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An ultrahigh vacuum system Vanandel, Hendrikus Willem H. 1963

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AN ULTRAHIGH VACUUM SYSTEM by  HENDRIKUS WILLEM H. VANANDEL B . S c , University of B r i t i s h Columbia, 196.2  A THESIS SUBMITTED .IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE -  i n the Department of PHYSICS  We accept this thesis as conforming to the, required standard  THE UNIVERSITY OF BRITISH COLUMBIA October, 1963  . I n the  presenting  r e q u i r e m e n t s f o r an  British  mission  for reference  for extensive  p u r p o s e s may  be  of  w i t h o u t my  written  Department o f  by  fulfilment  of  d e g r e e at. the  University  of  Library  study.  the  Head o f my  i s understood  Physics Columbia,.  1963.  agree  this thesis for  permission.  October,  s h a l l make i t f r e e l y  I further  that  or  c o p y i n g , or  shall  per-  scholarly  Department  that  for f i n a n c i a l gain  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, Canada. Date  in partial  the  copying of  It  this thesis  that  and  granted  representatives.  cation  advanced  Columbia, I agree  available  his  this thesis  not  be  by publi-  allowed  ABSTRACT  An ultrahigh vacuum system has been constructed f o r the purpose of f i l l i n g discharge tubes with gases without introduction of impurities. ultimate pressure of 6 x 10-10  An  mm. of Hg. has been reached before f i l l i n g .  Two tubes have been constructed and f i l l e d with Neon without contamination.  ii  critical  ACKNOWLEDGMENT  I should l i k e to express my thanks to Dr. R. A. Nodwell whose guidance arid encouragement has been of great help i n the completion of this project; to Mr. John Lees for his many hours of glassblowing and for h i s helpful advice on vacuum technique; to Mr. John Turner for a l l his time  devoted  to the design and construction of the electronic units; to Alex Fraser and other members of the technical s t a f f f o r their co-operation, and to the members of the Plasma physics group f o r their continued interest and assistance.  v  TABLE OF CONTENTS Page ABSTRACT  i i  ACKNOWLEDGEMENT  v  INTRODUCTION  1  CHAPTER I  -  THEORY Introduction  4  1. Mechanisms causing pressure change a.  Pumping with external pumps  5  b.  Backstrearning  9  c.  Adsorption  10  d.  Desorption  13  e.  Diffusion  15  f.  Permeation  17  g.  Ion pumping  18  2. Pressure equation CHAPTER II -  19  APPARATUS  32  1. Pumps  32  a.  Backing pump  32  b.  D i f f u s i o n pump  32  .  2. Valves  34  3. Gauges  35  a.  McLeod gauge  b.  Ionization gauge  c.  Discharge gauge  d.  O i l manometer  35 i  36 43 47  4. Gas handling system  47  5. Oven  48  6. Electronics  49  CHAPTER I I I - EXPERIMENTAL PROCEDURE AND RESULTS BIBLIOGRAPHY  50 57  - iii -  Page 59  ILLUSTRATIONS 1.  General arrangement of the vacuum system  59  2.  D i f f u s i o n pump  60  3.  Baffle  60  4.  O i l Manometer  60  5.  Neon f i l l i n g  6.  Valve e x t e r i o r  61  7.  Graph showing l e a k r a t e v e r s u s c l o s i n g torque f o r a l l metal v a l v e s  61  8.  D i s c h a r g e gauge  62  9.  Graph showing s t r i k i n g and e x t i n c t i o n v o l t a g e as a f u n c t i o n o f a i r p r e s s u r e f o r a t y p i c a l d i s c h a r g e tube  62  10.  C a l i b r a t i o n curve f o r d i s c h a r g e gauge  63  11.  C a l i b r a t i o n curve f o r d i s c h a r g e gauge  64  12.  Ion gauge c o n t r o l u n i t  65  13.  Ion c u r r e n t a m p l i f i e r u n i t  66  system  60  - iv -  INTRODUCTION It is well known that the presence of small amounts of impurity greatly affects the spectroscopic emission of most gases and vapors.  It is gener-  ally observed that the introduction of a foreign gas may drastically reduce both the intensity of resonance radiation and the coefficient of absorption for many electronic transitions (1).  The extent to which the impurity  affects the gas under study depends on the amount and the nature of the impurity (2).  Also, a particular impurity w i l l affect one transition more  than another. In this laboratory, experiments performed on the absorption properties of Neon showed that the coefficient of absorption from excited states is c r i t i c a l l y dependent on the purity level of the Neon gas.  With the existing  f a c i l i t i e s for the preparation of a Neon discharge tube it was not possible to prevent serious contamination of the Neon.  For this reason it was decided  that an ultra - high vacuum system would be built which could be used for the preparation of gas discharge tubes.  This thesis deals with the apparatus and  techniques involved in the construction and operation of such a system. aspects of ultra - high vacuum theory are also discussed.  Some  The project  described served as a link in a chain of experiments performed in this laboratory in order to measure transition probabilities in excited Neon. One possible reason for the decrease of intensity of resonance lines in the presence of a foreign gas is the depopulation of excited energy states of atoms of the major constituent because of collisions with the foreign gas atoms or molecules.  Instead of giving up its energy in radiation, the  excited atom transfers some or a l l of its excitation energy to the foreign gas, which carries it off as kinetic energy. - 1-  The probability for this to  - 2 -  occur i s obviously related to the probability that a c o l l i s i o n w i l l occur between the two atoms.  Hence those states with long lifetimes are more  e a s i l y depopulated than those with very short l i f e t i m e s , and correspondingly the effect of the foreign gas on the transitions from very shortlived states is smaller than on those from states with a r e l a t i v e l y long l i f e t i m e , for states with the same c o l l i s i o n cross section.  The absorption properties of a gas are of course s i m i l a r l y affected by impurities.  The e f f e c t may be p a r t i c u l a r l y severe i n the case of absorption  taking place from a metastable state to a higher state.  The  metastable  states have a l i f e t i m e of the order of milliseconds or more, and the probab i l i t y for a c o l l i s i o n to occur i s very high.  Hence the population density  of metastable states may be lowered to such an extent by the that the absorption becomes very weak indeed.  Meissner (3)  contaminant reports i n a  paper on the absorption of excited Neon that even the smallest traces of Hydrogen present i n the Neon discharge decreased the absorption d r a s t i c a l l y . Ladenburg (4) i n a paper on the anomolous dispersion of excited Neon confirms this and shows that only under extremely pure conditions i s one able to observe effects dependent on s u f f i c i e n t population of metastable  states,  such as absorption and anomolous dispersion involving t r a n s i t i o n s from these states.  The experimental arrangement formerly used i n this laboratory for absorption measurements was as follows.  A discharge tube with windows on  the ends was permanently attached to a conventional vacuum system served by an o i l d i f f u s i o n pump.  ^  -6 mately 10  The system was pumped down to a pressure of approxi-  mm.  of Hg. after'which i t was  a few millimeters.  f i l l e d with Neon to a pressure of  Powdered Uranium was employed as a Hydrogen getter.  A  potential difference of about a k i l o v o l t was applied across the electrodes of the discharge tube to give a steady orange glow of Neon.  A continuum f l a s h  background source was used and the absorption of this l i g h t by the Neon discharge photographed on a spectograph.  Since absorption lines of excited Neon  f a i l e d to appear i n s u f f i c i e n t intensity on the spectrograph under the most varied experimental conditions, i t was care was  suggested  that although the greatest  taken to avoid contamination of the Neon, the system used was  s u f f i c i e n t l y pure for the measurements required.  This was  not  further confirmed  by the fact that upon continued use, the color of the discharge began to deteriorate.  Balmer lines of Hydrogen were also c l e a r l y v i s i b l e on the  spectrograph whenever a picture of the Neon discharge by i t s e l f was  taken.  With the use of ultrahigh vacuum techniques i t was hoped that these d i f f i c u l ties could be avoided.  '  The Neon used i n this project was  the purest available commercially  and  was reported to have the following impurities present: Helium  80 parts per m i l l i o n  Oxygen  50  "  "  "  Nitrogen  10  "  "  "  Hydrogen  10  "  "  "  1  "  "  Water  Hence the quality of the system used had to be such that the l e v e l of impurity did not r i s e s i g n i f i c a n t l y above the quoted values due to the transfer of the Neon to the discharge tube and i t s subsequent use. the rest of this thesis, this requirement vacuum techniques.  As w i l l become apparent i n  necessitated the use of u l t r a high  CHAPTER I THEORY Introduction There exist many mechanisms which may be responsible for the change in pressure in a particular system making use of the removal or introduction of matter in the gas phase.  The most important ones are the following, assuming  leaks have been eliminated: a.  Pumping with rotary or diffusion type pumps.  b.  Backstreaming from the pumping units.  c.  Adsorption of molecules to the walls of the container.  d.  Desorption of molecules from the walls of the container.  e.  Diffusion of molecules from the interior of the walls of the container.  f.  Permeation of molecules through the walls of the container.  g.  Electrostatic entrapment of ions. There are a number of ways in which these processes may be classified;  external pumping, adsorption and ion entrapment are mechanisms leading to a decrease in pressure, while the others a l l tend to increase the pressure in the system; whereas one may at any time choose to turn off the pumps, the other processes go on at a l l times and can not be simply controlled.  Pumping  and backstreaming always occur at the same time, and adsorption and desorption likewise.  Desorption, diffusion, and permeation together are usually referred  to as "outgassing". In any particular system, not a l l these processes are equally important. The relative importance of the different mechanisms generally depends on many parameters of the system, such as the pumping speed of the external pumps, the temperature, the geometry of the wall material (e.g. surface to volume ratio),  - 5 and the nature and condition of the surfaces exposed to the vacuum.  Because  many of the processes invariably occur at the same time i t i s often d i f f i c u l t to distinguish one from the other, and therefore not much quantitative i n f o r mation i s available which would enable one to predict the behavior of a vacuum system accurately.  The d i f f i c u l t y i n predicting the system's behavior  i s increased by the fact that no one system i s i d e n t i c a l to another as far as i t s reactions to adsorption, desorption, d i f f u s i o n , and permeation i s concerned.  These mechanisms are always a function of the previous history of  the wall material of the system.  However, simple calculations based on data  obtained experimentally under more or less controlled conditions can give a f a i r l y good estimate of the r e l a t i v e order of magnitude of the d i f f e r e n t processes i n any p a r t i c u l a r experimental situation.  In what follows these  processes w i l l be discussed i n some d e t a i l i n turn.  For each of the  processes an expression w i l l be derived which gives the rate of change of pressure as a function of time i f only that p a r t i c u l a r process were i n operation.  After that the processes w i l l be combined i n one equation giving the'  pressure as a function of time, i n terms of a l l the d i f f e r e n t parameters ' connected with the separate mechanisms.  It must be emphasized, however, that  a l l the calculations only serve to at best give a semiquantitative picture of the behavior of a vacuum system. book on vacuum technique.  References can be found i n any standard  (See f o r example (5)  or (6).  For the adsorption  and outgassing processes, which are p a r t i c u l a r l y of importance vacuum technique, a good reference i s (7)  and also (8)).  for ultrahigh  The l a t t e r two also  describe the general requirements for ultrahigh vacuum. 1.  Mechanisms Causing Pressure Change.  a.  Pumping with External Pumps. The process of pumping with an external pump i s nothing more than the  - 6 coming to equilibrium of a gas which has an externally maintained gradient set up i n i t .  pressure  Consider therefore the following idealized system.  A  T  T  X  nr  Enclosures I and I I (with volumes  and V^, pressures p^ and p£, number  densities n^ and n£), characterized by temperature T, are joined by an aperture of area A. appears at A.  The stepfunction pressure gradient of height p£ - p^  We assume that p£  >  Pp  From the Kinetic Theory, for a gas of density n molecules per cc., the number of molecules s t r i k i n g unit area of a wall per unit time i s given by  V  CD  where k i s Boltzmann's constant, m i s the mass of one molecule, and T i s the absolute Temperature. Hence we can say i n the above model that the net number of molecules, v>  2)  , coming through the aperture A per unit time i n the preferred direc-  t i o n (II to I) i s given by  21  We define the Conductance C of the aperture as the net number of molecules traveling through the aperture i n the preferred d i r e c t i o n per unit time  - 7 -  per unit density difference.  r  -  Thus  = A  V A >  M  The dimensions of C are e a s i l y seen to be<-il_  J  We now derive an expression for C i n terms of p^, P2,  V^, and V^.  From  the elementary gas laws,  (3)  so that, at constant  dl  to  Temperature,  . __  dv->.  As -defined,  i s the number of molecules t r a v e l i n g from II to I per second; hence i t i s equal to the rate at which molecules leave II minus the rate at which molecules enter I I ; that i s  kT  - 8 By equation (3), the difference in densities, n2 - n^, is given by  Hence,  We now specialize this model to the case of pumping by defining enclosure I to be an ideal pump, having the properties that  Vv = O  and  —& = O  This defines C as the pumping speed S, and we have  From this we obtain the equation for the pressure rate of change due to pumping in enclosure II, dropping the subscripts,  4fc  _  d t  s  V  f,. r  The idea of pumping speed can be very naturally extended to include a l l mechanisms for the removal of gas out of or influx of gas into a system; i t is then defined by the equation  (5)  Q  where Q is a measure of the flow of gas into or out of the system; the; dimensions of Q are pV per unit time.  In this thesis we shall use the rather  convenient pV unit of mm. of Hg. - l i t e r s , abbreviated mm. - l i t e r s .  Thus an  i n f l u x of ga's Q of 10 mm. - l i t e r s per second means that for a one l i t e r system the pressure w i l l r i s e 10 mm. i n one second.  This quantity Q i s par-  t i c u l a r l y useful i f the rate of gas i n f l u x into a system i s constant. We note from this derivation that the rate of evacuation for any type of pump involving the k i n e t i c flow of gas through an o r i f i c e i s d i r e c t l y proport i o n a l to the quantity  /  • This means, for example, that Hydrogen i s  pumped five times as fast as Nitrogen.  We also see that the minimum cross-  sectional area of the system opening on the pumping side sets an upper l i m i t to the pumping speed of the system; for Oxygen at room temperature this value i s approximately liters/sec. ;  1 1  A  n  i  r  i  l  i n cm  2  Equation (4) i s of course only true for an ideal pump.  We have neglected  the drag introduced by the walls of the tubing to the pump, and also the backstreaming which i s always present to some extent i n every pump.  The f i r s t  effect i s not so important for our purposes; this merely changes the e f f e c t i v e pumping speed at any p a r t i c u l a r point i n the system.  The second e f f e c t w i l l  be discussed i n the next section. b.  Backstreaming from External Pumps. No p r a c t i c a l pumps s a t i s f y the defining requirement of the ideal pump,  v i z . \>> zsr O  , and - T ^  1  = O . In practice, a steady flow of gas i s present  going from the pump to the system.  This flow.is generally constant i n time  and independent of the pressure i n the system.  We must consequently rewrite  equation (4) as follows:  ^  -  b  +  Ok  (6)  - 10 Where  Q. ^  i s the quantity of gas flowing back i n units of pV/t.  equivalent way  An  of expressing this i s by w r i t i n g  a-  Here p„ must be defined as  s  a n  d  c a n  be seen to be the ultimate pressure  attainable with any p a r t i c u l a r pumping speed S and backstreaming i n f l u x  Q^.  Once again we emphasize that this i s only applicable to a system i n which one may  ignore a l l other effects (e.g. mechanisms c - f , page 1).  Often this i s  not the case i n p r a c t i c e , and the ultimate pressure l i e s considerably higher than p . u  c.  Adsorption. One Of the most important  phenomena occuring i n ultrahigh vacuum systems  i s adsorption of gases to the walls of the system.  Generally the d i s t i n c t i o n  i s made between physical adsorption, where gas molecules are held to the wall material by r e l a t i v e l y weak Van der Waals type forces, and chemical adsorption, where molecules combine chemically with the wall material. process i s often accompanied by d i s s o c i a t i o n of gas molecules,  The  latter  and the bonds  between the gas and wall material are usually much stronger than those of physical adsorption.  In this thesis we s h a l l treat physical and  adsorption i n the same general way,  chemical  although s t r i c t l y speaking i t i s not  correct to do so, since as mentioned, chemical adsorption i s often a more than one step process.  Since this would involve us i n too many d e t a i l s and special  cases at the expense of c l a r i t y , and since the assumption i s not c r i t i c a l ,  we  s h a l l assume that physical and chemical adsorption occur i n the same manner, the only difference between them being the energy of adsorption. tion appears j u s t i f i e d for c a l c u l a t i n g orders of magnitude.  The assump-  - 11 The pumping action due to adsorption depends on three things i n general:  -  Pressure.  (This determines the rate at which molecules  strike the wall.) Surface area of the wall. Sticking probability.  (The p r o b a b i l i t y that on s t r i k i n g  the wall a molecule w i l l be adsorbed.)  We can thus write for the number of molecules adsorbed per second,  =IN-  = c  A  w  d  Where c i s the s t i c k i n g p r o b a b i l i t y , A i s the available surface area, and  v(p)  =  Y\(p) J.2 kT i r m  (See equation  i s the number of molecules s t r i k i n g a unit area per unit time. equation  (1)  )  Hence, using  ( 3 ) , we obtain,  =  _ k j dN V  d T t  It i s convenient  to express  V  dt  and therefore  _ kT  A N  A  V ^  i n terms of the pressure; using the gas law  we obtain  vftf  =  IW-  . The quantity cA S .  - 12 -  /  (4))which we  i s a pumping speed, (c.f. equation  call  Although i t s form i s quite similar to the pumping speed derived for  external pumps, i t d i f f e r s from the l a t t e r i n two important  respects.  First  of a l l , the area A here refers to the e f f e c t i v e surface area of the wall material, whereas for the external pump A stands for the smallest crossect i o n a l area of o r i f i c e to the pump. magnitude larger than the l a t t e r . p r o b a b i l i t y factor c. l a r system.  The former i s usually several orders of  The>other difference i s the s t i c k i n g  This quantity i s d i f f i c u l t to determine for a particu-  I t depends f i r s t of a l l on the gas-wall material combination.  Furthermore, as can be expected for an adsorption process, i t depends on the surface coverage of the wall material. et a l . (9), Schafer and Gerstacker,  Measurements have been made by Foner  (10), and Becker (11).  s p e c i f i c a l l y with chemical adsorption.  The l a t t e r deals  The results can be generalized by  saying that for a clean surface c i s of the order of 1 for most combinations, varies d i r e c t l y as the percentage of the surface not yet covered during the formation of the f i r s t monolayer, and drops rather r a p i d l y for second and higher order monolayers.  For a clean surface i t can be seen that S  sents a very formidable pumping speed.  a  repre-  However, systems being pumped down  from atmospheric pressure do not have clean surfaces i n the above sense, and the pumping action i s therefore of l i t t l e or no significance.  In most unbaked  systems, on the other hand, as we s h a l l see i n the next section, the reverse process of desorption i s then more prevalent. pumping can play a very important l a t e r i n connection with the other  role.  In a baked system adsorption  This w i l l be discussed i n more d e t a i l  processes.  To give an idea of the numbers involved i n adsorption, the following table has been prepared.  We assume that i n an equilibrium situation at any  pressure a monolayer of gas i s adsorbed; this corresponds to a surface density  - 13 of molecules of the order of 5 x l O l  4  molecules per cm.^.  The last column  then gives the r a t i o of molecules i n the adsorbed phase (Na) to those i n the gas phase (Ng), for a spherical container.  The large r a t i o s at low pressures  should serve to convince any sceptic of the importance of the surface effects i n ultrahigh vacuum technology.  If one were to liberate one monolayer of gas  TABLE I p mm. 1 IO"  N mol./cm^  Ng mol./cc.  Hg  3 3 x 10l 6  lo-n  6  5 x 10l  *a/  N g  7. 5 x 10"  4  3 3 x 10l°  5 x  10  i 4  7. 5.x  3 3 x 10  5 x  10  1 4  7. 5 x 10  5  10  3  3  8  from the surface of a one l i t e r sphere into a perfect vacuum, the pressure would r i s e to 7.5 x 10"3 torr (1 torr = 1 mm.  of Hg.).  It i s therefore of  great importance to study the process of desorption as well as adsorption. This w i l l be done next. d.  Desorption. As indicated above, the process of adsorption i s always accompanied by  spontaneous  desorption of molecules from the surface of the wall material.  The average time that an adsorbed molecule remains on a surface i s given approximately by  where E^ i s the energy of a c t i v a t i o n , corresponding to the gas-wall material combination, T i s the absolute temperature, and t  Q  i s the period of thermal  o s c i l l a t i o n of the adsorbed molecule- normal to the surface. about 10"I  3  seconds.  Normally t  Q  I t follows from the above equation that the rate at  is  - 14 which molecules leave the surface on the average i s given by  d_N  NUC-t) -(-id)(10)  0  RT  -e.  -t  where N ( t ) i s the number of molecules per square centimeter adsorbed to the a  surface at any time t.  In terms of changes i n pressure, we obtain  4£ ^>Tf dt  A  ^  r  "Ir  k T ^ M A V  (ID  It should be noted at this point that the rate of r i s e i n pressure due to desorption depends exponentially on the quantity E<j/T.  To give an idea of  the orders of magnitude involved Table II has been prepared, which gives the rate of r i s e i n pressure for a one l i t e r spherical system (due to desorption only) as a function of a c t i v a t i o n energy, at room temperature, when the surface coverage i s equal to one monolayer of gas, which i s approximately lol^  5 x  molecules/cm . 2  TABLE II  Kcal./mole.  dp dt mm./second.  1  1.42  x  10  5  1.83  x  10  10  4.20  15  1.05  x  10"  20  2.6  x  10~  7  4  3  7  The importance of the temperature and a c t i v a t i o n energy i n the desorption process i s c l e a r l y brought out by this table. t i c e are:  Typical values for Ed i n prac-  Physical adsorption (inert gases - glass e.g.)  1-10  kcal. per  - 15 mole; chemical adsorption (Oxygen, Hydrogen - metals e.g.)  10 - 50' kcal per  mole.  e.  Diffusion. The d i f f u s i o n process i s also of great importance i n ultrahigh vacuum  work.  The mechanism i s as follows.  Gases entrapped  i n the i n t e r i o r of the  glass or metal during manufacture or absorbed on long storage diffuse out of the material and into the vacuum system, thereby causing a change i n the pressure.  A good general reference to the d i f f u s i o n process i s Barrer (14).  Much early work on this subject was performed by Sherwood (12) and more recently by Todd (13).  The l a t t e r did extensive measurements on the d i f f u -  that the p r i n c i p a l constituent given o f f by the  sion out of glass and found  glass was water vapor at a rate which varies as the inverse square root of the time:  a  Here  (12)  i s the amount of water vapor given o f f by the d i f f u s i o n process i n  units of pV.  The point t = 0 corresponds  to the atmosphere.  to the glass or metal being exposed  The quantity k(j i s related to the d i f f u s i o n c o e f f i c i e n t  by the r e l a t i o n  k where C  Q  =  d  C D  (13)  0  i s the i n i t i a l concentration of the gas i n the s o l i d , and D i s the  d i f f u s i o n c o e f f i c i e n t of the p a r t i c u l a r gas-solid combination. function of temperature:  D. =  D is a  Q  Do  e  RT  (13a)  - 16 where D  i s a c o n s t a n t and Q i s the heat o f d i f f u s i o n i n c a l . / m o l e .  Q  We can combine e q u a t i o n s (13) and (13a) and w r i t e i n g e n e r a l  _ E  where K  •  E a r e c o n s t a n t s a s s o c i a t e d w i t h the g a s - s o l i d combination.  Q>  Todd  (13) g i v e s f o r d i f f u s i o n o f water vapor out o f b o r o s i l i c a t e g l a s s the v a l u e s  -5 O  mm.W  /Gf*\JS*c  E - 9020  cal/  m o l e  C o r r e s p o n d i n g t o the d i f f u s i o n p r o c e s s , the r a t e o f r i s e i n p r e s s u r e i s g i v e n by the r e l a t i o n  A g a i n an o r d e r o f magnitude c a l c u l a t i o n w i l l r a t u r e i n the d i f f u s i o n p r o c e s s .  show the importance  o f the tempe-  Suppose an unbaked system i s pumped down  ."for 4 hours and c l o s e d o f f from t h e pumps.  The p r e s s u r e r i s e a t room tempe-  r a t u r e d u r i n g the next 4 hours due t o d i f f u s i o n u s i n g the above e q u a t i o n s would c o r r e s p o n d t o 2 x 10"^ t o r r .  We assume a one l i t e r  spherical  system.  Now suppose t h a t w h i l e pumping t h e system down we had baked the e n t i r e at  system  750° K. f o r 4 h o u r s , and upon c o o l i n g had c l o s e d the system o f f from the  pumps.'  D u r i n g the next 4 h o u r s , the p r e s s u r e would r i s e by o n l y 2 x 10-10  t o r r due t o d i f f u s i o n . o f the dP dt  versus  The f i g u r e s speak f o r themselves.  Because the s l o p e  curve i s so much s t e e p e r a t 750° K. than a t 300° K.,  4 hours o f out g a s s i n g a t b a k i n g temperature has the same e f f e c t as a p p r o x i 8 mately  10  hours a t room temperature.  T h i s corresponds t o about  1000 y e a r s !  Another a s p e c t o f d i f f u s i o n worth c o n s i d e r i n g i s the f a c t t h a t i t goes on f o r  -1/2 a very long time; Todd (13) reports that the d i f f u s i o n follows the t ' law for the order of a year even at 800 f.  K.  Permeation. The permeation of gases through solids i s of fundamental importance i n  that i t usually sets the l i m i t oh the vacuum obtainable f o r any p a r t i c u l a r system.  The phenomenon has been extensively studied by Norton (15).  The  process involves several steps and i$ a combination of the l a s t three mechanisms discussed (adsorption, desorption, and d i f f u s i o n ) .  Atmospheric gases  are adsorbed to the exterior surface of the walls where they dissolve into the wall material.  They are then diffused through the wall material, and  subsequently desorbed into the vacuum.  In some cases the gas dissociates on  adsorption (e.g. Hydrogen permeation through s t e e l ) , and the permeation then takes place i n the atomic state.  In such cases the permeation rate varies as  the square root of the pressure difference. glass, no d i s s o c i a t i o n generally takes place.  In the case of permeation through The permeation rate i s then  given by the emperical r e l a t i o n ,  kp i s a constant depending both on the material of the walls and the temperature.  I t varies with temperature according to the r e l a t i o n  k, = C e where Q i s the Heat of permeation.  *  T  <16)  Equation (15) i s analogous to the well  known heat transfer equation; p^ i s the p a r t i a l pressure outside the system (atmospheric) while P2 i s the vacuum pressure, A the surface area, and d the  - 18 thickness of. the wall material. i s constant. mation.  In normal situations, p£  <.<.  Pp  while  Hence the rate of i n f l u x i s constant to a very good approxi1  We have a corresponding rate of change of pressure given by  5v  V 5V  -  It turns out that of the atmospheric ly through glass.  K  P  v ^  <17)  gases, Helium permeates most rapid-  This i s not only borne out by the measurements of Norton  (15) but Alpert and Buritz showed i n a very interesting experiment (16) that the ultimate lower l i m i t on the pressure i n a ultrahigh vacuum system i s set by the permeation rate of Helium through glass, which they measured to be approximately  5 x 10-13 mm.  liters/sec.  This i s s t i l l a very small rate, but  important f o r very high vacuum work.  g.  Ion pumping. Ion pumping i n a vacuum system i s achieved i n the following manner.  Electrons from a source are caused to accelerate M.n an e l e c t r i c f i e l d u n t i l s5  they have enough energy to ionize atoms and, molecules.  The positive ions  thus formed are then c o l l e c t e d on a negatively charged electrode.  Special  pumps have been designed u t i l i z i n g ion pumping, but they were not used i n this project.  However, even an i o n i z a t i o n gauge acts l i k e a pump because i t s  operation depends on the c o l l e c t i o n of ions on a negatively charged electrode. The construction and operation of the i o n i z a t i o n gauge w i l l be discussed i n d e t a i l l a t e r , and therefore we s h a l l not go into the d e t a i l s of the pumping mechanism now.  The decrease i n pressure due to the ion pumping can be  described by an equation very similar to equation (4)  (18)  - 19 where  i s the pumping speed due to ion pumping.  An expression for  terms of the parameters of the gauge w i l l be derived l a t e r .  in  It should be  noted that there i s l i t t l e backstreaming i n an ion pumping arrangement such as the i o n i z a t i o n gauge.  Hence the gauge lowers the pressure u n t i l the out-  gassing rate of the system i s equal to the pumping rate of the gauge.  This  i s of p a r t i c u l a r importance i n well baked systems f o r which the outgassing rate i s small.  2.  Pressure Equation. Having b r i e f l y discussed each of the important mechanisms i n the  evacuation process, we s h a l l now make a quantitative estimation of a vacuum system's behavior, by combining these mechanisms into one set of equations. We obtain the following:  RT  A summary of the symbols used i s given below. V  i s the volume of the system.  S  i s the pumping speed of the external pumps.  -  S  a  20 -  is the pumping speed due to adsorption. is the pumping speed of the ion pumps.  A  is the area exposed to the vacuum.  t  is the period of oscillation of adsorbed molecules in a direction  Q  normal to the surface. N (t) is the number of adsorbed molecules per cm. of surface area, 2  a  k is Boltzmann's constant. T is the absolute temperature, m is the mass of one molecule of gas. is the energy of adsorption, kd is the constant associated with diffusion (equation 13, 13a.) kp is the constant associated with permeation (equation 16.) c is the sticking probability (equation 7.) P  a t  is the partial pressure outside of the system of the permeating gas.  d is the thickness of the wall material. Qb is the backstreaming rate. Equations (19) cannot be solved simply as they stand; i f N (t) is eliminated, a  we obtain a second order equation in p which is nonlinear.  The nonlinearity  is the result of the fact that the pumping action of the walls is dependent on both the pressure and the number of atoms already adsorbed. of this process varies with the condition of the vacuum.  The importance  It cannot be neg-  lected when in some way the wall surfaces are made free of adsorbed gas for a short period of time and then left to adsorb molecules from the gas phase of the system.  This occurs when the system is heated to some temperature above  room temperature and then left to cool. out.  Later considerations w i l l bear this  However, when the system is pumped down from atmospheric pressure and  has not been baked, the walls are not likely to do any pumping because many  -  21  -  monolayers of gas are already adsorbed and the s t i c k i n g probability c i s therefore small.  Hence i n such a case i t i s j u s t i f i a b l e to neglect the terms  associated with adsorption pumping.  This w i l l be done i n our c a l c u l a t i o n ,  which applies to pumping on a system s t a r t i n g from atmospheric pressure.  We  s h a l l also neglect the f % dependence i n the d i f f u s i o n term, and assume the d i f f u s i o n to be constant i n time.  The reason for this i s that a solution i n  closed form can not be obtained i f the t~h dependence i s l e f t i n . The importance of this term i s not i n the time dependence but rather the temperature dependence, as i s borne out by the sample c a l c u l a t i o n on page (\6).  Hence we write the following equations instead of equation (19)  j&  _ hLW  . -T&  <»)  Solving (21) for N ( t ) , we obtain a  a)  =  (22)  N.(O)  jut*  We define the following quantities:  OC  -  (3 .  i  (s  A  k T  +  s ) t  :^g>  22 -  - i d  We then obtain, substituting (22) i n (20), using the above d e f i n i t i o n ,  We now solve this equation with the i n i t i a l condition  Mo}  =  |po  oft  We rearrange equation (23) and multiply by  Then So that  *L  .  This gives  - 23 Hence  Hence the solution i s  As can be seen from this equation, the time independent part determines the ultimate pressure.  ^  ^  _X  _  i s a measure of the i n f l u x of gas  due to d i f f u s i o n and permeation, while speed of the system.  ,  (X  i s a measure of the pumping  can take on many d i f f e r e n t values depending on  the material of the vacuum chamber.  In the system used f o r this project,  the wall material was mostly glass, although metal valves were used.  For the  purposes of c a l c u l a t i o n we s h a l l assume that we have a one l i t e r glass system of surface area 1000 cm^. changes the s i t u a t i o n .  Later we s h a l l see how the presence of metal parts  We s h a l l also assume that we are pumping on this  system with a pumping speed of one l i t e r per second at a temperature of 300° K (approximately room temperature).  We f i r s t determine  - 24 From Todd's measurements (13) on Pyrex glass, we have i n i t i a l l y ,  •  =  I - 4-. x  io "  l\leYi/c^\itc.  mm,  U  (t = 1 sec)  ( 3 0 0  ° K )  According to experiments of Alpert, Buritz, and Rogers (17), the major i n f l u x of gas due to permeation of glass i s i n the form of atmospheric Helium, and from measurements of Norton (15) we calculate  —  m m Ute.r3 / c m ^ c  9 x \o  for unit pressure difference and thickness 1 mm.  Again,  parameter of the system used; here we s h a l l assume that compared to the d i f f u s i o n and permeation influxes.  i s very much a i s very small  In practice this i s not  true, c e r t a i n l y not at room temperature, unless very e f f i c i e n t traps are used.  However, assuming  to be small does show up other limitations of the  vacuum system that are not so obvious. The p a r t i a l pressure of Helium i n the atmospher  Y  For  = ( l . 4 . 1 o -  ,  l +  i s 5 x 10-3 mm.  4 , 5  a one l i t e r system with surface area 1000 cm  2  <  i o ' '  5  so we get  ) v  m  M  '  1  / ^  then, we obtain  The permeation rate i s much smaller then the d i f f u s i o n rate for an unbaked system and can be neglected at this stage.  Once the system has been  baked, however, the d i f f u s i o n rate i s much smaller (see page  ), and then  - 25 the permeation rate i s much more important. V JL  Since  i s the pressure at t = cO , we s h a l l c a l l i t p  Since 0(  pressure.  p  u  the ultimate  •» 1 for our sample system, we have  •- 1.4 x IO"  8  mm. of Hg.  Several points should be noted at this stage.  F i r s t of a l l , no mention  was made of any nongaseous contaminants which may be on the glass, such as grease or o i l films deposited while the glass was being handled.  These con-  taminants may have a very high vapor pressure compared to the value of p  u  quoted above, and set a corresponding l i m i t on the pressure that can be achieved.  Considerable amounts of gas may also be trapped i n the contaminat-  ing f i l m and the release of this gas, which i s not necessarily governed by one of the described processes, can cause the pressure to stay high for considerable length of time.  Our figures are therefore applicable only to  systems which are not contaminated phere f o r some time.  beyond having been exposed to the atmos-  A second point i s that while p  u  sets a l i m i t on the  pressure that can be achieved, we do not know how long i t takes to reach this pressure  u n t i l we have evaluated the time dependent part of equation (24).  This may i n fact be a very long time, depending on the values of  and X  .  In order to give an idea of the time dependence of the pressure i n the system, a table has been prepared giving the value of the pressure as predicted by equation (24) at times t = 15 minutes, sample system used above. desorption.  t = 24 hours and t = 10 days for the  The parameter which i s varied i s E^, the energy of  The reason for doing this i s that E  s o l i d combinations.  d  varies for d i f f e r e n t gas-  Measured values range from 20 c a l . per mole (the heat of  vaporization of l i q u i d Helium) to several hundred thousand c a l . per mole (e.g.  - 26 the a c t i v a t i o n energy of oxygen on Titanium i s 236 kcal. per mole.) quantitative data i s available for glass-gas systems. report E  d  Not much  Tuzi and Gkamoto (18)  for water on glass i n high vacuum apparatus to be between 13 and 40  kcal. per mole.  Because the values of E  d  for the various gases and vapors i n  the system are not well known, we have calculated the pressure time dependence for various values of E , i n order to see for which values of E d  d  the desorp-  t i o n process i s important from the point of view of reaching high vacuum quickly.  One d i f f i c u l t y i n computing the values of the table i s the choice of a suitable i n i t i a l value of N , a  the number of molecules adsorbed.  Briggs  gives a figure f o r water vapor on glass of the order of 5 x 1 0 ^ while the number of Nitrogen molecules per cm  2  molecules/cm  2  i s given as 5 x IO* mol./cm 4  This i s in-agreement with observations by Todd (13), who the gas desorbed from glass on heating i s water. be 5 x 10*6.  (19)  2  reports that 997. of  Hence we have taken Na(0)  to  The table i s given on page (27).  The results of the calculations show that i f our assumptions are correct, there exists a d e f i n i t e range of desorption energies for which the outgassing process impedes the speedy production of high vacuum.  This range l i e s between  20 and 30 kcal. per mole for the p  I f the p  u  calculated above.  be lower, this range i s correspondingly extended.  u  happens to  Q u a l i t a t i v e l y , these results  mean that for low energies of desorption, the molecules are pumped o f f the walls i n a very short time because they are bound by very weak forces.  For  high energies, on the other hand, the molecules are so t i g h t l y bound that no' appreciable desorption takes place.  It i s for the middle range of energies  that desorption takes place at a rate which keeps the vacuum of low quality for long times.  It i s l i k e l y that water vapor and other active  atmospheric  Cont'd on page 28.  - 27 TABLE (III) P (t --- 15 min.) P (t = 24 hrs.) P (t = 10 days) (mm. of Hg.) )mm. of Hg.) (mm. of Hg.) 1  kcal./mole  Pu  Pu  p  10 kcal,. /mole  Pu  Pu  Pu  20 kcal./mole  Pu  Pu  Pu  Pu  Pu  Pu  Pu  Pu  Pu  21 kcal./mole  1.8 x  22 kcal./mole  4.0 x lO-^  23 kcal./mole  2.0 x IO"  24 kcal./mole  6.0 x IO"  5  1.1 x IO"  6  25 kcal./mole  1.5 x  10"  5  6.0 x 10"  6  26 kcal./mole  2.4 x 10"  6  2.0 x IO"  6  4. 1 x  10"  7  27 kcal./mole  4.5 x  10-  7  4.5 x  10"  7  3. 3 x  10"  7  28 kcal./mole  8.9 x IO"  8  8.9 x IO'  8  29 kcal./mole  2.5 x  IO"  8  2.5 x IO"  8  2. 5 x IO"  8  30 kcal./mole  1.7 x  10-8  10-8  1. 7 x IO'  8  31 kcal./mole  10-4  u  4  1.7 x  Pu  Pu  Pu Pu  8. 9 x.,10-8  Pu  Table showing the time dependence of the pressure as a function of E<j, the energy of desorption, for a glass vacuum system at 300° K., Volume 1 l i t e r , surface area 1000 cm^,  i n i t i a l surface coverage 5 x 10*6 mol./cm ,-with a  pumping speed of 1 l i t e r / s e c .  2  p  i s the ultimate pressure under these con-  u  ditions and i s equal to 1.4 x IO"  8  mm.  of Hg.  /  - 28 gases are bound to the glass with energies i n this middle  The calculations show the importance work.  range.  of the parameter  i n high vacuum  I f we know E<j for a p a r t i c u l a r gas, we can see whether this gas i s  going to be present to any extent i n the residual gas once high vacuum has been reached.  I t may be, for example, that i n a p a r t i c u l a r system a l l  atmospheric gases are desorbed rapidly except one, say Hydrogen. a l gas may  The residu-  then be 90% Hydrogen, while i n the atmosphere Hydrogen i s only  present i n very small amounts.  While this i s not l i k e l y for glass systems,  and probably not to such a large" extent i n most systems, i t must nevertheless be r e a l i z e d that " s e l e c t i v e " pumping can occur i n this manner, and that the composition of the residual gas i s not governed so much by the composition of the gas before pumping commenced, but rather by the mechanisms involved i n f the pumping process.  Because so l i t t l e quantitative data i s available that  can be applied i n general to a vacuum system, i t i s very d i f f i c u l t to predict what the composition of the residual gas w i l l be.  It i s needless to say that  such a prediction would have been of some interest for our system. We s h a l l next discuss what happens when the system as a whole i s heated by means of an oven.  This process, c a l l e d "baking"  i s one of the most  e f f e c t i v e ways of reaching pressures which are less than 10-8 (ultra-high vacuum).  mm.  Q  f Hg.  We can no longer solve equation (24) i n the simple  manner as before, because the temperature However, i n order to see why  i s now also a function of time.  the vacuum improves,  i t i s s u f f i c i e n t to analyze  what happens q u a l i t a t i v e l y during and after baking.  F i r s t of a l l , the term  V — (16).  becomes several orders of magnitude smaller, as was The high temperature  shown on page  causes so much water vapor to diffuse out of the  glass, that the d i f f u s i o n rate at subsequent lower temperatures becomes  - 29 n e g l i g i b l e compared to the permeation rate.  The l a t t e r , which also changes  with temperature, comes back to i t s o r i g i n a l value on cooling rather than a lower value, because i t does not depend on how much gas i s i n the glass mater i a l , but rather on how much can go through the material. o Suppose we bake for s i x hours at 700  K.  Upon cooling k^, the constant  associated with d i f f u s i o n from inside the g l a s s , - i s calculated to be 2 x 10"^" mm.liters/cm^  sec.  for He as before.  kp i s unchanged, and equal to 9 x IO""*  4  mm.liters/cm sec. 2  Hence p , the ultimate pressure i s given by  -14  -13  £u  We can at this stage not neglect Q^; no matter how well we trap the backstreaming o i l ,  i s bound to be larger than 4 x 10*13 mm.  liters/sec.  Alpert (20) shows that exposure to the pumps eventually destroys the u l t r a high vacuum rather than maintain i t .  Hence the system i s usually closed upon  cooling and ion pumping used to obtain and maintain an ultrahigh vacuum.  The  process of ion pumping w i l l be described i n a l a t e r section, but we can say now that the backstreaming rate from an ion pump i s much smaller than that of a d i f f u s i o n pump, so that again we can neglect Q^, compared to the i n f l u x due to permeation.  This was experimentally shown by Alpert and Buritz (16).  Thus the permeation process i s now the most important, and sets the fundamental l i m i t on the vacuum.  The surface gases, which f o r unbaked systems caused slow net pumping speeds, are rapidly desorbed at the baking temperature.  The range of values  - 30 of Ejj, which w i l l cause d i f f i c u l t i e s i n pumping at 700° K. i s now shifted to <C  45 kcal. the  E^  <C  70 kcal.  Hence any gas or vapor with value of E^ i n  troublesome range between 20 and 30 kcal./mole, w i l l be rapidly desorbed  at such a high temperature.  Although on cooling some gas w i l l be readsorbed,  much, of the desorbed gas w i l l be pumped away while the system i s s t i l l warm and hence the surface coverage a f t e r cooling w i l l be several orders of magnitude smaller than before, with a corresponding lower pressure. The presence of metals i n the vacuum system can a l t e r the behavior significantly.  The desorption process i s not much d i f f e r e n t from that asso-  ciated with glass, although, as mentioned before, metals tend to have more nongaseous contaminants on them, which may a l t e r the desorption. sion out of metals i s of more importance, however.  The d i f f u -  Gases dissolve i n the  metal i n large quantities when the metal i s cast, and when exposed to a vacuum these gases d i f f u s e out.  Again, the rates of d i f f u s i o n vary greatly  with the p a r t i c u l a r glass metal combination. An extensive account of diffusion of gases i n s o l i d s i s given by Barrer (14), and-it would not do the subject any j u s t i c e to discuss i t i n this thesis.  We can say i n general, however,  that the l i g h t e r gases (Hydrogen, Helium) d i f f u s e the fastest, and particul a r l y at elevated temperatures this d i f f u s i o n may cause serious i n f l u x of gas. In order to i l l u s t r a t e  tihese points we use equation (13) and (13a) to compute  kd (see equation (14)), for some gas-metal combinations at room temperature, before, baking. Barrer (14).  Data for the d i f f e r e n t gas-metal combinations are taken*"out of We must choose a t y p i c a l value for C'0,  of gas dissolved i n t h e metal. take C  Q  to be .1 cc(NTP)y  Typically C  Q  cc  o  f  Since this varies for d i f f e r e n t samples, we m  e  i s between 10-5 c c /  We then obtain:  the i n i t i a l concentration  t l.  Th*- gives a rough upper l i m i t to k . 8  a  c c  a n  d  d  #  icc/  c c  (see (7), p. 356).  - 31 TABLE IV Gas metal Combination  k(j (upper l i m i t ) (mm. liters/cm^sec)  N - Fe  10~  H2 - Ni  3 x IO"  6  H  6 x IO"  6  H  For  1 x  2  - Fe  2 - P  14  4 x 10"^  d  comparison, the value obtained for d i f f u s i o n of water vapor out of  pyrex glass was 1.4 x 10"H  mm.  liters/cm  2  sec.  The r e s u l t s show that i f  sizable pieces of metal are i n the system, the d i f f u s i o n of Hydrogen out of these metals may be of importance. k  d  I t must be remembered that the values of  quoted 'are an upper l i m i t , and that actual values may be lower, depending  on the concentration of the gas i n the metal.  Baking w i l l lower the value,of  k^ by several orders of magnitude, as shown before.  CHAPTER I I  The apparatus used for the production of ultrahigh vacuum w i l l be describee! next.  A schematic diagram of the general arrangement of the system i s  i  given i n figure (1).  1.  Pumps.  a.  Mechanical backing Pump. A two stage rotary vane pump of pumping speed 1% l i t e r per second was  used to bring the pressure down to about 10 microns bf Hg.  The reason for  having t h i s type of mechanical pump i s to provide a suitable backing pressure for the d i f f u s i o n pump (see below), which w i l l not operate at pressures above 100 microns of Hg. b.  Diffusion Pump. The d i f f u s i o n pump i s one of the most widely used instruments for the  production of ultrahigh vacuum.  Generally the d i s t i n c t i o n i s made between  two types, the mercury d i f f u s i o n pump and the o i l d i f f u s i o n pump. have certain advantages and disadvantages. mercury pumps are:  Both types  The p r i n c i p a l advantages of the  a) the pump f l u i d can be e a s i l y trapped at l i q u i d  Nitrogen temperature, thereby reducing the backstreaming rate, and b) the pump f l u i d i s stable;  i . e . no products are produced by thermal breakdown.  However, O i l d i f f u s i o n pumps are more widely used i n spite of the fact that they do not have the above-mentioned less dangerous. efficient  advantages because the pump f l u i d i s  Also, recent development of low vapor pressure o i l s and  trapping systems have greatly reduced the backstreaming problems.  In this project, be-described  an o i l d i f f u s i o n pump was used.  briefly.  - 32 -  \  -  I t s mode of operation w i l l  - 33 Figure (2) i l l u s t r a t e s the design of the o i l d i f f u s i o n pump used.  The  casing i s water cooled and an e l e c t r i c heater for the evaporation of o i l i s i n s t a l l e d i n the base.  The j e t system consists of three concentric tubes  terminating i n nozzles, together with an annular nozzle system as shown. The o i l vapor r i s e s i n the tubes and issues from the nozzles at supersonic speeds. The gases from the vacuum chamber diffuse into the j e t of o i l vapor and are trapped by the o i l droplets.  The o i l subsequently condenses on the water-  cooled walls of the pump, where the trapped a i r i s liberated and pumped o f f by the backing pump.  The umbrella shaped j e t of o i l vapor issuing from the  nozzles acts as a diaphragm i n that i t keeps gas molecules i n the high pressure area below the j e t from escaping back into the vacuum system. For a more complete account of the physics involved i n the operation of the d i f f u s i o n pump see f o r example Dushman (5).  In order to reduce the backstreaming from this pump a s p e c i a l l y designed b a f f l e was i n s t a l l e d at the vacuum side of the pump which u t i l i z e d water cooling to condense any vapors that escaped from the pumping unit into the vacuum system.  Although such a b a f f l e lowers the pumping speed of the pump,  i t s use i s essential i n order that the system w i l l not be contaminated with o i l from the pump.  The ultimate pressure attainable with the pump i s also  lowered because of the reduced backstreaming.  Figure (3)  shows the design of  the b a f f l e . Even though the backstreaming i s reduced by means of the water-cooled b a f f l e , small quantities of o i l vapor w i l l s t i l l f i n d their way into the vacuum system.  As was shown by an experiment performed by Alpert (20),  prolonged pumping with a d i f f u s i o n pump, even though properly baffled, w i l l ultimately deteriorate the vacuum i n a baked system because of small deposits  - 34 of o i l on the walls of the vacuum chamber.  In order to minimize this effect  a trap was b u i l t into the system near the pumps, which was f i l l e d with p e l l e t s of a l k a l i metal alumino-silicate^more commonly known as Zeolite.  This  substance i s porous and acts as a very e f f e c t i v e pump for o i l vapor.  Biondi  (21) reports on the use of this material as an o i l trap and gives some data on i t s use for ultrahigh vacuum applications. and reactivated at 400° C.  The Zeolite may be degassed  Other kinds of traps, although not used for this  project,can be very e f f i c i e n t i n removing backstreaming o i l .  One simple  design i s the copper f o i l trap (see for example (8), page 656).  Clean copper  has an e f f i c i e n t adsorbing surface for backstreaming o i l vapor even at room temperature. instance  The e f f i c i e n c y of nearly a l l traps i s increased by cooling, f o r  with l i q u i d Nitrogen; while this may be of great advantage i n  obtaining a lower pressure, cold traps have the drawback that they must be kept cold at a l l times.  I f the trap i s allowed to warm up the evolved gas  may s p o i l the clean surface conditions of the chamber obtained with baking. For  this reason no l i q u i d a i r traps were used i n this project.  2.  Valves. Because the vacuum system had to be baked i n i t s entirety, no greased  stop cocks could be used to i s o l a t e the various compartments of the system. Not only does the grease melt at elevated temperatures, but also the vapor pressure of the best vacuum grease i s usually well above the ultrahigh vacuum range at room temperature.  Consequently bakeable metal valves were used  which employ no grease i n the sealing process.  These valves, manufactured  by the Granville P h i l i p s Co., are, designed to withstand temperatures up to 450° C. and can be closed to a leak rate of less than 1 x I O * -  B r i e f l y , their design i s as follows.  4  mm!1./sec.  The seal i s produced by a \ inch  diameter monel nosepiece carrying a very small c a r e f u l l y machined s i l v e r  - 35 gasket which seats i n a s p e c i a l l y shaped groove i n the monel valve  body.  The nose piece, mounted i n the center of a f l e x i b l e n i c k e l diaphragm i s moved i n and out of the sealing groove by a driver mechanism which i s designed to apply the high forces required for a metal to metal seal.  Metals i n contact  with the vacuum are monel, n i c k e l , s i l v e r , Kovar and very low vapor pressure brazing alloys containing gold, n i c k e l , s i l v e r and copper. exterior of the valve i s given i n figure ( 6 ) .  A diagram of the  R e l a t i v e l y large torques are  required to seal the valve; therefore the valve brackets were r i d g i d l y mounted on a s t e e l bar which i n turn was fastened to the oven base.  Because  the valves were r i d g i d l y mounted, allowance had to be made for the fact that with heating or cooling stresses might develop i n the glass tubing which could cause breakage of the glass.  In order to avoid this each valve was  f i t t e d with f l e x i b l e glass bellows i n order to reduce any stress. When baked, the valves had to be open i n order to prevent fusing of the sealing gaskets.  For this purpose the d r i v i n g screw was taken o f f and a  special clamp attached to keep the valve open during bakeout.  Figure ( 7 )  gives a graph of the leak rate versus closing torque for the valves used. 3.  Vacuum gauges. Four types of vacuum pressure gauges were used i n t h i s project. a)  The McLeod gauge (1 x I O  b)  The Ionization gauge (1 x IO" - 1 x I O  c)  A relaxation o s c i l l a t o r type discharge gauge (1-10 mm. of Hg.)  d)  An o i l manometer (.1 - 10 mm. Hg.)  - 2  - 1 x IO" mm. of Hg.) 5  4  - 1 0  mm. of Hg.)  These gauges w i l l be described i n turn. a)  McLeod Gauge.  This gauge was put on the system i n i t i a l l y for the purpose of testing  - 36 the behavior of the system before baking.  The gauge uses Mercury to measure  pressures between .1 and 10"6 mm. of Hg:  For a d i s c r i p t i o n of the design and  operation of the McLeod gauge see (6) page 77. Several points should be r e a l i z e d i n connection with this gauge. I t s chief advantage i s that i t gives an absolute measure of the pressure over a considerable range; f o r this reason i t i s very widely used for the c a l i b r a t i o n of other gauges.  On the other hand, i t reads pressure of noncondensible  vapors only, since any gas that condenses e a s i l y at room temperature i s removed from the gas phase during the reading.  Also, the instrument i s not  very convenient i f many readings are to be taken i n a short period of time, since i t takes a minute or so to l e t the mercury r i s e and f a l l .  For the same  reason leak testing, which i s very conveniently done with e l e c t r i c a l l y operated gauges (such as the P i r a n i or Ionization gauge), i s very d i f f i c u l t with the McLeod gauge.  A l i q u i d a i r trap must always be used together with  the McLeod gauge i n order to prevent Mercury vapor from contaminating the system. b)  Ionization Gauge.  The ionization gauge, hereafter abbreviated ion gauge, i s the most widely used instrument f o r the measurement of pressures of less than 10~ mm. of Hg.  4  In order to understand the design of the i o n i z a t i o n gauge as  used today, i t i s i n s t r u c t i v e to review some e a r l i e r designs, and to see why they had to be modified. triode.  The ionization gauge used before 1950 resembles a  The filament, placed i n the center of a c y l i n d r i c a l l y symmetric  arrangement, serves as a source of electrons which travel to a p o s i t i v e l y charged g r i d (about 200 v o l t s ) .  On their way to the g r i d these electrons  are capable of i o n i z i n g gas atoms and molecules.  These ions i n turn are  collected by the c y l i n d r i c a l outer electrode, which encloses the entire  - 37 electrode structure and i s negatively charged to about 50 v o l t s .  The number  of ions produced per unit electron current i s assumed to be proportional to the gas density, and hence the current to the ion c o l l e c t o r i s used as an indication of the pressure.  Thus for a constant electron accelerating  voltage, (which must be i n excess of the ionization potential of the gas molecules), the number of positive  ions formed should vary l i n e a r l y with  pressure according to the r e l a t i o n .  where i  i s the ion current, i _ i s the electron current to the g r i d , and p i s  the pressure i n mm.  of Hg.  The constant varies with d i f f e r e n t gaseS; a '  t y p i c a l value for k i s 10 (mm.  of Hg.)"^.  A normal operating electron  current i s 10 milliamps; hence the ion current at <a pressure of 10"^ mm.  Hg.  i s about 10"*9 amps.  Prior to 1948 no one had recorded a pressure of less than 10" an ion gauge of the design described above.  8  mm.  with  There i s good reason to believe,  however, that pressures well below this value had been obtained, even though the gauge read a higher pressure(22). l i m i t of the gauge was  A theory to account for this lower  f i r s t proposed by Nottingham (23), and l a t e r substan-  t i a t e d by the experimental work of Bayard and Alpert (24), Lander (25), and Metson (26).  The explanation was'as follows.  I t was  suggested that there  exists a residual current to the c o l l e c t o r of the ion gauge which i s completely independent of pressure.  This current i s caused by soft X-rays which are  created when the electrons s t r i k e the grid.  The X-rays, being intercepted by  the c o l l e c t o r , i n turn cause photoelectrons to be released from the c o l l e c t o r . These photoelectrons travel to the p o s i t i v e l y biased grid, and hence  - 38 constitute a current of the same sign as the ion current.  This X-ray current  at normal operating voltages i s about 1 to 2 x 10" amps; hence a reading of 7  less than 10-8 mm,  Q  f Hg. would be impossible to obtain.  In order to over-  come this d i f f i c u l t y the Bayard - Alpert type gauge was developed.  This  gauge i s now almost exclusively used, and was used for this project.  The design of the Bayard - Alpert type ion gauge grew out of experiments to prove the X-ray hypothesis of Nottingham.  A design was needed i n which  the c o l l e c t o r would riot intercept as many of the X-rays that were emitted from the grid.  Hence the electrode structure was inverted; the c o l l e c t o r was  made a thin wire i n the center, with the grid around i t , and the filament on the very outside.  In this way the s o l i d angle which the ion c o l l e c t o r pres-  ents to the X-rays from the grid i s at least one hundred times as small as that for the e a r l i e r design.  By making the wire of extremely small cross-  section this can be s t i l l improved.  Gauges are now manufactured which are  linear down to approximately 10-H mm. Hg.  The l i n e a r i t y of the ion gauge  was c l e a r l y shown i n experiments carried out by Alpert and Buritz (16).  One of the features of the ion gauge, besides i t s c a p a b i l i t y of measuring pressure,is that i t removes gases from the gas phase of the system while in operation.  This pumping action, b r i e f l y mentioned e a r l i e r i n this thesis,  (see page ( I S ) ) , has both advantages and disadvantages i n ultrahigh vacuum work.  If one wishes to obtain the lowest possible pressures i n the system,  the pumping action of the gauge i s of great advantage.  At the point where  backstreaming from the d i f f u s i o n pump becomes important, the system can be :  closed and further evacuated u t i l i z i n g the pumping action of the ion gauge. This pumping action i s a disadvantage when a steady state i s required; unless measurements are taken i n very small time i n t e r v a l s , the operation of the  - 39 ion gauge w i l l a l t e r the condition of the vacuum.  This i s further compli-  cated by the fact that immediately after the gauge i s turned on a small amount of gas i s desorbed from the gauge filament which also momentarily may change the  condition of the vacuum.  Because these e f f e c t s are of importance i n many  applications, the pumping action w i l l be discussed b r i e f l y  i n what follows:  The removal of gases from the system with the ion gauge predominantly takes place i n any of the following three ways:  a)  The surfaces of the electrodes may remove gases by physical or  chemical processes. For example, the hot filament may c a l l y active gases r e s u l t i n g i n gas removal.  interact with chemi-  Such a process accounts for the  rapid removal of oxygen, which forms oxides of Tungsten at the hot filament. b)  The negatively charged c o l l e c t o r w i l l trap the ions which account  for the pressure measuring ion current.  This mechanism i s the predominant  one for inert gases and also important for most molecular gases. c) the  The electrons traveling to the grid w i l l dissociate molecules and  r e s u l t i n g "active" atoms may be removed from the gas phase by attachment  or combination at the walls of the gauge. For  a l l three mechanisms, the rate of pressure reduction i s expected to  be proportional to the pressure; hence we write for any p a r t i c u l a r mechanism  (26)  &JL *  s  t n e  pumping speed for the i  c  n  mechanism.  mechanisms going on at the same time:  We have then for the three  - 40 Let S = ( s  a  + Sjj + S | c  It is convenient to define X" , the characteristic  pumping time  Thus  We must evaluate the different "t'S in order to get an estimate of the importance of the pumping action.  (a) is essentially a process as described  under adsorption earlier; an upper limit to the pumping speed is given by S = 11 A liters/sec. a  (see Equation (2))  where A is surface area of electrode in cm .  For a typical gauge, this sur-  2  face-area is approximately 0:. 2 cm .  Hence the pumping speed for this mecha-  nism is limited to 2 liters per second.  In practice the pumping speed is  lower because not a l l molecules incident on the electrode react and are captured.  The probability for reaction is usually much less than unity.  Tungsten at 2300° C.  For  the probability for reaction with an oxygen molecule is  about one tenth so that the pumping speed for this mechanism is of the order of 0.2 liters per second. For the ion entrapment at the collector (b), a first approximation of the ion pumping speed can be made by assuming that a l l the ions incident on the collector are permanently removed from the volume. vi  .  '  can be found in the following manner. molecular density  The ion pumping speed ''•!'  We write equation (26) in terms of the  - 41 -  dn  Since VS^. "  dN i s the t o t a l removal rate of molecules, — , we have dt  dN But ~ i s related to the ion current i dt c  srt  _  - -<  a A  where a i s the number of ions/unit i . c  Hence using equation (25), rewriting i t  in terms of n, we obtain  b "~  ~  Y\  where b i s the number of molecules/unit pressure per l i t e r at room temperature.  I f p Is i n mm. Hg. , k i n (mm. Hg .")"*", and i  i n amperes, then we, have  Thus at a normal operating current of 10 milliamps, with k equal to 10, we get a maximum pumping speed f o r ion pumping:  5b  = , 02.  \\ters  /sexi,  - 42 Considerably higher pumping speeds due to ion pumping have been observed. According to Alpert (20) this i s due to the ionization of gas outside the accelarating g r i d , with subsequent charged walls of the gauge.  c o l l e c t i o n of the ions at the negatively  This may increase the above pumping speed by as  much as a factor of ten. Not much quantitative data i s known about ( c ) , the removal of dissociated atoms.  Generally the crossections for d i s s o c i a t i o n of molecules due to  electron impact are smaller than those f o r ionization, so that we may conclude that the pumping speed i s correspondingly lower.  The f i r s t two processes  seem to be the more important ones. The c h a r a c t e r i s t i c pumping times associated with these two are respect i v e l y f o r chemical r e m o v a l ^ L ^ l second, and f o r ion pumping (X-^ 10 seconds. It i s therefore clear that during measurements of this order of time the condition of the vacuum w i l l change appreciably.  The c h a r a c t e r i s t i c time  associated with chemical removal can be lengthened by operating at a lower filament temperature, while the time  "C^ corresponding to ion pumping i s  lengthened by operating at a smaller grid current.  Hence i f accurate instan-  taneous values of the pressure are required i t i s recommended  that one operate  at a grid current of say 0.1 mi 11 lamp with as low as possible filament emission.  By taking the pumping speeds into account, one can make f a i r l y  accurate instantaneous measurements, and i f necessary, using the above equations, corrections to the measurements can be made to account for gauge pumping.  In this connection i t should be mentioned that a continuously recording  instrument for measuring taneous pressure.  the ion current s i m p l i f i e s the estimation of instan-  By observing the shape of the pressure versus time curve as  traced out by the instrument, one can estimate the t o t a l pumping speed of the  -  43 r  ion gauge quite accurately i f the sysfem i s closed.  Such an instrument was  used for this project. Of course one cannot continue lowering the pressure i n d e f i n i t e l y by means of ion gauge pumping i n a closed system; after a c e r t a i n time the rate of reduction of pressure i s balanced by the rate of r i s e of pressure due to outgassing of the system or re-emission of the i o n i c a l l y pumped gases.  For a  p a r t i c u l a r l y clean system the ultimate pressure that can be attained i s l i m i t ed by the permeation of Helium through the walls of the system, and this process coupled with the pumping action of the gauge f o r a t y p i c a l small system leads to an ultimate pressure of about 10*11 mm. t r a n s i t i o n from I O  - 8  mm.  to 1 0 H mm. _  of Hg. (16). The  i s generally accomplished  only after  baking, with the system closed from the pumps and the ion gauge or other ion pump operating continuously. c  -  Discharge Gauge. The Neon which was to be used i n the preparation of the absorption tube  had to be.inserted to a known pressure i n the 1 to 10 mm.  range.  I t was very  important i n the measurement of this Neon pressure that the gas not be contaminated by the pressure measuring device.  For this reason i t was not  desirable to use a mercury type gauge because this would necessarily introduce small amounts of mercury vapor into the Neon, thereby contaminating i t . Not many pressure gauges exist which are capable of measuring pressure accur a t e l y i n this range without introduction of impurities into the system.  One  type of gauge often used i s the so c a l l e d capacitance manometer (20), but bakeable commercially made gauges of this type are very expensive. gauges which u t i l i z e  Glass  the f l e x i b i l i t y of glass for the measurement of pressure  differences are also used.  This type of gauge was not thought to be very  suitable f o r this project since i t had to withstand atmospheric  pressure and  - 44 at the same time be capable of measuring small pressure differences i n the millimeter range.  These two requirements  together make the design of such a  gauge d i f f i c u l t . Hirsch (27) reports on an alternating discharge gauge of r e l a t i v e l y design which seemed to be very suitable for our purposes.  simple  The gauge works on  the p r i n c i p l e that the difference between the f i r i n g voltage and extinction voltage of a D.C. discharge i s a function of the pressure.  I t s design and  operation w i l l be described b r i e f l y . Figure (8)  shows a schematic diagram of the gauge together with the  c i r c u i t necessary f o r operation.  A potential difference of about 1000 volts  is applied which charges the capacitor C through the r e s i s t o r R.  The capaci-  tor w i l l charge u n t i l i t has reached a voltage Vf which i s s u f f i c i e n t to s t a r t a discharge i n the small, discharge tube.  This causes the capacitor to  discharge u n t i l i t has reached a potential difference V s u f f i c i e n t to maintain the discharge.  e  which i s no longer  The capacitor i s then recharged  R from the power supply and the cycle repeats i t s e l f .  through  The signal from this  relaxation type o s c i l l a t o r i s picked up from the small r e s i s t o r r arid f e d to a counter or oscilloscope so that i t s frequency may be observed.  Curves of s t r i k i n g and extinction voltage versus pressure f o r a t y p i c a l discharge tube f i l l e d with a i r are given i n figure (9).  I t can be seen  q u a l i t a t i v e l y that the frequency of the signal i s i n some way proportional to the difference of the s t r i k i n g and extinction voltage.  During the charging  part of the cycle, the voltage on the capacitor and hence across the discharge tube i s given by the r e l a t i o n  - 45 -  so that we may write  and  During the discharge, we assume that the resistance of the discharge tube i s constant and equal to R^,  and that the capacitance of the tube i s n e g l i g i b l e  compared to C; we then have the voltage across the capacitor governed by the relation  so that hence the t o t a l time taken by one cycle i s given by  Rewriting this i n terms of A frequency of o s c i l l a t i o n  , which we define as Vf - V , we get for the e  - 46 -  At low pressures, when  becomes very small, the frequency no longer  follows the simple logarithmic dependence as predicted by equation (27). This i s due to the fact that the internal resistance of the discharge tube on f i r i n g i s no longer constant but r i s e s considerably, while the capacitance of the discharge tube also increases. ultimate lower l i m i t .  Hence the gauge should not be used to i t s  The departure from equation (27) manifests i t s e l f i n a  sharp drop i n frequency where a logarithmic r i s e would be  expected.  The range of pressures that can be measured with a discharge gauge of the kind described above i s determined  by the geometry of the tube, the  nature of the gas, the voltage applied, and the values of the components i n the c i r c u i t .  The lower l i m i t of the gauge i s reached when for a p a r t i c u l a r  gauge geometry the f i r i n g voltage and extinction voltage of the gas under consideration are equal, while the upper l i m i t i s reached when the f i r i n g voltage exceeds the applied voltage.  By decreasing the width of the d i s -  charge gap, the gauge w i l l be able to measure higher pressures using a given applied voltage because the f i r i n g voltage i s lowered.  Similarly the range  i s also extended by increasing the applied voltage. C a l i b r a t i o n curves for the discharge gauge used i n this project are given for a i r and Neon i n figures (ID,; 11).  It should be mentioned that while the  c a l i b r a t i o n for a i r was quite reproducible, the Neon c a l i b r a t i o n could not be reproduced unless the s t r i c t e s t purity conditions were f u l f i l l e d .  This can  be attributed to the fact that the f i r i n g and extinction voltages of the noble gases change considerably with very small amounts of impurity present. design of the discharge tube used i s i l l u s t r a t e d i n figure (8). i s d i f f e r e n t from the one used by Hirsch (27), who  The  This design  used c y l i n d r i c a l l y sym-  metric electrodes. I t seemed to work equally well and i s a l o t simpler.  The  inter-electrode capacity of this design i s also smaller than the c y l i n d r i c a l  -47 design of Hirsch.  -  This fact should increase the useful range of the gauge.  Because electromagnetic noise from the lines influenced the operation of  i  i  i the gauge s i g n i f i c a n t l y , a small aluminum box was used to shield i t .  This  'i  was especially d) i  important when Neon pressure was  to be measured,  O i l Manometer.  i  .  A isimple U tube type o i l manometer was used for the c a l i b r a t i o n of the  i  discharge gauge.  This manometer u t i l i z e d low vapor pressure o i l (n-butyll  phthalate) to measure pressures between .1 and 10 mm. used to prevent the o i l from contaminating the system.  Hg.  A z e o l i t e trap was  The U tube was  t i a l l y evacuated at both ends and then sealed o f f at one end.  ini-  Figure (<$.)  shows the o i l manometer.  I 4)  ' • •  Gas Handling System. Figure (1) shows a schematic diagram of the gas handling system.  The  glass part of the apparatus, which formed the central part of the vacuum system, was divided into four main compartments by the metal valves and contained vacuum gauges, the gas supply, and the large discharge tube which was to be prepared for the spectroscopy experiment.  (In order to avoid confusion,  the l a t t e r w i l l be c a l l e d the discharge tube from here on*, the small-discharge tube used to measure pressure w i l l be c a l l e d the discharge gauge.) design of this part of the system was  The  such as to leave the system as v e r s a t i l e  as possible so that l a t e r i t could be adapted to prepare other gas tubes, possibly with mixtures of gases i n them.  The essential features of the system  w i l l be discussed i n what follows. The Neon bottle was. attached to the system at compartment IV.  The  manufacturer had supplied the bottle with one breakseal which could be opened after evacuation by dropping a steel b a l l on the seal, thereby breaking i t .  - 48 This i s the usual method for the introduction of gas into a vacuum system; the s t e e l b a l l i s moved from a small compartment above the seal by means of a magnet and i s then allowed to drop on the seal.  In this way, however, the  Neon bottle could be used only once between bakings because the metal valves could only be baked while they were open.  Therefore several additional break-  seals were i n s t a l l e d i n p a r a l l e l i n the manner shown i n figure (15).  The  bottle could be closed l a t e r by closing a c o n s t r i c t i o n i n the glass above the seal with a hot flame.  In this way the Neon bottle could be used several  times over, each time using a d i f f e r e n t breakseal.  The bottle i t s e l f was not  baked i n order not to introduce further impurities into the Neon. sure inside the Neon bottle would also become too high.  The pres-  The bottle was there-  fore suspended by i t s glass outlet tube underneath the oven base. The discharge tube was attached to volume I I I , so that any mixing could be done i f necessary i n volume II or I.  Before the discharge tube was  attached, the McLeod gauge was attached to volume I I I for testing of the vacuum system at pressures i n excess of 10~& mm.  This gauge was l a t e r removed  before the f i r s t bakeout. 5.  Oven. The oven used to bake the system was made i n the form of a double walled  16" x 32" x 16" hood which f i t t e d over an asbestos plate base on which the vacuum system was mounted.  The walls of the oven were 1/16 th inch aluminum  plate separated by asbestos spacers. the inner and outer walls. oven on i t s walls.  Glass wool was used as insulation between  Four 750 watt elements were mounted;inside the  The hood could be raised and lowered by means of a wheel  and axle system mounted above the apparatus.  The wiring inside the oven  consisted of stranded n i c k e l wire wrapped i n asbestos ribbon for insulation. The temperature inside the oven was controlled by a Fenwall Thermoswitch which  - 49 operated a relay to turn the oven power o f f and on.  A f t e r equilibrium had  been established this switch controlled the temperature to within a few centigrade degrees at 400° G. was  The duty cycle of the oven at this temperature  about 30 seconds on to one minute o f f .  100 - 500° C. oven. The  The temperature was  monitored by a  thermometer inserted through a small hole i n the side of the  asbestos plate base was  reenforced by means of a 1/16  th inch  aluminum sheet with the whole base mounted firmly to a dexion table.  The  oven hood enclosed v i r t u a l l y a l l of the system when lowered; the parts enclosed are indicated by the dotted l i n e on figure (1~). 6.  Electronics. The control unit for the i o n i z a t i o n gauge i s shown i n figure (12.).  The  unit i s designed to d e l i v e r a l l the necessary voltages to the electrodes, and to control these such as to keep the g r i d current constant at any set l e v e l independent of the pressure  i n the tube.  This i s done by means of a f e e d -  back network which r e f l e c t s any changes i n the g r i d current as a change i n the opposite d i r e c t i o n i n the filament emission.  The very minute c o l l e c t o r  current i s amplified and read on a continuously recording ammeter. current amplifier is,shown i n figure (13).  This ion  CHAPTER III EXPERIMENTAL PROCEDURES AND RESULTS In this section the general procedure for a t t a i n i n g ultrahigh vacuum w i l l be described, together with some remarks on general high vacuum techniques. After the vacuum system has been assembled and b u i l t , the f i r s t job i s to test for large leaks.  For this purpose i t i s convenient  to have a mano-  meter i n the micron to millimeter range since usually pinhole leaks i n solder joints or glassware show up i n this region of pressure. ware can be most e a s i l y detected using a Tesla c o i l ;  A leak i n the glass-  the discharge produced  in the p a r t i a l vacuum w i l l show a very bright spot at the location of the leak.  Several leaks i n the glassware of our system were located i n this  Leaks i n metal sections of the system are much harder to detect.  way.  If the  system has various compartments, the leak can usually be traced to one gener a l area.  The parts of the system under suspicion i n that area (usually  joints) can then be tested i n the following manner.  I f elsewhere i n the  system a v i s i b l e glow discharge can be maintained, a change of colour i n the discharge w i l l be observed when some acetone i s sprayed on the metal i n the v i c i n i t y of the leak.  In this way  the leak can be traced to a very small  area on the metal, by careful use of the acetone.  I f an e l e c t r i c a l l y  operated  gauge i s used, such as a p i r a n i gauge (6, page 81) or ion gauge, the organic vapor introduced into the leak w i l l cause a sudden change i n the reading, so that i t i s not necessary  to observe the colour of a discharge.  The ion gauge  can of course only be used i f the leak i s quite small, since i t should not be operated at pressures i n excess of 10  . ram. Hg.  It should be emphasized that a r i s e i n pressure occuring a f t e r the pumps are isolated from the system does not always indicate a leak.  This should be  clear from the considerations put forth i n the early part of this thesis; - 50 -  - 51 desorption, d i f f u s i o n and permeation take place at a l l times. d i f f i c u l t to distinguish processes.  Often i t i s  a r e a l leak from a r i s e i n pressure due to the above  One way of t e l l i n g i s by p l o t t i n g a pressure versus time curve.  I f there i s a leak, the pressure should r i s e l i n e a r l y for an i n d e f i n i t e period of time after the system i s closed.  Outgassing, on the other hand,  should show signs of saturation. Eventually, the desorption process w i l l be balanced by adsorption, and then the only s i g n i f i c a n t r i s e i n pressure w i l l be due to d i f f u s i o n , which follows a th law.  Hence the pressure versus time  curve w i l l l e v e l o f f for the outgassing processes.  One may nevertheless have  to wait a long time before t h i s leveling o f f occurs, p a r t i c u l a r l y for a " d i r t y " system which has not been baked.  With the system free of a l l leaks, the following procedure was used to reach ultrahigh vacuum.  The system was evacuated  to about 6 x IO" mm. Hg. 3  by means of the rotary backing pump, and the d i f f u s i o n pump turned on.  After  about twenty minutes the d i f f u s i o n pump would be warm and the pressure would f a l l rapidly to about 10"^ mm.  During this warmup time the bakeout clamps  (see figure 6) were put on the valves. open at a l l times during bakeout.  The clamps serve to keep the valves  The system was then checked with a Tesla  c o i l to make sure that the pressure was low enough for the ion gauge to be turned on. This precaution had to be taken i n order to avoid oxidation of the ion gauge filament.  After the ion gauge was turned on the pressure would  r i s e i n i t i a l l y and come to an equilibrium value of approximately 5 x 10-5 mm. of Hg.  This high .pressure i s the r e s u l t of the fact that for the unbaked  system the ion gauge electrodes and envelope liberate large quantities of gas. The pressure i n the unbaked system could be improved by c a r e f u l l y outgassing the gauge (this process w i l l be described l a t e r ) , but since the whole system was going to be baked anyway this was generally not done at this stage.  - 52 After a few minutes the ion gauge was turned o f f again, the wires removed from the oven area, and the oven hood lowered over the system.  In order to  avoid strains i n the glassware caused by too rapid heating, i t was necessary to raise the temperature inside the oven slowly.  Therefore the thermoswitch  was always set at i t s lowest value (about 50° C.) before the oven elements were f i r s t switched on.  The temperature inside the oven was then raised  slowly by p e r i o d i c a l l y setting the thermoswitch at a higher switching temperature.  Usually the t r a n s i t i o n from room temperature to 400° C. was made i n  about ten steps over a period of about 1% to 2 hours. subsequent bakeout the pumps were l e f t running.  During this time and  The system was usually l e f t  to bake f o r three or four hours, although once i t was baked for seven hours to make sure a l l surface gases had been desorbed.  The system was then l e f t  to cool with the oven' l i d i n place u n t i l a temperature of about two hundred degrees was reached.  This usually took about two hours.  The oven l i d was  then raised a l i t t l e at the time and after some twenty minutes removed a l l together.  A fan would be placed near the system to speed up the f i n a l cooling.  While the system was s t i l l warm to the touch but s u f f i c i e n t l y cool to be * handled, the ion gauge was connected to the control c i r c u i t and the pressure would drop to about 2 x IO" mm. 8  The grid of the ion gauge would then be  butgassed by passing 10 amperes through i t for some ten minutes.  The pressure  would then drop another factor of f i v e to about 4 x 10"^ mm. Hg.  Repeated  outgassing of the grid i n some cases lowered the pressure a l i t t l e more. During a l l this time the fan was blowing onto the system, thereby keeping the  i o n i z a t i o n gauge envelope cool.  would r i s e by about a factor of two. the  I f the fan was removed, the pressure I f at this point valve # 1 was closed,  pressure would drop quite rapidly and l e v e l o f f at about 1 x 10*9,  generally.  The best vacuum obtained with valve # 1 closed was 6 x 10-10  m m >  - 53 of Hg.  Due to possible errors i n c a l i b r a t i o n this value could be out by a  factor of two either way, but c e r t a i n l y no more.  It i s possible to s p o i l the vacuum temporarily or permanently i n many ways.  Outgassing  of the grid for too long a period or at too high a tempera-  ture w i l l liberate material from the grid i t s e l f which w i l l raise the pressure.  The best way to outgas the grid i s to do i t f o r short periods of time  (say five minutes) with the valve to the pumps open and to close this valve as soon as the grid has cooled down.  In this way the grid surface has no  time to adsorb any backstreaming o i l vapor from the pumps.  Leaving the  system exposed to the pumps f o r long periods of time after i t has cooled also deteriorates the vacuum.  For this reason the system was never l e f t to cool  off overnight, i n spite of the fact that the cooling process i s quite lengthy.  After the lowest possible pressures had been reached, Neon was introduced into the system with the valve to the pumps closed.  The Neon could be  l e t into the system by breaking the breakseal on the bottle (see figure ( 5 ) ). Usually valve 4 was closed before the seal was broken; i n this way the Neon would only expand into the small volume IV.  By opening valve 4 very care-  f u l l y , the Neon could be leaked into the other parts of the system very slowly.  After enough Neon had been leaked i n to maintain a discharge on the  large discharge tube, the electrodes of the tube were outgassed by passing a larger than usual discharge current through them.  This undoubtedly contami-  nated the Neon, but since valve 4 was closed at this stage, this d i r t y Neon could be pumped o f f , and fresh pure Neon leaked i n as before v i a valve 4. A s t r i k i n g c h a r a c t e r i s t i c of the Neon gas was that i t d i d not s t i c k to the walls of the system l i k e a i r does; even though the Neon pressures were as high as 10 mm. Hg., i t vzas always possible to reach ultrahigh vacuum again  - 54 without baking, by pumping the Neon o f f and outgassing the grid. The gauge which was intended f o r the measurement of Neon pressure after the tube had been f i l l e d had to be calibrated i n Neon before i t could be used. Because the c a l i b r a t i o n had tio take place under the same purity conditions as the f i l l i n g of the tube (see apparatus description page (16)), this posed a problem, since any other gauge used f o r . c a l i b r a t i o n would introduce impurities.  It was f i n a l l y suggested that the minimum amount of impurity  would be introduced i f an o i l manometer were used, f i l l e d with very low vapor pressure o i l , and connected  to the system v i a a tube f i l l e d with Z i o l i t e  pellets.  After the o i l manometer was put on the performance of the vacuum system changed s l i g h t l y .  The lowest pressure ever attained with the o i l manometer  in the system was of the order of 1 x 10**^ mm. Hg.  In general the system  performed quite well i n spite of the presence of the o i l .  The manometer was  placed outside the oven area, which meant that a part of the system was not baked.  The c a l i b r a t i o n curves f o r the discharge gauge are given i n figures (11,10).  The c a l i b r a t i o n was found to change s l i g h t l y i f the Neon were l e f t  in the o i l - c o n t a i n i n g part of the system for any length of time (say overnight).  Therefore i t was deemed best to measure the pressure of the Neon as  soon as possible a f t e r the leaking i n , and to seal the tube afterwards.  immediately  In this way the contamination of the Neon was kept to a minimum.  The second tube of Neon was prepared a f t e r c a l i b r a t i o n of the discharge gauge with the o i l manometer s t i l l i n the system, to provide a check on the pressure.  The pressure at which the tube was sealed o f f was 1.3 mm. Hg.  This  was the optimum pressure f o r the absorption experiment (see Ladenburg (4) ).  For  both tubes prepared, the pressure before f i l l i n g was i n the 10-9 range.  The tubes prepared were both used i n the absorption experiment and showed much improvement over the system used before.  The absorption l i n e s ,  which were nearly nonexistent with the o l d system, showed up stronger and also i n greater number.  The H ^  l i n e was barely v i s i b l e when emission  spectra of the Neon were taken, indicating that traces of Hydrogen were s t i l l present.  However, i t was very much weaker f o r the prepared tubes than i t had  ever been under similar conditions with the o l d system, indicating that the Hydrogen content of the discharge tube was much less than before.  This  Hydrogen i s probably that which was i n the Neon i n i t i a l l y (see page ( 3 ) ). The experiments with the absorption tubes are s t i l l  i n progress, and there-  fore i t cannot be said with certainty at this stage whether the Neon was pure enough for our purposes.  Indications are, however, that this i s the case. i  There are, of course, ways i n which the system can be improved.  I t seems  l i k e l y that with careful experimentation the ultimate pressure can be lowered somewhat, p a r t i c u l a r l y when the o i l manometer i s removed again.  This mano-  meter was l e f t on for the time being to check the c a l i b r a t i o n of the discharge gauge at some l a t e r time.  I f i n the preparation of more discharge  tubes i t appears that the l i m i t a t i o n on the purity i s not i n the ultimate pressure attainable, but rather i n the manufactured Neon bottle  itself,  various methods of further p u r i f i c a t i o n could be b u i l t into the system.  One  such method which seems p a r t i c u l a r l y promising i s p u r i f i c a t i o n by cataphoresis (28) .  The removal of Hydrogen u t i l i z i n g Uranium powder as described by Dieke  (29) could also be t r i e d . In the series of experiments c a r r i e d out i n this laboratory f o r the measurement of t r a n s i t i o n p r o b a b i l i t i e s i n excited gases, i t i s l i k e l y that  many tubes of the kind described above w i l l have to be prepared, not only f i l l e d with Neon, but also with other gases.  I t i s hoped that the system  b u i l t f o r this project, together with the experience gained, w i l l be of some use for this purpose and i n the general interest of s c i e n t i f i c endeavor i n the future.  - 57 -  BIBLIOGRAPHY 1.  M i t c h e l l , A.C.G. , and Zemansky, M.W.,  "Resonance Radiation and Excited  Atoms", Cambridge University Press, 2nd. ed. , 1961. 2.  Stuart, H., Zeits. fur Physik 32, 262 (1925)  3.  Meissner, K.W.,  4.  Ladenburg, R., Zeits. fur Physik 48, 32 (1928)  5.  Dushman, S., " S c i e n t i f i c Foundations of Vacuum Technique", John Wiley  Ann. d. Phys. _76, 124 (1925)  & Sons, 1949. 6.  Yarwood, J . , "High Vacuum Technique", John Wiley 6c Sons, 1961.  7.  Redhead, P.A., Hobson, J.P., and Kornelson, E.V., i n "Advances i n Electron Physics" V o l . 17, p 323, Academic Press, London and New York, 1962.  8.  Alpert, D., i n "Handbuch der Physik", (Flugge, ed.) Vol. 12, p 609, Springer, B e r l i n , 1958.  9.  Foner, S.N. et a l . , J- Chem. Phys. 31_»  5  4  6  (1959)  10.  Schafer, K. , and Gerstacker, H., Z. Elektrochem. 60,  11.  Becker, J.A. i n "Structure and Properties of S o l i d Surfaces", (Gomer &  874 (1956)  Smith, eds.) p 459, Univ. of Chicago Press., 1952. 12.  Sherwood, R.G. Phys. Rev. 12, 448 (1918)  13.  Todd, B.J., J . of Appl. Phys. 26, 1238 (1955)  14.  Barrer, R.M. , "Diffusion i n and through Solids", Cambridge Univ. Press, London & New York, 1951.  15.  Norton, F.J., J. of Appl. Phys. 28, 34 (1957)  16.  Alpert, D. , andBuritz,  17.  Alpert, D. and Buritz, R.S., and Rogers, W.A., 868 (1954) .  R.S., J. of Appl. Phys. 25, 202 (1954) J. of Appl. Phys. 25,  - 58 -  18.  Tuzi, Y. and Okamoto, H. , J . Phys., Soc. Japan 13, 960 (1958)  19.  Briggs, L , J. of P. Chem. 9, 617 (1905)  20.  Alpert, D., J. of Appl. Phys. 24, 860 (1953)  21.  Biondi, M.A.,  22.  Nottingham, W.B.,  J. of Appl. Phys. 8, 762 (1937)  23.  Nottingham, W.B.,  MIT Conference on Physical Electronics (1947)  24.  Alpert, D., and Bayard, R.T,, Rev. of Sc. Instr. 21, 571 (1950)  25.  Lander, J.J., Rev. of Sc. Instr. 21, 672 (1950)  26.  Metson, G.H.,  27.  Hirsch, E.H., Rev. of Sc. Instr. 32, 1373 (1961)  28.  Riesz, R., and Dieke, G.H.,  29.  Dieke, G.H. and Cunningham, S.P., J. of Opt. Soc. Am. 42, 187 (1952)  Rev. of Sc. Instr. 30, 831 (1959)  B r i t . J. of Appl. Phys., 2, 46 (1951)  J. of Appl. Phys. 25, 196 (1954)  discharge tube # 3 valve -Vol. I l l  I baked area  I  4J discharge gauge  -Vol. I. Vol. I I # 1 valve  l  # 4jvalye|. trap  ,  i  ion gauge 2 valve  l_  \ " Vol. IV J  T  o i l manometer  Neon bottle  FIGURE 1 General arrangement of the system  (schematic)  - 60 cooling water  to system  K pump  '  FIGURE 2 '/.  Diffusion pump (schematic diagram)  77777// FIGURE # U tube o i l manometer  I  I.  FIGURE 3 Baffle .  cooling water  Neon FIGURE 5  Schematic- of Neon f i l l i n g system  - 61 -  rn~T 11  TT  •I 16—»-|  FIGURE 6 Valve with driver  Valve with bakeout clamp  Torque i n f t . l b s . FIGURE 7 Graph of closing torque versus leak rate for the a l l metal valves  -62.-  250 M -AM/VW-  i '4"  GAUGE  FIGURE 9 Discharge gauge circuit  3«  "4" igauge geometry  300-Voits  1 Pressure in mm. Hg.  FIGURE 9 Graph of striking and extinction voltage versus pressure  - 63 -  - 64 -  2.ff  \  z  3  Pressure i n mm.  A-  5  6  7"  8  9  of Hg. FIGURE 11  C a l i b r a t i o n curve for the discharge gauge; V  13 Q  600 V. (Neoi^  1 1_  " A V *  470-TL  1— j  -\ooyf J_  /  ^ - B A Y A R D  IN  —-WY  47Q-K-  1 6 2 7  - A L P E R T  T U B E  I -TO  lc  -V  ISO  ' — i — \ A A A  I  ;  4-TL ( l O O  K—"L—lc •  2IO  MONITOR  V.  w)  H  400  269 ex  FIGURE 12 Ion gauge control unit  oH T  + I S O V.  REG.  2.00  ION COULECTOR  C K 3 I 2 A  M  X _ ISO V .  262  0-2  3.3  I IOOA  K  -vvw*-  MA  METER £ 3.9 K -A/VW*—  FIGURE 18 Ion Current amplifier  820X1-  I  40 lit  

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