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

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H E B E T A R A Y S OF R A D I U M E A N D A N T I M O N Y 1 2 4 Peter Lindenf eld A Thesis Submitted in Partial Fulfilment of the Requirements for the degree of Master of Applied Science in the Departement of Physics THE UNIVERSITY OF BRITISH COLUMBIA April 1948 ABSTRACT A thin-lens beta-ray spectrometer i s described and a brief analysis of i t s operation i s given. A coincidence Geiger-Mueller counter to be used with this instrument i s also described. The spectrometer has been calibrated with a line 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 plotting 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. It i s shown that the van der Held plot reduces to the.Fermi plot for this 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 level diagram of a gamma-ray transition figure 2 Energy level diagram of a simple beta and gamma-ray transition -figure 3 Simple beta-ray spectrum figure 4 Energy level diagram of double beta and .gamma-ray transition 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 figure 10 Coincidence counter Plate 3 Photograph, of spectrometer Plate 4 figure 8 Diagram of spectrometer Plate 5 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 particular branch of this study and i s concerned with the beta and gamma rays emitted during the disintegration of radioactive isotopes. When a. negative beta particle i s emitted from a nucleus of atomic number Z, the nucleus i s changed to one of atomic number Z 1. Similarly the emission of a positive beta particle lowers the atomic number by one unit. Very often a gamma ray accompanies such a transition. The present explanation of this process assumes f i r s t of a l l , i n accordance with quantum theory, that nuclei can exist only in discrete energy states or energy levels, and that i n a transition between the levels of a nucleus of * atomic number Z the energy difference i s accounted for by the emission or absorption of a gamma ray. An energy level diagram for this case i s shown in figure 1. Such a picture i s f u l l y i n accord with the observed li n e structure of gamma rays. If this picture i s now extended to beta transitions, serious d i f f i c u l t i e s arise. An energy level diagram here i s of the type shown in figure 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 particle and then drops to the ground state by the emission of a gamma ray. We would expect then that a l l the beta particles emitted in such a transition 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 distribution of the form shown i n figure'3, where the maximum energy E 0 i s equal to the energy of the disintegration. To overcome this d i f f i c u l t y without giving up the law of conservation of energy requires the postulation of another particle, the neutrino, which carries away the difference between the energy of the beta particle and the disintegration energy E 0. Such a.particle 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 far 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 calculation of the probability of emission of an electron and a neutrino which share the energy E Q. According to this theory the probability of emission of an electron whose energy l i e s between E and E + dE i s F(Z,E)dE - G 2 t # E ^ ~ l (E Q- E ) 2 ^ % ^ ) (Zpft^l f*ir(i+f+iy)I^E where G i s a constant IMJis a matrix element of the transition 3. $ - j/i - oi.2z2 - 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 energy units i n H I Q C 2 and momentum units i n moC.) For any one bet a-ray. spectrum this can be reduced to where N i s proportional to the intensity of beta radiation of energy E rj - momentum • 2 - l This relation.can be readily checked for any spectrum i f IT i s taken as the observed count of beta particles and i s plotted against E. This should result i n a straight l i n e for "allowed" transitions with the intercept on the E-axis being equal to E 0 . Such a curve i s called a Eermi plot and i s of great importance of beta-ray spectroscopy: i f i t turns out to be a straight l i n e i t adds considerable weight to the Eermi theory; regardless of theoretical considerations, however, i t w i l l be an important method of finding E 0 by extrapolation, i f at least the' portion of the spectrum near E 0 can be plotted as a straight l i n e . It should be pointed out here that the determination of E D by inspection from the spectrum as i n figure 3 i s very 4. unreliable, partly because of the small angle at which the curve approaches the E - axis, and partly because of the low counting rates in the neighbourhood of E . o The f i r s t Eermi plots showed considerable deviation from-the straight l i n e relation, and in 1935 Konopinski end UhlenbeckA •'. proposed a modification of the theory, which led to the new straight line relation N At f i r s t this seemed to f i t the experimental curves better, but as more accurate determinations were made the original Eermi theory was found to be more generally successful. The agreement i s far from perfect, but in many cases the Eermi plot approximates a straight line., especially near the endpoint; In an attempt to get better correlation with experimental spectra, van der Held^ 4) proposed using a linear combination of the terms arising 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 for lower;; *The ratio of K-capture to positron emission for 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, in 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 line relationship i s -^-hL-—^ ^ constant c turns out to be approximately equal to 1.8 x 10" 3 times the mass number for the sample curves made by van der Held. Whether this proportio-nality i s of theoretical significance can not be decided at this 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 in the two other types of plot. This i s of particular importance i n the resolution of complex beta-ray spectra. Consider an energy level diagram as.in figure 4 , where the emission of the beta particles may leave the daughter nucleus i n one of two different states. To each of the two possible transitions w i l l correspond a simple distribution as in figure 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 figure 5 . The resolution becomes very simple i f there i s a method of plotting separate beta groups as straight lines. If this can be done a straight line of a certain slope corresponds to each of the two beta groups, so that the plot of a double spectrum has the form shown i n figure 6 . Section b can be subtracted,' leaving a straight line 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 fact that in a double spectrum (see figure 4) a beta ray of group a w i l l in general be accompanied by a gamma ray, or followed by i t after a time interval 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. If the recording apparatus i s arranged in such a way that a beta particle registers only when i t i s accompanied by a gamma ray, no beta particles of energy greater than E Q a can be recorded. The chief d i f f i c u l t y of this method l i e s i n the relatively"low intensity available i n coincidence studies. It i s obvious from this discussion that present theories of beta disintegration are not too satisfactory. It is the hope of beta-ray spectroscopists that as more investigations of energy level schemes are made, and as the methods of measurement become more precise, the shortcomings of the present theories w i l l appear more definitely,"thus leading to refinements in the theories, or possibly to "a completely new theory to explain this 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 right angles to the plane of the electron path. If 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 in Mev can he found from the relation Hp = -y x IO4 l/£:CE> l . 0 2 ) gauss -c^r, 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 for the. distribution curve. Another method is to introduce various thicknesses of absorbers i n the path of the rays and" measuring the loss in intensity 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 rule and i s used normally for 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 relative intensity 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 particles i n magnetic and electric f i e l d s . The earliest and most common type of beta-ray spectrometer i s the 180 degree or -rr-type. Here the rays PL. *TC Z F ; 3 . IO 8. describe circular paths in a magnetic f i e l d just as in the cloud-chamber determinations mentioned on page 6, so that i f the radius of curvature i s held fixed by suitable limitation of the beam (see figure 7) different energy bands can be made to arrive at" the counter by varying the -strength of the magnetic f i e l d . •-. If the magnetic f i e l d i s in. the same direction as the i n i t i a l velocity of the electrons, as in a long solenoid, they w i l l execute a spiral motion in that direction, returning to the axis after a certain distance. Here again different 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 thin lens type^ 6). Since this i s the type used in the present investigation, i t w i l l be desribed i n detail under •Description of Spectrometer'. DESRIPTION OF THE SPEC UROMETER. A photograph of the spectrometer i s shown on plate three. Eigure 8 on plate four i s a" schematic representation. A i s a brass tube 7-f- inches in 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 diffusion 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 register on the Geiger-Mueller counter J. The lead collimating baffle D allows only a conical shell of beta particles to pass through in the direction of the counter. In the region of the magnet B they are deflected in spiral paths, returning to the axis at the counter window. Such beta particles are said to be 'focussed'. The second lead baffle E i s not necessary to limit the beam further but i s introduced to reduce scattered beta and gamma radiation that without i t would increase the background counting rate. "For this purpose the baffles G and H have also been found to be quite effective. The magnet i s wound in four sections -with number 10 wire with alternate layers of water cooling coils. The resistance of each section i s about one ohm. The current i s supplied by a 5 kva motor-generator and i s stabilized by an electronic regulator, using 38 6AS7 triddes i n parallel. The current i s measured as the potential 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 coils are used to compensate for the vertical 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 is attached with a drop of silver solder. At the end of wire B i s a small glass bead. The window C i s of thin 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/cm2, 1 0 . although i t was found possible to s p l i t the mica to less than one mg/cm2. 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 built by the Atomic Instrument Company. The scaler i s connected to a mechanical register made by the Cyclotron Specialties Company. In analogy to optics the spectrometer may be said to consist of a thin lens, an object and an image, with the image distance equal to the object distance and equal to twice the focal length of the lens. The focal length of the lens i s determined by the f i e l d current,. Just as i n the optical case there i s a'chromatic dispersion, i . e. rays of different energies w i l l be focussed at different points on the axis. The original beam i s thus separated into rays of different energies and only a small bundle of such rays reaches the counter for any one value of current through the c o i l . The momentum interval of the focussed rays as a percentage of their average momentum i s called the resolving power, and i n this 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 baffles so that from the equation Hp o 1/3 x 1 0 4 \/i(E + 1. 0 2 ) gauss-cm there i s thus a direct correspondence 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 line of known Hp. Since no iron i s present the magnetic f i e l d strength i s directly proportional to the current through the c o i l . Hence for each value of current, beta rays of a definite 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 this background i s an important limit to the accuracy of the determination, i t i s important to keep i t as low as possible. Up to the present this has been done by lead baffles as described earlier. It 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 figure 10. A beta ray coming at the proper angle w i l l pass through the window W and register in both counters. A gamma ray, however, w i l l liberate a secondary electron which w i l l register i n the f i r s t counter but has only a small probability of going in the right direction to pass into the second counter and registering 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 total count- background counter A 9468 77 counter B 3829 108 coincidences 3439 16 It w i l l be seen that the coincidence counting rate and the counting rate of counter B are not greatly different so that i t i s expected that the introduction of this double counter w i l l not appreciably affect the normal counting rate. The background for 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 this counter in the spectrometer before this time. CALIBRATION The spectrometer was calibrated with the IP-line of thorium B, for which Hp i s 1385.6 gauss-cm. This l i n e has been determined very accurately ( 7» 8), and i s now widely used as a secondary standard. The source was prepared by precipi-tation as a sulfide from a thorium nitrate solution, and put on a backing of mica. It was covered with a drop of collodion solution to hold i t in 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 for decay. Two sources were used, but only the second calibration was successful. The f i r s t source had a 13. considerable amount of inert material added to i t as carrier, with the result that the source was so thick that scattering broadened the line to the extent of being almost indistinguish-able 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. It has been studied by many i n v e s t i g a t o r s ^ 9 ' 1 0 ' 1 1 ) , yet there i s considerable variation i n the reported endpoints. Most of the d i f f i c u l t y seems to be due to the fact that the Eermi plot, instead of being a straight line, 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 for 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 lines very closely, but his method of plotting i s somewhat more complicated than for the other two cases. (See page 5 ) It i s necessary to get a preliminary value of the endpoint from the Eermi plot, and also to use a constant c, determined from, the slope of the line obtained when' f-~^-~A for the Eermi plot.) It should be noted,that the Eermi theory is. plotted against (E Q- E) . 14. i s a special case of the vW^ffeWtheory, with c equal to zero. Van der Held showed that for 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 metallic radium D in 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 did not interfere. Since the energies in the region to be studied were near 1 Mev no particular 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 in determining the endpoint from this i s apparent. The "Fermi plot i s shown on plate 6 . Its 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 line, disregarding the last three points. With a three per sent resolving power of the instrument these points w i l l be shifted to the right 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 in drawing the straight line. The endpoint i s then at 1.16 Mev. The constant c has been taken as 210 x 1.8 x 10~3 which i s the average value-obtained by van der ..Held for radium E. B. The Beta-Ray Spectrum of Antimony 124 !: The beta-ray spectrum of antimony 124 has been (12-21) T T reported on by about ten research groups . until 15. recently-it seemed to consist of two beta-ray groups, the f i r s t with an endpoint between .50 and .74 Mev, and the second between 1.53 and 2.54 Mev. It seemed desirable to determine these endpoints more closely, and a sample of antimony 124 was therefore ordered for the present investigation. In ..January 1948, however, two reports were published 1* 2 Q f 2 1 ^, giving the endpoints of f i v e beta-ray groups emitted by this isotope. With this complication the investigation of this spectrum became even more interesting. The source was prepared by irradiating antimony trioxide with slow neutrons in the heavy water pile at Chalk River. The antimony trioxide was specially purified in 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, silicon, manganese, strontium and barium definitely absent. The irradiated oxide had a specific activity of one millicurie per gram. The source strength used was a few microcuries, on a backing of mica of thickness less than 1 mg/cm2. The source was covered by a thin film of collodion, prepared by the method of Backus^ 2 2). A l l readings were corrected for 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 in reproducing parts of the spectrum, and i t was necessary to normalize two sections of the spectrum, since the intensity had dropped by about 2 per 16. cent i n the one case and by about 15 per cent i n the second. The possibility 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 irradiation of the sample. The spectrum i s shown-on plate 9. It shows a conversion li n e at ,57 8 Mev, corresponding to a gamma ray .energy of .61° Mev. This li n e has also been reported by Kern, Zaffarno and M i t c h e l l ^ 2 0 who obtained a gamma-ray energy of .603 Mev. A Eermi plot i s shown on plate 8. It consists of four straight line sections, with endpoints agreeing well with those reported earlier this year. A tabulation of results i s shown below: (energies in Mev) Kern, Zaffarno*, Cook. Langer Mann, Lindenfeld M i t c h e l l ^ 2 0 ; (21) .47 .50 .50 ± .02 .63 .68 .65 ± .03 .98 .98 .90 ± .06 1.58 1.50 — 2.31 2.37 2.43 ± .03 There .is an indication of a group with, an endpoint near 1.5 Mev, but i t seems to be very weak:. It must be stressed that these endpoints are derived wholly from the Eermi plot, and therefore depend entirely 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. 2 .-A plot, of ( l 0 - E ) 2 gave approximately a straight line with a slope either zero or at least very small. 17. Thus the van der Held plot for this case reduces to the Fermi plot. The van der Held plot was also plotted using c a 124 x 1.8 x 10~u , hut i t showed no indication of being composed of straight line sections. This shows the importance of determining c separately for each spectrum, since the proportionality to the mass number, as suggested by van der Held, does not seem to hold in 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 for this case i s not straight. The van der Held plot i s apparently better i n this respect. The endpoint from the Eermi plot i s 1.18 Mev, from the van der Held plot 1.16 Mev. The Eermi plot for antimony 124 shows four end-points at .50, .65, .90, and 2.43 Mev. A f i f t h group, reported by Kern, Zaffarno, Mitchell^ 2 0) and by Cook, Langer^ 2 1) i s too weak for significant evaluation in this experiment. The van der Held constant c i s near zero for 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 P L . A . T E a I O O O C U R R E N T (Vol*. .IAZS o« T H I S S C A L S Hp* (306 3 - * * . > 18 ACCTOWLEDGTafTOS The beta-ray spectrometer and auxiliary 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 for 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. for the protection of the experimenters. Mrs. B. E. Speers of the Department of Chemistry has purified the antimony trioxide and prepared the two thorium B sources. Dr. A. M. Crooker has made the spectroscopic analysis of the antimony trioxide. 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 gratefully acknowledged. I would li k e to express my special thanks to Dr. K. C. Mann, under whose encouraging and always helpful expert guidance this research was carried out. 19. REFERENCES E. Eermi Zeitschrift 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 Scientific Instruments, 15, 178, 1944 C. D. E l l i s Proceedings* of the Roys! Society, A 138, 318, 1932 K. C. Wang Zeitschrift fur Physik, 87, 633,.1934 L. M. Langer and M. D. \Whitaker "Physical Review, .51, 713, 1937 C. M. Witcher Physical Review, 60, 32, 1941 J. S. 0'Conor Physical Review, 52, 303, 1937 A. C. G. Mitchell, 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. Miller 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. Mitchell Bulletin 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. Mitchell-Bulletin of the American Physical Society, Jan. 1948 C. S. Cook and L. M. Langer Bulletin of the American Physical Society, Jan. 1948 J'. Backus Physical Review, 68, 59, 1945 J. J. Livingood and G. T. Seaborg Physical Review, 55, 415, 1939 

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