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WILSON, Robert Gray, 1934- NUCLEAR ENERGY LEVEL SCHEMES AND SYSTEMATICS IN THE HEAVY RARE-EARTH REGION.

The Ohio State University, Ph.D., 1961 Physics, nuclear

University Microfilms, Inc., Ann Arbor, Michigan NUCLEAR ENERGY LEVEL SCHEMES AND SYSTEMAT1CS

IN THE HEAVY RARE-EARTH REGION

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

By

ROBERT GRAY WILSON, B.Sc.

The Ohio State University 1961

Approved by

S ) 2 . L A d v ise ^ Department of Physics and A stronom y ACKNOWLEDGMENTS

I wish to acknowledge the experienced guidance of my adviser, Professor M. L. Pool, in the execution of this work.

Appreciation is expressed to Mr. R. P. Sullivan for his assistance in the electronic phases of this research. Thanks are due to my associate, Mr. G. G. Staehle, for his congenial collaboration on one of the publications and his major share in another of which he is the senior author. TABLE OF CONTENTS

I. INTRODUCTION ...... 1

II. SUMMARIES OF PUBLICATIONS ...... 2 in. APPENDIXES ...... 14

IV. BIBLIOGRAPHY ...... 5 3

V. AUTOBIOGRAPHY ......

iii ILLUSTRATIONS

Publication* Page Figure Caption 164 1 1827 1 Half-life decay curves for Tm .

1 1828 2 Gamma-ray spectrum of Tm ^^,

1 1828 3 Energy level scheme for the decay of Tm ^^, 162 1 1828 4 Energy level scheme for the decay of Tm .

2 1 Low energy gamma-ray spectrum of Yb*^.

2 2 Hi energy gamma-ray spectrum of Yb*^. 165 2 3 Yb coincidence spectra. 165 2 4 Energy level scheme for Er . „ 165 2 5 Gamma-ray spectrum of Tm

t. 165 . ^ 2 6 Tm coincidence spectra.

3 263 1 Gamma-ray spectrum of Tm ^^. 166 3 264 2 Tm coincidence spectra.

3 265 3 Energy level scheme for the decay of Tm ^^. 168 4 1 Gamma-ray spectrum of Tm ™ 168 4 2 Tm coincidence spectra, 168 4 3 Energy level scheme for the decay of Tm . 167 5 1296 1 Gamma-ray spectrum of Yb 167 5 1297 2 Energy level scheme for the decay of Yb 168 6 227 1 Energy level scheme for the decay of Lu .

r The publications are listed in Appendix 1. IV P age F ig u re C aption 170 7 9 8 1 1 Gamma-ray spectrum of Lu . 170 7 9 8 2 2 Energy level scheme for the decay

8 1844 1 Gamma-ray spectrum of Lu*^*. 171 8 1845 2 Coincidence spectrum of Lu 171 8 1845 3 Coincidence specrta of Lu 171 8 1846 4 Energy level scheme for the decay i

8 1848 5 Energy level scheme for the decay 169 172 9 1068 1 Half-life decay curve for Lu 172 9 1068 2 Gamma-ray spectrum of Lu 172 9 1069 3 Coincidence spectra for Lu 172 9 1070 4 Energy level scheme for the decay

10 808 1 Gamma-ray spectrum of Lu*^. 173 10 809 2 Energy level scheme for the decay 174 11 517 1 Gamma-ray spectrum of Lu 174 11 518 2 Energy level scheme for the decay

v TABLES ic P age T able C aption

1 1827 1 Composition of enriched erbium samples

1 1828 2 Internal conversion coefficient data 165 2 1 Data concerning transitions in Er 165 2 2 Coincidence information for Tm 165 2 3 Data concerning energy levels in Er _ 166 3 263 1 Data concerning transitions in Er

3 264 2 Internal conversion data for erbium

3 265 3 Coincidence information for Tm ^^

t_ . „ 168 4 1 Data concerning transitions in Er

4 2 Coincidence information for Tm ^^

5 1296 1 Coincidence information for Yb*^ 167 5 1297 2 Data concerning transitions in Tm

7 981 1 Coincidence information for Lu*^ 171 8 1844 1 Data concerning transitions in Yb

8 1845 2 Coincidence information for Lu*^*

8 1847 3 Internal conversion data for ytterbium

1 6 9 8 1849 4 Data concerning transitions in Yb

8 1849 5 Energy ratios for rotational bands 172 9 1068 1 Data concerning transitions in Yb

vi Publication Page Table Caption

9 1068 2 Internal conversion data for ytterbium 172 9 1069 3 Coincidence information for Lu 173 1 0 808 1 Coincidence information for L>u 173 1 0 809 2 Data concerning transitions in Yb INTRODUCTION

Thirty radionuclides of erbium ( 6 8 ), thulium (69)» ytterbium

(70), lutetium (71), and hafnium (72) with half-lives between two minutes and 600 days have been produced and studied prim arily by gamma-ray scintillation and gamma-gamma coincidence techniques.

Samples of heavy rare-earth oxides enriched in the stable mass numbers were irradiated with 6 -Mev protons, 17- and 24-Mev helium nuclei, and thermal neutrons. Approximately 130 irradiations were performed and about twice as many samples prepared and examined in a period of two and one-half years. Among the thirty activities 162 examined, four are previously unreported, ^^Tm (77 minutes),

^T m ^^ (2. 0 minutes), ^Y b*^ (9. 3 minutes), and ^L u*^ (7.1 minutes); and two previously reported, were shown not to exist, 171 172 ^jLu (600 days) and yjku (4*0 hours). The goal of this research was the construction of energy level schemes for the decays of these radionuclides and their daughter nuclei and an interpretation of the observed systematics. The experimental results of this work and the interpretations are reported in eleven publications in The

Physical Review and the Bulletin of the American Physical Society in I960 and 1961.

1 SUMMARIES OF PUBLICATIONS

162 Tm

Erbium oxide enriched to 35.1 percent in the mass number

162 was irra d ia te d with 6 -Mev protons. A previously unreported activity decaying by electron capture with a half-life of 77 minutes 162 was produced and assigned to Tm . The observed radiations were the erbium K x ray and gamma rays of 102 and 236 kev. All 162 three of these radiations are in coincidence. Energy levels in Er are assigned at 0 (0+), 102 (2+), and 338 (4+) kev,and the ground 162 state of Tm is tentatively assigned a spin of 3-. Electron capture branches occur to the 102-kev level (87%) and the 338-kev level (13%). The occurrence of some positron decay could not be ruled out because of the existence of the 1 1 2 -minute positron activity 18 of F .A search for other electromagnetic radiation between 0 and 3000 kev yielded negative results.

Erbium oxide enriched to 14. 1 percent in the mass number

164 was irradiated with 6 -Mev protons. A previously unreported activity decaying by both electron capture and positron emission with a half-life of 2. 04 minutes was produced and assigned to Tm ^^, The observed radiations were the erbium K x ray, annihilation radiation, a 2.9-Mev positron, and a 91 -kev gamma ray. No other gamma rays with energies between 0 and 3000 kev were detected. The half-life of this activity precluded coincidence measurements.

Although the 2. 9-Mev positron requires that the decay energy 164 164 of Tm is at least 3.9 Mev, only one excited state in Er (04) is indicated by these observations and occurs at 91 kev (2+). The relative intensities of the radiations imply that electron-positron 164 decay branches occur to the ground state of Er (57%) and to the

91-kev level (43%). This situation suggests that there may be two positrons of energies differing by 9 1 kev in the particle radiation spectrum. A coincidence measurement is being planned in order to attempt to prove or disprove this postulate. The best assignment of spin for Tm ^^ is 1 + with even parity favored by the short half- life.

_ 165 Tm

Thulium 165 (29 hours) was studied following the decay of 165 Yb (9. 3 minutes) which was produced by the irradiation of erbium oxide enriched in the mass number 162 with 24-Mev helium nuclei.

The complex gamma ray spectrum was analyzed with the aid of a previously reported analysis of the conversion electron spectrum by other workers. Gamma-gamma coincidence measurements were performed and used to aid in the construction of a complicated energy level scheme for erbium 165 with twenty levels at 0 (5/2-),

47.2 (5/2+), 77.2 (7/2-), 117. 8 (7/2+), 243.3 (3/2-), 296. 5 (5/2-),

297. 8 (1/2-), 356. 9 (3/2-), 384. 7 (5/2-), 507. 5 (3/2+), 564. 2 (5/2-),

574. 3 (7/2+), 589.8 (5/2+), 608.7 (7/2-), 664. 2 (3/2-), 699. 3 (3/2+),

854. 7 (1/2+), 1051. 3, 1250. 6 , and 1428. 8 (3/2-) kev. This energy level scheme accounts for 63 transitions following the decay of 165 Tm , the strongest of which are 47. 2 (20. 6 %), 54. 5 (26. 1%),

77. 2 (7. 2%), 243. 3 (41%), 297. 8 (15. 5%), and 807. 4 ( 8 . 0% ).

The primary electron capture branches occur to the levels at 243. 3

(5.5%), 297.8 (42.5%), 356.9 (11. 1%), 854.7 (13.0%), and 1428.88

(7. 2%) kev. The evidence strongly favors a spin assignment of l / 2 + for Tm^^,

T m 1 6 6

Erbium oxide enriched to 72. 9 percent in the mass number

166 was irradiated with 6 -Mev protons,and an activity decaying by electron capture and a small amount of positron emission with a half-life of 6 . 6 9 hours was produced and its assignment to confirmed. The complex gamma-ray spectrum was analyzed and w ©X© extensive gamma-gamma coincidence measurements^performed.

The measured endpoint energy of the particle radiation spectrum the was 2090 kev. Inflight of a recent investigation of the positron spectrum which shows that there are two components of 1 9 2 0 and 30 1220 kev. The former higher energy electron spectrum is now

believed to be caused by the conversion electrons from the intense

2088, 2060, and 1873 kev transitions. This paper concerning the

positron spectrum also gives evidence that the positron decay occurs

to the two 2+ levels at 80. 6 and 787 kev, while the gamma-gamma

coincidence measurements of this work were interpreted to mean

that positron decay occurs to the level at 265 kev. What the coincidence

spectrum does prove is that positron annihilations do precede the

depopulation of the 265-kev level. A better explanation now of this

observation is that the annihilation radiation in coincidence with the

transitions depopulating the 265-kev level is caused by the annihil*

ation of positrons formed by pair production from the 1875-kev transition which populates this level. Additional information

supporting all of these conclusions is the result of the recent

measurement^ of the ground state spin of Tm ^^ which is 2 .

A re-analysis of all of the existing data now leads to an energy

level scheme for the decay of Tm ^^ with levels in at 0 (0 +),

30 S. Chojnacki, Yu. Norseev, Z. Preibisz, J. Wolowski, andJ.

Zylicz, Polska Akedemia Nauk Instytut Badan Jadrowych No. 177/

I-A (I960), and to be submitted for publication in Acta Physica

P o lo n ica .

^ J.C . Walker and D. L.. Harris, Phys. Rev. 121, 224 (1961). 6

80. 6 (2+), 265. 0 (4+), 547 ( 6 +), 787. 2 (2+), 860. 7 (3+), 957.4 (4+),

1015 (3-), 1247 (3+), 1451 (1-), 1462 (2-). 1509 (2-), 1456 (3-),

1855 (1+), 1942 (2+), 2137.6 (3-), and 2166 (2+) kev. This scheme accounts for 38 transitions and the two positron branches mentioned above, and for eight electron capture branches^ the strongest of which are to the levels at 2166 (11%) and 2137. 6 (63%) kev. The decay energy of T m ^^ (2-) is 3020 kev according to this scheme.

T m 1 6 8

The irradiation of erbium oxide enriched to 76. 9 percent in the mass number 168 with 6 -Mev protons produced an activity decaying with a half-life of 8 6 days by electron capture. Extensive gamma- gamma coincidence measurements led to the confirmation of a level 168 scheme for the decay of Tm reported during this investigation.

Some additional information was obtained and an analysis of all 168 existing information indicates levels in Er at 0 (0), 79. 8 (2+),

264.4 (4+), 549.0 (6 +), 822.2 ( 2 +), 896.9 (3+), 996.3 (4+), 1095 (3-),

1244 (3+), and 1543 (3-) kev. The primary electron capture branches are to the levels at 1095 (38) and 1543 (41%) kev. Twenty-eight 168 transitions in Er are accounted for by this scheme. 7

Y b 1 6 5

When erbium oxide enriched to 14. 1 percent in the mass

number 162 was irradiated with 24-Mev helium nuclei, a previously

unreported activity decaying by both electron capture and positron

emission with a half-life of 9. 3 minutes was produced. Because the 165 daughter of this activity is the previously assigned 29-hour Tm , 165 this 9. 3-minute activity was assigned to Yb . The radiations

observed in this new activity were the thulium K x ray, annihilation

radiation, a strong gamma ray of 80 kev, weaker gamma rays of

1090, 1275, and 1440 kev, and composite gamma radiation in the

ranges 635 to 755 and 780 to 845 kev. Further analysis of this

activity must be carried out before an energy level scheme for the

decay of Tm ^^ can be constructed.

Y b 1 6 7

The irradiation of erbium oxide enriched to 35. 1 percent in

the mass number 164 with 24-Mev helium nuclei produced the pure 167 electron capture activity of Yb . The measured half-life of this

activity was 17. 7 minutes. Three gamma rays with energies of 106,

113, and 176 kev were resolved in this low energy activity in addition to the thulium K x ray. The results of gamma-gamma coincidence measurements and an anlysis of the conversion electron spectrum by other workers were used to construct an energy level 167 scheme for Tm with levels at 0 (l/2+), 10.4 (3/2+), 116. 5 (3/2+),

142. 3 (5/2+), 179.4 (7/2+), and 292. 7 (7/2-) kev. Two electron capture branches were established, 8 9 percent and 1 1 percent to the levels at 292. 7 and 179.4 kev respectively.

T 168 Lu

Ytterbium oxide enriched to 30. 9 percent in the mass number

168 was irradiated with 6 -Mev protons. A previously unreported activity decaying prim arily by electron capture with a half-life of 168 7. 1 minutes was produced and assigned to Lu . The radiations observed in this activity were the ytterbium K x ray, stronger gamma rays of 87, 900, and 987 kev, weaker gamma rays of 1410,

1800, and 2130 kev, and only possibly a small amount of annihil­ ation radiation. Gamma-gamma coincidence measurements were not performed for this short half-life activity. However, an energy 168 level scheme for Yb has been constructed with levels at 0 (0+),

87 (2+), and 987 (2+) kev. The relative gamma-ray intensities imply large electron capture branches to each of these three levels. Ytterbium oxide enriched to 81.4 percent in the mass number

170 was irradiated with 6 -Mev protons. An activity decaying by electron capture and possibly a small amount of positron emission 170 with a half-life of 2. 05 days was produced and assigned to Lu

A very complex gamma-ray spectrum extending past 3 Mev was o b se rv e 4 and gamma-gamma coincidence measurements were attempted. Because of the complexity of the spectrum, only a few clear results were obtained and only a partial energy level scheme could be constructed. After this work was submitted for publication, the results of an analysis of the conversion electron spectrum were reported. With the aid of these data, a not in proof was added which stated the level energies more accurately and changed one spin assignment. An examination of all of the data since the 170 publication of these two papers has led to suggested levels in Yb

0 (0+), 84.3 (2+), 277.7 (4+), 940.7, 1136, 1141.2, 1231.6, 1308.6,

1382.4, 1482. 5,1515. 1, 1537.4 (3+), 1766, 1926, 1939.0, 2123, 2359,

2512, 2740, 2767, 2836, 2956, 3348, 3367, 3389, and 3421 kev.

This scheme accounts for about 80 of the 90 reported transitions.

The gamma-ray spectrum analysis showed that all of the higher energy transitions were weak compared with the 84. 3-kev 170 transition,and hence that a major fraction of the decay of Lu 10 170 occurs to the 84. 3-kev level in Yb . Therefore, a ground state 170 spin of 2 is suggested for Lu

Lui 171

Before this investigation was begun, two activities with half- 171 lives of 8 days and 600 days had been assigned to Lu . In this study, ytterbium oxide enriched to 93. 8 percent in the mass number

171 was irra d ia te d with 6 -Mev protons^ and the resulting activity was observed for about 320 days. A strong component with a half-life of 8 . 28 days was followed until it was no longer detectable. The sample was examined periodically for 320 days but no 600-day activity attributable to Lu*^* was observed.

The 8 . 28-day activity is strongly predominated by a 740-kev gamma ray, but a careful analysis of the gamma-ray spectrum revealed five more gamma rays in addition to the K x ray and the

2Kx ray summation peak. Gamma-gamma coincidence measure­ ments were performed where possible. With the aid of these data 171 and the results of Coulomb excitation measurements on Yb and 171 an analysis of the conversion electron spectrum of Lu by other 171 workers, an energy level scheme was constructed for Yb with levels at 0 (1/2-), 6 6 . 7 (3/2-), 75.9 (5/2-), 95. 1 (7/2+), 122.4 (5/2-),

167. 3 (9/2+), 208. 1 (7/2-), 230. 5 (7/2-), 246. 5 (9/2-), 317. 3 (9/2-), 11 350, 835 (7/2-), 862, 935 (9/2+), and 949 (7/2+) kev. This icheme 171 accounts for 30 of 31 transitions reported for the activity of Lu

Relative intensity calculations show that only three significant electron capture branches exist, to the 835 (84%), 935 (10%), 171 and 949 (4%) kev levels. The ground state spin of Lu is either 7/2+ or 9/2- with 7/2+ being favored.

T 172 Lu

Irradiation of ytterbium oxide enriched to 95. 9 percent in the mass number 172 with 6 -Mev protons produced the 6 . 7 0-day 172 electron capture activity of Lu , but not the previously reported

4. 0-hour positron activity. The complex gamma-ray spectrum was analyzed,and extensive gamma-gamma coincidence measurements were performed. These data together with conversion electron information reported by other workers have led to the establishment 172 of levels in Yb at 0 (0+), 78.7 (2+), 260.2 (4+), 540 ( 6 +), 1172.7

(3+), 1263.3 (4+), 1376.0 (5+), 1546 or 1700 (3), 1663. 0 (3-), 1749

(5-?), 1803.0 and 2073. 5 (4+) kev. Approximately two-thirds of the electron capture decay occurs to the 2073. 5-kev level. A highly populated vibrational-rotational band with K-3 has been established 172 in Yb with the vibrational state at 1172. 7 kev. To this author's

4- knowledge, this is the first such assignment. The properties of such 12 a system of states can now be determined from a close examination 172 of the data for the decay of Lu

L » 1 7 3

Ytterbium oxide enriched to 92. 6 percent in the mass number

173 was irradiated with 6 -Mev protons. A long half-life activity 173 ('V 600 days) was produced and assigned to Lu . This activity decays purely by electron capture. A number of discrepancies in 173 the existing data for Lu were resolved by the examination of the

gamma-ray spectrum and by the results of gamma-gamma coincidence 173 measurements. Levels in Yb were established at 0 (5/2-), 78. 7

( 7 / 2 -), 179. 5 (9/2-), 351. 1 (7/2+)** and 633 (5/2+) kev, and all ten

possible transitions observed as gamma rays. The electron capture decay to the 633-kev level is by L capture only, and few if any 173 disintegrations occur to the ground state of Yb . The long half- 173 life and mode of decay of Lu implies a ground state spin of

9/2- r for Lu , 173

Lui 174

The irradiation of ytterbium oxide enriched to 98.4 percent in the mass number 174 with 6 -Mev protons produced the previously 174 * assigned 165-day activity of Lu . Examination of the electro­ 13 magnetic spectrum and a search for particle radiation in the very pure Lu*^* resulting from this irradiation showed that L u*^ decays by electron capture only and not by negatron decay also,as previously reported. Both the nuclei reached by the double decay 174 are stable. The gamma-ray spectrum of Lu is simple; it consists of the ytterbium K x ray and gamma rays of 76. 6 and 1228 174 kev which are in coincidence. Three levels in Yb are populated by this decay, the ground state (0+) (31%), and levels at 76.6 (24)

(59%) and 1305 (0+) (10%) kev. The high-energy level is populated by electron capture from the L shell only. This decay scheme 174 strongly favors the assignment of 1- for the ground state of Lu , which seems to conflict with the decay energy and half-life. This discrepancy has recently been resolved, however. The 165-day 174 half-life for Lu is actually that of an excited state with spin 6 - which decays to the ground state by the emission of three low energy and highly converted transitions. The half-life of the ground state is therefore unknown but^less than 165 days, probably much less. It is interesting to note that the collective model predicts a 174 doublet of states for Lu with spins of 1- and 6 - one of which should be the ground Sate and the other, an isomeric state.

T he sp in 6 - isomeric state decays through a 3-, 2-, 1 - rotational band in this odd-odd nucleus. APPENDIX I

This appendix is composed of a bibliographical listing of the publications, the summaries of which comprise the preceding

sectio n .

1 4 15 Publications'1,

1. Phys. Rev. 120, 1827-29 (I960), Radioactive Decay of 162 and Tm .

2. Bull. Amer. Phys.^Soc. 6, (1961), Radioactive Decay of

Yb*^, Tm ^^, and Er ^ and Energy Level Scheme of E r^^.

Also to be submitted to The Physical Review for publication.

3. Phys. Rev. 119, 262-266 (I960), Radioactive Decay of Tm ^^.

4. Bull. Amer. Phys. Soc. , 255 (I960), Radioactive Decay 166 J m 168 of Tm and Tm 1 67 5. Phys. Rev. 120, 1296-97 (I960), Radioactive Decay of Yb 168 6 . Phys. Rev. 118, 227-228 (I960), Radioactive Decay of Lu 170 7. Phys. Rev. 120, 980-982 (I960), Radioactive Decay of Lu 171 8 . Phys. Rev. 120, 1843-50 (I960), Radioactive Decay of Lu 171 16Q and Level Schemes for Yb and Yb 172 9. Phys. Rev, 118, 1067-72 (I960), Radioactive Decay of Lu 173 10. Phys. Rev. 117, 807-810 (I960), Radioactive Decay of Lu 174 11. Phys. Rev. 117, 517-519 (I960), Radioactive Decay of Lu .

All of these publications bear the name of a co-author, M. L. Pool,

my adviser, and one (2) bears the name of an associate, G. G. Staehle.

I APPENDIX n

This appendix is composed of reprints of the publications, the summaries of which comprise the preceding section.

16 17

Reprinted from T h e P h y sica l R eview , Vol. 120, No. 5, 1827-1829, December 1, 1960 Mated h V . U

Radioactive Decay of Tm1M and Tm1®*

R. G. Wilson and M. L. P ool Dtpartmtnl oj Pkyiici and Astronomy, Ohio Stott University, Columbus, Ohio (Received June 6,1960)

Erbium oxide enriched to 35.1% and 14.1% in the man numbers 164 and 162, respectively, were irradiated with 6-Mev protons. Activities decaying by electron capture with half-lives of (2.04±0.10) minutes and (77±4) minutes were produced and are assigned to TmIM and Tm1*, respectively, by the identification of the erbium A x ray, comparison with the activities produced by similar proton irradiations of each of the other enriched isotopes of erbium, and by the existence in the 2.04-minute activity of a prominent gamma ray with the energy of the first excited level in Er1** determined from the decay of Ho1*. The gamma- ray spectrum of Tm1* consists of only the erbium A x ray, a gamma ray of 91 kev, and annihilation radi­ ation ; and that of Tm1*, of the erbium A x ray and gamma rays of 102 and 236 kev. The three radiations observed in the decay of Tm1* are all in coincidence. Energy level schemes for the decay of these two new activities are proposed and branching ratios are estimated from relative intensities.

EXPERIMENTAL RESULTS due to the longer lived thulium activities in the sample LL of the enriched isotopes of erbium have been as shown in Fig. 1. A irradiated with 6-Mev protons. The well known The low-energy portion of the gamma-ray spectrum activities of Tm1TO, Tm1", Tm117, and Tm11® were of Tm1" is shown in Fig. 2. Only the erbium K x ray, produced from the enriched erbium isotopes with the a prominent gamma ray of (91 ±2) kev, and annihila­ same mass numbers by (p,n) reactions. The irradiated tion radiation were observed on the 2.04-minute half- samples of erbium oxide enriched in the mass numbers life in the range 0 to 3000 kev. The relative numbers 164 and 162 contained two activities differing from the of K x rays, 91-kev gamma rays, and annihilation other known thulium activities. Table I lists the radiation photons are 100:13.5:176, respectively. The percentages of erbium isotopes in the samples used. relative number of positrons can be obtained by The activity resulting from the irradiation of enriched dividing the number of counts in the spectral distri­ erbium 164 clearly showed a 2-minute component, and bution of the annihilation radiation by two. The factor that of enriched erbium 162, a 77-minute component. of two was confirmed with Na*. An unsuccessful The x ray in these activities was shown to be the K attempt was made to measure the energy of the x ray of erbium by comparison with known erbium K positron in the 2-minute activity of Tm1" by the x rays. The 2-minute activity exhibited a prominent method of plastic scintillation spectrometry. gamma ray of 91 kev which is the same as the energy The observed gamma-ray spectrum of Tm1" consists of the first excited level of erbium 164 determined by a of the erbium K x ray and gamma rays of (102±2) and study of the activity of Ho1" by Brown and Becker.1 (236±4) kev. No other gamma rays with energies It is assumed that these two new activities also resulted from (p,n) reactions with the enriched isotopes. For these reasons, the 2-minute activity is assigned to TmIM and the 77-minute activity is assigned to Tm1*. -e | |*“ I MINUTE The half-lives of Tm1* and Tm1" are (2.04±0.10) minutes and (77±4) minutes, respectively, as measured by following the decay of the individual K x rays, gamma rays, and annihilation radiation with a 100- channel scintillation spectrometer. The 2.04-minute 91-KEV t 1.93 MIN value was obtained by the subtraction of a base line

T able I. Composition of the enriched erbium oxide samples.

K-X RAYi Enriched Percentages of erbium isotopes* 2.12 MIN erbium Natural comprising enriched samples isotope percentages 162 164 166 167 168 170 164 1.56 <0.2 35.1 47.4 9.8 6.2 1.5 TIME 162 0.136 14.1 9.0 40.0 17.1 14.6 5.2 Fio. 1. Decay curves for Tm** obtained with a 100-channel analyzer. A similar curve for the annihilation radiation yielded • Supplied by the Stable Isotopes Division ot Oak Rld|e National the value of 2.04 minutes. The curve for the A x ray has been Laboratory. plotted one decade low. The base line for the A x ray is due to the A x ray in the 7.7-hour Tm1* activity and that for the 91-kev > H. N. Brown and R. A. Becker, Phys. Rev. 96,1372 (1954). gamma ray is the channel number where the gamma ray existed. 1827 18

1828 R . G . WILSON AND M. L. POOL

MTM'*KITr MIN) I-

// 13%/ / Fig. 2. Gamma-ray / W t spectrum of Tm‘“ I / IODINE obtained with a 1) Fio. 4. X 2 inch NafTl) energy level (16* crystal and a 100- for the decay of channel analyser. u 3BC 4+ Tm. »■

l O t t +

0 0 + En •• between 0 and 3000 kev were observed on the 77- minute half-life. The presence of the 112-minute posi­ tron activity of F1* prevented positive observation of assumed to be of £2 character. Because the 236-kev positrons in the 77-minute activity. The relative gamma ray is in coincidence with the 102, a level at numbers of the radiations observed in the spectrum of 338 kev is implied. Because the energy ratio of these Tm10 are approximately 100:20:10 =K x ray: 102y: two levels is that common for the first two rotational 236y. Gamma-gamma coincidence measurements were levels in even-even nuclei, the level at 338 kev is given performed with a circuit of resolving time 2r= 1.5 psec. the spin assignment 4+. The relative numbers of 91-, All three observed radiations are in coincidence. 102-, and 236-kev transitions shown in Figs. 3 and 4 were obtained by correcting the observed relative DISCUSSION numbers of gamma rays for internal conversion using Figures 3 and 4 show proposed decay schemes for the data displayed in Table II.* Tm1® and Tm1®. The 91-kev gamma ray observed in The rotational spin assignments for the low lying Tm1® probably depopulates the first ground-state levels in Er1® and Er1® and the branching ratios shown rotational level of Er1M and is therefore assumed to be in Figs. 3 and 4 suggest possible spin assignments of £2. The first rotational level of Er1® has not been 1— and 3— for Tm1® and Tm1®, respectively. The established by Coulomb excitation but is expected to decay scheme for Tm1® as presented in Fig. 4 accounts occur at about 100 kev. The 102-kev gamma ray of for the relative number of K x rays observed within Tm1® probably depopulates this level and is also the experimental error which in this case is larger than for Tm1® because of the lower enrichment of Er1®. If L capture is ignored in the case of Tm1®, there is little TM,M U;04 MIN) I- or no evidence of an electron capture transition to the EO 50% ground state of Er1®. However, if L capture is signifi­ f*50% cant, the possibility of such a transition does exist. If the choice of 3 for die spin of Tm1® is correct, then this ground-state decay has A /=3 and must then compete with two transitions for which A /= 1. The ground-state transition is then probably highly retarded and is not shown in Fig. 4.

T ablx II. Internal conversion coefficient data for the £2 traniitions of 91, 102, and 236 kev in erbium. The a ’a are from reference 2.

Ey «(*) a(Lt) a(Ln) a(Liu) ■ d O N,/N, 91 1.28 0.120 1.05 1.05 1.10 5.60 4.37 102 1.00 0.094 0.606 0.591 0.615 3.91 3.91 236 0.100 0.011 0.014 0.010 0.016 1.15 11.5

* M. E. Rose, Inltmai Contortion Coojlcimts (North-HoUand Fio. 3. Proposed energy level scheme for the decay of TmIM. Publishing Company, Amsterdam, 1958). 19

RADIOACTIVE DECAY OF TmIM AND Tm"» 1829

In the decay of both Tm1M and TmlM to the even- These levels are characteristically found at 750 to even isotopes of Er1M and Er1M, highly populated 1000 kev and decay to the levels of the ground-state members of a K —2+ vibrational band are observed.1 rotational band by the emission of gamma rays of 600 to 900 kev. No gamma rays in this energy range were * K. P. Jacob, J. W. Mihelich, B. HarmaU, and T. H. Handley, Phys. Rev. 117, 1102 (I960); R. G. Wilson and M. L. Pool, observed with the scintillation spectrometer following Phyi. Rev. 119, 262 (1960). the decay of either Tm1*4 or Tm,“. 20

Reprinted from The P h ysica l R eview , Vol. 119, No. 1, 262-266, July 1, 1960 Printed In U. S. A.

Radioactive Decay of Tm,##

R. G. Wilson and M. L. P ool Department of Phyfics and Astronomy, The Ohio State University, Columbus, Ohio (Received January 25,1960)

Erbium oxide enriched to 72.9% in the 166 mass number was irradiated with 6-Mev protons. An activity decaying by electron capture and positron emission with a half-life of 7.69±0.05 hours was produced by a (p,n) reaction and its assignment to Tm1** confirmed. The observed activity consists of the K x ray of erbium, gamma rays with energies of 81, 184, 289, 405, 460, 598, 674, 694, 707, 759, 782, 788, 878, 1052, 1179,1276,1351,1589,1874, and 2058 kev, annihilation radiation, and particle radiation with an end-point energy of 2090±40 kev. Gamma-gamma coincidence measurements and consideration of the energies and relative numbers of the observed radiations have led to the assignment or confirmation of energy levels at 81 (2+), 265 (4+), 554 (6+), 788 (2+), 863 (3+), 959 (4+), 1248 (2), 1317 (5), 1462 (0+), 1547 (3+), 1701 (4+), 1894 <5+), 2139 (3), and 2168 (0) kev in Er*‘*. The 2139-kev level is highly populated by electron capture and the positron transitions occur to the 265 (4+)-kev level. The positions of the observed radiations and the branching ratios of electron capture are shown in a proposed energy level scheme.

INTRODUCTION kev in Er***.* One group of workers has reported gamma N activity decaying 99+ % by electron capture rays in this activity with energies of 80, 180, 670, 800, A and <1% by positron emission with a half-life and possibly 1320 kev.' Another group has reported of 7.7 hours has been assigned to Tm1**.1 Gamma radia­ gamma rays with energies of 80,180, 690, and 780 kev tion of approximately 1.7 Mev was detected in tjiis in this activity and have postulated an energy level of activity and the positron end-point energy was found 780 kev (2+) in addition to the 265 (4+)- and 81 (2+)- to be 2.1 Mev. Conversion electron measurements fol­ kev levels.4 The assignment of an 80.6-kev level resulted lowing the proton irradiation of natural ytterbium oxide have led to the assignment of transitions with energies * J. W. Mihelich, B. Harmatz, and T. H. Handley, Phys. Rev. of 80.7, 154.6, 184.7, 194.8, and 215.4-kev and to the 108,989 (1957). »W. E. Nervik and G. T. Seaborg, Phys. Rev. 97,1092 (1955). postulation of energy levels of 81 (2+) and 265 (4+) •G. M. Gorodinskii, A. N. Murin. V. N. Pokrovskii, B. K. Preobrashenskii,and N. E. Titov, Doklady Akad. Nauk (S.S.S.R.) 1G. Wilkinson and H. G. Hicks, Phys. Rev. 75, 1370 (1949). 112,405 (1957) [translation: Soviet Phys.-Doklsdy 2,39 (1957)1 21

263 RADIOACTIVE DECAY OF Tra>"

T able I. Relative numbers of gamma rays, Ny, conversion electrons, JVM, and corresponding transitions, Ni, in the activity of TmIM for gamma-ray energies, £,, expressed in kev.

(« K X Ey Ref. N» N, K x ray 100 80.7 2,6 9 16000 86 •W 154.1 2,6 50 184.4 2,6 20 2200 31 193.2 2,6 240 t o t t 214 2,6 140 289 a <1 299 6 17 <1 347 6 22 <1 405 6 a 77 <1 460 6 a 89 ~ 1 595 6 598 6 1.5 }33 1.5 674 6 1 26 1.6 694 6 f6 45 2.7 707 6 J 27 1.7 759 6 763 6 }27 }2.2 •0 0 1600 ■Xooo 782 6 Ir 46 3.8 •0 788 6 J 37 3.0 878 1.0 1.0 Fig . 1 1. Gamma Gamma-ray spectrum of 7.7-hour Tm1" measured with a 1052 ~1 ~1 3X3 inch crystal on a 100-channel scintillation spectrometer. 1179 6 4.0 27 4.0 1276 6 5.7 32 5.7 1351 1.1 1.1 from Coulomb excitation experiments.6 The energies 1589 a <1 1874 17 17 and relative numbers of conversion electrons from 22 2058 6 15 30 transitions in the activity of Tm1** have been reported.* 2087 6 }39 6 9

EXPERIMENTAL RESULTS * Observed in coincidence spectra only. Erbium oxide enriched to 72.9% in the 166 mass number was irradiated with 6-Mev protons. The com­ 182*2, 600*5, 695*5, 780*5, 880*8, 1050*10, position of the remaining portion is as follows in percent: 1180*8, 1275*8, 1350±12, 1875*15, and 2055*12 <0.1 Er1®, 0.1 Er1*4, 17.7 Er1*7, 8.5 Er1*8, and 1.5 Er170. kev in addition to the erbium K x ray. Gamma-gamma The atomic number of the resulting activity was de­ coincidence measurements have shown that in addition termined by the identification of the erbium K x ray, to the gamma rays mentioned above, there exist weaker which was compared with the known K x rays of ter­ gamma rays with energies of 285, 405, 460, and 1590 bium, dysprosium, thulium, ytterbium, lutetium, and kev, and that the 695- and 780-kev peaks are each com­ tantalum emitted from radioactive Dy16*, Tb1*0, Yb1**, posed of more than one gamma ray. No gamma ray Tm170, Hf176, and WIM, respectively. Ion-exchange sepa­ with an energy greater than 2200 kev and an intensity ration was deemed unnecessary. greater than 1% of that of the 2055-kev gamma ray The mass number of the activity was determined by exists. A peak in the gamma-ray spectrum at 510 kev identifying and comparing the relative amounts of this was shown to be annihilation radiation by its strong activity and the well-known activities of Tm170, Tm1**, 180 deg self-coincidence. Weak particle radiation with and Tm1*7 in similar proton irradiations of enriched an end-point energy of 2090*40 kev was observed by Er1** and Er'«*. the use of a Geiger tube with aluminum absorbers. The half-life of the activity produced by the proton All of the reported radiations emitted from Tm1M are irradiation of enriched Er1** is 7.69*0.05 hours as listed in Table I with the conversion electron references. measured by following the decay of the gamma-ray The figures in the third column of Table I are the rela­ spectrum for over five half-lives with a 100-channel tive numbers of counts under the spectral distributions scintillation spectrometer. The original assignment of of the observed gamma rays after correction for crystal the 7.7-hour activity of Tm1** is therefore confirmed. efficiency. The figures in the fourth column are the rela­ Figure 1 shows the observed gamma-ray spectrum of tive numbers of conversion electrons as reported in Tm1** which includes gamma rays with energies of 81* 1, reference 6. The relative numbers of the 81- and 182-kev gamma rays were corrected for internal conversion7 * E. L. Chupp, J. W. M, DuMond, F. J. Gordon, R. C. Jopson, using the data in Table II. The numbers in the last and H. Mark, Phys. Rev. 112, 518 (1958). * A. A. Bashilov, la. Gromov, G. M. Gorodinskii, B. G. column of Table I are the relative numbers of the transi­ Dzhelepov, et al., Proceedings of the Second United Nations Con­tions. In cases where several gamma rays were observed ference on the Peaceful Uses of A tomic Energy, Genera, 1958 (United Nations, Geneva, 1958), U. S. Atomic Energy Commission micro­ 1 M. E. Rose, Internal Conversion Coefficients (North-Holland card A/CONF/15 P2477. Publishing Company, Amsterdam, 1958). 22

R . G , WILSON N D M . L . POOL 264

Table II. Internal convenion data for erbium. The a’i are the fifth column of Table 1 ,107, is the total number of from reference 7. Nr/N, and Nt/Nk are the ratioe of the total number of transition* to the number of gamma ray* and AC-con­ K x ray producing events of which only 100 are observed verted transition*, respectively, for gamma-ray energies, Ey, ex­ because the /T-fluorescence yield in erbium is 0.932.1 pressed in kev. Table III is a tabulation of the gamma-gamma coinci­ dence information obtained for the activity of TmIM Ey ax a n asi a n air Nr/Ny Nt/Nk with a coincidence circuit of resolving time 2r*» 1.5X10- * 81 1.60 0.61 1.90 1.90 1.84 8.4 5.2 sec and two lfX2 inch crystals used at either 90 or 182 0.21 0.021 0.042 0.035 0.042 1.35 6.4 180 deg. Three typical coincidence spectra are shown in Fig. 2. Curve A shows that only the K x ray and the 81-kev gamma ray are in coincidence with the 2055-kev in one peak, the numbers in the third column were gamma ray and that there are a sufficient number of divided using the data in the fourth column. All of the K x rays compared to the number of 81-kev gamma rays numbers in the last column have been adjusted to read to allow a very high percentage of K x rays to precede in percentages of disintegrations. The first number in the 2055-81-kev cascade. Curve B shows that theK x ray and the 81- and 182-kev gamma rays are in coin­ cidence with the 180 deg self-coincident annihilation radiation. This curve shows that there is approximately a one to one relationship between the number of 81- and 182-kev transitions in coincidence with the annihilation radiation after correction for internal conversion. Curve C shows that theK x ray and gamma rays with energies of 81, 285, (510), 600, 695, 1050, 1180, and 1875 kev 182 511 are in coincidence with the 182-kev gamma ray.

DISCUSSION Figure 3 shows an energy level scheme for the decay of TmIM which is consistent with the experimental evi­ dence of this investigation. It accounts for 27 out of the 28 observed radiations listed in Table I. The assignment of a 265-kev (4+) level is now con­ firmed because (1) strong coincidences are observed between the 81- and 182-kev gamma rays, (2) there exists no 265-kev crossover transition, (3) strong tran­ sitions occur from higher levels to this and the 81-kev (2-)-) levels as shown by the gamma-gamma coincidence measurements, and (4) as noted by others, this level is predicted for the second ground-state rotational level. The possibility of a 550-kev (6-1-) rotational level is suggested by the existence of a weak 285-kev gamma ray seen only in the coincidence spectrum gated by 182-kev NONSNCTNUM COINClMNCC events. The existence of a 2139-kev level in Er1*4 which is highly populated by electron capture transitions from Tm'M is clearly shown by the strong coincidences ob­ served between the 2055-kev gamma ray and only the 81-kev gamma ray and between the 182- and 1875-kev gamma rays. Further evidence of the 2139-kev level is seen by the coincidences observed between transitions from this level to lower excited states and transitions from these states to levels in the ground-state band. ENERGY Because a 2087-kev transition is reported in reference iooo isoo tooo 6 in addition to a 2058-kev transition, it is assumed that Fig. 2. Typical gamma-gamma coincidence spectra obtained the 2055-kev peak in the gamma-ray spectrum seen in for the activity of Tm1M with a coincidence circuit of resolving time 2rvl.5xlO~* sec. Gamma energies are in kev. Individual Fig. 2 is composed of both of these transitions. The spectra are coincidences gated by 2055-kev events for curve A, by 511-kev events for curve B, and by 182-kev events for curve C. •A. H. Wapstra, J. G. Hijgh, and R. Van Lieshout, Nuclear Curve C shows the 1590- and 285-kev gamma rays which can be Spectroscopy Tables (North-Holland Publishing Company, Am­ seen only in the coincidence spectrum. sterdam, 1959). 23

265 RADIOACTIVE DECAY OF T m '"

T able III. Gamma-gamma coincidence information for the activity of Tm1**. Deaignationi are gamma-ray energiea in kev.

675 765 K 81 184 215 285 405 460 511 600 695 780 880 1050 1175 1275 1350 1590 1875 2055 x my 705 785

ye* yes yes yes ye* yea yes ye* yes yes yes yes ye* ye* KXIP81 ye* no yes yes yea yea w ye* ye* ye* yea yea t82 yea yea yea yes yea ye* yea no no yea yea no no yea ye* no 511 yea yea yes no yea no no no no no no no no* no no no 695 y » yes yes yea no w no yea w no yes no yea no no no 780 y « yes no no yes yes no yea no no no no ye* yes no no no 1175 ye* yea y« no yea no ye* no no no no no no no 1275 yea yea ye* no no yes no no no no no no no no 1350 yea yea yes no yes ye* no no no no no no no no 2055 yea yes no no no no no no no no no no no no no no no no no coincidence information then shows that both of these of the ground state of the band and I is the spin of transitions occur to the 81-kcv level. Therefore a level at excited levels. 2168 kev is tentatively assigned with a possible spin of 0 The strong transitions from the 2139-kev level to the because no other transitions from this level are observed. 24- and 44- levels of the ground-state rotational band A 788-kev level with a spin of 1 or 2 is implied because and the weaker transitions to the three levels of the (1) strong 707- and 788-kev transitions exist in the 2 rotational band favor the assignment of spin 3 for activity of Tm "#, (2) the 695-kev peak is strongly in the 2139-kev level. A spin of 4 is made less favorable by coincidence with the 81-kev gamma ray, while the 780- the spin assignment made for the ground state of Tm1*4. kev peak is only weakly in coincidence with the 81-kev nTm»j'" is an odd-odd nucleus in the region of ellipti- gamma ray, and (3) a 1350-kev gamma ray is in coinci­ cally deformed odd-odd nuclei. The collective nuclear dence with both the 695- and 780-kev peaks. Reason (2) model predicts a doublet of states for such a nucleus, implies that the 780-kev peak is composed of at least one of which is the ground state. The measured ground- two gamma rays, one of which is stronger and not in state spins of i*Tm144 and siDy,,1*1 are J4- and f —, coincidence with the 81-kev gamma ray and one which is weaker and in coincidence with the 81-kev transition. A level with a spin of 3 or 4 is implied at 863 kev because (1) a 782-kev gamma ray is in coincidence with J - 3 3373 the 81-kev but not with the 182-kev, (2) the 598-kev 10%JRTTT\ 64% gamma ray is in coincidence with the 182-kev, and (3) the 1276-kev gamma ray is in coincidence with the 782-kev. Ip I 11 A 959-kev level of spin 3 or 4 is implied because the ■mu 878-kev gamma ray is in coincidence with only the 81- IIII and 1179-kev gamma rays and the 695-kev peak is in coincidence with the 182- and 1179-kev gamma rays. This situation also requires that the 695-kev peak be composed of more than one gamma ray. The weak 405- kev gamma ray observed in coincidence with the 182- kev gamma ray may then occur between this level and the 6+ level of the ground-state band. The 788-, 863-, and 959-kev levels seem to comprise a rotational band with K~2 and hence have spins of 24-, 34-, and 44-, respectively. If K were greater than 2, transitions within the rotational band (AA=0) would be more highly j j 3 g favored than transitions to the ground-state band (AK>3). If K were 1, the 598-kev transitions would be replaced by an 863-kev transition and the observed strong coincidences between the 182- and 598-kev gamma rays would not occur. That these three levels do comprise a rotational band is favored by the ratio of the energy differences between them. This ratio is 171/75 >“2.3 which is in the range of values predicted by the FiC. 3. Proposed energy level scheme for the decay of formula £ « [ / ( / 4 - l ) —/#(/o4-l)] whereh is the spin Tm,N. Energy level designations are in kev 24

R . G . WILSON AND M L. POOL 26 respectively. Spins of 2— and 3— are therefore pre­ imply that there is approximately a one to one relation dicted for ifTm» 7 IM. The choice of 3— for the ground ship between the 81- and 182-kev transitions followin state of Tm1M is consistent with the population of the the positron decay. Positron decay transitions to th two levels with spin 4+, one of which is populated ground state of E r14® are precluded by the large A/ an* directly by electron capture decay, and of the 6+ (0+) AK. The population of the 265rt3-kev level by th ground-state rotational level. 2090d:40-kev positron places the ground state of Tm11 The value of K for the 2139-kev level is not easily 3375±45 kev above the ground state of Er1*4. chosen. The high population of this level by electron The relative amounts of electron capture decay showi capture from the now assigned 3 — ground state of Tm 14® in Fig. 3 were obtained by subtracting from the 107 / implies the possibility of a single-particle state with x rays in the fifth column of Table I, K x rays resultin) K =3. However, then the strong transitions to the mem­ from K conversion and one K x ray for every electroi bers of the ground-state band for which the change in K capture transition required to balance the differenci is 3 seem inconsistent. The possibility of A = 0 is con­ between the number of transitions from a given leve sistent with transitions to the ground-state band but and transition into the same level. As seen in curve A seems to be inconsistent with the high population of the of Fig. 2, a large number of K x rays exists in the coinci 2139-kev level by electron capture. dence spectrum gated by 2055-kev events. Enough A A level at 1317 kev of possible spin 5 is suggested by x rays are present to account for K conversion of th< the 1050-kev gamma ray, which is in coincidence with 81-kev transitions required to produce the number ol the 182-kev gamma ray and by the existence of a 763- 81-kev gamma rays observed and to allow one K x raj kev transitions reported in reference 6. Levels at 1248 to precede every 2055-kev gamma ray which gated the (2), 1462 (0), and 1547 (3) kev are suggested by the observed 81-kev gamma rays. Therefore the 2139-ke\ gamma-gamma coincidence information and the exist­ level of ErIM must be populated largely by K capture, ence of transitions with energies of 214, 299, 347, 460, L capture to all of the levels of Er14® is assumed negligible 674, and 759 kev reported in reference 6. The 460-, 674-, with respect to K capture. and 759-kev gamma rays are observed in the coincidence At the end of the section on Tm1®® in reference 4, a spectra gated by the gamma peaks which include the rough calculation of the branching ratios of electron 707- and 788-kev gamma rays. capture to the 80- and 264-kev levels of Er1*® was made. The transitions with energies of 154, 193, and 347 are The inconsistency mentioned there of these ratios with not observed in the gamma-ray spectrum but are shown in Fig. 3 as comprising a rotational band with K—3. A the predicted spin for Tm1®® is now removed because rotational band is chosen because the 347-kev line can this investigation shows that the 80- and 265-kev levels serve as a crossover transition and the energy ratio is are largely populated from higher levels in Er14®. then just that predicted and observed for rotational bands. K = 3 is chosen because transitions from the ACKNOWLEDGMENTS excited levels of the band occur to the lower members One of us (R. G. W.) is grateful to the National of the band rather than to other states. Science Foundation for the grant of a fellowship which The strong coincidences observed between the annihi­ lation radiation and only the 81- and 182-kev gamma enabled the completion of this research. Appreciation is rays led to the conclusion that the positron decay of expressed to R. P. Sullivan of the Department of Physics Tm14® populates the 265-kev (4+) level of Er14®. The and Astronomy for assistance in the electronic phases of relative numbers of the 81- and 182-kev gamma rays this research and to the Office of Naval Research for in the coincidence spectrum gated by 511-kev events support in obtaining the enriched isotopes. 25

Reprinted from T h e P hysical Review , Vol. 120, No. 4, 1296-1297, November IS, 1960 M u d kV .L A .

Radioactive Decay of Ybm

R. G. Wusom and M. L. Pool Department of Physics and Astronomy, Ohio StaleUniter sitw\ Columbus, Ohio (Received M ay 27,1960)

Erbium oxide enriched to 35.1% in the mass number 164 was irradiated with 17- and 24-Mev alpha particles. An activity decaying by electron capture with a half-life of (17.7dfc0.2) minutes was produced and its assignment to Yb1

INTRODUCTION crystal and a 100-channel analyzer. The thulium K N activity decaying by electron capture with a x ray and gamma rays with energies of 106, 113, and A half-life of 18.5 minutes has been assigned to 176 kev are seen. The 106-kev gamma ray is weaker Yb1*7 by Handley and Olson.1 The conversion electron than the 113 but its existence is clearly shown by coinci­ spectrum of this activity has been examined and a decay dence measurements. No gamma rays with energies scheme proposed on the bases of transition energies and greater than 200 kev were observed nor was any multipole orders by Harmatz, Handley, and Mihelich.1 annihilation radiation. This substantiates the conclusion In the present investigation, the gamma-ray spectrum of that no positron emission occurs in the decay of Yb1*7.1 Yb117 has been examined with a 100-channel scintilla­ The relative numbers of counts in the spectral distribu­ tion spectrometer and gamma-gamma coincidence meas­ tions of the observed radiations are 100:38:8 =K x ray: urements performed. 106 and 113-y: 176y. The relative number of K x rays can be corrected for fluorescence by division by 0.935.' EXPERIMENTAL RESULTS Table I is a summary of the gamma-gamma coinci­ dence information obtained for the activity of Yb1*7 Samples of erbium oxide enriched to 35.1% in the with a coincidence circuit of resolving time 2r= 1.5 /usee. mass number 164 were irradiated with 17- and 24-Mev The lower curve in Fig. 1 shows the gamma-ray spec­ alpha particles. The composition of the remaining trum in coincidence with the 176-kev gamma ray. This portion in percentages is 0.2 Er1**, 47.4 Er1**, 9.8 Er"7, spectrum shows the 106-kev gamma ray which is largely 6.2 Er1M, and 1.5 Er170. The atomic number of the resulting activity was determined by the identification of the thulium K x ray which was compared with the TM K X RAY known K x rays of terbium, thulium, ytterbium, lutetium, and tantalum, emitted from radioactive Dy1M, Yb1", Tm170, Hf17», and W‘«, respectively. 113 The daughter of the 18.5-minute activity was the NONCOINCIDENCE well-established Tm1*7, and its assignment to Yb1*7 is l \ S SPECTRUM therefore confirmed. However, the half-life of Yb1*7 as determined by following the decay of the two gamma peaks and the K x ray for five half-lives with a 100- ITS channel scintillation spectrometer is (17.7±0.2) minutes. The gamma-ray spectrum of Yb1*7 is shown in the top curve of Fig. 1 as observed with a If- X 2-inch Nal(Tl) COINCIDENCE 8PECTRU T able I. Gamma-gamma coincidence information for the activity of Yb,,T. 0ATE0 BY 176 •AMMA RAY

£ x-ray 106 113 176

K x-ray yes yes yes yes ISO 200 106,113 yes yes yes yes 50 176 yes yes no no Fio. 1. Gamma-ray spectrum of Yb1*7 measured with a 1|X2 inch Nal(Tl) and a 100-channel analyzer.

1T. H. Handley and E. L. Olson, Phys. Rev. 94, 968 (1954). 'A. H. Wapstra, G. I. Niigh, and R. Van Lieshout, Nuclear 7 B. Harmatz, T. H. Handley, and J. W. Mihelich, Phys. Rev. Spectroscopy Tables (Nortn-Holland Publishing Company, 114,1082 (1959). Amsterdam, 1959). 26

1297 RADIOACTIVE DECAY OF Yb“ * obscured by the 113-kev gamma in the noncoincidence YB (ITT MIN f - f [523Ip ! spectrum. The existence of the 106-kev radiation is also shown by the self-coincidence of the 106-113 kev peak. Nine transitions have been observed in the conversion electron spectrum and some multipole orders assigned.1 A tentative assignment of £1 was made to the 113-kev transition. The gamma-ray spectrum substantiates this [523] assignment and also leads to the conclusion that the 176-kev transition is also £1. It is estimated from the gamma-ray and coincidence spectra that the 106-kev J gamma is about one-third as strong as the 113-kev 179.4 f* 4 [4 gamma ray while the respective numbers of conversion electrons observed in reference 2 are in about the inverse ratio. The ratio of the number of 113- to 176-kev gamma "7142.3 ^ + | rays observed in this investigation is the same as that calculated from the data of reference 2 and the internal H6.5 A • A conversion coefficients calculated by Rose,4 when both transitions are assumed to be £1.

Table II. Relative number of transitions, JV,, gamma rays, N y, K-converted transitions, Nt, and multipole orders, M. 0., for the gamma-ray energies of Yb"7, £,, expressed in kev. The data are obtained from the results of this investigation and Harmatz et al.* and Rose.k 10.4 + ■K2L M. O. N, N„ A Ey Ny „TM'*r iMDAYS> ° W M AT x-ray 7920 79204d (10.4) (3/l+£2) 3180 0 Fig . 2. Proposed energy level scheme for the decay of Yb’47. 25.8 (3/1+£2) 165 2 0 37.1 A/l(15%)-f-/i2(85%) 105 2 0 62.9 3/l(99+%)+£2(

Reprinted from T h e P hysical R eview , Vol. 118, No. 1, 227-228, A pril 1, 1960 M ated la U. S. A.

Radioactive Decay of Lu 188

R. G. W ilson and M. L. Pool Department of Physics and Astronomy, Ohio State University, Columbus, Ohio (Received October 26, 1959)

Ytterbium oxide enriched to 30.9% in the 168 mass number was irradiated with 6-Mev protons. An activity decaying by electron capture with a half-life of 7.1 ±0.2 minutes wu produced and asaigned to Lu1**. The activity consists of gamma rays with energies of 87±1, 900=k7, 987±7, 1410±20, 1800±40, 2130±60 kev in addition to the ytterbium K x ray. An energy level scheme tor this decay is presented.

TTERBIUM oxide enriched to 30.9% in the 168 activity of Lu1** proceeds from the first rotational level Y mass number was irradiated with 6-Mev protons. to the ground state of Yb1**. Thus an 87-kev 2+ level The initially resulting activity is assigned to Lu1** by is tentatively assigned to Yb1**. Because the energy the identification of the ytterbium K x ray and by difference between the 900- and 987-kev gamma rays comparison with the activities produced by similar is the same as that of the now assigned first rotational proton irradiations of each of the other enriched level of Yb1**, a 987-kev level of spin 1 or 2 is tentatively isotopes of ytterbium. Each of these irradiations assigned to Yb1**. produced the well known lutetium activity with the 7 iLu»71M is in the region of ellipticaily deformed same mass number as the irradiated ytterbium isotope odd-odd nuclei. Shell theory predicts spins of 1— and by a (p,n) reaction with no evidence of other reactions. 6 — for this nucleus using the measured spins of hLu17* The initial activity observed following the irradiation and MDy97IM which are 7/2+ and 5/2—, respectively. of Ybiu was different from all of the activities produced Because gamma rays corresponding to transitions by the irradiations of the other enriched isotopes of between rotational levels in Yb1** above the first are not ytterbium. It is assumed that this activity was also observed in the activity of Lu1**, the choice of 1— is produced by a (p,n) reaction. favored for the ground state of Lu1**. The observed activity of Lu1** consists of the ytter­ Assuming the 87-kev transition to be E%, its K, L\, Lt, bium K x ray and gamma rays with energies of 87=±= 1, U, and M internal conversion coefficients are 1.20,0.13, 9 0 0 ± 7 ,987±7,1410+20,1800+60, and 2130+80 kev. From an analysis of the decay of the annihilation radiation in the gamma-ray spectrum of this activity, 7|L u 'M (7.i m in ) it is concluded that if positron radiation exists in the i- activity of Lu1**, it results from less than 1% of the « 7 T T L — disintegrations of Lu1** and that the mode of decay of EC Lu1** is therefore essentially by electron capture to Yb1**. The half-life of Lu1** is 7.1+0.2 minutes as nr- o n /// measured by following the decay of the individual 1 1 gamma rays for over six half-lives with a scintillation • spectrometer. The approximate ratios of the relative F ig . 1. Prop energy level scheme 1,2 ■ 987 KIV numbers of the observed radiations in the activity of for the decay of Lu1** after correction for crystal counting efficiency are Lu'“ . K x ray: 87-kev y:900-kevir: 987-kev 7 *=100:7.5:10:13. 40%'j i The remaining three gamma rays are weak. 1 1? The energies of the established first rotational levels // •00 •00 1 10 of even-even nuclei in the region of Yb1** are between .*•*. ; 34 X 76 and 95 kev; in particular, those of the other even- 2 + 87 k e v even nuclei of ytterbium are 84, 79, 77, and 82 kev in ^ * d J 0 + 0 order of increasing mass number. It therefore seems ISO b probable that the 87-kev transition observed in the TO.Y 1 28

228 R . G . WILSON AND M. L. POOL

1.42, 1.42, and 1.41 as calculated from Rose’s data.1 of electron capture to the levels of Yb1** were obtained The ratios of the total number of transitions to the by determining the difference between the number of number of gamma rays and the number of /f-converted transitions from each level and the number of transitions transitions are then 6.6 and 5.5, respectively. Assuming into the same level. The number of K x rays remaining internal conversion of the high-energy transitions of after correcting for internal conversion and fluorescence Lu11* to be negligible, the ratios of the relative numbers was used as the relative number of electron capture of transitions in Yb1** are 49:10:13, respectively. The transitions to the ground state of Yb1**. relative number of K x rays can be corrected for There is no information currently available in the fluorescence by dividing the 100 K x rays observed by literature concerning the radioactive decay of Lu1** nor the K fluorescence yield in ytterbium which is 0.937.1 have any energy levels been established in Yb1** by The result is 107. Applying the ratio 5.5 to the 87-kev Coulomb excitation. The natural abundance of the 168 transitions implies that approximately 9 of the 107 K mass number in ytterbium is only 0.14%. x rays result from internal conversion of the 87-kev transition and approximately 98 result from K capture ACKNOWLEDGMENTS to the levels of Yb1**. One of us (R.G.W.) is grateful to the National Figure 1 shows a proposed energy level scheme for Science Foundation for the grant of a fellowship which the decay of Lu1M. The approximate branching ratios enabled the completion of this research. Appreciation 1M. E. Rose, Internal Contortion Coefficients (North-Holland is expressed to R. P. Sullivan of the Department of Publishing Company, Amsterdam, 1958). Physics and Astronomy for assistance in the electronic 'A . H. Wapstra, G. J. Niigh, and R. Van Lieshout, Nuclear Spectroscopy Tables (North-Holland Publishing Company, phases of this research and to the Office of Naval Re­ Amsterdam, 1959). search for support in obtaining the enriched isotopes. 29

Reprinted from T h e P h y sic a l R ev iew , Vol. 120, No. 3, 980-982, November 1, 1960 Printed in U. S. A.

Radioactive Decay of Lu 170

R. G . W ilson and M . L. P ool Department of Physics and Astronomy, Ohio State University, Columbus, Ohio (Received April 4,1960)

Ytterbium oxide enriched to 81.4% in the 170 mass number was irradiated with 6-Mev protons. An activity decaying by electron capture with a half-life of (2.05±0.05) days was produced and assigned to Lu1” by the identification of the ytterbium K x ray and by comparison with the activities produced by similar proton irradiations of each of the other enriched isotopes of ytterbium. The observed activity consists of the ytterbium K x ray, gamma rays with energies of 84, 193, 245, 1010, 1030, 1165, 1275,1415, 2035, 2365, 2665, 2890, and 3085 kev, and a small amount of annihilation radiation. Because no particle radiation exists, the mode of decay of Lu11* is solely by electron capture to Yb17*. Gamma-gamma coincidence measurements have led to the postulation of levels in Yb17* at 2120 (0), 2365 (1 or 2), 3170 (3), and 3395 (1 or 2) kev in addition to the previously assigned 84 (2+), and 278 (4+) levels. A partial energy level scheme with approximate electron capture branching ratios is proposed.

INTRODUCTION even isotopes of ytterbium using the natural oxide.* 1.7-DAY half-life activity decaying by electron The 84- and 278-kev levels mentioned above have been A capture and including gamma radiation of about designated as the first two rotational levels of Yb170.* 2.5 Mev has been assigned to Lu170.1 Conversion electron The 84.2-kev transition in Yb170 is also observed measurements following the proton irradiation of following the decay of Tm17#. natural ytterbium oxide have resulted in the assignment EXPERIMENTAL RESULTS of 84.2- and 193.5-kev transitions to the activity of Lu17# and 84- and 278-kev levels in Yb170.1 The half-life Ytterbium oxide enriched to 81.4% in the 170 mass of this activity was reported as 1.9 days. Twenty-five number was irradiated with 6-Mev protons. The conversion electron energies of approximately 2-day composition of the remaining protion is as follows in half-life found in a lutetium fraction obtained by the percent: 0.07 Yb1**, 7.8 Yb171, 4.8 Yb'», 2.3 Yb17*, irradiation of tantalum with 660-Mev protons have 3.1 Yb174, and 0.73 Yb17*. The atomic number of the been presented.* The corresponding transitions were resulting activity was determined by the identification associated with the combined activities of Lu1" and of the ytterbium K x ray which was compared with the Lu170 because of their similar half-lives. In an activity known K x rays of europium, terbium, thulium, of 1.5- to 2-day half-life produced in the same way as ytterbium, lutetium, and tantalum emitted from that of the preceding reference, 23 transitions were radioactive GdIM, Dy1M, Yb1", Tm170, Hf17*, and observed from conversion electron measurements.4 Two W1*1, respectively. Ion-exchange separation was deemed of these were attributed specifically to the activity of unnecessary. Lu170 and 13 associated with the combined activities of In order to determine the mass number of the Lu1" and Lu170. Gamma rays of energies 83, 190, 245, activity, similar proton irradiations were performed on and 2040 kev have been assigned to the activity of each of the other stable enriched isotopes of ytterbium Lu170 produced by the irradiation of natural lutetium and the resulting activities intercompared. The proton oxide with 250-Mev betatron bremsstrahlung.* The irradiations of Yb17*, Yb174, Yb17*, Yb177, and Yb171 measured half-life of the 2040-kev gamma ray was produced the well established activities of Lu17*, Lu174, (2.2±0.7) days and the 83-, 190-, and 245-kev gamma Lu17*, Lu17*, and Lu171, respectively. The initial activity rays were in coincidence with the 2040-kev gamma ray. produced by the irradiation of Yb17# was not observed No energy levels have been determined specifically in in any of the activities produced by the irradiations of Yb170 by Coulomb excitation although a composite the other enriched isotopes of ytterbium. After sufficient decay of this initially observed activity, identifiable level of about 78 kev has been assigned to all of the amounts of the activities of Lu171 and Lu17* were observed. These activities are attributable to the 7.8% ' G. Wilkinson and H. G. Hicks, Phys. Rev. 81, 540 (1951). * J. W. Mihelich, B. Harmatz, and T. H. Handley, Phys. Rev. and 4.8% of Yb171 and Yb171, respectively, existing with 108, 989 (1957). the enriched Yb170. *Iu. G. Bobrov, la. Gromov, B. G. Dzhelepov, and B. K. The half-life of the activity resulting from the Preobrazhenskii, Izvest. Akad Nauk S.S.S.R. Ser. Fiz. 21, 940 (1957) [Translation: Bull. Acad. Sciences U.S.S.R. 21, 942 proton irradiation of Yb17# is (2.05±0.05) days as (1957)]. determined by following the straight-line decay of the 4 V. M. Kel’man, R. Ya. Metskhavarishvili, B. K. Preobrazhen­ skii, V. A. Romanov, and V. V. Tuchkevich, Zhur. Eksp. i Teoret. gamma-ray spectrum for six half-lives on a 100-channel Fiz. 35, 1309 (1958) [Translation: Soviet Phys.—JETP 35(8), scintillation spectrometer. 914 (1959)]. 4 L. T. Dillman, R. W. Henry, N. B. Gove, and R. A. Becker, • G. M. Temmer and N. P. Hey den burg, Phys. Rev. 100, 150 Phys. Rev. 113, 635 (1959). (1955). 980 30

981 RADIOACTIVE DECAY OF Lu1"

T a b l e I. Gamma-gamma coincidence information for the activity of Lu170. .

K x ray 84 193 245 511 1010,1030 1165 1275 * 1415 2035 2665 84 yes no yes yes yes yes yes yes yes 511 ves yes yes yes yes 1010 and 1030 ves yes W yes yes 1275 no yes yes yes 2035 yes yes no yes yes yes no no no

L and K x rays of ytterbium were detected in the the 84-kev. Levels at 2365 and 3395 kev are suggested activity of Lu170 with a Geiger tube used with aluminum by the coincidences of 245-, 1030-, and 1275-kev and beryllium absorbers. Figure 1 shows the gamma-ray gamma rays with the 2035-kev gamma ray. The spectrum which is complex and extends past 3 Mev. 245-kev transition is placed lower only because a Some gamma rays are resolved in the gamma-ray 2365-kev level allows the placement of the 2365-kev spectrum and others have been observed in gamma- gamma ray from this level to the ground state. A spin gamma coincidence measurements. The observed assignment of 1 or 2 then seems consistent for both of gamma rays have energies of 84±1, 193±5, 245±5, these levels. A possible level at 3170 kev with spin 3 is 1010± 20, 1030± 20, 1165±30, 1275±20, 1415±30, suggested because the energy difference between the 2035± 15, 2320±40, 2665±20, 2890±40 and 3085±60 2890- and 3085-kev gamma rays is 195 kev, just the kev. No evidence of particle radiation was found by energy of the second ground-state rotational transition. the method of plastic scintillation spectrometry nor by The ratio of the number of counts under the spectral the use of a Geiger tube with aluminum and beryllium distribution of the K x ray to that of the 84-kev absorbers. A small amount of annihilation radiation gamma ray is 100/10.2. The K, L\, Lu, Lux, M, and does exist in the gamma-ray spectrum but was shown AT7 internal conversion coefficients® for an 84-kev El by absorption measurements with a plastic scintillation transition in ytterbium are 1.30, 0.14, 1.76, 1.76, 1.75, spectrometer to be the result of the annihilation of and 0.50, respectively. The ratios of the total number of positrons produced by the high-energy gamma radia­ tion. The mode of decay is therefore essentially by electron capture to Yb170 although the ground state of 1010 Lu170 is at least 3400 kev above that of Yb170 as discussed later. 1279 As seen in Fig. 1, the low-energy portion of the J4 I0 ' gamma-ray spectrum consists of the ytterbium K x ray and a strong 84-kev gamma ray. Two coincidence sum peaks are also observed; one is the sum of two coincident K X-BAY K x rays and the other is the sum of 84-kev gamma rays and K x rays. These peaks were shown by absorption measurements to be coincidence sum peaks. .ESCAPE Gamma-gamma coincidence information was obtained PEAK for the activity of Lu170 with a coincidence circuit of 64 resolving time 2 t = 1.5 /isec and two ljx2-inch crystals. Table I is a tabulation of this coincidence MSO information.

DISCUSSION 2KX 84+KX ITO The existence of a weakly populated level at 278 kev (4+) in Yb170 is further confirmed here because a weak 193-kev gamma ray is in coincidence with the 84-kev gamma ray. The ratio of the number of 193- to 84-kev transitions was calculated to be 0.055 from the data of references 2 and 7 which is consistent with the fact that the 193-kev gamma ray can be seen only in the coincidence spectrum. A level with a spin of 0 is !•Fio.10 , 1. Gamma-ray spectrum of LuIW as measured with a a proposed at 2120 kev because (1) the 2035-kev gamma 3X3 inch crystal and a'100-channel scintillation spectrometer. ray is in coincidence with the 84-kev but not with the Sample was centered on flat face of crystal. 193-kev, (2) no 2120-kev crossover transition is 7 Obtained from the data of reference 2. observed, and (3) the relative number of the 2035-kev * M. E. Rose, Internal Conversion Coefficients (North-Holland transitions is greater than any other transition except Publishing Company, Amsterdam, 1958). 31

R . G . WILSON AND M. L. POOL 982

and the high-energy radiations emitted in the decay of Lu'T0 (2.05 Days) JL Lu170 require an energy difference of about 3395 kev 71 EC between the ground states of Yb170 and Lu170. Because -35% approximately two-thirds of the electron capture disintegrations of Lu170 occur to the 84 kev 2 + level of Yb170, a spin of 2 is favored for the ground state of Lu170. 3395 1,2 Three other lutetium activities previously reported 3170 3 upon10 yield information about the energy level struc­ ture of the even-even isotopes of ytterbium with mass numbers 168, 172, and 174. The ground-state spins of all four of the radioactive lutetium isotopes, 168, 170, 2 3 6 5 1,2 172, and 174, are important factors in determining which t4»l 2120 0* parts of the even-even level structure of the ytterbium isotopes are excited. Lu17* has a ground state with a spin of 4, the decay of which excites the first three members of the ground-state rotational band, a K= 3 vibrational- rotational band at 1172 kev, and an apparently single­ particle level at 2072 kev. Lu174 and Lu1M have ground- 6 5 % state spins of 1 — and their decays excite a much smaller part of the level structure of the daughter isotopes; 1 only the first member of the ground-state rotational band, a 0 + vibrational level at 1305 kev in Yb*74, and a - 278 4 + level at 987 kev in Yb14* which is probably the ground state of the K= 2+ vibrational band. ■A 8 4 2 + As discussed above, Lu170 may have a ground-state — 0 0 + spin of 2 and is at least 3.4 Mev above the ground ITO state of Yb170. A departure from the portion of the level to YB structure which is excited occurs in this case. The first F ig . 2. Proposed partial energy level scheme for the decay of Lu1™.11 two members of the ground-state band are the only levels below 2 Mev which are excited.11 The complex, transitions to the number of corresponding gamma rays low-intensity, and high-energy gamma-ray spectrum and to the number of -converted transitions are 8.2 of Lu170 seems to result from the population of low-spin and 6.3, respectively. Because of K fluorescence yield levels in Yb170 with energy greater than 2 Mev. A in ytterbium is 0.937,* 107 K x-ray producing events minor closed shell or magic number occurs when the were necessary in order for 100 to be observed. one-hundredth neutron fills the HVt state. The (p,n) The 10.2 84-kev gamma rays result from 10.2X8.2 reaction with Yb170 requires the removal of this “closed = 84 transitions which also produce 84/6.3 = 13/l x-rays shell” neutron which may account for the energy by K conversion. If internal conversion of the other difference of 3.4 Mev. weak transitions is neglected, 107—13= 94 K x rays result from K capture to the levels of Yb17#. These ACKNOWLEDGMENTS figures indicate that a large fraction of the electron One of us (R.G.W.) is grateful to the National capture transitions result in an 84-kev transition. It is Science Foundation for the grant of a fellowship which estimated that approximately two-thirds of the electron enabled the completion of this research. Appreciation capture transitions of Lu170 occur to the 84-kev level is expressed to R. P. Sullivan of the Department of of Yb170, the remainder populating higher-eneigy levels Physics and Astronomy for assistance in the electronic or occurring directly to the ground state. Some flexibility phases of this research and to the Office of Naval Re­ must be allowed in this division because the fraction search for support in obtaining the enriched isotopes. of L capture is not known nor are the positions of the numerous but weak high-energy transitions certain. 10 R. G. Wilson and M. L. Pool, Phys. Rev. 117 , 517 (1960): The intensities of all of these high-energy radiations 118 , 227 (1960); and 118 , 1067 (1960). are small compared with the 84-kev, and the amount 11 Note added in proof. Ninety conversion electron transitions between 0 and 3000 kev have recently been reported [B. Harmatz, of L capture should be a minimum because of the large T. H. Handley, and J. W. Mihelich, Phys. Rev. 119, 1345 (I960).] decay energy. for the activity of tu 770. A large number of levels below 2 Mev probably do exist in Yb170 which are weakly populated from above. Figure 2 shows a proposed partial energy level scheme A weak crossover transition from the 2123-kev level is observed for the decay of Lu170. The coincidence information which suggests that the spin of this level is 2 rather than 0. More accurate energies for the levels shown in Fig. 2 are 84.2, 277.7, • A. H. Wapstra, G. J. Niigh, and R. Van Lieshout, Nuclear 2123, 2359, 2956, and (3389 and 3421) kev. Other possible levels Spectroscopy Tables (North-Holland Publishing Company, are suggested at 941, 1141, 1232, 1483, 1515, 1537, 2512, 2767, Amsterdam 1959), 2836, and 3348 kev. 32

Reprinted from T m P h y sic a l R ev iew , Vol. 120, No. 5, 1843-1850, December 1, 1960 PriaMd la U. S. A.

Radioactive Decay of Lum and Level Schemes for Yb 171 and Yb18*

R. G. W ilson and M. L. P ool Department of Physics and Astronomy, Ohio Stats University, Columbus, Ohio (Received March 14,1960; revised manuscript received August 31,1960)

Ytterbium oxide enriched to 93.8% in the mass number 171 and 949 (7/2) kev account for thirty of thirty-one transitions was irradiated with 6-Mev protons. An activity decaying by reported for this activity. Transition probabilities and branching electron capture with a half-life of (8.28±0.04) days was produced ratios for the election capture decay have been calculated. and its assignment to Lu171 confirmed by the identification of the Eighty-four percent of the disintegrations of the 7/2+ £404] ytterbium K x ray and by comparison with the activities produced ground state of Lu1" occur to the 7/2— £514] state at 835 kev by similar proton irradiations of each of the other enriched in Yb1". The previously reported 600-day activity of Lu,n was isotopes of ytterbium. The radiations observed in this activity not found. An energy level scheme for the decay of Lu,w (1.5 days) were the L and K x rays of ytterbium and gamma rays with as reported by other workers is proposed with levels in Yb1* at energies of 72-76, 668±7, 740±3, and 841 ±8 kev. Because 0 (7/2+ £633]), 24.2 (1 /2 - £521]), 70.9 (9/2+ 7/2), 87.0 neither particle nor annihilation radiation exists in this activity, (3 /2 - 1/2), 99.3 (5 /2 - 1/2), 161.7 (11/2+ 7/2), 191.4 (5/2— the mode of decay is solely by election capture to Yb1". Gamma- £512]), 244.0 (7 /2 - 1/2), 264.5 (9 /2 - 1/2), 278.7 (7/2 - 5/2), gamma coincidence measurements were performed for the ob­ 389.7 (9/2 - 5/2), 523.1 (11/2 - 5/2), 570.5 (5/2+ £642]), served radiations. An energy level scheme has been constructed 648.2 (7 /2 - £514]), 962.2, 1451.6 (9/2+ £624]), 1456 (9/2), for the decay of Lu,n using the results of the gamma-gamma and 1465 (9/2) kev and with less certainty, at 488.9 (11/2— 1/2), coincidence measurements, an energy analysis of the conversion 798,870,953, and 973 kev. This scheme accounts for 53 observed electron data, and calculations of transitions intensities for various transitions following the decay of Lu11*. The ground state of trial multipole orders. Levels in Yb1" at 0 (1/2— £521]), 66.7 LuIM is probably the 9/2 £514] orbital also assigned to the (3 /2 - 1/2), 75.9 (5 /2 - 1/2), 95.1 (7/2+ £633] r> 10-‘ sec), ground state of Lum. This scheme is based upon an energy 122.4 (5 /2 - £512]), 167.3 (9/2+ 7/2), 208.1 (7/2 - 5/2), 230.5 analysis of the reported transitions, intensity calculations for (7 /2 - 1/2), 246.5 (9 /2 - 1/2), 317.3 (9/2 - 5/2), (350), 835 various trial multipole orders, and analogy with the scheme (7 /2 - £514]), 862 (possibly 5/2+ £642]), 935 (9/2+ £624]), proposed for Yb1".

INTRODUCTION Dmitriev, and Preobrazhenskii.* Conflicts are evident WO activities decaying by electron capture with between the reported conversion electron and gamma- half-lives of (8.5+0.2) days and ~600 days have ray spectra. In some cases difficulty was encountered been assigned to Lu1T1 by Wilkinson and Hicks.1 because of the coexistence of the 7-day Lu177 with the Coulomb excitation experiments by Elbek, Nielsen, 8-day Lu171. In this investigation, in which enriched and Olesen* have led to the assignment of levels at 67 isotopes of Yb171 and Yb1” were irradiated with protons, and 76 kev in Ybm, which has a ground-state spin of it was possible to examine separately the gamma-ray 1/2, measured by Cooke and Park.* The 67-kev level spectra of Lu171 and Lu1”. Agreement is good with the has also been observed by Smith et al* following the conversion electron spectrum of Lu171 which was also 18~ decay of Tm171. obtained with enriched isotopes and reported by Several references exist concerning the conversion Harmatz, Handley, and Mihelich.7 electron spectrum*-7 and one concerning the gamma-ray EXPERIMENTAL RESULTS spectrum* of the 8-day Lu171. The half-life has been reported as 8.1 days7 and two conflicting partial energy Ytterbium oxide enriched to 93.8% in the 171 mass number was irradiated with 6-Mev protons. The levels schemes for Yb171 have been proposed by Harmatz, composition of the remaining portion in percents is: Handley, and Mihelich7 and by Gromov, Dzhelepov, 0.05 Yb1**, 0.04 Yb1", 3.4 Yb1”, 0.93 Yb1”, 1.17 Yb1”, and 0.23 Yb17*. The atomic number of the resulting 1 G. Wilkinson and H. G. Hicks, Phys. Rev. 81, 540 (1951). ' B. Elbek, K. O. Nielsen, and M. C. Olesen, Phys. Rev. 108, activity was determined by the identification of the 406 (1957). ytterbium K x ray which was compared with the known * A. H. Cooke and J. G. Park, Proc. Phys. Soc. (London) A69, 282 (1956). K x rays of europium, terbium, thulium, ytterbium, 4W. G. Smith, R. L. Robinson, J. H. Hamilton, and L. M. lutetium, and tantalum emitted from radioactive Gdlu, Langer, Phys. Rev. 107, 1314 (1957). Dy1", Yb1**, Tm1", Hf171, and W1*1, respectively. Ion- ‘ fu. G. Bobrov, K. la. Gromov, B. S. Dzhelepov, and B. K. Preobrazhenskii, Izvest. Akad. Nauk S. S. S. R. Ser. Fiz. 21, 940 exchange separation was deemed unnecessary. (1957) translation: Bull. Acad. Sciences U. S. S. R. 21, 942 In order to determine the mass number of the activity, (1957)]. similar proton irradiations were performed on each of •V. M. Kel’man, R. Ya. Metskhvarishvili, B. K. Preobraz­ henskii, V. A. Romanov, and V. V. Tuchkevich, Zhur. Eksp. i the other stable enriched isotopes of ytterbium and Teoret. Fiz. 35, 1309 (1958) translation: Soviet Phys.—JETP the resulting activities intercompared. The well-known 35(8), 914 (1959)]. activities of Lu17*, Lu1”, Lu1", and Lu1” were produced 1 B. Harmatz. T. H. Handley, and J. W. Mihelich, Phys. Rev. 114, 1082 (1959). ' K. Ia. Gromov, B. S. Dzhelepov, A. G. Dmitriev, and B. K. 1 A. I. Lebedev, A. N. Silant’ev, and I. A. lutlandov, Izvest. Preobrazhenskii, Izvest. Akad. Nauk S. S. S. R. Ser. Fiz. 21,1573 Akad. Nauk S. S. S. R. Ser. Fiz. 22, 839 (1958) translation: (1957) translation: Bull. Acad. Sciences U. S. S. R. 21, 1562 Bull. Acad. Sciences U. S. S. R. 22, 833 (1958)]. 33

1844 R. G. WILSON ND M, L. POOL by the irradiation of Yb”', YbIM, Yb,7,f and Ybm, T abu I. Data concerning the transitions in Ybin respectively, with no evidence of reactions other than following the decay of Lum. {p,n). The activity produced by the irradiation of Con­ Gamma- Transition enriched Yb171 was different from all of these activities. version Reason ray probability A small amount of the activity of Lu171 was observed Tran­ electron for intensity (per 100 with the activity of Lu171 and is attributed to the 3.4% sition refer­ Multipole M.O. (this disinte­ energy ences of Yb17* existing with the enriched Ybm. order choice work) grations) The half-life of LuI7> is (8.28±0.04) days as measured (9.2)* (MI+E2) a 27 (16.0)» (MI+E2) a 0.8 by following the straight-line decay of the strongest 19.2 7 £1 b 93 gamma ray, 740 kev, for over seven half-lives with a 27.3 7 £l(+A/2) a 0.6 100-channel scintillation spectrometer. The assignment 46.S 7 A/1+ £2 (25%) c 0.84 55.7 5.6.7 A/l+£2(2%) c 4.0 of the 8.3-day activity to Lu171 is therefore confirmed. 66.7 5.6.7 A/1+ £2 (39%) b 32 One sample of enriched Yb171 was irradiated with 72.2 5,6,7 A/1-f £2(8%) c ,d | (1.2) 17.5 6-Mev protons for a long period of time in an attempt (£1) 5.5 43 75.9 5,6,7 £2 b 1 (4.3) 68 to build up any long half-life Lu171 activity which might 85.7 5,6,7 A/1+£2 a 3.5 exist. The gamma-ray spectrum of this sample after 91.4 5,6,7 M2 a 0.9 109.2 7 M1+E2 c 1.8 320 days of decay can be entirely accounted for by the 142 7 ~ 0 J spectrum of the long-lived Lu171, which was produced 154.6 7 A/1+ £2 b 0.15 by a (p,n) reaction with the 0.93% Yb17' existing with 163.8 7 £2 b 0.64 170.6 7 £2 b 0.05 the enriched Yb*71. No long half-life gamma activity of 183 7 ~0.05 Lu171 exists with an intensity greater than 10~7 that of 194.9 7 £2 c 0.3 499 7 the 8-day activity. The long half-life activity previously 518 6,7 A/1+£2(10%) c, a 0.7 assigned to Lu171 is best attributed to an impurity. 627 6,7 M l+£2(10%) c, a 1.5 The L and K x rays of ytterbium were detected in 668 6,7 £ 1+ A/2 (50%) d,c 10 12.5 689 6,7 £l+A/2 a 0.8 the activity of Lu171 with a Geiger tube used with 713 6,7 A/1+£2(10%) c, a 2.1 aluminum and beryllium absorbers. By use of the 740 6,7 £ 1 + A/2 (38%) d, c 50 68 scintillation spectrometer, gamma rays of 72-76, 668 768 6,7 A/l+£2 a \ » la 2.0 782 6,7 A il+£2 c, a/ 2.6 ±7, 740±3, and 841 ±8 kev in addition to the ytter- 786 7 $0.1 795 7 $0.1 827 7 (El+A/2) a $0.1 841 6.7 Ml +£2(15%) d, c 5 6.8 ! life K X-RAY 854 6.7 Ml +£2(20%) c 1.1 AT x ray 100(107)' 140

• Implied by level echeme. k See reference 7. ■ Required by Inteailty calculations and by fit with conversion electron data. 4 Required by gamma-ray data. * Observed only in coincidence measurements ' Corrected for fluorescence [A. H. Wapstra. G. J. Nllgh, and R. Van Liestiout, Nuclttr Sptdrouopy Tablta (North-Holland PubUshlni Com­ pany, Amsterdam, 1959)1 740 KKX bium K x ray were found as shown in Fig. 1. A peak at 112 kev was shown by absorption measurements to be the sum peak of'two coincident K x rays. No gamma ray with an energy greater than 860 kev exists in the activity of Lu171 in an amount greater than 0.1% of ■ o that of the 740-kev gamma ray. This upper limit is in agreement with the conversion electron data of reference 7 and in conflict with the previously reported gamma- ray spectrum.* The gamma-ray spectrum of Lu17* has already been reported.10 It is now clear in nearly all cases which of the transitions reported in references 5 and 6 belong to the individual activities of Lu171 and Lu17*. No evidence of positron radiation was found in the activity of Lu171 by the method of plastic scintillation spectrometry, by the use of a Geiger tube with alumi­ 0 COO 400 400 400 num and beryllium absorbers, nor by a search for Fro. 1. Gamma-ray spectrum of 8.28-day Lu,n. “ R. G. Wilson and M. L. Pool, Phys. Rev. 118,1067 (1960). 34

RADIOACTIVE DECAY OF Lu'*1 1845

T a b u I I . Gamma-gamma coincidence information for the activity of Lum. Y r K X-RAY 768, ,171 A x ray 72-762 A x 668 740 782 841

COINCIDKNCI A x ray yes yea yea yes yea yea RPE6TRUM 2 A xrays yea yea w yea no yea no 668 yea yea yea no no no 740 yea no no no no no IRO* REOMETRY 841 yea no no no no no

shows the gamma-ray spectrum in coincidence with two coincident K x rays. The upper curve is the noncoinci­ Fto. 2. Gamma- dence spectrum. This coincidence spectrum is inter­ gamma coincidence NONCOINCIOENCE spectrum of Lutn preted to mean that the 668-kev gamma ray and 740 gated by two coin­ possibly either or both of the 768- and 782-kev gamma cident A x rays. rays are in coincidence with at least one highly K- converted low-energy transition in addition to the K x ray resulting from preceding K capture; and that the 740- and 841-kev gamma rays are in coincidence with IRR no more than one K x ray. The 668-kev gamma ray, as shown in Fig. 3, is in coincidence with the 72-76-kev peak but the 740-kev is not. Nothing but the single K x ray is in coincidence with the 740-kev gamma ray.

DISCUSSION A ground state (K—l/2) rotational band which is EWEROV. IN KEV consistent with the results of the Coulomb excitation 400 ROO experiments1 has been formulated for Yb171 by Harmatz et al.7 The scheme presented in reference 9 is not annihilation radiation in the gamma-ray spectrum. considered here because it differs from the results of Therefore, the mode of decay is solely by electron the Coulomb excitation experiments and contains a capture to Yb171. The transitions in Yb171 following the decay of Lu171 are listed in Table I with the appropriate conversion 180* GEOMETRY electron references from which the transition energies are obtained. In the fifth column are presented the YB K X RAY relative numbers of counts under the spectral distri­ NONCOINCIOENCE butions corrected for crystal efficiency of the observed GATE*.6 4 8 - 6 7 5 KEV gamma rays. These intensities were obtained with a IODINE GATE: 720-760 KEV 3 X 3-inch shielded Nal(Tl) crystal. The multipole -

1846 R. G. WILSON AND M. L. POOL crucial transition, 90.6 kev, which has been shown in sitions,7 and from the results of calculations of transition these investigations” to belong to the 7-day activity intensities for various trial multipole orders assumed of Luin. The former ground-state band has been used for all of the observed transitions. The transition as the basis for the proposed energy level scheme of probabilities listed in the last column of Table I are Yb1T1 shown in Fig. 4. This scheme has been constructed also shown in Fig. 4. These figures have been used to with the aid of the coincidence information for the calculate the branching ratios of electron capture from gamma rays of Lum described in the Experimental Lum to the levels of Ybm and are shown in Fig. 4. Results, from an energy analysis of the reported tran­ The relative number of K x rays corrected for fiuo-

(8.26 DAYS) f f (*04] iL

949 i + tf* 935 f + [624]

862 f + §542] 835 f " [514]

II 317.3 { - $ 246.51 ^ SB* 230.5 -f- -J- —j W 's '* ® % 9 l 208.1 - f-

167.3 f + f I I 122.4 | - [512]

95.1 \ +|633|<

0 t - i [521] E Iff K NnA

Fio. 4. Proposed energy level scheme for the decay of Luin. RADIOACTIVE DECAY OF Lu1’* 1847 rescence accounts for about 95% of the electron capture 72 kev in Yb171 is displayed in Table III. A mixture of transitions after subtraction of K x rays from K 92% MX and 8% £2 provides an excellent fit for the conversion of the transitions in Yb171. L capture data of Table III while no mixture of £1+Af2 can probably accounts for the remaining disintegrations. account for the relative numbers of observed Lu and Intrinsic orbital assignments have been made for the Lni conversion electrons. Pure £1 involves a 50% error levels of Ybin where appropriate, following the system in these numbers. Two other arguments against an of Mottelson and Nilsson1* and are shown in Fig. 4. assignment of £1 for the 72-kev transition exist. As A discussion of some of the details of the level scheme shown in Table II, the calculated relative number of for YbITt is given below. 72-kev transitions when added to the corresponding As seen in Fig. 1, the gamma-ray spectrum of Lum number of 740-kev transitions yields a number of is dominated by the transitions of 740, 668, 841, and populations of the 95-kev level which is 20% greater 72-76 kev in addition to the K x ray. The energy than can be carried away by both of the 67- and 76-kev difference between the 740- and 668-kev transitions is transitions to the ground state. Trial multipole order 72 kev and a 72-kev transition is observed in both the calculations for the intensities of the transitions of 740, conversion electron7 and gamma-ray spectra. The 668, and 841 kev when combined with the corresponding interpretations given above for the results of the gamma-ray intensities, lead to the conclusion that the gamma-gamma coincidence measurements require that 740- and 668-kev transitions are £l+ J/2 and the the 72-kev transition is highly K converted and that 841-kev transition is Ml+£2. The conclusion that the the 740- and 841-kev transitions are not in coincidence 668- and 740-kev transitions are both of the same with the 72-kev, while the 668- and either or both of multipole order indicates that the 72-kev transition is the 768- and 782-kev transitions are in coincidence with J#l+£2. As discussed by others,7 another weaker the 72-kev transition. An energy analysis of the tran­ rotational band with K— 5/2 and composed of the sitions shows that the 841- and 768-kev transitions transitions of 86, 109, and 195 kev, probably exists in differ by about 72 kev and that if the 854-kev transition Yb171. The three remaining low-energy transitions of is combined with the 782-kev, three pairs of transitions Lu171, 27, 46.5, and 55.7 kev,7 serve to link this band can be matched with the 72-kev transition in a manner with the two other bands as shown in Fig. 4. The which explains the result's of the coincidence measure­ multipole order admixtures given in Table I for the ments. The coincidence measurements also imply that transitions of 45.6 and 55.7 kev provide excellent fits these six transitions are not in coincidence with the with the relative numbers of L conversion electrons,7 76-kev transition to the ground state of Yb171 within while no mixtures of £1+Jlf2 are satisfactory. The the instrumental resolving time. A low-lying isomeric parity of this band, the ground state of which is at state (7/2+ [633]) is expected1* to exist in Yb171. If 122 kev, is therefore odd. The 5/2— [512] orbital, this state is placed a t 95 kev, the strong 19-kev £ 1 7 which is the ground state of Ybin, is expected to occur and the weaker 91-kev transitions serve as the link in Yb171 and has been assigned to the level at 122 kev. between the ground-state band and these three pairs of Three transitions, all originating from the previously transitions. Some of the isomerism of the 95-kev level established level at 835 kev with negative parity, serve is removed by the occurrence at lower energy of the to populate the three states of this £= 5/2 band. The second and third members of the ground-state band implied multipole orders of these three transitions are but enough hinderance is apparently provided by the Afl+£2. When trial multipole order intensity calcu­ difference in K of three units to make the lifetime of lations are made for these three transitions, it is found the 95-kev level longer than 10~* second. that mixtures of about 10% £2 with 90%Ml provide The establishment of the 7/2+ [633] orbital at 95 the number of populations of this band which are kev implies levels at 167 kev depopulated by the 72- carried away by the three transitions which depopulate and 91-kev transitions and at 835, 935, and 949 kev the 122-kev ground state. Any appropriate mixtures of which lead to the three pairs of high-energy transitions EX+M2 result in more populations of this band than differing by 72 kev. It is instructive to note at this are carried away. point that these three high-energy levels are also depopulated by transitions to other levels according to T able III. Internal conversion data for the transition of 72.2 kev the scheme of Fig. 4. in Ybln following the decay of 8.28-day Lu1TI, Because the energy of the 72-kev transition is typical Number of of rotational transitions in this region, the possibility Internal conversion coefficients* conversion that the 167-kev level is the 9/2 rotational state Shell £1 £2 MX M l electrons* associated with the 7/2 [633] orbital at 95 kev is K 0.554 1.52 6.32 59.9 » 5 0 considered. The 72-kev transition would then be Lt 0.0540 0.167 0.799 15.7 90 Afl+£2. Internal conversion data for the transition of La 0.0176 2.77 0.0653 1.63 30 Lui 0.0219 2.87 0.0102 4.34 22

11B. R. Mottelson and S. G. Nilsson, Kgl. Danske Videnskab. • See reference 11. Selakab, Mat.-Fys. Medd. 1, No. 8 (1959). 1 See reference 7. 37

1848 *• C. WILSON AND M. L. POOL

1 _ r.. .1 7lL u (15 DAYS 1

' M 7 7 7 / 1 1 ~ ////// EC

/ >1 «v. •». 1469 1496 14515$ + (b24J ^ / II

= = m ~ wz zS I I

648.2 $ - [5,-0

5705 i + [6423

523.1 |

±>— i------

389.7f - j. I I I I

2648 f - i — h i t 4 4 . 0 f - ^ _ jL L *

1914 |- [ 5 I 2 J ,61.7 £ +

7059 f + -$

24.2^--[52j J r ^ ,,

0 T + [633] 70 DAYS) E I ff K F io. 5. Proposed energy level scheme for the decay of Lu«. 38

RADIOACTIVE DECAY OF Lu,7‘ 1849

T a b u IV. Data concerning transitions in Yb1" level of Yb171, which is depopulated by a strong 740-kev following the decay of Lu1". gamma ray which places the system in a spin 7/2 semi-iSomeric state 95 kev above the spin 1/2 ground Con­ version Reason state of Yb171. This state at 95 kev is depopulated by a electron for strong 19-kev radiation which leads to the two ground- Transition refer­ Multipoleb M.O. Transition1** state transitions at 67 and 76 kev. A strong 9-kev energy ences order choice intensity radiation is then implied but has not yet been observed (12.3}* (A/l+£2) a £6* unless it is the “poorly discernible” 11-kev transition 20.5)* A/1+£2) a V* 24.2 6,7 £3 c 23*(»11 calc.) mentioned in reference 9. 62.8 6,7 A/l+£2(18%) c 18 A proposed level scheme for Yb1*7, which has been 70.9 A/l+£2(32%) a 32 75.0 £2 c 5.3 constructed with the aid of a previously proposed 87.4 V A/1 +£2(7%) a 24 partial scheme,7 an energy analysis of the transitions 90.8 V A/l+£2(6%) a 4.0 reported for the activity of Lu1",7 intensity calculations 92.1 7 A/1 a, d 2.0 104.4 7 A/1 a, d 2.1 for various trial multipole orders assumed for the 110.9 6,7 A/l+£2(10%) a 8.0 transitions, and from analogy with the scheme proposed 133.5 7 A/1+(£2) a 0.6 for Yb171, is presented in Fig. 5. This scheme accounts 144.6 7 A/l+(£2) c 1.3 157.0 £2 c 2.9 for 53 of the transitions observed following the decay 161.7 £2 c 0.6 of Lu1'*. Information concerning the transitions in 165.2 6,7V £2 c 4.2 167.2 7 £2 a 0.4 Yb1M for which multipole order choices have been made 191.4 £1 d 36 is displayed in Table IV. As for Yb171, where the 498.3- £2 a 0.9 admixture percentages could not be calculated from 244.4(244.9) V7 £2 c 0.08 259 7 A/1+£2 a 0.90 the conversion electron data, the percentage of quadra- 292 6,7 £l+A/2 a 3.8 pole radiation was arbitrarily assumed to be 30% for 370 6.7 A/1+£2 a 1.9 the calculations. The multipole order choices are based 379 6.7 £l+A/2 a 16.5 404 7 A/1+£2 a 0.50 upon assignments to some of the levels in Yb1" of 457 6,7 A/l+£2 a 1.5 intrinsic orbitals and their rotational states according 471 7 £l+A/2 a 4.0 484 7 El+A/2 a 2.0 to the system of Mottelson and Nilsson.17 Some of the 489 7 A/l+£2 a 0.26 multipole orders are assigned in reference 7. Because 549 7 A/l+£2 a 0.71 intrinsic level assignments could be made with some 577 7 £l+A/2 a 5.1 648 7 £l+A/2 a 1.3 degree of certainty for only some of the levels proposed 881 7 £2 a 1.0 in Yblw, Table IV contains only a partial list of the 890 7 A/1+£2 a 1.8 962 7 A/l+£2 a 7.4 1062 7 EI+M2 a 7.1 T able V. Energy ratios for rotational bands in the rare-earth 1173 7 £l+A/2 a 3.6 region. AU energies, £, are in kev. 1187(2) 7 E1+M2 a 9.2 1207 7 £l+A/2 a 1.6 At 1290 7 A/1+£2 a 1.0 £x+i/£jt+i Nucleus £ e+i £jr+t 1380 7 A/1+£2 a 1.5 3/2 2.400 calculated 3/2-* 2.43 Gd‘“ 60 146 • Implied by level echeme and confluent with converiion electron data. 3/2+ 2.40 TbIM 65 156 b All unftated multipole order admixturea are arbitrarily choaen aa 70% 3/2-* 2.40 Gd1" 54.5 131.0 dipole and 30% quadrupole for intenaity calculations 3/2+ 2.37 Xb>« 60.8 143.9 • See reference 7. • Required by trial multipole order intenaity calculations. 3/2+* 2.37 Tb‘» 58.0 137.5 • Per 100 diaintegrationa if electron capture to ground state is negligible. 3/2+ 2.37 Tb'“ 57 135 3/2— 2.44 W" 83 203 5/2 2 286 calculated It is now evident that there exists a negative parity 5 /2 - 2.27 DylU 77.5 175.6 S/2-* 2.29 Dy'“ 74 170 level at 835 kev in Yb171 which is highly populated by 5 /2 - 2.27 Yb1" 87.4 198.3 the electron capture decay of Lu171 to which has been 5 /2 - 2.27 Yb”1 85.7 194.9 assigned the 7/2+ [404] orbital.17 The 7/2— [514] 5/2-* 2.28 Yb1™ 78.7 179.5 5 /2 - 2.28 HP™ 81.6 186.0 orbital is expected17 to occur at a higher energy in Yb171. 5/2+ 2.27 Re1" 114.4 259.8 This orbital has been assigned to the 835-kev level 5/2+* 2.29 Re1" 125 286 5/2+ 2.25 Re1" 134 301 because it is consistent with the high population by 7/2 2.222 calculated electron capture from Lu171 and accounts well for the 7/2-* 2.21 Ho1" 94.8 210 mode of depopulation of this level. Tentative assign­ 7/2+* 2.21 Er1" 78 172 7/2+ 2.28 Yb1" 70.9 161.7 ments for the levels at 935 and 949 kev are shown in 7/2+* 2.21 Lu1" 113.8 251.1 Fig. 4 and less certain levels at 862 and 350 kev have 7/2-* 2.21 HP" 113.0 249.7 been suggested in order to account for the remaining 7/2+* 2.21 Tal« 136 301 9/2 2.182 calculated transitions reported for Lu171.7 9/2+* 2.17 HP™ 121 262 According to the scheme of Fig. 4, the primary mode of decay of Lu171 is by electron capture to the 835-kev ■ Measured. 1850 R. G. WILSON AND M. L. POOL transitions reported for the activity of Lu1". Intensity transition and would involve no asymptotic quantum calculations assuming both M I+ E 2 and E \+ M 2 number changes greater than one unit. This level is admixtures for the other transitions shown in Fig. 5 seen to be depopulated by a rather strong transition have been made, however, and rough estimates of the to the 161.7-kev level with spin 11/2, the highest spin relative population of the unassigned levels have been and highest K level of lower energy. A second weaker made. From all of the calculations, branching ratios for transition occurs, as might be expected, to the lower the electron capture decay of Lu1* have been estimated rotational level. and are shown in Fig. 5. Electron capture to the ground During the work on the construction of the level state of Yb1M has been assumed negligible for this schemes for Yb171 and Yb1**, a possible method for aid calculation. If this mode of decay is significant, then in the determination of the rotational structure of all of the other relative intensities must be adjusted nuclei in this region was investigated. The results of for it. However, the positions and calculated intensities the investigation are presented below. The method of the higher energy radiations imply that little if any involves the use of the theoretical formula for the electron capture occurs to the members of the three rotational energies, E « [ / ( / + 1 ) —K (A+l)] for K low-lying rotational bands, ^-electron capture cannot > 1/2. The formula for K «* 1/2 and a discussion of the occur to the K—1/2 band and the intensities calculated fit of experimental data with the predictions of the for the transitions populating the members of this band formula are given by Harmatz et al.1 and will not be account well for the intensities of the transitions internal discussed here. The above formula predicts ratios for to this band. the energies of rotational levels which are not very The ground state of Lu1** is expected to be either the different for different values of K. However, as seen 7/2+ [404] orbital or the 9/2— [514] orbital, both of in Table V, the experimental energy ratios seem to be which occur as ground and first excited states of other consistent enough to determine the value of K for a lutetium isotopes. The three rotational bands in Yb171 pair of rotational transitions in spite of the small exhibit the spin 9/2 state and the 7/2— [404] orbital differences. The experimental data of Table V is taken has been chosen for the ground state of Lu171. The same from references 7 and 12 and from the Nuclear Research three rotational bands in Ybu' exhibit the spin 11/2 Council Data Sheets. Only one exception is seen; the member. The choice of the 9/2— [514] orbital for the 7/2+ band in Yb1**, which fits better the K = 5/2 ratio. ground state of Lu1** therefore seems better. Levels in The 7/2+ [633] assignment is fairly certain for this Yb1** with probable spins of 9/2 are populated directly band, however, because of the spins of other nuclei by electron capture which favors this choice for Lu1**. with 99 neutrons and because of the well-studied decay The 1.5-day half-life of Lu1** which is six mass numbers of Yb1** (see reference 12). from the stability line, may be an indication of the higher spin ground state. ACKNOWLEDGMENTS If the ground state of Lu1** is the 9 /2— [514] orbital and if there does exist a 798-kev level in Yb1** which is One of us (R.G.W.) is grateful to the National Science depopulated as shown in Fig. 5, then this level at 798 Foundation for the grant of a fellowship which enabled kev could be the 11/2— [505] orbital specifically the completion of this research. Appreciation is ex­ discussed by Mottelson and Nilsson.1* These authors pressed to R. P. Sullivan of the Department of Physics feel that this orbital should be populated in this region and Astronomy for assistance in the electronic phases if conditions permit. The population of this orbital by of this research and to the Office of Naval Research for the electron capture decay of Lu1*9 would be an allowed support in obtaining the enriched isotopes. 40

Reprinted from T h e P h y sic a l R ev iew , Vol. 118, No. 4, 1067-1072, May 15, 1960 Primed la U. S. A.

Radioactive Decay of Lu17*

R. G. W ilson and M. L. P ool Deportment of Physics and Astronomy, Ohio State University, Columbus, Ohio (Received October 5, 1959; revised manuscript received January 11, 1960)

Ytterbium oxide enriched to 95.9% in mass number 172 was consideration of the relative numbers of the radiations observed irradiated with 6-Mev protons. An activity decaying by electron in the activity of Lum have led to the assignment of energy levels capture with a half-life of (6.70±0.04) days was produced and its at 530 (6+), 1172 (3+), 1263 (4+), 1375 (5+), 1662 (3 -), assignment to LuIM confirmed by the identification of the ytter­ (1699), and 2072 (4+) kev in Ybln in addition to the previously bium K x ray and by comparison with the activities produced by known 78.7 (2+)- and 260.2 (4-f )-kev levels. The positions of all similar proton irradiations of the other enriched isotopes of of the observed radiations and some observed only in conversion ytterbium. The 4.0-hour positron activity previously assigned to electron measurements are shown in a proposed energy level Lum was not observed and is best attributed to an impurity. The scheme for the decay of Luin. Approximate branching ratios for observed activity of Lum consists of the L and K x rays of ytter­ the electron capture disintegrations of LuIT< are also shown in the bium and gamma rays with energies of 79, 91, 113, 182, 203, 270, level scheme. Few, if any, electron capture transitions of Luln 324,373, 490, 527, 697, 809,900, 912, 1093, 1402, and 1583 kev. occur to the ground and first excited states of Ybm. Of the two Because no positron radiation exists in the activity of Lu11*, tbe predicted spins for the ground state of Luln, 4— is more consistent mode of decay is solely by electron capture to Yb17t. Gamma- with the proposed energy level scheme. gamma coincidence measurements, energy considerations, and

INTRODUCTION specifically to the activity of Lu171.* Elimination of these WO activities have been assigned to Lu171; one de­ latter transitions from the composite groups of the two caying by electron capture with a half-life of 6.7 preceding references and combination with those of daysT and one decaying by positron emission with a half- reference 2 yields a group of transitions with a maximum life of 4.0 hours.1 Absorption measurements led to the energy of about 1100 kev which should comprise the conclusion that the 6.7-day activity includes gamma 6.7-day activity of Lu171. radiation of about 1.2 Mev and that the energy of the Coulomb excitation of natural ytterbium oxide has positron in the 4.0-hour activity is about 1.2 Mev. Eight led to the assignment of a 79-kev level to Yb171.4,7 transition energies have been assigned to the 6.7-day Gamma rays of energies 79, 113, 181, 203, 325, 370, activity following the proton irradiation of natural 525, 820, 900, and 1090 kev were associated with the electron capture decay of the 6.7-day activity of Lu171 ytterbium oxide.1 These data were reported to suggest produced by the irradiation of natural lutetium oxide energy levels of 78.7 and 260.2 kev in Ybm. Twenty- with 250-Mev betatron bremsstrahlung.* Energy levels nine conversion electron energies have been measured of 373.1, 576.9,901.5,1082, and 1990 kev were assigned in an activity with a half-life of 7 to 8 days found in the to Yb171 in addition to the previously reported 78.7- lutetium fraction produced by the irradiation of tan­ and 260.2-kev levels. These assignments were made on talum with 660-Mev protons.* The corresponding transi­ the bases of some gamma-gamma coincidence measure­ tions were associated with the activities of Lum and ments and energy considerations. Gamma rays of Lu,n which have similar half-lives, but no specific energies 0.076,0.18,0.40,1.09,1.49, and 1.79 Mev have assignments were made. In a lutetium activity produced been observed following the 0“ decay of the 64-hour in the same way as that of the preceding reference and Tm171 activity.* Recently the enriched isotopes of having the same half-life range, 47 transitions were ob­ ytterbium have become available and it is the purpose of this paper to discuss the results of an investigation served by conversion electron measurements.4 The of the activity of Lu171 produced by proton irradiations transitions were again all associated with the activities of the enriched isotopes of ytterbium. of Lu171 and Lu171 but some assignments to the indi­ vidual activities were made. A recent investigation of EXPERIMENTAL RESULTS conversion electron energies following the proton irra­ Ytterbium oxide enriched to 95.9% in the 172 mass diation of enriched isotopes of ytterbium has resulted number was irradiated with 6-Mev protons. The com- in the assignment or confirmation of 30 transitions 1B. Harmatz, T. H. Handley, and J. W. Mihelich, Phys. Rev. > G. Wilkinson and H. G. Hicks, Phys. Rev. 81, 540 (1951). 114, 1082 (1959). 1 J. W. Mihelich, B. Harmatz, and T . H. Handley, Phys. Rev. * G. M. Temmer and N. P. Heydenburg, Phys. Rev. 100, 150 108,989 (1957). (1955). * lu. G. Bobrov, la. Gromov, B. G. Dzhelepov, and B. K. Preo­ 7 E. L. Chupp, J. W. M. DuMond, F. J. Gordon, R. C. Jopson, brazhenskii, Izvest. Akad. Nauk S.S.S.R. Ser. Fiz. 21,940 (1957) and H. Mark, as reported in D. Strominger, J. M. Hollander, and [translation: Bull. Acad. Sciences U.S.S.R. 21, 942 (1957)]. G. T. Seaborg, Revs. Modem Phys. 30, 585 (1958). 1V. M. Kel’man, R. Ia. Mctskhvarishvili, B. K. Preobrazhen- * L. T. Dillman, R. W. Henry, N. B. Gove, and R. A. Becker, skii, V. A. Romanov, and V. V. Tuchkevich, Zhur. Eksp. i Teoret. Phys. Rev. 113, 635 (1959). Fiz. 35,1309 (1958) [translation: Soviet Phys.—JGTP 35(8), 914 ' D. R. Nethaway, M. C. Michel, and W. E. Nervik, Phys. Rev. (1959)}. 103,147 (1956). 1067 41

1068 R. G. WILSON AND M. L. POOL

Tablk I. Relative number* of gamma ray*, Ify, and corn* ■ponding transitions, Ntnm, In the activity of Lu'1* for gamma-ray s. energies, Elt expressed in kev. v too-*** OAMMA-RAYS S .T4 D A Y S (kev) References N r Nlm. \ Fio. 1. Decay of 78.7 £2 2,4 8.1 82 three intense high- 90.6 £2 2,4 ~ 3 20 energy gamma rays in 112.8 E2(+Afl) 2,4 <1 and <3 3 the activity of Lu1™. 181.5 E2 2.4 21 29 I M D A Y S 203.4 £2 2,4 > 5 8 •IO- i i v OAMMA RAY \ \ 270 2.4 4 5 324 2,4 <1 <1 373 3 3 -H R -1 DAY \ 399* *4 410* 4 TIME 490 4 1 1 527 4 3 3 697 4 4 4 position of the remaining portion is in percent: 0.02 809 4 17 17 Yb1**, 0.08 Yb1™, 1.18 Yb‘T\ 1.47 Yb1", 1.14 Yb1", and 900 4\ 28 912 37 9 0.19 Yb17*. The atomic number of the resulting activity 1002* V was determined by the identification of the ytterbium 1093 4 50 50 K x ray which was compared with the known K x rays 1402 2 2 1583 3 3 of europium, terbium, thullium, ytterbium, lutetium, £ x ray 100 107 and tantalum emitted from radioactive Gd1M, Dy1**, Yb1*, Tm 17#, Hf1", and W1*1, respectively. Ion-exchange * Not observed to the gamma-ray spectrum. separation was deemed unnecessary. In order to determine the mass number of the ac­ tivity, similar proton irradiations were performed on The original assignment of the 6.70-day activity to each of the other enriched stable isotopes of ytterbium Lu1" is therefore confirmed. A careful search was made and the resulting activities intercompared. None of the following two irradiations of enriched Yb1" for the 4.0 activities produced by these similar irradiations of the hour positron activity also previously assigned to Lu1". other enriched isotopes of ytterbium was found in No 4-hour annihilation radiation was observed in the identifiable quantity in the activity produced by the gamma-ray spectrum of the activity resulting from these irradiation of enriched Yb1”. The activity obtained by irradiations. It seems most probable that the 4-hour the proton irradiation of enriched Yb1" was found in positron activity is attributable to an impurity. barely identifiable amounts in the activities produced by L and K x rays of ytterbium were detected in the the irradiations of enriched Yb171 and Yb1". This may activity of Lu1" with a Geiger tube used with aluminum be explained by the 3.4 and 2.3% of Yb1" existing with and beryllium absorbers. Figure 2 shows the observed the enriched Yb171 and Yb1", respectively. gamma-ray spectrum (to 1200 kev) of Lu1" which in­ The half-life of the activity resulting from the proton cludes gamma raj's with energies of 79±1, 90±5, irradiation of Yb1" is (6.70±0.04) days as determined by following the decay of the three most intense high- T able II. Calculations for relative numbers of low-energy energy gamma rays for 5J half-lives as shown in Fig. 1. transitions in the activity of Lu17*. Columns 2, 3, and 4, respec­ tively, are the ratios of the total number of transitions to the number of gamma rays, A-converted transitions, and transitions converted in the * shell as indicated in column 5. The a’s were obtained from Rose.* N t are the relative total numbers of transi­ tions obtained from the products of the values In column 4 by the u s jo - o a y Lu,T* appropriate relative numbers of conversion electrons given by Mihelich et al.b

B,(kev) (1 +Za)/at Fio. 2. Gamma-ray O+fcO/l (1 +Za)/at i N, spectrum of 6.70-day 78.7 10.1 7.2 4.2 Lt 4200 Lu'7* (to 1200 kev). This 90.6 6.2 5.4 5.0 U ,L x 1000 spectrum is a composite 112.8 3.1 4.0 7.7 Lt, Lt 175 of spectra taken at dif­ 4.0 K 140 ferent gains. Peak desig­ 181.5 1.4 6.4 6.4 K 1350 nations are in kev. A 26 Lt 1690 3 X 3-in. crystal was 31 Lt 1400 used. 203.4 1.3 8.5 8.5 K 340 39 Lt 550 52 Lt 415 270.1 1.1 16 16 K 300

• M. B. Rose. Inltrnal Conitriion CotJUknti (North-Holland Publishing Company, Amsterdam, 195S). 500KEVT000 * See reference 2. 4 2

RADIOACTIVE DECAY OF Lu*»» 1069

T a b l x III. Coincidence data for Lum. Gamma-ray energies in kev.

Kx 79 91 113 182 203 270 373 527 697 810 900 912 1093 1402 1583 Kx yes yes yes yes yes yes yes yes yes yes yes yes yes yes yea yea 79 (91) yea no yes yes yes yes yes yes yes yes yes w yes yes yes yea 113 yea yes yes w yes no yes no yes 182 (203) yes yes yes no yes yes w w yes w w yes no yea no 270 yes yes yes yes no 373 yes yes yes no yes yes yes no yes no no no yes yes 527 (490) yes yes yes yes yes yes yes no no no no yes yea 697 yes yes yes yes w yes yes no no yes yes 810 yes w yes no yes no no no yes yea 900 (912) yes yes yes yes no no no no no no(yea) yes yea 1093 yes yes yes yes no yea no yes yes yes yes yes no no no no

113*2, 182*2, 203=4=7, 270*3, 324*5, 373*3, 697-, 810-, and 900-kev gamma rays but in coincidence 490* 7, 527*4, 697*5, 809*5, 900*7, 912*7, with the 1093- and 912-kev gamma rays. Curve D shows 1093*4, 1402*15, and 1583*10 kev. No evidence of that the 373-, 527-, 697-, 809-, and 900-kev gamma rays positron activity was found in Lum by the method of are in coincidence with the 1093-kev gamma ray. This plastic scintillation spectrometry, by the use of a Geiger coincidence spectrum was interpreted to show the rela­ tube with aluminum and beryllium absorbers, nor by a tive amounts of the 809 and 900 gamma rays in co­ search for annihilation radiation in the gamma-ray incidence with the 1093 gamma ray and hence in the spectrum. Therefore the mode of decay of Lu1” is solely absence of the 912 gamma ray. The ratio of the heights by electron capture to Yb1”. of the 809 and 900 photopeaks was corrected for energy The fourth column of Table I gives the relative number of counts under the spectral distribution after correction efficiency and found to be 1.00/1.64. The ratio of the for crystal efficiency of the observed radiations listed heights of the 900+912 gamma rays to the 809 gamma in the third column. The value 82 in the last column of ray in the noncoincidence spectrum is 37/17 as shown Table I was obtained by multiplying 8.1 in the third in Table I. From these two ratios, the ratio of 900 to 912 column by 10.1 in the second column of Table II. The gamma rays is 3.1. A similar calculation was made using value 29 was obtained in a similar manner. The values the coincidence spectrum gated by the 900, 912 peak. 20, 3, 8, and 5 in the last column of Table I were ob­ The result gave a range of values which includes that tained by multiplying, respectively, the appropriate obtained above. numbers in the last column of Table II by the ratio 82/4200. The last number in the fifth column of Table I,

107, is the number of K x-ray producing events of which YB K X only 100 are observed because the fiT-fluorescence yield YB K X in ytterbium is 0.937.“ Table III is a tabulation of the coincidence informa­ tion for the activity of Lu1” obtained with a coincidence circuit of resolving time 2t=3X10“# sec and with two lj-inch by 2-inch Nal(Tl) crystals used at either 90° or 180°. This information was obtained from the analy­ sis of 25 coincidence measurements some of which lasted for 14 hours. Figure 3 shows four typical coincidence spectra. The upper curve in each case is the noncoinci­ dence spectrum. Curve A shows the 91-kev gamma ray which is obscured by the 79-kev gamma ray in the non­ coincidence spectrum and that the 91-, 113-, and 182- kev gamma rays are in coincidence with the 79-kev gamma ray. Curve B shows the 203- and 270-kevgamma rays which are obscured in the noncoincidence spectrum and that the 79-, 113-, 203-, and 270-kev gamma rays are in coincidence with the 182-kev gamma ray. Curve C shows clearly that the 527- and 373-kev gamma rays Fig. 3. Typical gamma-gamma coincidence spectra obtained are in coincidence and that this cascade is parallel to the 2t«3X1

DISCUSSION tion and the 90G-1093-kev cascade both occur to the Figure 4 shows a proposed energy level scheme for the ground state of Ybm , (4) the 79- and 1093-kev transi­ decay of Lum . Energy levels of 530 (6+ ), 1172 (3+ ), tions have been observed following the decay of 1263 (4 + ) 1375 (5 + ), 1662 ( 3 -) , (1699), and 2072 Tmm but the 900-kev transition has not. For these (4+) kev are assigned to Yb17* in addition to the pre­ reasons levels of 1172 and 2072 are assigned to Ybin. viously known 78.7 (2+ )- and 260.2 (4+)-kev levels. The 900-kev gamma ray is in coincidence with the These new assignments are made on the bases of gamma- 912-kev gamma ray and the latter transition fits be­ gamma coincidence measurements and by consideration tween the 1172- and 260-kev levels. The 809-kev gamma of the energies and the relative numbers of the gamma ray is in coincidence with the 1093-kev gamma ray and rays and of the rules governing transitions among levels the composite 900- and 912-kev peak although more in the region of elliptically deformed nuclei, particularly predominantly with the former, implying that the the rules of K forbiddenness. The accuracy of the 809-kev transition is in coincidence with only one of energies of these levels has been obtained by the use of the two transitions in the composite peak. The 809-kev the energies of the transitions in Ybin resulting from gamma-ray coincidence spectrum shows both the 79- the conversion electron measurements of references 2 and the 91-kev gamma rays but the 91-kev more pre­ and 4. Because this energy level scheme differs greatly dominantly. It is observed that the sum of the energies from the one previously proposed as mentioned in the of the 809- and the 91-kev gamma rays is 900 kev. Thus Introduction, an explanation of the scheme resulting the 809-91-kev cascade parallels the 900-kev transition from this investigation is given below. with their orders as yet undetermined. The 91-kev The 1093- and 900-kev gamma rays are intense and transition has been designated as £» from conversion in coincidence in agreement with reference 8. Four argu­ electron measurements* and is highly internally con­ ments for placing the 1093-kev transition immediately verted. This explains why the 91-kev gamma ray is preceding the 79-kev transition, and the 900-kev transi­ weaker than the 809-kev gamma ray in the gamma-ray tion therefore preceding the 1093-kev transition im­ spectrum. The 697- and 203-kev gamma rays are in co­ plying levels of 1172 and 2072 kev in Yb17* are (1) the incidence and their energies total 900 kev. The 113-kev 1093- and 900-kev gamma rays are both in coincidence gamma ray and the 91- and 697-kev gamma rays are in with the 79-kev gamma ray, (2) the 900-kev gamma ray coincidence, and the 203-kev gamma ray is in coinci­ is, and the 1093-kev gamma ray is not in coincidence dence with neither the 91- nor the 113-kev gamma rays. with the 182-kev gamma ray, (3) the number of K The 113- and 203-kev gamma rays have been designated x rays after accounting for K conversion of the low- as Ei from conversion electron measurements.* The energy transitions in Yb17* is not sufficient to account 79-, 113-, and 203-kev transitions may be assumed to for K capture to the levels of Yb172 if the 79-kev transi- be transitions among the levels of an excited rotational band with the 1172-kev level as its ground state.These levels are then alt populated by transitions from the ,Lu,M<«.roosrs) 2072-kev level. Thus levels of 1263 and 1375 kev are 4 - 4 - proposed as members of an excited rotational band in Yb17*. The 373-, 527-, and 1093-kev gamma rays are all in 6% coincidence and the energies of the first two total 900 kev. Thus there is another cascade between the 2072- 3- 0 - 1862 and 1172-kev levels. The 373- and 527-kev gamma rays have nearly the same intensity, however their order

5+ 3+ 1373 has not been proved. The existence of a weak 324-kev 4 + 3+ 1263 transition in both the observed gamma-ray spectrum 3 + 3+ 1172 and the conversion electron spectrum of reference 4 6% favors the placement of the 373-kev transition higher because the energy difference between the implied 1699- kev level and the 1374-kev level is 324 kev. Thus a level 16% of 1699 kev is tentatively assigned to Yb17*. 6 + 0 + 330 The low intensity 1402- and 1583-kev gamma rays are both observed in the coincidence spectrum of the 79-kev gamma ray but only the lower energy one is ob­ served in coincidence with the 182-kev gamma ray. A level of 1662 kev is therefore implied from which transi­ tions occur to the first and second excited levels of the ground state rotational band. It should be noted that F ig . 4. Proposed energy level scheme for the decay of Luln. Energy level designations are in kev. these are the only observed transitions which cross over 44

RADIOACTIVE DECAY OF Lu1’* 1071 the 1172-kev level. This 1662-kev level is also favored relative amounts shown in Fig. 4 and because no transi­ by the existence of 410- and 399-kev transitions in the tions occur from this level to those of the ground-state conversion electron spectrum of reference 4. These two rotational band. These latter transitions are highly K transitions may occur between the 2072- and the 1662- forbidden. kev levels and between the 1662- and the 1263-kev The choices of 7= 3— and £ = 0 — are made for the levels, respectively. One other transition, 1002 kev, ob­ 1662-kev level because (1) transitions occur from this served in the conversion electron spectrum of reference level to the 2+ and 4 + levels of the ground-state ro­ 1 but not in the gamma-ray spectrum has been fitted tational band, (2) these transitions cross over the into the level scheme of Ybm. £ = 3 + rotational band implying a low value for £ , Only the 270-kev gamma ray remains to be placed (3) this level is not strongly populated by electron in the level scheme of Ybm. This gamma ray is in co­ capture as would be expected consistent with the spin incidence with the 182-kev gamma ray. The theoretical assignment for the ground state of Luin. If this choice ratios for the excited ground-state rotational energies is correct, then the level with I— 1— and £ = 0 — is not of even-even nuclei in this region of elliptically deformed expected to be populated. nuclei are 1.00:3.33:7.00 for the first, second, and third 7 iLu,oi'72 is an odd-odd nucleus in the region of ellip­ levels, respectively. The experimentally observed values tically deformed odd-odd nuclei. The collective model of these ratios are consistently smaller or about predicts a doublet of states for such a nucleus, one of 1.00:3.30:6.75. These later ratios predict levels of 260 which is the ground state. 7iLu17‘ and roYbioi171 have and 531 kev in Yb177 for the second and third ground- measured ground-state spins of 7/2+ and 1/2—, re­ state rotational levels. The 260-kev level has previously spectively. Therefore jiLuioi177 would be expected to been established. Because the 270-kev gamma ray is in have spins of 3— and 4—. The choice of 7= 4 — and coincidence with the 182-kev gamma ray and the place­ therefore £ = 4 — for the ground-state spin of Lu177 is ment of the 270-kev transition immediately preceding made because of the population by electron capture the 182-kev transition results in a level with the energy from Lu17* to the 4+ (0+), 6 + (0+), 5+ (3+), and predicted for the third rotational level, a 530-kev level 4 + (4+) levels of Yb177. The level scheme of Fig. 4 with a spin of 6+ is assigned to Yb177. accounts for all of the radiations observed in this in­ The spins of the excited levels of Yb177 other than the vestigation and includes transitions corresponding to ground-state rotational band will now be considered. all 8 of those reported from conversion electron meas­ The rotational band with a ground-state energy of 1172 urements in reference 2 and for 18 of those of reference kev is assigned the £ quantum number 3+ and hence 4. The coincidence data of this investigation agree in spins of 3+ , 4 + , and 5 + to the 1172*, 1263-, and 1375- some part with those of reference 8 but the few cases of kev levels for the following reasons: (1) transitions from disagreement have resulted in very different energy level the 1172-kev level occur to the 2+ and 4+ ground-state schemes for the decay of Lu174. However, all of the rotational levels only, (2) no transitions are observed gamma rays reported in reference 8 were observed in from the 1263- and 1375-kev levels to the members of this investigation. the ground-state rotational band but rather to the 1172- The bases for the spin and parity assignment made kev ground-state level of this band, (3) the observed for the levels of Yb177 populated by the electron capture ratio of the energies of these levels above the 1172-kev of Lu177 have been given. The multipole orders for the ground state is exactly the same as the theoretical ratio transitions between levels of Fig. 4 are shown in the of the energies of the levels of a rotational band with third column of Table I. All of those multipole orders £ = 3 + . The theoretical ratio is 2.25:1.00 from the which have been determined from conversion electron formula £i

1072 R. G. WILSON AND M. L. POOL of 2— and 3— are predicted for the ground state of AOClfOWLBDOMBirrS Tmm by the method described above. Tmin with either One of us (R.G.W.) is grateful to the National of the two predicted spins and at sufficiently high energy Science Foundation for the grant of a fellowship which might be expected to decay into the 2 + level of the enabled the completion of this research. Appreciation ground-state rotational band, to the 1172-kev 3+ level, is expressed to R. P. Sullivan of the Department of and to the 1— and 3— levels of the JC« 0— band which Physics and Astronomy for assistance in the electronic would produce the observed radiations mentioned in phases of this research and to the Office of Naval the Introduction. Research for support in obtaining enriched isotopes. 4 6

Reprinted from T he P hysical R eview , Vol. 117, No. 3, 807-810, February 1, 1960 Printed In U. S. A.

Radioactive Decay of Lutetium 173

R. G. Wilson and M. L. P ool Department of Physics and Astronomy, Ohio State University, Columbus, Ohio (Received August 24, 1959; revised manuscript received October 12. 1959)

Ytterbium oxide enriched to 92.6% in the 173 mass number mately 179 and 282 kev. Because no positron radiation exists in was irradiated with 6-Mev protons. An activity with a half-life the activity of Lu171, the mode of decay is solely by electron of 6254:50 days was produced and its assignment to Lu1™ con­ capture to Yb'7*. Gamma-gamma coincidence measurements firmed by the identification of the ytterbium K x-ray, gamma rays have lead to the assignment of a 633-kev level with a probable corresponding to transitions among the energy levels established spin of |+ and the confirmation of a 351-kev level in Yb'™ in in Yb17* by Coulomb excitation of the separated isotope, and the addition to the previously known 179.5- and 78.7-kev levels. activities produced by the similar proton irradiations of the other The disintegrations of Lu1™ to the 633-kev level are by L capture enriched isotopes of ytterbium. The observed activity of Lu17* only. Branching ratios for the electron capture transitions of Lu17* consists of the L and K x-rays of ytterbium and gamma rays with to the levels of Yb17> and approximate relative probabilities for energies of 79±1,101 ± 1 ,172±2, 273±2,349±4, —*50, 556±4, the transitions in Yb1™ are given in a proposed energy level and 630±4 kev and two other gamma rays not resolved in the scheme for the decay of Lu,7J. The choice of 9/2- for the ground- gamma-ray spectrum but observed in gamma-gamma coincidence state spin of Lu17* is the most consistent with the proposed energy measurements. These two gamma rays have energies of approxi­ level scheme.

INTRODUCTION total of six internal conversion transitions and seven N activity decaying by electron capture with a gamma-rays were reported in the activity of Lu’71 in A half-life of approximately 500 days has been these four references. Conflicting coincidence data, assigned to Lu17'.1 Energy levels of 78 and 181 kev were branching ratios, and energy level schemes for the observed in Yb17* by Coulomb excitation of the decay of Lu17* were presented. separated isotope.1 Spins of 7/2 and 9/2, respectively, In a recent paper, gamma rays of energies 22, 79, were assigned to these levels. Earlier Coulomb excitation 113, 145, 176, 274, 335, 440, 550, and 640 kev were of natural ytterbium had resulted in the assignment of associated with the activity of Lu173.10 The half-life of 78- and 180-kev levels to Yb17*.* The measured ground- this observed activity produced by the irradiation of state spin of Yb171 is 5/2.4 natural lutetium oxide with 24-Mev betatron brems- Recently a number of papers concerning the radio­ strahlung was quoted as 1.4 years. Some gamma- activity of Lu171 produced by various reactions have gamma coincidence data were tabulated for these appeared but significant discrepancies exist among the gamma rays. reported data. Conversion electron measurements on an Discrepancies on the following points are evident in approximately 450-day half-life activity produced by the available information concerning the radioactive the proton irradiation of natural ytterbium oxide decay of Lu173: (1) the value of the half-life, (2) the resulted in the assignment of 78.8-, 100.9-, 171.5-, and position of the 272-kev transition, (3) the existence of 272.7-kev transitions to the activity of Lu173.6 transitions of energy greater than 351 kev and a few of Conversion electron measurements and gamma-ray lower energy and the position of these in the energy studies have been made by four groups of workers on level scheme, (4) the amount of electron capture an activity in a lutetium fraction produced by the transitions to the levels of Yb173, especially to the ground irradiation of tantalum with 660-Mev protons.*- * The state. half-life reported by these workers for this activity The enriched isotopes of ytterbium have recently designated as Lu171 varies from 150 to 200 days. A become available. It is the purpose of the present investigation to resolve the discrepancies listed above 1 G. Wilkinson and H. G. Hicks, Phys. Rev. 81, 540 (1951). by discussing the results of an examination of the * Elbek, Nielsen, and Olesen, Phys. Rev. 1#8, 406 (1957). activity of Lu173 produced by the proton irradiation of * G. M. Temmer and N, P. Heydenburg, Phys. Rev. 100, 150 (1955). the enriched isotopes of ytterbium. 4 A. H. Cooke and J. G. Park, Proc. Phys. Soc. (London) A69, 282 (1956). EXPERIMENTAL RESULTS * Mihelich, Harmatz, and Handley, Phys. Rev. 108,989 (1957). •Bobrov, Gromov, Dzhelepov, and Preobrazhenskii, Izvest. Ytterbium oxide enriched to 92.6% in the 173 mass Akad. Nauk S.S.S.R. Ser. Fiz. 21,940 (1957) [ColumbiaTechnical number was irradiated with 6-Mev protons. The Translation (942)]. 7 Gorodinskii, Murin, Pokrovskii, and Preobrazhenskii, Izvest. composition of the remaining portion is as follows in Akad. Nauk S.S.S.R, Ser. Fiz. 21, 1004 (1957) [Columbia Tech­ percent: 0.05 Yb1*8, 0.05 Yb170, 0.44 Yb171, 2.33 Yb173, nical Translation (1005)]. * Dzhelepov, Preobrazhenskii, and Sergiyenko, Izvest. Akad. 4.31 Yb174, and 0.38 Yb17*. The atomic number of the Nauk S.S.S.R. Ser. Fiz. 22, 795 (1958) [Columbia Technical resulting activity was determined by the identification Translation (789)]. of the ytterbium K x-ray which was compared with * Gorodinskii, Murin, Pokrovskii, and Preobrazhenskii, Izvest. Akad. Nauk S.S.S.R. Ser. Fiz. 22,818 (1958) [Columbia Technical “ Dillman, Henry, Gove, and Becker, Phys. Rev. 113, 635 Translation (812)]. (1959). 807 47

808 R. G. WILSON AND M. L. POOL the known K x-rays of europium, terbium, thullium, following the decay of the K x-ray and the 79- and ytterbium, lutetium, and tantalum emitted from 172-kev gamma rays for 200 days. It is certain that the radioactive GdlM, Dy“», Yb1*, Tm170, HP71, and half-life of this observed activity is not in the 170- to W1M, respectively. Ion-exchange separation was deemed 200-day range and the original assignment of the unnecessary. 500-day activity to Lu17* is confirmed. In order to determine the mass number of the L and K x-rays were detected with a Geiger-tube activity, similar proton irradiations were performed on used with aluminum and beryllium absorbers. Figure 1 each of the other enriched stable isotopes of ytterbium shows the observed gamma-ray spectrum of Lu17* which and the resulting activities intercompared. When includes 79±1-, 101±1-, 172±2-, 273±2-, 349±4-, Yb174 was irradiated, the well established 165-day ~450-, 556±4-, and 630±4-kev gamma-rays in activity of Lu174 was produced. When Yb171 was addition to the previously mentioned ytterbium K irradiated, the above activity was not observed but x-ray. Two other gamma rays not resolved in the a longer half-life substance was found. This activity gamma-ray spectrum but observed in gamma-gamma secured from Yb17* was not observed in any of the coincidence measurements have energies of 179 and activities produced when the remaining enriched 282 kev. No evidence of positron activity was found in isotopes of ytterbium were irradiated with protons. Lu17* by the method of plastic scintillation spectrometry, The half-life of the activity resulting from the proton by the use of a Geiger-tube with aluminum and beryl­ irradiation of Yb'7* is 625±50 days as determined by lium absorbers, nor by a search for annihilation radia­ tion in the gamma-ray spectrum. Therefore, the mode of decay of Lu171 is solely by electron capture to Yb177. 52.4-KEV YTTERBIUM K X-RAY

T ablk 1. Gamma-gamma coincidence data for the activity of Lu171. Energies are expressed in kev.

273, 172, ITS 630 556 349 28 2 179 101 79 K x-ray > e K x-ray no VCS yes yes yes yes yes yes u (625 0) 79 no weak no yes weak yes no •¥ ■*» > 101 no no no no weak no N X 172,179 no no no no weak 273,282 no no > weak 349 no no no 556 no no 630 no

Table I displays the gamma-gamma coincidence information for the activity of Lu17* obtained by the use of a coincidence circuit of resolving time 2r=1.5 asec. In addition to the data shown in the table, the 273-kev gamma ray is in coincidence with two coin­ cident K x-rays; and the 172, 179-kev peak is in coincidence with a weak 450-kev gamma-ray. One of the two coincident K x-rays in coincidence with the 273-kev gamma-ray originates from K capture preceding the 273-kev transition and the other, from K conversion of the 78.8-kev transition. 78.7- and 179.5-kev levels in Yb177 have been established by Coulomb excitation of the separated isotope.7 A 351-kev level has been assigned to Yb177 by other workers from the observation of a 351-kev transition and from conversion electron F" coincidences between the 78.7- and 272.5-kev transi­ tions.7 This assignment is now confirmed by gamma- gamma coincidence measurements and the observation of a gamma ray of approximately 351 kev. Because the

3 0 0 6 0 0 observed 630-kev gamma ray is in coincidence with no ENERGY IN KEV other gamma ray, a 556-kev gamma ray is in coin­ cidence with the 78.7-kev gamma ray, a 450-kev Fig . 1. Gamma-ray spectrum of the 625-day activity of Lu171 measured with a 3 in.X3 in. Nal scintillation crystal employing gamma ray is in coincidence with the 172, 179-kev a geometry which minimized summation affects. gamma peak, and a 282-kev gamma ray is in coincidence 48

RADIOACTIVE DECAY OF Lu‘»» 809

ITS with the 273-kev gamma ray, a 633-kev level is assigned 16260) to Yb171. The 630-kev gamma ray and the K x-ray are not in coincidence. On the basis of the intensity of the coincidences between the 556-kev gamma ray and the 6 3 3 Kiv K x-ray, it is concluded that if five percent or more of the electron capture transitions of Luin to the 633-kev level of Yb17* were by K capture, then coincidences between the 633-kev gamma-ray and the K x-ray could have been detected. The K x-ray in coincidence with the 556-kev gamma-ray is the x-ray resulting from K conversion of the coincident 78.7-kev transition. 351.1k iv The spins of the 351.1-, 179.5-, and 78.7-kev levels of Yb17* have been designated as 7/2+,7-7 9/2—, and 7/2—, respectively, and the measured ground-state spin of Yb17* is 5/2—. An examination of the relative 179.3k iv probabilities of the transitions leaving the 351-kev level of Yb17* shows that the most probable transition 76.7 k iv

T able II. Relative number of gamma rays, N y, corresponding transitions, Ntnm, and K converted transitions, Nt, in the activity of Luln for energies, £ ,, expressed in kev. R,, are internal conver­ sion transition energies with references. N tnm/Ny and lf tnam/Nt, are derived from Rose’s tables * F ig . 2. Proposed energy level scheme for the decay of Luin. The percentage associated with each transition is the percentage .Vuani/ .Vnu/ of total disintegrations proceeding through that transition. £«.k R eference Sy Ny Vtfmn* .V* St K x-ray 100 100 79±1 78.7, M, 8,6,5 5.0 11.4 57.0 1.9 30 five which were derived from Rose’s tables.11 Conversion 101 ± 1 100 8, .1/, 8,6,5 3.6 4.53 16.3 1.5 11 electron energies and multipole orders corresponding 172±2 171.6, Ei 8,6,5 3.1 1.08 3 3 15 0.2 to the observed gamma rays are listed in the second ~ 179 170.5, 8,6 1.0 1.40 1.4 6.4 0.2 273±2 272.4, Ei 8,6,5 17.1 1.02 17.5 51 0.3 column of Table II and the appropriate references in — 282 0.30 0.3 column three. Column eight shows the approximate 349±4 351.1, Ei 8 0.47 1.02 0.5 60 0 ~ 4 5 0 0.09 0.1 relative number of K x-rays resulting from internal 556± 4 0.60 0.6 conversion of the transitions in Yb171. The combined 6 3 0 ± 4 1.1 ... 1.1 . * . * * • 172, 179-kev peak was divided by using the ratio of 172- to 179-kev transitions from reference 8. A rough * Sec reference X X. b Note: Internal conversion transition energies corresponding to the four division of the combined 273, 282-kev peak was obtained unreferenced gamma rays have recently been reported [see Harmatx, from analysis of gamma-gamma coincidence data. Handley, and Mihelich, Phys. Rev. 114, 1082 (1959)]. Internal conversion of the transitions from the 633-kev level may be considered negligible for the involves no spin change but does involve a parity following calculations. The approximate percentages change, and the second most probable transition of electron capture to the five energy level of Yb171 involves a spin change of one and a parity change. were obtained by accounting for the K x-rays observed The choice of 5/2+ for the spin of the 633-kev level in the activity of Lu17*. L capture to the other four of Yb171 leads to exactly the same set of conditions for levels was considered negligible with respect to K the first and second most probable transitions from this capture. One K x-ray was subtracted for every K level. Although this argument for a spin assignment to capture required to balance the difference between the 633-kev level is of questionable validity, it is the the number of transitions from an excited level and the number of transitions to the same level. These best which can be given with the available information. differences were used as the relative number of electron The choice of 9/2— for the ground-state spin of Lu17* capture transitions from Lu17* to the levels of Yb17*. is the most consistent with the proposed energy level K x-rays were then subtracted to account for K scheme. conversion of the transitions in Yb17*. The number of The fourth column of Table II gives the relative L capture transitions to the 633-kev level of Yb17* number of counts under the spectral distribution after were determined from the number of transitions leaving correction for crystal efficiency for the observed radia­ this level. All the relative numbers for electron capture tions listed in the first column. The sixth column and transitions in Yb17* were then adjusted to yield a gives the relative number of the corresponding transi­ 11 M. E Rose, Internal Conversion Coefficients (Nortb-Holland tions calculated by using the ratios listed in column Publishing Company, Amsterdam, 1958). 49

810 R. G. WILSON AND M. L. POOL total of 100% for the electron capture transitions to the to occur but these two peaks disappear rapidly when five levels of Yb17*. The percentages of electron capture the K x-ray is absorbed. The conclusion is that the to the 633-, 351.1-, 179.5-, and 78.7-kev levels and the 113-kev peak is the coincidence sum peak of two ground state of Yb17* are thus approximately 4, 35, 24, K x-rays, and the 335-kev peak is that of a 273-kev 37, and 0, respectively. gamma ray and a K x-ray. The results of this investiga­ tion show that few if any electron capture transitions DISCUSSION of Lu17* occur to the ground state of Yb17*. The discrepancies enumerated in the introduction The gamma-gamma coincidence information obtained are now considered. The 625-day half-life of Lu171 is in this investigation agrees with that mentioned in the closest to the originally assigned value of 500 days. introduction10 for the cases where the gamma rays of The 273-kev transition is not to the ground state of the two investigations correspond. In particular, the Yb17* but originates from 351-kev level. Transitions of 172- and 273-kev peaks were each in coincidence with energy greater than 351-kev do exist in the activity of themselves and the 556-kev gamma ray was in coin­ Lu17> and their positions in the energy level scheme cidence with the 79-kev gamma ray only. are shown in Fig. 2. The 22-, 113-, 145-, and 335-kev gamma rays mentioned in the introduction are not ACKNOWLEDGMENTS observed by in the gamma-ray spectrum of Lu17* nor One of us (R.G.W.) is grateful to the National have any of them been observed by conversion electron Science Foundation for the grant of a fellowship which measurements. A 22-kev gamma ray was shown not to exist in an amount greater than two percent of the enabled the completion of this research. Appreciation is K x-ray by absorption measurements with a scintillation expressed to R. P. Sullivan of the Department of spectrometer. Peaks at about 113 and 335 kev are Physics and Astronomy for assistance in the electronic observed in the gamma-ray spectrum of Lu177 when a phases of this research and to the Office of Naval Re­ geometry is used which allows coincidence summation search for support in obtaining the enriched isotopes. Reprinted from The Physical Review, Y7ol. 117, No. 2, 517-519. January 15, I960 Printed in U. S. A.

Radioactive Decay of Lutetium-174

R. G. W ilson and M. L, P ool Department o] Physics and Astronomy, Ohio State University, Columbns, Ohio (Received July 20, 1959; revised manuscript received September 17, 1959)

Ytterbium oxide enriched to 98.4% in the 174 mass number was irradiated with 6-Mev protons. An activity of approximately 165-day half-life was produced and assigned to Lu174 by the identification of the ytterbium K x-ray and of the activities produced by similar proton irradiations of the other enriched isotopes of ytterbium. The observed activity of Lu174 consists of the L and K x-rays of ytterbium and 76.6- and 1228-kev gamma rays which are in coincidence. Because no beta radiation exists in the activity of Lu174, the mode of decay is solely by electron capture to Yb174. Approximately 31% of the disintegrations of Lu174 are to the ground state of Yb174. In addition to the 76.6-kcv level of YV74, there is a 1305-kev level with a spin of 0 + . The transitions of Lu174 to the 1305-kcv level of Yb174 are by L capture only and the percentages of electron capture to the 76.6- and 1305-kev levels of Yb174 arc approximately 59 and 10. respectively. A spin of 1- is assigned to the ground state of Lu174.

INTRODUCTION activity, similar proton irradiations were performed on N activity of half-life 165±5 days has been each of the other enriched stable isotopes of ytterbium A assigned to lutetium-174 and from absorption and the resulting activities intercompared. Because the measurements it was concluded that the mode of decay 165-day activity was not found in any of the other of this activity is approximately 80% by electron cap­ resulting activities, its assignment to the 174 mass ture and approximately 20% by 0.6-Mev (3~ emission.1 number is confirmed. Also observed with the 165-day From conversion electron measurements following the activity was a very small amount of a longer half-life proton irradiation of natural ytterbium oxide, a 76.6- activity corresponding to the only long half-life activity kev, E t (2-|— >0+), transition was assigned to Lu174.* resulting from the similar proton irradiation of ytter­ In a long half-life activity found in samples of natural bium oxide enriched in the 173 mass number. This lutetium oxide irradiated with 16- and 24-Mev betatron weaker activity is therefore attributed to the small bremsstrahlung, activity of maximum end-point amount of Ybm existing in the enriched Yb174. energy 1.2 Mev was found and attributed to Lu174 and L and K x-rays were detected with a Geiger tube to the naturally radioactive Lu174 existing in the used with aluminum and beryllium absorbers. Figure 1 samples.1 The gamma energies assigned to Lu174 were shows the observed gamma-ray spectrum of Lu174 77, 84, 113, 176, 230, 275, 990, and 1245 kev. The 77- which includes 76.6- and 1228±3 kev gamma rays in and 1245-kev gamma rays were in coincidence. Gamma addition to the previously mentioned ytterbium K energies of 84, 203, and 306 kev were in coincidence with beta energies greater than 250 kev. The 203- and 52.4-KE V YB K X-RAY 306-kev gamma rays were attributed to the natural Lu17' in the samples and the 84-kev transition was assigned to Hf174 as its first excited level populated by the0~ decay of Lu174. 76.6 KEV

EXPERIMENTAL RESULTS Ytterbium oxide enriched to 98.4% in the 174 mass number was irradiated with 6-Mev protons. The com­ 1228 KEV position of the remaining portion is as follows in percent: u 10' 0.01 Yb188, 0.03 Yb1TO, 0.13 Yb171,0.33 Yb17*, 0.78 Yb178, and 0.36 Yb17*. The atomic number of the activity was determined by the identification of the ytterbium K 10* ■ x-ray which was compared with the known K x-rays of europium, terbium, thullium, ytterbium, lutetium, and tantalum emitted from radioactive Gd1M, Dylw, Yb1*, Tm17#, Hf17*, and W181, respectively. Ion-ex­ 400 800 1200 change separation was deemed unnecessary. ENERGY IN KEV In order to determine the mass number of the Fio. 1. The gamma-ray spectrum of the activity produced by proton irradiation of ytterbium oxide enriched to 98.4% in the 1 G. Wilkinson and H. G. Hicks, Phys. Rev. 81. 540 (1951). 174 mass number and containing 0.78% of the 173 mass number * Mihelich, Harmatz, and Handley, Phys. Rev. 108,989 (1957). is shown by the solid line. The dashed line shows the spectrum **r' Dillman, " - Henry, " Gove, ~ and Becker, Phys. Rev. 113, 635 ^ after subtraction of the activity produced from the 173 mass (1959). number. 517 51

518 K. G. WILSON AND M. L. POOL

x-ray. Fortunately, the latter gamma-ray energy lies state. It follows that the percentages of electron capture between the two accurately known gamma-ray energies are approximately 31 to the ground state, 59 to the of Co", 1332.5±0.3 and 1172.8+0.5 kev.4 No evidence 76.6-kev level, and 10 to the 1305-kev level. The ratio of beta activity was found in Lu174 by the method of of the number of L to K x-rays in the activity of Lu174 plastic scintillation spectrometry nor by the use of is approximately 1.4. Geiger tube with aluminum and beryllium absorbers. No annihilation radiation exists in the gamma-ray DISCUSSION spectrum. The 76.6- and 1228-kev gamma rays were Because the 1245-kev gamma ray mentioned in the shown to be in coincidence by the use of a gamma- Introduction and the 1228-kev gamma ray observed in gamma coincidence circuit of resolving time 2 t = 1.5 this investigation were each in coincidence with the ftsec. Therefore a 1305-kev level exists in Yb174. After 76.6-kev gamma ray, it seems probable that they are correction for crystal counting efficiency, the ratios of the same gamma ray. As can be seen in Fig. 1, the 84-, the number of radiations emitted by Lu174 are A x-ray: 113-, 176-, 230-, 275-, and 990-kev gamma rays men­ 76.6-kev y: 1228-kev y= 100:6.2:10.7. tioned in the Introduction do not appear in the activity The conversion electron data mentioned in the intro­ of Lu174. The 113-, 176-, and 275-kev gamma rays are duction give the relative number of A', L\, Lt, Lit and M best attributed to the activity of Lu17*. As also pre­ conversion electrons resulting from the 76.6-kev transi­ viously mentioned, the 84-kev gamma ray was found tion. The K, Lt, Lt, Lt, and M internal conversion along with 203- and 306-kev gamma rays in coincidence coefficients, 1.52,0.17,2.80,2.87, and 2.71, respectively, with energies greater than 250 kev. The activity for a 76.6-kev Et transition in ytterbium were obtained and the 203- and 306-kev gamma rays were attributed from the internal conversion coefficient data calculated to the naturally radioactive Lu17* which has 89-, 203-, by Rose.* The above two sources of information yield a and 306-kev gamma rays in coincidence with a 1.2-Mev value of 11.2 for the ratio of the number of 76.6-kev activity.* Since there is no 0“ activity in Lu174, it transitions to the number of 76.6-kev gamma rays. seems probable that the 84-kev gamma ray also Therefore the ratio of the number of conversion elec­ orginates in the Lu17* activity. trons to the number of gamma rays resulting from the Figure 2 is a proposed energy level scheme for the 76.6-kev transition is 10.2. Also obtained is the value decay of Lu174. This scheme is consistent with all of of 7.37 for the ratio of the total number of 76.6-kev the experimental data. It accounts for all of the radi­ transition to the number of A-converted transitions. ations observed in the activity of Lu174 and does not The ratio of the number of counts under the spectral imply any radiations which are not observed. The distribution of the K x-ray in coincidence with the 76.6-kev 2+ level has been designated as the first 1228-kev gamma ray to the number of 76.6-kev gamma- rotational level of the even-even »oYb174 nucleus. Be­ ray counts also in coincidence with the 1228-kev gamma cause no decay occurs to higher rotational levels and ray was 1.5. The theoretical A-conversion coefficient there is no 1305-kev crossover transition, the assignment for the 76.6-kev transition is 1.5. There is some error of 0 + spin for the 1305-kev level of Yb174 is strongly associated with both the experimentally measured and favored. riLuioa174 is an odd-odd nucleus in the region of the theoretically calculated ratios but within these elliptically deformed odd-odd nuclei. Shell theory pre­ errors the two ratios are the same. This implies that dicts a doublet state for such a nucleus, one of which is no A x-rays precede the 1228-kev transitions and hence the ground state while the other is usually the first that the electron capture transitions of Lu174 to the excited state. nLu17* has a measured spin of 7/2+ and 1305-kev level of Yb174 are by L capture only. The Ybioa173 has one of 5/2—. Therefore 7iLuio»174 would be above factor 11.2 for the 76.6-kev transition was used expected to have spins of 1— and 6—. It is obvious to obtain the ratio of the number of A x-rays: 76.6-kev transitions: 1228-kev transitions= 100:69.5:10.7. Inter­ t yL uIT4(IM0) nal conversion of the 1228-kev transition may be con­ sidered negligible. If the probability of L capture to the i / r 76.6-kev level of Yb174 is considered small relative to A 10V ! / EC 0+ 1309 Kev capture, 69.5—10.7 = 58.8 A x-rays result from electron I 1 capture transitions to this level. The 69.5 76.6-kev / / I Fio. 2. Proposed energy-level transitions yield 69.5/7.37=9.5 A x-rays from internal i scheme for the decay of Lu174. conversion. If the probability of L capture to the ground state of Yb174 is also considered small relative to A / / 3I* capture, there remain 100— 58.8— 9.5 = 31.7 A x-rays / I* which are attributable to electron capture to the ground e + -T j* 1 76.6 KEV 0 + —1— *- 0 4 Lindstrbm, Hedgran, and Alburger, Phys. Rev. 89 , 1303 (1953). * M. E. Rose, Internal Conversion Coefficients (North-Holland • J. R. Arnold, reported in Strominger, Hollander, and Seaborg, Publishing Company, Amsterdam, 1958). Revs. Modern Phys. 30, 585 (1958). 5 2

RADIOACTIVE DECAY OF Lu” ' 519 that a spin of 1— should be assigned to the ground ACKNOWLEDGMENTS state of Lu174. One of us (R.G.W.) is grateful to the National An isomeric state in Lu174 of 75-/isec lifetime and Science Foundation for the grant of a fellowship which 133 kev above the ground state has been observed.7 enabled the completion of this research. Appreciation is It seems possible that this is the other member of the expressed to R. P. Sullivan of the Department of predicted doublet. Physics and Astronomy for assistance in the electronic ’ C. L. Hammer and M. G. Stewart, Phys. Rev. 106, 1001 phases of this research and to the Office of Naval Re­ (1957). search for support in obtaining the enriched isotopes. BIBLIOGRAPHY

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29. Lindstrom, Hedgran, and Alburger, Phys. Rev. 89, 1303 (1953).

30. S. Chojnacki, Yu. Norseev, Z. Preibisz, J. Wolowski, and J.

Zylicz, Polska Akademia Nauk Instytut Badan Jadrowych No.

177/l-A (I960), and submitted for publication in Acta Physica

P o lo n ic a .

31. J.C. Walker and D. L. Harris, Phys. Rev. 121, 224, (1961). AUTOBIOGRAPHY

I, Robert G. Wilson, was born in Wooster, Ohio, on April

7, 1934, and was graduated from Wooster High School in 1952. I began my undergraduate training with a scholarship in the Engineering

Physics Program at Cornell University but received the Bachelor of Science Degree in Physics from the Ohio State University in 1956.

My entire graduate experience has been in the Department of Physics and Astronomy of the Ohio State University. During the first two years I was employed by Battelle Memorial Institute of Columbus, Ohio, and attended the University on the Institute's educational program.

The third year was spent as a research assistant to Professor M. L.

Pool and the fourth and fifth years, as a departmental teaching assistant. I was granted a National Science Foundation Fellowship and an Ohio State Graduate School Fellowship for the summers of

1959 and I960 respectively, during which times I engaged in research toward a dissertation suitable for the degree Doctor of Philosophy.

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