38 Shape Coexistence in Neutron-Deficient Pb Isotopes 1
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J. Penninga, W. H. A. Hesselink, A. Stolk , H. Verheul , J. K. Ho , J. van Κlinken , H. J. Riezenbos, M. J. A. de Voigt , and A. Zemel 2
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Natuurkundig Laboratorium, Vrije Universiteit Amsterdam, the Netherlands Kernfysisch Versneller Instituut, Groningen, the Netherlands
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In search for rotational bands on low lying intruder states in Pb the Hg(α,xn) Pb and the Os( C,4n) Pb reactions have been studied. On the basis of e -γ and γ-γ coincidence data levels with spin up to J =6 and J=14 have tentatively been assigned to bands on the J =0 and J =11 excited states in Pb. Furthermore, the g-factor of the J =11 isomer in Pb has been measured. The experimental value g = 0.96(8) indicates that this state has a proton 2p-2h configuration. 188
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In many nuclei near single closed shells low lying intruder states have been observed. These intruder states are known to be due to particle-hole excitations across the closed shell. The additional particle-hole degree of freedom causes dramatic changes in the properties of the nuclear states. It is for example a characteristic feature of the intruder states that they act as band heads for rotational bands. Recently Heyde et al. have reviewed the experimental evidence for shape coexistence in odd A nuclei and the theoretical approaches which are made to describe the experimental data [HEY83]. Also in several even mass nuclei there is evidence for shape coexistence. A nice example are the rotational bands on ^=0 intruder states in the even mass Sn isotopes [BR079]. Van Duppen et al. have recently reported on the observation of low lying νΙ =0 states in neutron deficient Pb isotopes [DUP84]. The latter states are similar to those in the Sn nuclei and their observation is thus not unexpected. The systematic trend of the observed Ι = 0 states through the various Pb isotopes and Sn isotopes is the same. However, the excitation energy of the intruder states in Pb is much lower than in the Sn isotopes. This is due to the larger number of valence neutrons causing a stronger resi dual interaction between the proton particles and holes and the neutrons. In this respect it is of interest to investigate the protoperties of the rota tional bands which will likely occur on top of ^=0 intruder states in Pb. In S n rotational bands were excited up to J^=\2 states. The experimental study on rotational bands on ν1 =0 intruder states in the Pb isotopes is however more difficult than in Sn. In the first place the rotational bands in the neutron midshell nuclei of Sn could be studied by means of the (a,2n) reaction, whereas in Pb these nuclei are far away from the stability line. In the second place neutron (^13/2^ with angular momentum ^Γ=10 ,12 occur at relatively low excitation energy. This implies that the predicted excitation energy for band members with spin J>10 is more than 1 MeV above the Y-rast line. These states will therefore not likely be excited in fusion-evaporation reactions. One might however have some hope to populate the lower part of the bands with reactions induced by light ions. In search for rotational bands on j W states in " we have measured coincidences between γ-rays following the Hg(a,xn) reactions and conversion electrons corresponding to E0Q+_^Q+ transitions in these nuclei. The α-particle energy was 75 MeV and 2 1 92 MeV, respectively. The me tallic Hg (98 % enriched) target was produced by evaporating mercury oxalate onto a C-backing. The averaged target thickness was about 0.7 mg/cm . The γ-ray spectra (cf. Fig. 1) were taken with two ( Pb) and four ( Pb) +
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P b
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0097-6156/86/0324-0252$06.00/0 © 1986 American Chemical Society Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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38.
PENNINGA E T AL.
Neutron-Deficient Pb Isotopes
253
x10 160
1 9 8
Hg(a,6n)
1 9 6
Pb
120 3
O u u. 80 o or u m Σ 3 Z
40
t 200
100
E ( y
Fig.
1
500
400
300
S p e c t r u m o f γ-rays f o l l o w i n g t h e
700
600
keV) 1 9 8
Hg(a,6n)
1 9 6
Pb
reaction.
Ge d e t e c t o r s e a c h p r o v i d e d w i t h a c y l i n d r i c a l BGO + N a l Compton s u p p r e s s i o n s h i e l d . The c o n v e r s i o n e l e c t r o n s were measured w i t h two s p e c t r o m e t e r s e a c h c o n s i s t i n g o f a m i n i o r a n g e f i l t e r and a S i ( L i ) d e t e c t o r . The t r a n s m i s s i o n o f t h e f i l t e r was o p t i m i z e d f o r d e t e c t i o n o f e l e c t r o n s w i t h e n e r g i e s i n t h e r a n g e 0.9- 1.3 MeV. F o r P b t h e i n t e n s i t y o f t h e EO 0+ + O J t r a n s i t i o n i n t h e e l e c t r o n s p e c t r u m was a b o u t t h e same as f o r t h e L - e l e c t r o n s o f t h e 2^ •> 0^ t r a n s i t i o n . T h i s i n d i c a t e s t h a t t h e i n t e n s i t y r a t i o I + _^ Q+ /1^+ _^ Q ^ J Q - 3 Thus, the expected electron-gamma c o i n c i d e n c e 2 1 / 1 1 i n t e n s i t y i s a b o u t a f a c t o r 1 0 s m a l l e r t h a n t h e γ-γ c o i n c i d e n c e i n t e n s i t y f o r t h e s t r o n g e s t γ-ray t r a n s i t i o n s , t a k i n g i n t o a c c o u n t a ε = 0 . 0 3 f o r t h e e l e c t r o n spectrometers. For P b t h i s f a c t o r i s even 3. 1 0 . F i g . 2 shows t h e r e s u l t s f o r P b (the data a n a l y s i s f o r Pb i s i n p r o g r e s s ) . I n a d d i t i o n t o t h e 288, 307 and 754 keV t r a n s i t i o n s w h i c h h a v e a l s o b e e n o b s e r v e d by Van Duppen e t a l . i n t h e 3-decay o f B i i s o t o p e s we have f o u n d e v i d e n c e f o r a 413 keV and a 563 keV t r a n s i t i o n . These γ-rays a r e c l e a r l y v i s i b l e i n t h e c o i n c i d e n c e s p e c t r u m b e t w e e n d e l a y e d γ-rays and d e l a y e d c o n v e r s i o n e l e c t r o n s . T h i s does n o t e x c l u d e t h a t t h e y a r e a l s o p r e s e n t i n t h e prompt-prompt s p e c t r u m . I f t h e i n t e n s i t y i n t h e l a t t e r s p e c t r u m w o u l d a p p r o x i m a t e l y be t h e same as i n t h e d e l a y e d s p e c t r u m t h e s e γ-rays w o u l d have b e e n e s caped f r o m o u r o b s e r v a t i o n due t o t h e h i g h e r b a c k g r o u n d i n the prompt s p e c t r u m . We have t e n t a t i v e l y a s s i g n e d t h e 413 keV and t h e 565 keV t r a n s i t i o n s t o t h e d e c a y o f a J^=b and J =6" " l e v e l s o f t h e r o t a t i o n a l band b u i l t on t h e J =0 i n t r u d e r s t a t e on t h e b a s i s o f t h e c o i n c i d e n c e i n t e n s i t y and t h e t r a n s i t i o n e n e r g i e s ( c f . F i g . 3 ) . The r e g u l a r i n c r e a s i n g t r a n s i t i o n e n e r g i e s b e t w e e n t h e members o f t h e p r o p o s e d band a g r e e w i t h t h o s e o f t h e g r o u n d s t a t e band i n P t . A s i m i l a r o b s e r v a t i o n has b e e n made f o r t h e Sn i s o t o p e s . It w i l l be very d i f f i c u l t to o b t a i n f u r t h e r evidence f o r the r o t a t i o n a l band on t h e J™=0 state i n P b . Α γ-γ c o i n c i d e n c e measurement t o e s t a b l i s h t h e c o i n c i d e n c e r e l a t i o n s b e t w e e n t h e p r o p o s e d i n t r a b a n d t r a n s i t i o n s seems 1 9 6
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Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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NUCLEI O F F T H ELINE O F STABILITY
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H g ( a, 6nY
