Lawrence Berkeley National Laboratory Recent Work
Title NUCLEAR SPECTROSCOPIC STUDIES OF POLONIUM-204
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Author An, Jiann-Min.
Publication Date 1965-08-05
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[E [f[[l)~§~ 0 ~ ~~~lf~mJ TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Diuision, Ext. 5545 NUCLEAR SPECTROSCOPIC STUDIES OF POLONIUM-204 Berkeley, California DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. UCRL-16324 U1~VERSITY OF CALIFORNIA Lawrence Radiation Laboratory Berkeley, California J AEC Contract No. W-7405-eng-48 NUCLEAR SPECTROSCOPIC STUDIES OF POLONIUM-204 Jiann-min An August 5, 1965 (M. S. Thesis) -iii- NUCLEAR SPECTROSCOPIC STUDIES OF POLONIUM-204 Contents Abstract ..... v I. Introduction 1 II. Experimental Methods 3 A. Preparation of Targets 3 B. Bombardment Characteristics· 3 C. Methods of Separation and Chemical Purification· 4 1. Vaporization . 4 2. Wet Chemistry . 6 III. Measurements · · . 8 A. Alpha-particle Spectra .. •, 8 .10 B. Gamma-spectra· .. · · · . ' 1. Half-life Determination and Isotope Assignment . ..• 10 2. Relative Intensities . ·13 , C. Alpha-particle Branching Ratio .13 \Discussion .22 ' Acknowledgments ... .22 References . 23 .. ' -v- NUCLEAR SPECTROSCOPIC STUDIES OF PONONIUM-204 • Jiann-min An La~rence Radiation Laboratory University of California Berkeley, California August 5, 1965 ABSTRACT Sources of Polonium-204 ~ere derived from the electron capture 12 decay of Astatine-204 made by bombarding Au197 ~ith c in the Heavy Ion I Linear Accelerator at Berkeley. physical and chemical methods ~ere adjusted to make the sources as pure as possible for the measurements. Using scintillation crystals coupled to a 400 channel pulse-height 204 analyzer, an investigation of some of the nuclear radiations of Po has . 204 been made .. Ga~~a-rays from Po ~ere identified by their half-life; their energies and relative intensities ~ere carefully determined and the alpha branching ratio ~as measured . .. -1- I. INTRODUCTION • In the course of research on the radioactivity of various urani~~ ' and thorium minerals, Pierre and Marie Curie discovered polonium in 1898. This ~as the first ne~ element to be discovered as a result of its radio activity. They found that poloniu..rn behaved similarly to bismuth to form an insoluble sulphide compound and subsequently ~ere able to separate them through different volatility by vacuum sublimation. In the early days, the ~ork on polonium ~as complicated by the extremely small quantities available and sometimes by.incomplete separation of radio-active impurities, so that it ~as not until 1906 that the identification of polonium ~as definitely established. Ho~ever, the chemistry of polonium has been placed on a sound basis only ~ithin the last fifteen years. Polonium-210 is a natural product in the uranium-radium decay chain. Trace amounts of polonium ~ere usually obtainable from aged radon tubes and from aged radiu..rn salts. No~, ~ith the intense neutron fluxes in nuclear reactors, polonium can be economically prepared by the thermal neutron 20 210 210 capture of bi~muth, Bi 9(n,y) Bi ~>Po . This process is currently used for the production of milligram quantities of polonium. Templeton, l 206 p 207 d p 208 b Ho~ l and an d Per l man have produced Po , o an o y bombarding lead or bismuth in a cyclotron ~ith 20-MeV deuterons or 40 MeV heliu..rn ions ,2 209 in 194 7. Kelly and Segre have found Po · . By using higher energy beams in the 184-inch cyclotron at Berkeley, Karraker and Templeton3 have pro- . . 205 204 203 202 duced the llghter lSotopes of Po , Po , Po and Po . Through 4 200 improved measurements Karraker, Ghiorso and Templeton have reported p0 201 and Po . Still lighter polonium isotopes have also been studied. The isotopes ~ith mass number belo~ 200 had .been studied by s~edish scientists of the Nobel Institute ~ith heavy-ion bombardments and in Uppsala ~ith proton bombardments of bismuth.5 Polonium-204 has been observed and reported by different ~orkers.3' 6 '• 204 The decay relations of Po are as follo~s: -2- Pb204m / 68 min • G· / ~r1>/ l I.T. E.C. E.C. 4 ~ Bl. 204 E. C. "V 20h At20 ----'? Po204 ·-----', ------~--> Pb ·(stable) 9 min 3.5 hr 12 hr a '¥I E.C. E.C. 00 Pb200 ,. Hi (stable) 18 hr 204 197 In the present investigation, Po was produced by bom'oarding Au with 12 c in the Heavy Ion Linear Accelerator (Hilac). The polonium samples used in this measurement were chemically_purified from "bismuth and lead by the method developed by Kimura7 and co-workers. The ga~ma rays from the polonium samples were measured with the recently developed germanium crystal detectors. It was the prime objective of the present study to 20h provide information for the further study of the decay scheme of Po -3- .. II . EXPERIMENTAL METHOD A. Preparation of Targets The target consisted of a gold foil in front of which there was interposed an alu.rninu.rn foil as absorber.. By using different thickness of absorber11 in the beam of particles from the accelerator, we could reduce the energy of the particles to some desired value before it reached the gold foil. From known11 range-energy relationships we could choose the proper thickness of the gold foil and hence to control the energy band to 12 . which the target atoms were exposed. Hoff, Asaro and Perlman have given the relative yields for astatine isotopes produced in carbon ion bombard ments of gold. With those data tog,ether with the excitation ftmctions for 1 13 (c12, xn) reactions on Au 97, the desired sources could be prepared without difficulty. B. Bombardment Characteristics Using the branching ratios and half-lives of different astatine 1 isotopes, Thomas, Gordon, Latimer and Seaborg 3 have calculated the cross 12 sections and given the excitation functions for (c , xn) reactions on Au197 which are shown in Fig. l. In the present work, our interest is in the 5n reaction. However, the energy range which would give the maximu.rn 5n reaction is not suitable for this bombardment, because a substantial 205 205 amount of At and its undesirable daughter Po would be produced. . 205 . From the literature we know that Po has a half-life of 1.5 hours; 204 203 202 Po 3.5 hours; Po 42 min; Po 43 min. Therefore we chose the beam 20 energy of 90 MeV to 100 MeV in order to avoid the 4n reaction. The Po 3 202 and P o produced in this bombarQrnent would decay much faster than the •• 204 Po and hence would not disturb the experiment. This energy range did give very satisfactory results. We did not observe any radiation from 20 the contaminant Po 5. -l+- J3omoard.rnents "Were carried out "With J3erkeley Heavy Ion Linear A~celerator (Hilac) "Which accelerates heavy ions to 10.4 MeV per nucleon. The caroon ion oeam intensities used "Were in the range of 2 to 3 microamperes. The entire assemoly served as a Faraday cup for monitoring the oeams. It "Was cooled to avoid melting the target. The oomoarding times ranged from 20 to 60 min. C. Methods of Separation and Chemical Purifitation 1. Vaporization Method Follo"Wing the oomoardment, the target holder "Was removed from the Hilac and disassemoled under a hood. The gold foil ·Has removed and trans·· ferred into a glove oox and placed inside a small quartz cup "Which "Was heated "With an induction heating coil. The gold foil "Was melted and the volatile species, astatine, polonium and others "Were collected on platinum plate cooled "With dry ice. This platinQrn plate "Was removed from the glove oox and placed on a hot plate 'Which "Was preheated to a temperature of aoout 380°-390° c. A "bottomless aluminum cup "Was placed over the sample and part of the activity from the, first platinum plate "Was vaporized onto a second platinum plate "Which "Was in turn cooled oy dry ice. At this temperature only astatine "Was vaporized and left polonium and "bismuth 9n the first platinQrn plate. No poloniQrn or "bismuth acti ylties "Were ooserved initially on the second platinum.plate follo"Wing the second vaporization: T"Wenty minutes after the second vaporization the second platinum plate "Was flamed slightly or heated on the hot plate at a temperature not higher than 400° C. J3y this means "We got rid of the astatine activity and left its polonium daughters on the platinum plate. T"Wenty minutes after the second vaporiza- 204 205 tion most of the At has decayed a"Way, out At "With a half-life of 25 min "Would still remain in suostantial amount. This is a "Way to hold do"Wn •• the c01i.tamin~nt polonium~205. This :ttlatinum p:tate was k@pt waiting for 5 hours for all the polonium-203, polon~um-202.and most · of the poloniQrn- ( 205 to decay a"Way. Then a third -vaporization "Was made at 8: temperature of 440 o C. At this temperature most of the poloniQrn "Would oe vaporized av1ay -· and "bismuth and lead "Would stay. The polonium e.cti vity "Was collected ontb' -5- 100~ 10 ..c E .C: 0 .,_ (.) rJ) rJ) 0,_ u E c 12 ( Ia b ) ( Mev) MU-252.<10 •• Fig. l. Excitation functions for c12,xn reactions on Aul97. ~ J -6- a third platinum plate. The activity on this plate served as the source for alpha-spectra and gamma-spectra measurements. Many attempts were made oy this stepwise vaporization method. It was found that this method worked well foi?. · astatine but not for polonium. The reason for this may be due to the fact that at 440° C the vaporization of polonium was quite incomplete and at higher temperatures bismuth would go with it. In all the -attempts, the activities collected were too low and sometimes contaminated badly with bismuth activity. 2. Wet Chemistry Method Since polonium forms a number of complex halides in solution, it was expected that some of these compounds could be extracted into organic solvents. Kimura and co-v70rkers 7 found that poloniu.'ll could not be extracted from HCl or HBr solutions with diethyl ether or isopropyl ether. However, polonium was extracted with isopropyl ether or methyl isobutyl ketone from HCl solutions containing KI. In the present work, the chemical :procedures were 1 as follows. Following the bombarQ'Uent (60 min), the target was removed from the Hilac and disassembled under a hood. The target gold foil was not melted until 5 hours after the bombarQ'llent, because most of the shorter lived polo~iu.'ll isotopes would decay away within the 5 hour per~od. Then the gold foil was melted as before in a glove box, and the activities were collected on.platinum plate, mainly polonium, bismuth and lead. The platinu.'ll plate was removed from the glove box and washed with l cc of solu tion 0.25 M in KI and 3 N in HCl, 4 ml of isopropyl ether was then added to this solution and poloniu.'ll was extracted into the organic phase v7i th gentle swirling, while lead and bismuth remained in the aqueous :phase. Polonium was s·crubbed from the organic phase by shaking with l ml of • t! 3 N HCl solution; The resulted HCl solution was free from bismuth arid lead and contained :polonium alone. This HCl solution was evaporated to dryness on a platinum plate and served as the source for the various measure ments. The separation of polonium from bismuth and lead was very good. We -7- can see this oy comparing the ga~~a ray spectrum of the poloniQ~ and oismuth fractions shown in Figs. 4 and 6. A yield of approximately 80 percent was ootained and time required for this separation was aoout 15 minutes. ,I • -8- III . MEASUREMENTS A. Alpha-particle Spectra The :polonium fraction was _measured in an al:ph,a-:particle ioniza tion chamber. This ionization chamber utilizeda grid to shield the collecting electrode from :positive ions. A gas mixture of 93% argon and ·7% methane was used. Through a :preamplifier, discriminator, and amplifiers, the :pulse was sent into a 400 channel :pulse-height analyzer. Alpha par ticles have a counting efficiency of 42 :percent when counted in this grid chamoer. Al:pha-:particle spectra were obtained as shown in Fig. 2. PoloniR~-208 and :poloniR~-210 standardswereused for energy calibration. Two al:pha-:particle :peaks)· one with energy 5.375 MeV andthe other with 5.581 MeV were observed in this measurement. The decay was followed for 24 hours. It was found that a:l:pha-:particles with energy of 5.375 MeV decayed with a half-life of 3.5 hours and the alpha-particles with energy of 5.581 MeV decayed with a half-life of 43 min. The decay curves are shown in Fig. 3. From the :previously·re:p~rted data (see Taole I) the . . 204 202 5.375 MeV :peak belongs to Po and the 5.581 MeV peak belongs to Po . 204 Karraker and Tem:pl~ton observed Po in 1950. These early experiments were designed to estaolish the decay :produc~s and mass assign ment. They separated its daughter activity at a seq_uence of eq_ual time intervals~ the intervals corresponding approximately to the half-life of the :parents. A similar experiment was also done to identify the alpha decay daughter and fix the mass number. Their data together with other information are listed in Taole I. 202 The assignment of Po was oased on all the :previous information 202 given. The alpha oranching ratio of Po has oeen reported oy Stoner and 8 206 Hyde to be 2% from their experiments on the alpha-decay of Em . The • !• 203 alpha "branching of Po has not oeen reported yet) but it would appear to be much smaller than Po202 . Therefore we would assign the 5.581 MeV 202 20 to be Po . in spite of the similar half-life of Po 3 and Po202 . -9- Table I. Al~ha s~ectra data. Isoto~e Energy (MeV) Half-life Reference nQ~ber Po204 5.37 3.8 hr (3) -5·35 ± 0.05 3.5 ± 0.6 hr (6) 5.39 3.6 ± 0.2 hr (10) 5.375 3.5 ± 0.01 hr -This·, 'WOrk Po202 5.580 ± 0.010 44 ± 7 min (12) . ' 5.57 50 ± 10 min (10) 5.57 ± 0.05 51 ± 3 min (8) 5.61 ± 0.05 56 ± 5 min ( 9) 5.581 43 ± 4 min This vJOrk I>' -10- B. Ga~'Ua-suectra t t b Information on the gamma-rays daua~ o~~·p o204 has no ye een reported. The recently developed germanium crystal detector does provide a powerful tool in gamma-ray analysis. A germaniu.'TI crystal detector with 3 em by 2 em by 0.9 em dimension was used for the present measurements. A relative efficiency curve of the detector had been carefully made. A 22 ' Na standard with known disintegration rate was used in determining the absolute efficiency. All .the ga~'Ua-rays observed and their half-lives and possible assignments are listed in Table II. The gamma-spectrum is shown in Fig. 4. 1. Half-life Determination and Isotope Assignment The decay of all the ga~'Ua-rays has been followed. It was.found that there are eleven gamma rays in the polonium fraction which decayed 204 with approximately the half-life of Po and are therefore assigned to this isotope. They are listed in Table II, and their decay curves are shown in Fig. 5. It should be noted that these ga~'Ua rays were not observed in any substantial intensity in the bis~uth and lead fraction ' (Figs. 4 and 6). This latter fraction was obtained directly by vaporiza- tion from the melted gold target foil and hence contained other radio active elements beside bismuth and lead. In Fig. 4, (the gamma spectra of the polonium fraction) only two 204 intense ga~'Ua peaks ~f Bi have been labeled (375 keV and 899 keV). No 204 attempt has peen made to label all Bi ga~'Ua rays which grow into the polonium fraction and decay with a half-life of about 12 hours. 202 Ga~'Ua-rays of energy 421 keV and 961 keV seem to be due toBi 202 which is made by the electron capture decay of Po From the decay curve in Fig. 7, we see that these ga~'Ua rays grow into the poloniu.'TI • Cl,• fraction and then decay with a half-lifeof abou~ 98 minutes.as would be expected. Gamma-rays of these energies had been previously reported by Herrlander and Bergstr6m15 who assigned them to Bi202 . l ~ ~ ... ~·· .. I I ' } I r I ~ ~ .~ ~ I I I \ .,. ~- ~- I I 'f. l 4 t 10 Pol04 ...."' 5·375 Mn "' ,.-, ~ to3 :IE { \ / ~ I : ~ 0 ' 1-'. r. • 1-' ' \ .... ! \ 101 "'z ;' \ Po ::> I t 0 2 5·581 Mew (.) 10 J ~ ••• ./ '. .~"":-, ./ '\_ ../ :~ ;-? ·\~ ,: .. ~ v ., / ~ ·:-- , \ .. - ... ' ·~ .· .. :\ . .,, -~.··.· I ·:\ , .•,.. ,, ," / 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 CHANNEL NUMBER MU-36UO Fig. 2. Alpha .. spectrum of chemically purified.Po sample. \ ·-' lt ~ 3 10 Haif life • 43 mill (/) J z 5·581 Mev. I ::> l-' 0 I\) u I 10 0 1100 1200 1300 1400 1500 MINUTES Fig. 3· Decay curves of alpha groups observed in the MU-3H78 chemically purified Po sample. 6 .. A '- ' ...... ~ ...... _.... ~. _. __ ....,.,, __ ~"' -13- Two gamma rays ofshorter half-life were observed. The gam.rna ray decay curves are. shown in Fig. 8. The 707 keV gamma ray decayed with approximately 44 min half-life and the 1087 keV gam.rna ray decayed . with approximately a 45 min half·-life. These gamma rays might be due 20 202 to Po 3 or Po . This remains a question for· further. investigation. 2. Relative Intensities In measuring the intensities of these gamma rays, a chemically purified polonium .fraction sample was used. The gam.rna spectrQrn from this sample (as shown in Fig. 4) was observed with a germanium crystal detector connected to a 400 channel pulse height analyzer. The various peaks were integrated and the necessary· corrections for count ing efficiencies were made. The results are shown in Table III. Compar isons were made among those gam.rna rays which appear to be from the same . 204 isotope. The eleven gam.rna rays of Po were compared relative to the 268 keV gamma ray which was taken as one hundred. C. Alpha-particle Branching Ratio For the branching ratio·measurement, the sample used was the chemically purified poloniu.rn fraction previously discussed. The measure ment started about 50 minutes after the chemical separation. A gam.rna spectru.rn is shown in Fig. 4. The 899 keV gamma ray of 204 B J.· ·was· . prevJ.ous· 1 y kn own t o. have an J.n. t ensity of 100 percent. T he 899 keV peak decayed with a half-life 11.9 ± 1.5 hours after most of 204 ( . the Po has gone about 12 hotrrs after the chemical separation). 204 Since Bi comes fromthe electron capture decay of-Po204 . if we can ) . 204 find out the initial alpha intensity of Po (i.e. alpha intensity of Po 2~ at the time of chemical separation), and the initial Po 2~ electron capture intensity which would lead to the obs.erved intensity of the 899 keV gam.rna ray at time t, we can calculate ·the branching ratio. Then we have: -14·- Table II. Ga~~a rays observed in this work. (.' Possible assignment Energy(keV) Half-·life measured ( Po204 140 205 ± 6 min 202 210 ± 5 min 268 208 ± -4 min 305 210 ± 6 min 316 218 ± 14 min 537 215 ± 4 min 680 212 ± 11 min 764 210 ± 2 min 883 215 ± 3 min 1022 215 ± 4 min 1037 220 ± 15 min Bi202 421 99 ± 4 min 961 97 ± 5 min p 203 p 202 o or o (?) 707 44 min 1087 45 min . ( '-< q v ·~ I! Kc1 Ka1 5 10 r II Kt Po••• UOICI'I I A Po••• ~- 10J .I I I :E 1-' Vi I !04 81 ••• Bi Po••• Po••• ~ HIK1'1 421 h'f SlfKe'l .. AI .I I ••• <..) 3 If Po i 204 10 Po 10t!Jll'l I 201 tOl Po or Po 1 11 2 10 L---~-~-~--~-~-----~sfo~--~e~o----~~~o~o~~~~z~o----7147.0~--~~6~0~--~~~eo=---~zo~o=---~z~2~0--~2~4~o~--~z~s~o----~z~e~o--~3~~o 3~----3..18~0----4_JOO CHANNEL N.UMBER MU-36H6- Fig. 4. Gam.rna ·spectrum of the chemically purified Po sample. 1 1 1 1 1 1 1 1 (1 1 1 1 1 1 1 'ft • 2.0!1! 6 Mlr\ I \40 K•• 1 1 1 Tl.• •2.'ot5~'" z.o?. t 1'\ • 2,\0 min!764K<" 1 1 ,.t .z,~!' "''n . I 893 K •• 1 1 1 1 1 1 1 1 1 zo4 1 Fig. 5· necaY curves of the gamma raYS of Po from the cnemicall:i 1 1 piJ.rified ?o se.m.ple. 1 1 1 "-/ ~ _;; ~ ' 204 200 R> • Pb (?) 200 I I'll I ., (?) ~;204 1-' t -'--1 z I :::> 0 (?) u 9821<.. I 2 ~· 10 10 / 400 CHANNEL NUMBER •MU-36475 Fig. 6. Ga~~a spectrQ~ of Bi and Pb fraction. I · t I -·---- Holf life • 97 t 5 min 961 Ktw --- 3 ', --- 10 --- ',, I Ul ...... 1-' .... CP z I :::> ', Half life"' 99± 4 min 0 u 4 Zl Kev 2 ...... 10 ' ' ' ' .... Half lift • 4 3 min 101 ', '',~:~:!helical) ', ' ' ' ' ' ' 0 360 400 440 480 520 560 600 640 680 800 MINUTES MU.J64H Fig. 7. Decay curves of gamma rays of Bi 202 in the chemically -purified Po sample. (The dashed lines and circles are hypothetical and based on a 43 min half life pare~t aDd the growing of a 99 min daughter. /~ 6 .... -~ __ , ~ -:;>· 14' I t I l I I i, ' \ t I I i ., 1- z ::::> 0 u I f-' ~ 2 I 10 10 50 100 150 200 250 750 MINUTES MU -36H7 Fig. 8. Decay curves of shorter half-life gamma rays in the chemically purified Po sample. - M------~--~---~-'-·------~-- ·--- ••··---·- ~-·--·~-··- ~·· -20- Table IIT. Relative intensities. . ~· Energy Efficiency ·.Relative counting Relative intensity (keV) rate ( Garn.ma-rays o~ Po204 140 2.6 X 10-3 23764 66 202 1.42 X 10-3 2620 13 268 8. 6 'X 10 -4 11827 100 -4 305 6.9 X 10 1600 17 -4 316 6.5 X 10 1733 19 . 6 -1~ 537 · 2. ·x 10 1857 52 -4 680 1.71 X 10 917 l~o 4 764 1.38. X 10- 810 43 4 883 l.l X 10- 1523 101 5 1022 8.9 X 10- 937 77 1037 8.8 X 10'"5 339 28 i 202 Gamma-rays of Bi -4 421 3.9 X 10 6oo 100 961 9.6 X 10-5 233 158 Garmna-rays o f p o 203 or p o 202 (?) 707 l. 6 X 10 -4 159 100 1087 8.4 X 10-5 93 111 -21- 204 Po · (al~ha intensity)t=O Alpha branching ratio =·-'------.,.----__:::_:::______-.c.._ 204 Bi (899 keV total intensity)t=t ga~~a X -~it -kpot e e = {o.66 ± 0.007) %. A grid chamber ~as used for the alpha measurement ~ith counting efficiency 22 . . 6 of 42%. A kno~n source of Na containing (1.06 ± 0.6) X 10 d~m ~as used for the absolute efficiency calibration. A conversion coefficient correc tion (0.85%) for the 899 keV ga~~a ray ~as also made. fir • -22- IV. . DISCUSSION 204 .The alpha particl~ branching ratio of Po has been estimated by Karraker and Templeton) to be 1% based on their experiment of comparison of expected reaction cross sections and actual yields. Latimer, Gordon 16 and Thomas have plotted the log partial alpha.half-life of even-even polonium isotope versus the inverse square root of the effective alpha decay energy .. They have been able to predict a value of 0.0063 ± 0.0016. Our result of 0.0066 ± 0.0007 in the present work is in good agreement with their calculated value. There is a vast amount of work on the energy levels remaining to be done. High resolution y-y and e -y coincidence experiments in par ticular would be worth further investigation. ACKNOWLEDGMENTS I would like to express my gratitude to Dr. Frank Asaro and Professor Isadore PerL~an, under whose continued help and guidance this work was conducted. The discussions with Dr. William C. McHarris are appreciated as well as the opportunities to share bombarding time with. Mrs. Janis M. Dairiki. Special thanks to my wife, Florence for her inspiration and patience. This work was performed under the auspices of the U. s. Atomic Energy Co~~ission. -23·· REFERENCES 1. D. H. Templeton, J. J. Howland, and I. Perlman, Phys. Rev. 72, 758 (1947). 2. E. L. Kelly and E. Segre," Phys. Rev. 75, 999(1949). 3. D. G. Karraker and D. H. Templeton, Phys. Rev. 81, 510 (1951). 4. D. G. Karraker, A. Ghiors~ and D. H. Templeton, Phys. Rev. 83, 390 (1951). 5. S. Rosenblum and H. Thren, Compt. Rend. 239, 1205, (1954). 6. W. E. Burcham and B. C. Haywood, The Proceedings of the Physical Society (London), ~' 862 (1956). 7. · K. Kimura and T. Ishimori, Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic Energy, Geneva (United Nations, Geneva, 1958) Vol. 28, pp. 151, P/1322. 8. A. ltJ. Stoner and E. K. Hyde, J. Inorg. Nucl. Chem. ~' 77 (1957). 9. W. E. Burcham, The Proceedings of the Physical Society (London), 67A, 555, 733 (1954). 10. 'W. Forsling and T. Alvager, Ark. Fys. 19, 353 (1961). ll. ,E. L. Hubbard, University of California Lawrence Radiation Laboratory I . 'Report UCRL-9053 (1960). 12. R. W. Hoff, F. Asaro, and I. Perlman, University of California Lawrence Radiation Laboratory Report UCRL-7211 (1963). 13. T. D. Thomas, G. E. Gordon,. R. M. Latimer, and G. T. Seaborg, University of California Lawrence Radiation Laboratory Report UCRL-9950 (1961). 14. P. Thoresen, Private co~~unication (1964) 15. J. A. McDonell, R. Stockenda~, c. J. Herrlander and I. Bergstr~m, Nucl. Phys. 2_, 513 (1957). 16. R. M. Latimer, G. E. Gordon, and T. D. Thomas, University of California Lawrence Radiation Laboratory Report UCRL-9217 (1960). This report was prepared as an account of Government sponsored work. Neither the United States, nor the Com mission, nor any person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, appa ratus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any infor mation, apparatus, method, or process disclosed in this report. 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