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Application of a new technique to the measurement of stark shifts Thiessen, Edwin George 1973

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APPLICATION OF A NEW TECHNIQUE TO THE MEASUREMENT OF STARK SHIFTS by EDWIN GEORGE THIESSEN B. Sc., University of Saskatchewan, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of PHYSICS We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1973 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by h i s representatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of P h y s i c s The University of B r i t i s h Columbia Vancouver 8, Canada Date A p r i l 19. 1973 ABSTRACT A v a r i a t i o n of the c l a s s i c a l canal ray source has been developed for the study of the Stark E f f e c t i n p o s i t i v e ion l i n e s . Stress was placed on the measurement of small l i n e s h i f t s rather than the attainment of large e l e c t r i c f i e l d s . To t h i s end a s e n s i t i v e s h i f t measuring apparatus employing a l i n e a r neutral density f i l t e r to translate the l i n e s h i f t into a change i n i n t e n s i t y was b u i l t . S h i f t s were measured to an accuracy of about .02 A° i n the following l i n e s : He I 5016 A°, 7281 A°, 6678 A°, 3889 A° and Ar I 4272 A°, 4266 A°. No s h i f t s were detected i n Ar II 4727 A° and 4806 A°. A discussion of prospective improvements to be made i n the apparatus i s also included. With these improvements, the apparatus should be capable of measuring s h i f t s to an accuracy of .001 A°. i i TABLE OF CONTENTS P a 9 e ABSTRACT i i LIST OF FIGURES V LIST OF TABLES v i i ACKNOWLEDGEMENTS v i i i Chapter 1 INTRODUCTION 1 1.1 H i s t o r i c a l Introduction 1 1.2 Theory 3 1.3 Present Work 7 Chapter 2 APPARATUS 9 2.1 Ion Beam and High F i e l d System . . . . 10 2.1.1 Hollow Cathode Ion Source . . . 11 2.1.2 Ion Accelerating Lens 13 2.1.3 Stark F i e l d Plates 17 2.1.4 Vacuum System 20 i i i Chapter Page 2.2 Data A c q u i s i t i o n System 23 2.2.1 Op t i c a l System 24 2.2.2 Linear Transmission Wedge . . 25 2.2.3 El e c t r o n i c s 31 3 EXPERIMENTAL PROCEDURE 41 3.1 Stark Source 41 3.2 Opt i c a l Alignment 42 3.3 Data A c q u i s i t i o n 43 3.4 Calculations 44 4 CONCLUSIONS 48 4.1 Introduction 48 4.2 Problems with Stark Source 49 4.3 Problems with Data A c q u i s i t i o n System 51 4.4 Data Handling Improvements 52 4.5 Al t e r n a t i v e Method of Measurement . . 53 4.6 Concluding Remarks 54 BIBLIOGRAPHY 56 i v LIST OF FIGURES Figure Page 1. Theoretical T r a n s i t i o n Considered . . . . 4 2. Schematic of Ion Beam and High F i e l d System 10 3. Detailed Assembly of Hollow Cathode Ion Source 12 4. Ion Lens Schematic . 14 5. Ion Lens Assembly 16 6. Detailed Assembly of Stark F i e l d Plates 18 7. Vacuum System Schematic . . . . . . . . . 21 8. Data A c q u i s i t i o n System Block Diagram . . 23 9. Ideal Transmission Wedge and S l i t P r o f i l e 26 10. Wedge Production Schematic . . . . . . . 27 11. Experimental Wedge and Reference P r o f i l e s 29 12. Wedge C a l i b r a t i o n Curve 30 13. El e c t r o n i c s Schematic 31 v Figure Page 14. Photomultiplier Tube Pre-amplifier C i r c u i t 33 15. Photon Pulse Amplifier Discriminator C i r c u i t 36 16. Counter Modifications 37 17. Response Curve of Photon Counting El e c t r o n i c s 39 v i LIST OF TABLES Table Page 1. Experimental S h i f t s 46 2. Comparison of Measured S h i f t s with Previous Experiments and with Theoretical Values 47 v i i ACKNOWLEDGEMENTS I wish to thank my supervisor, Dr. A.J. Barnard, for his h e l p f u l comments during the course of t h i s experiment and f o r his patient assistance during the writing of the thes i s . I would also l i k e to thank other members of the group, p a r t i c u l a r l y Dr. B. Ahlborn, Dr. J . Meyer, and Dr. R. Morris, for stimulating discussions held with them during the course of t h i s work. I would l i k e to express my appreciation to Messrs. D. Sieberg and J. Zangeneh fo r t h e i r help i n the el e c t r o n i c s design and Mr. J . Bosma for his help during the design of the Stark source and wedge apparatus. Yi e l d i n g to threats of marital tumult, I hereby acknowledge typing and e d i t o r i a l assistance rendered by J. Thiessen. F i n a n c i a l assistance from the National Research Council of Canada i s g r a t e f u l l y acknowledged. v i i i Chapter One INTRODUCTION 1.1 H i s t o r i c a l Introduction* The e f f e c t of an external e l e c t r i c f i e l d on atomic spectra was discovered by Stark i n experiments done on hydrogen i n 1913. In the same year, Bohr developed his theory of the atom but i t was not u n t i l 1916 that Schwarzschild and Epstein, working independently, explained the s p l i t t i n g of the hydrogen l i n e s using the Bohr theory. The Stark e f f e c t was also the f i r s t a p p l i c a t i o n of Schroedinger's wave mechanics theory. This work was done independently by Schroedinger and Epstein i n 1926. Two years a f t e r Stark's discovery, Koch observed the Stark e f f e c t i n the two electron system of helium. In He, however, the e f f e c t was much more complicated, with l i n e s being s h i f t e d to smaller and larger wavelengths with and without s p l i t t i n g i n t o p o l a r i z e d components. Also i n many-electron systems, the s e l e c t i o n rules i n v o l v i n g L£ were broken i n the presence of an e l e c t r i c f i e l d . Bohr could explain some aspects of the He spectrum using * A l l h i s t o r i c a l information up to 1939 comes from a review a r t i c l e by Verleger (1939). 2 eccentric o r b i t s but a more accurate de s c r i p t i o n was l e f t to Schroedinger's wave mechanics and l a t e r to Dirac's theory of quantum mechanics. Between the two world wars research on the Stark e f f e c t was extremely active with many improvements being made i n t h e o r e t i c a l and experimental aspects of the problem. Both the canal ray and Lo Surdo types of sources were improved by various authors and the f i e l d strength was pushed further up. In 1930 von Traubenburg and Gebauer attained a f i e l d of 1100 kV/cm and i n 1936 Bomke attained a f i e l d of 1300 kV/cm. For the purposes of t h i s experiment i t i s i n t e r e s t i n g to note the work done on helium and argon during t h i s period. Foster, between 1924 and 1936, d i d a f a i r l y exhaustive study of He both t h e o r e t i c a l l y and experimentally. Ryde, beginning i n 1932, studied the Stark e f f e c t i n about 160 l i n e s of Ar from 8000 - 4200 A° i n f i e l d s from 55 to 155 kV/cm. During the Second World V7ar, there was very l i t t l e a c t i v i t y on the Stark e f f e c t . A f t e r the war, further work was immediately begun, but most of the i n t e r e s t was i n more complex molecular systems. The only immediate postwar work of i n t e r e s t i n t h i s experiment i s that done by Minnhagen, who, i n 194 8, studied about 80 l i n e s i n the v i s i b l e Ar I I 3 spectrum and measured s h i f t s on 20 of them. Measurements of the s h i f t s of a few He I l i n e s were also included i n t h i s paper. In 1949 he measured numerous le v e l s of Ar I, correcting some of the e a r l i e r work of Foster and Horton. In the 1950's, Stark broadening measurements on plasma l i n e s began to be popular and t h i s work has been a major factor i n the recent r e v i v a l of i n t e r e s t i n the Stark e f f e c t i n atoms. Bonch-Bruevich and Khodovoi (1967) have written a review paper on the most recent methods used i n Stark e f f e c t experiments. 1.2 Theory The theory f o r the Stark e f f e c t has been well known f o r years, but a b r i e f summary of i t i s presented here as t h i s w i l l lead e a s i l y into the reasons why further work on the e f f e c t i s important. The notation used w i l l be the same as that i n Condon and Shortley (1951). Consider a t r a n s i t i o n between two le v e l s with angular momentum quantum numbers J,M and J o,M Q. J',M' are the same quantum numbers for one of the l e v e l s l y i n g close to the upper l e v e l J,M. 4 F i g u r e 1. T h e o r e t i c a l T r a n s i t i o n C o n s i d e r e d T h e h a r a i l t o n i a n o f t h e a t o m i n t h e e l e c t r i c f i e l d xs H = H0 t e < f Z w h e r e H i s t h e h a m i l t o n i a n i n z e r o f i e l d , a n d o E i s t h e e l e c t r i c f i e l d ( w h i c h i s i n t h e z -d i r e c t i o n ) . I t w i l l b e a s s u m e d t h a t t h e s h i f t o f t h e l o w e r l e v e l i s n e g l i g i b l e i n c o m p a r i s o n t o t h e u p p e r l e v e l s h i f t s o t h a t t h e c h a n g e i n t r a n s i t i o n e n e r g y f r o m t h e s t a t e |0JM^ t o t h e s t a t e | @0 X Mc^ ^ s g i v e * i b y t h e s h i f t i n t h e u p p e r l e v e l . F r o m s t a n d a r d p e r t u r b a t i o n t h e o r y , i f o n e a s s u m e s j A ET J « | E flJ ~ ^ f l ' j ' L t h e f i r s t o r d e r c h a n g e i n t h e 5 energy i s given by < P J M l e £ 2 | p J M > = 0 since e^z i s odd. The second order perturbation i s where <^|3 j | | -A- || ft J'^ i s the reduced matrix element. For the case of a p a r t i c u l a r atom, further assumptions can usually be made. For example, i n the case of He, LS coupling can be assumed and the reduced matrix element becomes < ^ L 5 j | U | | o , ' L ' 5 ' j - ' > Assuming that the atom consists of a core plus an o p t i c a l electron, we can write and <*(- = t LTSTAL Going back to the expression for A E, we now have A E ( J , M j = tZ X h Vis <t where X indicates a sum over a l l primed quantum numbers and J> i s the largest of Jl and Jl . C T 2 ^ >f' depends on the r a d i a l wave functions and i s given by 7 A. The above c a l c u l a t i o n was done f o r the s p e c i a l case o f He, but f o r other atoms, by making o t h e r assumptions, the e x p r e s s i o n f o r AE may be reduced t o a s i m i l a r e x p r e s s i o n i n v o l v i n g o t h e r r a d i a l f u n c t i o n s . dominating the s h i f t o f the J,M l e v e l , the sum reduces t o one term and we get a value f o r C T * ^ ,g' . T h i s v a l u e can be used i n S t a r k broadening c a l c u l a t i o n s and as a check on the r a d i a l wave f u n c t i o n R . (r) u s i n g These 0" f u n c t i o n s are u s u a l l y o b t a i n e d i n the Coulomb approximation. 1.3 P r e s e n t Work We are now a t the stage where a r a t i o n a l e f o r the p r e s e n t work can be put forward. In the p r e v i o u s work done by Minnhagen (1948), the s h i f t s o f some Ar I and Ar I I l i n e s were measured. I f these s h i f t s are compared w i t h t h e o r e t i c a l v a l u e s o b t a i n e d I f t h e r e i s o n l y one c l o s e by l e v e l , J'jM', 8 using <T2 i n the Coulomb approximation, the agreement i s o f t e n poor. Yet these same <JZ 's are used w i t h the utmost of confidence i n Stark broadening c a l c u l a t i o n s . A systematic check of a l l the cr 2 values used i n these c a l c u l a t i o n s i s o b v i o u s l y r e q u i r e d , but i n order to"do t h i s , Stark s h i f t s of a l a r g e number of l i n e s need to be a c c u r a t e l y measured. Minnhagen has measured some o f the l e v e l s r e q u i r e d but the l e v e l s w i t h the s m a l l e r s h i f t s could e i t h e r not be measured at a l l by him or were measured i n a c c u r a t e l y . Thus a more accurate determination of the smaller Stark s h i f t s of many Ar II l i n e s i s i n order and t h i s i s the o b j e c t i v e of the present experiment. The technique used i s a new one developed by R. M o r r i s (1972) and c o n s i s t s of p l a c i n g a l i n e a r n e u t r a l d e n s i t y f i l t e r i n the e x i t plane of a monochromator. The l i n e to be measured i s p l a c e d on the centre of t h i s l i n e a r " t r a n s m i s s i o n wedge" and when the high f i e l d i s a p p l i e d t o the atoms, the s h i f t of the l i n e i s t r a n s l a t e d i n t o a change i n i n t e n s i t y . This technique i s capable of r e s o l v i n g s m a l l e r l i n e s h i f t s than the simpler method of measuring the a c t u a l s h i f t s o f the l i n e s . 9 Chapter Two APPARATUS The apparatus required to measure the small Stark s h i f t s of many Ar II l i n e s consists, broadly speaking, of a l i g h t source and a data a c q u i s i t i o n system. The l i g h t source i s an apparatus which produces a beam of p o s i t i v e ions i n a high e l e c t r i c f i e l d . The d e t a i l s of the design and construction of t h i s source are presented i n section 2 . 1 . The data a c q u i s i t i o n system i s described i n section 2 . 2 . It consists of a lens system to gather the l i g h t from the ion beam, a monochromator and wedge apparatus to translate the l i n e s h i f t into a change i n i n t e n s i t y , and a photon counting e l e c t r o n i c s system to measure the i n t e n s i t y change. 10 2.1 Ion Beam and High F i e l d System ANODE REGION Figure 2 . Schematic of Ion Beam and High F i e l d System The section of the apparatus which produces the p o s i t i v e ions i n the high e l e c t r i c f i e l d consists of a hollow cathode discharge tube i n which the ions are produced, an ion accelerating lens which extracts the ions through a s l i t i n the end of the cathode, and a high f i e l d region i n t o which the ion beam i s shot. Gas i s continuously pumped from the i n l e t i n the discharge tube, through the s l i t i n the hollow cathode, i n t o the high f i e l d region and from there, out of the system. 11 2.1.1 Hollow Cathode Ion Source The ion source (see F i g . 3) consists of a water cooled aluminum anode and cathode separated by a pyrex discharge tube about 50 cm long and 3.0 cm i n diameter. The cathode, instead of being plane, has a hole of 10 mm diameter i n i t and into t h i s hole i s pushed a c y l i n d r i c a l aluminum i n s e r t of I.D. 7/32 i n . This i n s e r t must be cleaned or even replaced p e r i o d i c a l l y due to heavy p i t t i n g by the discharge. The plane face of the cathode i s covered by a lava c y l i n d e r , as shown i n F i g . 3, to prevent sputtering of the aluminum by ion bombardment. Gas i s leaked into the discharge tube near the anode and flows out through a s l i t at the base of the hollow cathode i n s e r t . The discharge between anode and grounded cathode i s run at about 400 v o l t s and 45 mA i n a pressure of about 1 Torr. A b a l l a s t r e s i s t o r of 25 KP, and 225 watts i s i n series with the discharge tube to provide some current s t a b i l i t y . The diameter and length of the hollow cathode was determined p a r t l y by t r i a l and error and p a r t l y by reference to previous authors (Mark and Wierl (1929), Thornton (1935), Foster and S n e l l (1937), Ryde (1938), Ryde (1942), Maissel (1958), Steubing and Schaeder (1936)). The approximate siz e was determined by consulting previous authors and then A N O D E ASSEMBLY C A T H O D E ASSEMBLY Figure 3 . Detailed Assembly of Hollow Cathode Ion Source 13 in s e r t s of d i f f e r e n t I.D.'s were used u n t i l the brightness of the discharge i n the hollow cathode and i n t e n s i t y of ions from i t were maximized. 2.1.2 Ion Accelerating Lens A l l previous authors who used the canal ray method placed a s l i t at the end of the hollow cathode and the emerging beam of ions and neutrals was strong enough f o r them to study the Stark e f f e c t i n the neutral atoms. Most of these authors do not give d e t a i l s of exposure times and films used, so i t i s d i f f i c u l t to know how intense t h e i r beams were. This experiment was f i r s t attempted with simply a s l i t at the end of the hollow cathode. A f a i r l y bright beam was obtained before the Stark f i e l d plates were put into place, but with the plates i n place, only a dim glow was seen between them. To solve t h i s problem, an ion lens was inserted between the hollow cathode and the f i e l d p l a t e s . The lens used i s a simple three element e l e c t r o s t a t i c lens s i m i l a r to the type used i n the Nier Gun mass spectrometer ion source. The theory for t h i s lens i s described by Barnard (1953) and i s summarized below. The lens consists of three p l a t e s , A, B, and C, at 0 v o l t s , V^, and V^. The separations between the plates are d. and d 9 as shown i n F i g . 4. 14 Figure 4. Ions Lens Schematic The voltages and are set such that the f i e l d E 2 i n the second region i s larger than the f i e l d E^ i n the f i r s t . Equipotential surfaces from the region BC then bulge into the region AB as shown. Ions drawn from the l e f t of plate A h i t these equipotential l i n e s at an acute angle and t h e i r path i s bent towards the axis of the lens. The object of the lens i s to accelerate the beam coming through the s l i t i n A and focus i t on the s l i t i n C. The f o c a l length of the lens i s given by / d, dz ~ ZV, 15 In t h i s expression, the following assumptions have been made: (i) the s l i t width i s small i n r e l a t i o n to the plate separation; ( i i ) the s l i t i s i n f i n i t e l y long; ( i i i ) the plates are t h i n with respect to the s l i t width; (iv) the ions begin with zero k i n e t i c energy; and (v) space charge e f f e c t s are neglected. If d^ = d 2 , the r e l a t i o n between and V 2 to have the ions focus on the e x i t s l i t i n plate C i s = 6 V^, For the lens used i n t h i s experiment, d^ = d 2 = .120 i n . The voltage V 2 was made as high as possible ( 8 kV) so that the ions would get as f a r as possible i n t o the high f i e l d region between the Stark plates before h i t t i n g the negative p l a t e . The lens i s shown i n F i g . 5. I t consists of the e x i t s l i t fastened to the end of the hollow cathode using set screws and two .025 i n thick s t a i n l e s s s t e e l plates held to the e x i t s l i t by two nylon screws protruding from i t . These screws pass through the centre plate and thread into two round nuts spot welded to the end p l a t e . The plates are kept apart by baked lava spacers which have been threaded on the outside to increase the tracking distance between the plate s . The spacers between the centre and end plates come 16 up around the nuts, again to increase the tracking distance. Smooth unbaked lava spacers were used at f i r s t , but these tracked very badly when they became only s l i g h t l y d i r t i e d by the beam. The baked threaded spacers take much longer before tracking occurs. insert Figure 5 . Ion Lens Assembly The s l i t s i n a l l three plates were cut using a spark erosion cutter running at slow speed to keep the 17 edges of the s l i t s from becoming ragged. The dimensions of the hollow cathode e x i t s l i t , centre s l i t , and f i n a l s l i t are r e s p e c t i v e l y , .014 x .100 i n , .040 x .160 i n , and .030 x .150 i n . The high tension leads were fix e d to the centre and f i n a l plates by laying the wire at the edge of the p l a t e , wrapping n i c k e l f o i l around i t and over a small section on both sides of the p l a t e , and spot welding the wire to the n i c k e l f o i l and the n i c k e l to the s t a i n l e s s s t e e l p l a t e . Before i n s t a l l a t i o n , the plates were polished with #600 emery paper, 4/0 emery p o l i s h i n g paper, and a metal p o l i s h . 2.1.3 Stark F i e l d Plates The main c r i t e r i o n i n the design of the high f i e l d region was to keep the distance between a l l exposed high voltage metal parts and grounded parts as small as possible, thereby keeping the breakdown voltage between them high. The Stark f i e l d plates i n p o s i t i o n i n the high f i e l d region are shown i n the two views of F i g . 6. The grounded plate i s bolted to a platform and the negative high voltage plate i s supported from i t by three s t e e l pins. The pins are f i x e d to the grounded plate with set screws and s l i d e i n holes i n lava pieces bolted to the negative p l a t e . The fourth support i s l e f t out to allow a clear view of the ion beam where i t enters the region S E C T I O N A - A Figure 6. Detailed Assembly of Stark F i e l d Plates 19 between the two p l a t e s . Behind the negative plate i s a grounded aluminum s h i e l d which serves a dual purpose: one i s to keep the distance from the back of the negative plate to the nearest grounded object small, and the other i s to support the mechanism for moving the negative p l a t e . This mechanism consists of an i n s u l a t i n g threaded rod which i s kept fi x e d i n p o s i t i o n (from the back of the support) and i s turned i n a tapped hole i n the negative p l a t e . Both the negative and grounded plates were machined out of the same piece of aluminum and cut apart only a f t e r the lava pieces had been attached and support holes d r i l l e d . This was done to ensure that the high f i e l d faces of both plates were as p a r a l l e l as possibl e . The high voltage i s fed to the negative plate by an insulated wire coming through the holes where the fourth pin would have been. On the end of the wire i s a piece of brass which f i t s into the fourth hole i n the negative plate and i s held i n by a set screw. This whole assembly i s contained i n a brass housing with two viewing ports so that the beam between the high f i e l d plates can be viewed from both sides. One further point concerning the high f i e l d region i s worthy of note. The major problem i n getting i t operating properly was the prevention of breakdown from the f i n a l (8 kV) accelerating plate and the high voltage f i e l d 20 p l a t e . One method used was to coat some of the exposed metal parts with a l i q u i d porcelain c a l l e d Sauereisen. The points where the wires attach to the accelerating plates and the s t e e l support pins are two of the parts which were so coated. A l o t of time was wasted using t h i s technique u n t i l i t was f i n a l l y r e a l i z e d that the breakdown was being somehow enhanced by the Sauereisen (probably by outgassing from i t ) . In the f i n a l version, the only material added to the f i e l d plate region was some vacuum epoxy (Ultra Torr) used to strengthen the junction points between the high voltage leads and the accelerating p l a t e s . The best breakdown prevention method was found to be c a r e f u l p o l i s h i n g of a l l high voltage parts and attention to keeping the distances between them and grounded parts small. 2.1.4 Vacuum System The system (see F i g . 7) i s d i f f e r e n t i a l l y pumped by a CVC MCF 300 d i f f u s i o n pump (pumping speed 270 1/sec) and backed up by a Welch 1397 mechanical pump (pumping speed 7 1/sec). A l i q u i d nitrogen cold trap and a b u t t e r f l y valve separate the d i f f e r e n t i a l pump from the system, giv i n g a pumping speed of about 100 1/sec at the valve. The conductance of the glass elbow to the system i s about 250 1/sec (assuming molecular flow) giving a pumping Figure 7 . Vacuum' System Schematic i - 1 2 2 speed at the o u t l e t of the f i e l d plate region of about 70 1/sec. The number of ions i n the f i e l d plate region increases very quickly with an increase i n pressure i n the discharge tube, but t h i s , of course, increases the background pressure i n the f i e l d plate region and the l i k e l i h o o d of high voltage breakdown. Thus the object of the pumping system i s to maintain as large a pressure d i f f e r e n t i a l as possible between the discharge tube and the f i e l d plate region. When running Ar i n the system, i t was necessary to mix i n about one t h i r d He (by volume) to r a i s e the breakdown voltage i n the f i e l d plate region and to decrease sputtering of the cathode. The mixing of the two was done i n a mixing tank at about atmospheric pressure. The gas flowed through a needle valve from t h i s tank to the discharge tube. One problem encountered i n designing the vacuum system was a tendency f o r the discharge, e s p e c i a l l y at lower pressures, to go to the gas i n l e t needle valve or the discharge tube roughing l i n e instead of to the hollow cathode. To prevent the discharge to the needle valve, a c o i l of poly - f l o tubing about 1 m long was added to the gas i n l e t l i n e and to prevent i t s going to the roughing l i n e , a glass stopcock was placed between the discharge tube and the metal roughing l i n e . This valve was, of course, closed 23 during an experimental run. 2.2 Data A c q u i s i t i o n System The data a c q u i s i t i o n system (see F i g . 8) consists of three main parts: (i) an o p t i c a l section consisting of a simple lens system and a monochromator with two e x i t s l i t s ; ( i i ) a l i n e a r transmission wedge behind one s l i t ; and ( i i i ) a photon counting e l e c t r o n i c s system to measure the i n t e n s i t i e s at both e x i t s l i t s . wedge p.m. tube and pre-amp source wedge 7 •A o reference p.m. tube and pre-amp monochromator counter amplifier discriminator counter amplifier discriminator Figure 8. Data A c q u i s i t i o n System Block Diagram 24 2.2.1 Optical System The lens system used consists of an objective lens of f o c a l length 195 mm and diameter 36 mm placed such that the centre of the ion beam l i e s at i t s f o c a l length and an imaging lens of f o c a l length 178 mm and diameter 34 mm to image the p a r a l l e l l i g h t from the f i r s t lens on the entrance s l i t of the monochromator. This lens matches the f-number (f 6.8) of the monochromator and the Stark source was designed to have approximately the same f-number. Between the two lenses i s a dove prism to rotate the horizontal image from the ion beam into the v e r t i c a l plane of the monochromator entrance s l i t . The monchromator i s a Spex 1702 3/4 metre instrument with a Bausch and Lomb grating blazed at 1 um and having 1200 l i n e s per mm. The resolution of the instrument i s .030 A° i n t h i r d order with 6 um s l i t s . The monochromator i s also equipped with a motor driven scan with scan speeds from .5 A°/min to 5000 A°/min. For t h i s experiment the monochromator was modified by the addition of a camera tower mounted through the hole provided i n the top of the cover. A p a r t i a l l y s i l v e r e d mirror s p l i t s the beam into two parts sending part to the o r i g i n a l Spex e x i t s l i t mounted on the top of the camera tower and the r e s t to a Hilger s l i t replacing the o r i g i n a l 25 e x i t s l i t . The transmission wedge i s mounted just behind t h i s s l i t i n a housing which allows i t to be positioned properly with respect to the s l i t . The two e x i t s l i t s were made p a r a l l e l to the entrance s l i t and focussed by standard techniques. 2.2.2 Linear Transmission Wedge The l i n e a r transmission wedge technique i s described i n f u l l d e t a i l by Morris (1972). A short account of the production and use of the wedge i s included here for completeness. F i r s t a general d e s c r i p t i o n of the technique i s i n order. A neutral density f i l t e r whose transmission varies l i n e a r l y from 0 to 1.0 over the width of the l i n e (including most of the l i n e wings) i s placed i n the e x i t plane of a monochromator and the e x i t s l i t i s opened to the width of t h i s "wedge". The transmission of the combined wedge and s l i t i s shown i n F i g . 9. The l i n e whose s h i f t i s to be measured i s placed i n the centre of the wedge. When the l i n e i s s h i f t e d (in t h i s experiment by app l i c a t i o n of an e l e c t r i c f i e l d ) , the s h i f t i n p o s i t i o n of the l i n e i s translated into a change i n i n t e n s i t y of the l i g h t transmitted by the wedge. As shown i n F i g . 9, the s h i f t of the l i n e i s small compared to the l i n e width which i n t h i s 26 experiment i s the instrument width. S h i f t s which could not be resolved simply by using the monochromator can be resolved using the transmission wedge. Figure 9. Ideal Transmission Wedge and S l i t P r o f i l e A p e r f e c t l y l i n e a r transmission wedge would be very d i f f i c u l t to produce, but a good approximation can be made as follows: a fin e grain spectroscopic plate (Kodak 64 9 F) was exposed to an extended l i g h t source with a knife edge between the plate and the source (see F i g . 10). 2 7 d 1 LIGHT SOURCE KNIFE EDGE SPECTROSCOPIC PLATE Figure 10. Wedge Production Schematic x=0 x=r6 At an a r b i t r a r y p o s i t i o n x between x = 0 and x = r9, the exposure i s so that the exposure varies l i n e a r l y with p o s i t i o n . By changing the exposure time and the developer strength and developing time (thereby changing the slope and shape of the H and D curve of the p l a t e ) , t h i s l i n e a r exposure function was made into an approximately l i n e a r transmission function. This was done simply by t r i a l and e r r o r : many exposures were made and d i f f e r e n t developer strengths and E = 28 times used. The plates produced were then scanned using a microdensitometer and the best one selected. The width of the wedge desired i s dictated by the instrument width which i n t h i s experiment (using a 20 um entrance s l i t ) was about 40 um or .21 A° i n second order. This was determined experimentally by scanning the He I 3889 A° Ge i s s l e r l i n e using 20 um entrance and e x i t s l i t s and using h a l f the width of the p r o f i l e at one tenth maximum. The wedge used i s approximately li n e a r from T = .80 to T = .25 and the length of t h i s portion i s about 100 um or .5 A° i n second order. The values of the parameters used i n the production of t h i s wedge were: d = 1.625 i n R = 15 i n r = .25 i n Exposure: 6 sec to a 60 W bulb behind a ground glass screen Developer: Kodak D-19 d i l u t e d with 2 parts water Developing time: 3 min The wedge was mounted a few thousandths of an inch behind the e x i t s l i t i n a s p e c i a l housing. The wedge was supported from the bottom and could be rotated about t h i s lower pivot point by a d i f f e r e n t i a l screw attached to the top. This means of support allowed the wedge to be aligned 29 p a r a l l e l to the e x i t s l i t . The aluminum plate on which the wedge was mounted could be positioned accurately behind the e x i t s l i t using a 2 i n b a r r e l micrometer screw. The alignment of the wedge and reference s l i t s was done by scanning the He I 3889 A° Geissler l i n e across the s l i t s at 5 A°/min and averaging the amplified and discriminated photon pulses to give an analog signal which was fed to a Hewlett - Packard 7034 A XY p l o t t e r (see next section) . The optimum wedge p o s i t i o n and s l i t width were determined by many scans with d i f f e r e n t s l i t widths and wedge positions u n t i l the best p r o f i l e was obtained. The wedge and reference p r o f i l e s f i n a l l y used are shown i n F i g . 11. The reference s l i t width was chosen because the Figure 11. Experimental Wedge and Reference P r o f i l e s s l i t edges tended to f r o s t up when the photomultiplier Figure 12. Wedge Ca l i b r a t i o n Curve tubes were cooled. (This problem i s described more f u l l y i n section 4.3). The f i n a l wedge c a l i b r a t i o n was done by scanning the 3889 A° He Ge i s s l e r l i n e very slowly ( . 5 A°/min) across the s l i t and counting with both counters gated on for 1 sec at i n t e r v a l s of 10 sec by the c i r c u i t r y described i n the next section. The c a l i b r a t i o n curve i s shown i n Fi g . 12. 2.2.3 El e c t r o n i c s The e l e c t r o n i c s consists of two i d e n t i c a l systems, one for each e x i t s l i t . Each system has one dry ice cooled photomultiplier tube, a photon pulse pre-amplifier contained i n the photomultiplier tube housing, a pulse amplifier -discriminator, and a counter. (See F i g . 13). PRE-AMPLIFIER AMPLIFIER DISCRIMINATOR COUNTER PHOTOMULTIPLIER TUBE Figure 13. Ele c t r o n i c s Schematic 32 The photomultiplier tubes are EMI 9558 B's having S - 20 photocathodes, risetimes of about 15 nsec and f a i r l y low dark currents. Since the signal strength i n t h i s experiment was so low, s p e c i a l consideration was given to minimizing the dark current. Several techniques were used. The f i r s t i s the standard technique of placing an e l e c t r o s t a t i c s h i e l d at cathode p o t e n t i a l around the tube against the glass. The tube was also washed i n lukewarm water to remove a l l skin o i l s and other d i r t ; during and a f t e r a thorough r i n s i n g , i t was not touched d i r e c t l y with the hands. The r e s i s t o r s i n the dynode chain of the tube were chosen to l i m i t the chain current to about 150 uA because no more i s needed for photon counting while any larger current generates heat i n the tube housing, increasing the dark current. The metal photomultiplier tube housing was surrounded by a PVC c y l i n d r i c a l tube and the annulus between the two was f i l l e d with crushed dry ice to cool the tube to below -40° C. The dry i c e container was made f a i r l y a i r t i g h t so that the evaporation of the CC^ created a p o s i t i v e pressure i n i t . This was used to blow evaporated cold CC^ in t o the photomultiplier tube housing from the back, thus cooling i t f a s t e r and also preventing fogging of the photocathode window. The photomultiplier pre-amplifier (Fig. 14) i s 33 needed to give the current and voltage gain required to feed the 500, cable to the amplifier-discriminator c i r c u i t . The c i r c u i t i s a modified version of one use previously by Camm (private communication). The anode s i g n a l i s fed d i r e c t l y to the base of the f i r s t t r a n s i s t o r of a two t r a n s i s t o r negative feedback amplifier providing both current and voltage gain. The negative feedback gives the c i r c u i t a low input impedance (a few ohms), thus enabling i t to follow very f a s t pulses. From the emitter of the second t r a n s i s t o r the pulse enters an emitter follower to provide t e s t 1K W v — i 10 pf to anode r + 20 v. (5.6K) 1K R \ (15K) f < 3.3 K R 2 (15K)-3.3K •2.7K 2N 2369 3 2N2218 '4.7K 2 N 2369 1N914 2.2 220. 1.8K-> 1N751A* r = -2.2 4 7 < JL f Figure 14. Photomultiplier Tube Pre-amplifier C i r c u i t 34 more c u r r e n t g a i n . In t h i s experiment, the output p u l s e was c a p a c i t i v e l y coupled t o a v o l t a g e d i v i d i n g network c o n s i s t i n g o f a 220ft and a 47ft r e s i s t o r i n s e r i e s . T h i s network was r e q u i r e d t o cut down the o v e r a l l v o l t a g e g a i n to a v o i d s a t u r a t i n g the i n p u t a m p l i f i e r o f the a m p l i f i e r -d i s c r i m i n a t o r c i r c u i t . The two m o d i f i c a t i o n s made to the c i r c u i t f o r t h i s experiment were t o change the g a i n and t o change the t e s t i n p u t to s i m u l a t e a p h o t o m u l t i p l i e r tube source. The g a i n of the pre-amp i s p r o p o r t i o n a l t o the va l u e o f R^, the feedback r e s i s t o r ( F i g . 14). R^ was decreased from 15 Kft t o the p r e s e n t v a l u e o f 3.3 Kft, d e c r e a s i n g the g a i n by a f a c t o r o f about 4.5. To m a i n t a i n the proper b i a s l e v e l s i n the f i r s t two t r a n s i s t o r , the v a l u e s o f R^ and R 2 had to be changed by the same p r o p o r t i o n . Because o f the low i n p u t impedance o f the c i r c u i t , the p h o t o m u l t i p l i e r tube c u r r e n t p u l s e s a t the i n p u t are not v i s i b l e u s i n g p r e s e n t o s c i l l o s c o p e s . For t h i s reason, a t e s t i n p u t was p r o v i d e d . The i n p u t c o n s i s t s o f a d i f f e r e n t i a t i n g network (the 10 pf c a p a c i t o r and the "few ohm" i n p u t impedance o f the c i r c u i t ) p l u s a s e r i e s 1KP. r e s i s t o r . T h i s 1KP. r e s i s t o r determines the time i n which the c u r r e n t p u l s e i s i n j e c t e d i n t o the base o f the i n p u t t r a n s i s t o r . With a c a p a c i t a n c e o f 10 p f , an i n p u t p u l s e 35 of 100 - 200 mV gives a current pulse of 10 electrons i n about 15 nsec to the base of the input t r a n s i s t o r . This simulates c l o s e l y a sin g l e photon pulse from the photomultiplier tube. From the photomultiplier tube housing, the pulse was fed v i a a 50n cable to the amplifier-discriminator c i r c u i t (see F i g . 15).* The input amplifier i s an MC 1445 wideband amplifier IC with a voltage gain of 10 and a maximum output pulse of 500 mV. The inverted output pulse i s fed to the i n v e r t i n g input of an MC 1710C d i f f e r e n t i a l comparator IC. This c i r c u i t operates i n a s i m i l a r manner to a Schmitt t r i g g e r . When the input pulse becomes more negative than the negative reference voltage d i a l e d on the 500° potentiometer, the output goes from a low to a high state and remains high for a time determined by the RC series feedback network (providing the input pulse i s short compared with t h i s time). Thus the MC 1710C acts as a voltage discriminator, g i v i n g a standard 40 nsec, 1.5 V output pulse for every input pulse over the threshold l e v e l . From the comparator, the pulse goes through a Darlington pair emitter follower and into the 50 ft cable to the counters. At the comparator output there i s also an averaging network to provide an analog output to operate the XY p l o t t e r . * The idea f o r the design of the discriminator section of t h i s c i r c u i t was obtained from a colleague at ISAS, U. of S., Saskatoon. The design i t s e l f was done p a r t l y at U. of S. and p a r t l y at U.B.C. Figure 15. Photon Pulse Amplifier Discriminator C i r c u i t 37 I t consists of a low pass f i l t e r formed by a 3.3 Kft r e s i s t o r i n series with a .22 uf and 1000 pf capacitor network. The two counters are 40 MHz Analog D i g i t a l Research CM 40A's which have been modified s l i g h t l y to enable them to be gated simultaneously and for long integration times. The modifications are shown i n F i g . 16. I sec pulses spaced 10 sec apart for wedge calibration I min or 3 0 sec integration for data acquisition time mark generator push I button • lOpf =p MC 8601 monostable multivibrator 78pf ground :50K counter counter mercury solenoid Figure 16. Counter Modifications Both modifications consist of a l t e r i n g the t o t a l i z e function of the counters. Normally, placing the counter function switch to t o t a l i z e grounds the counter gate, allowing pulses to be counted while i t remains grounded. 38 In the modified counters, instead of going to the function switch, the counter gates are brought to a double throw switch enabling them to be con t r o l l e d externally i n two ways. One switch p o s i t i o n connects the counter gates to a mercury solenoid switch controlled by a push button. When the button i s pushed, both counter gates are grounded and the counters count u n t i l the button i s released. This method was used i n the 1 min and 30 sec integration times required i n the Stark data gathering. The other switch p o s i t i o n connects the counter gates to the output of an MC 8601 monostable multivibrator which, when h i t by a pulse from the time mark generator, grounds the counter gates f o r a time determined by the external RC constant chosen. This method of gating the counters was used f o r obtaining the wedge c a l i b r a t i o n p r o f i l e . The He I 3889 A° Geissler l i n e was scanned across the wedge at a rate of .5 A°/min and the time-mark generator -multivi b r a t o r c i r c u i t r y triggered the counters for 1 sec at 10 sec i n t e r v a l s . The response of the monochromator plus e l e c t r o n i c s was observed to be l i n e a r up to a count rate of about 350 KHz. At a rate of 4 00 KHz, the error due to non-linear response was about 3% (Fig. 17). Above 400 KHz the error due to 40 non-linearity i s considerable. This n o n - l i n e a r i t y at high count rates r e s u l t s from two factors. The f i r s t i s pulse pile-up. With a discriminator output pulse width of about 45 nsec, the pulse p a i r r esolution of the c i r c u i t was 80 nsec. Therefore i f two pulses a r r i v e within 80 nsec of one another only one of them i s counted. The p r o b a b i l i t y of t h i s happening follows the negative exponential d i s t r i b u t i o n (assuming the pulses are random and hence follow Poisson s t a t i s t i c s ) . At a 400 KHz count rate and 80 nsec dead time t h i s p r o b a b i l i t y i s 3 %. The second fa c t o r i s the s h i f t of the pulse base l i n e at high count rates. The c i r c u i t r y used i s a l l a.c. coupled and the e f f e c t of the coupling capacitors i s to cause the output pulses from them to have a mean value of zero v o l t s . Thus when the pulse rate increases, the baseline s h i f t s down, decreasing the peak pulse voltage and therefore e f f e c t i v e l y increasing the discriminator voltage. So i f the pulse rate goes up by a c e r t a i n percentage, the count rate (number of pulses through the discriminator) goes up by a smaller percentage. This e f f e c t becomes noticeable at about 500 KHz count rates. In t h i s experiment, the data count rates were much less than 350 KHz and during wedge c a l i b r a t i o n s the Geissler tube voltage was decreased u n t i l the He I 38 89 A° l i n e gave a count rate of less than 350 KHz. Chapter Three EXPERIMENTAL PROCEDURE In t h i s chapter the data gathering procedure (as opposed to the set t i n g up procedures) i s described. In the f i r s t two sections, dealing with the Stark source and the o p t i c a l system, only s p e c i a l procedures are d e t a i l e d , as the rest i s self-explanatory a f t e r reading Chapter Two. More d e t a i l i s given i n the data gathering and calc u l a t i o n s sections (3.3 and 3.4, r e s p e c t i v e l y ) . 3.1 Stark Source Before s e t t i n g the gas flow, the ion lens accelerating voltages, and the f i e l d plate voltage at t h e i r operating values, an "electrode conditioning" procedure was necessary. F i r s t the needle valve from the mixing tank was opened to about two thir d s of i t s operating value and the glow discharge started. The centre and f i n a l accelerating plates of the ion lens were then raised to -.75 kV and -4 kV. After about 10 min these voltages could be raised to t h e i r 41 42 operating values of -1.5 kV and -8 kV respectively and over about the next half hour, the mixing tank needle valve could be opened to i t s operating aperture. (This term, "operating aperture", though i t sounds very o f f i c i a l , i s arrived at completely e m p i r i c a l l y . I t i s the maximum needle valve se t t i n g which doesn't produce sparking i n the f i e l d plate region). The length of time required f o r t h i s "electrode conditioning" i s shorter i f the source has been used often i n the previous week or so and a good deal longer i f the source i s being run f o r the f i r s t time a f t e r being at atmospheric i n a i r . 3.1 Op t i c a l Alignment Before each experimental run, the alignment of the optics was checked.* The check was done by removing the wedge photomultiplier tube and shining a 650 W Quartz lamp backwards through the monochromator. (This method was preferred over laser line-up techniques because, with the la s e r , care must be taken that the beam h i t s the centre of the mirrors and grating i n the monochromator and because the las e r gives no information on the focus of the system). The image of the s l i t was seen between the Stark f i e l d plates and i t s p o s i t i o n adjusted by r a i s i n g or lowering the lens * The reason for t h i s i s that the time taken to do the check i s only a few minutes whereas, i f the monochromator or source had been inadvertently knocked out of alignment and not realigned, a whole day of data gathering would be wasted. 43 nearest the source. The focus was checked by the method of parallax. The f i n a l focussing was done by looking at the image of the beam on the entrance s l i t of the monochromator and the f i n a l p o s i t i o n i n g of the image was done by r a i s i n g or lowering the objective lens to maximize the count rate of one of the spectral l i n e s of the gas being used. 3.3 Data A c q u i s i t i o n Once the source was operating, the o p t i c a l alignment checked, and the photomultiplier tubes cooled, data could be taken. F i r s t the l i n e to be studied was positioned on the wedge by noting the maximum wedge si g n a l and scanning the l i n e across the wedge u n t i l t h i s s i g n a l was cut i n h a l f . Zero f i e l d readings of both t o t a l signals and dark currents were then taken. Without touching the monochromator scan, -2 kV and -5 kV readings were taken. With the Ar-He mixture at each Stark voltage s e t t i n g , dark current and t o t a l s i g n a l ( i . e . s i g n a l plus dark current) readings were alternated. Usually 4 dark current and 3 t o t a l signal readings were taken. When He alone was run, the dark current was usually stable enough that only one reading of dark current and one of the signa l was required. The count integration time was usually 1 min but was lowered to 30 sec i f the si g n a l was unusually strong. To c a l i b r a t e the e l e c t r i c f i e l d at the two voltage 44 settings and to check that the apparatus was working properly, the s h i f t s of a few i s o l a t e d l i n e s of He I were measured. The l i n e s chosen were 5016 A°, 7281 A°, 6678 A°, and 38 89 A°. The f i r s t three were done i n f i r s t order and the l a s t i n second order. To check f o r r e p e a t a b i l i t y , 5016 A° was repeated, t h i s time i n second order, and 3889 A° was repeated, again i n second order. The Ar-He mixture was then run and the s h i f t s of the following l i n e s were measured: Ar I 4266 A° and 4272 A°, Ar II 4727 A° and 4806 A°, He 3 889 A°. A l l were measured i n second order. The measurements were repeated with Ar I 4266 A° and 4272 A°, Ar II 4727 A°. The other two plus He I 5016 A° and 6678 A° would also have been measured i n the second run, but at t h i s point, the lava spacers i n the ion lens began to track and no further data could be taken. 3.4 Calculations The t o t a l signals and dark currents obtained f o r each l i n e were averaged and the net signals calculated. The r a t i o of wedge signal to reference sig n a l was calculated f o r each voltage s e t t i n g . The problem then arose that the zero f i e l d r a t i o s (the r a t i o s with the l i n e simply placed i n the middle of the wedge) were not equal for the d i f f e r e n t l i n e s . This was at t r i b u t e d to the differences i n p o l a r i z a t i o n of the d i f f e r e n t l i n e s and the d i f f e r e n t l i g h t throughputs 45 from entrance s l i t to wedge s l i t and entrance s l i t to reference s l i t for d i f f e r e n t p o l a r i z a t i o n s . To get around t h i s problem the sign a l r a t i o s f o r 2 and 5 kV were subtracted from that f o r zero f i e l d and the differences were expressed as percentages of the zero f i e l d r a t i o . The equivalent s h i f t s on the c a l i b r a t i o n curve were calculated by multiplying these percentages by the r a t i o at the h a l f way point of the wedge c a l i b r a t i o n curve. The corresponding wavelength s h i f t s could then be read o f f the c a l i b r a t i o n curve or calculated using the slope of the curve. Both methods were used f o r comparison. Table 1 shows the s h i f t s obtained from the graph and from the slope and also the "experimental s h i f t " . This "experimental s h i f t " i s a combination of the s h i f t from the graph and the s h i f t from the slope. I f the s h i f t was less than .06 A°, the s h i f t from the slope was taken as the experimental s h i f t and i f the s h i f t was greater than .06 A°, the s h i f t from the graph was taken as the experimental s h i f t . Table 2 shows how the s h i f t s obtained i n t h i s experiment compare with previous experimental values and with theore t i c a l values (calculated i n the Coulomb approximation by A.J. Barnard). The previous experimental values were scaled q u a d r a t i c a l l y to the f i e l d s used i n t h i s experiment. 46 Table 1. Experimental S h i f t s LINE STARK VOLTAGE SHIFT (FROM GRAPH) SHIFT (FROM SLOPE) SHIFT (ADOPTED VALUE) (kv) (A°) (A°) (A°) He I 2 -.038 -.038 -.038 5016 A° 5 -.263 -.248 -.263 He I 2 .034 .032 .032 7281 A° 5 .116 .107 .116 He I 2 .056 .051 .051 6678 A° 5 .265 .264 .265 He I 2 .028 .027 .027 3389 A° 5 .070 .067 .070 Ar I 2 .020 .018 .018 4266 A° 5 .051 .049 .049 Ar I 2 .019 .019 .019 4272 A° 5 .034 .034 .034 Ar II 2 .002 .001 .001 4727 A° 5 .010 .010 .010 Ar II 2 .007 .005 .005 4806 A° 5 .009 .009 .009 47 Table 2. Comparison of measured s h i f t s with previous experiments and with t h e o r e t i c a l values LINE FIELD SHIFTS (A°) (kV/cm) This . Experiment Previous Experiments Theoretical He I 19.2 .032 .014 7281 A° 50.5 .116 — .095 He I 19.2 .051 — .039 6678 A° 50.5 .265 — ..273 He I 19.2 .027 .004 LB .004 3889 A° 50.5 .070 .031 LB .027 Ar I 19.2 .018 .004 Ml — 4266 A° 50.5 .049 .029 Ml — Ar I 19.2 .019 .003 Ml — 4272 A° 50.5 .034 .014 Ml — Ar II 19.2 .001 .00 M2 -.055 x 10~ 3 4 727 A° 50.5 .010 .00 M2 -.38 x 10~ 3 Ar II 19.2 .005 .00 M2 -.066 x 10~ 3 4806 A° 50.5 .009 .00 M2 -.46 x 10~ 3 LB Landolt-Boernstein (1950) Ml Minnhagen (1949) M2 Minnhagen (1948) Chapter Four CONCLUSIONS 4.1 Introduction As seen i n Table 2, the s h i f t s obtained i n t h i s experiment agree with those calculated or measured previously only to within about .02 A° for He and .03 A° for Ar. If we assume that the only source of error i s the Poisson noise of the photons incident on the photomultiplier tubes, the accuracy should be .003 A° for the He I l i n e s and .01 A° for the Ar I and Ar II l i n e s . This leads to the conclusion that there must be other sources of error. These sources of error were e a s i l y recognized during the experiment and the improvements required to correct them are quite straightforward. There was no time to carry out these improvements fo r t h i s experiment but they should c e r t a i n l y be made before attempting any further work. In the following sections, the sources of error and improvements which could be made to reduce them are discussed. Concluding remarks are made i n the l a s t section. 48 49 4 . 2 Problems v/ith Stark Source The major source of error can be seen by examining the dark current data. This data shows an increase i n the "apparent" dark current when the Stark voltage was increased and a higher dark current for the Ar-He mixture than f o r s t r a i g h t He gas. The increase i n dark noise was due to r . f . noise produced by low current sparking i n the Stark source. These low current breakdowns were v i s i b l e i n the source and the r . f . noise produced by them could be seen on the 50 n cables from the photomultiplier tube pre-amps (with the a i d of a Tektronix 585 o s c i l l o s c o p e ) . There are two improvements which would eliminate t h i s problem. Both consist of decreasing the background pressure i n the f i e l d plate region thus increasing the breakdown voltage. The obvious way to do t h i s would be to decrease the discharge tube pressure, but t h i s would also decrease the number of ions produced. The goal, then, i s not j u s t to decrease the f i e l d plate region pressure, but to increase the pressure d i f f e r e n t i a l between the discharge tube and the f i e l d plate region. The f i r s t improvement i s the simplest and most obvious: use a faster d i f f u s i o n pump and increase the s i z e of the pumping l i n e to the f i e l d plate region. The second i s more complicated: instead of j u s t 50 one stage of d i f f e r e n t i a l pumping, use two stages. The Stark source apparatus would then consist of three separate sections: the high pressure (a few Torr) hollow cathode discharge region; a second region containing only the s l i t at the botom of the hollow cathode and an ion accelerating lens; and a t h i r d region joined to the second by a small s l i t and containing a second ion lens and the Stark f i e l d plates. The second and t h i r d regions would each have i t s own d i f f u s i o n pump system, r e s u l t i n g i n a much lower pressure i n the f i e l d plate region. If the pressure d i f f e r e n t i a l between the hollow cathode discharge region and the f i e l d plate region were increased by either of the two methods mentioned above, a higher pressure could be tolerated i n the discharge tube. This would lead to the production of more ions i n the hollow cathode and the ion s i g n a l would increase, enabling one to use a narrower monochromator s l i t width. The r e s u l t i n g narrower instrument p r o f i l e would allow one to use a steeper wedge and hence the small ion l i n e s h i f t s would be easier to measure. A second problem with the present Stark source i s that the ion beam gets only about 2 mm into the high f i e l d region before being dragged to the negative plate. Thus at l e a s t half of the beam l i e s i n the non-uniform fringe f i e l d of the plates and cannot be used for measurement. This 51 problem could be p a r t l y overcome by using the two stage d i f f e r e n t i a l pumping system described i n the l a s t section. Because the f i e l d plate region i n t h i s system would have a very low background pressure, the f i n a l accelerating plate voltage and hence the v e l o c i t y with which the ions are shot into the high f i e l d region could be increased. Thus they would t r a v e l further into the region before h i t t i n g the negative plate. 4.3 Problems with Data A c q u i s i t i o n System Another major source of error was the f a c t that, a f t e r running f o r an hour or so, the edges of both e x i t s l i t s and the surface of the wedge became p a r t i a l l y f rosted, thus changing the slope and width of the wedge. In t h i s experiment, the f r o s t was p a r t i a l l y cleared o f f the reference s l i t by stroking a pipe cleaner across i t p e r i o d i c a l l y . However, t h i s i s at best a clumsy solution and c e r t a i n l y does nothing for the wedge and wedge s l i t . This problem could be eliminated through use of a properly designed hermetically sealed dry ice cooled photomultiplier tube housing. These are avail a b l e commercially. Improvements could also be made i n the photomultiplier tubes themselves. The dark current data taken shows that one photomultiplier tube had a much higher dark current than 52 the other and that both had f a i r l y high dark currents even at dry i c e temperatures. There are tubes available with inherently lower dark currents than the EMI 9558B or lower dark currents achieved by r e s t r i c t i n g the photocathode surface. I t i s also possible to have tubes of a p a r t i c u l a r type selected for low dark current. 4.4 Data Handling Improvements Several improvements could be made to the data ' a c q u i s i t i o n system which would increase both i t s accuracy and e f f i c i e n c y . The dark noise fluctuations caused by the r . f . noise from the Stark source were a problem mainly because they were on a time scale of the order of the integ r a t i o n time. The accuracy of the data taken could by improved even i n the face of t h i s r . f . noise by chopping the l i g h t into the monochromator at a few hundred Hertz. The output from the photomultiplier tubes would then alternate between t o t a l s i g n a l and dark current at a rate much faster than the r . f . noise f l u c t u a t i o n s . This s i g n a l could be e a s i l y demodulated to feed the t o t a l s i g n a l and dark current to d i f f e r e n t counters. Once the accuracy of the experiment has been improved, the Stark s h i f t s of large numbers of l i n e s of 53 many elements and ions can be measured using t h i s technique. Without some automation of the data a c q u i s i t i o n , t h i s could be extremely tedious. In the automated system, data could be transferred from the counters to magnetic tape at the push of a button and the i n t e n s i t y changes could be translated into l i n e s h i f t s by a computer using a wedge c a l i b r a t i o n curve previously fed into i t . 4.5 A l t e r n a t i v e Method of Measurement Once the problems of f r o s t on the s l i t s and wedge, spurious and f l u c t u a t i n g dark currents, etc. are solved, the wedge technique should be able to measure s h i f t s to an accuracy of .001 A°. Before proceeding with the improvements d e t a i l e d above, one should ask whether there i s another, perhaps simpler, technique which can a t t a i n or surpass t h i s accuracy. One such technique i s the use of a Fabry-Perot etalon i n series with a monochromator. This method has a few advantages over the wedge technique. Its resolution can be .001 A° or better, a number of l i n e s can be measured at once, and, since a spectroscopic plate i s used as a detector, the method i s immune to the e f f e c t s of r . f . noise from the Stark source. Its major disadvantage i s that i t cuts down the i n t e n s i t y considerably and the i n t e n s i t y which does get 54 through i s spread into numerous rings, leaving a very low flux for the plate to detect. This single disadvantage could render the Fabry-Perot technique useless i n t h i s experiment, but since i t i s simple to set up i t i s c e r t a i n l y worth a t r y i n future work. 4.6 Concluding Remarks After such a gloomy discussion of the myriad sources of error encountered, one might be tempted to question whether i t i s possible, even without these a d d i t i o n a l errors, to measure Stark s h i f t s to an accuracy of .001 A° using t h i s technique. This question should c e r t a i n l y be pursued. I f we assume that improvements made i n the apparatus allow us to close the entrance s l i t to 10 um, we v / i l l have a l i n e width of .05 A°. I t w i l l probably be somewhat bigger than t h i s , so the wedge w i l l have to be .1 A° wide. Take the maximum value for the wedge to reference sig n a l r a t i o , s/S, to be 1 and consider the useable l i n e a r portion of the wedge to be from s/S = .25 to s/S = .85. To measure a change i n wavelength of .001 A° using t h i s wedge we must measure the wedge and reference signals to an accuracy of .5%. To a t t a i n t h i s accuracy with the only source of noise 3 being Poisson noise, we must integrate u n t i l we have 90 x 10 signal counts (with a signal to noise r a t i o of 10). With 55 the s i g n a l strengths encountered i n t h i s experiment, the inte g r a t i o n time would have to be a few minutes. This length of time i s c e r t a i n l y not unreasonable. Thus we can conclude that an experiment incorporating the improvements outlined above should produce s h i f t measurements of the required accuracy. 56 BIBLIOGRAPHY Barnard, G.P., 1953, Modern Mass Spectrometry, London In s t i t u t e of Physics. Bonch-Bruevich, A.M. and V.A. Khodovoi, 1968, Sov. Phys. Uspekhi, 10_, 637. Condon, E.U. and G.H. Shortley, 1951, The Theory of Atomic  Spectra, Cambridge U. Press. Dushman, Saul, 1962, S c i e n t i f i c Foundations of Vacuum  Technology, John Wiley. Foster, J.S. and H. S n e l l , 1937, Proc. Lond. Phys. Soc. A, 162, 351. Howatson, A.W., 1965, An Introduction to Gas Discharges, Pergamon Press, London. Landolt-Boernstein, 1950, Zahlenwerte u. Funktionen, 1:1 Springer Verlag, B e r l i n . Llewallyn-Jones, F. , 1966, The Glov; Discharge and an Introduction to Plasma Physics, Methuen and Co., London. Maissel, L. , 1958, J . Opt. Soc. Amer., 4_8 (11), 853. Mark, H. and R. Wierl, 1929, Z e i t s c h r i f t f. Physik, 53, 528. Meyer, Paul L., 1966, Introductory P r o b a b i l i t y and  S t a t i s t i c a l Applications, Addison-Wesley. Minnhagen, L., 1948, Arkiv f. Mat., Astr. o. F y s i k, 35A(16). Minnhagen, L., 1949, Arkiv f. Fysik, 1(20), 425. Minnhagen, L. , 1963 , Arkiv f. Fysik, 25_, 203. Morris, R.N., 1972, Ph.D. Thesis, University of B.C. 57 Ryde, N. , 1932, Z e i t s c h r i f t f. Physik, 77, 515. Ryde, N. , 1933, Z e i t s c h r i f t f. Physik, 109 , 108. Ryde, N., 1941, Ark. f. Mat., Astr. o. Fysik, 28B(2). Steubing, W. and J.A. Schaeder, 1936, J . Appl. Phys. 37 (6) , 2405. Thornton, R.L., 1935, Proc. Lond. Phys. Soc. A, 150, 259. Verleger, H., 1939, Ergebnisse der Exakten Naturwissenschaften, 18, 99. Von Engel, A., 1965, Ionized Gases, Oxford U. Press. Weston, G.F., Cold Cathode Glow Discharges, I l i f f e 3ooks Ltd., London. Zatzick, M.R., 1970, "Applying D i g i t a l Techniques to Photon Counting", Research and Development, Nov., 1970. Zatzick, M.R., "Photomultiplier Tube Selection and Housing Design f o r Wideband Photon Counting", Applications Note 71021, S.S.R. Instruments Co. 

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