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IRREGULARITIES IN THE YRAST LINE OF 156Er
T. Byrski, F. Beck, C. Gehringer, J. Merdinger, Y. Schutz, J.P. Vivien
To cite this version:
JOURNAL DE PHYSIQUE CoZZoque Cl 0, suppl6ment au n012, Tome 41, de'cembre 1980, page C10-98
IRREGULARITIES IN THE YRAST LINE
OF
l s 6 ~ rT. Byrski, F.A. Beck, C. Gehringer, J.C. Merdinger, Y. Schutz and J.P. Vivien.
Centre de Recherches NucZe'aires
e t
Universitg Louis Pastern, F-67037 Strasbourg Cedex, France.Abstract.- The variation of the moment of inertia vs the rotational frequency has been investigated at high spin values using the 1 4 1 p r ( 1 9 ~ , 4ny) 1 5 6 ~ r reaction. In addition to the backbending at I = 1 2 3 , a second one has been found at I = 26 h. Calculations performed by the Warsaw group with the HFB cranking model suggest a probable neutron effect.
As p o i n t e d o u t i n several i n v i t e d t a l k s 1,2) a t t h i s Conference, band c r o s s i n g phenomena have given r i s e t o a l a r g e amount o f experimental and t h e o p e t i c a l work i n t h e l a s t few years. Thus t h e f i r s t backbending near s p i n value 12 has been e x t e n s i v e l y s t u d i e d i n t h e r a r e e a r t h r e g i o n and can be understood as a r e s u l t o f weak c o u p l i n g between two i n t e r a c t i n g bands, the ground s t a t e band c r o s s i n g g e n e r a l l y w i t h an a1
i
gned two quasi p a r t i c l e neutron band. Several t h e o r e t i c a l c a l c u l a t i o n s have p r e d i c t e d t h e existence o f a second i r r e g u l a r i t y i n the y r a s t band near s p i n value 26, which can be r e l a t e d t o r o t a t i o n a l alignment, C o r i o l i s a n t i p a r i n g o r gapless s u p e r c o n d u c t i v i t y . Experimentally a second anomaly has been observed as an upbending i n t h e y r a s t l i n e o f 1 5 8 ~ r by the Berkeley group 3), and we have observed another more pronounced one4) i n 160yb. The observation o fa
second d i s c o n t i n u i t y f o r these N = 90 n u c l e i f i t s w e l l the p i c t u r e o f Qn o s c i l l a t o r y behavior o f t h e c o u p l i n g between the two i n t e r a c t i n g bands as suggested i n Ref.5 and discussed i n one o f t h e i n v i t e d papers2) o f t h i s Conference.I n
t h e present work our purpose was t o searchf o r t h e existence o r non existence o f a second backbending w i t h r e s p e c t t o t h e v a r i a t i o n i n neu
-
t r o n number, and f o r t h i s we have chosen t o study t h e 1 5 6 ~ r nucleus.The decay scheme o f the 1 5 6 ~ r nucleus had been p r e v i o u s l y e s t a b l i s h e d 6, up t o I' = 24' ; t h e onset o f t h e f i r s t backbending i s a t I ~ = 12'. To
produce t h i s nucleus, t h e r e a c t i o n s 1 2 8 ~ e ( 3 2 ~ , 4 n f 5 6 ~ r and 1 4 1 ~ r ( 1 9 ~ , 4 n ) 1 5 6 ~ r have been i n v e s t i g a t e d a t
several bombarding energies. We have p r e f e r r e d t o use the second one f o r which t h e r a t i o o f t h e 4n channel t o t h e 3n o r 5n channel i s more pronounced.
A f l u o r i n e bombarding energy o f 95 MeV has been choosen f o r t h e experiments. The experimental set-up f o r t h e y-y coincidences c o n s i s t e d o f an a r r a y o f t h r e e Ge(Li) d e t e c t o r s l o c a t e d a t
+
90°,-
90' and 0" w i t h r e s p e c t t o the beam d i r e c t i o n . T h i s a r r a y p e r m i t s an increase of t h e counting r a t e by t h e s i - multaneous measurement of t h r e e p a i r s o f coincidences and a l s o t o measure DC0 r a t i o s . These a r e e s s e n t i a l f o r t h e d e t e r m i n a t i o n of t h e m u l t i p o l e o r d e r o f t h e h i g h s p i n decay t r a n s i t i o n s s i n c e r e l i a b l e angular d i s t r i b u t i o n s a r e n o t f e a s i b l e f o r these weak y - r a y l i n e s.
I n F i g . i are r e p o r t e d t h e s i n g l e s s p e c t r a
z 4 S LOO U
.
V) C 0 C H A N N E LFig.1: S i n g l e s spectra obtained w i t h gates s e t on y-rays i s s u i n g from t h e h i g h - s p i n s t a t e s o f t h e y r a s t band
obtained w i t h gates s e t on t h e y-rays a r i s i n g i n t h e decay o f t h e 14', 18', 20+, 22' and 24' s t a t e s , as w e l l as
a
sum s p e c t r a w i t h gates s e t on y-rays decaying t h e 20', 22' and 24' s t a t e s . The spectrum obtained w i t h t h e t r a n s i t i o n decaying t h e 16' s t a t e has n o t been r e p o r t e d s i n c e t h i s t r a n s i t i o n has t h e same energy as t h e 6'-
4' t r a n s i t i o n . Beside t h e y-ray l i n e s i s s u i n g a l l from t h e e x c i t e d s t a t e s o f t h e y r a s t band up t o I" = 24'; t h e r e appears i n each spectrum a l i n e o f 766 keV which i s d i f f e r e n t from the 765 keV l i n e known t o belong t o t h e odd n e g a t i v e p a r i t y band6), s i n c e t h e s p e c t r a do n o t show t h e presence o f any o t h e r t r a n s i t i o n o f .this band. A 765 keV y-ray l i n e could a l s o be populated by t h e 1 4 1 ~ r ( 1 9 ~ , 3 n ) 1 5 7 ~ r re a c t i o n o r by the r a d i o a c t i v i t y 1 5 6 ~ r +156~o +156~y b u t these possi-
b i l i t i e s had t o be excluded s i n c e the decay t r a n s i - t i o n o f t h e f i r s t e x c i t e d s t a t e from these n u c l e i
(266 keV i n both cases) i s n o t seen i n these spectra. Therefore t h e 766 keV y - r a y l i n e i s f n t e r - preted, on t h e b a s i s o f t h e r e l a t i v e i n t e n s i t y and the DC0 r a t i o s , as t h e decay o f t h e I" = 26' S t a t e of 1 5 6 ~ r . This would be evidence f o r the existence o f a s t r o n g backbending i n t h e y r a s t band o f t h i s nucleus
I n Fig.2 are shown t h e spectra obtained i n coincidence w i t h t h e y-rays decaying t h e odd nega- t i v e p a r i t y band. As a c o n f i r m a t i o n o f our assign- ment, we can see t h a t t h e decay y-rays from t h e
C H A N N E L
c10- 100 JOURNAL DE PHYSIQUE
12' and h i g h e r s t a t e s appear o n l y i n t h e s p e c t r a obtained i n coincidence w i t h t h e 766 keV y - l i n e . I n t h i s spectrum we can see some o t h e r l i n e s , due t o coincidences w i t h t h e 765 keV t r a n s i t i o n i n 1 5 7 ~ r , b u t a l s o a 766 keV y - l i n e . Windows placed r e s p e c t i v e l y on t h e l e f t - h a n d s i d e (765 keV) and on t h e r i g h t h a n d s i d e (766 keV) o f t h e gate peak show a f e e d i n g p a t t e r n o f t h i s l i n e s i m i l a r t o t h e one o f t h e h i g h s p i n s t a t e s o f t h e y r a s t band. Accordingly and a l s o because o f t h e energy and o f the r e l a t i v e i n t e n s i t i e s observed, one can assume t h e e x i s t e n c e o f two c o i n c i d e n t 766 keV y-ray l i n e s i n t h e y r a s t band. So a second 766 keV l i n e i s t e n t a t i v e l y considered as t h e decay y-ray o f t h e 28' s t a t e
.
The Warsaw group7) has done t h e same k i n d o f c a l c u l a t i o n w i t h t h e Hartree-Fock-Bogolyubov (HFB) c r a n k i n g method f o r
1 5 6 ~ r
as t h e y have p r e v i o u s l y s u c c e s s f u l l y done f o r t h e i n t e r p r e t a t i o n o f o u r experimental r e s u l t s i n 160yb. L e t us f i r s t compa- r e t h e r e s u l t s o b t a i n e d i n 160yb and 1 5 6 ~ r w i t h t h e HFB cranking method performed a t t h e S t r u t i n s k y e q u i l i b r i u m deformation (B2 = 0.23, ,B4 = 0.02). A1 though t h e f i r s t backbending e f f e c t appears f o r both n u c l e i as t h e r e s u l t o f a c r o s s i n g o f t h e two quasi p a r t i c l e neutron band w i t h t h e ground s t a t e band, i n c o n t r a s t t o 160yb case f o r 1 5 6 ~ r t h e two quasi p a r t i c l e p r o t o n band does n o t cross w i t h t h e y r a s t band f o r t h i s deformation. However a backben- d i n g e f f e c t due t o t h e c r o s s i n g between t h e y r a s t band and t h e two quasi p r o t o n band can be obtained by l o w e r i n g the deformation from B2=0.23 t o B2=0.17. But i n t h i s case t h e backbending would occur a t a rotationa,l freqiency t o o low as compared t o t h e experiniental value, and the values o f the moment o f i n e r t i a thus c a l c u l a t e d remain 15-20%
t o d tiiqh,. Study o f t h e e f f e c t of B4(hexadecapole) deformation does n o t a1 t e r c a l c u l a t e d r e s u l t s s i g n i f i c a n t l y .A l t e r n a t i v e explanations suggested by t h e HFB c a l c u l a t i o n s performed a t the S t r u t i n s k y e q u i l i b r i u m deformation a r e :
-
A c r o s s i n g o f t h e y r a s t band w i t h one o f t h e two 4qp neutron bands which approach t h e y r a s t band r e l a t i v e l y c l o s e l y a t t h e backbending s p i n value-
According t o t h e c a l c u l a t i o n s t h e neutron p a i r i n g collapses around I = 24 h and t h e corresponding y r a s t l i n e bends down, producing a cusp i n t h e s p i n r e g i o n i n which the y r a s t band and the f o u r quasi p a r t i c l e band would otherwise have crossed.Beside t h e experimental decay scheme o f t h e 1 5 6 ~ r y r a s t band r e p o r t e d i n Fig.3, we have d i s p l a - yed t h e v a r i a t i o n o f t h e moment o f i n e r t i a versus
t h e r o t a t i o n a l frequency f o r t h e h i g h e r s p i n value p a r t o f the decay scheme. The c a l c u l a t e d moment o f i n e r t i a a t t h e backbending p o i n t correspond t o t h e p a i r i n g c o l l a p s e hypothesis b u t t h e general t r e n d I I I I I I I - ( 1 1 - ~ ( 1 - 2 1 ] / 2
-
EXPT
o
- H F B C
'Fig.3. Decay scheme o f t h e y r a s t band o f 156~r and ,comparison o f t h e HFB c a l c u l a t i o n s w i t h t h e
would be very s i m i l a r i f we had considered the rance o f p r o t o n o r neutron e f f e c t s . But the good case o f t h e c r o s s i n g o f the y r a s t band w i t h a agreement o f the HFB c a l c u l a t i o n s w i t h the f o u r quasi neutron band., experimental r e s u l t s shown i n F i g u r e 3, suggest
I n conclusion the second backbending e f f e c t more l i keley a neutron e f f e c t . observed i n 1 5 6 ~ r can be i n t e r p r e t e d as an occu-
REFERENCES
1) R .Bengtsson, I n v i t e d t a l k a t t h i s Conference. 5) R.Bengtsson,
I
.Hamarnoto and B .R.Mottelson, 2 ) A.Faessler, I n v i t e d t a l k a t t h i s Conference. Phys . L e t t .738
(1978) 259.3 ) I .Y .Lee, M.M.Aleonard, M.A.Deleplanque, 6) A.W.Sunyar, E.Der Mateosian, D.C.Kistner,
Y.El-Masri, J.O.Newton, R.S.Simon, R.M. A.Johnson, A.H.Lumpkin and P.Thieberger, Diamond and F.S.Stephens, Phys.Rev.Lett.38 Phys . L e t t
.S
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