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A low energy Beta-ray spectrometer Brown, Harry 1948

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3 3 7 3 7 <- 6 A LOW ENERGY BETA-RAY. SPECTROMETER by Harry Brown A thesis submitted in partial fulfilment of the requirements for the degree of MASTER OF ARTS in the department of PHYSICS The University of British Columbia April, 1948 ACKNOWLEDGEMENT This research has been carried out under a grant-in-aid from the National Research Council to Dr. K. C. Mann of the Physics Department, University of British Columbia. The author i s the holder of a National Research Council studentship for the year 1947-48, The author gratefully acknowledges the.continued advice and assistance of Dr. K. C. Mann under whose direc-tion the project was established. Further acknowledgement i s made of the assistance given by Dr. J. G. Hooley of the Department of Chemistry, University of British Columbia, in the preparation of the Radium D solution and of the valuable help and advice of Mr. A* Fraser and Mr. Wm. Pye of the Physics Department in the machining and glass blowing connected with the project. t ABSTRACT The c o n s t r u c t i o n and p r e l i m i n a r y t e s t s o f a Beta-ray spectrometer designed t o work a t en e r g i e s below 100 Kev i s d e s c r i b e d . The instrument i s of the s m a l l s e m i - c i r c u l a r f o c u s s i n g type u s i n g f o u r counter p a i r s . The d e s i g n i s f o r reasonably h i g h r e s o l u t i o n and h i g h t r a n s m i s s i o n a n g l e . The magnetic f i e l d i s produced by a combination o f plane c o i l s designed t o m a i n t a i n f i e l d homogeneity t o b e t t e r than 1%. The magnet c u r r e n t i s h e l d to 1 p a r t i n 10,000 by means o f an e l e c t r o n i c c i r c u i t composed o f D.C. and A.C. a m p l i f i c a t i o n stages c o n t r o l l i n g the g r i d s o f a bank o f 6AS7 double t r i o d e s i n s e r i e s w i t h the magnet windings. The counter windows and source backing are c o l l o -d i o n f i l m s o f the order o f 0.1JA . Work on t h i s instrument i s c o n t i n u i n g . TABLE OF CONTENTS Page I. INTRODUCTION * 1 I I . APPARATUS A. The Spectrometer . . . . . . 7 B. The Magnet 13 C. The Current C o n t r o l Apparatus 16 I I I . RESULTS A. T e s t s o f the U n i f o r m i t y o f the Magnetic F i e l d 19 B. T e s t s o f the Current C o n t r o l Apparatus 20 C. T e s t s o f the Spectrometer . 22 IV. CONCLUSION 25 • V . BIBLIOGRAPHY 21 TABLE OF ILLUSTRATIONS FIGURE F a c i n g Page 1 I n t e r n a l C o n s t r u c t i o n o f the Spectrometer 9 2 The F i e l d o f a Plane C i r c u l a r C o i l . . . . 14 3 C o n s t r u c t i o n o f the Magnet . . . . . . . . . 15 4 Schematic Diagram o f Current C o n t r o l C i r c u i t 16 5 Block Diagram o f Current C o n t r o l C i r c u i t . . .. 16 6 V a r i a t i o n o f Magnetic F i e l d i n Spectrometer 20 PLATE Page I The Spectrometer P a r t i a l l y Assembled . . . 28 I I The Magnet and F i e l d T e s t i n g C o i l s . . . . 29 I I I The Current C o n t r o l Panel 30 IV The Magnet and Spectrometer Assembled . . . . 31 V The 6AS7 Regulator C h a s s i s . 32 A LOW ENERGY BETA-RAY SPECTROMETER I.. INTRODUCTION In extending the range of ordinary methods of Beta-ray spectroscopy to low energies a number of d i f f i c u l t i e s arise. The most important of these are as follows: (a) The scattering of electrons from parts of the ap-paratus increases greatly at lower electron energies. From theoretical considerations the scattering, i n the two processes important at low energies, ionization and elastic scattering, varies inversely as the energy to the f i r s t power, at least, and directly as the atomic number of the scattering material. Experimentally the work of Sehonland"1" on the scattering of cathode rays has shown that approximately this holds true. (b) The detection of low energy Beta particles also presents d i f f i c u l t i e s . I f the ordinary Geiger counter i s used the 'window' must be made extremely thin i n order to B.J.F. Schonland, Proc.Roy.Soc. A113. 87, 19-6. 2. allow the passage of the particles into the counter chamber. 2 Even here scattering i s s t i l l the major problem as Crowther has found that Beta particles are strongly scattered by films so thin that absorption i s inappreciable. The method of mounting the radioactive source be-comes very important at low energies. Unless i t i s mounted on a very thin film an excess of low energy electrons i s caused by scattering from the support. The work of •z 4 Flammersf eld-7 and Tyler has shown the importance of this back scattering. (c) At low energies i t becomes d i f f i c u l t to measure the deflecting magnetic f i e l d to the accuracy required. In an ordinary large scale spectrometer fields of less than 30 gauss may be required to focus electrons of 10 Kev energy. Thus calibration to one percent entails measurement of a fraction of a gauss. (d) At these low f i e l d strengths the effect of extra-neous magnetic fields becomes important. In particular, compensation for the earth's magnetic f i e l d i s c r i t i c a l par-ticu l a r l y when i t has a component perpendicular to the focussing f i e l d . For these reasons very few authors have quoted-re-sults for Beta spectra below 100 Kev. Any measurements that have been obtained i n this region"have been the result 2J.A. Crowther, Proc.Roy.Soc. A80, 186, 1908. ^A. Flammersfeld, Zeits.fur.Physik 112, 727, 1939. 4A. W. Tyler, Phys.Rev. jj6, 125, 1939. of special techniques. The work of Backus<5 on the Beta spectrum of Cu^ illustrates this. In investigating this spectrum down to 5 Kev he used an electrostatic focussing spectrometer. His counter windows, and source backing were collodion films about 0*1 micron thick. The small amount of work done below 100 Kev i s due entirely to the experimental d i f f i c u l t i e s involved and not because of any lack of importance. On the contrary i t is essential for the development of consistent nuclear theory that reliable information be available i n the low energy region. The quantum mechanical equation for the energy spec-trum of Beta particles does not differentiate between posi-trons and electrons with the exception of an expression re-lated to the Coulomb f i e l d of the nucleus. This expression i s negligible except at low energies. Even on a classical picture i t i s evident that an electron spectrum should have some intensity even at zero energy while a positron spectrum should drop to zero intensity at an energy equivalent to the potential energy of a positron at the 'surface 1 of the nucleus. Backus^ in his investigation of the electron and positron spectra of Cu^ found a definite deviation from the low energy positron to electron ratio given by nuclear theory. A knowledge of the low energy spectra of many isotopes i s essential to the development of a satisfactory nuclear theory. V. Backus, Phys.Rev. 6_8, £9, 194.5. 4. In addition i t i s highly probable that there are numerous undiscovered Beta groups whose end points are below 100 Kev. Also the energy and intensity of Gamma rays i n this region are uncertain and require further investigation. For these reasons i t was decided to build a small spectrometer expressly to measure Beta spectra below 100 Kev. It could also be used to measure Gamma spectra in this region since the work of Robinson^ on photo effect's of X-rays below 10 Kev shows that the 'photo-electron radiator' technique of Beta-ray spectroscopy can be applied even at very low energies.. The design of the spectrometer was governed by the following factors: (a) In order to use current control apparatus already built for the large spectrometers magnetic focussing was chosen - u t i l i z i n g the. well known semi-circular focussing property of a magnetic f i e l d . (b) An air-cored magnet must be used i f possible so the f i e l d would be a linear function of the current - thus only one point calibration would be required. (c) The spectrometer had to be as small as possible to give a small radius of electron path, to limit the size of the f i e l d producing coils and so that the entire apparatus could be lined up parallel to the earth's magnetic f i e l d in order to eliminate the need for compensating co i l s . A lower limit i s imposed by the minimum size of the Geiger counters. ^H. Robinson, Proc.Roy.Soc. 104, 453, 1923. (d) A l l surfaces in the spectrometer must be made of a substance of low atomic number i n order to reduce the scat-tering of electrons. (e) In order to eliminate the use of heavy lead baffles to prevent gamma ray background the counters should be so arranged that a coincidence technique could be used i f neces-sary . (f) As large as possible a solid angle of the radiation from the source should be utiliz e d so that weak sources could be investigated. (g) As a preliminary figure a resolution of 1% was l a i d down, but the design of the spectrometer was to be such that changes in the baffle system, etc., could readily be made. (h) The technique developed by Ba c k u s f o r thin counter windows was to be used i f possible. As the spectrum of Radium D has been found to con-si s t of Beta's and 'Gamma's of very low energy, this substance was chosen to make preliminary tests of the instrument. Radium D has been investigated by several workers, a l l using rather special techniques. G. von Droste'' using a Wilson cloud chamber and magnetic deflection, together with measurements of path lengths, obtained a series of y_? lines a l l attributable to the action of one Y line of 47.2 Kev energy. Richardson and Leigh-Smith^ distributed Radium D as -*J. Backus, loc. c i t . ^G. von Droste, Zeits.fur.Physik 84, 17, 1933. 8 H.O.W. Richardson & A. Leigh-Smith, Proc.Roy.Soo. l60. 454, 1937. a gas in the form of tetra-methyl lead throughout a cloud chamber. The Beta spectrum was then calculated from electron path lengths and range-energy measurements. Lee and L i b b y 9 using magnetic deflection and aluminum abosrbers found a p r i -mary Beta-ray distribution of very low energy with an end point of 25 .5 ± 1 Kev. Considerable work on the Gamma radia-tion of RaD has been done by Tsien San-Tsiang, F r i l l e y , et. a l . 1 0 , who report the existence of several nuclear Gamma rays of various intensities. By a number of methods, such as crystal analysis, cloud chamber path.lengths, absorption measurements, etc., they show the existence of seven Gamma lines of energies from 7 . 3 Kev to 6 5 Kev. 9 D.D. Lee and W. V. Libby, Phys .-Rev. £5_, 2 5 2 , 1939. 1 0 T s i e n Sah-Tsiang, Comptes Rendus 2 1 6 , 7 6 5 , 194-3* Oxiang Te TchaO, J. Suruque, Tsien San-Tsiang, Comptes Rendus 217, 5 3 5 , 194-3. Tsien San-Tsiang, Comptes Rendus 2 1 0 , 5 0 3 , 1944. M. F r i l l e y , Comptes Rendus 2 1 8 , 50TT 194-4. Tsien San-Tsiang and C. Marty, Comptes Rendus 2 2 0 , 6 8 8 , 1 9 4 5 . Tsien San-Tsiang and C. Marty, Comptes Rendus 2 2 1 , 1 7 7 , 1 9 4 5 . Tsien San-Tsiang, Phys.Rev. 69_, 3 8 , 1 9 4 6 . 7. I I . APPARATUS The apparatus may c o n v e n i e n t l y he d i v i d e d i n t o t hree p a r t s , the spectrometer i t s e l f , the magnet, and the c u r r e n t c o n t r o l apparatus. A. THE SPECTROMETER The d e s i g n of the spectrometer i t s e l f Is governed by requirements ( c ) , ( f ) and (g) above. The b a s i c d e s i g n i s as f o l l o w s : The source i s mounted on a v e r t i c a l t h r e a d i n the c e n t e r o f a c i r c l e . Symmetrically spaced on t h e c i r -cumference of t h i s c i r c l e t h e r e are f o u r Geiger counters. By the a c t i o n o f a magnetic f i e l d p e r p e n d i c u l a r t o the plane 'of- the c i r c l e , e l e c t r o n s emitted by the source t r a v e l i n c i r -c u l a r paths. B a f f l e s are p l a c e d so t h a t o n l y e l e c t r o n s whose paths have diameters equal to the d i s t a n c e from t h e source to the counter window w i l l r e a c h the counter. The e n t i r e apparatus i s e n c l o s e d i n a c y l i n d r i c a l box which i s evacuated. P l a n and s e c t i o n views o f the spectrometer are shown i n F i g . 1. The a c t u a l dimensions "were c a l c u l a t e d i n i n c h e s because o f d i f f i c u l t i e s w i t h machine work and m a t e r i a l s a v a i l a b l e . The e n c l o s i n g box i s a brass c y l i n d e r o f 6_; inches 8. inside diameter with walls i inch thick. . The end plates are of 5/16 inch brass. Soft rubber rings are used as vacuum seals. A one inch copper pipe passes through the center of one end plate (hereafter called the base plate) for connection to the vacuum pump. Through the wall of this pipe there are several small tubes for counter f i l l i n g and evacuating, elec-t r i c a l connections and vacuum gauge. * One inch above the base plate there is mounted a 1/8 inch brass disc to which are fastened the baffle system and the source holder. When a l l four counters are used a U shaped bracket holds the thread on which the course i s de-posited, vertically above the center of this plate. This may possibly lead to d i f f i c u l t y i n some instances because of back-scattering from the thread. An alternative method which has been used in preliminary tests using only one counter i s to mount a thin (.004 inch) brass sheet with a . 0 1 0 inch slot directly over the center. The source i s formed by evaporating a solution of RaD chloride on a thin collodion f i l m so that the source covers an area 1 inch by J inch. The film i s then mounted behind the brass sheet so that electrons passing through the slot are focussed on the counter window. This method eliminates the back-scattering entirely and i n addi-tion gives a much greater intensity of Beta radiation. The distance from source to counter window is 2 . 4 inches so that the electrons travel in paths whose radius is 1 .2 inches ( 3 . 0 5 cm.). The baffles are cut to 1 . 2 1 2 inches radius, 1% greater. I I n making the b a f f l e s , advantage was taken o f the f a c t t h a t each p a i r form approximately a quadrant o f a c i r c l e . For the i n s i d e b a f f l e a . s o l i d c y l i n d e r 2 inches l o n g was turned on the l a t h e to the exact diameter. I t was then cut i n t o q u a r t e r s and the a b s o r b i n g grooves cut i n w i t h a shaper. The o u t s i d e b a f f l e s were made from a hollow c y l i n d e r t u r n e d to the exact i n s i d e diameter and handled the same way. These b a f f l e s are shown i n F i g . 1(a). The counters were made from f o u r b l o c k s o f b r a s s 1-1/2 inches l o n g . Two p a r a l l e l h o l e s 5/8 i n c h e s diameter were d r i l l e d through eaah o f these on c e n t e r s 21/32.inch a p a r t . The b l o c k s were then trimmed down t o a reasonable s i z e w i t h the shaper. P a r t i c u l a r care was taken w i t h the f a c e i n which the window was t o be s e t . I n the c e n t e r l i n e of t h i s f a c e o n l y .030 inches o f metal was l e f t . The i n s i d e s u r f a c e s were then p o l i s h e d w i t h emery paper and crocus c l o t h u n t i l q u i t e smooth. Next a narrow s l o t 3/4- inches l o n g and .035 i n c h e s wide was cut through the f r o n t f a c e o f the f i r s t counter, p a r a l l e l to the a x i s o f the h o l e . T h i s s l o t i s covered w i t h the c o l l o d i o n window. The p r e p a r a t i o n and mounting of these ' windows w i l l be d e s o r i b e d l a t e r . G l a s s caps, as shown i n F i g . 1(b), are then s e a l e d to the ends o f the f i r s t hole and a .005 i n c h tungsten w i r e extended through the c e n t e r . . I f the source being i n v e s t i -gated emits no Gamma rays or o n l y v e r y weak ones, t h i s counter may be mounted i n p o s i t i o n on the base p l a t e and c o n n e c t i o n s 2/3 S c a l e FIGURE 1 . INTERNAL CONSTRUCTION OF THE SPECTROMETER * 10. made t o the f i l l i n g system and s c a l i n g c i r c u i t s . However, i f the source emits s t r o n g Gamma r a y s , a r e c t a n g u l a r h o l e _• i n c h by f i n c h should be cut through the w a l l between the two h o l e s , t h i s s l o t f o r m i n g the window f o r the second counter. Glass cap's a r e then s e a l e d t o i t and the c e n t e r wire i n s e r t e d . The two counters are then connected i n a ' c o i n c i d e n c e ' c i r -c u i t , i . e . a count i s r e g i s t e r e d o n l y i f an e l e c t r o n t r a v e r s e s both c o u n t e r s . T h i s u s u a l l y happens on l y f o r e l e c t r o n s whioh are f o l l o w i n g the c i r c u l a r path through the b a f f l e s . The use o f t h i s c o i n c i d e n c e method on oth e r spectrometers has r e s u l t e d i n a g r e a t l y reduced background count caused by Gamma rays from the source, s c a t t e r e d e l e c t r o n s , and cosmic r a y s . The counters and t h e i r arrangement are shown i n F i g . 1 (c). Because o f the t h i n n e s s o f the c o l l o d i o n f i l m s i t i s necessary t o evacuate the counters and the vacuum en-c l o s u r e s i m u l t a n e o u s l y , as a pr e s s u r e d i f f e r e n t i a l g r e a t e r than 10 cms o f mercury w i l l r u p t u r e the window. A f t e r the e n t i r e system i s evacuated t o 10"^ mm the counters are con-nected t o the f i l l i n g system by means of the stopcock ((d) F i g . 1) and f i l l e d to 5 cms p r e s s u r e w i t h a mixture o f 1/7"alcohol and 6/7 argon. The f i l l i n g system i s standard except t h a t a b a l -l a s t f l a s k o f about 4 l i t e r s c a p a c i t y i s i n c l u d e d as the mix-tu r e o f argon and a l c o h o l s l o w l y d i f f u s e s through the counter window,. I n t h i s way runs o f s e v e r a l hours c o u l d be made 1 1 • without the characteristics of the counter changing appre-ciably. The collodion windows are prepared as follows: Collodion i s diluted with an equal volume of amyl acetate. Two drops of this solution are dropped in rapid succession into a pan of d i s t i l l e d water. The solution spreads in a thin film over a circular area about one foot in diameter of the surface of the water and dries in a few seconds. It can then be cut into sections by sawing a wire through i t with a rapid up and down motion. The sections can be l i f t e d out by means of a rectangular wire frame which is s l i d under a sec-tion of the film and l i f t e d gently so that the film f a l l s on both.sides of the frame. Thus the fil m i n i t s f i n a l state consists of two thicknesses of the original film. In this way a more uniform film, free from pinholes, i s produced. It has been found that when the films dry, they are sometimes marked with straight lines. Care should be taken to select films that are clear as the marks seem to indicate breaks i n one side of the film. The film i s mounted in the following way: The front face of the counter i s covered with a thin coating of vacuum wax (Apiezon Qj so that the color of the brass shows through the wax. A thin sheet of brass .004 inches thick is cut to the size of the counter face and a slot cut i n i t .010 inches wide and % inch long, so that this slot is ap-proximately in the middle of the slot i n the counter wall when the brass sheet i s placed exactly on the f l a t face. . 1 2 . Then one s i d e o f t h i s brass sheet i s covered w i t h a s i m i l a r f i l m o f wax. The counter i s h e l d f i r m l y so t h a t the f l a t f a c e i s h o r i z o n t a l and a f i l m l a i d ..on the wax so t h a t i t breaks around the edges o f the f a c e . The brass sheet i s p l a c e d on the f i l m , wax s i d e down, so t h a t the two s l o t s c o i n c i d e . (A b r i g h t l i g h t s h i n i n g through the s l o t i n the counter i s o f g r e a t a s s i s t a n c e i n a l i n i n g , a l s o l a t e r t o check t h a t no wax flows over the s l o t s . ) The brass sheet can be moved around to l i n e up the s l o t s without damaging the c o l l o d i o n f i l m i f no p r e s s u r e i s a p p l i e d . When the alinement i s completed, the brass sheet i s h e l d f i r m l y i n p l a c e w i t h a b l u n t o b j e c t and a heated g l a s s r o d i s run over the s u r f a c e to melt the wax and weld the two i n s i d e s u r f a c e s t o g e t h e r . Then the edges are covered w i t h a s m a l l amount o f wax. When the wax i s thoroughly c o o l the counter i s t e s t e d w i t h a p r e s s u r e d i f f e r e n t i a l of 7 t o 1 0 cms o f mercury. I t i s then i n s t a l l e d i n the spectrometer w i t h the s l o t a t the f o c u s o f the e l e c t r o n beam. ' When the f o u r counters and the source are i n s t a l l e d the spectrometer i s connected t o the magnet and the vacuum system and i t i s ready f o r o p e r a t i o n . The e n t i r e spectrometer i s made out o f brass w i t h the e x c e p t i o n of the copper t u b i n g and the o u t e r b a f f l e s which were made o f bronze as brass was not a v a i l a b l e i n the c o r r e c t s i z e . I n o r d e r to p r o v i d e a s u r f a c e o f low atomic number and thus reduce the s c a t t e r i n g o f e l e c t r o n s as much 13. as possible a l l exposed surfaces were coated with a t h i n f i l m of vacuum sealing was. B. THE MAGNET The spectrometer requires a uniform magnetic f i e l d over a c y l i n d r i c a l volume" 12 cms. i n diameter and roughly 2 cms. deep. An a r b i t r a r y l i m i t of one peroent was set on the v a r i a t i o n of the f i e l d over t h i s region. F i e l d strengths of 75 to 360 gauss were required to measure spectra from 5 Kev to 100 Kev (Hr 230 to 1100 gauss-cms). Further r e s t r i c t i o n s were imposed by the l i m i t a t i o n s of the current control c i r c u i t . I t was determined by experi-ment that the larger the load resistance the greater the t o t a l power available, but the .less the accuracy of contro l . Thus a compromise had to be made and a resistance of approximately 12 ohms and a current of 10 amperes was chosen i n order to give the maximum f i e l d of 360 gauss and the best possible c o n t r o l . I t i s usual to use a pair of Helmholz c o i l s to pro-vide a uniform magnetic f i e l d over a considerable volume, but i t was f e l t that i n t h i s case, since the uniform f i e l d was required only over a t h i n disc shaped volume, that i t could be provided by a c o i l or c o i l s i n one plane.• For a c i r c u l a r c o i l of radius a the f i e l d H Q at the center i s _LZ__L . where I i s the a . current through the c o i l . Con-14. s i d e r the p o i n t P i n the plane o f the c o i l a t a d i s t a n c e T h i s i s an e l l i p t i c i n t e g r a l and must be o b t a i n e d from t a b l e s . A curve p l o t t i n g Hp / H Q a g a i n s t x/a i s shown i n F i g . 2 . By use o f t h i s curve i t was determined t h a t three c o p l a n a r c o i l s , one o f +14000 ampere t u r n s o f r a d i u s 1 7 . 5 cms, a second o f - 2 7 0 0 ampere t u r n s o f r a d i u s 1 1 cms and a t h i r d o f +120 ampere t u r n s o f r a d i u s 8 . 5 cms would g i v e a f i e l d o f approximately 3o0 gauss a t t h e i r common c e n t e r , w i t h a maxi-mum inhomogeneity o f 0 . 2 8 f . over an a r e a o f r a d i u s 6 cm. (The minus s i g n on the second c o i l i n d i c a t e s t h a t the c u r r e n t i n t h i s c o i l i s i n the o p p o s i t e sense to the c u r r e n t i n the o t h e r two c o i l s . These c a l c u l a t i o n s , however, were f o r i d e a l c o i l s and would not n e c e s s a r i l y h o l d f o r c o i l s o f a f i n i t e c r o s s -s e c t i o n such as i s necessary to c a r r y the c u r r e n t r e q u i r e d . As a maximum c u r r e n t o f 1 0 amperes had been dec i d e d 15. upon, coils of 1400, 270, and 12 turns were needed. A pre-liminary calculation indicated that $14 wire was the smallest that would result in the required resistance of 12 ohms. By t r i a l and error calculations the c o i l arrange-ment shown in Fig. 3 was selected as giving a reasonably uni-form f i e l d . There are two possible causes for the deviation of the f i e l d of this magnet from the f i e l d of the ideal magnet discussed above. The f i r s t i s the f i n i t e width of the coils i n the radial direction, the other i s the f i n i t e thickness of the c o i l s . The f i n i t e width of the coils in the radial direc-tion was taken into account i n calculations by dividing the outer c o i l into 4 annular rings, the next into two annular rings and leaving the inner c o i l as one. For each of these annular rings, by use of the graph Fig. 2, the f i e l d at the center and at r = 1, 2,- 3> 4, 5> 6 cms was calculated as a fraction of the f i e l d H 0 at the center of an ideal c o i l of 14000 ampere turns and 17.5 cms. The resultant f i e l d ob-tained by adding the individual fields algebraically for each value of r was equal to 70 ± 1% o f .H Q . It i s evident because of the symmetry of the magnet that the f i n i t e thickness of the ©oils w i l l have only a negligible effect on the direction of the f i e l d i n the region considered. And further the uniformity of the f i e l d w i l l not be changed over the small central region since i n any plane through this region and perpendicular to the axis of the magnet the turns ratio of the coils i s unchanged. The y.'///////////////77. 1400 270 Turns 12 Turns Turns m 1/4 Scale FIGURE 3. CONSTRUCTION OF THE MaGNET 16. only possible way that the thickness of the coils could i n -fluence the f i e l d would be to change i t s over-all value. Consider a solenoidal c o i l of radius a with n turns per unit length and carrying a current i . The f i e l d at the point P on i t s axis i s given by Hp = 2 7rni (cos 0 2 - cos 0 _ ) where 0 i and 0 2 equal one half of the angles subtended at the point P by the ends of the solenoid. In the worst possible case for the magnet constructed 0 _ = 78°, 0 2 = 1 0 2 ° which gives. Kp = .832 7T n i . The total current around the loop i s 2nia tan 12° and i f this were concentrated in a single turn the f i e l d H at the center would be «* .848 7r n i . Thus H p - * 8 5 2 - 982 The f i n i t e thickness reduces the f i e l d by 1.8%. Therefore the f i e l d given by this magnet i s equal to 69 * 1% of the f i e l d H Q at the center of an ideal c o i l of radius 17.3 cms and of 14000 ampere turns. C. THE CURRENT CONTROL APPARATUS The schematic and block diagrams of the current control apparatus are shown i n Figs. 4 and 5 respectively. The system i s essentially that used by Dr. L. G. E l l i o t t i n FIGURE 4. SCHEMATIC DIAGRAM OF CURRENT CONTROL CIRCUIT I M a g n e t j 500 m f . i i G e n e r a t o r it i R u b i c o n P o t e n t i o m e t e r 3 t a . •tes. C a l i b r a t i o n C e l l C5 |Ga. C a l i b r a t i o n l v a n o m e t e r A . C . A m p l i f i e r 6AS7 B i a s T u b e (6L6) D . C . A m p l i f i e r a n d P h a s e S e n s i t i v e D e t e o t o r 6AS7»S S t a n d a r d B a t t e r y J L F I G U R E 5. B L O C K D I A G R A M O F CURRENT CONTROL C I R C U I T 17. the N a t i o n a l Research C o u n c i l l a b o r a t o r i e s a t Chalk R i v e r , w i t h some m o d i f i c a t i o n s r e q u i r e d by d i f f e r e n c e s o f equipment. There are a c t u a l l y two c o n t r o l c i r c u i t s i n the ap-paratus w i t h d i f f e r e n t frequency ranges. The ' D . C c i r c u i t w i l l handle f r e q u e n c i e s from zero to about 10 c y c l e s per second; the ' A . C c i r c u i t w i l l handle f r e q u e n c i e s from 10 c y c l e s per second to about 1000 c y c l e s per second. F r e -quencies above t h i s can be removed by means o f s u i t a b l e f i l t e r networks. I n the D.C. c o n t r o l c i r c u i t the v o l t a g e developed by the magnet c u r r e n t i n p a s s i n g through a s m a l l r e s i s t o r (.08 ohms) i s balanced a g a i n s t a v o l t a g e from a Rubicon potentiometer which i n t u r n can be c a l i b r a t e d by a standard c e l l . The d i f f e r e n c e i n these two v o l t a g e s i s converted i n t o a 60 c y c l e square wave by means o f a Brown c o n v e r t e r . T h i s i s a m p l i f i e d about 100 db. then r e c t i f i e d by means o f a 'phase s e n s i t i v e d e t e c t o r * and the r e s u l t a n t D.C. v o l t a g e a p p l i e d to the g r i d s of the r e g u l a t o r tubes. The phase s e n s i t i v e d e t e c t o r i s merely a f u l l wave d e t e c t o r b i a s e d by a 60 c y c l e v o l t a g e i n order t o ensure the proper p o l a r i t y i n the D.C. output v o l t a g e . The r e g u l a t o r tubes are t h i r t y - e i g h t 6AS7 twin t r i o d e s on a separate c h a s s i s . The g r i d s , p l a t e s , and cathodes, r e s p e c t i v e l y o f these tubes are connected i n p a r a l l e l w i t h the e x c e p t i o n o f a g r i d 'stopper' _of 1000 ohms i n each i n d i v i d u a l g r i d . The f i l a m e n t s are connected i n s e r i e s , i n two p a r a l l e l banks o f 19 tubes, a c r o s s the 115 v o l t A.C. 18. l i n e s . The t o t a l r a t e d p l a t e c u r r e n t c a p a c i t y o f these tubes i s r o u g h l y 10 amperes hut they appear to work s a t i s f a c t o r i l y up to 18 amperes. The A.C. c o n t r o l c i r c u i t c o n s i s t s of a.normal nega-t i v e feedback l o o p . A.C. f l u c t u a t i o n s a c r o s s the magnet are a m p l i f i e d by a 6AC7, then by a 6L6 and a p p l i e d d i r e c t l y t o the g r i d s o f the 6AS7 Ts i n the proper phase to reduce the f l u c t u a t i o n s . To prevent o s c i l l a t i o n due to phase s h i f t the response o f one stage i s reduced g r e a t l y a t h i g h f r e q u e n c i e s ( g r e a t e r than 1000 c y c l e s per second). F l u c t u a t i o n s o f h i g h e r frequency than t h i s are e l i m i n a t e d to a g r e a t extent by a 500 mf condenser a c r o s s the generator t e r m i n a l s . I n a d d i t i o n i t was found necessary to bypass each t e r m i n a l o f the generator to ground w i t h .1 mf condensers connected d i r e c t l y to the brushes. L a r g e r condensers than t h i s r e -duced the commutator 'hash* even f u r t h e r but r e s u l t e d i n s p o r a d i c o s c i l l a t i o n s of the system. 19. I l l . RESULTS A number o f t e s t s were c a r r i e d out to check the u n i f o r m i t y o f the .magnetic f i e l d and to check the o p e r a t i o n of the c o n t r o l c i r c u i t . I n a d d i t i o n the o p e r a t i o n o f the counters was t e s t e d i n v a r i o u s ways. A. TESTS OF THE UNIFORMITY OF THE MAGNETIC FIELD The u n i f o r m i t y o f the f i e l d was checked e x p e r i -m e n t a l l y i n the f o l l o w i n g way: Two i d e n t i c a l c o i l s o f 100 t u r n s of #28 wire were wound on b a k e l i t e forms so t h a t the i n s i d e diameter was 1 cm and the c o i l windings occupied a square o f 0.5 cm s i d e . The t w o . c o i l s were connected i n .series and to a b a l l i s t i c s galvanometer. With a measured c u r r e n t of 4 amperes through the magnet one c o i l was p l a c e d a t the c e n t e r of the magnet and t h e ' o t h e r a t ' i n f i n i t y 1 . The magnet c i r c u i t was broken and the d e f l e c t i o n of the galvanometer observed. Then the c o i l s were interchanged and the process repeated. The d e f l e c t i o n s d i f f e r e d by l e s s than 0.5f». Then one c o i l was f i x e d at the c e n t e r o f the magnet and the ot h e r , i n o p p o s i t e sense to the f i r s t was p l a c e d a t a p o i n t i n the r e g i o n t o be occupied by the spectrometer w h i l e the magnet c u r r e n t was i n t e r r u p t e d . The galvanometer 20. d e f l e c t i o n s observed were thus p r o p o r t i o n a l to t h e d i f f e r e n c e i n the f i e l d s a t the two p o i n t s occupied by the c o i l s , and by comparing the d e f l e c t i o n t o t h a t due to one c o i l alone the percent v a r i a t i o n o f the f i e l d c o u l d be found. A l a r g e number o f readings were taken f o r v a r i o u s p o s i t i o n s o f the second c o i l * The r e s u l t s are g i v e n by curves A i n F i g . 6. T h i s shows t h a t the maximum h o r i z o n t a l v a r i a t i o n o f the f i e l d was about 2% and t h a t the f i e l d i n c r e a s e d s t e a d i l y as the d i s t a n c e from the c e n t e r i n c r e a s e d . As the i n n e r c o i l o f 12 tu r n s was ve r y c l o s e to the spectrometer and thus had a f a i r l y l a r g e e f f e c t on the f i e l d f o r the l a r g e r v a l u e s o f T a t r i a l was made w i t h t h i s c o i l d i s c o n n e c t e d . The r e s u l t s are shown by the curves B i n ' F i g . 6. The maximum v a r i a t i o n over a c y l i n d r i c a l r e g i o n 6 cms i n r a d i u s and 2 cms t h i c k i s seen .to be 1% which i s the value o r i g i n a l l y s p e c i f i e d . I t was t h e r e f o r e decided to use the c i r c u i t w i t h the i n n e r c o i l d i s c o n n e c t e d f o r the pre s e n t , but i n the f u t u r e f u r t h e r measurements w i l l be conducted w i t h d i f f e r e n t numbers o f tu r n s on the i n n e r c o i l . B. TESTS OF THE CURRENT CONTROL APPARATUS An a m p l i f i e r o f about 90 db. g a i n was connected t o an o s c i l l o s c o p e . Using a v o l t a g e d i v i d e r an A.C. s i g n a l o f .001 v o l t s r.m.s* (.0028 v o l t s peak to peak) was f e d i n t o the a m p l i f i e r and a s i g n a l h e i g h t of 3 inches was observed on the o s c i l l o s c o p e s c r e e n when the g a i n c o n t r o l was s e t a t the 1/2 maximum g a i n p o i n t . . The l i n e a r i t y o f the g a i n c o n t r o l 2 1 . was checked and no c o r r e c t i o n s were necessary t o the d i a l s e t t i n g . Thus a t f u l l g a i n a s i g n a l o f . 0 0 1 v o l t s peak should g i v e a d e f l e c t i o n o f 2 . 2 inches on the screen. The a m p l i f i e r i n p u t was then connected a c r o s s the standard r e s i s t a n c e i n the magnet c i r c u i t . When the c o n t r o l c i r c u i t was i n o p e r a t i o n and magnet c u r r e n t f l o w i n g random p u l s e s o f a maximum h e i g h t o f 1 i n c h were observed. T h i s i s e q u i v a l e n t to a peak v o l t a g e o f .0004 v o l t s . These surges were very narrow, l e s s than 100 micro-seconds, and the frequency was about 100 times per second. No apparent change i n the amplitude o f these surges was n o t i c e d between magnet c u r r e n t s o f 50 m;a. t o 10 amperes so l o n g as c o n t r o l was maintained. At l e s s than 50 m.a. the system s t a r t e d to o s c i l l a t e because of the e f f e c t o f the sm a l l condensers c o n n e c t i n g the generator brushes t o ground. When these were, d i s c o n n e c t e d the system d i d not o s c i l l a t e f o r any v a l u e o f the c u r r e n t , but the v o l t a g e surges mentioned above i n c r e a s e d i n amplitude by a f a c t o r o f 2 or 3 , I t i s evid e n t t h a t these random surges are due to commutator 'hash'. A c u r r e n t o f 50 m.a. develops .004 v o l t s a c r o s s the standard r e s i s t a n c e w h i l e 10 amperes develops . 8 v o l t s . Thus the p u l s e s v a r i e d from 1 p a r t i n 10 to 1 p a r t i n 2 0 0 0 . T h e i r maximum r a t i o f o r c u r r e n t s to be used i n the magnet i s about 1 p a r t i n 5 0 . F u r t h e r i t i s thought t h a t the a c t u a l magnet c u r r e n t does not f o l l o w the form o f the v o l t a g e a c r o s s the standard r e s i s t a n c e s i n c e the magnet has approximately 1 Henry inductance. The h i g h frequency components o f the 22. v o l t a g e are probably s h o r t - c i r c u i t e d a c r o s s the magnet by the i n t e r - t u r n c a p a c i t y . As f a r as D.C. or low frequency f l u c t u a t i o n s are concerned none o f g r e a t e r than .1 inches were observed on the o s c i l l o s c o p e s c r e e n and a t low c u r r e n t s they were not d e t e c -t a b l e . They c e r t a i n l y c o u l d have been observed i f as g r e a t as .05 i n c h e s . Thus the D.C. c o n t r o l i s ac c u r a t e to a t l e a s t 1 p a r t i n 10,000 over the e n t i r e u s a b l e c u r r e n t range. C. TESTS OF THE SPECTROMETER The e n t i r e apparatus was assembled and mounted on • a s m a l l t a b l e so t h a t the magnetic f i e l d o f the c o i l was p a r a l l e l t o the ear t h ' s magnetic f i e l d . Connections were then made to the vacuum pump and to the counter f i l l i n g system. The apparatus was found to be reasonably vacuum t i g h t , but s p e c i a l care was r e q u i r e d i n the p o s i t i o n i n g o f the rubber s e a l i n g r i n g s i n order t o ensure a t i g h t j o i n t . For p r e l i m i n a r y t e s t s only one counter and one s e t of b a f f l e s were i n s t a l l e d as f a i r l y l a r g e source s t r e n g t h s were a v a i l a b l e . A t e s t o f the counter was made w i t h the window completely s e a l e d w i t h a brass p l a t e . A source e m i t t i n g s t r o n g gamma ra y s was used and the counter when f i l l e d appeared t o operate e f f i c i e n t l y , w i t h a p l a t e a u o f approximately 75 v o l t s s t a r t i n g a t about 850 v o l t s . A source o f Radium D was then put i n the s p e c t r o -meter u s i n g the a l t e r n a t i v e method d e s c r i b e d i n I I A. The counter, now w i t h a c o l l o d i o n v/indow i n s t a l l e d , was mounted 23. i n the spectrometer and a l i n e d w i t h the b a f f l e system, and connections made to the hi g h v o l t a g e power supply and to the s c a l i n g c i r c u i t s . A g r e a t d e a l o f d i f f i c u l t y was then encountered. The mixture o f argon and a l c o h o l used f o r f i l l i n g the counters appeared t o l e a k through the window even though i t was a i r -t i g h t as f a r as c o u l d be a s c e r t a i n e d . On s e v e r a l o c c a s i o n s 5 cms o f a i r was allowed t o e n t e r the counter without changing the degree o f vacuum i n the spectrometer, y e t when t h i s was pumped out and the counter f i l l e d w i t h 5 cms of argon and a l c o h o l mixture the vacuum gauge immediately i n -d i c a t e d a l e a k . These l e a k s were so l a r g e t h a t i t was i m p o s s i b l e to m a i n t a i n the 3 cms pr e s s u r e i n the counter even w i t h the b a l l a s t f l a s k a t t a c h e d to the f i l l i n g system. As a r e s u l t when the v o l t a g e was a p p l i e d to the counter, i t went i n t o almost continuous d i s c h a r g e w i t h the p o s s i b l e e x c e p t i o n o f a few s h o r t i n t e r v a l s when i t appeared to be o p e r a t i n g n o r m a l l y . F u r t h e r attempts were c a r r i e d out to make the counter operate s u c c e s s f u l l y . A s l i g h t l y t h i c k e r window was i n s t a l l e d and extreme care taken t o ensure t h a t the p r e s -sure d i f f e r e n t i a l a c r o s s the window was kept t o a very low l e v e l at a l l times. T h i s was done by ' t h r o t t l i n g ' the vacuum pump w i t h a clamp on the rubber hose c o n n e c t i o n . None o f these attempts have as y e t been s u c c e s s f u l and consequently no readings of the Beta-ray spectrum o f Radium D have been taken. I t i s f e l t , however, t h a t suc-c e s s f u l o p e r a t i o n o f the apparatus depends o n l y on the de-velopment o f the proper technique o f d e a l i n g w i t h the c o l l o -d i o n f i l m s . A t the time o f w r i t i n g , counters have been • prepared w i t h a p p a r e n t l y l e a k p r o o f windows and work i s con-t i n u i n g . 25. IV. CONCLUSION While the spectrometer as a u n i t has not yet been t e s t e d s u c c e s s f u l l y , the p r i n c i p l e s i n v o l v e d are sound, and as i t now stands the apparatus should g i v e s a t i s f a c t o r y measurements o f low energy Beta-ray s p e c t r a , once the proper techniques of mounting the c o l l o d i o n windows have been de-veloped and mastered. However, c e r t a i n improvements and m o d i f i c a t i o n s w i l l i n c r e a s e the f l e x i b i l i t y o f the spectrometer w h i l e o t h e r s w i l l make i t e a s i e r o f o p e r a t i o n . A h i g h e r v o l t a g e magnet power supply would enable the range o f the spectrometer to be extended almost to 200 Kev. The l i m i t o f course depends on the temperature r i s e o f the copper i n the magnet but water c o o l i n g c o i l s c o u l d r e a d i l y be a t t a c h e d . I t w i l l probably be necessary to develop a s i m p l e r method o f a t t a c h i n g the c o l l o d i o n windows to the counters i n order t o reduce the danger o f b r e a k i n g the f i l m s w h i l e s e t t i n g up the counter and a t the same time making the t e c h r niques i n v o l v e d l e s s p a i n s t a k i n g and d e l i c a t e . The u n i f o r m i t y o f the magnetic f i e l d can q u i t e r e a d i l y be improved- to almost any d e s i r e d degree of accuracy by changing the t u r n s r a t i o of the c o i l s . I t may a l s o be necessary to clamp the s i d e s o f the magnet tog e t h e r more f i r m l y as a s l i g h t warping developed when the c o i l s were being wound. The development of t h i s spectrometer i s b e i n g con t i n u e d and when i t i s s u c c e s s f u l l y operated i t should prove to be a v a l u a b l e t o o l f o r the i n v e s t i g a t i o n o f low energy Beta- and Gamm-ray s p e c t r a . 27. V. BIBLIOGRAPHY BOOKS H. A. Bethe E. R u t h e r f o r d , J . Chadwick, "Elementary N u c l e a r Theory", John Wiley & Sons, 1947. C. D. E l l i s , " R a d i a t i o n s from R a d i o a c t i v e Substances". REFERENCES 1 . B. J . F. Schonland, 2 . J . A. Crowther, 3. A. Flammersfeld, 4 . . A. W. T y l e r , 5. J". Backus, 6. H. Robinson, 7. G. von Droste, 8. H. 0. W. Richardson & A. Leigh-Smith, 9. D. D. Lee & W. F. L i b b y , 1 0 . T s i e n San-Tsiang, Ouang Te Tchao, J . Suruque, & T s i e n San-Tsiang, T s i e n San-Tsiang, M. F r i l l e y , T s i e n San-Tsiang, & C. Marty, T s i e n San-Tsiang, & C. Marty, T s i e n San-Tsiang, Proc.Roy.Soc. A113. 87, 1 9 2 6 . Proc.Roy.Soc. A 8 0 , 186, 1908. Z e i t s . f u r P h y s i k 1 1 2 , 727, 1939. Phys, Rev. __6_, 1 2 3 , 1 9 3 9 -Phys.Rev. 68, 5 9 , - 1 9 4 3 . Proc.Roy.Soc. 104, 4 5 5 , 1 9 2 3 . Z e i t s . f u r P h y s i k 8_4, 17, 1 9 3 3 . Proc.Roy.Soo. l 6 0 , 4 5 4 , 1 9 3 7 . Phys.Rev. _5_5, 232, 1 9 3 9 . Comptes Rendus 2 1 6 , 7 6 5 , 1 9 4 3 . Comptes.Rendus 2JL7, 535, 1943. Comptes Rendus 21br, 503, 1944. Comptes Rendus 2__T, 505, 1944. Comptes Rendus 2j_0, 688, 1945. Comptes Rendus 221,. 177, 1945. Phys.Rev. 6__, 3&T1946. PLATE I. THE SPECTROMETER PARTIALLY ASSEMBLED PLATS I I . THE MAGNET AND FIELD TESTING COILS PLATE IV. THE MAGNET AND SPECTROMETER ASSEMBLED PLATE Y. THE 6AS7 RECrUI_'LTOR CHASSIS 

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