12), we see that the resolution depends exponentially upon the ratio of V/T, thu s justifying conclusion (5) of page 30. 46 ~ C\:) ~ ~ ~ ~ "" ~ 0) ~ C;) ' C) 0\ ~ ~ 0 t'-.. C() <5 ti ~ <:) I ~ c5 t'r) <:) C\l c ~ C) <:::> ~ f\J t :::Jcu ~ '~ !( 47 1 23 ~4 ZS Ma.ss-~ 48 FIELD PENETRATION Both in the 1rype I a..11.d in the 1J.1:ype I I source, it is desirable to know thedepth of penetration of the field between Sand G int o the region C (Figure 16 . )· We shall approximate the actual arrangement by the slotted pole piece of Plg11re 30a, wh:i.ch :i.s electrical- ly equivalent to Pigure 30b. The Schwartz transf or- . f'·or l'- igure .3 Ob is . ma t ion given in 'l • w • H0 0 t a"GJ..C 1 • .....:1 ~11!L1. " ,..., J.Jyn- amic giecti~icity 11 , by Dr. \7illiam H.. Smythe (page 29~) and is (1) where g = x + iy, '~V = U + iV and I is a constant. In this c Gse , V is the potent:l.al function. Prom Figure 30b and equation (1) we get· the following conc:U tions: = o, x = o, u y If y ~o.o, v = o. ( 2e.) - o. ( ~3b) V.+V 0 • (2c) Putting (f~b) &.nd ( 2 c) into (1) and noting that the second term of (1) rema ins finite for V = V0 , we see the.t (3) Therefore , if x = O, equation (1) bec omes 49· ---------~- ZA Figure 30d r;gure 30/J _50 (4) The field on the x = 0 e.:x:is is 1)'// dv/ · == £"Y = - J;/x .. o ~ COStf:Vo_ -y.1A.z + 13,zcosi!,n ;JVo ( 5) For _the sake of an appr oximate calculation, let -2A = 5/16n, 2B c 5/lon, V = 4000 volts. lience A • 5/02n, B 9 5/32", V0 = 2000 volts. li1igur•e 31 shows the plot of V and IEyl a.s a function of the distance y. The position y = B = 5 corres- ponds to the top surface of the Plate, S, of ~igure 16 . .51 40 2.0 ·8 .9 ·6 .(j .4 .7 .z .6 3.0 .5 .{j .4 -6 ,3 .4 .z .z ./ 2-·0 / .O ~ ~ ~ ~ ........... ~ 'CJ ............ Q) potenf 10..I ~ I 'ti ~ ~ ~ /'1c/d ~ ~ l·O s ·8 .4 .(j .J .4 .z .z •I o.o I ,£!, 3 4 5 6 7 8 y - _j unit - JZ - It I r1gure 31 9 10 II IZ 13 14 52 AC KN 0 ~-~LEGJ:!J':~ BNT l he author wishes to express his sincere appre- 11 c:lB.t :l.on to Dr. William H. Smythe, whose construct i ve s uBge stions greatly aided the progress of the rlesearch. 53 BIBLIOGRAPHY 1. Aston,nhlass Spectra and Isotopes.ti 2. -.JJ· .. R 1_. Sim7:;"' th e, u .:>-Ca ~ . t ic . McGraw-Hill, 1939. 3. w. Walker, "Ergebnisse der Exakten Naturwissenschaffen, XVIII Band, 1939. 1. B. F .. Bathe and A. J. Dempster, .Astrophy. J. 75, 34 -39 ( 1 932) . 2. H. R. Crane, C.C. Lauritsen and S. Soltar, Pl~fs . Rev. tJ:5, 507. 3. Ann Catherine Davi es , Roy . Soc . Proc. 1 55A, 123124 {]\Iay , 1936) • 4. A. J. 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