HIGH SPIN ISOMERS IN 153Er

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HIGH SPIN ISOMERS IN 153Er

D. Horn, G. Young, C. Lister, C. Baktash

To cite this version:

D. Horn, G. Young, C. Lister, C. Baktash. HIGH SPIN ISOMERS IN 153Er. Journal de Physique

Colloques, 1980, 41 (C10), pp.C10-39-C10-42. �10.1051/jphyscol:19801004�. �jpa-00220623�

JOURNAL DE PHYSIQUE ColZoque C l O , suppliment au n012, !Tome 41, ddcembre 1980, page

C10-39

1 5 3

H I G H SPIN ISOMERS IN Er

D. Horn, G.R. Young, C.J. L i s t e r and C. Baktash.

Brookhaven National Laboratory, Upton, New York 11 973, U . S.A.

INTRODUCTION

The o b s e r v a t i o n of n u c l e a r behavior which conforms t o t h e s h e l l model a t l a r g e v a l u e s o f a n g u l a r momentum is s i g n i f i c a n t f o r s e v e r a l r e a s o n s , p a r t i c u l a r l y i n t h e N282 r e g i o n . I t c o r r o b o r a t e s t h e view t h a t , n e a r c l o s e d s h e l l s , h i g h s p i n s t a t e s may be formed by t h e alignment of s i n g l e p a r t i c l e a n g u l a r momenta; i t e l i m i n a t e s from c o n s i d e r a t i o n p o s s i b l e extremes of deformation and c o l l e c t i v i t y ; and f i n a l l y , i t p e r m i t s a p p l i c a t i o n o f t h e i n s i g h t and p r e d i c t i v e powers of t h e s h e l l model t o a new and f e r t i l e a r e a .

The N=85 i s o t o n e s with t h r e e n e u t r o n s o u t s i d e a major closed s h e l l , have provided s t r i k i n g examples o f ' s h p l l s t r u c t u r e a t h i g h s p i n s . For example, t h e s i g n a t u r e of v a l e n c e neutron s t a t e s and t h e i r coupling t o a l i g n e d proton c o n f i g u r a t i o n s i n l5'Ey h a s been d i s c e s s e d by

~ l e i n h e i n e

.

Such coupling may be expected t o g i v e r i s e t o isomeric s t a t e s . Indeed, s e v e r a l survey experiments have r e p o r t e d evidence of h i g h s p i n isomers i n 5 3 ~ r . 2-4 , Our subsequent i n v e s t i g a t i o n o f 1 5 3 ~ r confirmed two long-lived h i g h s p i n isomers, and e s t a b l i s h e d t h e i r p o s i t i o n s and decays.

EXPERIMENT

The experiments were performed a t t h e Brookhaven Tandem Van de Graaff. An a r r a y of f o u r NaI d e t e c t o r s i n delayed coincidence with a Ge(Li) d e t e c t o r was used t o provide a n e x t r a timing

1

153ER ISOMR EYPRSS LOOKING PROVPT

I 153ER 299.340.588.812 LOQKING ABOVE

$11 ^.

153ER ISOKR BYPASS LOOKING OBOE

1 ) Gated Y-Y coincidence s p e c t r a from 1 2 ~ + 1 4 4 ~ m a t 82 MeV a s d e s c r i b e d i n t h e t e x t . The l i n e s marked a r e : t o p (open c i r c l e s ) 299,340,588,712, and 812 keV; second from t o p ( c l o s e d c i r c l e s ) 349,357,363,418,747, and 910 keV, open c i r c l e s a s above; t h i r d from t o p ( l i n e s ) 195,344,403, 428,693,758, and 800; bottom (checks) 21 1 and 737.

4

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801004

C10-40 JOURNAL DE PHYSIQUE

p a r a m e t e r and enhance o u r o b s e r v a t i o n o f h i g h m u l t i p l i c i t y e v e n t s . Gamma-ray c a s c a d e s were i d e n t i f i e d from e x c i t a t i o n f u n c t i o n s and c r o s s bombardments o f 1 2 ~ + 1 4 4 ~ m

, " B + ~ ~ ~ s ~ ,

and 7 2 ~ + 1 2 4 ~ e . Angular d i s t r i b u t i o n s , Y -Y c o i n c i d e n c e s , and h a l f 1 i v e s were measured u s i n g t h e 1 4 4 ~ m ( 1 2 ~ , 7 n ) 1 5 7 ~ r r e a c t i o n a t 8 2 MeV w i t h a 1 4 4 ~ m t a r g e t o f 7mg/cm2 a r e a 1 d e n s i t y backed by a t h i c k l a y e r o f 2 0 8 ~ b . A d d i t i o n a l p u l s e d beam measurements were t a k e n a t 6 8 and 75 MeV t o p r o m p t l y p o p u l a t e t h e l o w e r i s o m e r and t o examine out-of-beam Y-y c o i n c i d e n c e s .

The uppermost segment o f F i g u r e 1 shows a c o i n c i d e n c e s p e c t r u m g a t e d by t h e r e q u i r e m e n t t h a t y - r a y s come i n prompt c o i n c i d e n c e w i t h t h e 299, 740, 588, o r 812 keV t r a n s i t i o n s . I n a d d i t i o n t o t h e d e l a y e d c a s c a d e r e p o r t e d by t h e Lund-Copenhagen c o l l a b o r a t i o n 5

,

t h e r e is a weaker

s e t o f l i n e s v i s i b l e . These a r e summed a s g a t e s i n t h e n e x t segment. T h i s c a s c a d e ( f i l l e d c i r c l e s ) is i n c o i n c i d e n c e w i t h a l l o f t h e l o w e r c a s c a d e ( o p e n c i r c l e s ) e x c e p t t h e 712 keV l i n e , and is s e e n t o b y p a s s t h e i s o m e r . T r a n s i t i o n s f e e d i n g t h e l o w e r i s o m e r a r e shown i n t h e t h i r d segment o f F i g u r e 1 , which i s g a t e d by t h e r e q u i r e m e n t t h a t Y-rays p r e c e d e t h e 299, 512, 588, 340, and 712 keV l i n e s i n t i m e . Among t h e gamma-rays v i s i b l e h e r e a r e t h e 195, 344, 403, 4 2 8 , 697, 758, and 799 keV l i n e s which h a v e been i n d i c a t e d by v e r t i c a l marks. F i n a l l y , t r a n s i t i o n s f e e d i n g t h e u p p e r i s o m e r a r e s e e n i n t h e bottom segment o f F i g u r e 1 , which i s g a t e d by t h e r e q u i r e m e n t t h a t t h e gamma-rays p r e c e d e t h e b y p a s s c a s c a d e .

,

The 7 3 3 and 211 keV l i n e s a r e marked.

isomer

d e c a y c u r v e s o b t a i n e d u s i n g a beam p u l s e d a t l vs i n t e r v a l s a r e shown i n F i g u r e 2.

The 4 1 8 keV c u r v e ( t o p ) , t a k e n a t 75 MeV, was c h o s e n t o d i s p l a y t h e d e c a y o f t h e u p p e r i s o m e r .

Combining i t s v a l u e w i t h t h o s e f o r t h e o t h e r l i n e s o f t h e c a s c a d e , one o b t a i n s T 1 / 2 = 2 7 0 ~ 2 0 n s . The 712 keV l i n e was s t u d i e d a t 6 8 MeV, b o t h t o minimize f e e d i n g from t h e u p p e r i s o m e r ( e s t i m a t e d a t 5-10% f o r t h i s e n e r g y ) and t o e l i m i n a t e c o n t a m i n a t i o n from a n o t h e r d e l a y e d 712 keV l i n e i n 1 5 2 ~ o , which a p p e a r s a t h i g h e r e n e r g i e s . I n c l u d i n g a d e l a y e d f e e d i n g o f -8% g i v e s a l i f e t i m e o f 380ns w i t h a n e s t i m a t e d , e r r o r o f '30ns. Note t h a t t h o u g h t h i s t r a n s i t i o n a p p e a r s t o be t h e one most d i r e c t l y d e e x c i t i n g t h e i s o m e r , i t s t i l l h a s a n o n - n e g l i g i b l e prompt component.

F o r a c l e a n l i n e w i t h c a r e f u l background s u b t r a c t i o n , t h i s s h o u l d i n d i c a t e t h e e x i s t e n c e o f a n unobserved t r a n s i t i o n d e e x c i t i n g t h e i s o m e r .

-1

153ER 712KEV L I N E

2) B a c k g r o u n d - s u b t r a c t e d c u r v e s showing t h e d e c a y r a t e s o f t h e u p p e r i s o m e r ( t o p

-

4 1 8 keV l i n e ) and

.

l o w e r i s o m e r ( b o t t o m

-

^{712 keV} l i n e ) w i t h h a l f l i v e s T1 /2=270+20ns and T1 /2=380z30ns, r e s p e c t i v e l y . The t i m e s c a l e i s 1.422ns p e r i n d i c a t e d c h a n n e l .

RESULTS

The l e v e l scheme i s shown i n Figure 3.

Ordering w i t h i n t h e v a r i o u s cascades was based on s i n g l e s i n t e n s i t i e s , r a t i o s o f prompt and delayed components, and coincidence c r o s s o v e r s . N e v e r t h e l e s s , some a m b i g u i t i e s i n o r d e r i n g remain.

The 285-394-294 keV sequence i s o n l y t e n t a t i v e l y e s t a b l i s h & d , t h e o r d e r i n g of .the 195-344-482 keV sequence i s n o t known, and t h e l o c a t i o n o f t h e 363 keV l i n e i s based on an i n t e n s i t y obtained by s u b t r a c t i n g t h e estimated amount of contamination from 1 5 3 ~ 0 , where i t i s a l s o p r e s e n t . Likewise, t h e 767 keV t r a n s i t i o n ' s a n g u l a r d i s t r i b u t i o n , showing a s u b s t a n t i a l Legendre polynomial a 2 c o e f f i c i e n t , could p o s s i b l y be dominated by t h e j 5 ' ~ o quadrupole t r a n s i t i o n , though we do i n f a c t a s s i g n i t a s an E2 i n I 5 ' ~ r . The o t h e r u n c e r t a i n m u l t i p o l a r i t y assignment i s t h a t o f t h e 749 keV t r a n s i t i o n . Since i t a p p a r e n t l y h a s no prompt f e e d i n g , t h e n u c l e a r alignment is a t t e n u a t e d by a 270 nanosecond h a l f l i f e and t h e r e s u l t i n g a n g u l a r d i s t r i b u t i o n c o e f f i c i e n t i s a2=0.0220. 02. Our assignment o f d i p o l e c h a r a c t e r t o t h i s l i n e i s t h u s t e n t a t i v e .

A 7 keV t r a n s i t i o n d e e x c i t i n g t h e T,/2=380ns isomer i s deduced from energy sums of gamma r a y s between t h e isomers. These gamma r a y s a l l feed t h e J =27/2- l e v e l and d i s p l a y t h e l i f e t i m e of t h e upper isomer. The decay o f t h e upper isomer i s exceedingly fragmented, with many p a r a l l e l branches. Only t h e most c e r t a i n cascades a r e shown h e r e . Moderately i n t e n s e l i n e s n o t d e f i n i t e l y placed i n c l u d e t h e 403, 489, and 800 keV t r a n s i t i o n s .

The e x c i t a t i o n energy of t h e 27011s isomer, 5.183 MeV, corresponds well t o t h a t o b t a i n e d from sum s p e c t r o m e t e r work by Bj6rnholm e t a l . ?

somewhat s h o r t e r than t h e i r suggested v a l u e . And& e t a l s o r e p o r t an isomer with e x c i t a t i o n energy - >5 MeV, Tl,,2=350'50ns, and estimated s p i n of 37/2.

DISCUSSION

The assignment of s h e l l model c o n f i g u r a t i o n s t o l e v e l s i n 1 5 3 ~ r i s perhaps b e s t discussed i n t h e l i g h t o f o t h e r N=85 i s o t o n e s . A schematic diagram of three-neutron l e v e l s i s shown i n Figure 4. The ~ = = 1 1 / 2 - and 15/2- l e v e l s i n 1 5 7 ~ r a r e

taken from work of t h e Lund-Copenhagen

c o l l a b o r a t i o n . 6 Levels o f 51 Dy and 1 4 9 ~ d , though s t u d i e d by many g r o u p s , a r e h e r e taken from References 1 and 7 , r e s p e c t i v e l y . The l e v e l p a t t e r n s , i n c l u d i n g t h e v ( f 7 / 2 )7/2-, 3 /2-, 15/2- l e v e l s and t h e v(hg/2 )CN ( f7/2 2 )0+, 2+, 4+, 6+

c o n f i g u r a t i o n s with c h a r a c t e r i s t i c j2 s h e l l s p a c i n g , confarm c l o s e l y t o t h a t expected i n t h e

(5.2020.25 M ~ V )

,

though o u r measured h a l f l i f e i s 3 ) Level scheme f o r 1533,.

c10-42 JOURNAL DE PHYSIQUE

s h e l l model. Note t h e lowering of t h e hgI2 energy r e l a t i v e t o t h a t of f7/2 a s proton number i n c r e a s e s . This would be expected f o r i n c r e a s i n g o b l a t e n e s s .

s t a t e s with h i g h e r a n g u l a r momenta t h a n Jn=21/2- appear t o be formed by coupling t o t h e

=(h,, /22)10+ l e v e l , with t h e Jn=25/2-, 27/2-, 71/2- and 35/2- s t a t e s corresponding t o a ( l o r )

@ ~ ( f ~ ~The ~ )25/2- . s t a t e might t h e n be explained a s t h e 5/2 p r o j e c t i o n o f (fyl2') f a l l i n g 1 5 I m . l below t h e 7/2 p r o j e c t i o n a s discussed f o r

Since n e i t h e r t h e l e v e l s n o r t h e s p i n s o f t h e 5'=29/2-, 37/2-, o r 37/2- s t a t e s a r e f i r m l y e s t a b l i s h e d , t h e analogy with corresponding l e v e l s i n I5lDy implied i n Figure 4 i s p u r e l y c o n j e c t u r e . The n a t u r e of t h e bypass cascade and h i g h e r s t a t e s is not y e t understood.

The JT=O+, 2+, 4+, 6+ spacing e x h i b i t e d by t h e

4) Schematic o f N=85 s h e l l s t r u c t u r e . S p i n s a r e i n d i c a t e d by 25 and a l l d i s p l a y e d l e v e l s a r e thought t o b e o f , n e g a t i v e p a r i t y . The l 5 3 ~ r . J n = l 1/2- and 15/2' l e v e l 3 a r e taken from Ref. 6.

Each nucleus h a s v ( f ) l e v e l s o f f s e t 5 i g h t and

~ ( h ~ / , f , / ~ ~ ) l e v e l s oZ4Zet l e f t . Above J =21/2- a coupling t o t h e proton c o n f i g u r a t i o n = ( h l /22)10+

i s expected.

V(f7/2 2 , c o n f i g u r a t i o n s l e a v e s open f o r t h e o r e t i c a l i n v e s t i g a t i o n t h e q u a n t i t a t i v e r e l a t i o n s h i p o f c o l l e c t i v i t y and d e v i a t i o n from p e r f e c t j 2 - s h e l l spacing. Both i n t h e s e n u c l e i and i n N=86 isotones8 t h e n e u t r o n m u l t i p l e t s e x h i b i t g r e a t e r compression i n energy and more s h a r p l y d e c r e a s i n g spacing above t h e coupling t o

71 +

t h e J = l 0 proton c o n f i g u r a t i o n than below i t . For '?'E,, i t must a t any r a t e be s a i d t h a t s h e l l e f f e c t s e x e r t t h e dominating i n f l u e n c e . The complex feeding p a t t e r n , t h e e x i s t e n c e o f a long-lived isomer of s p i n ;41/2, and t h e i r r e g u l a r energy s p a c i n g s i n t h i s nucleus a l l f a v o r t h e a l i g n e d s i n g l e - p a r t i c l e p i c t u r e

.

ACKNOWLEDGEMENTS

The a u t h o r s wish t o acknowledge p a r a l l e l s t u d i e s o f t h i s nucleus by t h e Lund/copenhagen c o l l a b o r a t i o n and a r e e s p e c i a l l y g r a t e f u l f o r d i s c u s s i o n s with D r . G.B. Hagemann prompting us t o improve our l i f e t i m e measurement f o r t h e lower isomer.

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D. J.C.M. Hageman, M. J . A . de Voight, and J.F.W. Jansen, Phys. L e t t . (1979) 301.

S. Bjbrnholm, J . Borggreen, 0. C h r i s t e n s e n , A. DelZoppo, B. Herskind, J . Pedersen, and G. S l e t t e n , Proc. Symp. on High-Spin Phenomena i n N u c l e i , Argonne

,

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S. Andre, J. Genevey, A. Gizon, J. Gizon, .J. J a s t r z e b s k i , 'J. a u k a s i a k , M. Moszynsk,i, and Z. P r e i b i s z , C o n t r i b u t i o n t o t h i s Conference.

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J. Lyttkens, H. Ryde, S. Strijmberg,

G.B. Hagemann, B. Herskind, and D.L. H i l l i s , Proc

.

^{I n t}

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.

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*

, Summer v i s i t o r from ORNL,

Oak

Ridqe, Tenessee

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