GAMMA-RAY TRANSITION ENERGY CORRELATIONS IN 156Er AND 160Yb

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GAMMA-RAY TRANSITION ENERGY

CORRELATIONS IN 156Er AND 160Yb

J. Vivien, Y. Schutz, F. Beck, T. Byrski, C. Gehringer, J.C. Merdinger

To cite this version:

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JOURNAL DE PHYSIQUE CoZloque C I O , suppZgment au n012, Tome 41, de'cembre 1980, page

C10-135

GAMMA-RAY TRANSITION ENERGY CORRELATIONS I N

l S 6 ~ r

AND

160yb

J.P. Vivien, Y . Schutz, F . A . Beck, T . Byrski, C . Gehringer and J . C . Merdinger.

Centre de Recherches NucZe'aires 67037 Strasbourg Cedez, France.

Extensive s t u d i e s l y 2 of the rotational nucleus 160yb

have shown the presence of two backbendings in the

y r a s t l i n e , as well a s the existence of i r r e g u l a r i -

t i e s in the side bands. The interpretation of t h e

level scheme i n terms of bandcrossing2 has been very

3 successful. The nucleus

1 5 6 ~ r ,

whose y r a s t band i s

very similar t o t h a t of 160yb up t o spin 24-5, has

4 been shown t o exhibit a sharp second backbending.

This second i r r e g u l a r i t y has been interpreted a s

being most l i k e l y due t o a neutrons e f f e c t . In

order t o complement t h e studies of the d i s c r e t e l i r e s ,

we looked a t t h e y-continuum via t h e

Y - Y

energy cor-

5 r e l a t i o n technique described by

B.

Herskind

.

The two nuclei 160yb and 1 5 6 ~ r

were respectively

produced

i n the heavy ion fusion reactions

14'sm

+

160 a t Einc

=

105 MeV and l 4 ' p r

+

"F

a t

Einc

=

100 MeV. In both reactions the 4n channel i s

prominent

w i t h

84

%

of the t o t a l cross section f o r

the production of 160yb and

-

70

%

f O r 1 5 6 ~ r

;

the

percentage f o r the

3

and 5n e x i t channels are respec-

t i v e l y

=

16

%

f o r l6'yb and 15

%

f o r

1 5 5 y 1 5 7 ~ r .

The

maximum angular momentum brought i n t o the compound

nucleus calculated according the formula deduced by

6 Baas f o r a liquid drop i s 5 2 % and 51

f i

respectively.

To record t h e

y - y

coincidence matrix, a s e t of three

Ge(Li) detectors was used. The detectors were care-

f u l l y chosen with nearly t h e same response function

and t h e i r gain properly matched t o be able t o sum

a l l the three possible +-y combinations. In the

resulting coincidence spectra, on1 y a small percen-

tage of the t o t a l number of events i s due t o photo-

peak

X

photopeak coincidences, t h e r e s t being due t o

Compton scattered events. In order t o remove the

Compton background from the spectra two methods can

be applied. One c o n s i s t s of unfolding the two dimen-

tional Ge(Li)

X

Ge(Li) spectra, a f t e r a scrupulous

study of the response function of each detector i n

s i t u . This procedure i s now operating but i s very

time consuming f o r t h e computer.

A

second method

5 calculates the deviation

aNi

from the average

coincident r a t e .

his

method does not yield the

absolute counts value, but enables the s t r u c t u r e of

t h e y -y coincidence f i e l d t o be reproduced q u i t e

e a s i l y . I t has been applied i n the present study.

In f i g . 1 and

2 ,

t o enhance the gross s t r u c t u r e

observed i n the two dimentional correlation spectra,

the energy resolution of the detectors has been

a r t i f i c i a l l y deteriorated from 2 keV t o 20 keV.

This procedure gives more evident spectra, but one

loses information f o r the f i n e structure. For t h a t

reason, t h e r e s u l t s obtained f o r d i f f e r e n t s l i c e s

perpendicular t o the 45O diagonal a r e taken from the

real correlation spectra.

The valley along the diagonal observed i n these spec-

t r a i s the signature of the rotational behaviour of

t h e nuclei. The width

W

of the valley i s d i r e c t l y

r e l a t e d t o the "collective" moment of i n e r t i a

3c,

t h a t i s t h e moment of i n e r t i a which corresponds t o

J

2

a rotational band, by the r e l a t i o n

W =

16

-h

/2?c.

F.S. stephens e t a17 have shown t h a t f o r intermedia-

t e nuclear deformation

(0-0.21

and f o r one o r a few

p a r t i c l e s i n a high

-

j

s h e l l , a coupling scheme

where

j

i s quantized along the rotation a x i s i s valid

(Rotational A1 ignment)

.

The following energy spec-

10

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JOURNAL DE PHYSIQUE

E

ll

5,- 5,

Fdwte

1 :

P o s X v e com&aAion npecttLurd

A f l ~ j

dot " " ~ b

and coWaponding

phoject.ionn penpendicdan. .to .the diagond. Fotr each n f i c e t h e

mean enengy v d u e

( E y l + Ey21

/2

-in

indicated.

% I

and

EUZ,

hs

and?iW6' ahe

Rhe

backbending dhequenciu i n t h e

yhdcl.t

band and

side ban&

h e , 5 p ~ c f i ~ & L j .

E

1,

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?i

2

trum i s then obtained E

=

e j a

+

-

( I - j a ) where ja represents t h e a l i g n e d angular momentum along t i l e r o t a t i o n a x i s and e j a i s t h e s i n g l e p a r t i c l e energy. The t r a n s i t i o n energy w i t h i n a band i s :

-

"

4 ( 1 - j a )

=

2

h w (1). From t h e c o r r e l a -

E~

-q

t'ion spectra one can deduce an average value o f

3c,

corresponding t o several bands w i t h d i f f e r e n t ja. Figures l and 2 d i s p l a y t h e bidimensional c o r r e l a - t e d spectra o b t a i n e d f o r 160yb and 1 5 6 ~ r , t o g e t h e r w i t h d i f f e r e n t s l i c e s perpendicular t o t h e 45' d i a g e n a l . The v a l l e y can be f o l l o w e d up t o 870 keV f o r 160yb and t o 920 keV f o r 1 5 6 ~ r . For low energy t r a n s i t i o n s , up t o

-

500 keV, the observed narrowing o f t h e v a l l e y f o r 160yb w i t h i n c r e a s i n g energy r e f l e c t s t h e behaviour o f t h e ground band moment o f i n e r t i a below t h e f i r s t backbending. Beyond t h e f i r s t backbending up t o 750 keV, t h e w i d t h o f t h e v a l l e y remairsalmost constant. The r i d g e which i s narrow below t h e f i r s t backbending becomes wider, i n d i c a t i n g a spread i n t h e moment o f i n e r t i a . One can then r e - present t h e y r a s t l i k e decay by several c o l l e c t i v e bands w i t h d i f f e r e n t moments o f i n e r t i a correspon- d i n g t o v a r i o u s degrees o f alignment o f t h e s i n g l e p a r t i c l e s . The d i s c o n t i n u i t i e s observed i n t h e d i f - f e r e n t bands o f t h e d i s c r e t e spectra, shows up i n the y-y c o r r e l a t i o n spectra as a f i l l i n g o f t h e v a l l e y . I n b o t h n u c l e i , t h e f i r s t backbending i s v e r y sharp and t h e r e f o r e does n o t cause any f i l l i n g a t t h e proper frequency. I n s t e a d one observes a f i l l in r l o f t h e v a l l e y i n 160yb a t a p o i n t corresponding t o t h e t r a n s i t i o n s : 589 keV (8' + 6') and a 586 keV (12' + 10') and i n 1 5 6 ~ r t o t h e t r a n s i t i o n s :

A t % W = 0.40 MeV i n 160yb and 0.38 MeV i n 5 6 ~ r t h e v a l l e y i s t o t a l l y f i l l e d up. A t h i g h e r energy t h e v a l l e y appears again and vanishes around 870 keV and 020 keV r e s p e c t i v e l y . The f a c t t h a t these s t r u c t u r e s i n t h e v a l l e y a r e v e r y pronounced i s understood i n terms o f multi-band crossing, i . e . t h e f a c t t h a t many bands a r e expected t o experience crossings a t t h e same frequencies as t h e i r r e g u l a r i t i e s observed i n t h e d i s c r e t e spectra. I n o r d e r t o reproduce t h e e f f e c t o f a multi-band crossing, we have constructed a computer simulated y y spectrum assuming t h a t t h e d i f f e r e n t bands experience a c r o s s i n g around t h e same frequency

hw

= 0.39 MeV. I n t h i s s i m u l a t i o n we have used r e a l i s t i c d e t e c t o r response f u n c t i o n s t o c o n s t r u c t t h e y-y two dimensional coincidence spectrum. Then we have a p p l i e d t h e data r e d u c t i o n technique discussed before t o e x t r a c t t h e c o r r e s - ponding c o r r e l a t e d spectrum. F i g u r e 3 d i s p l a y s t h e c o r r e l a t i o n spectrum r e s u l t i n g from such a simulation f o r 160yb.

1000

682 keV ( 1 2' -+ 10') and 710 keV (20' -P 18') (see

E,,(KeV)

p o i n t noted a i n t h e f i g u r e s ) . I n 160yb, two s t r o n g

_{F i g m e}

₃ _:_ShimLLeated

_{pob.itive cohheRdtion}

bridges appear %U,') and correspond t o t h e

**_{b p ~ o t * w t ~}**

_ANij

_don

_I6'yb.

frequencies o f t h e d i s c o n t i n u i t i e s observed i n t h e

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c10-138 JOURNAL DE PHYSIQUE

shown i n a backbending p l o t j n t h e upper l e f t p a r t 2. L.L. Riedinger, 0. Andersen, S. Frauendorf,

o f f i g u r e 3. J.D. Garret, J.J.Gaardh&je, G.B. Hagemann,

The observed end p o i n t o f t t i e v a l l e y (Fig.l,2) i s M. Guttormsen, B. Herskind, Y.V.Makovetzky, P.O. r e l a t e d t o t h e energy o f t r a n s i t i o n s d e - e x c i t i n g Tjom and J.C. Waddington, Phys. Rev. L e t t .

3

t h e h i g h e s t s p i n c o l l e c t i v e s t a t e s f e d i n t h e (1980) 568.

r e a c t i o n . The corresponding moments o f i n e r t i a

3. A.W. Sunyar, E. de Mateossian, O.C. K i s t n e r , deduced from t h e width' a r e 136

*

18 ( M ~ v ) - ' f o r

A. Johnson, A.H. Lumpkin and P. Thieberger, 160yb and 111 22 ( M ~ v ) - , ? fo r 1 5 6 ~ r . The maximum

Phys. L e t t .

625

(1976) 283 angular momentum a t t h e Cop o f t h e y r a s t cascade i s

4. F.A. Beck, T. B y r s k i , C. Gehringer, J.C.Merdinger, evaluated t o be I

=

44 TI f o r b o t h n u c l e i . We assumed

Y .Schutz and J.P.Vivien, C o n t r i b u t i o n t o t h i s t h a t each neutron takes away 1 . 5 5 and t h a t 4 s t a -

Conference. t i s t i c y-rays d o - e x c i t e t h e n u c l e i towards t h e y r a s t

r e g i o n each t a k i n g away 0.5

3

8. Using r e l a t i o n l, 5. B. Herskind, C o n t r i b u t i o n t o t h i s conference.

t h e corresponding s i n g l e p a r t i c l e a l i g n e d angular 6. R. Baas, Phys. Rev. L e t t . ,

39

(1977) 265. momentum i s found t o be

=

1 5 -Fi f o r y t t e r b i u m and 7. F.S.Stephens, R.M. Diamond and S.G.Nilsson,

=

17 R f o r erbium. These values a r e i n good agree- Phys. L e t t . 44 B (1973) 429.

ment w i t h t h e a l i g n e d angular momentum deduced from 8. J.P. V i v i e n , Y. Schutz, F.A. Beck, T. Byrski, t h e d i s c r e t e spectra beyond t h e second backbending : C . Gehringer and J.C. Merdinger, Phys. L e t t . i t has been evaluated t o be 17 6 i n 1 5 6 ~ r and a t 85B (1979) 325.

l e a s t 1 5 6 i n 160yb. References

l .

F.A. Beck, E. Bozek, T. B y r s k i , C. Gehringer,