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

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37  L£5 I  9  Sf  A8  Rs\\/z  THE VACUUM SYSTEM OF THE UNIVERSITY OF BRITISH COLUMBIA VAN DE GRAAFF GENERATOR AND A MASS SPECTROMETER LEAK DETECTOR  by E r i c Harvey Richardson  A thesis submitted i n p a r t i a l f u l f i l m e n t of the requirements f o r the degree of MASTER OF ARTS IN THE DEPARTMENT OF Physics  We accept this thesis as conforming to the standard required f o r the degree of Master of Arts:  Members of the Department of Physics  The University of B r i t i s h Columbia Vancouver, Canada May 1, 1951  THE VACUUM SYSTEM OF THE UNIVERSITY OF B R I T I S H VAN DE GRAAFF GENERATOR AND A MASS  COLUMBIA  SPECTROMETER  LEAK DETECTOR  ABSTRACT  The is  vacuum s y s t e m o f t h e V a n d e G r a a f f  described.  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 t u b e s a r e i n d i c a t e d . of  generator  The r e q u i r e d p e r f o r m a n c e  t h e s y s t e m a n d t h e methods o f a t t a i n i n g  c a l c u l a t i o n s being  i tare outlined,  given i n the Appendices.  Vacuum  pressure  gauges a r e d e s c r i b e d and t h e i n d i c a t e d p e r f o r m a n c e o f t h e system recorded.  Vacuum p r o t e c t i o n c i r c u i t s  are discussed.  Methods o f l e a k d e t e c t i o n a r e d i s c u s s e d a n d t h e University tor  of B r i t i s h  C o l u m b i a mass s p e c t r o m e t e r  described i n detail.  The t h e o r y  rometer w i t h a coterminous c r o s s e d field is  i s given.  An  detec-  o f a n a l y s i s by a  spect-  electric  and magnetic  operating procedure f o r the  described i n detail  cussed.  leak  a n d some r e s u l t s  recorded  spectrometer and  dis-  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 f o r guidance and assistance throughout this vrork. The author wishes to make i t clear that he had nothing to do with the o r i g i n a l 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  c r e d i t must go to Mr. A l Salone^in p a r t i c u l a r f o r h i s work i n making f i n a l , exhaustive vacuum tests of the tube sections and i n 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 i n constructing the mass spectrometer leak detector.  Credit must also go to Mr. John Lees, glass blower.  C O N T E N T S Page I II  Introduction The  Design  (a} The (b) The III  V  C o n s t r u c t i o n o f t h e Vacuum System  H i g h V o l t a g e Vacuum Tubes Pumps  2 5  P r e s s u r e Measurement . . . . . . . . . . . .  7  General P i r a n i Gauges I o n i z a t i o n Gauges McLeod -Gauges D a t a on P e r f o r m a n c e o f Vacuum S y s t e m  7 8 9 12 13  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 fb) W a t e r F a i l u r e ( c ) Power F a i l u r e  16 17 18  Leak D e t e c t o r s  20  (a^ G e n e r a l (b) Improved I o n Gauge L e a k D e t e c t o r s . (c) Mass S p e c t r o m e t e r L e a k D e t e c t o r s . . VI  2  . . .  (a) (b) (c) Id J (e) IV  and  1  A Mass S p e c t r o m e t e r L e a k D e t e c t o r f o r t h e Van de G r a a f f G e n e r a t o r (a) D e s i g n (i) (ii) (iii) (iv) (v)  26 26  General Ion Source A n a l y s e r and C o l l e c t o r Power S u p p M e s Gas F e e d  (b) T h e o r y (i} S t r a i g h t Through A n a l y s i s . (ii) Dispersion (c) O p e r a t i o n  20 22 24  30 30 32 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 Bibliography Appendix  I  Appendix  II  f o r Improvement  . . . . . .  47 48  I L L U S T R A T I O N S  Figure  Page  1.  S i d e V i e w o f Tube S e c t i o n  2.  Side  View o f  3.  Side  View of the  Analyser  4.  Side  View of  Collector Section  5.  C i r c u i t D i a g r a m o f D e f l e c t i o n P l a t e Power S u p p l y and I s o l a t i n g T r a n s f o r m e r s  28  Photographs of O s c i l l o s c o p e P e a k s on a t i m e B a s e  40  6. 7. 8.  ' "  t h e Mass S p e c t r o m e t e r I o n  the  Source  Section  2 25 27 27  Displays  of  Photographs of O s c i l l o s c o p e D i s p l a y s P e a k s on a V o l t a g e B a s e  of  Photographs of O s c i l l o s c o p e D i s p l a y s H e l i u m and A i r  of  Air Air 40 42  PLATES I. II.  Cementing P r e s s Pumping P o r t  3  Pots at  the Base o f  t h e Van  Graaff III. IV.  Vacuum S y s t e m a t  4 the  Pumps  5  Mass S p e c t r o m e t e r L e a k D e t e c t o r  and  Apparatus V. VI. VII. VIII.  Isolating The  26  T r a n s f o r m e r s and  Power S u p p l i e s  Source and  Analyser  O s c i l l o s c o p e Sweep V o l t a g e S u p p l y Oscilloscope  29 go,  D e f l e c t i o n P l a t e Supply  Wired Ion  de  35 at 37.  TA  B L E S  1.  R a t e d Pumping Speeds o f D i f f u s i o n Pumps  6  2.  R a t e d Pumping S p e e d s o f F o r e Pump  7  3  O u t g a s s i n g o f "Vacuum S y s t e m  4.  Differential  Tube P r e s s u r e s v s Pd L e a k  Voltages 5.  Performance  6.  U n a n a l y s e d Beam C u r r e n t s  7.  Mass S p e c t r o m e t e r O p e r a t i n g C o n d i t i o n s f o r the Spectra Displayed i n Figure 8 A c o m p a r i s o n o f M e a s u r e d and C a l c u l a t e d v a l u e s o f Vd f o r t h e c o l l e c t i o n o f N + a n d He*  8.  2  14  o f the Ion Source  15 40 40  43  44  1  THE  VACUUM SYSTEM OF THE UNIVERSITY OF B R I T I S H  COLUMBIA.  VAN DE GRAAFF GENERATOR AND A MASS SPECTROMETER LEAK DETECTOR  I.  If  Introduction  t h e V a n de G r a a f f  d e f i n e d beam, t h e p r e s s u r e  generator  i s t o produce a w e l l  o f t h e gas i n t h e system  w h i c h t h e beam p a s s e s must b e s u f f i c i e n t l y a p p r e c i a b l e number o f c o l l i s i o n s gas  molecules.  F o r example, a p r e s s u r e  The  through a path  calculation of this Even without  upper l i m i t  low to prevent  an  b e t w e e n beam p a r t i c l e s and  o f Hg i s r e q u i r e d t o p a s s 90 p e r c e n t collision  through  o f a b o u t 1.5 1 0 " ^ mms  of a proton  beam  without  l e n g t h o f 20 f e e t i n h y d r o g e n g a s . pressure  i s g i v e n i n Appendix I .  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 , a n  o f about 1 0 ~  4  mm  o f Hg p r e s s u r e  i s required  t h r o u g h o u t a l l p a r t s o f t h e h i g h v o l t a g e vacuum t u b e s t o 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 applied.  It is in a  h i g h v o l t a g e vacuum t u b e t h a t t h e beam i s a c c e l e r a t e d the v o l t a g e d e v e l o p e d by the g e n e r a t o r . t u b e must be c a p a b l e o f 2.8  sible the  This accelerator  65,000 v o l t s  across  i n c h i n s u l a t o r s a n d p o s s i b l y 250,000 v o l t s  during  surges.  of withstanding  through  The d i a m e t e r o f t h e t u b e s h o u l d  to hasten  outgassing  enough t o s t a n d  a pressure  be a s l a r g e a s p o s -  and k e e p t h e p r e s s u r e  l e a k from the i o n source.  each  The t u b e must be  down d e s p i t e sturdy  o f 200 l b s p e r s q u a r e i n c h  u n d e r vacuum a n d must be vacuum t i g h t u n d e r t h e s e  while  circumstan- .  ces.  II.  (a)  DESIGN AND  The H i g h V o l t a g e The  Graaff  CONSTRUCTION OF THE VACUUM  SYSTEM  Vacuum"Tubes  two h i g h v o l t a g e vacuum t u b e s o f t h e V a n de  generator  are sixteen feet i n height  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 . l a i n rings are glazed plane lel  on t h e c y l i n d r i c a l  ends a r e g r o u n d f l a t  t o w i t h i n 0.00\6 i n c h e s .  to s h i e l d  The  s u r f a c e s and t h e  t o w i t h i n 0.002 i n c h e s The s t e e l  porce-  and p a r a l -  e l e c t r o d e s a r e shaped  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  scattered  f r o m t h e beam and edges a r e r o u n d e d and p o l i s h e d t o e l i m i n a t e avoidable  c o r o n a 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  i s made up of four pairs of lenses and rings cemented at 400°F under compression. ure  1.  A t y p i c a l section i s shown i n F i g -  The portion of the electrodes to which porcelain i s  cemented i s made of t h i n stainless s t e e l and i s welded i n only three places to reduce stress i n the porcelain due to unequal expansion c o e f f i c i e n t s i n l i e u of using a cement having a lower setting temperature.  The cement used was V i n y l i t e  Resin Solution, Blend 571, manufactured by the Bakelite Corporation. Steel spacers are sealed between sections with f l a t Neoprene gaskets 3/64 inch thick and greased with high vacuum lubricant.  The s t e e l apacers have 0.005 inch ridges 3/8 inch  wide to prevent the gaskets from being drawn i n . Most spacers are 3/8 inch thick but about every tenth spacer was made f inch thick to keep the tube electrodes at the same l e v e l as the stack electrodes. The accelerator tube i s p a r a l l e l e d by the second high voltage vacuum tube, the d i f f e r e n t i a l 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 s u f f i c i e n t rate to maintain a calculated equilibrium pressure of about 10~ mm at the ion source. 4  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 v e r y much b e t t e r vacuum i n the a c c e l e r a t o r tube than would be p o s s i b l e without the a c t i o n o f the d i f f e r e n t i a l pumping tube.  C a l c u l a t i o n s i n v o l v e d a r e given i n  Appendix I I . From the bases o f the tubes to the d i f f u s i o n pumps the system i s c o n s t r u c t e d ports painted  o f l a r g e welded and f l a n g e d  with red g l y p t a l .  iron  Each o f 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 p l a t e v a l v e . valves  The e x t e r n a l p o r t i o n s  o f these  a r e v i s i b l e i n Pigtie• • 2. A holder  providing  compression and alignment  i t a t e d the cementing o f tube s e c t i o n s . s i o n " r o d and three an i r o n p l a t e .  shorter  An a x i a l "compres-  "alignment" rods a r e mounted on  Alignment o f the l e n s e s  i s produced by c i r -  c u l a r wedges dropped down the alignment rods.  A f t e r a com-  p l e t e s e c t i o n i s i n p l a c e a second p l a t e and a c o i l from a railway shaft.  spring  c a r a r e s l i p p e d down over the compression  A brass head threads down t h e s h a f t and a c t s on the  s p r i n g through b a l l b e a r i n g s . fit  facil-  The head i s r a d i a l l y bored t o  l e v e r i n g rods. P r e v i o u s to the baking o f an assembled s e c t i o n the  cement a p p l i e d t o the r i n g and l e n s surfaces  was d r i e d e i t h e r  by warming t o 150°F f o r a few hours or by a p p l y i n g  the cement  t h r e e days p r e v i o u s to the baking o f the p a r t i c u l a r s e c t i o n .  PLATE I I  Pumping P o r t  Pots  A t t h e B a s e o f t h e V a n de  Graaff  5  A very  thin film  o f cement was u s e d and b o t h s u r f a c e s  were  coated.  B e s t r e s u l t s were o b t a i n e d b y h e a t i n g  the sections  400°F a n d g r a d u a l l y c o o l i n g i n t h e c l o s e d o v e n f o r a b o u t  to  14 h o u r s t o room t e m p e r a t u r e .  To r e a c h  c o n n e c t i o n o f t h e oven t h e r m o s t a t s .  400°F r e q u i r e s  dis-  Bubbles remain i n the  cement i f a s e c t i o n h a s b e e n u n d e r - b a k e d ;  i f over-baked, t h e  cement i s s c o r c h e d and b r i t t l e .  Sections oven.  c a n be t a k e n t o p i e c e s i n a 150  degree  Cement i s removed by s o a k i n g t h e 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 tray.  (b)  The Pumps  Each o f the D i s t i l l a t i o n  P r o d u c t s MCF 700  pumps h a s a r a t e d  pumping s p e e d s g i v e n i n T a b l e 1.  lated  I I , these speeds a r e s u f f i c i e n t  a  i n Appendix  p r e s s u r e o f a b o u t 10"^  mm  tube under normal o p e r a t i n g  i s s e t above  conditions.  Because  city above  Also,  two l i q u i d  o f about 8 l i t r e s  t h e i n t a k e s o f each  visible  i n Plate I I I .  to maintain  o f i t s ex-  diffusion t h e vacuum  a i r t r a p s h a v i n g a combined  are located  t h e d i f f u s i o n pumps.  calcu-  i s u s e d i n t h e pumps.  pump t o r e d u c e t h e d i f f u s i o n o f o i l v a p o u r i n t o system.  As  a t the base o f t h e a c c e l e r a t o r  tremely low vapour pressure, O c t o i l A water b a f f l e  diffusion  capa-  i n t h e ends o f t h e p o r t s  The p o r t  ends  and pumps a r e  The pumps r e q u i r e a b a c k i n g p r e s s u r e  PLATE  III  Vacuum System a t  t h e Pumps  6  of 50 microns or l e s s .  TABLE 1 Rated Pumping Speeds of D i f f u s i o n Pumps Speed  Pressure  (CFM)  (MMS)  1400  lO"  4  1000  lO"  5  100  lO"  6  The common fore pump f o r the four d i f f u s i o n pumps i s a Kinney VSD 778. Table 2.  I t s rated pumping speeds are given i n  Calculations i n Appendix I I 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 d i f f u s i o n pumps o f f . A d r i e r i s placed on the a i r intake to prevent water vapour from entering the pump oil.  Also a silica-gel d r i e r i s set i n the tube between the  d i f f u s i o n pump and the Kinney pump.  7  TABLE 2 Rated  Pumping  Speeds o f F o r e  Speed  Pump  Pressure  (CFM)  (MMS)  27  760  22  200  14  .05  7  .01  D a t a o n t h e p e r f o r m a n c e o f t h e vacuum s y s t e m i s recorded  i n the next  section.  III.  (a)  PRESSURE MEASUREMENT  General  P r e s s u r e i n t h e vacuum s y s t e m o f t h e V a n d e G r a a f f generator ionization  i s measured b y a t o t a l o f f o u r P i r a n i a n d two VG-2 gauges.  "Forevac"  p r e s s u r e s o f t h e two s e t s o f  d i f f u s i o n pumps a r e m e a s u r e d b y two P i r a n i  gauges l o c a t e d b e -  tween t h e two d i f f u s i o n pump p a i r s a n d t h e v a l v e s l e a d i n g t o t h e f o r e pump. and  " H i v a c " p r e s s u r e s a r e m e a s u r e d b y two P i r a n i  two i o n i z a t i o n gauges s e t a t t h e b a s e s o f t h e vacuum  tubes.  The p l a t e v a l v e s s e p a r a t e  diffusion  pumps.  t h e h i v a c gauges f r o m t h e  8  An  i n d i c a t i o n of pressure  application of a tesla forevac  gauges.  ficiently (b)  coil  can be o b t a i n e d by t h e  t o t h e glow tubes next  No g l o w a p p e a r s  when t h e p r e s s u r e i s s u f -  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  Pirani  to the  pumps.  Gauges  The  Pirani  Gauges a r e m a n u f a c t u r e d by D i s t i l l a t i o n  Products I n c .  Measurement o f p r e s s u r e by means o f a P i r a n i is  o b t a i n e d by comparing  ated i n a bridge c i r c u i t .  two f i n e w i r e  gauge  resistances incorpor-  One o f t h e r e s i s t a n c e s i s i n a  vessel  s e a l e d a t h i g h vacuum and t h e o t h e r i s i n a n i d e n t i c a l  vessel  joined  t o t h e vacuum s y s t e m .  about the l a t t e r r a t e and thus  lower  i t s temperature  b e l o w 40  The  by  The v a r i a t i o n o f p r e s s u r e  Pirani  gauges s l o w l y move o f f c a l i b r a t i o n and adjustment  Pirani  of the bridge c i r c u i t  resis-  a c c u r a t e p r e s s u r e measurements.  r e a d i n g s depend n o t o n l y o n t h e p r e s s u r e  o f t h e gas b u t a l s o o n t h e m o l e c u l a r gas.  caused  the b r i d g e galvonometer i s approximately  to maintain reasonably The  The  microns.  thus r e q u i r e p e r i o d i c tances  and r e s i s t a n c e .  t o compensate t h e e f f e c t  i n room t e m p e r a t u r e .  with c u r r e n t through linear  o f gases  r e s i s t a n c e servers to increase i t s cooling  sealed v e s s e l i s designed fluctuations  The p r e s e n c e  heat  c o n d u c t i v i t y o f the  T h e r e f o r e , t h e i n t r o d u c t i o n o f a gas o r vapour o f d i f -  9  ferent  c o n d u c t i v i t y t h r o u g h a l e a k i n t h e vacuum  w o u l d be  detected  leak detector (c)  by  system  the P i r a n i which would thus s e r v e  as w i l l be  discussed  as  a  later.  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  t h e vacuum s y s t e m . p o s i t i v e and  c o l l e c t e d on  lision  of  the  g r i d and grid.  electrons with  gas  c o l l e c t e d a t the p l a t e or on  the  in  combination with  v o l t s negative.  the  The  the p l a t e .  the  grid voltage  The  p o s i t i v e ion current  p r o p o r t i o n a l to the  pressure,  but  the  gas.  T h i s d e p e n d e n c e makes t h e  tor  i n leak hunting  ficiently  prevents  a l s o d e p e n d s on  the  other  large into  eventually  region  negative  of the  to  volts  pass  f o r m e d by  molecules i n this  collector.  A  grid  plate before  P o s i t i v e ions  constant,  colare  voltage  electrons electrons  and from  i s almost d i -  factors  being  i o n i z a t i o n p r o b a b i l i t y of gauge u s e f u l as  when t h e p r e s s u r e  i n the  a  system i s  detecsuf-  low.  The sure  to the  p l a t e overcomes t h e r m a l e n e r g i e s  reaching rectly  i s m a i n t a i n e d a t a b o u t 150  e l e c t r o n s which f l o w  r e g i o n between t h e  being  grid  t h e p l a t e a t a b o u t 85  percentage o f the the  The  a t r i o d e open  i o n i z a t i o n gauge c a n n o t be  above 1 m i c r o n  d e s t r u c t i o n of  the  (10~  4  cms. o f Hg)  filament.  operated  without  at a  pres-  accelerated  10  B e l o w 0.5 m i c r o n t h e VG-2 h a s a s e n s i t i v i t y of. 32 m i c r o a m p e r e s p e r m i c r o n f o r a g r i d  for air  c u r r e n t o f 10 m i l -  liamperes. Thus,, t h e c o l l e c t o r  current corresponding  t o 10~§ms  o f Hg i s o n l y 0.32 m i c r o a m p e r e s w h i c h i s t o o s m a l l t o r e a d with  ease and a c c u r a c y  grid  current fluctuates considerably  lector by  on a p a n e l micr©ammeter.  current fluctuations.  a t e s d . c . a m p l i f i e r and a g r i d regulator. 1 Sands.  A similar  By k e e p i n g grid  circuit  operating voltages  positive,  The  a r e overcome  c o n t r o l u n i t which i n c o r p o r current s t a b i l i z e r  or emission  i s d e s c r i b e d by E l m o r e a n d  and t h e c o l l e c t o r a t z e r o  t h e i o n gauge c o n t r o l c i r c u i t  150  effectively  provides  position earthed  plate  and p l a t e 25 n e g a t i v e .  d.c. a m p l i f i e r i s b u i l t  6SC7 d o u b l e t r i o d e . grid  volts,  the r e -  o f t h e VG-2: c a t h o d e z e r o ,  around a b r i d g e  i n w h i c h two b r a n c h e s a r e made up o f t h e two h a l v e s  other  col-  t h e cathode a t 25 v o l t s p o s i t i v e , t h e  a t 175 v o l t s p o s i t i v e ,  quired  causing undesirable  These d i f f i c u l t i e s  t h e V a n de G r a a f f g e n e r a t o r  Also, the  One g r i d  circuit  of a  i s permanently earthed; the  i s c o n n e c t e d t o t h e common t e r m i n a l o f a s i x  selector switch. and t h e c i r c u i t  A t "Zero"  balanced  setting  the g r i d i s  by a d j u s t m e n t o f t h e 3K  r h e o s t a t which v a r i e s t h e r e l a t i v e magnitudes o f t h e r e s i s t ances o f t h e other  two b r a n c h e s o f t h e b r i d g e  circuit.  11  W i t h t h e s w i t c h a t " C a l " a b o u t 0.2 v o l t s i s a p p l i e d g r i d and t h e r e s i s t a n c e justed  to give  full  to the  i n s e r i e s w i t h t h e microammeter a d -  scale deflection.  At position  "1" f u l l  s c a l e d e f l e c t i o n c o r r e s p o n d s t o a p r e s s u r e o f 2 10~3 mms o f Hg;  that  i s , the g r i d i s switched  to a r e s i s t o r across  a voltage  producing f u l l  microamps  flows through the h i g h l y  At  insulated  collector  voltage grid  setting of the amplifier  i s displaced  I f t h e i o n gauge f i l a m e n t  by t h e  and g r i d v o l t a g e s  t u r n e d o f f , t h e b r i d g e may b e b a l a n c e d  to give  f l e c t i o n o n t h e microammeter w i t h t h e s e l e c t o r  zero de-  s w i t c h on pos-  4.  Grid regulation  current  o f t h e i o n gauge i s s t a b i l i z e d t h r o u g h  of the filament  emission.  A #273 Hammond  former i s connected i n s e r i e s w i t h t h e f i l a m e n t The i m p e d a n c e o f t h e #273 i s a f f e c t e d r e n t w h i c h d e p e n d s on t h e g r i d v o l t a g e which a c t a s load to  At position  produced a c r o s s t h e g r i d r e s i s t o r by p o s i t i v e i o n  current.  ition  lead.  the pressure corresponding to  s c a l e d e f l e c t i o n d r o p s by a f a c t o r o f t e n .  "4" t h e z e r o  are  s c a l e d e f l e c t i o n d e v e l o p e s when 64  each succeeding p o s i t i o n ,  full  which  t h e 6SJ7 p l a t e ,  resistors.  i o n gauge g r i d c u r r e n t (which i s a l s o  cur-  o f t h e two 2 A 3 t u b e s a r e connected  o f w h i c h i s d e t e r m i n e d by t h e  which produces  the p o s i t i v e bias  Thus a n i n c r e m e n t a l i n c r e a s e  transformer.  by t h e s e c o n d a r y  The 2 A 3 g r i d s  the voltage  trans-  t h e 6SJ7 g r i d  o n t h e i o n gauge  i n i o n gauge g r i d  voltage  filament).  current  increases  the plate  c u r r e n t o f t h e 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 The  corresponding  primary  decrease  i n secondary  current raises the  i m p e d e n c e o f t h e #273 and t h e c o n s e q u e n t  r e d u c t i o n i n the f i l a m e n t emission tends in  grid  tubes.  incremental  to offset  the r i s e  current. The  r e l a y i n t h e i o n gauge g r i d  c i r c u i t , when  ener-  g i z e d by a n e x 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 t h e g r i d a n d a p plies  the grid voltage across a r e s i s t o r  maintain or  The switches  thrown.  Out-gas s w i t c h s h o r t s t h e r e l a y the g r i d  the v o l t a g e drop  ment e m i s s i o n  McLeod  energizing  o f t h e 6SJ7 t o a r h e o s t a t w h i c h  between t h e cathode  6SJ7 g r i d v o l t a g e can be reduced  (d)  to  t h e e n e r g i z i n g c u r r e n t u n t i l t h e power i s s h u t o f f  t h e "Out-gas" s w i t c h  and  to the cathode  and e a r t h .  to permit  t o p r o v i d e 20 m i l l i a m p s  coil  spans  Thus, t h e  sufficient  i o n gauge g r i d  filacurrent.  Gauge  The  McLeod gauge g i v e s a n a b s o l u t e measurement o f  t h e p r e s s u r e o f t h e permanent gases p r e s e n t by compressing known v o l u m e o f g a s f r o m resultant  a  t h e vacuum s y s t e m a n d m e a s u r i n g t h e  pressure manometrically.  A g l a s s tube  t h e vacuum s y s t e m down t o a n , e v a c u a t e d that  t h e l e v e l o f Hg i n t h e t u b e  into  the reservoir.  runs  from  m e r c u r y r e s e r v o i r so  c a n be r a i s e d  by l e t t i n g a 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  t a p e r s upward i n t o a c a p i l l a r y Parallel similar  t o the c a p i l l a r y , capillary  t e n s i o n on liary  speed.)  c o l u m n s o f Hg.  When t h e l e v e l o f t h e Hg  reaches  a point opposite  t h e p r e s s u r e , P^  equals  t h e d i s t a n c e , h,  t h e Hg  l e v e l and  the c a p i l l a r y .  effect  t h e c l o s e d end  o f the compressed from the top  t h e v o l u m e , V^' Therefore,  gas  i f the o r i g i n a l  w h i c h i s the volume o f the b u l b ,  surface capil-  capillary  sealed i n cms  capilof  of the c a p i l l a r y  i s h times  a  to i n c r e a s e  i n the  of the  of  (This  a tube of normal diameter  pumping  i s V_  two  end.  tube narrows to  t o compensate t h e  o f the  i s b y - p a s s e d by  lary,  the o r i g i n a l  i n order  the l e v e l s  tube s e a l e d a t the top  Hg  down t o  the area, A ,  of  volume o f t h e  then,  the  gas  original  pressure,  P  P  as  of (e)  —5 10-  corresponds  I n t h e low  o f t h e Van  de  p r e s s u r e s , A/V  pressure  should  a  gauge c o n s t r u -  G r a a f f i o n gauges,  t o a n h o f 2.54  pressure  mms.  D a t a on P e r f o r m a n c e o f t h e Vacuum S y s t e m  Observations tem  to measure low  s m a l l as p o s s i b l e .  cted f o r c a l i b r a t i o n  2  V  Thus, i n o r d e r be  M> /\  a = bV a =  on  i n t h e S p r i n g o f 1951 (1)  are given  Pump down t i m e  15 m i n u t e s i f s y s t e m (2)  t h e p e r f o r m a n c e o f t h e vacuum  sys-  below.  f r o m 760  mms  t o 25  microns:  tight.  Ultimate pressure with backing  When pump warms, P i r a n i  readings  rise  pump: 7  t o a b o u t 18  microns.  microns.  14  (3) F o r e p r e s s u r e w i t h d i f f u s i o n pumps o p e r a t i n g : a b o u t 35 m i c r o n s  r e a d on P i r a n i  gauges.  (4) Time f o r d i f f u s i o n pumps t o b e g i n pumping being 45  t u r n e d o n : 30 m i n u t e s .  Total  time  after  t o r e a c h 3 10~^mms:  minutes. (5) P r e s s u r e s  below.  after' prolonged  The d i f f u s i o n pumps a r e s h u t  pumping a r e r e c o r d e d  o f f over  night.  TABLE 3  Outgassing  o f Vacuum  Pressure (mms)  System  T o t a l Pumping Time D i f f u s i o n Pumps (hours)  3 10"  5  1  1 10"  5  8  8 10  3 10~ Finally,  16  - 6  24  6  upon f i l l i n g  the l i q u i d  a i r traps,  a pres-  -6 s u r e o f 1 10  mm was r e a c h e d  i n both  tubes.  (6) P r e s s u r e s a t t h e t o p a n d b o t t o m o f t h e d i f f e r e n t i a l pumping  tube  r e c o r d e d below. pounds p e r s q u a r e  f o r various i o n source leak rates a r e  P r e s s u r e i n t h e h y d r o g e n b o t t l e was 100 inch.  T A B L E  Differential  Pd  4  Tube P r e s s u r e s v s Pd L e a k Voltages  Leak V o l t a g e  Pressure (mms Top o f Tube  10  ) Base o f 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.  (a)  Pressure As  VACUUM PROTECTION CIRCUITS  Rise p r e v i o u s l y m e n t i o n e d , d i f f u s i o n pumps and i o n -  i z a t i o n gauges s h o u l d he o p e r a t e d microns and 1 micron  a t pressures  respectively.  Operation  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  deterioration or destruction. have been d e s i g n e d apparatus  150  of the d i f f u -  Heating  ofi o n  g r e a t l y hastens  their  Vacuum p r o t e c t i o n c i r c u i t s  to switch o f f the respective pieces of  i n t h e event  Pirani  than  pump o i l t h r o u g h o u t  the system c a u s i n g a major c l e a n i n g problem. gauge f i l a m e n t s a t t o o h i g h a p r e s s u r e  less  o f dangerous p r e s s u r e  controlled  circuits  rises.  a r e 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. description of a satisfactory  T. S. Wang circuit  design.  gauges o f t h e V a n de G r a a f f a r e o p e r a t e d control circuit units.  In order  o f t h e two a c c e l e r a t o r t u b e s unit  i s wired  wiring  couple  The f o u r  b y two p o r t a b l e  that pressures  may be r e a d  dual  simultaneously,  each  Thus, t h e  o f t h e u n i t s makes them u n s u i t a b l e a s  f o r a-relay c i r c u i t .  gauge and a v e r y  Pirani  a t the bases  t o one h i v a c and one f o r e v a c gauge.  and p o r t a b i l i t y  triggers  has p u b l i s h e d a  I . Amdur  sensitive  relay  d i f f u s i o n pumps r a t h e r t h a n a P i r a n i  utilizes  a thermo-  f o r the p r o t e c t i o n of  control  circuit.  4  H. I . S. A l l w o o d  employs a G e i s s l e r d i s c h a r g e  tube  17  designed 150  to operate  from  atmospheric  p r e s s u r e down t o a b o u t  microns.  A very  simple d i s c h a r g e tube  consisting  of only a  K o v a r s e a l i s now i n c o r p o r a t e d i n t h e vacuum s y s t e m o f t h e High  Tension  s e t used  Graaff generator. maa. d . c . r e l a y 500  to test  t h e i o n s o u r c e o f t h e V a n de  The d i s c h a r g e t u b e ,  energizing c o i l ,  i n s e r i e s w i t h a 10  operates  v o l t s A. C. when t h e p r e s s u r e r i s e s  on  t o 150 m i c r o n s .  Immediately  t h a t the tube  transformer  i s c u t o f f t h u s p r e v e n t i n g damage t o t h e K o v a r ,  relay,  strikes,  satisfactorily  v o l t a g e t o t h e 500 v o l t  or transformer.  Protection of thermionic filaments i s provided i n Allwood's operates  circuit from  by a Penning  10mm  type d i s c h a r g e tube  o f Hg t o 1 m i c r o n  i n series with a  T h i s p r o t e c t i o n c a n a l s o be p r o v i d e d by a r e l a y t h e p o t e n t i a l o f t h e i o n gauge c o l l e c t o r current  amplifier. 5 Detector.  (b)  Water  Such a c i r c u i t  which  t r i g g e r e d by  a t the grid  i s used  relay.  of the  i n t h e G. E . L e a k  Failure. Water f a i l u r e  i n the cooling  pumps r e s u l t s , i n d e c o m p o s i t i o n d e s t r u c t i o n o f the heater series with a relay  system o f a l l d i f f u s i o n  o f t h e pump o i l and e v e n t u a l  elements.  i s an obvious  A "water s w i t c h " i n  remedy.  18  One container ciently of  o f the f i r s t  water  s u s p e n d e d on a c o i l  to c l o s e an e l e c t r i c  switches  s p r i n g vrtiich s t r e t c h e d c o n t a c t when t h e cup  sensitive  became  "micro-switches"  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 stream  of  full  Amdur employs a b e l l o w s  the p r e s s u r e o f water.  by  s w i t c h e s , however,  water i s f l o w i n g should  may  the  obstructed.  A s i m p l e and constructed testing  diverted  which i s i n f l a t e d  Such bellows  p r e s s u r i z e d even t h o u g h no  d r a i n be  e x e r t e d by a  are  water.  I.  inexpensive water r e l a y  t h e i o n s o u r c e o f t h e Van concentric cylinders  the presence  o f water t o enable  r e l a y which i s connected Water f l o w s  i n through  f l o w exceeds  the o v e r f l o w i s  de  Graaff.  i s reduced  used  for  sufficiently  by  e n e r g i z a t i o n o f t h e power  the i n n e r tube  water r i s e s  was  The r e s i s t a n c e  i n s e r i e s w i t h the water  the o u t f l o w through  the outer c y l i n d e r ,  Power  system  f o r t h e vacuum s y s t e m o f t h e a p p a r a t u s  between two  (c)  suffi-  water. At present, very  be  involved a leaking  resistance.  o r c y l i n d e r and  i f the  the l e a k a t the bottom between the c y l i n d e r s  of  until  reached.  Failure The  r e t u r n o f power a f t e r  damage t o t h e vacuum s y s t e m due  failure  to the r i s e  could r e s u l t i n pressure  in while  19  the to  power was o f f a n d / o r due t o t h e f a i l u r e come b a c k o n a f t e r t h e r e t u r n  small  pressure r i s e  could  o f power.  of the fore A  damage t h e i o n gauge  pump  comparatively filaments.  A l a r g e r p r e s s u r e r i s e due t o l e a k a g e t h r o u g h t h e f o r e  pump  m i g h t 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 t h e f o r e pump f a i l e d  t o come b a c k o n .  As  t h e tfinney f o r e  pump o f t h e V a n de G r a a f f i s  powered b y a t h r e e p h a s e m o t o r f e d b y a s t a n d a r d m a g n e t i c switch the return  o f power w o u l d n o t s t a r t t h e m o t o r .  it  necessary  was a b s o l u t e l y  to place  f u s i o n pumps t h e c o n t a c t s o f a r e l a y of  one p h a s e o f t h e f o r e pump m o t o r  be a p p l i e d  i n series with the d i f e n e r g i z e d by t h e v o l t a g e so t h a t  voltage  t o t h e d i f f u s i o n pumps u p o n t h e r e t u r n  after a failure until  t h e r e l a y was  Thus,  reset.  would n o t  o f power  V.  (a)  LEAK  DETECTION  General  I n any l a r g e vacuum s y s t e m can item  be e x t r e m e l y  tedious  before  assembly.  sections  and p l a c i n g  o f leaks  and t i m e c o n s u m i n g , s o t h a t  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  section)  the detection  checked  (e.g.  each each  tube  T h i s may b e done by p r e s s u r i z i n g  them u n d e r w a t e r o r p a i n t i n g o v e r  with  soap s o l u t i o n t o e n a b l e t h e l o c a t i o n o f l e a k s  by t h e a p p e a r -  ance o f bubbles.  i n this  Large leaks  c a n be d e t e c t e d  A f t e r a system i s assembled l e a k s l o c a l i z e d by i s o l a t i n g judicious  choice  of pressure of valves  sections  of valve  c a n be  positions)  and o b s e r v i n g  t h e change  w i t h i n o r w i t h o u t t h e s e c t i o n s when t h e p l a c e m e n t  The  spark from a T e s l a  enter  enough a n d c l o s e  coil  possible.  applied  t h e system t h r o u g h a h o l e ,  to a glass i f i t i s large  enough t o t h e e l e c t r o d e , m a k i n g t h e p u n c t u r e  clearly visible.  As t h e c o l o r o f t h e d i s c h a r g e  t u b e d e p e n d s o n t h e n a t u r e o f t h e r e s i d u a l gas, t i o n o f an organic tected  roughly  ( a p r o c e s s made e a s i e r by  i n t h e s y s t e m makes s u c h i s o l a t i o n  system w i l l  manner.  and t h e l e a k  Acetone, o r b e t t e r  vapour a p p l i e d localized ether,  o f a glow the absorb-  t o t h e system c o u l d  to the area  be d e -  of application.  f o r example, w o u l d change t h e d i s -  charge c o l o r t o bluish-white.  The s e n s i t i v i t y  o f the Pirani  gauge t o gas if  composition  the p r e s s u r e  i.e.,  can  f r o m 1 m i c r o n t o a b o u t 300 a gasket  Below 1 micron, leak detection. a l l y necessary  de  to prevent  reducing  A  during  Graaff.  leaks but  The  the v i s c o s i t y  of  f i l a m e n t s by the use  R.  B.  p a r a l l e l with  grid  only  the  col-  i o n gauges the  l o c a t i o n of by  t h e vacuum g r e a s e w h i c h i s t h e n  emission  as d e t e c t o r s  o f heated  on vacuum s y s t e m s  In a c o n t r o l c i r c u i t  to prevent  hour can  be  with  described  by  i s wired  in  whose p l a t e  regulation.  spurious  readily  tungsten  o f o x y g e n makes p o s -  through emission  principle  in  i n s e a l i n g the leaks p o s s i b l y  the p l a t e c u r r e n t of the d e t e c t o r diode.  The  be u s e d  enables  the f i l a m e n t o f a c o n t r o l d i o d e  m i c r o n . l i t r e per  bag  current i s especi-  the f i l a m e n t o f the d e t e c t o r diode  arrangement i s designed  in a  t h e vacuum t u b e s o f  increasing concentrations  current i s stabilized  Pirani,  seal.  j e t s o f oxygen. 7  Nelson  gas  gauge may  used with  ether not  r e d u c t i o n of the  of diodes  Coal  manner  f l u c t u a t i o n s i n the  e t h e r was  also often results  The  searching  of  range of the  ionization  the assembly of use  similar  joint.  spurious  j e t of  drawn i n t o p e r f e c t t h e  sible  the  in a  microns.  S t a b i l i z a t i o n of the  current.  as d e t e c t o r s Van  utilized  i s w i t h i n the p r a c t i c a l  i s u s e f u l f o r checking  lector  be  This  fluctuations i n  A leak of  0.76  detected.  i n v o l v e d i n the d e t e c t o r  described  22  8 by  W. C. W h i t e a n d J . S. H i c k e y  emits p o s i t i v e i o n s , the  emission  i s that red hot platinum-  even a t a t m o s p h e r i c p r e s s u r e ,  i s i n c r e a s e d markedly w i t h  o f t h e v a p o u r o f a h a l o g e n compound.  the p a r t i a l  ally tor  a i r o r any o t h e r  gas c o n t a i n i n g a h a l o g e n v a p o u r and t h e 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 . contains  pressure  The s y s t e m u n d e r i n -  vestigation i susually pressurized with suitable  and t h a t  General  information on detector  convenient  i n testing  Electric  of this  Bulletin  type  which i s e s p e c i -  r e f r i g e r a t i o n u n i t s as the detec-  i s s e n s i t i v e t o F r e o n gas i t s e l f .  Alternatively,  d e t e c t o r may be s e t i n t e r n a l l y b e t w e e n t h e d i f f u s i o n and no  f o r e pump a n d a s e a r c h i n g  "the  limit  the external  to small leaks  o f magnitude as t h a t obtained  pump  f o r i n t e r n a l use i s a v a i l a b l e .  concerning  of sensitivity  the  j e t a p p l i e d e x t e r n a l l y though  commercial d e t e c t o r designed  White and Hickey w r i t e ,  GEC-283  with  detector,  i s t h e same  t h e mass  order  spectrometer"  w h i c h i s a b o u t 0.57 m i c r o n . l i t r e s p e r h o u r f o r a 283 l i t r e 9 system a c c o r d i n g t o J a c o b s and Zuhr. • (b)  I m p r o v e d I o n Gauge L e a k  Detectors  10 B r u b a k e r a n d Wouk t o r y method o f i n d i c a t i n g  have d e v i s e d  a sensitive audi-  changes i n t h e c o l l e c t o r  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 are  a m p l i f i e d by a d . c .  voltage  a m p l i f i e r and used t o 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 amplifier.  current of  a loud  A change i n c o l l e c t o r  speaker through an audio current w i l l  either bring  23  on the o s c i l l a t o r  i f i t was  originally  t h e f r e q u e n c y i f t h e o s c i l l a t o r was arrangement  e n a b l e s one man  pressure of 4 x 10~  cms  9  biased  originally  to leak hunt.  o f Hg  c a n be  o f f or  change  operating.  The  A change i n p a r t i a l  noticed.  11  palladium  H. N e l s o n ' s  hydrogen i o n i z a t i o n  t u b e immersed  i n t h e vacuum s y s t e m and  ionization 1 0 " ^ mms  gauge i n w h i c h  o f Hg  down t o lO""* which  c  m  o f Hg  and  Preferentially which would thousand  The  s y s t e m i s pumped  to hydrogen  at  exclusively.  entrance to the system through a l e a k i t i s  admitted to the i o n i z a t i o n  gauge, t h e gas i n  then c o n t a i n a percentage o f hydrogen  times g r e a t e r  tect f a r smaller leaks  about  than the percentage o f hydrogen  gas o f t h e vacuum s y s t e m .  directly  to an  the p a l l a d i u m h e a t e d to 800°C  temperature i t i s permeable  I f hydrogen f i n d s  sealed  a  the p r e s s u r e has been reduced t o  with the a i d of a g e t t e r . s  gauge i n v o l v e s  I t follows  As  i n the  t h a t the d e v i c e can  t h a n c o n v e n t i o n a l i o n gauges  t o t h e vacuum s y s t e m .  one  de-  attached  t h e h y d r o g e n i s pumped  from  the system i t d i f f u s e s  t h r o u g h t h e h o t Pd and o u t o f t h e i o n  gauge a g a i n s t  mm  has  the 1 0 "  the advantages  4  a i r pressure.  investigation  tively  obtained pressure:  easily  t u b e s on a s m a l l s y s t e m a d i f f u s i o n and  gauge  o f l e a k h u n t i n g a t an extremely low p r e s s u r e  w h i l e the system under h i g h and  The h y d r o g e n  i s maintained at a i n testing  pump i s n o t e v e n  radio required  a minimum l e a k o f 10""* l i t r e . m i c r o n p e r s e c o n d c a n  detected.  rela-  be  24  (c)  Mass S p e c t r o m e t e r L e a k  The  Detectors  most s e n s i t i v e l e a k d e t e c t o r s  s p e c t r o m e t e r and a p r o b i n g 12  j e t of helium.  i n v o l v e a mass The b e s t  of  these  instruments  can detect  which contains  a b o u t 1 p a r t He i n 200,000 p a r t s o f a i r .  rarity and  o f He p l u s i t s h i g h  t h e p r e s e n c e o f He i n n o r m a l a i r  rate of diffusion  through  leaks  i t s u n i q u e e/m r a t i o ..are t h e f a c t o r s w h i c h r e s u l t  choice  i n the  o f He a s t h e p r o b e g a s . The  during  This  mass s p e c t r o m e t e r l e a k d e t e c t o r was  W o r l d War I I t o h a s t e n  developed  the s e a l i n g o f the extensive  vacuum s y s t e m employed i n t h e U n i t e d  S t a t e s Atomic  Energy  Project. The tremely  mass s p e c t r o m e t e r l e a k d e t e c t o r  s e n s i t i v e but also- enables  minimum o f t i m e a s s e a r c h i n g  i s n o t o n l y ex-  the l o c a t i o n of leaks  c a n be commenced w h i l e  u n d e r i n v e s t i g a t i o n i s a t f o r e pump p r e s s u r e s  in a  t h e system  and n o t y e t o u t -  gassed.  5 A c o m p l e t e , p o r t a b l e mass s p e c t r o m e t e r l e a k d e t e c t o r with 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 t h a n $4,000 f r o m al  Electric  which a l s o manufactures i o n resonant  Gener-  and B e n n e t RF  13  mass s p e c t r o m e t e r t u b e s .  The i o n r e s o n a n t  magnet p r o v i d i n g a u n i f o r m  field  B e n n e t RF v e l o c i t y  tube r e q u i r e s a  o f a b o u t 2000 g a u s s .  The  s e l e c t o r t u b e r e q u i r e s no magnet b u t a  somewhat h i g h e r and  there  d. c. a c c e l e r a t i n g v o l t a g e .  i s no r e c o r d o f i t s h a v i n g  T h i s t u b e i s new  been used i n l e a k  detec-  t i o n t h o u g h i t may p r o v e w e l l s u i t e d t o s u c h a n a p p l i c a t i o n . It  i s s e n s i t i v e t o 1 p a r t o f He i n 200,000 p a r t s o f a i r .  VI.  A MASS SPECTROMETER LEAK DETECTOR FOR VAN  (a)  DE  THE  GRAAFF GENERATOR  Design (i)  General  A s i m p l e cheap v e r s i o n o f a mass s p e c t r o m e t e r  leak  d e t e c t o r has "been d e s i g n e d f o r u s e w i t h t h e U n i v e r s i t y  of  British  cathode  ion tube  Columbia  Van  de  Graaff.  source plus a s t r a i g h t  I t uses  a simple cold  through a n a l y s e r , w i t h cathode  display.  The  spectrometer  is built  cathode,  the cathodes,  the metal p a r t s are b r a s s .  (ii) The F i g u r e 2.  Ion  a n a l y s e r , and  i n four  source, lower  The u p p e r  collector.  i o n source i s a r e f l e c t o r  What i s termed  cathode  sections:.ion Except f o r  source t y p e , as shown i n  the i o n source s e c t i o n c o n s i s t s  a c y l i n d r i c a l anode s e a l e d a b o v e and i s suspended  f l a n g e d a t the  by a k o v a r  s e a l from  o f t h e anode where a c o n n e c t i o n i s made t o t h e gas p a r a t u s t h r o u g h a ground s o u r c e by  glass  joint  a s t o p c o c k s u p p o r t e d by  s e p a r a t e d from  a glass  A c y l i n d r i c a l magnet made up ring  ray  sections f i t s  magnetized  as  around  t o copper  of three  the i o n source.  a u n i t and n e v e r  subsequently  of  base. the  top  feed  ap-  the i o n seal.  identical  The magnet  was  taken apart i n  PLATE IV  Mass S p e c t r o m e t e r L e a k and  Apparatus  Detactor  2  Figure  S i d e View of Ion  Source  31  ft)  ~*  /  >•  3 1  c  ! LUr B  B  •  — 1  A: B: C: D: Key:  Ion source s e c t i o n Cathode s e c t i o n Upper cathode Upper e l e c t r o d e o f f i r s t Blue i n k : Bed i n k : Green i n k : Pencil:  lens  brass except f o r aluminum glass rubber gaskets SCALE:  Kovar  FULL  SIZE  27  order  t o ensure t h e 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  a b o u t 200 g a u s s 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  c a t h o d e s e c t i o n by a g l a s s r i n g set  i n t h e cathode and i o n s o u r c e  t i o n was t u r n e d  rubber  flanges.  The c a t h o d e  lens.  The c o r e  r e m o v a b l e t o e n a b l e r e p l a c e m e n t by c o r e s  diameters.  A l l orifices  section i s sealed  (iii) The  gaskets sec-  f r o m one p i e c e o f aluminum a n d c o n t a i n s t h e  upper electrode o f the f i r s t is  sealed with  from the lower  o f the cathode of various  a r e 7/16 i n c h l o n g .  to the analyser  orifice  The c a t h o d e  by a second g l a s s  Analyser  and C o l l e c t o r  analyser  electrodes a r e contained  in a  spacer.  length  o f r e c t a n g u l a r wave g u i d e t u b e w h i c h i n c l u d e s t h e l o w e r e l e c trode  of the f i r s t  l e n s , t h e d e f l e c t i o n p l a t e s , and t h e  second l e n s , each e l e c t r o d e being supported  by k o v a r  magnet tsupported:.by  s e a l s as shown i n F i g u r e 3. an i n s u l a t e d s t a n d  section a t the d e f l e c t i o n p l a t e s . tion  separates  The with  the analyser  about the a n a l y s e r  A Corning  upper p l a t e o f the c o l l e c t o r  of v a r i a b l e width f i t s  p l a t e below which i s f i x e d asses  fits  A I700lgauss  g l a s s tube  sec-  from the c o l l e c t o r .  s i x | i n c h diameter holes  A slit  h e l d by h o r i z o n t a l r o d s  to increase  section i s d r i l l e d t h e pumping  speed.  above t h e c e n t r e h o l e o f t h e  a cylindrical  t h e F a r a d a y cup w h i c h i s s u p p o r t e d  s c r e e n w h i c h encompby a r o d f i x e d  to a  Figure S i d e View o f  3  Analyser  Section  A  ^3  B  u  lower electrode o f f i r s t lens. Deflection plates. S l e c t r o d e a of second l e n a . Material:  B r a s s e x c e p t f o r Kovar SCALE:  seals 1TJL1  SI  Figure S i d e View o f  4  Collector Section  r  •  brass except  f o r Kovar  I  1_J"  If  i  rt 11  11  II ii  A: B: C:  B  Slit Faraday Shield  Material:  cup  SCALE:  seal  FULL SI33  28  kovar s e a l s e t i n the side o f the c o l l e c t o r i n Figure  s e c t i o n a s shown  4 The b a s e o f t h e s e c t i o n i s s e a l e d t o t h e vacuum  system by a rubber  The  gasket.  collector  c a n be c o n n e c t e d t o e i t h e r a  galvano-  meter o r a n o s c i l l o s c o p e depending on whether o r n o t t h e d e f l e c t i o n p l a t e v o l t a g e h a s a n a . c . component. The applied v.  t o t h e sweep o f t h e o s c i l l o s c o p e t h r o u g h a 12,500  d . c . 0.25 m i c r o f a r a d (iv)  Power  The by  a 7000 v o l t  der is  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 c a n be  s e tacross  condenser and p o t e n t i a l  divider.  Supplies  voltages  o f the d e f l e c t i o n plates are provided  selenium  rectifier  which has a p o t e n t i a l  t h e secondary a t the transformer.  done by two 1 m i c r o f a r a d  divi-  Filtering  4000 v . d . c . c o n d e n s e r s i n s e r -  i e s b l e d by a t o t a l o f 44 megohms w h i c h draw a maximum o f 0.16 m i l l i a m p s  w h i c h p r o d u c e 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 analyser.  One o f t h e d e f l e c t i o n p l a t e s i s w i r e d  t i v e output  of the r e c t i f i e r  the p o t e n t i a l d i v i d e r voltages five  t o enable  i n the negative  provides  to the output o f  a p p l i c a t i o n o f various a. c.  d. c. v o l t a g e o f the r e c t i f i e r .  step rheostat capable  3500 v o l t s  and t h e o t h e r  to the p o s i -  of operating  across  the lower a. c. v o l t a g e s .  the h i g h e s t r h e o s t a t v o l t a g e  A  a maximum o f To s w i t c h  from  t o p o s i t i o n 6 a t t h e o t h e r end  o f t h e 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  t I A  T3  110 v o l t s 60 c y c l e s  110 v  TI  T2 V  J l T4  •B  ton  •C  T T5  Position 6  *7«i A: B: C:  D:  To i o n s o u r c e power s u p p l y . To p o s i t i v e d e f l e c t i o n p l a t e . To u p p e r e l e c t r o d e o f s e c o n d 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 casing. To n e g a t i v e d e f l e o t i o n p l a t e .  TI = T2 = T3 = M a l o n e y F i l a m e n t T r a n s f o r m e r ; s e c o n d a r y , 11 v, 15 a , i n s u l a t e d f o r 25,000 v o l t s . T4 = S u p e r i o r  Electric  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  rectifier.  Figure Circuit  5  Diagram o f D e f l e c t i o n and  Isolating  Plate  Power  Transformers  Supply  29  unplugging with  the  and  r e p l u g g i n g o f the l i n e  e a s i l y and  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 .  When t h e o u t p u t  o f the p o t e n t i a l d i v i d e r  See  the r e c t i f i e d  voltage.  e n c e o f 14,000 v o l t s An  i d e n t i c a l 7000 v o l t  rectifier  doubler  i s a p p l i e d to the f i r s t 0/3600 v o l t s  a t 20  and  As  giving  such  isolating  The  110  ing  t h e m a i n s v o l t a g e t o one  i s o l a t e d voltages feeds  15,000 v o l t s occur  a  isolating  i o n source  through  isolated  the l e a k .  of the p r i m a r i e s .  the f i r s t  l e n s and  collector  s t a n d o f t h e gas  One  and of  applythe  plate  supply.  r e a c h a maximum o f  i f a d i s c h a r g e does  i n t h e t u b e c o n n e c t i n g ;the i o n  feed apparatus  three  voltages  deflection  the i o n source  This discharge i s prevented  neces-  f o r 25,000 v o l t s  v o l t a g e were a v a i l a b l e ,  anode o f t h e i o n s o u r c e may  t h e gas  supply  t r a n s f o r m e r s when  insulated  the other feeds  above t h e  potential  voltage  v o l t s by w i r i n g t h e s e c o n d a r i e s i n p a r a l l e l  The  differ-  s u p p l i e d by v a r i a c s  t r a n s f o r m e r s were u s e d t o p r o v i d e two  power s u p p l i e s and  and  milliamps.  secondary  of  to  lens.  transformers  a 15 v o l t  without  a 7000 v o l t  transformer primaries are  w h i c h a r e t h e m s e l v e s f e d by sary.  6,  i s available.  s u p p l i e s t h e s e c o n d l e n s and  All  between z e r o  T h u s , a maximum v o l t a g e  divider  delivers  done  P l a t e VI.  i s at position  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 twice  rapidly  by  not source  insulating  with a bakelite  sheet.  the  The  Deflection Plate  S U P P I V  30  (v) Gas The spectrometer  Feed gas  and  feed apparatus includes  two  m e r c u r y manometer, a s i l i c a leak consisting needle valve.  500  c a n be d e t e r m i n e d  ml  designed  to t e s t  storage f l a s k s ,  gel drier,  of a flattened Stopcocks  was  copper  a Pirani  the an  open  gauge, and  tube i n s e r i e s w i t h  are arranged  so t h a t  b y a l l o w i n g t h e gas  t o f l o w i n t o a known  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 n o t n e c e s s a r y  o f t h e mass s p e c t r o m e t e r  of helium i n a i r . in  gas  A  s e p a r a t e m e c h a n i c a l pump p e r m i t s  the  changes  with the a c t i o n of  d i f f u s i o n pumps o f t h e s p e c t r o m e t e r vacuum  immediately  to determine  to d e t e c t v a r i o u s percentages  composition without i n t e r f e r i n g  T h i s gas  a  the l e a k r a t e  v o l u m e and  ability  a  the  system.  supply i n a c t i v e o p e r a t i o n w i l l  above the v a l v e above t h e K i n n e y  be  the  gas  Pump—closed  to  g i v e a h i g h enough p r e s s u r e f o r t h e i o n s o u r c e t o o p e r a t e .  (b)  Theory (i) If  accelerated electric forces F vely. pass  Straight  ions of velocity,  analysis V,  t h r o u g h a v o l t a g e , V,  and m a g n e t i c e  through  and  F  m  field,  and  having  t h e y a r e a c t e d u p o n by  o f the e l e c t r i c  through without  Q,  enter a coterminous,  and  I f t h e s e f o r c e s a r e e q u a l and straight  charge,  magnetic  fields  crossed  parallel respecti-  opposite the ions  deflection.  been  will  31  S.  cEQ/300 = 10 EQ where  F i s i n dynes E i n volts/cm & Q i n emu. A l s o F  =HQV  m  where H i s i n g a u s s and v i n c m s / s e c . F  = F m  e  x  E = 10  The in  If  8  H v  (1)  energy o f the i o n s  i n joules  8, emu; t h e e n e r g y i n e r g s i s 10 VQ. 1 0 V Q = | Mv , 8  M i n grams.  2  Therefore,.;, v = (2VQ10 / M ) Let be  i s 10VQ a s Q i s  2  (2)  t h e c h a r g e , Q, e x p r e s s e d i n e l e c t r o n i c  units  q.  Then, q = Q/e where e i s t h e c h a r g e o f a n in  electron  emu. and  Also,  Q = 1.6 1 0  M = m/6.023  10  q.  - 2 0  -23  where m i s t h e m o l e c u l a r wt. Substituting  f o r Q and M i n (2) ,8 , „ v = (2Vq 10 1.6  _-20„ 23 , si 10 6.023 10 /m)  e  v = 1.39 10 Substituting  x  V  x  (q/m)  2  2  (3)  (3) i n t o ( l )  .-2 - i E = 1.59 10 V (o/m) 2  z  H  (4)  32  For across  the  V = 7000 v o l t s and  deflection Vd  where 2.54  cms  (ii)  Vy  and  the  field  by  the  f  exerts  the  =  plate  separation.  Finally  5000 (q/m)*  (5)  equal F  v e l o c i t y , v,  acquires  .  my field  Let be  beam moves o f f  a horizontal Therefore  x  a v e r t i c a l force,  crossed  , the  component b e i n g V .  component F  the  axis  at  F  y  = F  resolved  into F  and  F_ = x  - F  F  mx m Y  (7),  HQVy =  considered =  F  - M  (6)  - M  and  = a cos  =  M dv —jj^y  dv x  d vy/ut  (HQ/M  t + jrf)  2  origin  the  2  (6)  ^  substituting for  (M/HQ  the  field,  d t  Integrating, v  magnetic  positive. 8  v  and  the  HQv - 10 EQ * x *  = HQv = y  Differentiating in  e  the  y  the upper boundary o f  my  component,  i n a d d i t i o n to the h o r i mx r e s u l t a n t f o r c e , F, e x e r t e d  the  downward d i r e c t i o n o f x b e i n g  the  F  x t a k e n on  voltage  = E/2.54  does not  vertical  zontal  gauss, the  Dispersion If  axis  = 1700  plates,  i s the Vd  H  )  dv^/ut  33  L e t t = 0 a t t h e o r i g i n where v y 0 = ff/2  Then, and  v  Differentiating  = ( a sinlHQt/M and s u b s t i t u t i n g  V As  = Q.  = 10  8  (8) f o r dv / d t i n ( 6 ) ,  E/H+a c o s HQt/M  (9)  v = v a t t = 0 , a  = v  Equations  -  10  E/H  8  (8) a n d ( 9 ) show t h a t  (10) the motion  o f a downward t r a n s l a t i o n o f c o n s t a n t v e l o c i t y with a circular  consists  i n combination  rotation of radius. r  = a/w = aM/QH  Integrating  (8), the h o r i z o n t a l  deflection,  y = aM/HQ ( l - c o s HQt/M) as  y = 0 when t = 0. If  the t o t a l  T i s t h e passage  deflection,  is  s, a t t h e l o w e r b o u n d a r y  s = aM/HQ ( l - c o s HQT/M) The  flected  r e l a t i v e motion  then  o f the f i e l d ( l l )  o f i o n s when o n l y s l i g h t l y d e -  from t h e a x i a l p a t h w i l l  the instrument. very  time through the f i e l d ,  F o r these ions,  determine  the dispersion of  i t i s obvious that only a  s m a l l p o r t i o n o f t h e c i r c u l a r m o t i o n w i l l have been ex-  ecuted b e f o r e t h e i o n s pass o u t o f t h e f i e l d , small,  a n d a p p r o x i m a t i o n s c a n be made.  i . e . , HQT/M i s .  Using  (11) a n d t h e r e l a t i o n  cos x = 1 - x / 2 l + . . . . 8  s = aM/HQ (HQT/2M) a p p r o x i m a t e l y 2  As  = v - 1 0 E / H f r o m (10) 8  a  s = | HQvT /! - $ IC^EO^/M 2  Also  v  (10).  =  v  a p p r o x i m a t e l y f r o m e q u a t i o n s (9)  and  T h e r e f o r e , T = L / v where L i s t h e l e n g t h o f t h e f i e l d  which  i s assumed e q u a l t o t h e l e n g t h o f t h e d e f l e c t i o n  Thus,  •  s = § HQL^/Mv - f 1 0 E Q L / M v 8  2  plates (12)  2  T h i s a p p r o x i m a t i o n c a n a l s o be d e r i v e d by assuming initially  t h a t F = F and v = v and making u s e o f t h e equay x  t i o n s . F = ma a n d s = | a t , s e t t i n g 2  F F  e  = Ma  m m  = 10 EQ * 8  e  = Ma m  = HQv  m  Whereupon, s = i a t m Substituting  from  2  - | a t e  2  = | HQL /Mv - § 1 0 E Q L / M v  ( 2 V ^ 1 0,8° / M )  (2) v =  s = § HL As  2  (Q/M)V  2  2  2  (2V10 )^ 8  s = (constant) L x = C L 2  8  -  § EL /2V 2  (13)  2  ds/dL = 2 C L = 2 s / L T h e r e f o r e , i f D (cms) i s t h e s e p a r a t i o n f r o m t h e c e n t r e o f t h e p l a t e s t o t h e c o l l e c t o r , -the d e f l e c t i o n S, a t the  collector i s 3 =  (2D/L)s  ;  35  The two  difference i n the deflections,  ions having  I  ratios  AS = I Converting cms,  Q /M and a a  Hl2((Q  )* -  s and s o f a b Q../E i s D O  /M^/teVlO )* 8  (13a)  t o q/m and t a k i n g H = 1700 g a u s s L = 2.54  a n d V = 7000 v o l t s , ( ( q /m ) 5 - d /m )£ ) a a D b  (14)  A S = 10.3 ( ( q /m )§ - (q./m. ) * ) a a b b  (15)  AS  = 0.456  F o r a D o f 30 cms,  As  a n example, t h e d i s p e r s i o n b e t w e e n H e  +  and i t s  n e a r e s t l i k e l y n e i g h b o r , (J*"*" i s S = 10.8  ((1/4)4  _  (a/12)i  )  = 10.8 (0.500 (0.408)  = 0.99 cm = 9.9 As  t h e beam d i a m e t e r  i n g by t h e r e s u l t s , (c)  mms i s much l e s s  such a d i s p e r s i o n i s q u i t e  judg-  satisfactory.  Operation As  a guide to f u t u r e operators o f the spectrometer  a s i t i s now s e t up f o r t e s t i n g 122,  than t h i s ,  the following  descriptions (l)  o n t h e vacuum t a b l e o f Room  operating data are given with  o f the apparatus  pertinent  and i t s perdormance.  The s p e c t r o m e t e r s h o u l d be e v a c u a t e d down t o a b o u t  0.1 m i c r o n w i t h t h e g r o u n d g l a s s  joint  connecting the spect-  Wired ion Source and Analyser  36  rometer  t o t h e gas f e e d a p p a r a t u s  i n p l a c e and t h e i o n s o u r c e  s t o p c o c k l o c a t e d between t h e s p e c t r o m e t e r joint  open.  The " e n d " s t o p c o c k w h i c h c o n n e c t s  gauge s e c t i o n o f t h e gas f e e d a p p a r a t u s leading  and t h e ground  t o the spectrometer  evacuated  by t h e f e e d pump.  (2)  Should  needle v a l v e about before closing visible  appar-  beyond i t c a n be  t h e s p e c t r o m e t e r be e v a c -  so a s n o t t o f l o o d  Gas i s f e d i n t o  the tube  o r t h e h o s e t o t h e gas f e e d  uated with the i o n source stopcock c l o s e d , opened v e r y g r a d u a l l y  the P i r a n i  to either  a t u s pump may b e c l o s e d a s t h e " f e e d " s y s t e m  glass  i t s h o u l d be  t h e vacuum  t h e i o n s o u r c e by opening  system.  the leak  a q u a r t e r o f a r e v o l u t i o n f o r a few seconds  t h r e e s i x t e e n t h s o f a t u r n o r s o . An e a s i l y  d i s c h a r g e between t h e l e a k a n d t h e i o n s o u r c e  should  o c c u r u p o n a p p l i c a t i o n o f a Tsesla t o t h e f e e d s t a n d i f t h e i o n source i s wired. l e a k i s easy of  t o work w i t h u s i n g t h e 1/16 i n c h d i a m e t e r  t h e cathode  used.  A p r e s s u r e o f 20 cms o n t h e h i g h s i d e o f t h e  though  When s e t t i n g  p r e s s u r e s up t o one atmospmiere h a v e b e e n  t h e s t o p c o c k s c a r e s h o u l d be t a k e n n o t t o  t u r n the " c e n t r a l " feed stopcock, which connects section  to either  orifice  the P i r a n i  the h i g h o r low pressure sides o f the l e a k ,  to  t h e 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 t h e end s t o p c o c k i s o p e n e d  to  the spectrometer.  (3)  The i o n s o u r c e power s u p p l y i s t u r n e d u p u n t i l  current flows. cathode  a  F l a s h - o v e r s s o m e t i m e s o c c u r i n s i d e between t h e  s e c t i o n a n d t h e i o n s o u r c e s e c t i o n b u t a r e sometimes  37  reduced i n frequency on decreasing the pressure by closing the needle valve a l i t t l e .  The flashes may  cease altogether  a f t e r prolonged operation.  Also,a permanent breakdown some-  times occurs between the upper cathode and the top of the anode, the kovar becoming s u f f i c i e n t l y contaminated  to have a  measurable resistance whereupon i t i s necessary to turn o f f 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 i o n source  i s working properly the current w i l l stop flowing upon d i s connection of the lower cathode.  Before performing this test  a c a r e f u l check was made to ensure that the lens voltages were o f f ; 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 d e f l e c t i o n plate supply i s slowly turned up with the potential divider set on p o s i t i o n 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 c a l i b r a t i o n curves.  The oscilloscope sweep  generator i s synchronized at a multiple of 60 cycles and the shielded c o l l e c t o r lead connected a t an input giving two stages of amplification and bled externally through a 2  PLATE O s c i l l o s c o p e V o l t a g e  S u p p l y  V I I I  E x t e r n a l a t  S w e e p  O s c i l l o s c o p e  58  megohm r e s i s t o r .  I f a voltmeter i s set between the analyser  and the p o s i t i v e d e f l e c t i o n plate which i s always a t a pure d. c. voltage, the a i r spectra should have appeared on the scope screen by the time the meter reads 1000 v o l t s .  I f the  pattern f a i l s to appear, i t i s necessary to check that a pickup voltage appears on the screen when the input i s touched with a finger; f a i l u r e usually means that the c o l l e c t o r lead i s shorted to a shield.  I f the scope i s functioning i t i s  necessary to check the ion source as described above, making sure that the lens voltages are o f f and the condensers d i s 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 def l e c t i o n voltage i s available at the o s c i l l o s c o p e . The r e l a t i v e magnitude of the voltage i n comparison with the d e f l e c t i o n plate a. c. voltage can be varied by the multiple p o s i t i o n switch to compensate f o r changes i n the d e f l e c t i o n plate a. c. voltage. (5)  Vary the positions on the a. c. potential d i v i d e r  of the d e f l e c t i o n plate supply so that small segments of the t o t a l mass spectrum can be examined.  In this way i t i s pos-  s i b l e to sweep the helium peak along i n which case any peak appearing on the screen during leak hunting' i s known to be helium.  39  (6)  When t h e a .  c. p o t e n t i a l d i v i d e r  g i v e a p u r e d.  c. v o l t a g e a c r o s s  current  f e d through a s e n s i t i v e  c a n be  ute  readings  the  spectrometer  the  e x t e r n a l l y generated  meter  (d)  are desired.  I t has  i n the  i o n source 5 and  given i n Tables  internally  Figure 6.  As  d i s p l a y e d on crests  to  set  c e n t e r i n g t h e peak adjusting for a  generated  6.  and  unanalysed  on  galvano-  o f v a r i o u s magnitudes  s c o p e sweep v o l t a g e a r e  a r e f e r e n c e , the a.  a pure d.  shown i n  c. d e f l e c t i o n v o l t a g e i s  raises  the  the  negative  the p o s i t i v e p l a t e which i s at  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 .  This  c . p o t e n t i a l o f t h e p o s i t i v e p l a t e i s m e a s u r e d and half  The  t o z e r o v o l t a g e b e t w e e n t h e p l a t e s as  same p o t e n t i a l as  cur-  using  t h e l o w e r t r a c e o f t h e d o u b l e beam s c o p e .  correspond  to the  beam  Oscilloscope traces for  c . v o l t a g e i s t h e n maximum p o s i t i v e and  plate  one  galvanometer i f a b s o l -  Results  Position 6 deflection voltages  d.  collector  proven convenient  sweep b e f o r e  to  reading.  rents are  a.  the p l a t e s , the  on a c e r t a i n peak by  Currents  an  i s switched  o f t h e a v e r a g e v o l t a g e d i f f e r e n c e , Vd',  equals  between  the  -i  plates. In diagram cycle  indicating  and  one  age  from zero  (a) o f F i g u r e 6  t h a t the  t h r e e peaks appear  spectrum d i s p l a y e d c o n s i s t s o f  h a l f peaks, t h a t i s , d u r i n g t o i t s maximum v a l u e  the  rise  during  i n plate  the f i r s t  per one  volt-  half  of  a  TABLE 5 Performance o f the I o n Source Orifice:  d i a m e t e r d = 0.25 length - 0.312 i n c h e s  Gas: a i r  Anode Voltage  Anode Current (m.a)  Upper Cathode Current (m.a)  Lower C a t h o d e Current (m.a)  T o t a l beam Current (microamps)  1110  2  0.81  1.15  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  .5  TABLE 6 U n a n a l y s e d 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 C u r r e n t : 4.5 m.a. F i r s t Lens V o l t a g e : 0 W i d t h o f C o l l e c t o r S l i t : 0.25 i n c h Second Lens (Volts)  T o t a l Beam C u r r e n t a t Collector (microamps)  Beam C u r r e n t i n t o F a r a d a y Cup  2030  100  3  4000  200  5.1  5250  250  6.7  (a)  Figure 6 Photographs of Qsnilloscc-pe Displays of A i r Peaks on a Time Base  Figure 7 Photograph of an Oscilloscope Display of A i r Peaks on a Voltage Base  41  c y c l e one beam p a s s e s o v e r beam r e a c h e s During  the s l i t  the c o l l e c t o r  An  t h e s e c o n d h a l f o f a c y c l e t h e s p e c t r u m i s swept i n  thus  resulting  resolves  t h e sweep o f t h e t h i r d  t h e c e n t r a l peak i n t o  (b) o f F i g u r e  spectrum appears on t h e  i n t h e d i s p l a y o f t h r e e peaks p e r c y c l e .  i n c r e a s e i n Vd completes  gram  and a second  a t t h e d e f l e c t i o n p l a t e v o l t a g e , 2Vd.  r e v e r s e o r d e r and a " r e f l e c t e d " screen  slit  two p e a k s a s  shown i n d i a -  8.  A f t e r f u r t h e i n c r e a s e i n Vd' t h e H e if  Helium i s present  displaced they  beam a n d  i n t h e gas m i x t u r e  +  peak a p p e a r s  and t h e a i r peaks a r e  towards t h e c e n t r e as would be e x p e c t e d .  a r e also diminished  v o l t a g e , having  i n height.  However,  A l s o t h e second l e n s .  b e e n a d j u s t e d when t h e a i r s p e c t r u m  first  a p p e a r e d t o g i v e maximum peak h e i g h t  and d e f i n i t i o n ,  be  of the helium  readjusted to increase the height  The  effect  In part  i s displayed i n parts  half  peak becomes g r e a t e r t h a n  o f t h e sweep c y c l e s .  displays  When t h e s e c o n d  doubly of  peak  of the " r e f l e c t e d "  the helium  peak o f t h e f i r s t  The o p e r a t i n g v o l t a g e s  f o r the  o f F i g u r e 8 a r e g i v e n i n T a f e t e ^ 7. When t h e e x t e r n a l 60 c / s A c sweep v o l t a g e  applied  8.  t h e a i r peak and t h e h e l i u m  assume e q u a l p r e d o m i n a n c e a n d t h e h e i g h t helium  peak.  ( a ) and (b) o f F i g u r e  ( a ) t h e H e l i u m peak i s d o m i n a n t .  lens v o l t a g e i s decreased,  c a n now  supply i s  to the h o r i z o n t a l plates o f the o s c i l l o s c o p e a t r a c e d spectrum appears, t h e second t r a c e being  the reflected  spectrum.  Figure 7 displays i n this  single that  new  42  way  the  cause  identical  the  same  spectrometer trace the  should  i s linear  be  mum  minimum a t at  the  i n Vd  linear  or  i n  right  pattern  rather  than  of  left  of a  i s used  6  (b).  f o r both  the x-axis of i s the  Bethe  the  scope  case f o r  from equation Therefor^./the x-axis  the'Voltage base"  the v o l t a g e end  i n Figure  time as  "time base."  (q/m)^ when  phase the  shown  the o s c i l l i o s c o p e ,  sweep  The a  sweep v o l t a g e  and  internal  a i r spectrum  5  i s used.  sweep  i s such  that  displayed  pattern  and  Vd a  is  maxi-  end.  UU  /v\. (b)  XL (d)  (c)  Figure  Photographs  Figure on  a voltage  8  base;  8  of Oscilloscope  (c) d i s p l a y s the helium  the  Displays  spectrum  of Figure  peak i s d e f i n i t e l y  8  (a)  predominant.  43  TABLE 7  Mass S p e c t r o m e t e r O p e r a t i n g Spectra Displayed  Conditions  f o r the  i n Figure 8  Ion Source Voltage: 1825 Anode C u r r e n t (m.a.): 1.5 Gas:  Mixture  Orifice:  o f a i r and  helium  d = 0.062 i n c h L = 0.312 i n c h  S p e c t r o m e t e r Sweep:  Position 6  Figure First  lens  voltage  Second l e n s  voltage  Vd' Oscilloscope  sweep  8  (a)  (b)  (c)  (d)  6800  6800  6800  4650  2400  5400  2400  3400  4400  4400  4400  4565  time base  time base  voltage base  voltage base  However, a c h a n g e o f t h e l e n s v o l t a g e  improves  the helium  p e a k a n d b r i n g s up t h e a i r p e a k a s shown i n F i g u r e 7 ( d ) . The  helium  position  peak i s s h i f t e d  beam, h a v i n g  a  i n comparison w i t h i t s  i n F i g u r e 8 (c) because the f i r s t  been decreased,  voltage  to the l e f t  (from  as g i v e n i n T a b l e  less  7, w h e r e u p o n t h e h e l i u m  energy, i s c o l l e c t e d  equation  l e n s v o l t a g e has  a t a lower  ( 4 ) . ) By m e a s u r i n g  deflection  t h e l e n g t h , S, o f  p a t t e r n a n d t h e d i s t a n c e , s, o f a c e r t a i n p e a k f r o m t h e  44  TABLE 8 A c o m p a r i s o n o f m e a s u r e d and v a l u e s o f Vd Pattern  Ion  f o r the c o l l e c t i o n Vdc  Ng  F i g u r e 7(d)  H  origin  (the l e f t  ?08  +  end  is  the average  is  the i n s t a n t a n e o u s v a l u e ) .  T a b l e 8, and  .426  t h e Vd  .417  i f t h e maximum Vd  calculated  as m e n t i o n e d  Thus, Vd  from  f o r Ng  equation  T a b l e 7.  0.01  collect  per c y c l e i s earlier.  = 2 Vd«  +  and (4)  He  (s/S).  i s t a k e n as +  are  o n l y t h e a m p l i f i c a t i o n p r o v i d e d by o f the o s c i l l o s c o p e ,  p a r t s o f a i r c a n be  detected.  (Vd' Vd I'ho Ng  +  compared  u s i n g the data '  stage a m p l i f i e r  ±  0.01,  v a l u e o f t h e d e f l e c t i o n p l a t e v o l t a g e and  t h e m e a s u r e d v a l u e s o f Vd  Using  ±  r e q u i r e d to  t h e massed a i r peak o f F i g u r e S ( d )  with values  +  Vdc/Vdm  5750 ± 1 0 0  T h i s v o l t a g e e q u a l s 2 Vd'  He  (Measured)  of the t r a c e ) ,  t h i s beam c a n be d e t e r m i n e d known.  + and  g  2125*50  2400  +  of N  Vdm  (Calculated) F i g u r e 7(d)  calculated  of  '  the  a b o u t 1 p a r t o f He  ,  two in  50  45  (e)  Interpretation of Results  Results  show a c o n s i d e r a b l e  the p r e d i c t i o n s o f theory meter. that  The d i s c r e p a n c y  the e l e c t r i c  nous n o r c o n f i n e d  and t h e o p e r a t i o n  one a n d a h a l f i n c h e s  is  one i n c h ; t h u s ,  cotermi-  dimension to the length o f  The p o l e  is  s e p a r a t i o n o f t h e magnet  and t h e d i a m e t e r o f t h e p o l e  face  t h e f l u x l e a k a g e i s o f t h e same o r d e r o f  magnitude as the c o n f i n e d  flux.  As t h e d e f l e c t i o n p l a t e s  a r e one i n c h s q u a r e and s e p a r a t e d i s very  to the f a c t  and m a g n e t i c f i e l d s a r e n e i t h e r i n vertical  between  of the spectro-  i s no d o u b t due c h i e f l y  the d e f l e c t i o n p l a t e s .  fect  discrepancy  b y one i n c h , t h e end e f -  considerable.  A possible explanation of the v a r i a t i o n o f the heights is  o f t h e a i r p e a k s w i t h Vd and t h e s e c o n d l e n s  t h a t t h e unsymmetric  p o t e n t i a l g r a d i e n t between t h e p o s i -  t i v e d e f l e c t i o n p l a t e and t h e lower lens affects  the focus  voltage  electrode o f the f i r s t  o f t h e beams and t h a t t h e 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 t h e beam i s d e f l e c t e d  stray f r o m t h e a x i s t o w a r d s t h e p o s i t i v e p l a t e by. t h e m a g n e t i c A  field.  Thus, t h e l e n s v o l t a g e  f o r best  focus  o f a beam d e -  pends o n Vd and o n 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 balanced voltage  around the a n a l y s e r v o l t a g e  a r e n o t always  which i s a l s o the  o f the lower e l e c t r o d e o f t h e f i r s t  l e n s and t h e  46  and the upper electrode of the second lens does not have a s i g n i f i c a n t effect on the focus as the a i r peaks continue to diminish as they move through the centre of the voltage sweep at which position the d e f l e c t i o n plate voltages  are  halanced. As seen i n Figures 7(a) and 8(b) the spectrum of the second h a l f of a sweep cycle i s not an exact r e f l e c t i o n of that of the f i r s t h a l f .  This d i s t o r t i o n may  be p a r t l y  due to the amplifier as i t i s more pronounced on d i f f e r e n t 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 e f f e c t on the exactness of the r e f l e c t i o n i n height correspondence.  Imperfect r e f l e c -  tion due to d i s t o r t i o n i n the d e f l e c t i o n voltage of the spectrometer was  thought to have been eliminated by tapping  this  voltage with n e g l i g i b l e phase s h i f t onto the scope x-plates i n preference to the use of a separate supply of sine v o l t age for the scope.  However, i f the d i s t o r t i o n i s not sym-  metrical a given voltage difference between the plates corresponding to the c o l l e c t i o n of a c e r t a i n beam may  be accom-  plished by d i f f e r e n t i n d i v i d u a l plate voltages at the  two  points of c o l l e c t i o n i n a cycle thus e f f e c t i n g the focus i n d i f f e r e n t degrees.  47  (f)  Suggestions f o r improvement A well shielded high gain amplifier must be b u i l t  i f the spectrometer i s to be used as a sensitive leak detector.  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 mm while that of the -4  spectrometer was no doubt higher.  The pressure at which the  G.E. Leak detector operates i s about 7 10~®mm. 0  A l t e r a t i o n of the ion source i s d e f i n i t e l y required f o r improved performance.  Redesign of the upper h a l f of the  source to resemble the cathode arrangement of the lower h a l f might permit r e l i a b l e 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 1.  Elmore, W. C ,  Sands, M., Electronics Experimental Techniques . McGraw-Hill Book Company, Inc., 1949, p. 397.  2 .  Wang, T.S.,  Indust. Engn. Chem. (Analvt. Edit.)(1945)  3.  4 5.  pp. 17,67. Amdur, I.,  Rev. S c i . Instr.. 18, 66 (1947)  Allwood, H.I.S., J . S c i . Inst. Phys. Ind. 28, 207-8 (1947) United States Atomis Energy Commission Instructions. Cei - 18293A, p. 17  6.  Pupp, W.,  Phvs. Z e i t s . 33, 530, (1932)  7.  Nelson, R. B., Rev. S c i . Instr . 16, 55, (1945)  8.  White, W.C.,  9.  Jacobs, R.B. and Zuhr, H.F., J . Applied Phvs.. 78, 34,  and Hickey, S., E l e c t r o n i c s . 21, 100, (1948)  (1947) 10.  Brubaker, W.M.  and Wouk, V.  Rev. S c i . Instr.17. 97,(1946)  11.  Nelson, H.,  Rev. S c i . Inst.. 16, 273 (1945)  12.  Thomas, H. A., Williams, T.W.,  and Hippie, J.A., Rev.  S c i . Inst. 17, 368, (1946) 13.  General E l e c t r i c ,  14.  Dushman, S.,  B u l l e t i n G.E.C. - 696. p. 6.  S c i e n t i f i c Foundations of Vacuum Technique. p. 43.  APPENDIX I ,  Scattering  Let  a beam o f e n e r g e t i c p r o t o n s  hydrogen gas. greater cules is  f r o m a P r o t o n Beam  As  the v e l o c i t i e s  than the v e l o c i t i e s  c a n be  hydrogen molecule w i t h i n d/2 collision.  o f the gas m o l e c u l e s ,  i n comparison and  The  the approach  the average,  the d i s t a n c e t r a v e l l e d  containing  one  m o l e c u l e .on  of  proton  d, o f  a proton  c a n be  considered a  will  o c c u r when t h e  a collision  by  the mole-  o f the centre of a  v o l u m e o f t h e c y l i n d e r o f r a d i u s d/2 is  diameter  w i t h the diameter,  o f the c e n t r e o f a molecule On  of  o f t h e p r o t o n s a r e much  considered (stationary.  negligible  e n t e r a volume  and  the proton,  l e n g t h , L, w h e r e equals  L  the volume  ::i-c C 7 r ? a - ; e ,  Thus, LT)'(d/2) =  1/N  where N i s t h e number o f per  •The mean f r e e L  Let protons  us  = 4/Nird  v.  5  path (1)  2  c o n s i d e r , a t t = 0,  of v e l o c i t y ,  car  molecules  a group o f N  L e t N d e n o t e t h e number o f  w h i c h have n o t been s c a t t e r e d  after  a period t.  tagged protons  ^50  Appendix I (cont'd)  The  c o l l i s i o n frequency i s v/L.  Then  dN = N ( v L ) d t Integrating N = N As  q  e"  s  a n d s e t t i n g v t = .3, /  current, I  Taking L  = I  .  L  I = Nev, e~  and L = s / l n  s/L  Q  (2)  s = 20 f e e t a n d r e q u i r i n g = 192 f e e t = 5 8 . 6  I / I = 0.9. Q  meters.  —8 T a k i n g d = 2.2 10  as o b t a i n e d from  electron  13 collision  experiments N = 4.5 10  11  molecules/cm  3  from ( l )  3 The number o f m o l e c u l e s p e r cm i n a g a s a t N.T.P. 23 4 19 3 i s 6.023 10 /2.24 10 = 2.63 10 /cm Therefore,  t h e p r e s s u r e e x e r t e d by 4.5 m o l e c u l e s a t  T = 0 p = 760 ( 4 . 5 1 0  1:L  /2.68 I Q  1 9  ) = 1.3 l- mm o f Hg. 5  ^ 5 /  APPENDIX I I  Times (i)  f o r Pump Down a n d E q u i l i b r i u m P r e s s u r e s  Formulae The  time i n minutes  required  t o pump down a v o l u m e  V f r o m P I t o P2 a t a c o n s t a n t pumping s p e e d o f S CFM f e e t p e r minute) r a t e much l e s s  (cubic  i f t h e system i s t i g h t and o u t g a s s i n g a t a  than S i s t = 2.5 (V/S) l o g P1/P2 = (V/S) I n P1/P2 ( l )  If  t h e 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 / S 1 ) I n (P1/P2) + ... + (1/Sn) I n (2)  (Pn/pn')) where S n i s t h e pumping s p e e d b e t w e e n P n a n d P n ' a n d n ' = n + 1 .  The c o n d u c t a n c e o f a. t u b e f o r a i r m e a s u r e d i n CFM a t t h e h i g h p r e s s u r e end o f t h e t u b e o f d i a m e t e r D i n c h e s and l e n g t h L i n c h e s i s F = 166 D / 5  if  ( L +: (4/3)D)  t h e p r e s s u r e a t one end o f t h e t u b e i s a n e g l i g i b l e  (3) frac-  t i o n o f t h a t a t t h e o t h e r end a n d i f t h e h i g h e r p r e s s u r e i s less  t h a n 2.5/D m i c r o n s .  52  APPENDIX I I (cont'd)  The  equilibrium pressure i n a P  (mm)  =  system,  Q/S  (4)  Where Q, t h e o u t g a s s i n g a n d / o r m e a s u r e d i n CFM a t 1 mm  The  1/F As  a n d S i n CFM a t P.  conductance F o r a s e r i e s  a n c e s F l , F 2 , ...  leak rate, i s  o f tubes o f conduct-  Fn,i s  = 1/F1  =  1/F2  +  ...  +  1/Fn  t h e conductances a r e measured a t t h e h i g h  pres-  s u r e ends o f t h e t u b e s a pump c a n b e c o n s i d e r e d a t u b e o f c o n d u c t a n c e S.  Thus, t h e pumping s p e e d S' a t t h e end o f a  t u b e o f c o n d u c t a n c e F when a c t e d o n by a pump o f s p e e d S i s 1/S' and  (ii)  S'  = =  Diffusion After  +  1/S  F S / ( F + S)  (5)  Pumps t h e d i f f u s i o n pumps h a v e b e e n t u r n e d o n f o r  a b o u t one h a l f h o u r intensity  1/F  the o i l begins t o b o i l  with increasing  a n d t h e s p e e d o f t h e pumps i n c r e a s e s i n s t a g e s t o  t h e r a t e d pumping s p e e d c a u s i n g  the p r e s s u r e t o drop i n  steps.  t h e t i m e t o pump down t h e  However, i n c a l c u l a t i n g  accelerator  t u b e a n d p o r t s f r o m 50 t o 10  assume t h a t  t h e pumps h a v e r e a c h e d t h e i r  the conditions o f equation ( l ) hold.  microns l e t us rated  s p e e d and t h a t  As two pumps a c t i n t h e  5 3  APPENDIX I I • (cont'd) tube, t h e average speed i s about 2 0 0 0  CFM.  (3),  t u b e i s a b o u t 3 6 0 CFM  the average F o f the a c c e l e r a t o r  From e q u a t i o n  t a k i n g D = 9 i n c h e s a n d L = 8 f e e t w h i c h i s one h a l f o f t h e height  o f the tube.  T h u s , f r o m e q u a t i o n ( 5 ) , S' = 3 0 5 CFM,  and f r o m e q u a t i o n ( l ) t a k i n g V = 1 2 c u b i c t  Taking  =.(12/305)  In 5 0 / 1 0 "  feet,  =  2  0.3  minutes  the conductance o f the i o n source  orifice  a s 1 / 5 CFM a n d t h e p r e s s u r e i n t h e s o u r c e a s 1 0 0 m i c r o n s ,  and  where S '  Q  =  (1/5) 0.1/1  P  =  Q/S'  =  2  10~  10" /180  =2  2  CFM  = 1  2  i s t h e speed o f the d i f f e r e n t i a l  (at 1  10~  pumping  4  mm)  mm  tube which  one h a l f o f 3 6 0 CFM a n d P i s t h e p r e s s u r e a t t h e i o n  is  s o u r c e when t h e d i f f e r e n t i a l pumping  pumping  speed o f t h e a c c e l e r a t o r  tube a c t s  a l o n e . The  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 accelerator  i s placed  over the top o f the  t u b e t o r e d u c e t h e f l o w o f gas i n t o  thus enable i t to m a i n t a i n a lower p r e s s u r e .  t h e tube and  I f this  orifice  a l s o h a s a c o n d u c t a n c e o f 1 / 5 CFM, t h e l e a k t h r o u g h t h e second o r i f i c e  into Q =  the a c c e l e r a t o r ( 1 / 5 ) 10  - 4  ''':.  as t h e p r e s s u r e above t h e o r i f i c e 10~  4  mm.  tube i s  = 2  10~  5  i n this  Also, P = Q / S ' = (20/S) 1 0 "  6  mm  CFM  case i s only  APPENDIX I I (cont'd)  Thus e v e n i f S' d r o p p e d  t o o n l y 2 CFM  top o f the tube approached  as the p r e s s u r e a t  t h a t a t the bottom  and  the  d i t i o n s o f e q u a t i o n (3) were no l o n g e r s a t i s f i e d , of 1 10~  5  mm  could  still  be m a i n t a i n e d .  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~^ CFM  Q = PS = 1 0 ~  a t 10~  mm 6  (iii)  t o be 0.2  mm  is  CFM  4  10~  4  CFM  the l e a k through the  as c a l c u l a t e d  above.  F o r e Pump Rated S  (average)  P r e s s u r e Range  (CFM)  Substituting into  pumps  a t the base o f the  This gives a service factor of 5 taking second o r i f i c e  con-  a pressure  The maximum Q a g a i n s t w h i c h t h e d i f f u s i o n  t u b e t a k i n g S t o be 100  the  (mms)  24  760—200  18  200—0.05  10  0.05—0.01  t h e above r a t e d  a v e r a g e pumping  e q u a t i o n ( 2 ) , t h e pump down t i m e f o r t h e  t u b e and 760 mms  i t s ports,  o f e s t i m a t e d v o l u m e 12  t o 50 m i c r o n s T = 2.3  (0.05 mm) 12  4000 + =8.3  minutes  differential  cubic f e e t ,  is  ( ( 1 / 2 4 ) l o g 3.8  *  (1/10) l o g 5)*  speeds  (1/18) l o g  from  APPENDIX I I (cont'd)  The maximum p e r m i s s a b l e Q a t 10 m i c r o n s S i s 10  CFM  a t 10 m i c r o n s  q which i s f i v e  = PS  times  the d i f f e r e n t i a l  =  given  that  is 0.1  CFM  g r e a t e r than the i o n source l e a k  tube as c a l c u l a t e d  above.  into  

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