36 Intruder States in Highly Neutron-Deficient Pt Nuclei Evidence from Lifetime Measurements?
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U. Garg , M. W. Drigert , A. Chaudhury, E. G. Funk , J. W. Mihelich , D. C. Radford , H. Helppi , R. Holzman, R. V. F. Janssens, T. L. Khoo , A. M. Van den Berg, and J. L. Wood3 25 ,
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26,
1
Physics Department, University of Notre Dame, Notre Dame, IN 46556 Physics Division, Argonne National Laboratory, Argonne, IL 60439 School of Physics, Georgia Institute of Technology, Atlanta, GA 30332
2 3
The recoil distance technique has been employed following the 154
34
184
Sm( S,4n) Pt reaction to measure the lifetimes of states in the ground state band (GSB) of the highly neutron deficient nucleus Pt. Lifetimes have been extracted for up to the 18 state in the GSB. The B(E2) values exhibit a marked increase between spins 2 and 10, suggesting a strong mixing between the GSB and a less collective band at low spin and, thus, providing indirect evidence for an "intruder" (6h-2p)O ground state. This is further confirmation of the shape coexistence occuring at low energy in this region. 184
+
+
The structure of very neutron-deficient Pt isotopes is most unusual and needs to be better clarified for a number of reasons: (1) There is an emerging picture of an extensive occurence of shape coexistence at low energy in the region near Z=82 and N=104 (see, for example,[HEY83,DUP84] and the references therein). This region needs to be carefully mapped away from closed shells to determine the systematics of bandhead energies and mixing matrix elements for the coexisting bands. The present view [WOO81] 4
Current address: Chalk River Nuclear Laboratories, Chalk River, Ontario K0J 1J0, Canada Permanent address: Lappeenranta University of Technology, Finland Current address: Rijks Universiteit Utrecht, Utrecht, the Netherlands
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0097-6156/ 86/ 0324-0239506.00/ 0 © 1986 American Chemical Society
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
240
NUCLEI OFF THE LINE OF STABILITY The
in Table data.
lifetimes obtained from a p r e l i m i n a r y analysis a r e summarized
1 which also includes the B ( E 2 ) values e x t r a c t e d from the lifetime
F i g . 2 s h o w s a plot of B ( E 2 ) ' s
depopulating
( i n W e i s s k o p f U n i t s ) v s . s p i n of t h e
state.
TABLE I Ε
I.
Ύ (keV)
τ
*
B(E2) §
B(E2)t
f
1
(W.U.)
(ps)
163
2+ •> 0+
272
4
+
363
6
+
-> 4
+
433
8
+
+
+ 2
Theory
(W.U.)
85.3
582±50
112±10
39± 3
200±17
96.5
9± 2
220±49
107.8
-> 6
+
3.2±0.5
261±41
115.8
-
8
+
1.7±0.4
309±73
120.7
+ 10
+
2.310.4
184±33
122.3
476
10
+
497
12
+
522
14
+
+ 12
+
2.0±0.4
166±34
104.6
555
16
+
+ 14
+
l-6±0.5
153±47
106.2
586
18
+
+ 16
+
1.3±0.8
144±90
107.8
* E r r o r s a r e c o n s e r v a t i v e estimates. f l n t e r n a l conversion has been taken values. §From
into account i n calculating B ( E 2 )
(KUM80) It i s clear from F i g . 2 that t h e r e i s a s i g n i f i c a n t i n c r e a s e i n t h e
B(E2)
values between spins 2
+
+
a n d 1 0 ( b y a f a c t o r o f >2.5). T h i s
vides an unequivocal
c o n f i r m a t i o n of t h e b e h a v i o r
mixing
+
between the 0
states a n d between the 2
bands and, thus, provides strong evidence 184 existence i n
pro
b e c a u s e of t h e
states i n the coexisting
f o r t h e p r e s e n c e of s h a p e c o -
P t . A s e p a r a t e a n d i n t r i g u i n g a s p e c t of o u r m e a s u r e m e n t s +
is t h e d e c l i n e i n B ( E 2 ) v a l u e s b e y o n d s p i n 1 0 . of t h e o n s e t of t r i a x i a l i t y . in
+
expected
This might be indicative
I tcoincides with thebeginning
t h e G S B a n d i s , p o s s i b l y , t h e r e s u l t of a l a r g e i n d u c e d
to t h e a d d i t i o n of a p a i r o f 1-^3/2 ° ±
uas
n
of b a c k b e n d i n g triaxiality due
i eutrons· S u c h b e h a v i o r
has recently
been observed
i n the deformed r a r e - e a r t h nuclei [FEW85]. Also, a possible 186 e m e r g e n c e of t r i a x i a l i t y h a s b e e n r e p o r t e d f o r Hg [ J A N 83]. Finally, 184
our B ( E 2 ) values i n calculated conserving
P t can be comoared
(Table
1) w i t h B ( E 2 ) v a l u e s
[KUM80] u s i n g a microscopic a p p r o a c h of v a r i a t i o n w i t h n u m b e r projection from a p a i r i n g - p l u s - q u a d r u p o l e
Hamiltonian.
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
36.
241
Intruder States in Highly Neutron-Deficient Pt Nuclei
GARG ET AL.
of t h e e v e n - m a s s
Os, P t , H g a n d Pb isotopes i s v e r y
p u z z l i n g i n that the
more deformed b a n d i s an e x c i t e d b a n d i n P b , H g a n d O s a n d a p p e a r s to be the g r o u n d b a n d i n P t ( f o r A
