A study of the level structure of 148Sm excited in the decay of 148Eu

Nuclear Physics A96 (1967) 97--114; ~ ) North-Holland Publishing Co., Amsterdam N o t to be reproduced by photoprint or microfilm without written permission from the publisher

A S T U D Y O F T H E L E V E L S T R U C T U R E OF 14SSm E X C I T E D I N T H E DECAY OF 148Eu J. E. CLINE National Reactor Testing Station, Idaho Fails, Idaho, Idaho Nuclear Corporation t

Received 21 December 1966 Abstract: The decay of 148Eu(54 d) to levels in 148Smhas been studied using the techniques of Si(Au), Si(Li), Ge(Li) and NaI(TI) spectrometry. Sources were made by roSin(p, 2n)a~SEu reactions and were chemically purified using ion-exchange techniques. Over 120 transitions have been identified as being associated with the decay of this nucleide and approximately 90 have been placed in a decay scheme containing 30 energy levels. Multipolarity assignments to many of the stronger transitions have been made on the basis of conversion coefficients measurements. RADIOACTIVITY 14SEu [from l~sSm(p,2n)]; measured Ev, Iv, lee, Y'7 coin; deduced cc, log ft. 14sSmdeduced levels, J, ~z. Enriched target, Ge(Li) detector.

[

]

1. Introduction Nucleides in the transition region between spherical and deformed equilibrium shapes often present interesting, although complicated and poorly understood, energylevel structures. With the advent of Ge(Li) spectrometers, it should now be possible to obtain considerably more information concerning these nuclei. The 14SSm nucleus within this region has been shown previously through studies 1,2) of the decay of 14Spm, 14Smpm and 14SEu to have a highly complex level scheme. This paper reports on the investigation of the decay of 54 d 14aEu, which decays by electron capture to 14aSm with a decay energy in excess of 3 MeV. Studies have been made using NaI(T1) and Ge(Li) gamma-ray spectrometers and Si(Li), Si(Au) and 180 ° permanent-magnet electron spectrometers. Coincidence measurements have been carried out using the gamma-ray spectrometers and a 256 x 256 channel multiparameter analyser. Nearly 100 of the 121 transitions identified in this work have been placed in a consistent decay scheme on the basis of the coincidence measurements and "energy-fitting" techniques. Multipolarity assignments have been made to m a n y of the g a m m a rays on the basis of the conversion-coefficient measurements. Spins and parities have been made to m a n y of the levels on the basis of these and previous measurements. A brief discussion of the possible interpretation is given. t Work performed under the auspices of the U.S. Atomic Energy Commission. 97

98

J . E . CLINE

2. Experimental measurements 2.1. SOURCE PREPARATION Sources of 148Eu were produced by a (p, 2n) reaction on samples of Sm203, enriched to 99 + ~o in 149Sm. Irradiations were performed using the 22 MeV proton beam of the ORNL production cyclotron. Samples were purified by rare-earth chemistry followed by an ion-exchange column separation with alpha-hydroxyisobutyric acid as the eluting agent. A slight contaminant of 24 d 147Eu w a s observed in early runs on the samples. After one year, the presence of 120 d 149Eu became evident. No other Eu activity or any other rare-earth activity became apparent even after 2 y of use of the sample. Samples for gamma-ray counting were made by evaporating the material onto "scotch-tape" backing. Sources for conversion-electron studies were made by elcctroplating from solution onto 0.15 mm diam. platinum wire. 2.2. GAMMA-RAYAND CONVERSION-ELECTRON MEASUREMENTS In figs. 1 and 2 are shown gamma-ray spectra of 148Eu taken with a 7.62 cm x 7.62 cm NaI(T1) spectrometer and with a 2.5 cm2x 8 mm Ge(Li) spectrometer, respectively. The Ge(Li) spectrometer incorporated a preamplifier which was internal to the cryostat. This system, previously reported 3.4), yielded resolutions (FWHM) of 0.75 keV at 100 keV and 1.7 keV at 1 MeV. With these resolution characteristics, 121 transitions were identified as being associated with the decay of 14SEu. Energy calibration of the 4096-channel spectrometer was accomplished through internal calibration using sources of 153Gd, 6SZn, 137Cs, say and the Sm X-rays. The values used for the energies of the transitions in these sources are, respectively, 97.43 and 103.18 keV for 153Gd, 1115.5 for 65Zn, 661.60 for 137Cs, 898.01 and 1836.08 for s a y and 40.12 for K~ in Sm. To determine the energies of the gamma rays, the channel positions of the peaks in these data were determined by a method of non-linear least-squares and were corrected for non-linear effects on the 4096-channel analyser. These techniques have previously yielded precisions in energy determinations corresponding to less than 0.1 channel. The photopeak detection efficiency, as a function of gamma-ray energy, has been determined for the Ge(Li) spectrometer by using sources calibrated with the NaI(T1) spectrometer in use at this laboratory for several years. From this efficiency curve and an integration under each of the peaks shown in fig. 2, relative intensities were obtained for all of the transition observed. Absolute intensities were obtained by assuming that all decays proceeded through the levels at 550 keV, an assumption which appeared justified by the coincidence measurements. Conversion-electron spectra were taken using a 1 mm thick Si(Au), a 2 mm thick Si(Li) and 180° permanent-magnet electron spectrometers. A spectrum of the conversion electrons taken with the Si(Au) spectrometer is shown in fig. 3. The resolution (FWHM) of this system was 1.5 keV at 100 keV and 2.3 keV at 1 MeV. An efficiency curve for this spectrometer was measured by using calibrated sources of 1°9Cd, 139Ce ' 54Mn' 113min and 2°7Bi. Relative intensities for the conversion electrons were

99

DECAY OF 148Eu

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Fig. 2. G a m m a - r a y spectrum o f Z4aEu taken with a 2.5 c m 2 × 8 m m Ge(Li) spectrometer. The counts in the first 2100 channels have been increased by a factor o f 10 so that the last 2000 channels could be s h o w n o n the s a m e plot.

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Fig. 3. Conversion-electron spectrum of X~SEu taken with a 1 cm z x 1 m m Si(Au) spectrometer. T h e energies and the assignments of the peaks are included in this plot.

20•0

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54CONVERSION-ELECTRON SPECTRUM

2000

102

J. E. CLINE TABLE 1 Energies and absolute intensities for the gamma rays observed in the decay of X*SEu

(keV)

22.41±0.10 67.634-0.10 98.404-0.10 116.3 4-0.2 121.184-0.12 166.2 4-0.3 182.8 4-0.2 189.6 ±0.2 215.9 4-0.2 241.5 4-0.1 243.6 4-0.2 252.3 4-0.3 279.1 4-0.5 287.9 4-0.2 296.0 4-0.3 310.0 4-0.3 311.4 4-0.1 319.1 4-0.3 377.5 4-0.5 413"91 413.91 4-0.1 432.6 4-0.1 437.2 4-0.3 468.4 4-0.3 481.2 4-0.5 495.2 4-0.3 501.3 4-0.1 505.2 4-0.5 516.7 4-0.3 528.5 4-0.3 550.204-0.5 553.2 4-0.1 571.904-0.05 587.6 4-0.5 590.1 4-0.2 595.2 4-0.3 599.5 4-0.2 602.6 4-0.3 611,264-0.05 620.0 4-0.1 629.904-0.05

( ~o offl-decays)

0.09 4-0.03 0.10 4-0.03 0.0064-0.003 0.05 -t-0.02 0.11 4-0.3 0.10 -t-0.3 1.04 0.23 0.05 0.06 0.22

4-0.10 4-0.05 4-0.02 4-0.02 4-0.05

0.22 -t-0.05 1.14 4-0.10 0.05 4-0.02 / ~18"6 4-0.5 2.79 4-0.20 0.49 0.16 0.25 0.75 0.28 0.40 0.19 99.00 17.1 9.1 0.08 0.32 0.20 0.51 0.23 19.3 1.0 70.9

4-0.08 4-0.05 4-0.06 4-0.13 4-0.07 -4-0,10 4-0.05 4-0 4-0.5 4-0,3 4-0.03 4-0,08 4-0,06 4-0,10 4-0,16 4-0,5 4-0.1 4-3.0

654.34-0.1 657.04-0.5 667.74-0.5 669.94-0.2 675.44-0.5 683.24-0.1 690.74-0.5 714.84-0.1 719.64-0.3 725.74-0.1 734.84-1.0 756.64-0.5 770.44-0.2 780.24-0.5 799.24-0.2 870.0-4-0.1 895.84-0.1 903.94-0.2 906.94-0.3 915.34-0.1 924.9-t-0.2 930.54-0.1 938.2-4-1.0 949.74-0.2 964.24-0.4 967.34-0.1 980.04-0.5 989.74-0.2 1013.94-0.2 1034.14-0.1 1043.94-1.0 1047.54-0.3 1066.84-0.2 1069.24-0.4 1082.24-0.5 1089.34-0.5 1097.54-0.7 1104.34-0.2 1107.94-0.3 1113.84-0.3 1126.94-0.5

1.314-0.15 0.174-0.06 0.134-0.05 0.344-0.08 0.084-0.03 1.064-0.10 0.084-0.03 1.544-0.12 0.114-0.03 12.2 4-1.0 0.064-0.03 0.194-0.05 0.385:0.07 0.084-0.03 0.344-0.07 4.9 4-0.5 0.554-0.10 0.344-0.07 0.154-0.04 2.4 4-0.2 0.274-0.06 2.4 4-0.2 0.114-0.04 0.21+0.06 0.204-0.07 2.9 4-0.3 0.134-0.04 0.394-0.08 0.414-0.08 7.9 4-0.7 0.044-0.02 0.204-0.07 0.294-0.08 0.224-0.08 0.174-0.07 0.204-0.07 0.104-0.04 0.455:0.09 0.104-0.03 0.134-0.04 0.104-0.03

1146.94-0.1 1151.74-0.5 1156.44-0.5 1165.34-0.5 1180.54-0.5 1183.34-0.1 1194.14-0.5 1207.44-0.2 1219.74-0.5 1222.04-0.5 1230.04-0.5 1236.64-0.2 1267.14-0.5 1276.54-0.5 1309.84-0.2 1328.54-0.2 1344.64-0.1 1353.64-0.2 1362.64-0.2 1409.04-0.6 1454.34-0.3 1460.54-0.2 1493.24-0.6 1503.04-0.3 1512.04-0.5 1521.94-0.3 1536.14-0.3 1543.1 4-0.2 1547.34-0.4 1551.74-0.5 1560.74-0.2 1565.0±0.6 1621.54-0.1 1635.34-0.3 1650.44-0.1 1664.54-0.5 1677.84-0.1 1748.14-0.5 1776.94-0.3 1940.04-0.5 1974.24-0.5 2173.24-0.3

1.9 4-0.2 0.084-0.03 0.04-t-0.02 0.084-0.03 0.304-0.08 1.7 4-0.2 0.134-0.04 0.594-0.12 0.084-0.03 0.09-4-0.03 0.034-0.02 0.394-0.09 0.094-0.03 0.074-0.03 0.464-0.09 1.224-0.15 3.3 4-4-0.3 0.474-0.09 0.554-0.10 0.104-0.03 0.25±0.07 1.124-0.12 0.074-0.03 0.134-0.04 0.044-0.02 0.114-0.03 0.084-0.03 0.714-0.13 0.084-0.03 0.024-0.01 0.874-0.10 0.034-0.02 4.6 4-0.5 0.184-0.4 3.7 4-0.5 0.084-0.03 0.424-0.08 0.024-0.01 0.044-0.02 0.054-0.03 0.034-0.02 0.224-0.06

The estimated uncertainties in the values have been included. Although these estimates are subjective, it is felt that they represent a realistic appraisal of the uncertainties involved in the techniques which were used.

103

DECAY OF 14aEu

TABLE 2 Conversion-electron energies and intensities E~ 98.4 116.3 166.2 189.6 241.5 287.9 311.4 413.9

432.6 468.4 550.2 553.2

571.9 611.3 629.9

654.3 683.2 714.8 725.7

870.0 895.9 915.3 930.5 967.3 1034.1 1146.9 1183.3 1344.6

Ee

Assignments

51.5 91.1 114.8 119.3 142.7 194.6 234.2 241.0 264.5 304.1 367.0 406.6 412.4 385.7 425.3 421.5 503.3 506.3 543.0 546.0 549 552 525.0 564.4 604 583.0 622.5 628.4 607.4

K L M K K K L K K L K L M K L K K

1 . 9 × 1 0 -2 8 ×10 a 2.5 × 10 .2 1 . 2 × 1 0 -1 1 . 2 × 1 0 -2 1 . 9 × 1 0 .2 1 . 9 × 1 0 .2 8.6 × 10 .3 2.4 × 10 -x 3.2 X 10 -3 9.5 x I 0 -a 2.4 × 10 -2 4.5 × 10 -3 2.6 × 10 -2 9.9 × 10 -1

L

636.3 667.9 678.8 718.4 725.7 823.1 862.7 848.9 868.4 883.6 920.4 987.2 1026.8 1100.0 1136.4 1297.7

le(Si)

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Multipolarity assignment

/ 2.8 × 10-x~

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K K L K L M K L K K K L M K L K K K K K L K K K

1.9 × 10 -8 4 . 5 x 1 0 -2 5 . 4 x 1 0 -a 4.0 x 10 -1 5.4 x 10 -2 1 . 6 × 1 0 -2 1.1 × 10 -8

2.6 × 10 -2 5 X l 0 -2 5 × 1 0 -a 4.2 x 10 -1 6.0 X 10 -2 1 . 9 × 1 0 -~ 2 × 10 -8 6 × 1 0 -a 1.2 × 10 -2 1.6 X 10 -8 6.6 X 10 -8 1 x 10 -8

2.4 × 10 -a 2 . 3 × 1 0 -a 2 . 8 x 1 0 -4 5.7 × 10 -a 8.0 x 10 -4 2 . 5 × 1 0 -4 1 x 10 -1 3 × 1 0 -a 8.6 x 10 -a 7.5 × 10 -a 4.7 X 10 -a 5 × 10 -4

M 1 ( 9 . 0 × 10 -a) ( 3 . 0 × 1 0 -a) M 1 (8.2 × 10 -a) M 1 (7.4 X 10 -a) E 2 ( 4 . 2 × 10 -a) (6.8 x 10 -4)

1.8 × 10 -2

2.9 × 10 -a 2.0 X 10 -4 2.1 × 10 -a 2.1 X 10 -a 1.9 x 10 -a 2.0 x 10 -2 1.9 × 10 -a 1.6 x 10 -4 1 . 6 × 1 0 -a 1.5 × 10 -a 9.4 × 10 -4

E 2 ( 2 . 8 × 10 -a) (4.3 X 10 -4) E 2 ( 2 . 6 × 10 -a) E 2 ( 2 . 4 X 10 -a) E 2 ( 2 . 3 X 10 -2) E 2 ( 2 . 2 X 10 - s ) E 2 ( 1 . 9 x 10 -2) (2.7 X 10 -4) E 2 ( 1 . 5 8 X 1 0 -a) E 2 ( 1 . 4 0 X 10 -a) E 2 ( 1 . 1 0 X 10 -a)

9.0 × 10 -a 1.1 X 10 -8 4.9 × 10 -2 3.7 x 10 -a 9.2 × 10 -4 1.4 × 10 -8 1.0 X 10 -3 1.2 × 10 -a 5.1 x 10 -2 4.4 X 10 -2 5.7 x 10 -2 1.5 x 10 -a 1.3 x 10 -a 2 . 9 X I 0 -a 2.6 × 10 -2 3.1 × 10 -a

1.3 × 10 -1 2 . 3 × 1 0 -3 2 × 1 0 -3 3 × 1 0 -2 4.5 × l 0 -2 2.1×10 1 3 ×10 2 4 ×10 2 2 ×10 2

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E l (2.6 x 10 -a) E 1 ( 2 . 3 × 1 0 -3) ( 3 . 1 × 1 0 -4) E 2 ( 5 . 6 x 10 -a) a) (9.8 X 10 -4)

E1 E2

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E2

E2 E2 E2 E2 E2 E2 E1

a) U s e d f o r e l e c t r o n i n t e n s i t y n o r m a l i z a t i o n . I n c l u d e d i n t h i s t a b l e a r e t h e a s s i g n m e n t s o f t h e e l e c t r o n s , m e a s u r e d c o n v e r s i o n coefficients, a n d multipolarity assignments. Along with the multipolarity assignments are given the theoretical conv e r s i o n coefficients c o r r e s p o n d i n g t o t h e s e a s s i g n m e n t s .

~4

)6

Fig. 4. Gamma-ray spectrum of14smpm, 1*sPm taken with a 2.5 cm~x 8 mm Ge(Li) spectrometer

D E C A Y O F l a SEu

105

obtained using this curve. Absolute intensities were obtained by assumming that the 550 keV and the 630 keV are pure E2 transitions, an assumption justified by previous authors 1,2). Internal-conversion electrons were also observed photographically with 180 ° magnetic spectrographs. The data were used to identify the low-energy transitions that were present as well as to determine energies and some electron intensities. The

c o (..) UJ

Z

Z 0

0

I

I

20

40

I

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60 80 Chonnel Number

100

120

140

Fig. 5. Spectrum of gamma rays coincident with the 414 keV transition. The recorded spectrum was taken with a 7.62 cm × 7.62 cm N a I f I ' l ) detector. The gating detector was Ge(Li).

intensities were determined using a method reported by Slatis 5), where it is assumed that log I o / I = C log ( E + 1), where E is the exposure (in electrons/cm2), Io the light intensity at any unexposed portion, I the intensity at any particular exposed portion and C a constant to be determined from different exposure times.

106

J.E. CLINE

The data from the gamma-ray and conversion-electron measurements are summarized in tables 1 and 2. Table 1 lists the gamma-ray energies and intensities and the uncertainties which have been assigned to these values. Although these uncertainties are rather subjective in nature, it is felt that they are reasonably conservative. The 22.41, 67.63 and 215.9 keV transitions were observed only with the magnetic spectrographs and therefore no intensities can be given for these gamma rays. The doublet nature of the peak at 414 k e y was established through coincidence measurements and through a comparison of the relative intensities of the 414 and 432 keV transitions as 105,

104

o

Z

1J 0

20

40

60

80

100

120

~40

160

180

Channel Number

Fig. 6. Spectra of gamma rays coincident with 553 and 550 keV transitions. The gates used in taking these spectra were on the low side and on the high side of the peak at ~ 5 5 0 keV for the spectra coincident with the 550 and with the 553 keV transitions respectively.

observed in the decay of 14SEu and of 148mpm. These two transitions have been shown in the decay of 14SmPm, to depopulate the same level in 148Sm, a level which is also excited in the decay of 148Eu. A spectrum of ~4Spm-~4S=pm taken with the Ge(Li) spectrometer is shown in fig. 4. Energies obtained from an analysis of these data served to corroborate the energy determinations of the transition observed in the decay of ~4SEu. Table 2 lists the transition energies, electron energies, electron assignments, electron intensities, measured conversion coefficients and the most probable multipolarity

DECAY OF laSEu

107

assignments for the transitions. Included also are the values of the conversion coefficients calculated by Sliv 6) for these multipolarities and the multipolarities given by Baba et al. 2). The only differences in the assignments in the present work and those reported by Baba occur in the assignment for the 714.8 and for the 1344.6 keV transitions. The reason for these discrepancies lies only in the gamma-ray intensities and not in the electron intensities. This is not surprising since accurate analysis of gamma-

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Spectrum Coincident With- " 572-keY Transition

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100

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130

Fig. 7. Spectrum of gamma rays coincident with the 572 keV transition. ray intensities would have been almost impossible without the resolution afforded by the germanium devices. Table 2 includes the electron intensities derived from both the Si(Li) and magnetic spectrometers. The agreement between these two intensity determinations is felt to be indicative of the uncertainties to be associated with these values. Gamma-gamma coincidence measurements were carried out using a 256 x 256

108

~. E. CL[NF.

channel multi-parameter analyser. Measurements were made with two NaI(TI) detectors and also with one NaI(T1) and one Ge(Li) detector. Due to the highly complex nature o f this decay, only the latter type o f measurements proved to be of much value. Because o f counting-rate considerations, a two-Ge(Li) coincidence system was not used in this experiment. The Ge(Li) spectrometer used for these measurements 105 E

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Spectrum Coincident With 611 keV Transition Contribution To Spectrum From Random Coincidences And From Coincidences With Cornpton Distributions From Higher Lying Gamma Rays Has Been Removed, 104

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160

Fig. 8. Spectrum of gamma rays coincident with the 611 keV transition.

was a 4 c m 3 detector having a resolution of ~ 2.5 keV (FWI-IM) at 100 keV and 4 keV at 1 MeV. Some o f the more pertinent coincidence spectra are shown in figs. 5-12. In the taking o f these data, the detectors were operated at 180 ° with respect to one another and the source was located between them at a distance of 10 cm from the front face o f the 7.62 cm x 7.62 cm NaI(T1) detector and 4 cm from the front face

D E C A Y OF 1£8Etl

109

of the Ge(Li) detector. The contribution to each spectrum from coincidences with the Compton distribution from higher-energy gamma rays in the gating detector has been removed. Contributions to these spectra from random coincidences are less than 5 ~o. Included on each spectrum is a partial level scheme showing the interpretation of the coincidence data. Wherever possible, the coincidence spectrum was analysed by spectrum stripping techniques for the intensities of the constituents. The values obtained, normalized to the absolute intensity of the gating gamma ray, have been included in 10 s

-•0,553keV "~ 414 keV- !q ~ •

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SpectrumCoincident With 650-keV Transition Contribution From Random Coincidences And From Coincidences Coo, 1 Lying Gamma Rays Hove Been Removed. I

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Channel Number Fig. 9. Spectrum of gamma rays coincident with the 630 keV transition. these partial decay schemes. In most cases, the intensity analyses permitted a unique assignment for the order of the transitions. Careful analyses of the data taken coincident with the 611 and with the 630 keV gamma rays were made. These yielded considerable information as to which of the major transitions fed these transitions and which fed the 550 keV level directly. In many cases, the intensity information from these data enabled one to decide between gamma rays which were unresolved in the spectrum taken with the NaI(T1) detector. The spectrum coincident with the 550 keV

110

J.~. CLINE

transition, except for contributions from coincidences with the 553 keV transition, was found to be identical with the singles spectrum. The two spectra presented in fig. 6 represent spectra coincident with the low-energy and with the high-energy sides of the 550-553 keV doublet peak.

I

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.' ! 80

100

Fig. 10. S p e c t r u m o f g a m m a rays coincident with t h e 725 keV transition.

In addition to the information provided by the spectra shown, the coincidence data provided information on the remainder of the stronger transitions. Data taken coincident with the 654, 683, 915, 967, 1034, 1622 and 1650 keV transitions showed only the presence of the 550 and 630 (611) keV transitions, indicating that these transitions directly fed either the level at 1161 or the level at 1180 keV.

D E C A Y O F z i SEu

l l 1

3. Presentation of decay scheme The proposed decay scheme for t4SEu is presented in fig. 13. Two categories of placement of transitions and levels in the scheme are shown. Transitions and levels represented by solid lines are those established either by conclusive coincidence information or from the previously reported 1) decay of 14SPm and 148=Pm. Those 10 3,

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112

J.E. CLrt,rE

levels, it is highly likely that some of the transitions placed on the basis of energy differences are simply the result of random chance. Thus, the dashed transitions and levels are included in the scheme with some reservation. Spins and parities which are shown on the proposed level scheme are those which have been assigned by other measurements 1,2) or through the few unambiguous

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114

J.E. CLINE 4. Discussion

In general, the proposed level scheme is consistent with that proposed by Baba e t al. 2). With the exception of the m a n y additional levels proposed above 2 MeV,

and the more precise energies, the only difference lies in the parity assignment to the level at 1894.9 keV. F r o m the present work, the 714.8 keV and the 1344.6 keV transitions have been identified as being E1 and E2, respectively. Thus, the level at 1894 keV, from whence these transitions originate, must have positive parity. Most of the levels which have been identified in this experiment probably correspond to levels reported by Kenefick and Sheline 7) from (d, p) and (p, p') experiments and by Veje s) from (d, d') experiments. Interpretations of the level structure of 14SSm have been made primarily by Baba e t al. 2), Bhatt 9) and Lipas lo). Due to the incomplete and ambiguous nature of the information available on this nueleide, no attempt is made here to elaborate further on these possible interpretations. However, it should be pointed out that the level at 1173 keV, included by Lipas lo) in his discussion of the two-phonon quadrupoleoctupole excitations has been shown by the present study as well as by Baba e t al. 2) to have positive parity. It thus cannot be interpreted as a quadrupole-octupole state. I take pleasure in acknowledging the assistance of L. D. McIsaac for his work in purifying the sources, of R. G. Helmer for assistance in the magnetic spectrograph measurements and of J. G. Prather for assistance in making the measurements using the Si spectrometers. References 1) Nuclear Data Sheets, compiled by K. Way et al. (Printing and Publishing Office, National Academy of Sciences - National Research Council, Washington, 25 D.C.) 2) C. V. K. Baba, G. T. Ewan and J. F. Suarez, Nuclear Physics 43 (1963) 285 3) K. F. Smith and J. E. Cline, IEEE Trans. Nucl. Sci. NS-13 (1966) 468 4) R. L. Heath, W. W. Black and J. E. Cline, IEEE Trans. Nucl. Sci. NS-13 (1966) 445 5) H. Slatis, Ark. Phys. 22 (1962) 517 6) L. A. Sliv and I. M. Band, AEC-tr-2888 (1956) 7) R. A. Kenefick and R. K. Sheline, Phys. Rev. 133 (1964) B25 8) E. Veje, private communication to C. W. Reich 9) K. H. Bhatt, Phys. Lett. 17 (1965) 282 10) P. O. Lipas, Nuclear Physics 82 (1966) 91