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The vacuum system of the University of British Columbia Van de Graaff generator and a mass spectrometer.. 1951

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L£5 37 I 9 Sf A 8 Rs\\/z THE VACUUM SYSTEM OF THE UNIVERSITY OF BRITISH COLUMBIA VAN DE GRAAFF GENERATOR AND A MASS SPECTROMETER LEAK DETECTOR by Eric Harvey Richardson A thesis submitted in partial fulfilment of the requirements for the degree of MASTER OF ARTS IN THE DEPARTMENT OF Physics We accept this thesis as conforming to the standard required for the degree of Master of Arts: Members of the Department of Physics The University of Br i t i s h Columbia Vancouver, Canada May 1, 1951 THE VACUUM SYSTEM OF THE UNIVERSITY OF BRITISH COLUMBIA VAN DE GRAAFF GENERATOR AND A MASS SPECTROMETER LEAK DETECTOR ABSTRACT The vacuum system o f the Van de G r a a f f generator i s d e s c r i b e d . Techniques i n the c o n s t r u c t i o n o f the h i g h v o l t a g e vacuum tubes are i n d i c a t e d . The r e q u i r e d performance of the system and the methods of a t t a i n i n g i t are o u t l i n e d , c a l c u l a t i o n s being g i v e n i n the Appendices. Vacuum p r e s s u r e gauges a r e d e s c r i b e d and the i n d i c a t e d performance o f the system recorded. Vacuum p r o t e c t i o n c i r c u i t s a r e d i s c u s s e d . Methods of l e a k d e t e c t i o n are d i s c u s s e d and the U n i v e r s i t y o f B r i t i s h Columbia mass spectrometer l e a k d e t e c - t o r d e s c r i b e d i n d e t a i l . The theory o f a n a l y s i s by a s p e c t - rometer w i t h a coterminous c r o s s e d e l e c t r i c and magnetic f i e l d i s g i v e n . An o p e r a t i n g procedure f o r the spectrometer i s d e s c r i b e d i n d e t a i l and some r e s u l t s recorded and d i s - cussed. ACKNOWLEDGMENTS This work was financed from the Van de Graaff grant of the National Research Council of Canada. The author i s pleased to express his gratitude to Dr. J. B. Warren for guidance and assistance throughout this vrork. The author wishes to make i t clear that he had nothing to do with the original design of the vacuum system of the Van de Graaff and that the majority of the work on the system v/as done by others. Special credit must go to Mr. Al Salone^in particular for his work i n making f i n a l , exhaustive vacuum tests of the tube sections and in assembling and sealing the high voltage vacuum'tubes. The author washes to acknowledge the work of Mr. "A. J. Fraser of the Physics Department machine shop in constructing the mass spectrometer leak detec- tor. Credit must also go to Mr. John Lees, glass blower. C O N T E N T S Page I I n t r o d u c t i o n 1 I I The Design and C o n s t r u c t i o n o f the Vacuum 2 System (a} The High V o l t a g e Vacuum Tubes . . . 2 (b) The Pumps 5 I I I Pressure Measurement . . . . . . . . . . . . 7 (a) General 7 (b) P i r a n i Gauges 8 (c) I o n i z a t i o n Gauges 9 Id J McLeod -Gauges 12 (e) Data on Performance o f Vacuum System 13 IV Vacuum P r o t e c t o r C i r c u i t s 16 (a} P r e s s u r e R i s e 16 fb) Water F a i l u r e 17 (c) Power F a i l u r e 18 V Leak D e t e c t o r s 20 (a^ General 20 (b) Improved Ion Gauge Leak D e t e c t o r s . 22 (c) Mass Spectrometer Leak D e t e c t o r s . . 24 VI A Mass Spectrometer Leak D e t e c t o r f o r the Van de G r a a f f Generator 26 (a) Design 26 ( i ) General ( i i ) Ion Source ( i i i ) A n a l y s e r and C o l l e c t o r ( i v ) Power SuppMes (v) Gas Feed (b) Theory 30 (i} S t r a i g h t Through A n a l y s i s . 30 ( i i ) D i s p e r s i o n 32 (c) O p e r a t i o n 35 C O N T E N T S (cont'd) Page (d) R e s u l t s 39 (e) I n t e r p r e t a t i o n o f R e s u l t s 45 ( f ) Suggestions f o r Improvement . . . . . . 47 B i b l i o g r a p h y 48 Appendix I Appendix I I I L L U S T R A T I O N S F i g u r e Page 1. S i d e View of Tube S e c t i o n ' " 2 2. S i d e View o f the Mass Spectrometer Ion Source 25 3. S i d e View of the A n a l y s e r S e c t i o n 27 4. S i d e View of the C o l l e c t o r S e c t i o n 27 5. C i r c u i t Diagram o f D e f l e c t i o n P l a t e Power Supply and I s o l a t i n g Transformers 28 6. Photographs o f O s c i l l o s c o p e D i s p l a y s o f A i r Peaks on a time Base 40 7. Photographs o f O s c i l l o s c o p e D i s p l a y s o f A i r Peaks on a V o l t a g e Base 40 8. Photographs o f O s c i l l o s c o p e D i s p l a y s o f Helium and A i r 42 PLATES I . Cementing Press 3 I I . Pumping P o r t Pots a t the Base o f the Van de G r a a f f 4 I I I . Vacuum System a t the Pumps 5 IV. Mass Spectrometer Leak D e t e c t o r and Apparatus 26 V. I s o l a t i n g Transformers and Power S u p p l i e s 29 V I . The D e f l e c t i o n P l a t e Supply g o , V I I . Wired Ion Source and A n a l y s e r 35 V I I I . O s c i l l o s c o p e Sweep V o l t a g e Supply a t O s c i l l o s c o p e 37. T A B L E S 1. Rated Pumping Speeds o f D i f f u s i o n Pumps 6 2. Rated Pumping Speeds o f Fore Pump 7 3 Outgassing o f "Vacuum System 14 4. D i f f e r e n t i a l Tube P r e s s u r e s vs Pd Leak Voltages 15 5. Performance o f the Ion Source 40 6. Unanalysed Beam Currents 40 7. Mass Spectrometer Operating C o n d i t i o n s f o r the S p e c t r a D i s p l a y e d i n F i g u r e 8 43 8. A comparison of Measured and C a l c u l a t e d v a l u e s o f Vd f o r the c o l l e c t i o n o f N 2+ and He* 44 1 THE VACUUM SYSTEM OF THE UNIVERSITY OF BRITISH COLUMBIA. VAN DE GRAAFF GENERATOR AND A MASS SPECTROMETER LEAK DETECTOR I . I n t r o d u c t i o n I f the Van de G r a a f f generator i s to produce a w e l l d e f i n e d beam, the p r e s s u r e of the gas i n the system through which the beam passes must be s u f f i c i e n t l y low to prevent an a p p r e c i a b l e number o f c o l l i s i o n s between beam p a r t i c l e s and gas molecules. F o r example, a p r e s s u r e o f about 1.5 10"^ mms of Hg i s r e q u i r e d to pass 90 pe r c e n t o f a p r o t o n beam without c o l l i s i o n through a path l e n g t h o f 20 f e e t i n hydrogen gas. The c a l c u l a t i o n o f t h i s p r essure i s g i v e n i n Appendix I . Even without c o n s i d e r a t i o n o f beam s c a t t e r i n g , an upper l i m i t o f about 1 0 ~ 4 mm o f Hg p r e s s u r e i s r e q u i r e d throughout a l l p a r t s o f the h i g h v o l t a g e vacuum tubes to p r e - 2 vent f l a s h o v e r s when h i g h v o l t a g e i s a p p l i e d . I t i s i n a h i g h v o l t a g e vacuum tube t h a t the beam i s a c c e l e r a t e d through the v o l t a g e developed by the generator. T h i s a c c e l e r a t o r tube must be capable o f withstanding 65,000 v o l t s a c r o s s each of 2.8 i n c h i n s u l a t o r s and p o s s i b l y 250,000 v o l t s d u r i n g surges. The diameter o f the tube should be as l a r g e as pos- s i b l e to hasten o u t g a s s i n g and keep the pr e s s u r e down d e s p i t e the l e a k from the i o n source. The tube must be st u r d y enough to stand a pr e s s u r e o f 200 l b s per square i n c h w h i l e under vacuum and must be vacuum t i g h t under these c i r c u m s t a n - . ces. I I . DESIGN AND CONSTRUCTION OF THE VACUUM SYSTEM (a) The High V o l t a g e Vacuum"Tubes The two h i g h v o l t a g e vacuum tubes of the Van de G r a a f f generator a r e s i x t e e n f e e t i n h e i g h t and c o n s i s t o f a s u c c e s s i o n o f p o r c e l a i n r i n g s and s t e e l l e n s e s . The porce- l a i n r i n g s a r e glaze d on the c y l i n d r i c a l s u r f a c e s and the plane ends a r e ground f l a t to w i t h i n 0.002 inches and p a r a l - l e l to w i t h i n 0.00\6 i n c h e s . The s t e e l e l e c t r o d e s a r e shaped to s h i e l d the p o r c e l a i n s u r f a c e s from p a r t i c l e s s c a t t e r e d from the beam and edges are rounded and p o l i s h e d to e l i m i n a t e a v o i d a b l e corona and f l a s h - o v e r s . Figure 1 Side /lew of a Tube Section Green ink: porcelain SCAIS: 1 / 3 SIZE 3 The tubes are assembled i n sections. Each section is made up of four pairs of lenses and rings cemented at 400°F under compression. A typical section i s shown i n Fig- ure 1. The portion of the electrodes to which porcelain i s cemented i s made of thin stainless steel and is welded in only three places to reduce stress i n the porcelain due to unequal expansion coefficients i n l i e u of using a cement hav- ing a lower setting temperature. The cement used was Vinylite Resin Solution, Blend 571, manufactured by the Bakelite Corpo- ration. Steel spacers are sealed between sections with f l a t Neoprene gaskets 3/64 inch thick and greased with high vacuum lubricant. The steel apacers have 0.005 inch ridges 3/8 inch wide to prevent the gaskets from being drawn i n . Most spa- cers are 3/8 inch thick but about every tenth spacer was made f inch thick to keep the tube electrodes at the same level as the stack electrodes. The accelerator tube is paralleled by the second high voltage vacuum tube, the differential pumping tube. The bulk of the gas which diffuses into the system through the ion source o r i f i c e i s pumped out through the tube at a sufficient rate to maintain a calculated equilibrium pressure of about 10~ 4 mm at the ion source. A second o r i f i c e i s set below the f i r s t at the entrance to the accelerator tube to reduce the flow of gas into the tube. This pumping arrange- PLATE I Cementing Press 4 merit produces a very much better vacuum i n the accelerator tube than would be possible without the action of the d i f f e r - e n t i a l pumping tube. Calculations involved are given i n Appendix I I . From the bases of the tubes to the d i f f u s i o n pumps the system i s constructed of large welded and flanged i r o n ports painted with red g l y p t a l . Each of the tubes i s acted on by two d i f f u s i o n pumps, and can be i s o l a t e d from the pumps by a plate valve. The external portions of these valves are v i s i b l e i n Pigtie• • 2. A holder providing compression and alignment f a c i l - i t a t e d the cementing of tube sections. An a x i a l "compres- sion" rod and three shorter "alignment" rods are mounted on an i r o n plate. Alignment of the lenses i s produced by c i r - cular wedges dropped down the alignment rods. Afte r a com- plete section i s i n place a second plate and a c o i l spring from a railway car are slipped down over the compression shaft. A brass head threads down the shaft and acts on the spring through b a l l bearings. The head i s r a d i a l l y bored to f i t levering rods. Previous to the baking of an assembled section the cement applied to the ring and lens surfaces was dried either by warming to 150°F for a few hours or by applying the cement three days previous to the baking of the p a r t i c u l a r section. PLATE I I Pumping P o r t P o t s A t the Base o f t h e Van de G r a a f f 5 A v e r y t h i n f i l m o f cement was used and both s u r f a c e s were coated. Best r e s u l t s were o b t a i n e d by h e a t i n g the s e c t i o n s to 400°F and g r a d u a l l y c o o l i n g i n the c l o s e d oven f o r about 14 hours to room temperature. To r e a c h 400°F r e q u i r e s d i s - c o n n e c t i o n o f the oven thermostats. Bubbles remain i n the cement i f a s e c t i o n has been under-baked; i f over-baked, the cement i s scorched and b r i t t l e . S e c t i o n s can be taken to p i e c e s i n a 150 degree oven. Cement i s removed by soaking the l e n s e s and r i n g s i n acetone i n an e s p e c i a l l y designed t r a y . (b) The Pumps Each o f the D i s t i l l a t i o n Products MCF 700 d i f f u s i o n pumps has a r a t e d pumping speeds g i v e n i n Table 1. As c a l c u - l a t e d i n Appendix I I , these speeds a r e s u f f i c i e n t to m a i n t a i n a p r e s s u r e o f about 10"^ mm a t the base of the a c c e l e r a t o r tube under normal o p e r a t i n g c o n d i t i o n s . Because of i t s ex- tremely low vapour press u r e , O c t o i l i s used i n the pumps. A water b a f f l e i s s e t above the i n t a k e s o f each d i f f u s i o n pump to reduce the d i f f u s i o n o f o i l vapour i n t o the vacuum system. A l s o , two l i q u i d a i r t r a p s having a combined capa- c i t y o f about 8 l i t r e s a re l o c a t e d i n the ends of the p o r t s above the d i f f u s i o n pumps. The p o r t ends and pumps a r e v i s i b l e i n P l a t e I I I . The pumps r e q u i r e a backing p r e s s u r e PLATE I I I Vacuum System a t the Pumps 6 of 50 microns or less. TABLE 1 Rated Pumping Speeds of Diffusion Pumps Speed (CFM) 1400 1000 100 The common fore pump for the four diffusion pumps is a Kinney VSD 778. Its rated pumping speeds are given in Table 2. Calculations in Appendix II show that this pump should evacuate the entire vacuum system from atmospheric pressure to 50 microns i n about 17 minutes and that i t should maintain an equilibrium pressure of 10 microns while the 1 system i s outgassing at/CFM (cubic feet per minute) as the rated pumping speed at this pressure i s 7 CFM. Such a low equilibrium pressure i s desirable i f the system i s to be l e f t overnight with the diffusion pumps off. A drier i s placed on the air intake to prevent water vapour from entering the pump o i l . Also a silica-gel drier i s set i n the tube between the diffusion pump and the Kinney pump. Pressure (MMS) l O " 4 l O " 5 lO" 6 7 TABLE 2 Rated Pumping Speeds of Fore Pump Speed P r e s s u r e (CFM) (MMS) 27 760 22 200 14 .05 7 .01 Data on the performance o f the vacuum system i s recorded i n the next s e c t i o n . (a) General P r e s s u r e i n the vacuum system o f the Van de G r a a f f generator i s measured by a t o t a l o f f o u r P i r a n i and two VG-2 i o n i z a t i o n gauges. "Forevac" p r e s s u r e s o f the two s e t s o f d i f f u s i o n pumps are measured by two P i r a n i gauges l o c a t e d be- tween the two d i f f u s i o n pump p a i r s and the v a l v e s l e a d i n g to the f o r e pump. "Hivac" p r e s s u r e s a re measured by two P i r a n i and two i o n i z a t i o n gauges s e t a t the bases o f the vacuum tubes. The p l a t e v a l v e s separate the h i v a c gauges from the d i f f u s i o n pumps. I I I . PRESSURE MEASUREMENT 8 An i n d i c a t i o n o f pr e s s u r e can be obtained by the a p p l i c a t i o n o f a t e s l a c o i l to the glow tubes next to the for e v a c gauges. No glow appears when the pre s s u r e i s s u f - f i c i e n t l y low f o r o p e r a t i o n o f the d i f f u s i o n pumps. (b) P i r a n i Gauges The P i r a n i Gauges are manufactured by Dis t i l l a t i o n Products I nc. Measurement of pr e s s u r e by means of a P i r a n i gauge i s o b t a i n e d by comparing two f i n e wire r e s i s t a n c e s i n c o r p o r - ated i n a b r i d g e c i r c u i t . One of the r e s i s t a n c e s i s i n a v e s s e l s e a l e d a t h i g h vacuum and the o t h e r i s i n an i d e n t i c a l v e s s e l j o i n e d to the vacuum system. The presence o f gases about the l a t t e r r e s i s t a n c e s e r v e r s to i n c r e a s e i t s c o o l i n g r a t e and thus lower i t s temperature and r e s i s t a n c e . The se a l e d v e s s e l i s designed to compensate the e f f e c t caused by f l u c t u a t i o n s i n room temperature. The v a r i a t i o n o f p r e s s u r e w i t h c u r r e n t through the b r i d g e galvonometer i s approximately l i n e a r below 40 microns. The P i r a n i gauges slowly move o f f c a l i b r a t i o n and thus r e q u i r e p e r i o d i c adjustment o f the b r i d g e c i r c u i t r e s i s - tances to m a i n t a i n reasonably a c c u r a t e p r e s s u r e measurements. The P i r a n i readings depend not only on the p r e s s u r e o f the gas but a l s o on the molecular heat c o n d u c t i v i t y o f the gas. T h e r e f o r e , the i n t r o d u c t i o n of a gas or vapour of d i f - 9 f e r e n t c o n d u c t i v i t y through a l e a k i n the vacuum system would be d e t e c t e d by the P i r a n i which would thus serve as a l e a k d e t e c t o r as w i l l be d i s c u s s e d l a t e r . (c) I o n i z a t i o n Gauges An i o n i z a t i o n gauge i s e s s e n t i a l l y a t r i o d e open to the vacuum system. The g r i d i s maintained a t about 150 v o l t s p o s i t i v e and the p l a t e a t about 85 v o l t s n e g a t i v e . A l a r g e percentage o f the e l e c t r o n s which f l o w to the g r i d pass i n t o the r e g i o n between the g r i d and the p l a t e b e f o r e e v e n t u a l l y being c o l l e c t e d on the g r i d . P o s i t i v e i o n s formed by c o l - l i s i o n o f e l e c t r o n s w i t h gas molecules i n t h i s r e g i o n a r e c o l l e c t e d a t the p l a t e or c o l l e c t o r . The n e g a t i v e v o l t a g e on the p l a t e overcomes thermal energi e s o f the e l e c t r o n s and i n combination w i t h the g r i d v o l t a g e prevents e l e c t r o n s from re a c h i n g the p l a t e . The p o s i t i v e i o n c u r r e n t i s almost d i - r e c t l y p r o p o r t i o n a l to the p r e s s u r e , o t h e r f a c t o r s being constant, but a l s o depends on the i o n i z a t i o n p r o b a b i l i t y of the gas. T h i s dependence makes the gauge u s e f u l as a d e t e c - t o r i n l e a k hunting when the p r e s s u r e i n the system i s s u f - f i c i e n t l y low. The i o n i z a t i o n gauge cannot be operated a t a p r e s - sure above 1 micron ( 1 0 ~ 4 cms. of Hg) without a c c e l e r a t e d d e s t r u c t i o n of the f i l a m e n t . 10 Below 0.5 micron the VG-2 has a s e n s i t i v i t y f o r a i r of. 32 microamperes per micron f o r a g r i d c u r r e n t o f 10 m i l - l i a m p e r e s . Thus,, the c o l l e c t o r c u r r e n t corresponding to 10~§ms o f Hg i s onl y 0.32 microamperes which i s too small to read w i t h ease and accuracy on a panel micr©ammeter. A l s o , the g r i d c u r r e n t f l u c t u a t e s c o n s i d e r a b l y causing u n d e s i r a b l e c o l - l e c t o r c u r r e n t f l u c t u a t i o n s . These d i f f i c u l t i e s a re overcome by the Van de G r a a f f generator c o n t r o l u n i t which i n c o r p o r - ates d.c. a m p l i f i e r and a g r i d c u r r e n t s t a b i l i z e r or emi s s i o n r e g u l a t o r . A s i m i l a r c i r c u i t i s d e s c r i b e d by Elmore and 1 Sands. By keeping the cathode a t 25 v o l t s p o s i t i v e , the g r i d a t 175 v o l t s p o s i t i v e , and the c o l l e c t o r a t zero v o l t s , the i o n gauge c o n t r o l c i r c u i t e f f e c t i v e l y p r o v i d e s the r e - q u i r e d o p e r a t i n g v o l t a g e s o f the VG-2: cathode zero, p l a t e 150 p o s i t i v e , and p l a t e 25 n e g a t i v e . The d.c. a m p l i f i e r i s b u i l t around a b r i d g e c i r c u i t i n which two branches are made up of the two halve s o f a 6SC7 double t r i o d e . One g r i d i s permanently earthed; the other g r i d i s connected to the common t e r m i n a l o f a s i x p o s i t i o n s e l e c t o r s w i t c h . At "Zero" s e t t i n g the g r i d i s earthed and the c i r c u i t balanced by adjustment o f the 3K r h e o s t a t which v a r i e s the r e l a t i v e magnitudes of the r e s i s t - ances o f the other two branches o f the b r i d g e c i r c u i t . 11 With the sw i t c h a t " C a l " about 0.2 v o l t s i s a p p l i e d to the g r i d and the r e s i s t a n c e i n s e r i e s w i t h the microammeter ad- j u s t e d to g i v e f u l l s c a l e d e f l e c t i o n . At p o s i t i o n "1" f u l l s c a l e d e f l e c t i o n corresponds to a p r e s s u r e o f 2 10~3 mms o f Hg; t h a t i s , the g r i d i s switched to a r e s i s t o r a c r o s s which a v o l t a g e producing f u l l s c a l e d e f l e c t i o n developes when 64 microamps flows through the h i g h l y i n s u l a t e d c o l l e c t o r l e a d . At each succeeding p o s i t i o n , the p r e s s u r e corresponding to f u l l s c a l e d e f l e c t i o n drops by a f a c t o r o f t e n . At p o s i t i o n "4" the zero s e t t i n g o f the a m p l i f i e r i s d i s p l a c e d by the v o l t a g e produced a c r o s s the g r i d r e s i s t o r by p o s i t i v e i o n g r i d c u r r e n t . I f the i o n gauge f i l a m e n t and g r i d v o l t a g e s are t u r n e d o f f , the b r i d g e may be balanced to give zero de- f l e c t i o n on the microammeter w i t h the s e l e c t o r s w itch on pos- i t i o n 4. G r i d c u r r e n t o f the i o n gauge i s s t a b i l i z e d through r e g u l a t i o n o f the f i l a m e n t emission. A #273 Hammond t r a n s - former i s connected i n s e r i e s w i t h the f i l a m e n t t r a n s f o r m e r . The impedance of the #273 i s a f f e c t e d by the secondary c u r - r e n t which depends on the g r i d v o l t a g e o f the two 2 A 3 tubes which a c t as l o a d r e s i s t o r s . The 2 A 3 g r i d s a re connected to the 6SJ7 p l a t e , the v o l t a g e o f which i s determined by the i o n gauge g r i d c u r r e n t which produces the 6SJ7 g r i d v o l t a g e (which i s a l s o the p o s i t i v e b i a s on the i o n gauge f i l a m e n t ) . Thus an i n c r e m e n t a l i n c r e a s e i n i o n gauge g r i d c u r r e n t i n c r e a s e s the p l a t e c u r r e n t o f the 6SJ7 and thus reduces the p l a t e v o l t a g e which i s the g r i d v o l t a g e o f the 2 A 3 tubes. The corresponding decrease i n secondary c u r r e n t r a i s e s the primary impedence of the #273 and the consequent i n c r e m e n t a l r e d u c t i o n i n the f i l a m e n t emission tends to o f f s e t the r i s e i n g r i d c u r r e n t . The r e l a y i n the i o n gauge g r i d c i r c u i t , when ener- g i z e d by an ex c e s s i v e c u r r e n t , d i s c o n n e c t s the g r i d and ap- p l i e s the g r i d v o l t a g e a c r o s s a r e s i s t o r to the cathode to ma i n t a i n the e n e r g i z i n g c u r r e n t u n t i l the power i s shut o f f or the "Out-gas" switch thrown. The Out-gas s w i t c h s h o r t s the r e l a y e n e r g i z i n g c o i l and switches the g r i d o f the 6SJ7 to a r h e o s t a t which spans the v o l t a g e drop between the cathode and e a r t h . Thus, the 6SJ7 g r i d v o l t a g e can be reduced to permit s u f f i c i e n t f i l a - ment e m i s s i o n to p r o v i d e 20 m i l l i a m p s i o n gauge g r i d c u r r e n t . (d) McLeod Gauge The McLeod gauge g i v e s an a b s o l u t e measurement o f the p r e s s u r e of the permanent gases p r e s e n t by compressing a known volume of gas from the vacuum system and measuring the r e s u l t a n t p r e s s u r e manometrically. A g l a s s tube runs from the vacuum system down to an,evacuated mercury r e s e r v o i r so t h a t the l e v e l o f Hg i n the tube can be r a i s e d by l e t t i n g a i r i n t o the r e s e r v o i r . The r i s i n g Hg e n t e r s a g l a s s b u l b which 13 tapers upward i n t o a c a p i l l a r y tube s e a l e d a t the top end. P a r a l l e l to the c a p i l l a r y , the o r i g i n a l tube narrows to a s i m i l a r c a p i l l a r y i n o r d e r to compensate the e f f e c t of s u r f a c e t e n s i o n on the l e v e l s o f the two columns o f Hg. ( T h i s c a p i l - l i a r y i s by-passed by a tube of normal diameter to i n c r e a s e pumping speed.) When the l e v e l of the Hg i n the c a p i l l a r y reaches a p o i n t o p p o s i t e the c l o s e d end o f the s e a l e d c a p i l - l a r y , the p r e s s u r e , P^ o f the compressed gas i n cms o f Hg equals the d i s t a n c e , h, from the top of the c a p i l l a r y down to the Hg l e v e l and the volume, V^' i s h times the area, A , of the c a p i l l a r y . T herefore, i f the o r i g i n a l volume o f the gas i s V_ which i s the volume of the b u l b , then, the o r i g i n a l p r e s s u r e , P a = PbV Va = M>2/\ Thus, i n order to measure low p r e s s u r e s , A/V a should be as s m a l l as p o s s i b l e . In the low p r e s s u r e gauge c o n s t r u - c t e d f o r c a l i b r a t i o n of the Van de G r a a f f i o n gauges, p r e s s u r e —5 of 10- corresponds to an h o f 2.54 mms. (e) Data on Performance of the Vacuum System Observations on the performance o f the vacuum s y s - tem i n the S p r i n g of 1951 are g i v e n below. (1) Pump down time from 760 mms to 25 microns: 15 minutes i f system t i g h t . (2) U l t i m a t e p r e s s u r e w i t h backing pump: 7 microns. When pump warms, P i r a n i readings r i s e to about 18 microns. 14 (3) Fore pressure w i t h d i f f u s i o n pumps o p e r a t i n g : about 35 microns read on P i r a n i gauges. (4) Time f o r d i f f u s i o n pumps to b e g i n pumping a f t e r being turned on: 30 minutes. T o t a l time to re a c h 3 10~^mms: 45 minutes. (5) Pressures a f t e r ' prolonged pumping are recorded below. The d i f f u s i o n pumps are shut o f f over n i g h t . TABLE 3 Outgassing o f Vacuum System Pr e s s u r e T o t a l Pumping Time (mms) D i f f u s i o n Pumps (hours) 3 1 0 " 5 1 1 1 0 " 5 8 8 1 0 - 6 16 3 10~ 6 24 F i n a l l y , upon f i l l i n g the l i q u i d a i r t r a p s , a p r e s - -6 sure o f 1 10 mm was reached i n both tubes. (6) Pressures a t the top and bottom o f the d i f f e r - e n t i a l pumping tube f o r v a r i o u s i o n source l e a k r a t e s a r e recorded below. Pre s s u r e i n the hydrogen b o t t l e was 100 pounds per square i n c h . T A B L E 4 D i f f e r e n t i a l Tube Pr e s s u r e s vs Pd Leak Vol t a g e s Pd Leak V o l t a g e P r e s s u r e (mms 10 ) Top o f Base of Tube Tube 130 9.6 16 120 6.5 11 110 4.2 7 100 2.5 4 80 1.6 2.8 0 0.76 1.8 16 IV. VACUUM PROTECTION CIRCUITS (a) P r e s s u r e R i s e As p r e v i o u s l y mentioned, d i f f u s i o n pumps and i o n - i z a t i o n gauges should he operated a t p r e s s u r e s l e s s than 150 microns and 1 micron r e s p e c t i v e l y . O p e r a t i o n o f the d i f f u - s i o n pump a t e x c e s s i v e p r e s s u r e s d i f f u s e s pump o i l throughout the system causing a major c l e a n i n g problem. Heating o f i o n gauge f i l a m e n t s a t too h i g h a pr e s s u r e g r e a t l y hastens t h e i r d e t e r i o r a t i o n o r d e s t r u c t i o n . Vacuum p r o t e c t i o n c i r c u i t s have been designed to s w i t c h o f f the r e s p e c t i v e p i e c e s o f apparatus i n the event o f dangerous p r e s s u r e r i s e s . P i r a n i c o n t r o l l e d c i r c u i t s a re o f t e n used f o r the 2 p r o t e c t i o n o f d i f f u s i o n pumps. T. S. Wang has p u b l i s h e d a d e s c r i p t i o n o f a s a t i s f a c t o r y c i r c u i t d e s i g n . The f o u r P i r a n i gauges o f the Van de G r a a f f are operated by two p o r t a b l e d u a l c o n t r o l c i r c u i t u n i t s . In order t h a t p r e s s u r e s a t the bases of the two a c c e l e r a t o r tubes may be read s i m u l t a n e o u s l y , each u n i t i s wired to one h i v a c and one f o r e v a c gauge. Thus, the w i r i n g and p o r t a b i l i t y o f the u n i t s makes them u n s u i t a b l e as t r i g g e r s f o r a - r e l a y c i r c u i t . I . Amdur u t i l i z e s a thermo- couple gauge and a very s e n s i t i v e r e l a y f o r the p r o t e c t i o n of d i f f u s i o n pumps r a t h e r than a P i r a n i c o n t r o l c i r c u i t . 4 H. I . S. Allwood employs a G e i s s l e r d i s c h a r g e tube 17 designed to operate from atmospheric p r e s s u r e down to about 150 microns. A v e r y simple d i s c h a r g e tube c o n s i s t i n g o f o n l y a Kovar s e a l i s now i n c o r p o r a t e d i n the vacuum system o f the High T e n s i o n s e t used to t e s t the i o n source o f the Van de G r a a f f generator. The d i s c h a r g e tube, i n s e r i e s w i t h a 10 maa. d.c. r e l a y e n e r g i z i n g c o i l , o perates s a t i s f a c t o r i l y on 500 v o l t s A. C. when the pre s s u r e r i s e s to 150 microns. Immediately t h a t the tube s t r i k e s , v o l t a g e to the 500 v o l t t r a nsformer i s cut o f f thus p r e v e n t i n g damage to the Kovar, r e l a y , or transformer. P r o t e c t i o n o f thermionic f i l a m e n t s i s p r o v i d e d i n Allwood's c i r c u i t by a Penning type d i s c h a r g e tube which operates from 10mm o f Hg to 1 micron i n s e r i e s w i t h a r e l a y . T h i s p r o t e c t i o n can a l s o be pro v i d e d by a r e l a y t r i g g e r e d by the p o t e n t i a l o f the i o n gauge c o l l e c t o r a t the g r i d o f the c u r r e n t a m p l i f i e r . Such a c i r c u i t i s used i n the G. E. Leak 5 D e t e c t o r . (b) Water F a i l u r e . Water f a i l u r e i n the c o o l i n g system of a l l d i f f u s i o n pumps r e s u l t s , i n decomposition o f the pump o i l and e v e n t u a l d e s t r u c t i o n o f the he a t e r elements. A "water switch" i n s e r i e s w i t h a r e l a y i s an obvious remedy. 18 One o f the f i r s t water switches i n v o l v e d a l e a k i n g c o n t a i n e r suspended on a c o i l s p r i n g vrtiich s t r e t c h e d s u f f i - c i e n t l y to c l o s e an e l e c t r i c c o n t a c t when the cup became f u l l of water. At present, v e r y s e n s i t i v e "micro-switches" a r e a v a i l a b l e which r e a c t to the p r e s s u r e exerted by a d i v e r t e d stream o f water. I . Amdur employs a bellows which i s i n f l a t e d by the p r e s s u r e of water. Such bellows switches, however, may be p r e s s u r i z e d even though no water i s f l o w i n g should the d r a i n be o b s t r u c t e d . A simple and i n e x p e n s i v e water r e l a y system was c o n s t r u c t e d f o r the vacuum system of the apparatus used f o r t e s t i n g the i o n source of the Van de G r a a f f . The r e s i s t a n c e between two c o n c e n t r i c c y l i n d e r s i s reduced s u f f i c i e n t l y by the presence of water to enable e n e r g i z a t i o n o f the power r e l a y which i s connected i n s e r i e s w i t h the water r e s i s t a n c e . Water flows i n through the i n n e r tube or c y l i n d e r and i f the f l o w exceeds the outflow through the l e a k a t the bottom of the outer c y l i n d e r , water r i s e s between the c y l i n d e r s u n t i l the o v e r f l o w i s reached. (c) Power F a i l u r e The r e t u r n of power a f t e r f a i l u r e c o u l d r e s u l t i n damage to the vacuum system due to the r i s e i n p r e s s u r e w h i l e 19 the power was o f f and/or due to the f a i l u r e o f the f o r e pump to come back on a f t e r the r e t u r n o f power. A comparatively s m a l l p r e s s u r e r i s e c o u l d damage the i o n gauge f i l a m e n t s . A l a r g e r p r e s s u r e r i s e due to leakage through the f o r e pump might r e s u l t i n d i f f u s i o n pump damage e s p e c i a l l y i f the f o r e pump f a i l e d to come back on. As the tfinney f o r e pump of the Van de G r a a f f i s powered by a thr e e phase motor f e d by a standard magnetic s w i t c h the r e t u r n o f power would not s t a r t the motor. Thus, i t was a b s o l u t e l y necessary to p l a c e i n s e r i e s w i t h the d i f - f u s i o n pumps the contacts o f a r e l a y e n e r g i z e d by the v o l t a g e o f one phase of the f o r e pump motor so t h a t v o l t a g e would not be a p p l i e d to the d i f f u s i o n pumps upon the r e t u r n o f power a f t e r a f a i l u r e u n t i l the r e l a y was r e s e t . V . L E A K D E T E C T I O N (a) General In any l a r g e vacuum system the d e t e c t i o n o f l e a k s can be extremely t e d i o u s and time consuming, so t h a t each item as f a r as p o s s i b l e i s s e p a r a t e l y checked (e.g. each tube s e c t i o n ) before assembly. This may be done by p r e s s u r i z i n g s e c t i o n s and p l a c i n g them under water or p a i n t i n g over w i t h soap s o l u t i o n to enable the l o c a t i o n o f l e a k s by the appear- ance o f bubbles. Large l e a k s can be de t e c t e d i n t h i s manner. A f t e r a system i s assembled l e a k s can be roughly l o c a l i z e d by i s o l a t i n g s e c t i o n s ( a process made e a s i e r by j u d i c i o u s choice o f v a l v e p o s i t i o n s ) and obs e r v i n g the change o f p r e s s u r e w i t h i n or without the s e c t i o n s when the placement of v a l v e s i n the system makes such i s o l a t i o n p o s s i b l e . The spark from a T e s l a c o i l a p p l i e d to a g l a s s system w i l l enter the system through a h o l e , i f i t i s l a r g e enough and c l o s e enough to the e l e c t r o d e , making the puncture c l e a r l y v i s i b l e . As the c o l o r o f the di s c h a r g e o f a glow tube depends on the nature o f the r e s i d u a l gas, the absorb- t i o n o f an or g a n i c vapour a p p l i e d t o the system c o u l d be de- t e c t e d and the l e a k l o c a l i z e d to the area o f a p p l i c a t i o n . Acetone, o r b e t t e r ether, f o r example, would change the d i s - charge c o l o r to b l u i s h - w h i t e . The s e n s i t i v i t y o f the P i r a n i gauge to gas composition can be u t i l i z e d i n a s i m i l a r manner i f the p r e s s u r e i s w i t h i n the p r a c t i c a l range of the P i r a n i , i . e . , from 1 micron to about 300 microns. C o a l gas i n a bag i s u s e f u l f o r checking a gasket j o i n t . Below 1 micron, the i o n i z a t i o n gauge may be used i n l e a k d e t e c t i o n . S t a b i l i z a t i o n o f the g r i d c u r r e n t i s e s p e c i - a l l y n ecessary to prevent spurious f l u c t u a t i o n s i n the c o l - l e c t o r c u r r e n t . A j e t o f ether was used w i t h the i o n gauges as d e t e c t o r s d u r i n g the assembly of the vacuum tubes o f the Van de G r a a f f . The use o f ether not o n l y enables l o c a t i o n o f l e a k s but a l s o o f t e n r e s u l t s i n s e a l i n g the l e a k s p o s s i b l y by r e d u c i n g the v i s c o s i t y o f the vacuum grease which i s then drawn i n to p e r f e c t the s e a l . The r e d u c t i o n of the emission o f heated tungsten f i l a m e n t s by i n c r e a s i n g c o n c e n t r a t i o n s o f oxygen makes pos- s i b l e the use of diodes as d e t e c t o r s on vacuum systems w i t h s e a r c h i n g j e t s o f oxygen. I n a c o n t r o l c i r c u i t d e s c r i b e d by 7 R. B. Nelson the f i l a m e n t o f the d e t e c t o r diode i s w i r e d i n p a r a l l e l w i t h the f i l a m e n t o f a c o n t r o l diode whose p l a t e c u r r e n t i s s t a b i l i z e d through emission r e g u l a t i o n . T h i s arrangement i s designed to prevent s p u r i o u s f l u c t u a t i o n s i n the p l a t e c u r r e n t o f the d e t e c t o r d i o d e . A l e a k o f 0.76 m i c r o n . l i t r e per hour can be r e a d i l y d e t e c t e d . The p r i n c i p l e i n v o l v e d i n the d e t e c t o r d e s c r i b e d 22 8 by W. C. White and J . S. Hickey i s t h a t red hot platinum- emits p o s i t i v e i o n s , even a t atmospheric press u r e , and t h a t the emission i s i n c r e a s e d markedly w i t h the p a r t i a l p r e s s u r e of the vapour o f a halogen compound. The system under i n - v e s t i g a t i o n i s u s u a l l y p r e s s u r i z e d w i t h a i r or any oth e r s u i t a b l e gas c o n t a i n i n g a halogen vapour and the d e t e c t o r probe a p p l i e d e x t e r n a l l y . General E l e c t r i c B u l l e t i n GEC-283 c o n t a i n s i n f o r m a t i o n on d e t e c t o r o f t h i s type which i s e s p e c i - a l l y convenient i n t e s t i n g r e f r i g e r a t i o n u n i t s as the d e t e c - t o r i s s e n s i t i v e to Freon gas i t s e l f . A l t e r n a t i v e l y , the d e t e c t o r may be s e t i n t e r n a l l y between the d i f f u s i o n pump and f o r e pump and a se a r c h i n g j e t a p p l i e d e x t e r n a l l y though no commercial d e t e c t o r designed f o r i n t e r n a l use i s a v a i l a b l e . White and Hickey w r i t e , concerning the e x t e r n a l d e t e c t o r , "the l i m i t o f s e n s i t i v i t y to small l e a k s i s the same order o f magnitude as t h a t obtained w i t h the mass spectrometer" which i s about 0.57 m i c r o n . l i t r e s per hour f o r a 283 l i t r e 9 system a c c o r d i n g to Jacobs and Zuhr. • (b) Improved Ion Gauge Leak D e t e c t o r s 10 Brubaker and Wouk have d e v i s e d a s e n s i t i v e a u d i - t o r y method o f i n d i c a t i n g changes i n the c o l l e c t o r c u r r e n t of i o n i z a t i o n gauge d e t e c t o r f l u c t u a t i o n s i n c o l l e c t o r v o l t a g e a r e a m p l i f i e d by a d.c. a m p l i f i e r and used to b i a s a r e l a x a - t i o n o s c i l l a t o r which feeds a l o u d speaker through an audio a m p l i f i e r . A change i n c o l l e c t o r c u r r e n t w i l l e i t h e r b r i n g 23 on the o s c i l l a t o r i f i t was o r i g i n a l l y b i a s e d o f f or change the frequency i f the o s c i l l a t o r was o r i g i n a l l y o p e r a t i n g . The arrangement enables one man to l e a k hunt. A change i n p a r t i a l p r e s s u r e of 4 x 10~ 9 cms o f Hg can be n o t i c e d . 11 H. Nelson's hydrogen i o n i z a t i o n gauge i n v o l v e s a p a l l a d i u m tube immersed i n the vacuum system and s e a l e d to an i o n i z a t i o n gauge i n which the p r e s s u r e has been reduced to 10"^ mms o f Hg with the a i d o f a g e t t e r . The system i s pumped down to lO""* c m s o f Hg and the p a l l a d i u m heated to 800°C a t which temperature i t i s permeable to hydrogen e x c l u s i v e l y . I f hydrogen f i n d s entrance to the system through a l e a k i t i s P r e f e r e n t i a l l y admitted to the i o n i z a t i o n gauge, the gas i n which would then c o n t a i n a percentage o f hydrogen about one thousand times g r e a t e r than the percentage o f hydrogen i n the gas of the vacuum system. I t f o l l o w s t h a t the d e v i c e can de- t e c t f a r s m a l l e r l e a k s than c o n v e n t i o n a l i o n gauges a t t a c h e d d i r e c t l y to the vacuum system. As the hydrogen i s pumped from the system i t d i f f u s e s through the h o t Pd and out o f the i o n gauge a g a i n s t the 1 0 " 4 mm a i r p r e s s u r e . The hydrogen gauge has the advantages o f l e a k hunting a t an extremely low p r e s s u r e w h i l e the system under i n v e s t i g a t i o n i s maintained a t a r e l a - t i v e l y h i g h and e a s i l y obtained p r e s s u r e : i n t e s t i n g r a d i o tubes on a s m a l l system a d i f f u s i o n pump i s not even r e q u i r e d and a minimum l e a k o f 10""* l i t r e . m i c r o n per second can be d e t e c t e d . 24 (c) Mass Spectrometer Leak D e t e c t o r s The most s e n s i t i v e l e a k d e t e c t o r s i n v o l v e a mass spectrometer and a probing j e t of helium. The b e s t o f these 12 instruments can d e t e c t the presence o f He i n normal a i r which c o n t a i n s about 1 p a r t He i n 200,000 p a r t s o f a i r . T h i s r a r i t y o f He p l u s i t s h i g h r a t e o f d i f f u s i o n through l e a k s and i t s unique e/m r a t i o ..are the f a c t o r s which r e s u l t i n the ch o i c e o f He as the probe gas. The mass spectrometer l e a k d e t e c t o r was developed d u r i n g World War I I to hasten the s e a l i n g o f the e x t e n s i v e vacuum system employed i n the U n i t e d S t a t e s Atomic Energy P r o j e c t . The mass spectrometer l e a k d e t e c t o r i s not onl y ex- tremely s e n s i t i v e but also- enables the l o c a t i o n o f l e a k s i n a minimum o f time as searc h i n g can be commenced while the system under i n v e s t i g a t i o n i s a t f o r e pump pre s s u r e s and not y e t out- gassed. 5 A complete, p o r t a b l e mass spectrometer l e a k d e t e c t o r - w i t h a s e n s i t i v i t y o f 1 p a r t He i n 100,000 p a r t s o f a i r i s a v a i l a b l e a t a p r i c e c o n s i d e r a b l y more than $4,000 from Gener- a l E l e c t r i c which a l s o manufactures i o n resonant and Bennet RF 1 3 mass spectrometer tubes. The i o n resonant tube r e q u i r e s a magnet p r o v i d i n g a uniform f i e l d o f about 2000 gauss. The Bennet RF v e l o c i t y s e l e c t o r tube r e q u i r e s no magnet but a somewhat h i g h e r d. c. a c c e l e r a t i n g v o l t a g e . T h i s tube i s new and there i s no r e c o r d o f i t s having been used i n l e a k d e t e c - t i o n though i t may prove w e l l s u i t e d to such an a p p l i c a t i o n . I t i s s e n s i t i v e to 1 p a r t o f He i n 200,000 p a r t s o f a i r . V I . A MASS SPECTROMETER LEAK DETECTOR FOR THE VAN DE GRAAFF GENERATOR (a) Design ( i ) General A simple cheap v e r s i o n o f a mass spectrometer l e a k d e t e c t o r has "been designed f o r use w i t h the U n i v e r s i t y of B r i t i s h Columbia Van de G r a a f f . I t uses a simple c o l d cathode i o n source p l u s a s t r a i g h t through a n a l y s e r , w i t h cathode ray tube d i s p l a y . The spectrometer i s b u i l t i n f o u r s e c t i o n s : . i o n source, lower cathode, a n a l y s e r , and c o l l e c t o r . Except f o r the cathodes, the metal p a r t s are b r a s s . ( i i ) Ion source The i o n source i s a r e f l e c t o r type, as shown i n F i g u r e 2. What i s termed the i o n source s e c t i o n c o n s i s t s of a c y l i n d r i c a l anode s e a l e d above and f l a n g e d a t the base. The upper cathode i s suspended by a kovar s e a l from the top o f the anode where a c o n n e c t i o n i s made to the gas f e e d ap- paratus through a ground g l a s s j o i n t separated from the i o n source by a stopcock supported by a g l a s s t o copper s e a l . A c y l i n d r i c a l magnet made up o f t h r e e i d e n t i c a l r i n g s e c t i o n s f i t s around the i o n source. The magnet was magnetized as a u n i t and never subsequently taken a p a r t i n PLATE IV Mass Spectrometer Leak Detactor and Apparatus Figure 2 Side View of Ion Source 31 ft) ~* / >• 3 1 c B — 1 ! LUr B • A: Ion source s e c t i o n B: Cathode s e c t i o n C: Upper cathode D: Upper e l e c t r o d e of f i r s t l e n s Key: Blue ink: brass except f o r Kovar Bed i n k : aluminum Green ink: g l a s s P e n c i l : rubber gaskets SCALE: FULL SIZE 27 order to ensure the g r e a t e s t p o s s i b l e f i e l d s t r e n g t h which i s about 200 gauss a t i t s c e n t r e . The i o n source s e c t i o n i s separated from the lower cathode s e c t i o n by a g l a s s r i n g s e a l e d w i t h rubber gaskets s e t i n the cathode and i o n source f l a n g e s . The cathode sec- t i o n was turned from one p i e c e o f aluminum and co n t a i n s the upper e l e c t r o d e o f the f i r s t l e n s . The core o f the cathode i s removable to enable replacement by cores o f v a r i o u s o r i f i c e d iameters. A l l o r i f i c e s are 7/16 i n c h l o n g . The cathode s e c t i o n i s s e a l e d to the a n a l y s e r by a second g l a s s spacer. ( i i i ) A n a l y s e r and C o l l e c t o r The a n a l y s e r e l e c t r o d e s a r e contained i n a l e n g t h o f r e c t a n g u l a r wave guide tube which i n c l u d e s the lower e l e c - t r o d e o f the f i r s t l e n s , the d e f l e c t i o n p l a t e s , and the second l e n s , each e l e c t r o d e being h e l d by h o r i z o n t a l rods supported by kovar s e a l s as shown i n F i g u r e 3. A I700lgauss magnet tsupported:.by an i n s u l a t e d stand f i t s about the a n a l y s e r s e c t i o n a t the d e f l e c t i o n p l a t e s . A Corning g l a s s tube s e c - t i o n separates the a n a l y s e r from the c o l l e c t o r . The upper p l a t e o f the c o l l e c t o r s e c t i o n i s d r i l l e d w i t h s i x | i n c h diameter h o l e s to i n c r e a s e the pumping speed. A s l i t o f v a r i a b l e width f i t s above the c e n t r e h o l e o f the p l a t e below which i s f i x e d a c y l i n d r i c a l s c r e e n which encomp- asses the Faraday cup which i s supported by a rod f i x e d to a Figure 3 Side View of Analyser S e c t i o n A B u ^3 lower e l e c t r o d e o f f i r s t l e n s . D e f l e c t i o n p l a t e s . S l e c t r o d e a of second l e n a . M a t e r i a l : Brass except f o r Kovar s e a l s SCALE: 1TJL1 SI F i g u r e 4 Side View of C o l l e c t o r S e c t i o n 1_J" If I i r • r t 11 11 II i i B A: S l i t B: Faraday cup C: S h i e l d M a t e r i a l : brass except f o r Kovar s e a l SCALE: FULL SI33 28 kovar s e a l s e t i n the s i d e o f the c o l l e c t o r s e c t i o n as shown i n F i g u r e 4 The base o f the s e c t i o n i s s e a l e d to the vacuum system by a rubber gasket. The c o l l e c t o r can be connected to e i t h e r a galvano- meter or an o s c i l l o s c o p e depending on whether or not the de- f l e c t i o n p l a t e v o l t a g e has an a. c. component. The d e f l e c t i o n p l a t e a . c. v o l t a g e p a t t e r n can be a p p l i e d to the sweep o f the o s c i l l o s c o p e through a 12,500 v. d. c. 0.25 m i c r o f a r a d condenser and p o t e n t i a l d i v i d e r . ( i v ) Power Su p p l i e s The v o l t a g e s o f the d e f l e c t i o n p l a t e s are pr o v i d e d by a 7000 v o l t selenium r e c t i f i e r which has a p o t e n t i a l d i v i - der s e t ac r o s s the secondary a t the transformer. F i l t e r i n g i s done by two 1 m i c r o f a r a d 4000 v. d. c. condensers i n s e r - i e s b l e d by a t o t a l o f 44 megohms which draw a maximum o f 0.16 mi l l i a m p s which produce a r i p p l e o f 0.03$. The common t e r m i n a l o f the condensers i s s e t a t the p o t e n t i a l o f the a n a l y s e r . One of the d e f l e c t i o n p l a t e s i s wired to the p o s i - t i v e output o f the r e c t i f i e r and the oth e r to the output o f the p o t e n t i a l d i v i d e r to enable a p p l i c a t i o n o f v a r i o u s a. c. v o l t a g e s i n the n e g a t i v e d. c. v o l t a g e o f the r e c t i f i e r . A f i v e step r h e o s t a t capable o f o p e r a t i n g across a maximum of 3500 v o l t s p r o v i d e s the lower a. c. v o l t a g e s . To sw i t c h from the h i g h e s t r h e o s t a t v o l t a g e to p o s i t i o n 6 a t the other end o f the o v e r - a l l p o t e n t i a l d i v i d e r n e c e s s i t a t e s manual T3 110 v T2 J l T4 T5 t A I 110 v o l t s 60 c y c l e s TI V ton T •B •C *7«i P o s i t i o n 6 A: To i o n source power supply. B: To p o s i t i v e d e f l e c t i o n p l a t e . C: To upper e l e c t r o d e of second l e n s , lower e l e c t r o d e o f f i r s t l e n s , and a n a l y s e r c a s i n g . D: To negative d e f l e o t i o n p l a t e . TI = T2 = T3 = Maloney Filament Trans- former; secondary, 11 v, 15 a, i n s u l a t e d f o r 25,000 v o l t s . T4 = Superior E l e c t r i c Powers t a t Type 20. T5•= Hammond Type 26182, 87 VA, 2500 v, c . t . S = s e l e c t o r .swi ton V = Selenium r e c t i f i e r . F i g u r e 5 C i r c u i t Diagram o f D e f l e c t i o n P l a t e Power Supply and I s o l a t i n g Transformers 29 unplugging and r e p l u g g i n g of the l i n e e a s i l y and r a p i d l y done w i t h the s p e c i a l i n s u l a t e d hook p r o v i d e d . See P l a t e VI. When the output o f the p o t e n t i a l d i v i d e r i s a t p o s i t i o n 6, the v o l t a g e between the p l a t e s o s c i l l a t e s between zero and twice the r e c t i f i e d v o l t a g e . Thus, a maximum v o l t a g e d i f f e r - ence o f 14,000 v o l t s i s a v a i l a b l e . An i d e n t i c a l 7000 v o l t r e c t i f i e r without a p o t e n t i a l d i v i d e r s u p p l i e s the second l e n s and a 7000 v o l t v o l t a g e d o u b l e r i s a p p l i e d to the f i r s t l e n s . The i o n source supply d e l i v e r s 0/3600 v o l t s a t 20 m i l l i a m p s . A l l transformer p r i m a r i e s are s u p p l i e d by v a r i a c s which a r e themselves f e d by i s o l a t i n g transformers when neces- s a r y . As i s o l a t i n g transformers i n s u l a t e d f o r 25,000 v o l t s and g i v i n g a 15 v o l t secondary v o l t a g e were a v a i l a b l e , three such transformers were used to p r o v i d e two i s o l a t e d v o l t a g e s of 110 v o l t s by w i r i n g the s e c o n d a r i e s i n p a r a l l e l and apply- i n g the mains v o l t a g e to one of the p r i m a r i e s . One of the i s o l a t e d v o l t a g e s feeds the f i r s t l e n s and d e f l e c t i o n p l a t e power s u p p l i e s and the other feeds the i o n source supply. The anode o f the i o n source may reach a maximum of 15,000 v o l t s above the c o l l e c t o r i f a d i s c h a r g e does not occur through the gas i n the tube connecting ;the i o n source to the l e a k . T h i s d i s c h a r g e i s prevented by i n s u l a t i n g the stand of the gas f e e d apparatus w i t h a b a k e l i t e sheet. The D e f l e c t i o n P l a t e S U P P I V 30 (v) Gas Feed The gas f e e d apparatus was designed to t e s t the spectrometer and i n c l u d e s two 500 ml storage f l a s k s , an open mercury manometer, a s i l i c a g e l d r i e r , a P i r a n i gauge, and a l e a k c o n s i s t i n g o f a f l a t t e n e d copper tube i n s e r i e s w i t h a needle v a l v e . Stopcocks are arranged so t h a t the l e a k r a t e can be determined by a l l o w i n g the gas to f l o w i n t o a known volume and o b s e r v i n g the r a t e o f i n c r e a s e o f p r e s s u r e w i t h the P i r a n i though such data i s not necessary to determine the a b i l i t y o f the mass spectrometer to d e t e c t v a r i o u s percentages o f helium i n a i r . A separate mechanical pump permits changes i n gas composition without i n t e r f e r i n g w i t h the a c t i o n o f the d i f f u s i o n pumps of the spectrometer vacuum system. T h i s gas supply i n a c t i v e o p e r a t i o n w i l l be the gas immediately above the v a l v e above the Kinney Pump—closed to g i v e a h i g h enough pressure f o r the i o n source to operate. (b) Theory ( i ) S t r a i g h t through a n a l y s i s I f i o n s o f v e l o c i t y , V, and charge, Q, having been a c c e l e r a t e d through a v o l t a g e , V, enter a coterminous, c r o s s e d e l e c t r i c and magnetic f i e l d , they a r e a c t e d upon by p a r a l l e l f o r c e s F and F o f the e l e c t r i c and magnetic f i e l d s r e s p e c t i -e m v e l y . I f these f o r c e s are equal and o p p o s i t e the ions w i l l pass s t r a i g h t through without d e f l e c t i o n . 31 S. cEQ/300 = 10 EQ where F i s i n dynes E i n vol t s / c m & Q i n emu. A l s o F m =HQV where H i s i n gauss and v i n cms/sec. I f F = F e xm 8 E = 10 H v (1) The energy of the i o n s i n j o u l e s i s 10VQ as Q i s 8, i n emu; the energy i n ergs i s 10 VQ. 10 8VQ = | Mv 2, M i n grams. Therefore,.;, v = (2VQ10 /M) 2 (2) be q. Then, L e t the charge, Q, expressed i n e l e c t r o n i c u n i t s q = Q/e where e i s the charge o f an e l e c t r o n i n emu. A l s o , and Q = 1.6 1 0 - 2 0 q. M = m/6.023 10 -23 where m i s the molecular wt. S u b s t i t u t i n g f o r Q and M i n (2) ,8 , „ _ - 2 0 „ 23 , si v = (2Vq 10 1.6 10 6.023 10 /m) e x x v = 1.39 10 V 2 (q/m) 2 (3) S u b s t i t u t i n g (3) i n t o ( l ) .-2 - i E = 1.59 10 V 2 (o/m)z H ( 4 ) 32 For V = 7000 v o l t s and H = 1700 gauss, the v o l t a g e a c r o s s the d e f l e c t i o n p l a t e s , Vd = E/2.54 where 2.54 cms i s the p l a t e s e p a r a t i o n . F i n a l l y Vd = 5000 (q/m)* (5) ( i i ) D i s p e r s i o n I f f does not equal F , the beam moves o f f the a x i s and the v e l o c i t y , v, a c q u i r e s a h o r i z o n t a l component, Vy the v e r t i c a l component being V x . T h e r e f o r e the magnetic f i e l d e x e r t s a v e r t i c a l f o r c e , F i n a d d i t i o n to the h o r i - mx z o n t a l component F . L e t the r e s u l t a n t f o r c e , F, e x e r t e d my by the c r o s s e d f i e l d be r e s o l v e d i n t o F and F and the o r i g i n x y taken on the a x i s a t the upper boundary of the f i e l d , the downward d i r e c t i o n o f x being c o n s i d e r e d p o s i t i v e . F = F - F = HQv - 10 8EQ = M dv (6) y my e * x * — j j ^ y F_ = F m Y = HQv v = - M dv x mx y d t x ^ D i f f e r e n t i a t i n g (6) and s u b s t i t u t i n g f o r dv^/ut i n ( 7), HQVy = - M (M/HQ d 2 v y / u t 2 ) I n t e g r a t i n g , v = a cos (HQ/M t + jrf) 33 L e t t = 0 a t the o r i g i n where v = Q. y Then, 0 = ff/2 and v = (a sinlHQt/M (8) D i f f e r e n t i a t i n g and s u b s t i t u t i n g f o r dv / d t i n ( 6 ) , V = 10 8 E/H+a cos HQt/M (9) As v = v a t t = 0 , a = v - 10 8 E/H (10) Equations (8) and (9) show t h a t the motion c o n s i s t s o f a downward t r a n s l a t i o n o f cons t a n t v e l o c i t y i n combination w i t h a c i r c u l a r r o t a t i o n of r a d i u s . r = a/w = aM/QH I n t e g r a t i n g ( 8 ) , the h o r i z o n t a l d e f l e c t i o n , y = aM/HQ ( l - cos HQt/M) as y = 0 when t = 0. I f T i s the passage time through the f i e l d , then the t o t a l d e f l e c t i o n , s, a t the lower boundary o f the f i e l d i s s = aM/HQ ( l - cos HQT/M) ( l l ) The r e l a t i v e motion o f i o n s when onl y s l i g h t l y de- f l e c t e d from the a x i a l path w i l l determine the d i s p e r s i o n o f the instrument. For these i o n s , i t i s obvious t h a t o n l y a ve r y s m a l l p o r t i o n o f the c i r c u l a r motion w i l l have been ex- ecuted b e f o r e the i o n s pass out of the f i e l d , i . e . , HQT/M i s . s m a l l , and approximations can be made. Using (11) and the r e l a t i o n cos x = 1 - x 8 / 2 l + . . . . s = aM/HQ (HQT/2M) 2 approximately As a = v - 10 8E/H from (10) s = | HQvT 2/! - $ IC^EO^/M A l s o v = v approximately from equations (9) and (10). T h e r e f o r e , T = L/v where L i s the l e n g t h o f the f i e l d which i s assumed equal to the l e n g t h o f the d e f l e c t i o n p l a t e s Thus, • s = § HQL^/Mv - f 10 8EQL 2/Mv 2 (12) Thi s approximation can a l s o be d e r i v e d by assuming i n i t i a l l y t h a t F = F and v = v and making use of the equa- y x tions.F = ma and s = | a t 2 , s e t t i n g F = Ma = 10 8EQ e e * F m = Ma m = HQv m m Whereupon, s = i a t 2 - | a t 2 = | HQL2/Mv - § 10 8EQL 2/Mv ; m e ,8 S u b s t i t u t i n g from (2) v = (2V^10°/M) 2 s = § HL 2 (Q/M)V (2V10 8)^ - § EL 2/2V (13) As s = (constant) L 2 x = C L 2 ds/dL = 2 C L = 2s/L Therefore, i f D (cms) i s the s e p a r a t i o n from the cen t r e o f the p l a t e s t o the c o l l e c t o r , -the d e f l e c t i o n S, a t the c o l l e c t o r i s 3 = (2D/L)s 35 The d i f f e r e n c e i n the d e f l e c t i o n s , s and s o f a b two ions having I r a t i o s Q /M and Q../E i s a a D O AS = I Hl2((Q )* - / M ^ / t e V l O 8 ) * (13a) Converting to q/m and t a k i n g H = 1700 gauss L = 2.54 cms, and V = 7000 v o l t s , AS = 0.456 ((q /m ) 5 - d /m )£ ) (14) a a D b For a D o f 30 cms, AS = 10.3 ((q /m )§ - (q./m. )* ) (15) a a b b As an example, the d i s p e r s i o n between He + and i t s n e a r e s t l i k e l y neighbor, (J*"*" i s S = 10.8 ((1/4)4 _ (a/12)i ) = 10.8 (0.500 - (0.408) = 0.99 cm = 9.9 mms As the beam diameter i s much l e s s than t h i s , judg- i n g by the r e s u l t s , such a d i s p e r s i o n i s q u i t e s a t i s f a c t o r y . (c) O p e r a t i o n As a guide to f u t u r e o p e r a t o r s o f the spectrometer as i t i s now s e t up f o r t e s t i n g on the vacuum t a b l e o f Room 122, the f o l l o w i n g o p e r a t i n g data are gi v e n w i t h p e r t i n e n t d e s c r i p t i o n s o f the apparatus and i t s perdormance. ( l ) The spectrometer should be evacuated down to about 0.1 micron w i t h the ground g l a s s j o i n t c onnecting the s p e c t - Wired ion Source and Analyser 36 rometer to the gas fe e d apparatus i n p l a c e and the i o n source stopcock l o c a t e d between the spectrometer and the ground g l a s s j o i n t open. The "end" stopcock which connects the P i r a n i gauge s e c t i o n o f the gas f e e d apparatus to e i t h e r the tube l e a d i n g to the spectrometer o r the hose t o the gas fe e d appar- atus pump may be c l o s e d as the " f e e d " system beyond i t can be evacuated by the fe e d pump. Should the spectrometer be evac- uated with the i o n source stopcock c l o s e d , i t should be opened very g r a d u a l l y so as not to f l o o d the vacuum system. (2) Gas i s f e d i n t o the i o n source by opening the l e a k needle v a l v e about a q u a r t e r of a r e v o l u t i o n f o r a few seconds b e f o r e c l o s i n g t h r e e s i x t e e n t h s o f a t u r n o r so. An e a s i l y v i s i b l e d i s c h a r g e between the l e a k and the i o n source should occur upon a p p l i c a t i o n o f a Tsesla to the f e e d stand i f the i o n source i s wired. A pressure o f 20 cms on the h i g h s i d e o f the l e a k i s easy to work wi t h u s i n g the 1/16 i n c h diameter o r i f i c e o f the cathode though p r e s s u r e s up t o one atmospmiere have been used. When s e t t i n g the stopcocks care should be taken not to t u r n the " c e n t r a l " f e e d stopcock, which connects the P i r a n i s e c t i o n to e i t h e r the h i g h o r low pre s s u r e s i d e s o f the l e a k , t o the h i g h p r e s s u r e s e c t i o n s w h i l e the end stopcock i s opened to the spectrometer. (3) The i o n source power supply i s turned up u n t i l a c u r r e n t f l o w s . F l a s h - o v e r s sometimes occur i n s i d e between the cathode s e c t i o n and the i o n source s e c t i o n but are sometimes 37 reduced in frequency on decreasing the pressure by closing the needle valve a l i t t l e . The flashes may cease altogether after prolonged operation. Also,a permanent breakdown some- times occurs between the upper cathode and the top of the anode, the kovar becoming sufficiently contaminated to have a measurable resistance whereupon i t i s necessary to turn off the voltage, remove the upper cathode connection, ground the anofledeand apply to the upper cathode a Tesla which usually cleans up the contamination and begins sparking across the outside of the kovar. After reconnection, i f the ion source i s working properly the current w i l l stop flowing upon dis- connection of the lower cathode. Before performing this test a careful check was made to ensure that the lens voltages were off; i t was found necessary to form the habit of applying a grounded wire to any metal part of the spectrometer before touching with the hands. (4) After checking the oscilloscope and turning on the lens voltages the deflection plate supply i s slowly turned up with the potential divider set on position 6. The f i r s t lens supply i s set at 60 meter ;divisions and the second at 0.1; the voltages corresponding to these scale readings can be obtained from the calibration curves. The oscilloscope sweep generator i s synchronized at a multiple of 60 cycles and the shielded collector lead connected at an input giving two stages of amplification and bled externally through a 2 PLATE V I I I O s c i l l o s c o p e E x t e r n a l S w e e p V o l t a g e S u p p l y a t O s c i l l o s c o p e 58 megohm resistor. If a voltmeter i s set between the analyser and the positive deflection plate which i s always at a pure d. c. voltage, the a i r spectra should have appeared on the scope screen by the time the meter reads 1000 volts. If the pattern f a i l s to appear, i t i s necessary to check that a pick- up voltage appears on the screen when the input i s touched with a finger; failure usually means that the collector lead is shorted to a shield. If the scope is functioning i t i s necessary to check the ion source as described above, making sure that the lens voltages are off and the condensers dis- charged. Even the gas feed apparatus should not be touched while the lens voltages are on; adjustment to the needle valve should be made with an insulated screw driver. An ex- ternal sweep voltage of the same pattern and phase as the de- flection voltage i s available at the oscilloscope. The rela- tive magnitude of the voltage in comparison with the deflec- tion plate a. c. voltage can be varied by the multiple posi- tion switch to compensate for changes in the deflection plate a. c. voltage. ( 5 ) Vary the positions on the a. c. potential divider of the deflection plate supply so that small segments of the total mass spectrum can be examined. In this way i t i s pos- sible to sweep the helium peak along in which case any peak appearing on the screen during leak hunting' i s known to be helium. 39 ( 6 ) When the a. c. p o t e n t i a l d i v i d e r i s switched to g i v e a pure d. c. v o l t a g e a c r o s s the p l a t e s , the c o l l e c t o r c u r r e n t can be f e d through a s e n s i t i v e galvanometer i f a b s o l - u t e readings a r e d e s i r e d . I t has proven convenient to s e t the spectrometer on a c e r t a i n peak by c e n t e r i n g the peak on the e x t e r n a l l y generated sweep b e f o r e a d j u s t i n g f o r a galvano- meter r e a d i n g . (d) R e s u l t s Currents i n the i o n source and unanalysed beam c u r - r e n t s are given i n Tables 5 and 6 . O s c i l l o s c o p e t r a c e s f o r P o s i t i o n 6 d e f l e c t i o n v o l t a g e s o f v a r i o u s magnitudes u s i n g an i n t e r n a l l y generated scope sweep v o l t a g e a r e shown i n F i g u r e 6 . As a r e f e r e n c e , the a. c. d e f l e c t i o n v o l t a g e i s d i s p l a y e d on the lower t r a c e o f the double beam scope. The c r e s t s correspond to zero v o l t a g e between the p l a t e s as the a. c. v o l t a g e i s then maximum p o s i t i v e and r a i s e s the n e g a t i v e p l a t e to the same p o t e n t i a l as the p o s i t i v e p l a t e which i s a t a pure d. c. p o t e n t i a l w i t h r e s p e c t to the a n a l y s e r . T h i s d. c. p o t e n t i a l o f the p o s i t i v e p l a t e i s measured and equals one h a l f of the average v o l t a g e d i f f e r e n c e , Vd', between the -i p l a t e s . I n diagram (a) of F i g u r e 6 three peaks appear per c y c l e i n d i c a t i n g t h a t the spectrum d i s p l a y e d c o n s i s t s of one and one h a l f peaks, t h a t i s , d u r i n g the r i s e i n p l a t e v o l t - age from zero to i t s maximum v a l u e d u r i n g the f i r s t h a l f of a TABLE 5 Performance of the Ion Source O r i f i c e : diameter d = 0.25 l e n g t h - 0.312 inches Gas: a i r Anode Anode Upper Cathode Lower Cathode T o t a l beam Vol t a g e Current (m.a) Current (m.a) Current (m.a) Current (microamps) 1110 2 0.81 1.15 .5 1250 2.7 1.10 1.50 20 1550 3.8 1.6 2.1 26 1770 5.6 2.35 2.35 33 2120 8.1 3.3 4.1 44 TABLE 6 Unanalysed Beam Currents d =0.25 1 = 0.312 i n c h Gas: a i r Anode v o l t a g e : 3100 Anode Current: 4.5 m.a. F i r s t Lens V o l t a g e : 0 Width of C o l l e c t o r S l i t : 0.25 i n c h Second Lens T o t a l Beam Current Beam Current ( V o l t s ) a t C o l l e c t o r i n t o Faraday Cup (microamps) 2030 100 3 4000 200 5.1 5250 250 6.7 (a) Figure 6 Photographs of Qsnilloscc-pe Displays of Air Peaks on a Time Base Figure 7 Photograph of an Oscilloscope Display of Air Peaks on a Voltage Base 41 c y c l e one beam passes over the c o l l e c t o r s l i t and a second beam reaches the s l i t a t the d e f l e c t i o n p l a t e v o l t a g e , 2Vd. During the second h a l f o f a c y c l e the spectrum i s swept i n r e v e r s e order and a " r e f l e c t e d " spectrum appears on the sc r e e n thus r e s u l t i n g i n the d i s p l a y o f three peaks per c y c l e . An i n c r e a s e i n Vd completes the sweep of the t h i r d beam and r e s o l v e s the c e n t r a l peak i n t o two peaks as shown i n d i a - gram (b) of F i g u r e 8. A f t e r f u r t h e i n c r e a s e i n Vd' the He + peak appears i f Helium i s presen t i n the gas mixture and the a i r peaks a r e d i s p l a c e d towards the centre as would be expected. However, they a r e a l s o d i m i n i s h e d i n h e i g h t . A l s o the second l e n s . v o l t a g e , having been a d j u s t e d when the a i r spectrum f i r s t appeared to g i v e maximum peak h e i g h t and d e f i n i t i o n , can now be r e a d j u s t e d to i n c r e a s e the h e i g h t of the helium peak. The e f f e c t i s d i s p l a y e d i n p a r t s (a) and (b) of F i g u r e 8. I n p a r t (a) the Helium peak i s dominant. When the second l e n s v o l t a g e i s decreased, the a i r peak and the helium peak assume equal predominance and the h e i g h t o f the " r e f l e c t e d " h e lium peak becomes g r e a t e r than the helium peak of the f i r s t h a l f o f the sweep c y c l e s . The o p e r a t i n g v o l t a g e s f o r the d i s p l a y s o f F i g u r e 8 are g i v e n i n Tafete^ 7. When the e x t e r n a l 60 c/s A c sweep v o l t a g e supply i s a p p l i e d to the h o r i z o n t a l p l a t e s o f the o s c i l l o s c o p e a s i n g l e doubly t r a c e d spectrum appears, the second t r a c e being t h a t of the r e f l e c t e d spectrum. F i g u r e 7 d i s p l a y s i n t h i s new 42 way t h e i d e n t i c a l a i r s p e c t r u m shown i n F i g u r e 6 ( b ) . B e - c a u s e t h e same sweep v o l t a g e p a t t e r n i s u s e d f o r b o t h t h e s p e c t r o m e t e r and t h e o s c i l l i o s c o p e , t h e x - a x i s o f t h e s c o p e t r a c e i s l i n e a r i n Vd r a t h e r t h a n t i m e a s i s t h e c a s e f o r f r o m e q u a t i o n 5 t h e i n t e r n a l sweep o r " t i m e b a s e . " T h e r e f o r ^ . / t h e x - a x i s s h o u l d be l i n e a r i n (q/m)^ when t h e ' V o l t a g e b a s e " i s u s e d . The p h a s e o f t h e v o l t a g e sweep i s s u c h t h a t Vd i s a minimum a t t h e l e f t end o f a d i s p l a y e d p a t t e r n a n d a maxi- mum a t t h e r i g h t e n d . UU /v\. ( c ) (b) XL (d) F i g u r e 8 P h o t o g r a p h s o f O s c i l l o s c o p e D i s p l a y s F i g u r e 8 ( c ) d i s p l a y s t h e s p e c t r u m o f F i g u r e 8 (a) on a v o l t a g e b a s e ; t h e h e l i u m peak i s d e f i n i t e l y p r e d o m i n a n t . 43 TABLE 7 Mass Spectrometer Operating C o n d i t i o n s f o r the Sp e c t r a D i s p l a y e d i n F i g u r e 8 Ion Source V o l t a g e : 1825 Anode Current (m.a.): 1.5 Gas: Mixture o f a i r and helium O r i f i c e : d = 0.062 i n c h L = 0.312 i n c h Spectrometer Sweep: P o s i t i o n 6 F i g u r e 8 (a) (b) (c) (d) F i r s t l e n s v o l t a g e 6800 6800 6800 4650 Second l e n s v o l t a g e 2400 5400 2400 3400 Vd' 4400 4400 4400 4565 O s c i l l o s c o p e sweep time time v o l t a g e v o l t a g e base base base base However, a change of the l e n s v o l t a g e improves the he l i u m peak and b r i n g s up the a i r peak as shown i n F i g u r e 7 ( d ) . The helium peak i s s h i f t e d to the l e f t i n comparison w i t h i t s p o s i t i o n i n F i g u r e 8 (c) because the f i r s t l e n s v o l t a g e has been decreased, as given i n Table 7, whereupon the helium beam, having l e s s energy, i s c o l l e c t e d a t a lower d e f l e c t i o n v o l t a g e (from equation (4).) By measuring the l e n g t h , S, of a p a t t e r n and the d i s t a n c e , s, o f a c e r t a i n peak from the 44 TABLE 8 A comparison o f measured and c a l c u l a t e d v a l u e s of Vd f o r the c o l l e c t i o n of N g + and He + P a t t e r n Ion Vdc Vdm Vdc/Vdm (C a l c u l a t e d ) (Measured) F i g u r e 7(d) Ng + ?08 2 1 2 5 * 5 0 .426 ± 0.01, F i g u r e 7(d) H + 2400 5750 ±100 .417 ± 0.01 o r i g i n (the l e f t end of the t r a c e ) , the Vd r e q u i r e d to c o l l e c t t h i s beam can be determined i f the maximum Vd per c y c l e i s known. T h i s v o l t a g e equals 2 Vd' as mentioned e a r l i e r . (Vd' i s the average v a l u e of the d e f l e c t i o n p l a t e v o l t a g e and Vd i s the instantaneous v a l u e ) . Thus, Vd = 2 Vd« ( s / S ) . I'ho Table 8, the massed a i r peak o f F i g u r e S(d) i s taken as Ng + and the measured values o f Vd f o r Ng + and He + a r e compared wit h v a l u e s c a l c u l a t e d from equation (4) u s i n g the data o f Table 7. ' ' , Using o n l y the a m p l i f i c a t i o n p r o v i d e d by the two stage a m p l i f i e r of the o s c i l l o s c o p e , about 1 p a r t of He i n 50 p a r t s o f a i r can be d e t e c t e d . 45 (e) I n t e r p r e t a t i o n o f R e s u l t s R e s u l t s show a c o n s i d e r a b l e d i s c r e p a n c y between the p r e d i c t i o n s o f theory and the o p e r a t i o n o f the s p e c t r o - meter. The disc r e p a n c y i s no doubt due c h i e f l y to the f a c t t h a t the e l e c t r i c and magnetic f i e l d s a r e n e i t h e r c o t e r m i - nous nor c o n f i n e d i n v e r t i c a l dimension to the l e n g t h o f the d e f l e c t i o n p l a t e s . The p o l e s e p a r a t i o n o f the magnet i s one and a h a l f inches and the diameter of the po l e f a c e i s one i n c h ; thus, the f l u x leakage i s o f the same order o f magnitude as the c o n f i n e d f l u x . As the d e f l e c t i o n p l a t e s are one i n c h square and separated by one i n c h , the end e f - f e c t i s very c o n s i d e r a b l e . A p o s s i b l e e x p l a n a t i o n o f the v a r i a t i o n o f the he i g h t s o f the a i r peaks w i t h Vd and the second l e n s v o l t a g e i s t h a t the unsymmetric p o t e n t i a l g r a d i e n t between the p o s i - t i v e d e f l e c t i o n p l a t e and the lower e l e c t r o d e o f the f i r s t l e n s a f f e c t s the focus o f the beams and t h a t the e f f e c t on a c e r t a i n beam i s i n f l u e n c e d by how f a r the beam i s d e f l e c t e d stray from the a x i s towards the p o s i t i v e p l a t e by. the Amagnetic f i e l d . Thus, the l e n s v o l t a g e f o r b e s t focus o f a beam de- pends on Vd and on q/m. That the d e f l e c t i o n p l a t e v o l t a g e s are not always balanced around the a n a l y s e r v o l t a g e which i s a l s o the v o l t a g e o f the lower e l e c t r o d e o f the f i r s t l e n s and the 4 6 and the upper electrode of the second lens does not have a significant effect on the focus as the air peaks continue to diminish as they move through the centre of the voltage sweep at which position the deflection plate voltages are halanced. As seen in Figures 7(a) and 8(b) the spectrum of the second half of a sweep cycle i s not an exact reflection of that of the f i r s t half. This distortion may be partly due to the amplifier as i t is more pronounced on different scopes and the pre-amplifier increases i t tremendously. However, part of the trouble must be i n the spectrometer as the second lens voltage has an effect on the exactness of the reflection in height correspondence. Imperfect reflec- tion due to distortion in the deflection voltage of the spec- trometer was thought to have been eliminated by tapping this voltage with negligible phase shift onto the scope x-plates in preference to the use of a separate supply of sine volt- age for the scope. However, i f the distortion i s not sym- metrical a given voltage difference between the plates cor- responding to the collection of a certain beam may be accom- plished by different individual plate voltages at the two points of collection in a cycle thus effecting the focus in different degrees. 47 (f) Suggestions for improvement A well shielded high gain amplifier must be built i f the spectrometer i s to be used as a sensitive leak detec- tor. Shielding .the rod supporting the shielded Fa:/raday cup might be necessary to reduce noise voltages when high gains are used. Also, a higher vacuum i n the spectrometer would probably reduce noise; the vacuum i n the pumping table on which the spectrometer sat was about 10-4mm while that of the spectrometer was no doubt higher. The pressure at which the G.E. Leak detector operates i s about 7010~®mm. Alteration of the ion source i s definitely required for improved performance. Redesign of the upper half of the source to resemble the cathode arrangement of the lower half might permit reliable operation. The upper flanges of the redesigned source would have to be small enough to allow the magnet to pass over. The magnet could be shortened to i n - sulate i t from the upper cathode through the centre of which the gas could be introduced. 48 BIBLIOGRAPHY Elmore, W. C , Sands, M., Electronics Experimental Tech- niques . McGraw-Hill Book Company, Inc., 1949, p. 397. Wang, T.S., Indust. Engn. Chem. (Analvt. Edit.)(1945) pp. 17,67. Amdur, I., Rev. Sci. Instr.. 18, 66 (1947) Allwood, H.I.S., J. Sci. Inst. Phys. Ind. 28, 207-8 (1947) United States Atomis Energy Commission Instructions. Cei - 18293A, p. 17 6. Pupp, W., Phvs. Zeits. 33, 530, (1932) 7. Nelson, R. B., Rev. Sci. Instr. 16, 55, (1945) 8. White, W.C., and Hickey, S., Electronics. 21, 100, (1948) 9. Jacobs, R.B. and Zuhr, H.F., J. Applied Phvs.. 78, 34, (1947) 10. Brubaker, W.M. and Wouk, V. Rev. Sci. Instr.17. 97,(1946) 11. Nelson, H., Rev. Sci. Inst.. 16, 273 (1945) 12. Thomas, H. A., Williams, T.W., and Hippie, J.A., Rev. Sci. Inst. 17, 368, (1946) 13. General Electric, Bulletin G.E.C. - 696. p. 6. 14. Dushman, S., Scientific Foundations of Vacuum Technique. p. 43. 1. 2 . 3. 4 5. APPENDIX I , S c a t t e r i n g from a P r o t o n Beam Le t a beam o f e n e r g e t i c protons enter a volume o f hydrogen gas. As the v e l o c i t i e s o f the protons are much g r e a t e r than the v e l o c i t i e s o f the gas molecules, the mole- c u l e s can be considered (stationary. The diameter o f p r o t o n i s n e g l i g i b l e i n comparison w i t h the diameter, d, o f a hydrogen molecule and the approach o f the c e n t r e o f a p r o t o n w i t h i n d/2 o f the c e n t r e o f a molecule can be c o n s i d e r e d a c o l l i s i o n . On the average, a c o l l i s i o n w i l l occur when the volume o f the c y l i n d e r o f r a d i u s d/2 and l e n g t h , L, where L i s the d i s t a n c e t r a v e l l e d by the proton, equals the volume c o n t a i n i n g one molecule .on ::i-c C7r?a-;e, Thus, LT)'(d/2) = 1/N where N i s the number of molecules per car 5 •The mean f r e e path L = 4/Nird 2 (1) L e t us c o n s i d e r , a t t = 0, a group o f N tagged protons of v e l o c i t y , v . L e t N denote the number o f protons which have not been s c a t t e r e d a f t e r a p e r i o d t . ^50 Appendix I (cont'd) The c o l l i s i o n frequency i s v/L. Then dN = N(vL) d t I n t e g r a t i n g and s e t t i n g v t = .3, N = N q e " s / L . As c u r r e n t , I = Nev, I = I Q e~s/L and L = s / l n (2) Taking s = 20 f e e t and r e q u i r i n g I / I Q = 0.9. L = 192 f e e t =58.6 meters. —8 Taking d = 2.2 10 as o b t a i n e d from e l e c t r o n 13 c o l l i s i o n experiments 11 3 N = 4.5 10 molecules/cm from ( l ) 3 The number of molecules p e r cm i n a gas a t N.T.P. 23 4 19 3 i s 6.023 10 /2.24 10 = 2.63 10 /cm Ther e f o r e , the p r e s s u r e exerted by 4.5 molecules a t T = 0 p = 760 (4.5 10 1 : L/2.68 I Q 1 9 ) = 1.3 l-5mm of Hg. ^ 5 / APPENDIX I I Times f o r Pump Down and E q u i l i b r i u m P r essures ( i ) Formulae The time i n minutes r e q u i r e d t o pump down a volume V from PI t o P2 a t a constant pumping speed o f S CFM ( c u b i c f e e t per minute) i f the system i s t i g h t and outg a s s i n g a t a r a t e much l e s s than S i s t = 2.5 (V/S) l o g P1/P2 = (V/S) I n P1/P2 ( l ) I f the p r e s s u r e range i s d i v i d e d up i n t o increments over which S i s constant^ t =V((1/S1) I n (P1/P2) + ... + (1/Sn) I n (Pn/pn')) (2) where Sn i s the pumping speed between Pn and Pn' and n' = n + 1 . The conductance o f a. tube f o r a i r measured i n CFM a t the h i g h p r e s s u r e end o f the tube o f diameter D in c h e s and l e n g t h L inches i s F = 166 D 5/ (L +: (4/3)D) (3) i f the p r e s s u r e a t one end o f the tube i s a n e g l i g i b l e f r a c - t i o n o f t h a t a t the other end and i f the h i g h e r p r e s s u r e i s l e s s than 2.5/D microns. 52 APPENDIX I I (cont'd) The e q u i l i b r i u m p r e s s u r e i n a system, P (mm) = Q/S (4) Where Q, the outgassing and/or l e a k r a t e , i s measured i n CFM a t 1 mm and S i n CFM a t P. The conductance F or a s e r i e s o f tubes o f conduct- ances F l , F2, ... Fn, i s 1/F = 1/F1 = 1/F2 + ... + 1/Fn As the conductances are measured a t the h i g h p r e s - sure ends o f the tubes a pump can be c o n s i d e r e d a tube o f conductance S. Thus, the pumping speed S' a t the end o f a tube o f conductance F when ac t e d on by a pump o f speed S i s 1/S' = 1/F + 1/S and S' = FS/(F + S) (5) ( i i ) D i f f u s i o n Pumps A f t e r the d i f f u s i o n pumps have been turned on f o r about one h a l f hour the o i l begins t o b o i l w i t h i n c r e a s i n g i n t e n s i t y and the speed of the pumps i n c r e a s e s i n stages to the r a t e d pumping speed causing the p r e s s u r e to drop i n s t e p s . However, i n c a l c u l a t i n g the time to pump down the a c c e l e r a t o r tube and p o r t s from 50 to 10 microns l e t us assume t h a t the pumps have reached t h e i r r a t e d speed and t h a t the c o n d i t i o n s o f equation ( l ) h o l d . As two pumps a c t i n the 5 3 APPENDIX I I • (cont'd) tube, the average speed i s about 2 0 0 0 CFM. From equation ( 3 ) , the average F of the a c c e l e r a t o r tube i s about 3 6 0 CFM t a k i n g D = 9 inches and L = 8 f e e t which i s one h a l f o f the h e i g h t o f the tube. Thus, from equation ( 5 ) , S' = 3 0 5 CFM, and from equation ( l ) t a k i n g V = 1 2 c u b i c f e e t , t = . ( 1 2 / 3 0 5 ) I n 5 0 / 1 0 " 2 = 0 . 3 minutes Taking the conductance o f the i o n source o r i f i c e as 1 / 5 CFM and the pr e s s u r e i n the source as 1 0 0 microns, Q = ( 1 / 5 ) 0 . 1 / 1 = 2 1 0 ~ 2 CFM ( a t 1 mm) and P = Q/S' =2 1 0 " 2 / 1 8 0 = 1 1 0 ~ 4 mm where S ' i s the speed of the d i f f e r e n t i a l pumping tube which i s one h a l f o f 3 6 0 CFM and P i s the p r e s s u r e a t the i o n source when the d i f f e r e n t i a l pumping tube a c t s a l o n e . The pumping speed of the a c c e l e r a t o r tube a t the i o n source i s t n e g l i g i b l e as a second o r i f i c e i s p l a c e d over the top o f the a c c e l e r a t o r tube to reduce the flow of gas i n t o the tube and thus enable i t to m a i n t a i n a lower p r e s s u r e . I f t h i s o r i f i c e a l s o has a conductance of 1 / 5 CFM, the l e a k through the second o r i f i c e i n t o the a c c e l e r a t o r tube i s Q = ( 1 / 5 ) 1 0 - 4 ' ' ' : . = 2 1 0 ~ 5 CFM as the p r e s s u r e above the o r i f i c e i n t h i s case i s onl y 1 0 ~ 4 mm. A l s o , P = Q / S ' = ( 2 0/S) 1 0 " 6 mm APPENDIX I I (cont'd) Thus even i f S' dropped to o n l y 2 CFM as the p r e s s u r e at the top o f the tube approached t h a t a t the bottom and the con- d i t i o n s o f equation (3) were no l o n g e r s a t i s f i e d , a p r e s s u r e o f 1 10~ 5 mm c o u l d s t i l l be maintained. The maximum Q a g a i n s t which the d i f f u s i o n pumps c o u l d m a i n t a i n a p r e s s u r e o f 10~^ mm a t the base of the tube t a k i n g S to be 100 CFM a t 10~ 6 mm i s Q = PS = 1 0 ~ 4 CFM T h i s g i v e s a s e r v i c e f a c t o r o f 5 t a k i n g the l e a k through the second o r i f i c e to be 0.2 1 0 ~ 4 CFM as c a l c u l a t e d above. ( i i i ) F ore Pump Rated S (average) P r e s s u r e Range (CFM) (mms) 24 760—200 18 200—0.05 10 0.05—0.01 S u b s t i t u t i n g the above r a t e d average pumping speeds i n t o e q u a t i o n ( 2 ) , the pump down time f o r the d i f f e r e n t i a l tube and i t s p o r t s , o f estimated volume 12 cub i c f e e t , from 760 mms to 50 microns (0.05 mm) i s T = 2.3 12 ((1/24) l o g 3.8 * (1/18) l o g 4000 + (1/10) l o g 5)* =8.3 minutes APPENDIX I I (cont'd) The maximum permissable Q a t 10 microns g i v e n t h a t S i s 10 CFM a t 10 microns i s q = PS = 0.1 CFM which i s f i v e times g r e a t e r than the i o n source l e a k i n t o the d i f f e r e n t i a l tube as c a l c u l a t e d above.

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