Nuclei Off the Line of Stability - American Chemical Society

43 Decay of

Cs to Levels of

145

1

Ba

145

1

2

3

3

3

J. D. Robertson, W. B. Walters , E. F. Zganjar , R. L. Gill , H. Mach , A. Piotrowski, H. Dejbakhsh, and R. F. Petry 4

5

1

Department of Chemistry, University of Maryland, College Park, MD 20742 Department of Physics, Louisiana State University, Baton Rouge, LA 70805 Physics Department, Brookhaven National Laboratory, Upton, NY 11973 Cyclotron Laboratory, Texas A&M University, College Station, TX 77643 Department of Physics, University of Oklahoma, Norman, OK 73019

2 3 4

Downloaded by TUFTS UNIV on June 3, 2018 | https://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch043

5

-

145

An investigation of the β decay of the 0.59s Cs was made at the TRISTAN on-line mass separation f a c i l i t y . The level scheme for Ba has been const ructed. The proposed spin and parity assignments are based upon transition multipolarities and γγ angular correlation measurements. 145

In 1950, Rainwater proposed that the discrepancy between the measured and calculated values of the quadrupole moments of some odd-A nuclei could be resolved by allowing those nuclei to take on a spheroidal shape. I t is now well-known that the properties of many nuclei can be described on a deformed quadrupole basis. Over the past three years, much attention has been focused on the question of whether or not some nuclei, in addition to breaking rotational symmetry, also break reflection symmetry in the i n t r i n sic frame. The nuclei in question are found in the Ra-Th region with A=220 -228 and a review of the extensive literature on this region can be found in references [LEA82], [IAC82], [GAI83], [NAZ84], and [LEA84]. In a recent paper, Leander et. a l . introduce the idea that nuclei with N=88-90 in the immediate vicinity of Ba might exhibit the same type of "octupole deformed" character that i s observed in the Z=88-90 region.[LEA85] Three reasons are given as to why this might be a new region which can also be described by a basis which breaks reflection sym metry in the i n t r i n s i c frame. F i r s t of a l l , the systematics of the ϋ = 1 " and 3" levels in the neutron rich e-e Ba isotopes suggest a minimum in the s p l i t t i n g of the K*=0 and 0" bands near B a . The Γ level in B a at 738 keV, in units of the f i r s t 2 l e v e l , is as low as the 1" levels in the Ra and Th isotopes. The presence of these low-lying negative parity states in the structure of the light actinides was one of the f i r s t indications that the Ra-Th region was octupole deformed.[M0L72] Secondly, the deformed-shell-model calculations of Nazarewicz et. a l . indicate that Z=56, as well as Ζ or N=88-90, is an optimal particle number for octupole deformation.[NAZ84] The Strutinsky type calculations predict an octupole deformed equilibrium shape for Ite and a potential which is very soft towards octupole deformations for B a . The results are independent of the three single-particle potentials employed in the calculations. Finally, the large differences observed between the experimental and theoretical masses calculated bv Moller and Nix in the Ra-Th region are also found in the region near Ba.[LEA82j The difference is greatest for B a and, as in the light actinides, can be accounted for by the extra binding energy obtained by including octupole correlations in the mass calculations. The ground state (g.s.) spin of B a has been determined to be 5/2 by laser spectroscopy at IS0LDE[MUE83]. The same group reports a g.s. magne t i c moment of -0.272n.m. and a spectroscopic quadrupole moment (Qs) of 1.15b. 145

π

+

1 4 6

1 4 6

+

1 4 4

145

1 4 5

1 4 5

0097-6156/86/0324-0284$06.00/0 © 1986 American Chemical Society Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

43.

Decay of Cs to Levels of

ROBERTSON ET AL.

Ba

145

285

145

From the measured g . s . spin and Qs value, Leander c a l c u l a t e s t h a t l ^ B a m^y be octupole deformed with an e q u i l i b r i u m value of &3"0.08 (32=0.172 and 34=0.069).[LEA85] These r e s u l t s , however, are not conclusive. From a s i m p l i f i e d view, the g.s. spin of 5/2 f o r B a can be explained by allowing the odd neutron to occupy the 5/2[521] Nilsson o r b i t a l w i t h 32"0·12. I n a d d i t i o n , the measured magnetic moment f o r the g . s . i s cons i s t e n t w i t h both 03=0 and £ 3 * 0 in Leander s calculations.[LEA85] Another i n d i c a t i o n of octupole deformation i n the s t r u c t u r e of Ba would be the presence of a pair (or more) of strongly perturbed opposite p a r i t y r o t a t i o n a l bands connected by c o l l e c t i v e El t r a n s i t i o n s . Even more s p e c i f i c a l l y , these bands would have a small energy s p l i t t i n g and the decoupling parameters f o r the two bands would be equal in magnitude but opposite i n sign.[RAG83] This type of c o l l e c t i v e s t r u c t u r e i s observed i n both R a and A c ( f i g . 1 ) . In the study of the 3~ decay of C s by Rappaport e t . a l . , only the 435.7 keV gamma was i d e n t i f i e d as an El transition.[RAP82] In t h e i r study, however, only 31 gamma rays were assigned to the decay of C s and they determined the level s t r u c t u r e of B a up to 785keV. A l a t e r study by uejbakhsh found 77 new gamma rays and defined l e v e l s up to 2.8MeV.[UEJ85] U n f o r t u n a t e l y , the l a t t e r i n v e s t i g a t i o n was not able to determine the m u l t i p o l a r i t i e s of any of the new t r a n sitions. 1 4 5

1

1 4 5

2 2 5

2 2 7

1 4 5


1 4 5

1 4 5

5/2--260 7/2+220

9/2-J22

3/2--22S

3/2+J37 11/2-.5Q2 9/2+J6? 5/21438

5/2+J80

3/2U5Q

7/2-321

K= 3/2± 13/21271

7/2HL25/2-J20

13/2+_2ii 11/2+187 9/2-127 ~ * 9/2+110

9/2+101

1 1 / r 1 9 9

7/2-69

, 3 /

1/2II55 3/2-J3Q K=1/2±

£

„ 1/2-ZS

2 + 4 3 , y . 32 9

1/2+_Q K=1/2±

3/2l_ K=3/2± 227 * 89

225,

Ra

Figure 1 .

A c

P a r i t y doublets i n

2 Z 5

R a and

2 2 /

Ac

[15,16j

EXPERIMENTAL PROCEDURE The study of the 3" decay of 0.59s C s was conducted a t the TRISTAN mass separator o n - l i n e to the high f l u x reactor a t BNL. A a e t a i l e d d e s c r i p t i o n of the TRISTAN f a c i l i t y can be found i n reference [ 6 I L 8 1 ] . The r a d i o a c t i v e samples were produced by f i s s i o n i n g a uranium t a r g e t integrated i n a p o s i t i v e surface i o n i z a t i o n source.[PI084] In t h i s experiment, the usual Re surface i o n i z e r was replaced w i t h a Ta i o n i z e r . The lower work f u n c t i o n of Ta ensured no independent production of Ba from the ion source a t a low power operation («1200°C). 1 4 5

Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

286

NUCLEI OFF THE LINE OF STABILITY

Two G e ( L i ) d e t e c t o r s and t w o HpGe d e t e c t o r s were used t o c o l l e c t s i n g l e s gamma s p e c t r a and 10° t h r e e p a r a m e t e r ΥΎ t c o i n c i d e n c e e v e n t s . The r e s o l u t i o n o f t h e germanium d e t e c t o r s r a n g e d f r o m 1.7 t o 2 . 2 keV FWHM a t 1332 k e V . I n a d d i t i o n , a LEPS d e t e c t o r w i t h 0 . 5 5 keV FWHM a t 122 keV was used i n t h e l o w - e n e r g y γ - r a y measurements. T a b l e 1 summarizes t h e r e s u l t s o b t a i n e d i n t h e a n g u l a r c o r r e l a t i o n measurements. The e l e c t r o n s p e c t r a and t h e γ - e c o i n c i d e n c e d a t a were a c q u i r e d u s i n g b o t h a S i ( L i ) and HpGe d e t e c tor. The r e s u l t s o f t h e c o n v e r s i o n e l e c t r o n measurements a r e g i v e n i n table 2.


T A B L E 1 - EXPERIMENTAL ANGULAR CORRELATION COEFFICIENTS Transition

A22:

A44

323->112->Q 368->199->0 317->435->0 155-*55->0

0.271Û.04 0.13±0.05 -0.2U0.09 0.22±0.08

0.2U0.05 0.0610.10 0.23*0.10

HQ

23.0 1.4 0.4 3.6

T A B L E 2 - EXPERIMENTAL INTERNAL CONVERSION COEFFICIENTS O'THEORXIO 2* EY(ke-V) 112.5-K 112.5-L 175.4-K 175.4-M 198.9-K 198.9-L 207.1-K 241.0-K 435.7-K 434.7-K

CXEXFX10 107.2±12.0 12.8±1.2 16.9±2.2 0 ?4±0 11 18.U2.6 3-U0.6 0->6.4 8.8±1.2 0.48±0.06 3610!)

2

58.0 8.4 17.2 0.52 12.3 2.0 11.0 7.3 1.5 1.4

C*E2 84.0 34.0 20.4 1.1 13.6 3.2 12.0 74 1.1 1.0

c*El 13.9 1.9 4.2 0.11 3.0 0.40 2.7 1.8 0.38 0.34

' C a l c u l a t e d f r o m NUT 21.225-230 (1978). EXPERIMENTAL RESULTS The l o w - e n e r g y p o r t i o n o f t h e l e v e l scheme deduced f o r i s shown i n figure 2. A s a t u r a t e d sample o f t h e A=145 i s o b a r s was c o l l e c t e d and c o u n t e d t o d e t e r m i n e t h e a b s o l u t e i n t e n s i t y o f t h e 175.4 k e V ^ - r a y ( 1 9 . 8 ± 2.4/100 decays). T h i s v a l u e , a l o n g w i t h t h e yt and c - e i n t e n s i t i e s , was u s e d t o c a l c u l a t e t h e 3 " f e e d i n g t o t h e g . s . and e x c i t e d l e v e l s i n ^Ba. The l a r g e u n c e r t a i n t y i n t h e 3 " f e e d i n g t o t h e 198.9 keV l e v e l i s a d i r e c t r e s u l t o f t h e l a r g e u n c e r t a i n t y i n t h e r e l a t i v e i n t e n s i t y o f t h e 198.9 keV γ-ray. 5 c g has a 12% d e l a y e d n e u t r o n b r a n c h [ R I S 7 9 ] and t h e m a j o r t r a n sition in B a i s a 199 keV - r a y . As a r e s u l t , t h e i n t e n s i t y o f t h e 198.9 keV t r a n s i t i o n i n ^^Ba c o u l d o n l y be d e t e r m i n e d f r o m t h e c o i n c i d e n c e data. Our v a l u e o f 54.7 f o r I199 i s l o w e r t h a n t h e 68.5 v a l u e p r e v i o u s l y r e p o r t e d by R a p p a p o r t e t . a l . [ R A P 8 2 ] , As n o t e d a b o v e , t h e g . s . s p i n o f ^ B a has been measured t o be 5 / 2 . The n e g a t i v e p a r i t y a s s i g n m e n t f o r t h e g . s . i s based upon t h e s y s t e m a t i c s o f t h e N=89 i s o t o n e s and t h e Z=56 i s o t o p e s . The g . s . J" o f b o t h N d and 51sm i s 5 / 2 " . The g . s . o f J * o f t h e odd-A Ba i s o t o p e s i s : 3 9 B a = 7 / 2 ~ , 4 l B a = 3 / 2 ' , and From t h e i n t e r n a l c o n v e r s i o n c o e f f i c i e n t s g i v e n i n t a b l e 2 , t h e f o l l o w i n g gammas were d e t e r m i n e d t o be M1/E2 t r a n s i t i o n s i n agreement w i t h 1 4

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IN IN IN

0 3 9 1 . 2 1.1 3 6 7 . 7 5.3 2 4 6 . 9 2.5 5 4 7 . 1 23.6 4 3 4 . 7 5.7 348.2 1 1 2 2 7 . 4 3.4 4 9 2 . 1 13.8 2 1 4 . 5 1.2 4 5 4 . 8 10.6 3 4 1 . 7 0.6 2 7 9 . 5 4.0 2 5 5 . 9 2.5 3 8 . 2 1.8 4 3 5 . 6 24.3 3 2 3 . 3 5.2 2 6 0 . 3 0.9 416.55.1 3 0 4 . 5 2.4 2 4 1 . 0 28.9 319.8 7 6 2 0 7 . 1 16.2 1 2 1 . 0 0.4 277.1 2 1 164.67.5 1 9 8 . 9 54.7 8 6 . 3 5.8 1 7 5 . 4 100.0 54.1

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*-W +W +

UIUI

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7 8 5 . 3 0.4 5 8 6 . 6 1.8 3 6 9 . 2 0.9 238.4 7 9 753.311.2 5 7 8 . 1 7.6 5 5 4 . 7 5.2 3 1 7 . 6 3.0 7 2 4 . 3 8.1 5 4 8 . 9 5.8 5 2 5 . 8 3.7 3 0 8 . 0 2.0 559.94.5 3 9 5 . 1 5.6 1 0 5 . 9 0.8 6 6 3 . 5 1.9 4 8 7 . 2 6.9 4 6 4 . 8 1.0

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2 452 429.1 13 172.0 4 3 611.2 54 4 3 5 . 7 1 0 2j 1 9 4 . 6 3.4 1 1 5 6 . 3 2.4

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288

NUCLEI OFF THE LINE OF STABILITY

the e a r l i e r work: 112.4, 175.4, 198.9, 241.0, and 454.7. Rappaport also reports t h a t the 86.3 and 238.4 3-rays are M1/E2 t r a n s i t i o n s . The non-zero A44 values f o r both the 368-199-0 cascade and the 155-455-0 cascade r u l e s out the p o s s i b i l i t y t h a t J* of the 199 keV level or the 455 keV level i s 3 / 2 " . In a d d i t i o n , the A22/A44 values f o r the 368-199-0 cascade l i m i t the spin of the 567 level to e i t h e r 1/2 or 3/2 once J* of the 199 level i s known not to be 3 / 2 " . The large A22 value of 0.24 f o r the 323-112-0 cascade, coupled w i t h the f a c t t h a t the 435 level i s 5 / 2 , rules out the p o s s i b i l i t y t h a t the 112 level has ϋ = 3 / 2 " ; the l a r g e s t A22 can be f o r a 5 / 2 * - 3 / 2 " - 5 / 2 " cascade i s 0.08. The only t r a n s i t i o n t h a t was determined to be El by the measured conversion c o e f f i c i e n t s was the 435.7 k e V y - r a y . Setting an upper l i m i t on the area expected f o r the 547.1 K-conversion peak in the electron spectrum indicates t h a t the 547 i s also an El t r a n s i t i o n . Likewise, an upper l i m i t of 0.064±0.008 f o r α f o r the 207.1 keV t r a n s i t i o n can also be established by s e t t i n g the mixing of the 175.4 keV t r a n s i t i o n to zero. This low α value suggests t h a t the 207.1 γ - r a y i s an El t r a n s i t i o n . A spin of 3/2" f o r the 435 level i s ruled out by the large negative A22 value f o r the 317-435-0 cascade. In a d d i t i o n , a l o g ( f t ) of approximately 6 f o r the 435 keV level makes the 3~ decay to t h i s level an allowed t r a n s i t i o n . Because the g . s . spin of C s i s 3 / 2 , t h i s rules out the p o s s i b i l i t y t h a t the spin of the 435 level i s 7 / 2 and subsequently l i m i t s the spin of the 435 level to 5 / 2 . +

π

κ


κ

1 4 5

+

+

+

CONCLUSION Clearly the r o t a t i o n a l band s t r u c t u r e observed i n the l i g h t odd-A a c t i nides i s not seen i n the s t r u c t u r e of B a . Like the N-89 isotones N d [ P I N 7 7 ] and Sm[C0076], the negative p a r i t y states in B a can not be grouped i n t o any obvious r o t a t i o n a l bands and seem to be very hard to explain i n the framework of the Nilsson model. The l e v e l s a t 319.8, 547.1, and 785.5 keV might be members of a strongly perturbed p o s i t i v e - p a r i t y r o t a t i o n a l band, but both the 319 and 547 appear to be 5/2 l e v e l s . Aparently the 435.7 keV level can not be a member of the p o s i t i v e p a r i t y band including the 547 and 785 because neither of these p o s i t i v e p a r i t y l e v e l s decay to i t . The absence of any c l e a r l y defined n e g a t i v e - p a r i t y r o t a t i o n a l bands in B a suggests that i t i s not strongly quadrupole deformed. The clear r o t a t i o n a l s t r u c t u r e observed in l**(àû 1 5 5 completely dissapears as the N=89 isotones cross the Z=64 s h e l l . This conclusion i s also supported by the 32 values calculated from the l i f e t i m e s of the f i r s t 2 states in B a and B a . The r a t i o 3 / 3 p · r e s p e c t i v e l y . This i s compared to the r a t i o of 12.2 and 10.7 observed in the quadrupole deformed nuclei G d and D y . Thus, although B a does not have the p a r i t y doublets expected for an octupole deformed nucleus, i t i s possible t h a t (because $2 * small) i t s s t r u c t u r e i s determined by higher orders of deformation; i . e . the s t r u c t u r e i s very dependent on 3 3 , 3 Δ , e t . . The great s i m i l a r i t y between the s t r u c t u r e of Ba, C e , and N d added to our i n a b i l i t y to understand these N=89 (and N=87) nuclei in the framework Nilsson model points to the need f o r extended t h e o r e t i c a l and experimental work in t h i s t r a n s i t i o n a l r e g i o n . 1 4 5

149

151

1 4 5

1 4 5

a n d

ΰν

+

1 4 4

1 4 6

f

2

o

r

1 4 4 β ά

a n d

1 4 6 β ά

i s

6

8

a n d

7

5

2 s

1 5 4

s

1 5 6

1 4 5

s o

1 4 5

1 4 7

1 4 9

ACKNOWLEDGMENTS We would l i k e to thank Dr. Leander f o r his helpful discussions. work has been supported by the U.S. Department of Energy.


This

43. ROBERTSON ET AL.

Decay of Cs to Levels of Ba 145

145

289


REFERENCES [LEA82] G.A. Leander, R.K. Sheline, P. Moller, P. Olanders, I. Ragnarsson, and A.J. Sierk, Nucl. Phys. A388 452 (1982). [IAC82] F. Iachello and A.D. Jackson, Phys. Lett. 108B 151 (1982). [GAI83] M. Gai, J.F. Ennis, M. Ruscev, E.C. Schloemer, B. Shivakumar, S.M. Sterbenz, N. Tsoupas, and D.A. Bromley, Phys. Rev. Lett. 51 646 (1983). [NAZ84] W. Nazarewicz, P. Olanders, I. Ragnarsson, J. Dudek, G.A. Leander, P. Moller, and E. Ruchowska, Nucl. Phys. A429 269 (1984). [LEA84] G.A. Leander and R.K. Sheline, Nucl. Phys. A413 375 (1984). [LEA85] G.A. Leander, W. Nazarewicz, P. Olanders, I. Ragnarsson, and J. Dudek, Phys. Lett. 152B 284 (1985). [MOL72] P. Moller, S.G. Nilsson, and R.K. Sheline, Phys. Lett. 40B 329 (1972). [MUE83] A.C Mueller, F. Buchinger, W. Klempt, E.W. Otten, R. Neugart, C. Ekstrom, and J. Heinemeier, Nucl. Phys. A403 234 (1983). [RAG83] I. Ragnarsson, Phys. Lett. 130B 353 (1983). [RAP82] M.S. Rappaport, G. Engler, A. Gayer, and I. Yoresh, Z. Physik. A305 359 (1982). [DEJ85] H. Dejbakhsh, private communication. [GIL84] R.L. Gill, M.L. Stelts, R.E. Chrien, V. Manzella, H. Liou, and S. Shostak, Nucl. Instr. and Meth. 186 243 (1981). [PIO84] A. Piotrowski, R.L. Gill, and D.C. McDonald, Nucl. Instr. and Meth. 22 1 (1984). [RIS79] C. Ristori, J. Crancon, K.D. Wunsh, G. Jung, R. Decker, and K.L.Kratz, Z. Physik. A290 311 (1979). [PIN77] J.A. Pinston, R. Roussille, G. Sadler, W. Tenten, J.P. Bocquet, B. Pfeiffer, and D.D. Warner, Z. Physik. A282 303 (1977). [COO76] W.B. Cook, M.W. Johns, G. Louhoiden, and J.C. Waddington, Nucl. Phys. A259 461 (1976). RECEIVED

August 1, 1986


Nuclei Off the Line of Stability - American Chemical Society

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