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The beta rays of radium E and antimony 124 Lindenfeld, Peter 1948

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HE  B E T A AND  RAYS  OF  R A D I U M  A N T I M O N Y  124  Peter Lindenf eld  A Thesis Submitted i n P a r t i a l Fulfilment of the Requirements f o r the degree of Master of Applied Science i n the  Departement of  Physics  THE  UNIVERSITY April  OF  BRITISH 1948  COLUMBIA  E  ABSTRACT  A thin-lens beta-ray spectrometer i s described and a b r i e f analysis of i t s operation i s given.  A coincidence Geiger-  Mueller counter to be used with t h i s instrument i s also described. The spectrometer has been calibrated with a l i n e of the thorium B spectrum, and used to obtain beta-ray spectra of radium E and antimony 124.  The experimental spectra have  been found to agree well with those previously published. Several methods of p l o t t i n g beta-ray spectra are described and applied to radium E and antimony 124. "From the Fermi plot the endpoint of the radium E spectrum appears to be at 1.18 Mev, from the van der Held plot at 1.16 Mev.  For antimony 124 four endpoints have been  determined from the Fermi plot at .50, .65, .90 and 2.43  Mev.  I t i s shown that the van der Held plot reduces to the.Fermi plot f o r t h i s spectrum.  TABLE OF CONTENTS page Beta-Ray Spectra  1  Techniques of Measurement  6  Description of Spectrometer  8  Coincidence Counter  11  Calibration  12  Experimental Results A. Beta-Ray Spectrum of Radium E  13 .  B. Beta-Ray Spectrum of Antimony 124  14  Conclusions  '  17  Acknowledgements  18  References  19 TABLE OF ILLUSTRATIONS  Plate 1 figure 1  Energy l e v e l diagram of a gamma-ray t r a n s i t i o n  figure 2  Energy l e v e l diagram of a simple beta and gamma-ray t r a n s i t i o n -  figure 3  Simple beta-ray spectrum  figure 4  Energy l e v e l diagram of double beta and .gamma-ray t r a n s i t i o n  figure 5  Double beta-ray spectrum  figure" 6  Eermi plot of double beta-ray spectrum  Plate 2 figure 7  180 degree beta-ray spectrometer  figure 9  Bell-type Geiger-Mueller counter  f i g u r e 10  Coincidence counter  Plate 3  Photograph, of spectrometer  Plate 4 figure 8 Plate 5  Diagram of spectrometer High-energy part of the-beta-ray spectrum of Radium E  Plate 6  Fermi plot of the beta-ray spectrum of Radium E  Plate 7  Van der Held plot of the beta-ray spectrum of Radium E  Plate 8  Fermi plot of the beta-ray spectrum of Antimony 124  Plate 9 •  Beta-ray spectrum of Antimony 124  THE BETA RAYS OF RADIUM E AND ANTIMOFY 124  BETA-RAY SPECTRA One of the most important methods of investigating the structure of atomic nuclei i s the study of nuclear radiation.  Beta-ray spectroscopy i s a p a r t i c u l a r branch of  t h i s study and i s concerned with the beta and gamma rays emitted during the disintegration of radioactive isotopes. When a. negative beta p a r t i c l e i s emitted from a nucleus of atomic number Z, the nucleus i s changed to one of atomic number Z  1.  S i m i l a r l y the emission of a p o s i t i v e  beta p a r t i c l e lowers the atomic number by one unit.  Very  often a gamma ray accompanies such a t r a n s i t i o n . The present explanation of t h i s process assumes f i r s t of a l l , i n accordance with quantum theory, that nuclei can exist only i n discrete energy states or energy l e v e l s , and that i n a t r a n s i t i o n between the l e v e l s of a nucleus of * atomic number Z the energy difference i s accounted f o r by the emission or absorption of a gamma ray.  An energy l e v e l  diagram f o r t h i s case i s shown i n f i g u r e 1.  Such a picture i s  f u l l y i n accord with the observed l i n e structure of gamma rays. If t h i s picture i s now extended to beta t r a n s i t i o n s , serious d i f f i c u l t i e s arise.  An energy l e v e l diagram here i s of the  type shown i n f i g u r e 2, where a nucleus'of atomic number Z changes to an exited nucleus of atomic number Z + 1 by the  emission of a negative beta p a r t i c l e and then drops to the ground state by the emission of a gamma ray.  We would expect  then that a l l the beta p a r t i c l e s emitted i n such a t r a n s i t i o n have the same energy E, equal to the energy difference ."between' the ground state of the parent nucleus and the exited state of the daughter  nucleus.  This does not agree with experiment;  the observed  beta rays show "a continuous energy d i s t r i b u t i o n of the form shown i n f i g u r e ' 3 , where the maximum energy E energy of the disintegration.  0  i s equal to the  To overcome t h i s d i f f i c u l t y  without giving up the law of conservation of energy requires the postulation of another p a r t i c l e , the neutrino, which carries away the difference between the energy of the beta p a r t i c l e and the disintegration energy E . 0  Such a . p a r t i c l e  would have to have an extremely small mass and no charge, making i t s detection most d i f f i c u l t .  So f a r no experiment  on  the existence of the neutrino has been decisive. At the present time the most comprehensive theory of beta disintegration i s that proposed by "Fermi ^  on the  basis of a quantum.mechanical c a l c u l a t i o n of the p r o b a b i l i t y of emission of an electron and a neutrino which share the energy E . Q  According to t h i s theory the p r o b a b i l i t y of  emission of an electron whose energy l i e s between E and E + dE i s F(Z,E)dE - G t # E ^ ~ l ( E - E ) ^ % ^ 2  2  Q  )  (Zpft^l  where G i s a constant IMJis a matrix element of the t r a n s i t i o n  f*ir(i+f+iy)I^E  3. $ - j/i - oi. z 2  2  -  1  Z i s the atomic number o< - 1/137 y'p i s the radius of the nucleus (The factors have been made dimensionless by choosing units i n  H I Q C  2  energy  and momentum units i n moC.)  For any one bet a-ray. spectrum t h i s can be reduced to  where N i s proportional to the i n t e n s i t y of beta r a d i a t i o n of energy E  rj - momentum •  2  - l  This relation.can be r e a d i l y checked f o r any spectrum i f IT i s taken as the observed count of beta p a r t i c l e s and i s plotted against E.  This should r e s u l t i n a straight  l i n e f o r "allowed" transitions with the intercept on the Eaxis being equal to E . 0  Such a curve i s called a Eermi plot  and i s of great importance of beta-ray spectroscopy:  if it  turns out to be a straight l i n e i t adds considerable weight to the Eermi theory; regardless of t h e o r e t i c a l considerations, however, i t w i l l be an important method of finding E by 0  extrapolation, i f at least the' portion of the spectrum near E  0  can be plotted as a straight l i n e . I t should be pointed out here that the determination of E  D  by inspection from the spectrum as i n f i g u r e 3 i s very  4. unreliable, p a r t l y because of the small angle at which the curve approaches the E - axis, and p a r t l y because of the low counting rates i n the neighbourhood of E . o The f i r s t Eermi plots showed considerable deviation from-the straight l i n e r e l a t i o n , and i n 1935 Konopinski end Uhlenbeck  •'. proposed a modification of the theory, which l e d  A  to the new straight l i n e r e l a t i o n N  At f i r s t t h i s seemed to f i t the experimental curves better, but as more accurate determinations were made the o r i g i n a l Eermi theory was found to be more generally successful.  The agreement i s f a r from perfect, but i n many  cases the Eermi plot approximates  a straight line., especially  near the endpoint; In an attempt to get better c o r r e l a t i o n with experimental spectra, van der Held^ ) proposed using a l i n e a r 4  combination of the terms a r i s i n g out of the Eermi and the K - U theories.  The Eermi plots are often straight near the  endpoint, while the K - U plots are straight f o r lower;;  *The r a t i o of K-capture to positron emission f o r Cadmium 107-lo9, as calculated from* the Eermi theory, . agrees well with experiment, while the K - U theory leads to a value sixty times too large. ^  On the other hand the h a l f - l i f e  values calculated by integrating the Eermi formula do not, i n general, agree with experimental values.  5  energies.  A graph combining  the two methods should therefore  be straight over a greater range.  In this case the function  which should be plotted against E to give a straight l i n e relationship i s  -^-hL-—^  approximately equal to 1.8 x 10"  ^ 3  constant c turns out to be times the mass number f o r the  sample curves made by van der Held.  Whether t h i s proportio-  n a l i t y i s of theoretical significance can not be decided at t h i s time.  For some elements i t has been shown that the  straight l i n e character of the plot i s retained over a larger region of energies than i n the two other types of p l o t .  This  i s of p a r t i c u l a r importance i n the r e s o l u t i o n of complex betaray spectra. Consider an energy l e v e l diagram as.in f i g u r e 4 , where the emission of the beta p a r t i c l e s may nucleus i n one of two d i f f e r e n t states.  leave the daughter  To each of the two  possible transitions w i l l correspond a simple d i s t r i b u t i o n as i n f i g u r e 3 , but i t w i l l be very d i f f i c u l t to separate two beta groups from a composite spectrum, which would have the form of f i g u r e 5 .  The resolution becomes very simple i f there  i s a method of p l o t t i n g separate beta groups as straight l i n e s . If t h i s can be done a straight l i n e of a certain slope corresponds to each of the two beta groups, so that the p l o t of a double spectrum has the form shown i n f i g u r e 6 .  Section  b can be subtracted,' leaving a straight l i n e whose intercept on the E - axis gives the endpoint of the f i r s t beta group. There i s one other method of resolving complex spectra: that of coincidence measurements.  This method i s  6.  based on the f a c t that i n a double spectrum (see f i g u r e 4) a beta ray of group a w i l l i n general be accompanied by a gamma ray,  or followed by i t after a time i n t e r v a l short compared to  the resolving time of the coincidence c i r c u i t .  The emission  of a beta ray of group b leaves the daughter nucleus i n the ground state without the emission of a gamma ray.  I f the  recording apparatus i s arranged i n such a way that a beta p a r t i c l e r e g i s t e r s only when i t i s accompanied by a gamma ray, no beta p a r t i c l e s of energy greater than E  Q  a  can be recorded.  The chief d i f f i c u l t y of t h i s method l i e s i n the r e l a t i v e l y " l o w i n t e n s i t y available i n coincidence studies. I t i s obvious from t h i s discussion that present theories of beta d i s i n t e g r a t i o n are not too s a t i s f a c t o r y . I t i s the hope of beta-ray spectroscopists that as more investigations of energy l e v e l schemes are made, and as the methods of measurement become more precise, the shortcomings of the present theories w i l l appear more d e f i n i t e l y , " t h u s leading to refinements i n the theories, or possibly to "a completely new theory to explain t h i s very important fundamental  process.  TECHHIQ.UES 07  MBASimTOTFIWT  The most direct method of measuring beta-ray energies i s to observe the curvature of cloud-chamber tracks i n a homogeneous magnetic f i e l d at r i g h t angles to the plane of the electron path.  I f the magnetic f i e l d i s H gauss and  7.  and the radius of curvature of the electron path i s p, then the energy E i n Mev can he found from the r e l a t i o n  Hp = -y x IO l/£:CE> l . 0 2 ) gauss -c^r, 4  Since the curv.ature of each track has to he measured separately a great many determinations have to be made i n order to arrive at a reasonably accurate shape f o r the. d i s t r i b u t i o n curve. Another method i s to introduce various thicknesses of absorbers i n the path of the rays and" measuring the loss i n i n t e n s i t y of the beta-ray beam.  I f R i s the absorber thick-  ness i n grams per square centimeter which completely  absorbs  •'•  (5)  beta rays up to an energy E Mev then according to Feather"* ' R>  .543 E - .161  This i s a purely empirical r u l e and i s used normally f o r energies above .4 Mev. For complex spectra the absorption and coincidence methods can be combined to correlate gamma rays with the proper beta-ray groups.  This i s a straightforward way of  estimating the decay scheme of an isotope, but i t i s d i f f i c u l t to use i t when the r e l a t i v e i n t e n s i t y of one of the beta groups- i s very small.  For the most accurate determinations,  however, beta-ray spectrometers are now universally used. These can be of various types, but a l l are based on the dispersion of charged p a r t i c l e s i n magnetic and e l e c t r i c fields. The e a r l i e s t and most common type of beta-ray spectrometer i s the 180 degree or -rr-type.  Here  the rays  PL.  F ; . IO 3  *TC Z  8. describe c i r c u l a r paths i n a magnetic f i e l d just as i n the cloud-chamber determinations mentioned on page 6, so that i f the radius of curvature i s held f i x e d by suitable l i m i t a t i o n of the beam (see f i g u r e 7) d i f f e r e n t energy bands can be made to a r r i v e at" the counter by varying the -strength of the magnetic f i e l d . •-. I f the magnetic f i e l d i s in. the same d i r e c t i o n as the i n i t i a l v e l o c i t y of the electrons, as i n a long solenoid, they w i l l execute a s p i r a l motion i n that d i r e c t i o n , returning to the axis after a certain distance.  Here again d i f f e r e n t  energies can be focussed at a counter, depending on the strength of the f i e l d . . A type of spectrometer using an inhomogeneous f i e l d i s the t h i n lens t y p e ^ ) . 6  Since t h i s i s the type used i n the  present investigation, i t w i l l be desribed i n d e t a i l under •Description of Spectrometer'.  DESRIPTION OF THE SPEC UROMETER. A photograph of the spectrometer i s shown on p l a t e three.  Eigure 8 on plate four i s a" schematic representation.  A i s a brass tube 7-f- inches i n diameter and 40 inches long, which i s evacuated to a pressure of about .001 mm Hg with a Cenco Megavac pump.  An o i l d i f f u s i o n pump i s also connedted  but.is used only where lower pressures are desirable. C i s a lead cylinder placed between E and J to absorb gamma rays , from the source E which would otherwise r e g i s t e r on the Geiger-Mueller counter J .  The lead collimating b a f f l e D  allows only a conical s h e l l of beta p a r t i c l e s to pass through i n the d i r e c t i o n of the counter.  In the region of the magnet  B they are deflected i n s p i r a l paths, returning to the axis at the counter window. 'focussed'.  Such beta p a r t i c l e s are said to be  The second lead b a f f l e E i s not necessary to  l i m i t the beam further but i s introduced  to reduce scattered  beta and gamma r a d i a t i o n that without i t would increase background counting rate.  the  "For t h i s purpose the b a f f l e s G and  H have also been found to be quite e f f e c t i v e . The magnet i s wound i n four sections -with number 10 wire with alternate layers of water cooling c o i l s . resistance of each section i s about one ohm.  The  The current i s  supplied by a 5 kva motor-generator and i s s t a b i l i z e d by an electronic regulator, using 38 6AS7 triddes i n p a r a l l e l .  The  current i s measured as the p o t e n t i a l drop across a manganin resistance of about .8 ohms with a Rubicon potentiometer. In order to "eliminate the effect of the  earth's  magnetic f i e l d the spectrometer i s aligned along the horizontal component of the earth's f i e l d , and two Helmholtz c o i l s are used to compensate f o r the v e r t i c a l component. The Geiger-Mueller counters are of the b e l l type, shown i n fi'gure 9. A i s a .020 inch tungsten wire to which the .005 inch tungsten wire B i s attached with a drop of s i l v e r solder.  At the end of wire B i s a small glass bead.  The window C i s of t h i n mica which i s sealed between the two brass plates D and E with a mixture of equal parts of beeswax and rosin;  The mica used had ,„a thickness of about two mg/cm , 2  10.  although i t was found possible to s p l i t the mica to l e s s than one mg/cm . 2  The glass envelope E was waxed to the base D with  Apiezon wax "W".  The counter was f i l l e d with a mixture of 9.5  cm argon and .7 cm ethyl alcohol. The pulse from the Geiger-Mueller counter goes to a cathode-coupled  preamplifier stage using a 6.CF6 miniature twin  triode, and then to a scale of 64 scaler b u i l t by the Atomic Instrument Company.  The scaler i s connected to a mechanical  r e g i s t e r made by the Cyclotron S p e c i a l t i e s Company. In analogy to optics the spectrometer may be said to consist of a t h i n lens, an object and an image, with the image distance equal to the object distance and equal to twice the f o c a l length of the lens.  The f o c a l length of the lens i s  determined by the f i e l d current,. there i s a'chromatic  Just as i n the o p t i c a l case  dispersion, i . e. rays of d i f f e r e n t  energies w i l l be focussed at d i f f e r e n t points on the axis. The o r i g i n a l beam i s thus separated into rays of d i f f e r e n t energies and only a small bundle of such rays reaches the counter f o r any one value of current through the c o i l .  The  momentum i n t e r v a l of the focussed rays as a percentage of their average momentum i s called the resolving power, and i n t h i s instrument i t i s about three per cent. The radius of curvature p  of the focussed rays i s  held constant by the position of the b a f f l e s so that from the equation  H p o 1/3 x 1 0  d i r e c t correspondence  4  \/i(E + 1 . 0 2 ) gauss-cm there i s thus a  between the strength H of the f i e l d and  the energy E of the rays registered by the counter.  Uo  I  \  .  _  _  •  11. attempt i s made to arrive at an absolute value of either H or p  and the instrument i s calibrated with a gamma ray conversion  l i n e of known Hp.  Since no i r o n i s present the magnetic f i e l d  strength i s d i r e c t l y proportional to the current through the coil.  Hence f o r each value of current, beta rays of a d e f i n i t e  energy or momentum are focussed on the counter.  COINCinENCE COUNTER In a l l work with the spectrometer there i s a steady background counting rate which i s caused to some extent by cosmic rays,, but mainly by scattered beta and gamma rays.Since t h i s background i s an important l i m i t to the accuracy of the determination, i t i s important to keep i t as low as possible.  Up to the present t h i s has been done by lead baffles  as described e a r l i e r .  I t i s proposed to reduce i t further by  using two connected beta-ray counters next to one another and operated i n coincidence.  See f i g u r e 10.  A beta ray coming at the proper angle w i l l pass the window W and r e g i s t e r i n both counters.  through  A gamma ray,  however, w i l l l i b e r a t e a secondary electron which w i l l r e g i s t e r i n the f i r s t counter but has only a small p r o b a b i l i t y of going i n the r i g h t d i r e c t i o n to pass into the second counter and r e g i s t e r i n g as a coincidence. Such a counter has been made and tested outside the spectrometer with a beta-ray source.  The following sample  readings w i l l serve to indicate i t s performance.  12. Source 4^- inches from window t o t a l count-  background  counter A  9468  77  counter B  3829  108  coincidences  3439  16  I t w i l l be seen that the coincidence counting rate and the counting rate of counter B are not greatly d i f f e r e n t so that i t i s expected that the introduction of t h i s double counter w i l l not appreciably a f f e c t the normal counting rate. The background f o r coincidences i s seen to be considerably reduced over that of a single counter. Due to mechanical d i f f i c u l t i e s i t has not been possible to test t h i s counter i n the spectrometer before t h i s time.  CALIBRATION The spectrometer was calibrated with the IP-line of thorium B, f o r which Hp i s 1385.6 gauss-cm.  This l i n e has  been determined very accurately ( » ) , and i s now widely used 7  as a secondary standard.  8  The source was prepared by p r e c i p i -  tation as a s u l f i d e from a thorium n i t r a t e solution, and put on a backing of mica.  I t was covered with a drop of collodion  solution to hold i t i n place.  This isotope has a h a l f - l i f e of  10.6 hours so that a l l readings had to be corrected f o r decay. Two sources were used, but only the second c a l i b r a t i o n was successful.  The f i r s t source had a  13. considerable amount of inert material added to i t as c a r r i e r , with the r e s u l t that the source was so thick that scattering broadened the l i n e to the extent of being almost indistinguishable from the background.  EXPER IMEFTAL  RESULTS  A.  The'"Beta-Ray Spectrum of Radium E  Radium E i s a good example of a beta-ray emitter with a simple spectrum.  I t has been studied by many  i n v e s t i g a t o r s ^ ' ' ) , yet there i s considerable v a r i a t i o n 9  1 0  1 1  i n the reported endpoints.  Most of the d i f f i c u l t y seems to be  due to the f a c t that the Eermi p l o t , instead of being a straight l i n e , i s concave upwards.  When the Konopinski - Uhlenbeck  modification of the Eermi theory was f i r s t announced i t was thought that i t was more successful f o r radium E, but as more accurate measurements were made i t was  shown that the K - U  plot drops sharply near the endpoint.  Thus extrapolated K - U  plots give endpoints which are much too high. Van der Held was able to get plots which approximate straight l i n e s very closely, but h i s method of plotting i s somewhat more complicated than f o r the other two cases. page 5 )  (See  I t i s necessary to get a preliminary value of the  endpoint from the Eermi p l o t , and also to use a constant c, determined from, the slope of the l i n e obtained when'  f-~^-~A  i s . plotted against ( E - E) . Q  f o r the Eermi plot.)  I t should be noted,that the Eermi theory  14.  i s a special case of the vW^ffeWtheory, with c equal to zero. Van der Held showed that f o r the few elements which he studied c was proportional to the atomic mass number A, with e/A -3 approximately equal to 1.8 x 10 • The source used was one of m e t a l l i c radium D i n equilibrium with i t s daughter products.  The beta and gamma  rays from radium D have very low energies (less than .05 Mev) so that they d i d not i n t e r f e r e .  Since the energies i n the  region to be studied were near 1 Mev no p a r t i c u l a r precautions to reduce scattering had to be taken. The high energy end of the spectrum i s shown on plate 5.  The d i f f i c u l t y i n determining the endpoint from t h i s i s  apparent.  The "Fermi plot i s shown on plate 6 .  I t s curvature  i s quite pronounced, so that extrapolation becomes unreliable. The endpoint i s at 1.18 Mev.  The van der Held plot (plate 7)  shows a straight l i n e , disregarding the l a s t three points. With a three per sent resolving power of the instrument these points w i l l be shifted to the r i g h t so that' the endpoint appears 1-g- per cent too high.  Since the s t a t i s t i c a l accuracy  of these points i s f a i r l y low, it seems best to disregard them i n drawing the straight l i n e .  The endpoint i s then at 1.16 Mev.  The constant c has been taken as 210 x 1.8 x 10~ which i s the 3  average value-obtained by van der ..Held f o r radium E. B.  The Beta-Ray Spectrum of Antimony 124  The beta-ray spectrum of antimony 124 has been (12-21) reported on by about ten research groups . until !:  TT  15.  r e c e n t l y - i t seemed to consist of two beta-ray groups, the f i r s t with an endpoint between .50 and .74 Mev, between 1.53  and 2.54 Mev.  and the second  I t seemed desirable to determine  these endpoints more closely, and a sample of antimony 124 was therefore ordered f o r the present investigation.  In  ..January 1948, however, two reports were p u b l i s h e d *  ^,  1  2 Q f2 1  giving the endpoints of f i v e beta-ray groups emitted by this isotope.  With t h i s complication the investigation of t h i s  spectrum became even more i n t e r e s t i n g . The source was prepared by i r r a d i a t i n g antimony t r i o x i d e with slow neutrons i n the heavy water p i l e at Chalk River.  The antimony t r i o x i d e was s p e c i a l l y p u r i f i e d i n the  department of chemistry, and spectroscopically tested.  The  spectroscopic analysis showed that the only impurity present was copper (.002 %) with sodium, calcium, tin"and iron probably absent and arsenic, lead, bismuth, lithium, potassium, manganese, strontium and barium d e f i n i t e l y absent.  silicon,  The  i r r a d i a t e d oxide had a s p e c i f i c a c t i v i t y of one m i l l i c u r i e per gram.  The source strength used was a few microcuries, on a  backing of mica of thickness l e s s than 1 mg/cm . 2  The source  was covered by a thin f i l m of collodion, prepared by the method of Backus^ ). 22  A l l readings were corrected f o r decay, assuming a (23) h a l f - l i f e of 60 days, as given by Livingood and Seaborg Some d i f f i c u l t y was  experienced i n reproducing parts of the  spectrum, and i t was necessary to normalize two sections of the spectrum, since the i n t e n s i t y had dropped by about 2 per  16.  cent i n the one case and by about 15 per cent i n the second. The p o s s i b i l i t y of some antimony 122 ( h a l f - l i f e 63 hours) being present can not be ruled out, although at least three weeks had elapsed after the end of i r r a d i a t i o n of the sample. The spectrum i s shown-on plate 9.  I t shows a  conversion l i n e at , 5 7 Mev, corresponding to a gamma ray 8  .energy of .61° Mev.  This l i n e has also been reported by  Kern, Zaffarno and M i t c h e l l ^ energy of .603 Mev.  who obtained a gamma-ray  2 0  A Eermi plot i s shown on p l a t e 8. I t  consists of four straight l i n e sections, with  endpoints  agreeing well with those reported e a r l i e r this year. tabulation of r e s u l t s i s shown below: Kern, Zaffarno*, Mitchell^ ;  (energies i n Mev)  Cook. Langer (21)  2 0  .47 .63 .98 1.58 2.31  A  Mann, Lindenfeld  .50 .68 .98 1.50 2.37  .50 .65 .90 — 2.43  ± .02 ± .03 ± .06 ± .03  There .is an i n d i c a t i o n of a group with, an endpoint near 1.5 Mev, but i t seems to be very weak:.  I t must be  stressed that these endpoints are derived wholly from the Eermi p l o t , and therefore depend e n t i r e l y on the accuracy of the Eermi theory.  They can not be regarded as well  established u n t i l they are confirmed by coincidence measurements. ' v.  .-A plot, of  2  ( l - E ) gave approximately a 2  0  straight l i n e with a slope either zero or at least very small.  17.  Thus the van der Held plot f o r t h i s case reduces to the Fermi plot.  The van der Held plot was also plotted using c a 124 x  1.8 x 10~  , hut i t showed no i n d i c a t i o n of being composed of  u  straight l i n e sections.  This shows the importance of  determining c separately f o r each spectrum, since the proportionality to the mass number, as suggested by van der Held, does not seem to hold i n a l l cases.  CONCLUSIONS The work on radium E has confirmed the work of many other investigators who have found that the Eermi plot f o r t h i s case i s not straight.  The van der Held plot i s  apparently better i n this respect.  The endpoint from the  Eermi p l o t i s 1.18 Mev, from the van der Held plot 1.16 Mev. The Eermi plot f o r antimony 124 shows four endpoints at .50, .65, .90, and 2.43 Mev.  A f i f t h group,  reported by Kern, Zaffarno, M i t c h e l l ^ ) and by Cook, 2 0  Langer^ ) i s too weak f o r s i g n i f i c a n t evaluation i n t h i s 21  experiment.  The van der Held constant c i s near zero f o r  antimony 124, so that van der Held*s suggestion that c i s proportional to the mass number appears-to be incorrect.  PL. ATT <»  ©  1.2.  Mev  PL.A.TE a  IOOO  C U R R E N T  (Vol*.  .IAZS  o«  THIS  SCALS  Hp* (306  3-**. >  18  ACCTOWLEDGTafTOS  The beta-ray spectrometer and a u x i l i a r y apparatus are part'of a grant-in-aid to Dr. K. C. Mann from the National Research Council of Canada.  The National Research  Council has also awarded a studentship to me f o r the session 1947-48. Two tons of lead have been donated by the Consolidated Mining and Smelting Company of T r a i l , B. C. f o r the protection of the experimenters. Mrs. B. E. Speers of the Department of Chemistry has p u r i f i e d the antimony t r i o x i d e and prepared the two thorium B sources. Dr. A. M. Crooker has made the spectroscopic analysis of the antimony t r i o x i d e . Mr. A. W. Pye has made the glass parts of the Geiger-Mueller counters and. the f i l l i n g  systems.  Their help i s g r a t e f u l l y acknowledged. I would l i k e to express my special thanks to Dr. K. C. Mann, under whose encouraging and always h e l p f u l expert guidance t h i s research was carried out.  19.  REFERENCES  E. Eermi Z e i t s c h r i f t fur Physik,  88, 161,  1934  E. J . Konopinski and G. E. Uhlenbeck Physical Review,  48, 7,  1935  H. Bradt, P. C. Gugelot, 0. Huber, H.  Medicus,  P. Preiswerk and P. Scherrer Physical Review,  68, 57, '1945  E. E. M. van der Held Physica,  8,' 196,  1941  N. Feather Proceedings of the Cambridge Philosophical Society, 34,  34, 599,  1938  M. Deutsch, L. G, E l l i o t t and R. D. Evans Review of S c i e n t i f i c Instruments,  15, 178,  1944  C. D. E l l i s Proceedings* of the Roys!  Society,  A 138, 318,  K. C. Wang Z e i t s c h r i f t fur Physik,  87, 633,.1934  L. M. Langer and M. D. \Whitaker "Physical Review, .51, 713,  1937  C. M. Witcher Physical Review, J. S. 0'Conor Physical Review,  60, 32, 52, 303,  1941 1937  1932  A. C. G. M i t c h e l l , L. M. Langer and P. W. McDaniel - Physical Review 57, 1107, 1940 E. B. Hales and E. B. Jordan Physical Review,  £2,, 553, 1942  E. B. Hales and E. B. Jordan Physical Review,  64, 202, 1943  L. C. M i l l e r and L. P. Curtiss Physical Review,  70, 983, 1946  W. E. Meyerhof and G. Scharff-Goldhaber Physical Review,  72, 273, 1947  M. L. Wiedenbeck and K. Y. Chu Physical Review,  72, 1164, 1947  E. T. Jurney and A. C. G. M i t c h e l l B u l l e t i n of the American Physical Society,  Jan. 1948  M. V. Scherb and C. E. Mandeville Bulletin, of the American Physical Society, Jan. 1948 B. D . Kern, D. J . Zaffarno, A. C. G. MitchellB u l l e t i n of the American Physical Society,  Jan. 1948  C. S. Cook and L. M. Langer B u l l e t i n of the American Physical Society, J'. Backus Physical Review, J. J . Livingood  68, 59, 1945  and G. T. Seaborg  Physical Review,  55, 415, 1939  Jan. 1948  

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