Nuclear energy states of 155Eu

I.E.I: [ 3.A [

Nuclear Physics A105 (1967) 698--704; (~) North-Holland Publishiny Co., Amsterdam Not to be reproduced by photoprint or microfihnwithout written permissionfrom the publisher

N U C L E A R E N E R G Y S T A T E S O F lSSEu G. P. AGIN +, C. E. M A N D E V I L L E *t and V. R. POTNIS t+

Kansas State University ttt, Manhattan, Kansas, USA 66502 Received 2 August 1967 Abstract: lsotopically concentrated xs4Sm has been irradiated in a research reactor to produce ~5~Sm which decays by beta emission to aSSEu. A total of 29 gamma rays observed in Ge(Li) detectors has been ordered in a level scheme which contains levels having the following excitation energies in keV, spins, and parities: 0(.~~-),79(½+), 105(:)-), 168(.~-), 246(~+), 307(~+), 614(.~-), 768(-~+), 782(~+), 876(~+), 1102(]-), 1264({-, ½+, -~-+) and 1302(½-, ½~, .~"). These states of ~55Eu have been assigned Nilsson numbers or rotational properties. RADIOACTIVITY xssSm [from 154Sm (n, ~)]; measured E.e,/./,y-j-coin. 155Eu deduced levels, J, ~. Enriched target.

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1. I n t r o d u c t i o n

T h e r a d i a t i o n s o f the 22 rain 155Sm have been studied i) in the course o f several recent investigations 2-7). K r a c i k et al. 2), e m p l o y i n g magnetic s p e c t r o g r a p h a n d scintillation spectrometry, r e p o r t e d the presence o f 13 g a m m a transitions which were interpreted to c o r r e s p o n d to de-excitation o f seven excited states in the residual nucleus t55Eu. Also c o m b i n i n g m a g n e t i c s p e c t r o g r a p h a n d scintillation spectrometry, F u n k e et al. 3) r e p o r t e d seven beta spectra and 25 g a m m a transitions to be associated with the disintegration o f l SSSm. These r a d i a t i o n s were considered to indicate the presence o f nine excited states in the nucleus o f 155Eu. A g i n et al. 4) observed 22 g a m m a rays from 155Sm in a d e t e c t o r o f G e ( L i ) , a n d c o m m e n t e d specifically u p o n the presence o f four excited states in ~55Eu. I n c o n t i n u e d s t u d y o f 155Sm, F u n k e et al. 5) detected 34 g a m m a transitions and located ten excited nuclear energy states in lSSEu. Reinvestigation by Potnis et al. 6), again using G e ( L i ) yielded evidence o f the presence o f 28 g a m m a transitions in ~55Eu. Sujkowski and Ungrin 7) have rep o r t e d 36 g a m m a rays to be emitted in the decay o f 155Sm, e m p l o y i n g G e ( L i ) a n d Si(Li) spectrometry. The presently r e p o r t e d d a t a m a y be regarded as resulting from an extension o f the m e a s u r e m e n t s o f ref. 6). + The content of this paper constitutes a portion of a thesis to be presented by G. P. Agin to Kansas State University in partial fulfillment of requirements for the degree of Doctor of Philosophy. +t Present address: Michigan Technological University, Houghton, Michigan 49931. ~++ Work supported in part by the U. S. Atomic Energy Commission. 698

155Eu N-tSCI.EAR STATES

699

2. The measurements To obtain sources of 155Sm, quantities of 154Sm203 (isotopic concentration 99.26 %) were successively exposed in amounts of 5 mg each for periods of 10 min in the KSU Triga M ark II reactor. Gamma-ray spectra of single counts were observed in a Ge(Li) detector of depletion layer thickness 4.2 mm. The relative intensities of the '

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The spectrum o f the softer gamma rays of 155Sm is shown in tig. 1. The spectrum of the harder gamma rays is presented in fig. 2. These data are ordered according to increasing quantum energy in table I, along with calculated relative intensities• In table 1 are also given for comparison the energies and relative intensities o f the g a m m a rays of 155Sm as reported in ref. 5).

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T w o Ge(Li) detectors were placed in coincidence, and coincidences were measured between the 141 keV gamma ray and some of the softer gamma rays of the spectrum. These data are shown in fig. 3. From this figure it is apparent that the 141 keV g a m m a ray is coincident with quanta at 105, 79, and 63 keV, and with the K-shell X-rays of europium. T o obtain these results, the gating pulse from a slow-fast coincidence circuit was provided by the 141 keV gamma, and the gamma-ray spectrum

155Eu N U C L E A R

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STATES

coincident with it was displayed upon a multi-channel analyzer. For the purpose of obtaining a necessary correction, a gating pulse was also taken from the region of energy just above 141 keV to observe any coincidences between the gamma rays of fig. 3 and the Compton distributions of gamma rays of high energy. These coincidences, as well as chance coincidences, were accumulated simultaneously with the total of coincidences of every variety, and subsequently subtracted from the total to give the genuine coincidences of fig. 3. The full energy peak at 79 keV is unduly intense because of the presence of backscattered radiation of the 105 keV quanta,

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3. Spectrum o f the g a m m a rays o f ~ s S m coincident with quanta o f energy 141 keV. Energies are in keV. These data were obtained w h e n two Gc(Li) detectors were placed in coincidence.

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Attempts to observe coincidence spectra generated by gating pulses of gamma rays at energies greater than 141 keV failed because of the poor detection efficiency of the coincident Ge(Li) detectors (depletion layer thicknesses 4.2 mm and 5 mm, respectively), and because of the fact that the hard gamma rays of t55Sin are few in number, 93 ~ of the beta disintegrations feeding the 105 keV level 5) of 155Eu.

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TABLE 1 Energies and relative intensities of the gamma rays of "ssSm

Ref. s) 26 62 79 105 135 141 168 203 229 246 280 307 429 462 509 522 570

(11 85) (4.5 q:l.5) (4.5 !-1.5) (2000--200) (45 4-5) (1.8 -_':0.3) (4.0 ~1.5) (1.2 4-0.2) (100) (0.4 :L0.1) (0.4 ::0.1) (0.4:0.1) (1.8)-0.2) (0.5 L-0.1) (4.4 4-0.3) (0.6 --0.1)

Ref. s)

Present results

63 (6.6 ":0.6) 79 ( ~ 0.5) 105 (2270±40) 141 168 203 229 246

(54.+_2) (1.7 J0.3) (1.0 ::0.2) (1.3 -!:0.1) (100)

307 429 462 509 522 570

(0.20:'0.05) (0.36±0.04) (2.03-+-0.05) (0.30-t-0.04) (4.334-0.06) (0.624-0.04)

602 630 647 663 676 710 768 860 931 996 1004 1050 1158 1197 1223 1262 1302

(0.4 _--0.1) (0.6 ri:0.1) (0.207i:0.7) (I.9 :'_-0.2) (0.18--0.07) (0.23.--_0.08) (0.22:i:0.08) (__<.0.1) (0.18.-'.-0.03) (0.4 :L0.2) (0.5 :±0.2) (__<_0.2) (0.33"-0.06) (0.16~::0.04) (0.7 4-0.1) (0.14t-0.05) (2.6 --0.3)

Present results

602 630 650 663 677 711 768

(0.39":0.04) (0.52±0.04) (0.30a-0.04) (1.81--0.05) (0.204-0.05) (0.204-0.05) (0.16'--0.05)

931 (0.374-0.06) 997 (0.264-0.05) 1000 (0.56_-~0.06) 1160 1200 1225 1264 1302

(0.30:!:0.04) (0.12t-0.03) (0.62-,_:0.03) (0.084-0.02) (2.11--0.06)

Energies are in keV. Relative intensitities are included between parentheses.

3. Discussion The data of table 1 show five gamma rays reported in ref. 5), which were not detected in the present investigation. Evidence for the gamma rays at 135, 860 and 1050 keV had been earlier obtained with application of coincident scintillation spectrometers 3), and the presence of the 280 keV gamma ray was observed s) in a single detector of Ge(Li). Since the results of table 1 also indicate that in the course of the present study, gamma rays of comparable or smaller intensity than the four above cited were indeed satisfactorily observed, no obvious explanation for the differing observations is apparent. Whether the presence of these four gamma rays 3, 5) might be related to contaminating 5) 152Eu is not certain. The presently unobserved fifth gamma ray is the one of energy 26 keV. Its intensity at the surface of the quantum sensitive Ge(Li) was too heavily reduced by preabsorbing materials such as lucite, aluminium and germanium. The lucite shield (thickness 0.64 cm) was introduced to stop hard beta rays from the source of 155Sm, and the Ge(Li) detector was housed in an aluminium cup of wall thickness 0.0384 cm. The "dead layer" of germanium before the sensitive volume of the detector had a thickness of 0.03 cm. The data of figs. 1-3 are summarized in fig. 4 where the nuclear level structure of 15SEu is given. The measured 8) spin values of the ground states of 15'Eu and lS3Eu are each ~. By analogy, the spin of the ground state of ~SSEu can also be considered to be 5. Gamma-gamma directional correlation studies 9) provide for the ground state and the excited states at 105 and 246 keV a spin sequence of ~, }, 3. Internal

lSSEu NUCLEAR STAIES

703

conversion studies 2. ~o) suggest the 105, 141 and 246 keV transitions to be E1 + M2, El + M2 and MI + E 2 in character. Both the correlation experiments and these measurements of the conversion coefficients are consistent with spin-parity assignments of ~+, zs.- and z5.+ for the 246 keV and 105 keV excited states, and the ground state. On the Nilsson diagram ~t), the spacing in energy of the ground state and the 105 and 246 keV levels corresponds to a value of the deformation parameter of ~SSEu of 5 + [413] for the about 0.2, which in turn indicates Nilsson quantum numbers of "z ground state of tSSEu and { - [532] and 3+ [411 ] as Nilsson quantum numbers for the states at 105 and 246 keV, respectively. Additional considerations of energy spacings

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o f the nuclear states result in the choice of ~- [523], ½+ [411 ] and 3.- [541] for Nilsson numbers of the states at 614, 768 and 1102 keV. Similarly, Nilsson numbers ½+ [420], ~+ [422] or ½- [550] could be assigned to either of the energy states at 1264 and 1302 keV. The state at 79 keV is interpreted as being the second member of a rotational band 12) based upon the ground state. The 168 keV state is interpreted as occupying an analogous position relative to the 105 keV Nilsson state. The 307 keV state is considered to be the second member of a rotational band founded upon the 246 keV single-particle state, and the 782 and 876 keV states are assumed to be the second and third members of a rotational band commencing with the 768 keV state of spin-

704

G. P, AGTNet al.

parity designation ½+. The energy spacings of the states of this particular rotational band yield a value of - 0 . 5 0 for the decoupling parameter 12). The arguments of the paragraphs immediately preceding provide the spin-parity assignments presented in fig. 4. The writers wish to express appreciation for discussion with Dr. P. M. Rinard concerning theoretical aspects of this problem and for the cooperation of Dr. Walter Meyer in carrying out irradiations. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12)

M. L. Pool and L. L. Quill, Phys. Rev. 53 (1938) 437 B. Kracik et al., Czech. J. Phys. BI3 (1963) 79 L. Funke et aL Nuclear Physics 70 (1965) 335 G. P. Agin e t al., Bull. Am. Phys. Soc. 11 (1966) 408 L. Funke et al., Nuclear Physics 88 (1966) 641 V. R. Pomis, G. P. Agin and C. E. Mandeville, Bull. Am. Plays. Soc. 12 (1967) 579 Z. Sujkowski and J. Ungrin, Bull. Am. Phys. Soc. 12 (1967) 714 J. M. Baker and F. 1. B. Williams, Proc. Roy. Soc. 267A (1962) 283 R. E. Sund, R. G. Arns and M. L. Wiedenbeck, Phys. Rev. 118 (1960) 776 L. C. Schmid and S. B. Burson, Phys. Rev. 115 (1959) 447 B. R. Mottelson and S. G. Nilsson, Mat. Fys. Skr. Dan. Vid. Selsk. 1, No. 8 (1959) A. Bohr and B. R. Mottelson, Mat, Fys. Medd. Dan. Vid. Sclsk. 27, No. 16 (1953)