30 Structure Transition in Heavy Y Isotopes 1
2
1
1
1
1
G. Lhersonneau, Richard A. Meyer , K. Sistemich, H. P. Kohl , H. Lawin , G. Menzen , H. Ohm , T. Seo, and H. Weiler 1
3
1
1
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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97
98
99
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
99
98
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. +
jjjjz^
5
1
Z r
#
B o t n
9 8 s r
a
n
d
1 0
Z r
a
r
e
9 8
9 7
Y
w n e r e
a
D a n d
w
l
t
n
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
