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The gamma rays of radium and its disintegration products Matthews, Frank Samuel 1948

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THE  GAMMA  RAYS  AND  OF  OF  RADIUM  I T S  D I S I N T E G R A T I O N  P R O D U C T S  by Frank Samuel Mathews  A Thesis Submitted i n P a r t i a l Fulfilment o f the Requirements f o r the degree of MASTER  OF ARTS  i n the Department of PHYSICS  THE  UNIVERSITY  OF  BRITISH  September, 19kS>»  COLUMBIA  ABSTRACT  A short history of gamma ray investigations i s given. Particular reference i s made to the use of beta-ray spectrometers i n these investigations, and a detailed description i s given of the thinlens beta-ray spectrometer and i t s a u x i l i a r y apparatus.  The energies  of the gamma rays of radium and of i t s equiblibrium disintegration products are determined by measuring the momentum of the photoelectrons ejected by these gamma rays from a lead radiator.  These energies agree  well with the values reported by E l l i s and Mann, and also agree with most of the previously unconfirmed values reported by Latyshev.  Evidence i s  given f o r the existence of a gamma ray (dOk Kev.) previously unreported. The energy calculations are based on a calibration using the F l i n e of thorium B (Ho = 1385.6 gauss-cm.)  TABLE OF CONTENTS Page  I.  HISTORICAL BACKGROUND 1 . The Radium Family . . . . . . . . . 2. Gamma Rays 3. Beta Ray Spectra k* Measuring Gamma Ray Energies  II.  EXPERIMENTAL METHOD 1. The Thin Lens Spectrometer. . . . . . . . . . . . . . 2. Spectrometer Alignment 3. Magnet Current Control U. Radioactive Source Arrangement. . . . . . . . . . . . 5. Detector Arrangement • 6 . Measurement of Gamma Ray Energies  11 Ik 15 18 19 21  in.  EXPERIMENTAL RESULTS 1 . Spectrometer Alignment. • 2 . Spectrometer Calibration 3. The Radium Family Gamma Ray Spectrum k. Comparative Results . .  23 25 2$ 27  1 3 k 6  . . . . . . . .  IV.  CONCLUSIONS  30  V.  BIBLIOGRAPHY  32  ILLUSTRATIONS  Figures  Page  1.  The Radium Family  2  2.  The E l e c t r o s t a t i c  3.  The Magnetic Semi-Circular Focussing Spectrometer  8  1;.  The Solenoidal Wound Electron Lens Spectrometer  9  5.  The Thin Lens Spectrometer.  6.  Magnet Current Control (Block Diagram). . . . . . .  16  7.  Magnet Current Control ( C i r c u i t Diagram)  17  8.  Source Arrangement  19  9.  B e l l Type Geiger-Mueller Counter.  20  7  Focussing Spectrometer  . . . . . . . 1 2  10.  Spectrometer Tube Alignment  . . 2 3  11.  E f f e c t of Compensator Current on Peak Shapes  2k  12.  Compensator C o i l E f f i c i e n c y  2k  13.  Radium Family Gamma Ray Spectrum  . . . . 2 6  Table 1.  Comparative Results  28  Thin Lens Spectrometer  13  Plate I.  THE GAMMA RAYS OF RADIUM AND OF ITS DISINTEGRATION PRODUCTS  I. 1.  HISTORICAL BACKGROUND  THE RADIUM FAMILY The presence of radioactivity in uranium ores, f i r s t detected  by Becquerel^in 1896, led Madame Curie(2)to the discovery and isolation of radium in I 8 9 8 . The emitted radiation was found to contain three components: beta rays, separated out by Giesel^)^ and Meyer and von Schweidler^) in 1899j alpha  raySj  detected by Rutherford^) in  1903j  and  gamma rays, a penetrating electromagnetic radiation, first discovered by villard^ ) in 1900 and named by Strutt(7)in 1903. 6  Further investigations showed that this radioactivity was an (1)  H. Becquerel. Comptes Rendus, 1 2 2 , 501. 689. (1896).  (2)  P. Curie, Mme. Curie, and G. Bemont, Comptes Rendus, 1 2 £ ,  (3)  F.O. Giesel, Ann. Phys. Chem.,  (U  S. Meyer, and E. von Schweidler, Phys. Zeits.,  1215,  (I898).  6 9 , 83!+, ( 1 8 9 9 ) .  (5) E. Rutherford, Phil. Mag., 5, 177,  1,  9 0 , (1899).  (1903).  (6)  P. Villard, Comptes Rendus, 13J),  (7)  R.J. Strutt (afterwards Lord Rayleigh.), Proc.Roy.Soc, 7 2 ,  1178,  (1900). 208, (1903).  2. atomic phenomenon, and that the rays were emitted when radium atoms (which are s l i g h t l y unstable) broke up spontaneously to form new elements.  I t was  found that radium i t s e l f emitted only alpha and gamma rays, but that i n the process of expulsion of the alpha p a r t i c l e s , another radioactive element was formed which i n turn decayed into alpha-active or beta-active daughter products, some o f which also emitted gamma rays.  In 1913, the Law of  Radioactive Group Displacement, formulated by R u s s e l l ^ , and i n more d e t a i l by F a j a n s ^ ) and Soddy^ ^, enabled most of the known f a c t s about 0  radium to be correlated, and the complete decay scheme to be worked out, as shown i n Figure 1*  THE  RADIUM  FAMILY.  ®  so ATOMIC WEIGHT  Figure 1*  (8) A. S. R u s s e l l . Chem. News.. 107, W, (1913). (9) K. Fajans, Phys. Z e i t s . , llj., 131, 136, (1913). (10) F. Soddy, - Chem. News., 10J., 97 (1913). - Jahrb. Radioaktivitat, 10, 188, (1913).  3.  2.  GAMMA RAYS In the Law of Radioactive Group displacement, r a d i o a c t i v i t y was  recognized as an actual disintegration o f the nucleus which, i n the alpha ray  case, consisted o f the emission of a helium nucleus, and i n the beta r a y  case of an electron from the nucleus. I t was not u n t i l 1922, however, that gamma rays were proven to originate i n s p e c i f i c a l l y nuclear processes.  In that year,  Ellis^^  attributed r e g u l a r i t i e s i n the energy d i s t r i b u t i o n o f the beta rays to the presence o f quantum radiations from the nucleus, describable i n terms of nuclear energy l e v e l s .  These quanta, o r gamma rays, i n turn ejected  photoelectrons from the electron s h e l l s of the atom, with energies, T, calculated from the E i n s t e i n ^ ) photoelectric equation Tehv -Et, where v  i s the frequency of the gamma photon and Eb i s the binding energy  of the electron i n i t s s h e l l . The reason f o r the presence o f energy l e v e l s , and hence o f excited states within the nucleus remained unexplained u n t i l 1925 when M e i t n e r ^ ) , and E l l i s and Wooster^-^) showed that gamma rays are only emitted from the nucleus, presumably during a nuclear reorganization, as the r e s u l t o f a previous ejection of an alpha or a beta p a r t i c l e .  Finally  i n 1 9 3 2 , E l l i s p r o v e d that the emission o f an alpha p a r t i c l e o r beta (11) C.D. E l l i s , Proc.Roy.Sec, A, 101, 1, (1922). (12) A. E i n s t e i n , Ann. d. Phys., 17_, 1 3 2 , (1905). (13) L. Meitner, Z e i t s . f . Phys., 3jt, 807, (1925). (IU) C.D. E l l i s and W.A. Wooster, Proc.Camb.Phil.Soc, 22, 8kh, (1926). (15)  C.D. E l l i s , Proc.Rpy.S6c, A, 1 3 6 , 3 9 6 , (1932).  p a r t i c l e with l e s s than the normal energy leaves the parent nucleus with an excess energy of e x c i t a t i o n which may be emitted afterwards as one or more quanta o f gamma r a d i a t i o n . Is  In t h i s way the Law of Conservation of Energy it-  extended to the nuclear domain.  In the radium family, the only alpha-active element mhieh also emits gamma rays i s radium i t s e l f .  On the other hand, a l l the beta-  emitters of the f a m i l y except radium C" and radium E emit one o r more gamma rays as shown i n Figure 1 and Table 1.  3.  BETA RAY SPECTRA E a r l y investigations o f the energies of beta rays from a  radioactive nucleus revealed a smooth d i s t r i b u t i o n of energies from zero up to a d e f i n i t e maximum which was c h a r a c t e r i s t i c of the element considered.  Superimposed upon t h i s continuous d i s t r i b u t i o n there were  u s u a l l y a number o f sharp peaks which a t f i r s t were thought t o indicate groups of monokinetic electrons emitted from the n u c l e i i . Rutherford and R o b i n s o n ( l ^ g h ^ e d  In 1913,  ^h % these monokinetic groups were, i n a  f a c t , a secondary e f f e c t o r i g i n a t i n g i n the electron s h e l l s , and f i n a l l y i n 1922, E l l i s ^ l - ' - ) , as previously mentioned, gave a f a i r l y complete explanation f o r the whole complex beta d i s t r i b u t i o n . * In the case o f beta a c t i v i t y t h i s statement only holds true i f the emission of a neutrino i s postulated. * * An element i s s a i d to emit gamma rays i f i t produces an excited daughter element which emits one or more gamma quanta i n decaying to the ground state.  (16) E. Rutherford and H.R. Robinson, P h i l . Mag., 26, 717, (1913)  Fill i s showed that i n beta ray disintegrations there are only two main phenomena, the emission o f the actual d i s i n t e g r a t i o n electrons from the nucleus (with a smooth energy d i s t r i b u t i o n ) , and the consequent emission of one o r more gamma photons from the excited daughter nucleus. In addition, however, the beta ray spectrum i s influenced by several secondary e f f e c t s associated with gamma emission.  Chief among them are  the following: (a) The r e l a t i v e l y frequent conversion of the gamma rays i n the extranuclear electrons of the same atom, and the consequent emission of photoelectrons or "conversion electrons" of c h a r a c t e r i s t i c energy T  =  hv - E . D  This phenomenon occurs even i n the thinnest lamina and i s  most prominent i n materials o f high atomic number. (b) Pair production^-'-?) ( f o r gamma ray energies above 1.02 Mev.). A gamma photon may be annihilated i n i t s interaction with matter, and produce an electron positron p a i r with t o t a l combined T  energy  h\> - 2 m c . 2  =  Q  The cross section f o r p a i r production increases with gamma ray energy and also with atomic number. (c) Comp ton scattering.  In 1923, Compton ^) showed that i n v  their passage through matter, gamma photons e j e c t f r e e or l o o s e l y bound electrons from the material, and a t the same time lose a p a r t of t h e i r own energy.  (This process i s completely separate from the photoelectric e f f e c t  i n which o r b i t a l electrons are ejected a t the expense o f the whole quantum). The Compton electrons are ejected most r e a d i l y by low energy photons and have an energy d i s t r i b u t i o n from zero up to an energy approaching that o f (17) C D . Anderson, Phys. Rev., [£, k9k, (1933). (18) A.H. Compton, Phys. Rev., 21, U82, (1923).  the  incident photon.  Compton scattering i s most important i n elements o f  low atomic number since the Compton scattering cross section increases l e s s r a p i d l y with absorber atomic number than does the photoelectric cross section.  k.  MEASURING GAMMA. RAY ENERGIES Energy determinations may be made on the gamma photons them-  selves, or t h e i r energies may be deduced by a study of the photoelectrons, electron positron p a i r s , or Compton electrons which they e j e c t . In the case o f low energy r a d i a t i o n s , most o f the investigators have used t y p i c a l X-ray methods.  A l v a r e z ^ 9 ) a p p l i e d the methods o f  " c r i t i c a l absorption l i m i t s " and the " t r a n s i t i o n e f f e c t " , and Abelson( °), 2  (21) Tsien'  f  (22) and F r i l l e y ^ 'made frequency, and hence energy determinations  using the bent c r y s t a l spectrograph.  In t h i s way Tsien discovered s i x  gamma rays o f radium D i n the range 7-50 Kev., and F r i l l e y discovered twenty-two gamma rays of radium B and C i n the range 50-770 Kev, In the study o f gamma rays of higher energies, almost a l l the investigations have centred around an e l e c t r o s t a t i c or magnetic analysis of the photoelectron, electron positron p a i r , or Compton electron o energies using some type of beta ray spectrometer.  The e a r l i e s t o f such  spectrometers was one introduced by Baeyer and Hahn( 3) i n 1 9 1 0 . 2  Electrons  from a s l i t source were deflected i n a magnetic f i e l d , the degree o f t h e i r deflection being a measure o f t h e i r energy. ( 1 9 ) L.Alvarez, Phys.Rev., ^ , 1*86, ( 1 9 3 8 ) . ( 2 0 ) P.H.Abelson, Phys.Rev., 5 6 , 7 5 3 , ( 1 9 3 9 ) . ( 2 1 ) S.T.Tsien, Phys.Rev., 6 £ , 3 8 , (1?1|6). (22) A . R . F r i l l e y , "These", P a r i s ,  (1928).  (23) H.Baeyer and O.Hahn, Physik., 1 1 , U 8 8 ,  (1910).  At the present time four main types of spectrometer are i n use: (a) The E l e c t r o s t a t i c Focussing Spectrometer shown i n Figure 2 . Based on a -theory developed by Hughes and Rojansky(2U)in 192$, successfully by Backus Cu^k.  i n 19k5  i t was  used  to measure the low energy gamma rays of  i t uses a r a d i a l , inverse f i r s t power, e l e c t r o s t a t i c f i e l d to  refocus a diverging bundle of electrons of the correct energy a f t e r they have been deviated through an angle of 127° 17'•  It i s particularly  useful with weak sources, since a f a i r l y large' s o l i d angle i s subtended between the source s l i t and the d e f l e c t i n g p l a t e s , but i t i s r e s t r i c t e d to use with low energy p a r t i e l e s because of the t e c h n i c a l d i f f i c u l t i e s involved i n producing the stronger e l e c t r o s t a t i c f i e l d needed to focus the more energetic p a r t i c l e s .  Figure 2 .  (2U) A.LW Hughes and V. Rojansky, Phys.Rev., _ t , 2b]*, (1925) (25) J. Backus, Phys.Rev., 6 8 , 5 9 ,  (19U5).  8. (b) The Magnetic Semi-circular Focussing Spectrometer shown i n Figure 3*  This instrument, developed by D a n y s z ^ ^ i n 1912,  and sub-  sequently improved by Robinson and R u t h e r f o r d ^ ^ a n d many others, i s s i m i l a r to the Baeyer and Hahn spectrometer of 1910 but possesses a f a r higher e f f i c i e n c y . the  Baeyer and Hahn depended on a narrow s l i t to give them  necessary r e s o l u t i o n , t u t i n so doing reduced the e f f i c i e n c y to a  point where only intense sources could be used.  Danysz increased the  e f f i c i e n c y tremendously by i n s t a l l i n g a f a r wider s l i t , but maintained the resolving power by focussing the electron beam.  A homogeneous magnetic  f i e l d i s applied perpendicular to the plane of the f i g u r e .  Electrons  possessing equal v e l o c i t i e s describe c i r c l e s of equal diameter, and i t can be seen from the diagram that, even with a r e l a t i v e l y wide s l i t , the p a r t i c l e s converge to an approximate focus a t the plate or Geiger tube.  Figure 3 .  (26) J.Danysz, Le Radium, £, 1,  (1912); 10, ii, (1913).  (2?) H.Robinson and E.Rutherford, Phil.Mag., 2 6 , 717,  (1913).  9. Using t h i s type o f instrument and measuring i n t e r n a l  conversion  and photoelectron l i n e energies, E l l i s and associates( ^)reported f i f t y 2  four gamma rays o f radium, and radium B, C, and D. With the same type o f spectrometer and measuring the energies of positrons formed by p a i r production i n lead, Alichanov and Latyshev^ ^ r e p o r t e d twelve gamma rays of radium C with energies greater than 1 . 0 2 Mev. (c) The Solenoidal Wound Electron Lens Spectrometer,  first  suggested by Kapitza, and constructed and used by Tricker(30) i n I92I4. i s  shown i n Figure U.  A solenoidal winding, surrounding the whole length  of the evacuated cylinder, serves as an electron lens, and focusses electrons of any desired energy on the Geiger tube detector.  F a i r l y weak  radioactive sources can be investigated with i t ••-•since, i n e f f e c t , the Geiger tube subtends a large s o l i d angle at the source.  I t i s more  e f f i c i e n t than type (b) but has a lower resolving power.  Figure k» (28) C D . CD. CD. CD.  E l l i s , Proc.Camb.Phil.Soc, 2 1 , 1 2 5 , ( 1 9 2 2 ) . E l l i s and H . W . B . Skinner, P r o c R o y . S o c , 105A. 1 6 5 , ( 1 9 2 U ) . E l l i s and W.A. Wooster, P r o c R o y . S o c , lHjA"7~276, U 9 2 7 ) . E l l i s and F.W. Aston, P r o c R o y . S o c , 129A, 180, (1930)  (29) A.I.Alichanov and G.D.Latyshev, CR.Acad.Sci., (U.R.S.S.), 2 0 , 1 1 3 , (1938). (30) R.A.Tricker, Proc.Camb.Phil.Soc, 2 2 , U5U, ( 1 9 2 U ) .  10. (d) The Thin Lens Spectrometer shown i n Figure 5 and Plate I . It was f i r s t suggested by K L e m p e r e r ^ ^ i n  1935 and developed to i t s  present state by Deutsch, E l l i o t t , and E v a n s ^ 2 ) .  A spectrometer o f t h i s  type, designed by E l l i o t t was used i n the present study, and i s described i n d e t a i l i n the following section.  I t has a higher e f f i c i e n c y than  type ( a ) , (b), or ( c ) , but a s l i g h t l y lower resolving power. The work with t h i s spectrometer was undertaken i n an attempt to correlate the findings o f E l l i s and Wooster, and Alichanov and Latyshev previously mentioned.  Similar work with t h i s spectrometer was undertaken  i n 19k7 by Mann and Ozeroff using a 10 m i l l i c u r i e source, but i t was deemed advisable to repeat the i n v e s t i g a t i o n using a 500 m i l l i c u r i e source and a more sensitive magnetic f i e l d control.  (3U  O.KLemperer, Phil^Mag., 2 0 , 5h5,  U935).  (32) M.Deutsch, L . G . E l l i o t t , and R.D.Evans, Rev.Sci.Instr., l£, 1 7 8 , (19kh)  11.  II. EXPERIMENTAL METHOD 1. THE THIN LENS SPECTROMETER This instrument consists essentially of an evacuated brass tube 8 inches in diameter and hQ inches long, surrounded at its center by a short water cooled magnet coil of number 10 wire, wound in four sections. The system is evacuated to a pressure of 10"^ m.m. Hg. using a Cenco Hypervac pump and a metal, water cooled, 20 liters/second o i l diffusion pump. Pressures are indicated on a Pirani gauge. Inside the tube are five lead baffles (see Figure 5.).  Baffles A, D, and E mask the counter  from any scattered radiations and hence reduce the normal background count, baffle C prevents gamma radiations of any energy from passing directly from source to counter, and baffle B defines a hollow cone of electrons emitted from the source into the field of the magnet. The spectrometer analyses an electron spectrum using the "chromatic aberration" of a short magnetic electron lens. For a given coil current, electrons of one particular energy in the hollow cone defined by baffle A are focussed by the field of the lens coil on the window of the Geiger counter. Electrons of other energies are focussed at other points on or near the axis of the tube and strike the tube wall or the baffles D and E. Since the focussed electrons always traverse a fixed path, and since a particle can only travel in a curved path of radius o in a magnetic field H i f i t has a momentum mv determined by the formula mv - Hep e being the electron's charge, i t follows that the momentum of -the focussed  Figure 5.  lit. electrons i s always d i r e c t l y proportional to the focussing magnetic f i e l d H.  Consequently, since the magnet c o i l contains no i r o n , the momentum of  the focussed electrons i s proportional to the c o i l current i t s e l f . The current i through the n turns of the focussing c o i l and the momentum o f the focussed electrons are related by the formula: f  a,  ke mv c ni.  where k i s a constant depending on the s i z e and shape of the c o i l , and f i s the f o c a l length, r e l a t e d to u the source distance from the center of the lens, and v the counter distance from the center of the lens by the thin lens formula:  '  1 = 1 +1  f  2.  u  v.  SPECTROMETER ALIGNMENT Any perturbations or abnormal inhomogeneities  i n the focussing  f i e l d cause defocussing of the electron beam over i t s long path. Consequently, c a r e f u l precautions had to be taken to ensure that the axis of the spectrometer tube was symmetrically placed with respect to the focussing f i e l d and that no extraneous magnetic f i e l d s were present. (a) The spectrometer tube i s provided with adjusting screws f o r moving i t r e l a t i v e to the magnet c o i l .  Each end of the tube was moved, i n  turn, h o r i z o n t a l l y and v e r t i c a l l y and set at the p o s i t i o n which gave a maximum beta p a r t i c l e transmission f o r a given c o i l current. (b) The perturbing e f f e c t of the h o r i z o n t a l component of the earth's magnetic f i e l d was eliminated by choosing the area i n the laboratory where the earth's f i e l d was most constant, p l a c i n g the spectrometer there, and a l i g n i n g the axis of the spectrometer tube with the  15. d i r e c t i o n of this h o r i z o n t a l component. (c) The spectrometer i s provided with two rectangular c o i l s , mounted h o r i z o n t a l l y above and below the spectrometer tube, and connected Current from a 250 v o l t D.C.  as Helmholtz c o i l s .  source was  supplied to  the c o i l s i n such a way as to compensate f o r the perturbing e f f e c t of the v e r t i c a l component of the earth's magnetic f i e l d . the adjustment of the compensator current was  The f i n a l c r i t e r i o n f o r  the shape of the photo-  electron peaks as measured on the spectrometer.  The current was  adjusted  to give maximum peak height, minimum peak width, and minimum d i s t o r t i o n .  3.  MAGNET CURRENT CONTROL Since the photoelectron peaks occupy a very small momentum  i n t e r v a l , i t i s necessary to hold the magnetic f i e l d steady to a t l e a s t .1 per cent when any accurate i n v e s t i g a t i o n of peak shapes i s made. The control apparatus f o r t h i s purpose i s shown i n block diagram i n Figure 6 and schematically i n Figure 7.  The system.is e s s e n t i a l l y that  used by Dr. L.G. E l l i o t t i n the National Research Council laboratories a t Chalk River, with a few modifications to allow the use of a grounded power supply. The D.C.  amplifier shown i n Figure 7 accepts a l l current  fluctuations with frequencies from zero up to about 10 cycles per second, the A.C.  amplifier i s sensitive to f l u c t u a t i o n frequencies from 10 cycles  per second up to about 1000  cycles per second, and f l u c t u a t i o n s more rapid  than 1000 per second are shorted out by the i+oOO microfarad condenser across the magnet c o i l . In the D.C.  control c i r c u i t the difference between a standard  reference voltage obtained from a standardized Rubicon potentiometer,  and  4 6 0 0 MF.  + A.C. AMPLIFIER. •  6AS7.s  5LS BIAS 220  VOLT  TUBE.  FOR  SUPPLY.  D.C. AMPLIFIER PHASE  AND  6AS7  SENSITIVE  DETECTOR.  RUBICON POTENTIOMETER. STANDARD CALIBRATION CELL.  CALIBRATION GALVANOMETER  RESISTANCE.  STANDARD CELL.  Figure 6  18. the control c i r c u i t voltage obtained across the .08 resistance i s converted into a 60 converter.  ohm manganin standard  cycle square wave by means of a Brown  This "error s i g n a l " i s amplified about 100  means of the "phase s e n s i t i v e detector",  db.,  r e c t i f i e d by  (a f u l l wave detector biased by a  60 cycle sine wave voltage i n order to ensure the correct p o l a r i t y of  D.C.  output voltage) and applied to the grids of the regulator tubes. The A.C.  control c i r c u i t i s merely a negative feedback loop.  fluctuations across the magnet are amplified by the 6AC7 and the  A.C.  and applied d i r e c t l y to the grids of the regulator tubes.  One  6L6,  stage of  this amplifier i s made i n s e n s i t i v e to high frequency fluctuations (over 1000  cycles per second), i n order to eliminate any o s c i l l a t i o n s i n the  c i r c u i t which might be caused by a phase s h i f t of the f l u c t u a t i o n s i g n a l . The regulator tubes are 38 6AS7 twin triodes connected i n p a r a l l e l , each plate having i t s own each g r i d i t s own  1000  ohm  100  ohm  'grid stopper'.  i s 10 amperes, but currents of 15  1  equalizing r e s i s t a n c e , 1  The t o t a l rated p l a t e  Tests with an oscilloscope  indicate that the control i s accurate to a t l e a s t .01  k.  current  amperes can be drawn f o r f a i r l y long  periods without any serious consequences.  current range 0-15  and  per cent over the  amperes.  RADIOACTIVE SOURCE ARRANGEMENT The experimental arrangement of the 500  shown i n Figure 8. according to the  The thickness of the aluminum capsule i s such that, Feather^^rule:  R(gms/cm ) = .5U3 2  (33)  m i l l i c u r i e source i s  N.Feather, Proc.Camb.Phil.Soc.,  E (Mev.) -  0.16  3ji, 599,. (1938).  19. even the most energetic primary beta p a r t i c l e or i n t e r n a l conversion electron i s absorbed.  At the same time' however, gamma rays of each  p a r t i c u l a r energy give r i s e to a continuous energy d i s t r i b u t i o n of Compton electrons i n the aluminum, and to photoelectrons of discrete energies i n the lead radiator, so that p assing into the spectrometer tube there i s a stream o f electrons with a very complex energy d i s t r i b u t i o n .  Figure 8. The lead radiator i s 3 millimeters i n diameter, and has a surface density o f 50 milligrams per square centimeter.  This density i s  an optimum value, and represents a compromise between a higher value, which would give better photoelectron peak intensity,and a lower value, which would give sharper photoelectron peaks.  5.  DETECTOR ARRANGEMENT The focussed electrons are detected by a b e l l type Geiger-  Mueller counter shown i n Figure 9. copper tube.  The cathode i s a 0.75 inch diameter  The anode i s a .005 inch tungsten wire on the end of which  i s a small glass bead.  The window i s of mica and i s sealed to the counter  20. with a cement made from equal parts of beeswax and r e s i n .  The mica has a  surface density of 2 . 9 milligrams per square centimeter and i s transparent to beta p a r t i c l e s with energies greater than 1 0 0 Kev.  A brass disc with a  k millimeter aperture masks the counter window, and improves the spectrometer resolving power  by keeping unfocussed electrons out of the  counter.  Figure 9 . The counter i s f i l l e d with a Trost mixture of 0 . 7 c m . (Hg) of ethyl a l c o h o l vapour and 9 . 3 c.m. of argon.  The plateau i f 1 3 0 v o l t s long,  commencing at 10k0 v o l t s , and has a gradient of 0 . U per cent per v o l t . A l l counts i n the succeeding investigation were made with a counter p o t e n t i a l of 1 0 6 0 v o l t s . The counter p o t e n t i a l i s supplied from a high voltage battery pack which has an output that can be varied from 63O v o l t s to 12lj0 v o l t s *  The resolving power of a spectrometer i s defined as the momentum i n t e r v a l of the focussed electrons as a percentage o f t h e i r average momentum*  21. i n 22§ v o l t steps.  The current drain through the Geiger tube during  discharge i s very low and tests with an e l e c t r o s t a t i c voltmeter show that the battery output i s extremely stable.  This counter supply voltage  s t a b i l i t y i s an absolute necessity, since fluctuations i n counter voltage cause changes i n the counting rate l e v e l and thus d i s t o r t the r e s u l t s . The output pulse from the G-M counter i s f e d into a twin triode, cathode-coupled preamplifier, and v i a a grounded g r i d output i n t o a scale of 61i scaling c i r c u i t and a mechanical r e g i s t e r .  Incorporated i n the scaler  i s a "pulse size discriminator" which determines  the minimum pulse size  which can cause a count.  Since the discriminator i s very sensitive to l i n e  voltage f l u c t u a t i o n s , power f o r the s c a l i n g c i r c u i t i s obtained from a Sola constant voltage transformer.  3h t h i s way, changes i n the counting  rate l e v e l due to a s h i f t i n discriminator bias are eliminated.  6.  MEASUREMENT OF GAMMA RAY ENERGIES Projected i n t o the spectrometer tube from the source and radiator  Is a stream of electrons with a complex energy d i s t r i b u t i o n .  The focussing  f i e l d current i s varied and the momentum spectrum plotted, f i r s t with the lead radiator i n place, and then with i t removed.  The f i r s t p l o t shows a  series o f photoelectron peaks superimposed upon a continuous Compton background, and the second p l o t shows only the Compton background. The difference between these two curves thus shows the momentum d i s t r i b u t i o n of the photoelectrons alone.  The energies o f the photoelectrons, and  consequently the energies of the o r i g i n a l gamma rays themselves can then be calculated. As was previously mentioned, the momentum of the focussed electrons i s proportional to the f i e l d c o i l current.  The current required  22. to focus photoelectrons of a known momentum i s therefore determined, and this one-point calibration serves for the whole spectrum.  Once the  photoelectron" momentum i s known, the photoelectron energy can easily be calculated from the formula: Hy •  /y/T(T*  1.02)  where  = mv i s the electron momentum i n gauss-cm. and T i s the kinetic e energy i n Mev. The gamma ray energy and the associated photoelectron energy are related by the formula: h»  =  T 4- E  D  where hv i s the gamma ray energy and E^-, i s the binding energy of the photoelectron i n i t s particular s h e l l .  The binding energies for the K, L,  M, and N shells of the lead r a d i a t o r a r e as follows: E^ E  87.6 _ = 15.8 =  K  bL E E  b M  Ml  =  Kev. Kev.-  3.85 Kev. - 0.89 Kev.  (3U) J«M. Cork, Radioactivity and Nuclear Physics, l o c . c i t . , 301.  23.  III.  1.  EXPERIMENTAL  RESULTS  SPECTROMETER ALIGNMENT (a) Adjustment of the spectrometer tube, r e l a t i v e to the magnet  c o i l i s shown i n Figure 10.  Counting rates, accurate to within 1.5 per  cent are plotted f o r various tube positions recorded r e l a t i v e to an a r b i t r a r y i n i t i a l setting.  The f i n a l p o s i t i o n of both source and counter,  end i s indicated by the broad arrow.  Figure 10.  (b) The effectiveness of the earth's f i e l d compensator i s shown i n Figures 11 and 12.  Figure 11 shows how variations i n compensator  current a f f e c t the p l o t t e d shape and i n t e n s i t y of the 0.162 Mev. K l i n e of radium.  The optimum current setting was taken as 1050 milliamperes.  2U.  0.40  1050  UA  1000  MA  COMPENSATOR CURRENT  0.45  POTENTIOMETER SETTING.  Figure  11.  Figure 12 shows the degree of earth's f i e l d compensation at various points along the spectrometer tube axis when the optimum compensator current i s flowing.  The incomplete compensation near the ends  of the spectrometer tube does not r e s u l t in' any serious defocussing.  The  focussing f i e l d perturbation at the source end can be neglected since i t only a f f e c t s unfocussed electrons, and the "counter end" perturbation i s not serious since the electrons t r a v e l only a short distance a f t e r being perturbed, and are therefore deviated only s l i g h t l y from t h e i r intended path.  ORIGINAL  VERTICAL FIELD  H  VERTICAL  FIELD  AFTER COMPENSATION.  MAGNET POSITION.  DISTANCE  ALONG  SPECTROMETER .  Figure 1 2 .  25. 2.  SPECTROMETER CALIBRATION In the determination of the c o i l current required to focus  l i n e of thorium B ( Ho  =  1385.6 gauss-cm.)^  was not a v a i l a b l e , the spectrometer was  .  Since a thorium B source  calibrated against the 1.77  Mev.  l i n e of radium C, which i n turn was c a l i b r a t e d against the F l i n e of thorium by Mann and Ozeroff ^ 6 ) ^  2$k7»  The 1.77  Mev. l i n e was chosen  because i t has the highest i n t e n s i t y of a l l the radium l i n e s , and because i t i s the radium l i n e on whose value E l l i s , Alichanov, and Mann agree most closely. 7116.2  The potentiometer reading which corresponds to the Ho value of gauss-cms f o r the 1.77  Mev. l i n e was found to be 0 . 7 7 i i v o l t s .  t h i s , by d i r e c t proportion, a l l the other Ho values are  3.  From  determined.  THE RADIUM FAMILY GAMMA RAY SPECTRUM Figure 13 shows the graph of the photoelectron peaks and Compton  background over the momentum range O-96OO gauss-cm.  The momentum scale i s  logarithmic so that the momentum i n t e r v a l at any point i s a constant f r a c t i o n of the t o t a l momentum a t that p o i n t .  Each point on the composite  curve and on the Compton background curve represents an average t o t a l count of at l e a s t  "U0,000.  The s t a t i s t i c a l accuracy of each of these curves i s  therefore * \ per cent.  The s t a t i s t i c a l accuracy a of the difference  curve i s given by the formula:  (35)  C . D . E l l i s . P r o c R o y . S o c , 138. 318. K.C.Wang, Z e i t s . f . Phys., 8 7 , 3 3 , 6  (36)  K.C.Mann and M.J.Ozeroff,  (1932). (193U).  "Thesis", U.B.C, ( l ? U 7 ) .  •  - -  27.  where b and c are the s t a t i s t i c a l errors i n the composite curve and the Compton background curve.  The photoelectron peaks i n the difference curve  therefore have a s t a t i s t i c a l accuracy of  0.7 per cent.  The gamma ray energies shown are calculated on the assumption that a l l the peaks are due to photoelectrons ejected from the K s h e l l of the lead atom ( E ^ 87.6 Kev.). K  I t w i l l be seen l a t e r that i n the case of  the 650 Kev. and 688 Kev. peaks t h i s assumption may not be j u s t i f i e d . The bracketed gamma ray energy values i n Figure 13 are values which are considered doubtful or u n r e l i a b l e because of the poor shape or low s t a t i s t i c a l weight of the plotted photoelectron peaks.  They are  therefore not included i n the following discussion.  k.  COMPARATIVE RESULTS Table I shows a comparison between the values found by previous  investigators and those found i n t h i s present study. of the l i n e s are also included.  Relative i n t e n s i t i e s  The i n t e n s i t i e s measured i n t h i s present  discussion have been corrected f o r the decrease i n cross-section f o r photoelectron production with increasing gamma ray energy, using recently (37)  p u b l i s h e d ' ' c r o s s - s e c t i o n curves. w  U37J C D . C o r y e l l , M.Deutsch, R.D.Evans e t a l , "The Science and Engineering of Nuclear Power," (Addison-Wesley), p. UO.  TABLE 1 E l l i s and associates Gamma-ray Energy  Constantinov & Latyshev(38) Gamma^ray Energy  Alichanov and Latyshev Gamma-ray Relative Energy Intensity  Mann and Ozeroff Relative Gamma-ray Intensity Energy  Mann and Mathews' Gamma-ray Relative Energy Intensity  -OL72  .0536 .0589  M89  .197 .205  .237  .21*5  .260 .275 .297 .332 .35U .389 .U29  11 *  .295  .314;  28  .359  .U28  6  .U78  s  3ei  .391  2.2 11  .503 .612 .773  .606  M  .933  1.13  1.2U8  1.390 1.U26  .598 .768  .766 1.12 1.23U  1.370 l.l+lli  1.10 1.21 1.29  18  1.39  h9  1.52 1.62  23  29  .209 .250  .289  .UU8  • U71  0.6**  55 11 78  1.11 1.22  33  I.I4O  22  .U28  .1*98  .518 .608 .779 .80I4  .91*0  1.11  1.2U 1.33  9.1  13 25 76 36 30 IU  5o 21  3.7  1.U1  18  1.53  2U  22  (continued on Page 29)  E l l i s arid associates Gamma-ray Energy  -  TABLE 1 (cont'd) Cons tantinov ' . Alichanov and - - . Mann and & Latyshev(38) Latyshev Ozeroff Gamma-ray Gamma-ray Relative Gamma-ray Relative Energy Intensity Energy Energy Intensity 1.69  1.778  2.219 2.5  1.761 2.200  1.75 1.82 2.09  2.20 2.1*2  :i7  100 17 15  ia 21  ,  1.77  100  2.19  22  (2.U)  -  Mann and Mathews Gamma-ray Relative Energy Intensity  1.68 1.77 1.87 2.09  '  2.2k  2.1*3 (2.8)  6 100 31 2k  20 18 —  Not corrected f o r photoelectric cross-section.  (38)  A.A.Constantinov and G.D.Latyshev, J.Phys.U.S.S.R., 5, 21*9, ( 1 9 U U .  ~  £  30.  IV.  CONCLUSIONS  The comparative chart i n Table I shows twenty-three gamma rays (energies between .209 Mev. and 2.J4.3 Mev.)  found i n t h i s investigation.  The lower end of the spectrum i s cut o f f a t 100 Kev. because of absorption i n the counter window, and therefore gamma rays with energies below 188 Kev. (100 Kev. plus the lead K s h e l l binding energy of 88 Kev.) are undetected. At the upper end of the spectrum the Compton background end point a t Mev.  indicates the presence of a gamma ray of energy 2.8 Mev.,  apparently too weak to show as a photoelectron l i n e .  2.2*6  which i s  This value i s  calculated from the formula: hi) (Mev.) = ... ' " -.51T T - <\| TtT+ 1.02;  cos $  where T i s the maximum Compton r e c o i l energy ( i n Mev.)  and 0, the  scattering angle i s taken as 0 . The twenty-three gamma rays reported here agree very c l o s e l y with the values reported by E l l i s , Mann and Latyshev. At low energies, the agreement with the values reported by E l l i s i s very good. Qrie.notlceably/ d i f f e r e n t value, (reported here as 250 Kev.) may w e l l be an unresolved p a i r (.245 Kev. & .260 Kev.).  Consideration o f  the peak shape (very broad compared with neighbouring peaks) makes t h i s suggestion p l a u s i b l e . At higher energies, the agreement with the findings o f Latyshev i s even more noticeable. A l l but one of h i s previously unconfirmed are reproduced here.  The 1.1*26 Mev.  values  gamma ray reported by E l l i s , however,  . 3 1 . i s noticeably absent i n these r e s u l t s and i n the r e s u l t s of the other  (19) investigators.  This absence has been explained by Theboud  vJ7  'as being due  to the f a c t that this p a r t i c u l a r gamma ray i s almost t o t a l l y i n t e r n a l l y converted.  In view of t h i s f a c t , detection of this gamma ray cannot be  expected i n an instrument  such as the thin lens spectrometer which meas-  ures energies of the secondary electrons. In the middle of the spectrum the r e s u l t s are l e s s conclusive. The .650 and .688 Mev.  l i n e s shown i n Figure 13 may a c t u a l l y be caused by  the L and M l i n e s associated with the strong .608 Mev. K l i n e . separation between the K (.608 Mev.)  and the M (.688 Mev.)  The  i s appreciably  correct, but the separation between the K and the L (.650 Mev.) i s considerably i n error. an energy of .80I4. Mev.  One new l i n e i s reported i n t h i s region.  I t has  and i s f a i r l y intense but may have been missed  previously because i t l i e s on a very steep portion of the composite curve.  (39)  G.D.Theboud, "These", P a r i s , (1925).  32.  V.  BIBLIOGRAPHY  P. H. Abelson  Phys.Rev., 3 6 , 753,  A. I. Alichanev and G. D. Latyshev  C.R. Acad.Sci.,(U.R.S.S.),20, 1 1 3 ,  L. Alvarez  Phys.Rev., 5 k , U86, ( 1 9 3 8 ) .  C. D. Anderson  Phys.Rev., i ^ , 4 9 4 , (1933).  J. Backus  Phys.Rev., 6 8 , 5 9 , ( 1 9 4 5 ) .  H. Baeyer and 0 . Hahn  Physik., 1 1 ,  H. Becquerel  Comptes Rendus, 122. 5 0 1 , 6 8 9 , ( 1 8 9 6 ) .  A. H. Compton  Phys.Rev., 2 1 , 1*82, ( 1 9 2 3 ) .  A. A. Constantinov and G. D. Latyshev  J.Phys. U.S.S.R., £, 21*9,  (1938)  488, (1910).  (1941).  R a d i o a c t i v i t y and Nuclear Physics, loc. c i t . , 301.  J. M. Cork C. D. C o r y e l l , M. Deutsch, R. D. Evans e t a l  P; Curie, Mme. G. Bemont  (1939).  Curie, and  "The Science and Engineering of Nuclear Power", (Addison-Wesley) Comptes Rendus, 1 2 J , 1215,  (1898),  J. Danysz  Le Radium, 9 , 1 , ( 1 9 1 2 ) . Le Radium, 1 0 , 1*, ( 1 9 1 3 ) .  M. Deutsch, L. G. E l l i o t t , and R. D. Evans  Rev.Sci.Instr., 1 5 , 1 7 8 ,  A. E i n s t e i n  Ann. d. Phys., 17, 132, (1905.  CD.  P r o c R o y . S o c , A, _101, 1, (1922). Proc.Camb.Phil.Soc, 21, 1 2 5 , (1922). P r o c R o y . S o c , I38, 318", (1932).  Ellis  (1944).  33.  C. D. E l l i s and F. W. Aston  Proc.Roy.Soc, 129A,  180,  (1930).  C. D. E l l i s and H. W. B. Skinner  Proc.Roy.Soc, 105A,  165,  (1921*).  C. D. E l l i s and W. A. Wooster  Proc.Camb.Phil.Soc, 22, 81*1*, (1926). Proc.Roy.Soc, III4A, 2?6, (192?).  K. Fajens  Phys.Zeits, IU, 131,  N. Feather  ProcCamb.Phil.Soc, 3ji, 599, (1938). Reports on Progress i n Physics, 2, 66, (19U0).  A. H. F r i l l e y  "These", Paris, (1928).  F. 0. Giesel  Ann.Phys.Chem., 69, 83I4, (1899).  A. L. Hughes and V. Rojansky  Phys.Rev., 3jt, 281;,  (1925).  0. KLemperer  Phil.Mag., 20, 51*5,  (1935).  K. C. Mann and M. J. Ozeroff  "Thesis", U.B.C., (191*7).  L. Meitner  Z e i t s . f . Phys., 3j*, 807,  S. Meyer, and E. von Schweidler  Phys.Zeits., 1,  H, Robinson and E. Rutherford  PhilMag., 26, 717,  A. B. Russell .  Chem.New., 107,  E. Rutherford  Phil.Mag.,_5, 177,  E. Rutherford and H. R. Robinson  Phil.Mag., 26, 717,  E. Rutherford, J . Chadwick, C. D. E l l i s  90,  1*9,  136,  (1913).  (1925).  (1899). (1913). (1913). (1903). (1913).  "Radiations from Radioactive Substances", (Cambridge), (1930).  J. D. Main Smith  "Chemistry and Atomic Structure", (Ernest Benn), London, (192ii).  F. Soddy  Chem.New., 107, 97, (1913). Jahrb. Radioaktivitat, 10, 188,  (1913).  L. F. Stranathan  "The P a r t i c l e s of Modern Physics", (Blakiston), Philadelphia, (191*2).  R. J. S t r u t t  Proc.Roy.Soc, 7_2> 208,  (1903.  34.  G. D. Theboud  "These", Paris, ( 1 9 2 5 ) .  R. A. Tricker  Proc.Camb.Phil.Soc, 2 2 , 4 5 4 , ( 1 9 2 4 ) .  S. T. Tsien  Phys.Rev., 6 £ , 3 8 , ( 1 9 4 6 ) .  P. V i l l a r d  Comptes Rendus, 130, 1178,  K. C. Wang  Z e i t s , f . Phys., 8 7 , 6 3 3 , ( 1 9 3 4 ) .  0O0  (1900).  ACKNOWLEDGEMENTS  The beta-ray spectrometer and a u x i l i a r y apparatus f o r t h i s study were provided out of a Grant-in-Aid o f Research to Dr. K. C. Mann from the National Research Council o f Canada. The auther i s indebted to Dr. S. E. Maddigan o f the B r i t i s h Columbia Research Council, who made available a 500 m i l l i c u r i e Radium sourcej to Mr. J . Bryden o f the Consolidated Mining and Smelting Company of Canada Limited, who arranged f o r a g i f t o f 1|060 pounds o f lead which was used i n the construction o f b a f f l e s f o r background reduction, and castles f o r personnel protection; and to Mr. A. W. Pye, who aided i n the construction o f end window beta counters and t h e i r associated f i l l i n g system. The author wishes to express h i s s p e c i a l thanks to Dr. Mann f o r the expert advice and i n s p i r i n g encouragement he has given while supervising the project.  

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