37 Shape Coexistence in
Au
185
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E. F. Zganjar , C. D. Papanicolopoulos, J. L. Wood , R. A. Braga , R. W. Fink , A. J. Larabee, M. Carpenter , D. Love , C. R. Bingham, L. L. Riedinger, and J. C. Waddington Downloaded via TUFTS UNIV on July 12, 2018 at 10:19:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70808 School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 School of Chemistry, Georgia Institute of Technology, Atlanta, GA 30332 Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996 Department of Physics, McMaster University, Hamilton, Ontario L85 4K1, Canada
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185
The β-decay of Hg -> Au has been studied following on-line mass separation at UNISOR/HHIRF. Transitions with strong EO components were observed feeding the h and h bands and are interpreted as resulting from the coupling of the h and h single-proton configurations in the Pt and Hg cores, respectively. 9/2
11/2
9/2
11/2
184
The f i r s t
186
i n d i c a t i o n o f shape c o e x i s t e n c e near Z = 8 2 , N=104 came f r o m
the observation
[B0N72] o f a l a r g e i n c r e a s e i n t h e mean-square c h a r g e
i n going from
Hg
1 8 7
pumping o f o n - l i n e
to
1 8 5
Hg.
s e p a r a t e d Hg i s o t o p e s a t t h e ISOLDE f a c i l i t y .
measurements [KUH77, B0N79] i n c l u d e d many i s o m e r i c ground s t a t e s o f 1 8 4 , 1 8 3 , 1 8 1 g H
state of
1 8 5
Hg
28 sec 1 3 / 2 Early
+
shows a l a r g e
interestingly,
e
in-beam
γ
-spectroscopy
on
1 8 4
188
184
clearly
as t h e
w h i l e t h e 55 sec 1 / 2 " g r o u n d
i n c r e a s e i n t h e mean-square c h a r g e r a d i u s , t h e
bands become deformed above t h e 2 + 188
Later
s t a t e s as w e l l
i s o m e r does n o t .
+
T a
radius
T h i s r e s u l t was o b t a i n e d [B0N76] by o p t i c a l
H g j
186
T a
^ 186
H g
a
n
d
H g and
state T a
1 8 6
Hg
showed t h a t t h e i r
[PR073, RUD76]. 184
H g
Studies o f the
decays a t UNISOR and ISOCELE
r e v e a l e d two c o e x i s t i n g bands w h i c h approach each o t h e r
d e c r e a s i n g mass [HAM75, B0N76, C0L76, C0L77, BER77, C 0 L 8 4 ] . work [MA 8 4 ] on energy r e l a t i v e
1 8 2
Hg
indicates
yrast
with
Recent
t h a t t h e deformed band c o n t i n u e s
in-beam
t o drop i n
t o t h e g r o u n d s t a t e b a n d , a l t h o u g h t h e r a t e o f approach
a p p e a r s t o have g r e a t l y
diminished.
These d a t a [HAM85] a r e shown i n f i g . 1 ,
0097-6156/86/0324-0245S06.00/ 0 © 1986 American Chemical Society
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
246
NUCLEI OFF THE LINE OF STABILITY
which i s presented here as the c l a s s i c example of a coexistence of levels b u i l t on d i f f e r e n t shapes. Shape coexistence appears to be a general of nuclei in the neutron d e f i c i e n t region near Z=82.
feature
Concurrently, but
ENERGY (MeV) , 0 12»
F i g . 1.
The energies of the
spherical and deformed states in even-even
1 8 2 _ 1 9 8
Hg.
Deformed
states f o r A > 188 have not been observed [MA 84, HAM85].
182
184
186
Î88
MASS
Î9Ô
Î92
194
Ϊ9ό
W~
NUMBER
independent of these developments, c o e x i s t i n g bands i n the odd-proton T i and Au isotopes were established by radioactive decay and in-beam reaction studies [HEY83]. For the even-even nuclei i n the region, one has the p o s s i b i l i t y t h a t a p a i r of p a r t i c l e s or holes can couple to quite d i f f e r e n t cores.
This has
been shown [DUP84] to occur even i n proton magic Pb isotopes, where excited θ£ deformed states are observed i n 192-198p^
w l
-
t n
a
continuous drop
i n energy such that the O2 level become the f i r s t excited state i n 192
pb
and 1 9 4
Pb.
Most t h e o r e t i c a l c a l c u l a t i o n s indicate [HAM85] t h a t the near-
spherical shape i s oblate and the well-deformed shape i s p r o l a t e , although t h i s i s not f i r m l y established experimentally.
For odd-A nuclei in the
region, i t then becomes possible to form m u l t i p l e shape c o e x i s t i n g configurations.
For example, i n the odd-mass Au isotopes the odd proton
p a r t i c l e can couple to the two configurations with d i f f e r e n t deformation i n the Pt cores, and the odd-proton hole to the two structures with d i f f e r e n t deformation i n the Hg cores. The energy level spacings of the 0 - 2 - 4 +
+
+
members of the shape
c o e x i s t i n g s t r u c t u r e s observed i n the even-even Pt isotopes do not provide as clear an i n d i c a t i o n of which band i s the more spherical as do the corresponding levels i n the even-even Hg isotopes.
Evidence from
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
37.
Shape Coexistence in
ZGANJAR ET AL.
247
Au
185
systematics indicates [W0081] t h a t f o r neutron d e f i c i e n t isotopes of Pt with A < 186, a deformed c o n f i g u r a t i o n i s present i n the groimd s t a t e . A d d i t i o n a l l y , a staggering of r o t a t i o n a l band parameters has been found [HAG78] f o r 176-182
Pt>
where the odd-mass Pt isotopes
appear more deformed than the even-mass ones. A consistent p i c t u r e of the d e t a i l s presented above, based on the idea t h a t nuclear deformation i s due to the residual force between valence neutrons and valence protons, has been proposed [W0082, HEY83]. In t h i s p i c t u r e , the e x c i t a t i o n of nucléons i n t o shell-model i n t r u d e r states leads to the coexistence of states with d i f f e r e n t deformations.
Shell-model i n t r u d e r states
f o r the odd-mass Au isotopes, f o r example, are presented i n f i g . 2. h ^ 9
and i
2
1 3
y
Note t h a t the
i n t r u d e r s drop r a p i d l y
2
as one goes more neutron d e f i c i e n t [ZGA80].
F i g . 2. Shell model states 185-195 in
In
A U e
1 8 5
A u the 5 / 2 ' member of the h ^ 9
2
intruder
band a c t u a l l y becomes the ground state [EKS76]. the hi , 9c
2
The corresponding data f o r the odd-mass T* isotopes also show and i
1 3
/
i n t r u d e r s , but in t h a t case the h ^ reaches a minimum
2
9
2
near ^ T j . and then rises again as the system becomes more neutron d e f i c i e n t [HEY83, HAM85]. from mid-shell
That the i n t r u d e r energy should increase as one moves away (N « 104 i n t h i s case) i s consistent with the above p i c t u r e
[W0082, HEY83].
Also shown i n f i g . 2 are the proton hole s t a t e s .
p a r t i c u l a r l y the unique p a r i t y h ^ n
2
state.
Note
The coexistence observed i n the
odd-mass Au isotopes can be understood as a r i s i n g from the coupling of the proton p a r t i c l e ( h ^ ) and the proton hole ( h ^ ) 9
2
u
2
to the d i f f e r e n t core
shapes in the even-even Pt and Hg isotopes, r e s p e c t i v e l y . schematically f o r the Pt Φ π Ι ι ^ 9
2
coupling i n f i g . 3.
This i s shown
These ideas were
stimulated by the study [ZGA81] of the low-energy s t r u c t u r e of UNISOR f o l l o w i n g the decay of couples to the
1 8 6
1 8 7
9» Hg. m
In t h a t case the h ^ 9
2
1 8 7
A u at
particle
P t core to form two bands with states of the same spin
connected i n many cases by t r a n s i t i o n s of E0+M1+... m u l t i p o l a r i t y [ZGA81].
American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036 Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
248
NUCLEI OFF THE LINE OF STABILITY
These two bands can be i n t e r p r e t e d couplings
1 8 6
(see f i g . 3) as a r i s i n g
[ 0 * π(2ρ-6η)] Θ π η *
Pt
9
t h e odd p r o t o n i s
i n the h
9
/
2
and
2
intruder
1 8 6
Pt
[0
orbital,
states
(2ρ-6η)
1 8 7
data i n d i c a t e
that
it
that
the ground s t a t e o f
configuration
resulting
intruding h ^
orbital
9
2
We i n i t i a t e d UNISOR f o r picture
the ground s t a t e o f
a careful
Hg
+
1 7 6
Hf(
0,7n)
reasons:
largely
of the
To e x p l o r e
9» Hg
1 8 5
further
m
the
and
f o r an i n - b e a m s t u d y [LAR85] o f 1 8 5
9Hg
1 8 7
Au
and, consequently,
(2p-6h)
into
the
decay
singles,
The
1 8 5
Hg
isotope
[ZGA81], t h i s as t h e
1 8 4
Pt
quality
Au.
separator.
6
0
+
7
1 8 5
produced
As p r e v i o u s l y and h o l e
density.
It
level
i s only
c o u p l e d w i t h an e x t r e m e l y c a r e f u l
( e . g . r u n n i n g c o i n c i d e n c e g a t e s , [ZGA81])
Au
and were mass
c o r e , leads to a large line
The decay in
time-sequence
coexistence of p a r t i c l e
to a very high spectral
data of great s t a t i s t i c a l
1 8 5
earlier
provide
s o u r c e s were 1
at
coexistence
to excited states
r e a c t i o n u s i n g 140 MeV beams o f
t h e odd p r o t o n as w e l l
procedure
(55s, 1/2")
and c o n v e r s i o n - e l e c t r o n
s e p a r a t e d w i t h t h e UNISOR o n - l i n e i n t h e s t u d y on
The
which i s blocked a n d ,
consists
and d e t a i l e d s t u d y o f t h e
and c o i n c i d e n c e measurements. 1 6
and
affected.
decay and t h a t o f t h e Orsay group [ B 0 U 8 2 ] ; and t o
(28s, 13/2 )
spectroscopy
for
Pt
where
principle
d e s c r i b e d a b o v e ; t o r e s o l v e t h e d i s a g r e e m e n t between o u r
were s t u d i e d by γ - r a y
by t h e
Pt
intruder
[ W 0 0 8 1 , ZGA81].
complementary i n f o r m a t i o n 1 8 5 m
1 8 6
f o r Au
(1ρ-4η)
from the promotion o f a p a i r o f protons
the f o l l o w i n g
d a t a on t h i s
of
π
t h e n be c o r r e s p o n d i n g l y
consequently,
The
and h = h o l e .
c o r e i s b l o c k e d due t o t h e P a u l i
1 8 6
of
respectively,
for is
π
are
Pt
Au will
(2p-6h)
(4η)
configurations
p=particle
the core c o n f i g u r a t i o n
and
The p r o t o n
π
When
near Z=82.
respectively.
and
i B b
.
2
proton configurations
and P t a r e π ( 3 η )
in the
9
A schematic view
intruder
configuration
the
η ^
π
the f o r m a t i o n of the
F i g . 3.
normal
from
τυ(4η)] θ
2
t h a t one can
observed states, density with analysis
obtain
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
37.
Shape Coexistence in
ZGANJAR ET AL.
r e l i a b l e and consistent r e s u l t s . two h y 9
2
Au
249
185
For example, an important feature of the
bands are the E0+M1+... t r a n s i t i o n s .
r e s u l t s on
Preliminary analysis of our
supports our e a r l i e r [ZGA81] i n t e r p r e t a t i o n of such
t r a n s i t i o n s as E0+M1+E2 i n t h a t these t r a n s i t i o n s in
1 8 7
1 8 5
A u and c o n t r a d i c t s the i n t e r p r e t a t i o n [B0U82]
A u have anomalous Ml components.
The Orsay
group [B0U82] were forced to conclude t h a t anomalous Ml components were involved since they obtained conversion c o e f f i c i e n t s larger than Ml or E2 f o r a number of t r a n s i t i o n s between l e v e l s of d i f f e r e n t s p i n .
Our
coincidence data were of s u f f i c i e n t s t a t i s t i c a l q u a l i t y t h a t we could q u a n t i t a t i v e l y d i s t i n g u i s h unresolved doublets.
In e s s e n t i a l l y a l l cases,
we were able to account f o r the anomalous i n t e r n a l conversion [B0U82] by l o c a t i n g another t r a n s i t i o n of nearly the same energy which connected states of the same spin and could thus contain an EO component. As i n the
1 8 7
A u study, t r a n s i t i o n s i n
were observed to feed the h ^ 9
2
and h ^ n
2
1 8 5
A u with strong EO components One example f o r each i s
bands
presented i n f i g s . 4 and 5, r e s p e c t i v e l y . Gate on 213KeV Xj
These t r a n s i t i o n s are therefore i n t e r p r e t e d as decays out of the hy
2
the h ^ 9
1427 KeV.
h y 9
2
2
and h ^ n
and h i y
2
2
and η ' ^ η
bands to
2
bands r e s p e c t i v e l y .
bands, of course, r e s u l t from
the coupling of the h ^ 9
2
and h ^ n
2
single
proton configurations to the excited 0 configurations i n the
427 KeV
respectively.
si n K
4ΩθΙ
1
->
The
1 8 4
P t and
1 8 6
+
H g cores,
These r e s u l t s are shown i n
f i g . 6. In summary,
'
l i e s i n a most
i n t e r e s t i n g region of shape coexistence where F i g . 4.
Portions of the
many d i f f e r e n t shapes are observed at low
e l e c t r o n and γ-spectra gated
energies.
on the 213 keV
1 8 5
γ
which i s i n
Recent in-beam work [LAR85] on
A u suggests t h a t these various shapes are
cascade with both the 323 and
made up of both oblate and prolate structures
427 keV t r a n s i t i o n s .
which can be understood as a r i s i n g from the
With E2
m u l t i p o l a r i t y f o r the 323,
competition between close l y i n g K=l/2
the 427 a(K) i s determined
(oblate) and K=ll/2 ( p r o l a t e ) states f o r
t o be 0.33 ± 0.03.
h
f o r Ml i s 0.25.
The «(K)
1 1 / 2
; and K=l/2 ( p r o l a t e ) and K=9/2 (oblate)
for h ^ . 9
2
While the r e s u l t s of these
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
250
NUCLEI OFF THE LINE OF STABILITY
G a t e on \ZVKevr
700
F i g . 5.
500
υ
P o r t i o n s o f t h e e l e c t r o n and γ-
s p e c t r a g a t e d on t h e 211 keV γ w h i c h i s
ο 300
in
4 9 2 KeV -
cascade w i t h b o t h t h e 461 and 492 keV transitions. G a t e on 211 KeV Y lEIectronslj
4 9 2
461,
1
H
t h e 492
0.14 ± 0 . 0 2 .
s t u d i e s are c o n s i s t e n t w i t h the e x p l a n a t i o n
W i t h E2 m u l t i p o l a r i t y a
(K) The
a
(K)
f o r Ml i s
[W0082, HEY83] o f
c o e x i s t e n c e based on t h e p r o m o t i o n o f a p a i r o f p a r t i c l e s orbital,
additional
experimental
and t h e o r e t i c a l
for
the
i s d e t e r m i n e d t o be 0.072.
shape
i n t o an
intruder
work i s needed t o
d e t e r m i n e w h i c h mechanism g i v e s r i s e t o shape c o e x i s t e n c e
fully
i n t h e Z=82
region.
405
Portions
Fig.
of
the
h ^ , 9
2
h ^ , u
2
hy
2
and
η
Ίι/2
bands
in 185
compared
to
the
corresponding transitions
0 , +
2
+
and
core-particle
0 ', +
couplings
2 '
levels
in
are
noted
with
+
^ Pt 4
and
dashed
1 8 6
Hg.
lines.
w i t h EO components a r e shown.
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
A u
The Only
37.
ZGANJAR ET AL.
Shape Coexistence in
Au
185
Acknowledgments Work s u p p o r t e d i n p a r t by t h e U . S . D e p a r t m e n t o f Energy C o n t r a c t s and G r a n t s DE-FG05-84ER40159 a t L o u i s i a n a S t a t e U n i v e r s i t y , DEAS05-80ER10599 and DE-AS05-76ER03346 a t t h e G e o r g i a I n s t i t u t e o f T e c h n o l o g y , DE-AS05-76ER04936 a t The U n i v e r s i t y o f T e n n e s s e e , and DE-AC05-760R00033 f o r UNISOR a t t h e Oak Ridge A s s o c i a t e d Universities.
References [BER77] [BON72] [BON76] [BOU76] [BOU82] [COL76] [COL77] [COL84] [DAB79] [DUP84] [EKS76] [HAG78] [HAM75] [HAM85] [HEY83] [KUH77] [LAR85] [MA 84] [PRO73] [PRO74] [RUD73] [WOO81]
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Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
