Intruder deformed bands in 110Sn and 112Sn

Volume 207, number 1

PHYSICS LETTERS B

9 June 1988

INTRUDER DEFORMED BANDS IN n°Sn AND n2Sn H. H A R A D A , T. M U R A K A M I , K. Y O S H I D A , J. K A S A G I Department of Physics, Tokyo Institute of Technology, Oh-Okayama, Meguro, Tokyo 152, Japan T. I N A M U R A and T. K U B O R1KEN (Institutefor Physicaland Chemical Research), Wako-shi,Saitama 351-01, Japan Received 23 September 1987; revised manuscript received 29 March 1988

Results from gamma-ray spectroscopy studies using five BGO anti-Compton spectrometers reveal the existence of distinct deformed bands on the high-spin region of t ~OSnand ~~2Sn. The bands are identified as those of the proton intruder configuration. A striking similarity of the band structure between ~ ZSn and ~ 6Xe is shown not only for the g.s. band but also for the v (hll/~ )2 aligned band and for the negative-parity band.

In the last few years, i n t r u d e r states in nearly single-closed shell nuclei have been well investigated both experimentally a n d theoretically. In e v e n - e v e n nuclei near Z = 50 and 82, these states are known as the 0J- states at very low excitation energies a n d several collective states have been found built on t h e m [1,2]. They are u n d e r s t o o d as proton excitations through the closed shell, i.e., excitations o f a proton pair from the closed shell to the next m a j o r shell. H e y d e et al. [ 3 ] have d e v e l o p e d a shell-model approach to these states to describe both the small excitation energy a n d the A-dependence. According to them, the q u a d r u p o l e - q u a d r u p o l e interaction between the valence protons (particles and holes), which is caused by the p a r t i c l e - h o l e excitation, a n d neutrons outside the closed shell plays a decisive role in lowering the excitation energy and in m a k i n g the average nuclear field deformed. Consequently, the intruder states in the proton-closed shell nuclei are considerably d e f o r m e d as c o m p a r e d to n o r m a l neutron-quasiparticle states. This large difference in def o r m a t i o n characterizes the shape coexisting feature in the spherical nuclei, and the i n t r u d e r states can be observed as distinguished d e f o r m e d bands a m o n g m a n y spherical states. It is o f particular interest to c o m p a r e the b a n d o f the i n t r u d e r configuration with that o f an adjacent nucleus which has the same n u m b e r o f valence pro0 3 7 0 - 2 6 9 3 / 8 8 / $ 03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division )

tons and neutrons, because if the p - n interaction among the valence particles is the d o m i n a n t factor in inducing deformation, one can expect an identical b a n d structure in both nuclei. Recent experiments have shown such similarity for 98Zr and I°2Ru [4], ll6Sn and 12°Xe [1,3], and 192pt and 196pb [2] nuclei. However, a c o m p a r i s o n has been m a d e only for a few low-spin m e m b e r s o f the ground-state (g.s.) band. It is highly desirable to make a more stringent test o f the identity o f b a n d structures. To m a k e such a test, we have studied the high-spin states in ~°Sn and ~L2Sn, which can easily be populated via the (160,4n) reaction unlike those in the heavier Sn isotopes. Low-spin m e m b e r s o f the def o r m e d b a n d in l l 2 S n have been known up to the 12 + state [ 1 ] and the properties o f the known yrast states are well r e p r o d u c e d by the generalized-seniority mixing calculation [5,6]. Thus, one can predict the excitation energies o f both the d e f o r m e d and spherical states in J J2Sn. The high-spin m e m b e r s o f the def o r m e d b a n d are predicted to lie very close to the calculated spherical yrast states. High-spin states in l~°Sn, which we reported before [7], have recently been confirmed by Viggars et al. [ 8 ]. In this p a p e r we shall discuss the band structure observed at high spins in 11°Sn together with that in ~J2Sn. The experiments were carried out at R I K E N using the 160 cm cyclotron. Levels o f l~°Sn a n d ~2Sn were 17

Volume 207, number 1

PHYSICS LETTERS B

1600 1400

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Fig. 1. Gamma-gamma coincidence spectra observed in the 98l°°Mo(t60,4n)]~°'tt2Sn reactions with five BGOACSs. The upper spectrum is gated by the 745 keV transition ( 12 +- 10+ ) in t t2Sn and the lower one gated by the 809 keV transitions in ' '°Sn. excited by the 9 8 ' t ° ° M o ( 1 6 0 , 4 n ) 110.]12Sn reactions at energies o f 72 and 76 MeV. Targets used were leadbacked enriched foils o f 98Mo (98% enrichment) and t°°Mo (96% e n r i c h m e n t ) . G a m m a - g a m m a coincidence and angular distribution m e a s u r e m e n t s were performed. Five sets B G O a n t i - C o m p t o n spectrometers (BGOACS) [9] were used for the g a m m a - g a m m a coincidence measurements. They were placed at 35 °, 90 °, 145 °, - 45 ° and - 135 ° with respect to the b e a m direction, and the distance between the target and the center o f the Ge detector was 15 cm. A collimator o f 4 cm thick heavy metal was placed in front o f each BGOACS. The angular distributions o f gamma-rays were measured using two sets o f B G O A C S in coincidence with neutrons detected with a neutron multiplicity filter [ 10 ] similar to the previous setup for the recoil distance m e a s u r e m e n t s [6]. Fig. 1 shows the g a m m a - r a y spectra observed by gating on the 745 keV transition in ~2Sn and on the 809 keV transition in ~°Sn. Distinct rotation-like b a n d transitions are seen in both spectra as indicated with solid lines. Fig. 2 shows relevant parts o f the de18

9 June 1988

cay schemes o f ~°Sn and ~2Sn obtained in the present work. Levels assigned to the m e m b e r s o f the intruder bands are drawn with thick, heavy lines in the decay schemes. The g a m m a - r a y angular distributions and g a m m a - g a m m a angular correlations are consistent with stretched-quadrupole transitions for all intra- and inter-band transitions but the 1039 keV, 1148 keV transitions in ~2Sn a n d the 1004 keV transition in 1t°Sn. The fact that the lifetimes o f the b a n d m e m b e r s o f ~2Sn are less than 30 ps [6] indicates that the cascading transitions are o f E2. The results for the levels below E~ = 5 MeV agree well with those reported previously [ 1 ]. The transitions shown are the most intensive ones and no levels other than the b a n d m e m b e r s have been found above Ex = 7 MeV. Therefore it is concluded that the yrast line changes from spherical states to d e f o r m e d states at a r o u n d J = 14 in both nuclei. F o r the ~~2Sn nucleus, the present results reveal the existence o f a distinct positive-parity b a n d up to the (22 + ) state. A new negative-parity b a n d built on the 13- state is also assigned up to the (21 - ) state. The previously known b a n d ( 6 + - 8 + - 1 0 + - 1 2 + ) is considered to correspond to the g.s. bands in J°SPd (proton 4h) and ~6Xe ( p r o t o n 4 p ) . (We shall call this b a n d " i n t r u d e r g.s. b a n d " . ) High-spin m e m b e r s o f the intruder g.s. b a n d have not been found because the b a n d crossing occurs at J " = 12 + and the intruder S-band becomes yrast states. This is consistent with the band crossing observed in Pd and Xe nuclei, which is considered to be caused by alignment o f a pair o f h~ ~/2 neutrons [ 11-13 ]. the newly observed negative-parity b a n d is also consistent with the existence of the negative-parity b a n d in Pd and Xe nuclei, which is considered to be built on a two-quasiparticle state involving one h~ ~/2 neutron [ 11,12 ], The 10 ~- state at 4680 keV, which is fed by the 12+ state, is not assigned to be a m e m b e r o f the intruder aligned b a n d ( S - b a n d ) . First, although the 12 ~- state decays to the 10 ~- state, the relative B (E2) strength to the 10 ~- state is seven times smaller than the one to the 10 + state o f the i n t r u d e r g.s. band. Second, the 10~- state decays exclusively to the 9 - state which is well interpreted as the spherical neutron two-quasiparticle state [5,6]. Therefore, the m a i n c o m p o n e n t o f the 10~state is considered to be the spherical one, i.e., neutron two quasiparticle configuration (h~l/2) 2 10 +. The excitation energy o f the 10~- state is also consis-

Volume 207, number I

PHYSICS LETTERS B

9 June 1988

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tent with the systematics o f the spherical (hi i/2)2 10 + states known in heavier Sn isotopes [ 5,14 ]. F o r the l l°Sn nucleus, a rotation-like b a n d above the J = 12 state at 6036 keV was also found. The band, whose m e m b e r s are connected by the stretched quadrupole transitions, forms the yrast lines in l l°Sn, as the i n t r u d e r aligned b a n d in ~~2Sn. We propose that the b a n d is the i n t r u d e r aligned b a n d in ~°Sn. The 5227 keV state ( I 0 <+)) seems to be the spherical ( P h i l / 2 ) 2 l0 + state as well as the one at 4680 keV in 112Sn"

The observed i n t r u d e r bands in ~l°Sn and ~i2Sn are c o m p a r e d to the bands in l°6pd6o [ 1 1 ] a n d 116Xe62 [13] in fig. 3; the 12 + state in n°6pd is placed at the same excitation energy as the 12 ~+ ) state in i lOSn' and the 6 + state in ~ 6 x e is placed at the same excitation energy as the 6 + state in ~2Sn. As is seen from the figure, the similarity o f the b a n d structures between

l l2Sn and l n6Xe is remarkable. Energies o f the corresponding levels in both nuclei agree quite well not only for the g.s. b a n d but also for the aligned bands. Although the negative-parity b a n d in l l-~Sn lies slightly higher than in l~6Xe ' the difference in the level energy for the 17- state is only 200 keV. It is noteworthy that the ratios o f the intraband transition energies i n l l 6 X e tO the corresponding ones in l l2Sn a r e almost unity. This suggests that the bands o f the proton 2 p - 2 h i n t r u d e r configuration in s12Sn is almost identical with those o f the proton 4p configuration in I 16Xe"

N o definite conclusion is drawn from the c o m p a r ison o f ll°Sn and i°6pd because o f the lack o f levels to be compared. Agreement o f the levels between l~°Sn and n°6pd is not so good as is the case for l n2Sn a n d l l6Xe. It should be noticed that the i n t r u d e r g.s. b a n d has not been found in zl°Sn in spite o f the exis19

Volume 207, number 1

PHYSICS LETTERS B

9 June 1988

._.__2g~?'] (21-)

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Fig. 3. Comparison of the intruder bands in ~°Sn and l l2Snwith the bands in t°6pd and 116Xe. tence of the intruder aligned band. One possible explanation for this p h e n o m e n o n is that the states with the intruder g.s. configuration does not exist as deformed-band members in l~°Sn. This is consistent with a potential energy surface calculation for the configurations with no n e u t r o n quasiparticle excitation [ 15 ]. The calculation shows the existence of a clear second m i n i m u m at a r o u n d fl= 0.3 for heavier Sn nuclei with A > 1 12 but does not in ' ~°Sn. Another explanation is that the branching ratio of the decay of the 12 + state to the 10 + state of the intruder g.s. b a n d is too small to be observed due to the small transition energy. If the ratio of the B ( E 2 ) value, B(E2; 12~- - 10~ ) / B ( E 2 ; 12i~ - 10~ ), isthe same as in ~'2Sn and the energy of the 12+ - 10 + transition is less than 300 keV, then the 12 + state exclusively decays to the 10i- state as observed in the experiment. In this case, however, the intruder g.s. b a n d should lie more than 1 MeV higher than in ~2Sn although the intruder aligned b a n d in 1~OSnlies only about 500 keV higher than in ~2Sn. In summary, we have observed deformed b a n d s in the high-spin region of ~°Sn a n d ~2Sn. I n ~12Sn, as20

signed are the intruder g.s. band, the intruder (h, 1/2) 2 aligned b a n d and the intruder negative-parity band. In 11°Sn, no intruder g.s. b a n d is found although the intruder (h i, / 2) 2 aligned b a n d is observed. The b a n d structure in 1~2Sn is f o u n d similar to that of l~6Xe by comparing the level energies. This conclusion gives definite evidence that the p r o t o n - n e u t r o n interaction induces quite similar b a n d structures in nuclei when the n u m b e r of valence protons (particles and holes) a n d neutrons is identical.

References [ 1] J. Bron et al., Nucl. Phys. A 318 ( 1979) 335. [2] P. Van Duppen et al., Phys. Rev. Len. 52 (1984) 1974. [3] K. Heyde et al., Phys. Rep. 102 (1983) 291; Proc. Intern. Symp. on In-beam nuclear spectroscopy (Debrecen, 1984) p. 151, Phys. Lett. B 155 (1985) 303. [4] R.A. Mayer et al., Phys. Lett. B 177 (1986) 271. [ 5 ] A. Van Poelgeest et al., NucL Phys. A 346 (1980) 70; G. Bonsignoriet al., Nucl. Phys. A 432 (1985) 389, [6] J. Kasagi et al., Phys. Lett B 176 (1986) 307. [7] J. Kasagi et al., Proc. Intern. Nuclear physics Conf. (Harrogate, 1986).

Volume 207, number 1

PHYSICS LETTERS B

[8] D.A. Viggars et al., Phys. Rev. C 36 (1987) 1006. [ 9 ] M. Fukuda et al., RIKEN Accel. Prog. Rep. 18 ( 1984 ) 152. [ 10 ] T. Murakami et al., Nucl. Instrum. Methods A 241 (1985) 172. [ 11 ] J.A. Grau et al., Phys. Rev. C 14 (1976) 2297; L.K. Kostov et al., Phys. Lett. B 123 (1983) 29.

9 June 1988

[ 12 ] A. Kerek et al., Z. Phys. A 317 ( 1984 ) 169. [ 13 ] V.P. Janzen et al., McMaster Accelerator Laboratory Annual Report (1984) p. 54. [ 14] P.J. Daly et al., Z. Phys. A 323 (1986) 245. [ 15 ] M. Sugita, preprint; and private communication.

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