Structure Transition in Heavy Y Isotopes - American Chemical Society

A good example is §^50» the isotone of 9 8 S r and 1 0 0 Zr, where an ... shell closures at Ζ = 40 (or 38) and Ν = 56 which is considerable in 9 6...
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30 Structure Transition in Heavy Y Isotopes 1

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G. Lhersonneau, Richard A. Meyer , K. Sistemich, H. P. Kohl , H. Lawin , G. Menzen , H. Ohm , T. Seo, and H. Weiler 1

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Institut für Kernphysik, Kernforschungsanlage Jülich, D-5170 Jülich, Federal Republic of Germany Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550 Reactor Research Institute, Kyoto University, Osaka, Japan

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The structures of the neutron-rich isotopes Y, Y and Y reflect with special clearness the rapid change of the nuclear shape at neutron number 60. The discovery of a new isomer in Y has provided evidence for the shell-model character of this nucleus even at high excitation energies while Y shows the properties of a symmetric rotor already in the ground state. The level pattern of the intermediate isotope Y indicates shape coexistence. 97

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The understanding o f the rapid s t r u c t u r e change o f t h e n e u t r o n - r i c h nuclei with A ~ 100 i s a f a s c i n a t i n g t o p i c f o r f a r - o f f - s t a b i l i t y s t u d i e s . A sudden t r a n s i t i o n from spherical t o deformed nuclear shapes takes place when the neutron number raises from 58 t o 60. This i s e s p e c i a l l y so i n the Sr and Zr isotopes where the energies o f the f i r s t excited 2 l e v e l s decrease by more than a f a c t o r of 5 between ^ S r g and §§ST5Q and between and 40 60 ° deformed i n t h e i r ground s t a t e s , but probably have c o e x i s t i n g spherical shapes at low e x c i t a t i o n energies. This f a c t and the r e s u l t s o f several experimental and t h e o r e t i c a l studies suggest t h a t the nuclei around A = 100 change t h e i r shapes r a p i d l y but t h a t they have complex p o t e n t i a l energy surfaces. In p a r t i c u l a r , these nuclei are supposed t o be s o f t w i t h respect t o γ deformations. However, r e ­ cent i n v e s t i g a t i o n s on odd-mass nuclei revealed properties o f c l a s s i c a l symmetric r o t o r s . A good example i s §^50» the isotone o f S r and Zr, where an extended ground-state band and several side bands have been found [PFE81, M0N82, W0H83, PET85, MEY85]. Now, new information on Y has been obtained at the f i s s i o n product separator JOSEF [LAW76] which indicates t h a t t h i s nucleus has shell model character even at high e x c i t a t i o n energies. Thus, no signs o f a p a r t i c u l a r softness are observed on e i t h e r side of the s t r u c t u r e t r a n s i t i o n i n t h e Y isotopes. Shape coexistence may e x i s t only i n the intermediate nucleus §§ 59 r o t a t i o n a l properties has been known f o r long t o e x i s t [GRU72, SIS76] based on an excited state o f 495 keV. +

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0097-6156/ 86/ 0324-0202S06.00/ 0 © 1986 American Chemical Society

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

1 0 0

30.

Structure Transition in Heavy Y Isotopes

LHERSONNEAU ET AL.

203

A new i s o m e r i n A level

scheme o f

been e s t a b l i s h e d cently, ered to

Y which contains

9 7

an i s o m e r i c

state with a h a l f - l i f e

9

state.

2

transition

rate

The the

The i s o m e r l i e s

of

about

decay

isomeric

quasiparticle basically

t h e most

with

spin

proposed

all

and

The d e p o p u l a t i o n

In t h e lower

b u t one member Similar

nuclei

at

core-coupled

shell (or

9

that

closures ^Sr).

tation

in

Shape c o e x i s t e n c e

in

The i s o t o p e with

half-lives

levels 8 \is

with

In studies

into a d y 5

2

and 0

well

with

s t a t e s o f t h e cores

+

exist

candidates

for

multiplet.

+

have

is

been

Nb

9 3

of

observed

[MEY77] w i t h

9 7

Y

in less

neutron-rich

two v a l e n c e

neutrons

o r b i t a l . Analoguously,

2

reflect

closures

the influence

i s considerable

are important

it

of the subin

even a t h i g h

i s o t o p e i s n e x t n e i g h b o u r t o deformed

9 6

Zr

exci­

nuclei.

Y remarkable

nucleus.

Several

3 ns and 8 \is have been o b s e r v e d

between

character

a t 495 k e V . I t

and v e r y

had

interest

little

isomeric

discovered

more

since

rotational

bands

states

i n i t . A band o f

staggering

been

F i g . 1 i s shown t h e p a r t

in

the study

are given of

precision

transitions

of

t h e decay

i s based on t h e

t h a n a decade ago have been

of the level of

scheme o f

t h e \is i s o m e r s

the γ which

inside

transitions allows

t h e band

band)

tions,

see F i g . 1 . A d d i t i o n a l

and Κ r a y s α

t o deduce (which

and t h e c o n v e r s i o n

have

observed

the mixing

coefficients

γ lines

for

ratios several

on t h e l i f e

of the

β"

decay

relative

determined

with

δ f o r t h e ΔΙ = 1

behaviour

have been o b s e r v e d

170 and 495 keV, and new i n f o r m a t i o n

Y which i s ob­

work

been

show t h e t y p i c a l

9 8

(results

i n [ S I S 7 6 , B E C 8 3 ] ) . Compared t o e a r l i e r

tional at

with three

odd-mass n u c l e i w i t h Ν > 6 0 .

intensities higher

+

o f t h e scheme t h e r e 2

states

Y i s a very

[GRU72] and has r e g a i n e d i n several served

part

+

Q

rotational

isomer

a

o f l e v e l s where t h e

7 9 2 , 912 and 990 keV a g r e e +

these s h e l l

9 8

9 8

is

neutron p a r t i c l e

and one p r o t o n i n t h e g y

Y , although t h i s

be u n d e r s t o o d

The i s o m e r

Ζ = 40 ( o r 38) and Ν = 56 w h i c h

Apparently, 9 7

Its

for a

2 7 / 2 " and t h e 162 keV t r a n s i t i o n

o f a hn/2

of

t h e new l e v e l s

at

1.00(19).

characteristic

can b e s t

Fig. 1.

of the 6 , 4 , 2

A ~ 1 0 0 . An example

concluded

is

o f the two-neutron c o r e . Thus, t h e energies

of the g g / 2 ®

beyond t h e Ν = 50 s h e l l is

S r . Re­

with higher spins i n ­

proceeds t h r o u g h a s e r i e s

intense γ t r a n s i t i o n s

Zr.

9 8

in

and p a r i t y

average energy d i f f e r e n c e s

Sr

9 7

Y has been d i s c o v ­

of E 3 > =

units

and i t s d e p o p u l a t i o n are

9 9 / 2 Photon i s c o u p l e d t o s t a t e s the

particle

consists of the conversion

neutron hole.

of

[BL085].

which

state

9 7

a t 3523 keV and i s d e p o p u l a t e d t h r o u g h a

2 single

state

configurations

s p i n s up t o 9 / 2 had

o f 144 ms i n

o f 162 keV w i t h a p r o b a b l e m u l t i p o l a r i t y

particle-to-hole

9 6

with

o f t h e β " decay

[LHE85] w h i c h decays t h r o u g h a sequence o f l e v e l s

the g /

transition

of

levels

[M0N76, PFE81] f r o m t h e s t u d y

of

for a

rota­

the γ t r a n s i ­

between t h e l e v e l s

times

of

a c h i e v e d f r o m γ - γ - t measurements.

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

l e v e l s was

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

of the

K forbiddenness

degree o f

in

/ 2

u*

9 7

2

Y

are populated

schemes o f t h e

π p 1/

β-decay

2

Tt g 9 / ® 2*

2

1

π g 9/ ®

2

ν

97 39'58

[MEY85].

[ R 0 E 7 8 ] . h = l o g ( H ) / n , where H i s

from

are taken

+

2

2

πς9/ ® v(h11/2.g7/2) π g 9/ β v(d5/ - g7 )

level

2

(21/ )

(27/f)

Ut. ms

(the hatched l e v e l s

1: Those p a r t s

isomers

Fig.

1 *.27.9 1336.3 1 319.6

3 3 6 1 .1

3523.3

=

283.8

«2.2

(9/2Ί

« M .

706.,''M.

975

the hindrance

of

the t r a n s i t i o n s

and η =

/ 2

=8.6us 2

2

-

K

f

- λ

the

coefficients

of

5

-TT.U22/]

]v[^113/ ][40^2]

v

99 39'60

i n t h e decay

5

f ;/2 tx[^22

conversion

Ν ~ 60, which are observed

0.83ns

,('321

['7/2*1

i n t h e β" d e c a y ) . The t h e o r e t i c a l

at

2

tl,

200 keV energy

Y isotones

100 Transition

v

98 39'59

Η

> 00

30.

Structure Transition in Heavy Y Isotopes

LHERSONNEAU ET AL. A reasonable

obtained with

fit

t o the energies

the rotational

the parameters

Κ = 2, E

A considerably

better

Ε = E

Although ed, of

o f t h e band

+A [ l ( I + l ) - K j

can be

+ Β [I(1+1)-K ]

2

Q

2

Q

the value

however,

possible

[PEK84] w i t h

o f A = 1 7 . 2 keV i s s m a l l

o f about

the

formula

90 % o f t h e r i g i d

rotor

( i t corresponds

value)

it

t o a mo­

i s not unexpect­

s i n c e a v a l u e o f A = 1 8 . 0 keV has been deduced f o r t h e ground s t a t e t h e odd-mass

neighbour

with the classical would

result

A = 9 9 . I f Κ = 1 o r Κ = 3 were used

rotational

formula then values

which are not compatible w i t h

The the

parity

with

ground

its

own

state

of

ground

of

decay

t h e members 9 8

into

[BEC83,SIS76]. tions

of

the available Y

o f t h e band

information.

since sight

state

of

indicate

9 8

the conversion

1 2 1 , 2 0 4 , 51 and 119 keV w h i c h

state

from

multipolarities

connect

nuclei unambi­

are assigned

+

into

are

this

probably

level

to

and

allowed

of the γ t r a n s i ­

t h e band

o f Ml a n d / o r

other Y.

head w i t h

the

E2 a n d , h e n c e , no p a r i t y

parity.

But i t c a n n o t be r u l e d o u t t h a t t h e 121 keV t r a n s i t i o n has a m u l t i p o ­ o f E l w i t h a s m a l l a d m i x t u r e o f M2 i n s t e a d o f M1/E2. A m i x i n g p a r a 2 9

larity meter

of δ

= 4 · 1 0 " ^ would a c c o u n t

transition

factors the

Sr

1

coefficients

c h a n g e . Thus t h e band head s h o u l d have p o s i t i v e

this

Zr

9 8

9 8

be d e t e r m i n e d

and p a r i t y

t h e β" decays

t h e ground

At f i r s t

cannot

Spin

band

for the f i t

o f A ~ 27 keV and ~ 12 keV

t h e knowledge a b o u t

i n t h i s mass r e g i o n . Hence, Κ = 2 i s proposed f o r t h e band i n guously

if

2

+ al.

2

ment o f i n e r t i a

o f t h e members

E=E

= 4 5 5 . 3 k e V , A = 1 7 . 2 keV and Β = - 2 2 eV a r e u s e d .

fit is,

+ A [l(I+l)-K ]

Q

formula

205

of

f o r both

and f o r t h e h a l f - l i f e

4.6 · 1 0

121 keV l i n e

the conversion

o f t h e 495 keV l e v e l

and 3 . 3 f o r t h e E l and M2 f r a c t i o n ,

7

has t h i s

character,

then the p a r i t y

coefficient (with

of

hindrance

respectively).

If

o f t h e band members

is

negative. Calculations nuclei ties,

namely

both

have

this

intruder

9 7

Sr

and

of the excitation

i n t h e A = 100 r e g i o n

and

{π[422 5 / 2 ] v [ 4 0 4

the

9 9

same

Zr,

see b e l o w .

the neutron

bital

f o r the 2

driving

neutron

configuration

+

alternative

proton-neutron While

band w h i c h

there

is

9/2]}2

+

o f t h e band heads o f odd-odd f o r both

pari­

and { π [ 3 0 3 5 / 2 ] v [ 4 0 4 9 / 2 ] } 2 "

which

configuration.

candidates

There

is

evidence

[MEY84]

causes a l s o i s o m e r i s m i n t h e odd-mass

Another

configuration

energies

[H0F84, MEY84] o f f e r

fact

of interest

originate with

from

i s , that

the ggy

the p o s s i b i l i t y

2

that

isotones

both the proton

single

particle

or­

of a strong deformation

-

interaction. little

doubt

about

the rotational

i s based upon t h e 495 keV s t a t e ,

the nature

properties

of the

of the levels

below QO

the is

band head and o f t h o s e w h i c h a r e o n l y not c l e a r .

able

that

though

most

These l e v e l s

populated

do n o t show a membership t o bands and i t

o f them a r e n o t o f r o t a t i o n a l

t h e energy

of

i n t h e β" decay o f

119 keV o f t h e f i r s t

character.

excited

state

is

In p a r t i c u l a r , is similar

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

yo

Sr

prob­ al­

to the

206

NUCLEI OFF THE LINE OF STABILITY

energy

of

the

odd-mass exists

first

nuclei

in

Y.

9 8

e x c i t e d members o f

it If

is

so t h e n

l y i n g members was f e d to

choose

strong

improbable in

a convenient

hindrance

of

it

rotational

that

would

an

be a s t o n i s h i n g

t h e decay o f t h e

value

of

more t h a n

a t 495 keV i n t o t h e ground

Κ for

10

bands o f t h e

unperturbed

for

1 0

that

state

none o f

i s o m e r s . Moreover

such

neighbouring

ground it

the is

band higher

difficult

a band w h i c h w o u l d e x p l a i n

the

direct

decay

from the

the

band

head

state. QQ

The

ground

corresponding

and

levels

first

excited

in the

isotones

character of the p a r t i c l e - v i b r a t o r The

lowest

levels

similarity clei ond

of

of

the

these

9 7

are

9 9

may r a t h e r

Zr

for

due

depicted

be r e l a t e d

which a J-2

in

probabilities

in spite

possibly

*°Y

and

are

observed t r a n s i t i o n

states

Sr

of

fig.

a low

lying

J-l

[MEY84].

The

indicates

of the differences to

2.

to

and

c o u p l i n g t y p e has been proposed

isotones

have t h e same s t r u c t u r e excited

states

striking

that

both

nu­

i n e n e r g y . The s e c ­

intruder

configuration

f r o m a deformed s h a p e . The s y m m e t r i c

rotor

The l e v e l 9 9

Sr

[PFE81,PET85] and

which the

" γ

scheme o f

9 9

Y

via

has been s t u d i e d b o t h t h r o u g h t h e β" decay

an

is

directly

populated

most

extended

rotational

isomeric

in

fission

symmetric

properties rotor.

accordance

with

deformation the

bands

nuclei.

Thus, the

of ε can

The

of

be

9 9

Y

suggest

the

accounted

half-life

of

for

the

keV

has been f o u n d that

configurations Also

2142

with

in

high

of spin

nucleus

contains

an odd-A

nucleus

s i d e bands have been o b s e r v e d .

predictions

0.3.

at

[M0N82,MEY85]. T h i s

band w h i c h

a t A ~ 1 0 0 . In a d d i t i o n , s e v e r a l The

state

of

can the

mixing in

this

Nilsson ratios

the

nucleus

be a s s i g n e d model

δ for

classical

isomer

at

2142

on

three

to

is

for

classical

the

bands

A ~ 100

in

and

t h e ΔΙ = 1 members picture

keV

a

all

is

of

a of

rotational

obviously

due

to

Κ

forbiddenness. Conclusions^ The

available

knowledge

t o p e s o f Y shows t h a t t h e s e n u c l e i 9 7

Y

has

properties

nucléons

which

beyond t h e c o r e

three-quasiparticle

9 6

the

neighbouring

have v e r y d i f f e r e n t

are

basically

determined

Zr

(or

The e x i s t e n c e o f t h e i s o m e r

character

9

^Sr).

indicates

that

Y

most

rotational

probably

has c o e x i s t i n g

band on t h e

rather of vibrational 9 9

Y

is

characterized

the investigated

nuclear

495 keV l e v e l

through

there

s o f t n e s s a g a i n s t d e f o r m a t i o n even a t h i g h e x c i t a t i o n 9 8

neutron-rich

is

the

no

valence of

particular

energy.

shapes w i t h

and o t h e r

iso­

structures:

a well

developed

l e v e l s w h i c h seem t o

be

nature. through t h e occurence o f

rotational

bands a l l

r e g i o n o f e n e r g i e s up t o 2 MeV as a s y m m e t r i c

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

over

rotor.

30.

Structure Transition in Heavy Y Isotopes

LHERSONNEAU ET AL.

These

results

demonstrate

considerable study

of

details

change

the of

isotopes

the

be even more still

of

shape

rapid at

vestigations

of

the

of

only

nuclei

at

one

neutron

A ~ 100

odd n u e l e o n numbers can p r o v i d e

in

shapes

the

The t r a n s i t i o n

in

the

Sr and Zr c h a i n s where t h e

and where t h e s h e l l - m o d e l

high e x c i t a t i o n

energies

has n o t y e t

of

the

level

schemes

of

the

causes

and t h a t

insight

Y isotopes Ν = 60

character

into seems

Y isotopes

and

to

a

the the to

isotones

of the Ν =

been t e s t e d . F u r t h e r

a r e , h o w e v e r , needed i n o r d e r t o c o n f i r m i n d e t a i l

interpretation similarly

with

a difference

nature

transition.

than

have c o e x i s t i n g

58 i s o t o n e s

that

the

207

the see

in­

proposed whether

r a p i d s t r u c t u r e changes o c c u r i n t h e Rb and Nb i s o t o p e s a t Ν ~ 6 0 .

F i g . 2: The lowest l e v e l s of the isotones of Y . The v a l u e s o f 9 8

i 2 f e x c i t e d s t a t e s and o f