High-spin structures in 112Sb

N U C L E A R PHYSICS A

ELSEVIER Nuclear Physics A 607 (1996) 350-362

High-spin structures in l l2Sb

A.K. Singh a, G. Gangopadhyay a, D. Banerjee a, R. Bhattacharya a, R.K. Bhowmik b, S. Muralithar b, G. Rodrigues b, R.P. Singh b,

A. Goswami c, S. Chattopadhyay c, S. Bhattacharya c, B. Dasmahapatra c, S. Sen c

a Department of Physics, University College of Science, Calcutta University, Calcutta-700 009, India b Nuclear Science Centre, New Delhi-110 067, India

c Saha Institute of Nuclear Physics, Calcutta-700 064, India

Received 9 February 1996; revised 29 May 1996

Abstract

A spectroscopic study of n2Sb has been carried out using the l°3Rh(12C,3n) reaction. The low energy excited levels are interpreted in terms of two-quasiparticle configurations arising from the proton and neutron motion in the q'rg7/2, 7"rd5/2, t.,g7/2, vds/2 and Ph11/2 shell model orbitals. A strongly coupled rotational band, most likely based on a deformed zrg~/~rg~/2 ® uhll/2 configuration, is identified at somewhat higher excitation energy. The B(M1 ) /B(E2) ratios for some of the transitions in the strongly coupled band indicate mixed nature of the intrinsic configuration.

Keywords: NUCLEAR REACTIONS 103Rh(12C,3n) ll2sb; E = 75 MeV; measured E~,; yy coin; DCO ratios; ll2sb deduced levels; j~r; B(MI )/B(E2) ratios.

1. Introduct ion

The nucleus is a unique many-body system which displays extreme richness in its

excitation spectra arising out of the single particle and collective modes of excitation and

their interplay. Experimental investigations carried out for the last several years using

heavy ion beams and large detector arrays have enabled us to study the behaviour of

nuclear systems with large number of valence nucleons, spanning wide range between

very low and very high excitation energy and rotational frequency. The study of evolution

of nuclear shapes, changing role of pairing and Coriolis forces, etc. with increasing

rotational frequency has enriched our knowledge about the interplay of collective and

non-collective modes of excitations in producing richness in nuclear spectra. Such studies

0375-9474/96/$15.00 Copyright (~) 1996 Elsevier Science B.V. All rights reserved PH S0375-9474(96)002552

A.K. Singh et al./Nuclear Physics A 607 (1996) 350-362 351

have been extended recently to nuclei near closed shell, especially in the Z = 50, A = 100

region, and very interesting results have been obtained [ 1-7]. Experimental data in the

Z = 50 and A = 110 region reveal that the spectra of the Sb and Sn nuclei in this mass region are non-collective in nature near low-spin and excitation energy, as expected.

However, with increasing spin and excitation energy, collective bands are developed

with moderate to large deformation through particle-hole excitations across the Z = 50

major shell. This opens up the possibility to study collective and non-collective effects

within the same nuclear system. More interestingly, it has been shown [8,9] that the collective bands observed in l°9311Sb offer a unique opportunity to investigate the gradual

transition from collective to non-collective behaviour within a specific configuration.

In the present work, we report the results of our investigation on the level structure of ll2Sb carried out using (HI, xpyn) reaction techniques. An earlier study [10] on this

isotope had established excited levels up to 1170 keV while tentative spin assignment

has been made for levels up to 796.4 keV and these levels have been interpreted to arise from two quasiparticle configurations. Very recently, a strongly coupled rotational band

built on the deformed (fl ~ 0.16) ~rg~/l~rg2/2 ® ~h11/2 configuration has been identified

in l°sSb by Cederk~ill et al. [ 11 ] based on their heavy ion fusion evaporation study and

total routhian surface calculations. To our knowledge no such band structure has earlier been identified in ll°Sb and ll2Sb.

2. Experimental methods and results

High-spin states in t l2Sb were populated through the l°3Rh(12C,3n) reaction at a beam

energy of 75 MeV, obtained from the 15UD Pelletron Accelerator of the Nuclear Science Centre (NSC), New Delhi. A thick (,~ 25 mg/cm 2) self-supporting rhodium foil was

used as target. The other dominant channels in this target-projectile combination were

found to be 4n and 3pn leading to the excited states of lllSb and lllSn, respectively. The results on the excited states of l llSn have already been reported [5]. Though the

presence of three competing channels introduces complexity in the gamma-ray spectra,

it is found that the majority of the gamma rays arising out of these nuclei are non- overlapping.

The gamma-gamma coincidence data were collected with a multiple detector array

consisting of nine Compton suppressed HPGe detectors along with fourteen BGO de-

tectors used as a multiplicity filter to reduce the radioactive background. The detectors were arranged in two groups of four detectors each at 99 ° and 153 ° and one detector at

45 ° with respect to the beam direction. The details of the experimental set up and data acquisition system can be found in Ref. [ 12]. A total of 42 million events corresponding to two or higher fold coincidences in HPGe detectors was recorded in List mode. Each coincidence event was qualified with the condition that at least one BGO detector of the multiplicity filter should fire. The pulse height of each detector was gain matched

to 0.5 keV/channel and the T-T coincidence data were sorted out into a 4096 × 4096 total E~,-E~, matrix. The energy spectra gated by T-rays of interest were generated from

352 A.K. Singh et al./Nuclear Physics A 607 (1996) 350-362

400

3O0

200

100

6OO

450

3O0

15C

1000

75O

500

250

0

1209

8O0

400

0 200 400 600 800 1000 12 O0 1400 1600 1800 2000 2200

CHANNELS

Fig. 1. Gated energy spectra of l l2Sb. Conversion gain = 0.5 keV/channel .

this matrix. Fig. 1 shows the coincidence spectra with gates on several y-rays.

The multipolarities of the observed y-rays were determined through the directional

correlation orientation (DCO) ratios. For this purpose a separate 4096 x 4096 matrix

was generated with the events recorded at 99 ° along one axis and those recorded at

153 ° along the other axis. The DCO ratio was determined as

RDCO(3,1) = l(3,J at 99 ° with 3'2 at 153°)/1(3,1 at 153 ° with 3'2 at 99 °) where a stretched AI = 1 transition was chosen as 3"2.

The energies, relative intensities and DCO ratios of the y-rays and the tentatively assigned spins and parities of the relevant levels of 112Sb are given in Table 1. Because

of the presence of a few overlapping 3,-rays arising out of the competing channels, the relative intensities have been determined from the total projected as well as individually

gated spectra. The DCO ratios can be divided into two groups, centered around the

values expected for stretched quadrupole (RDco --~2.0) and stretched dipole (RDco _~ 1.0) transitions. There are ambiguities in some cases which will be mentioned at appropriate places.

The level scheme of ll2Sb has been constructed from the 3"-3" coincidence data,

the y-ray intensities and the multipolarities of the 3,-rays, deduced from DCO ratio

measurements. A total of 38 y-transitions has been placed in the level scheme, shown in Fig. 2. The previously reported [ 10] 196.9 keV 3,-transition de-exciting the level at

1170 keV was not found in the present study. In practically all cases, an increasing spin with increasing excitation energy has been assumed in the present analysis. The presence of cross-over transitions in several cases provide an additional confirmation

A.K. Singh et al./Nuclear Physics A 607 (1996) 350-362

Table 1

Energy, intensity and DCO ratio of the gamma rays assigned to H2Sb

353

E z, Relative DCO

( keV ) intensity ratio

A s s i g n m e n t

E i ---* Ef

(keV) li'~ -.. lt3

72 a 183 4- 36 1.92 4- 0.38 1745 ---* 1673 166 a 245 4- 28 166 ~ 0

202 1690 4- 96 1.03 4- 0.04 1947 --, 1745 295 117 4- 13 0.72 4- 0.10 3692 + 3397

318 107 4- 12 0.73 4- 0.08 4010 + 3692

325 19104- 115 0.99 4- 0.04 2272 --~ 1947

335 a 1834-20 0.844-0.05 501 ---+ 166

353 1455 4- 89 1.00 4- 0.04 2625 ---* 2272

356 162 4- 16 1.54 4- 0.19 4366 --~ 4010 374 321 4- 25 0.92 4- 0.06 3379 ---* 3005

380 1000 1.00 4- 0.04 3005 ~ 2625

392 538 4- 38 1.05 4- 0.06 3397 ---* 3005

398 a 448 4- 26 0.79 4- 0.05 501 ~ 103

407 386 4- 29 0.96 4- 0.06 3804 ~ 3397

407 1673 ~ 1267

414 59 4- 9 0.86 4- 0.2l 4780 ---* 4366

425 124 4- 16 1.10 4- 0.31 3804 ---* 3379

442 a 662 4- 39 1.05 4- 0.07 1267 ---, 825

471 a 462 4- 27 1.09 4- 0.07 972 ~ 501

478 a 359 4- 22 0.72 4- 0.04 1745 ~ 1267 486 165 4- 18 1.31 4- 0.12 4290 ~ 3804

502 124 4- 17 4792 ~ 4290 528 h 72 4- l 1 1.12 4- 0.24 5320 ~ 4792

528 c 147 4- 20 2272 --~ 1745

678 290 4- 29 1.55 4- 0.32 2625 ~ 1947

701 a 321 4- 19 1.16 4- 0.11 1673 ~ 972 733 214 4- 23 1.60 4- 0.25 3005 --~ 2272

754 148 4- 21 1.46 4- 0.38 3379 ---* 2625 772 131 4- 20 2.33 4- 0.66 3397 + 2625

799 165 4- 23 2.00 4- 0.32 3804 ~ 3005 848 a 917 4- 53 1.41 4- 0.11 1673 ~ 825

893 29 4- 5 4290 --+ 3397 988 48 4- 22 4792 --~ 3804

1031 34 4- 7 5320 ~ 4290 1122 a 610 4- 36 1.90 4- 0.24 1947 + 825

8 ~ ---+ 7 -

4 + ____, 3 +

9 - 4 8 -

1 4 - ) ---+ ( 1 3 -

1 5 - ) ---, ( 1 4 - 10- - - . 9 -

5 + _.~ 4 + 11- ---* 10-

1 6 - ) --, ( 1 5 - 13- --~ 12-

12- ---* 11-

13- ~ 12-

5 + ~ 4 +

14- ---* 13-

8 - - - * 7 -

( 1 7 - ) ~ ( 1 6 - )

14- --~ 13- 8 - ---* 7 -

6 + ~ 5 +

8 - ---* 8 - 15- ---* 14-

16- -~ 15- 17- ~ 16-

10- ---*8- 11- ---~ 9 -

7 - --~6 +

12- ---+ 10-

13- --, 11- 13- ---* 11-

14- -+ 12- 7 - --- ,7-

15- --~ 13- 16- ----, 14-

17- ~ 15- 9 - 4 _ _ , 7 -

a Evaluated from projected spectrum (see text). b Only the intensity of ( 1 7 - ~ 1 6 - ) transition.

c Evaluated from branching ratio of 353 and 528 keV transitions in 295 keV gate.

o f s p i n a n d p a r i t y a s s i g n m e n t s . T h e 4 0 7 a n d 5 2 8 y - t r a n s i t i o n s o c c u r t w i c e i n t h e l e v e l

s c h e m e as e a c h o f t h e m i s o b s e r v e d in t he c o i n c i d e n c e s p e c t r u m g a t e d b y t h e y - r a y o f

t he s a m e e n e r g y .

T h e m a i n f e a t u r e o f t h e l e v e l s c h e m e is c h a r a c t e r i s e d b y t he p r e s e n c e o f s e v e r a l

l o w - l y i n g s t a t e s a r i s i n g f r o m t w o - q u a s i p a r t i c l e c o n f i g u r a t i o n s , as e x p e c t e d in an o d d -

o d d n u c l e u s n e a r c l o s e d s h e l l , a n d a r o t a t i o n a l b a n d w i t h i t s b a n d h e a d at 1 .745 M e V ,

354

14- I 425

13- t 3379 I

12- 754 374t

11-

A.K. Singh et al./Nuclear Physics A 607 (1996) 350-362

9-

I- 7- 6 I 478 [

1122 8- ~ 848

7- ~ 8- I 79s

456 6 + ~ 340 4+ z~? 3 +

17 [ [ 5320

528 I

- I 1031 4792 (17) 16 I 502 / 15- 988 f l 4290 (16)

48e (15) /

693 3804 (14)

799 407 I 13- l

t 392 772 T 3005 360 t 733 ~ 2625 353 678 10- l 2 2 7 2

325 ~ 1947 528 t 202 1745

I 172 4~7 1267 1673 701

6 + 825 I 972

5+ 471 t ~ 501

103 3~8 ~ 166 j ~ 103

o

I 4780

414 4368

316 4010 318

3692 2~5 3397

ll2sb

Fig. 2. Proposed level scheme of 112Sb. The 29 keV transition, feeding the isomeric level at 796 keV (8-), is not shown. The excitation energies in keV are given on the fight side of the levels.

most probably built on the 7rg9/~77"g2/2 Q ~'hll/2 configuration. The spin-parity of the

ground and excited states at 103, 166 and 501 keV are assigned on the basis of earlier

experimental data [ 10] and the results of a particle-quasiparticle calculation by van

Gunsteren et al. [13]. Our DCO ratio measurements of the 103 and 398 keV y-rays

are also consistent with these assignments. The 796 keV isomeric state was assigned a

spin-parity 8 - in an earlier theoretical study [ 13 ]. The presence of an isomeric 8 - state,

involving the hi 1/2 neutron orbital coupled to either the ds/: or the g7/2 proton orbital, is a characteristic of the odd-odd antimony isotopes from 132Sb down to ll:Sb. This

level in J l2Sb decays to the first excited 4 + state through emission of two successive

y-transitions of energies 456 and 237 keV. So there are two possibilities, either both the transitions should be quadrupole in character or the 237 keV transition is a dipole

and the 456 keV transition is an octupole. The theoretical results of van Gunsteren et al. [ 13] favour the former. So we have assigned spin-parity 6 + to the level at 340 keV.

In our prompt gated spectra (TAC width ~ 70 ns), the 456 and 237 keV y-rays are

not observed when the gates are set on any of the y-rays originating from higher lying

levels. On the other hand, they are observed in coincidence when the gate is made wide ( ~ 350 ns). The spin of the 972 keV level is assigned 6 because of the dipole character

of the 471 keV y-ray and parity positive because this state does not show any decay

branch to the lower lying negative parity isomeric state. The 202, 325, 353, 380, 392, 407, 486, 502, 528 keV y-transitions and the cor-

responding cross-over y-transitions (viz., 678, 733, 772 keV, etc.) show a stretched dipole and quadrupole character, respectively (Table 1). They form a rotational band


like structure based on the 1745 keV level. The states belonging to this band are as-

signed negative-parity primarily because negative-parity bands of similar structure based -1 2 on the 7"rg9/27rg7/2 ® uhl 1/2 configuration have earlier been identified in ll4.116Sb [ 7,14].

Recently, a similar band structure has been established in a lighter-mass antimony isotope, ~°8Sb [ 11]. A total routhian surface calculation (Fig. 5 of Ref. [ 11 ] ) indicates a

modest prolate deformation of/32 ----- 0.16 for this structure involving the promotion of

a g9/2 proton to the g7/2 orbital across the Z = 50 shell. No such rotational band has earlier been identified in 112Sb.

The negative parity of the levels belonging to this band is also consistent with the

decay mode of the 1947 keV level. This level is found to decay through the emission

of two y-rays having energies 202 and 1122 keV. The DCO ratio of the 1122 keV y-ray indicates a stretched quadrupole character for this transition and it feeds a level at

825 keV. Therefore the level at 825 keV is assigned a spin-parity of 7 - . The assignment

is consistent with the fact that no transition connecting this state to any of the lower lying positive parity states has been observed. The only possible decay mode of this level is

through a highly converted 29 keV transition (unobserved) to the lower lying isomeric

796 keV ( 8 - ) level. A wider gate ( ~ 350 ns) shows a coincidence between 1122 keV

y-ray with the previously known 456 and 237 keV y-rays [ 11]. In the coincidence

spectrum gated by the 478 keV y-ray (1745 keV ~ 1267 keV) presence of a weak 471 keV y-ray (1267 keV ---, 796 keV) along with the 442 keV y-ray lends additional

evidence for the existence of two very close-lying levels. The 1673 keV level also decays predominantly to the 825 keV level through emission of a 848 keV y-transition. The

stretched dipole character of the 701 keV y-ray and the mixed nature of the 848 keV

y-ray (Table 1) are consistent with the spin assignments of the 825, 972 and 1673 keV

levels. The Weisskopf estimate for the lifetime of the 1673 keV ( 7 - ) level is about 5 × 10 -12 s which is well within the prompt coincidence window of 70 ns. The proposed

level at 1267 keV is fed from the 1673 ( 7 - ) and 1745 ( 8 - ) keV levels. The DCO ratios of the corresponding y-rays (478 and 407 keV) are consistent with an assignment

of a spin-parity 8 - to this level. The band head at 1745 keV decays through the 72 keV y-transition. The DCO

ratio measurement indicates a stretched quadrupole character for this y-ray. However, according to the proposed level scheme the transition should be a stretched dipole. Since the proposed spin-parity of the levels at 1673 and 1745 keV levels is consistent with

the measured DCO ratios of several other y-rays e.g., 202, 701,848, 1122 keV, etc., we

think that the ambiguous DCO ratio of the 72 keV y-ray may be due to the uncertainty

in efficiency measurement in this energy region. Another interesting observation is the presence of the two very close lying 13- levels

in the level scheme at 3379 and 3397 keV, respectively. A pair of very close lying 13- levels are also observed in the level scheme of l°8Sb [11] and ll4Sb [14]. In

addition, a band like structure consisting of 295, 318, 356 and 414 keV transitions feeding the 3397 keV ( 1 3 - ) level is observed. The DCO ratios of the y-rays are found to lie between 0.7-0.9. Considering the experimental uncertainties in DCO ratios, these transitions appear to be of stretched dipole character. However, we have not observed


any cross-over transitions and hence the spin parity assignments for these levels are rather tentative.

3. Discussion

I lISb can be described as a proton in the d5/2 orbit The ground state (5/2 +) of 51 coupled to the 0 + state of the even-even ll°Sn core. The first excited state (7/2 +) of

I I ISb ' which corresponds to the lifting of the proton to the g7/2 state, lies at an excitation energy of 851 keV [4]. Eleven neutrons outside the N = 50 shell in 112Sb are occupying

mainly pd5/2 and vgT/2 orbits. Therefore, one would expect to get several low-lying states in l l2Sb arising from the coupling of "n'd5/2 configuration to the neutron quasiparticle

motion in d5/2/g7/2 orbits. At somewhat higher excitation energy one would expect several positive parity and negative parity states arising from the coupling of ds/2/g7/2, g7/2 and h11/2 proton, respectively, to the neutron quasiparticle motion. The theoretical study of Gunsteren et al. [ 13] also supports such a picture for the low-lying states of

light mass odd-odd Sb isotopes. The ground state and other excited states below 1 MeV in ll2Sb presumably belong to this category. The ground state (3 +) most probably

arises from the coupling of the yrd5/2 proton to the 7/2 + ground-state configuration of I llSn. Similarly, the 103 keV (4 +) first excited state arises from the coupling of the

7"rd5/2 proton to the first excited 154 keV 5/2 + state of lJlSn [15]. Other low-lying positive parity excited states at 166, 340, 501 and 972 keV arise presumably either

from 7"rd5/2 @ Pg7/2, 7"rd5/2 ® pd5/2, yrg7/2 @ Pg7/2, etc. configurations or their suitable admixtures. The presence of an isomeric 8- state at 796 keV is consistent with the systematics of such isomeric states in higher mass odd-odd Sb isotopes, arising from

the ~ds/2®vhl 1/2 configuration. Such 8 - states are observed at lower excitation energies in heavier odd-odd Sb isotopes [ 10]. The higher excitation energy in l l2Sb of this state

is connected with the energy of the phi 1/2 orbital which moves upward with decreasing neutron number as seen in the neighbouring odd-Sn isotopes. The observation of another isomeric negative parity state at 825 keV is interesting because no such pair of isomeric states have been observed in neighbouring odd-odd Sb isotopes. This state is likely

to be a member of the multiplet arising from rrd_~/2 ® phil~2, ~g7/2 @ Phll/2 or their admixtures as these are the only low lying two quasiparticle configurations leading to a negative parity state.

In addition to these two-quasiparticle states, one would expect to find several collective states arising from the coupling of the odd-proton and odd-neutron motion to the quadrupole vibration of the even-A Sn core near 2 + excitation energy (1.212 MeV) of l l °Sn. These levels should decay to the ground state through successive strong collective E2 transitions. However, no such level has been observed in the present work.

The strongly coupled negative parity rotational band based on the 1745 keV 8 - level --1 2 is most probably based on the ~g9/2~'g7/2 deformed proton-particle-hole configura-

tions [4] coupled to the / / h l l /2 neu t ron i.e. a configuration resulting from all the high j orbitals in the N = 4 and 5 shell. This band structure has earlier been observed in


60

4o

%

2O

v

- - . _ l k

0.2 0.3 0.4 0 5

u u i

~ i i I _

0.2 0.3 0.4 0.5

hco(MeV)

Fig. 3. Plots of the dynamical moment of inertia j(2) and relative alignment in n2Sb. Solid circles represent the transitions belonging to the even-spin sequence while the open triangles represent the transitions of the odd-spin sequence.

heavier odd-odd Sb isotopes. Very recently, a similar band structure has been observed

in msSb by Cederk~ill et al. [11 ]. They have performed total routhian surface (TRS)

calculation (Fig. 5 of Ref. [ 11]) based on a Woods-Saxon potential using the Cranked

Strutinsky shell correction approach. The results of their calculation show that the low- est lying deformed minimum that appears in the total routhian surfaces for l°SSb can

- I 2 be associated with the rrg9/2rrgT/2 ® Uhll/2 configuration. This minimum starts to de- velop at ho~ ~_ 0.18 MeV and the corresponding deformation parameters are/32 -~ 0.16, /34 ~ 0.04 and 3/-~ 1.0 ° and the observed strongly coupled band was assigned K = 5.

The 112Sb isotope contains four additional neutrons and the deformation of the band

head is predicted to increase with increasing neutron number [4]. Inspection of the Nilsson diagram (Fig. 4 of Ref. [ 11 ] ) for the neutron levels in this mass region near f12 ~ 0.16 shows that the last neutron of 112Sb would occupy U~ = 3/2 or 5/2 orbital

originating from the Vhtl/2 shell model state. This would give rise to a higher K-value (6 or 7) for the band head. We have calculated the relative alignment as a function

of increasing rotational frequency (Fig. 3), assuming K = 6 and a constant moment of inertia core with Jo = 23hZ/MeV [4] and using the standard expressions for calculating Ix and w [ 16]. The relative alignment plot does not show any abrupt change although the dynamical moment of inertia plot for the even-J states ( a = 0) shows irregular behaviour (Fig. 3). Cederk~ill et al. also observed a similar feature for their relative


1.5

1,0

~Q Z < 0 . 5

b-

o 0 .0

Z o

0 .5 0

O.. I

- - 1 . 0 [/9 <

1.5 0 . 0 0 . i 0 . 2 0 . 3 0 .4 0 .5 0 .6 0 . 7 0 .8 0 . 9 l .O

h a ) ( M e V )

Fig. 4. Quasiproton routhians for ll2Sb calculated at f12 ~ 0.16, f14 --~ 0 .04 and y _~ 1.0% The parities and signatures of the routhians are indicated in the following way: ( + , + 1 / 2 ) , solid; ( + , - 1 / 2 ) , dotted; ( - , + 1 / 2 ) , d a s h - d o t t e d ; ( - , - 1 / 2 ) , dashed.

alignment plot. The initial alignment obtained for this band in l l2Sb is smaller than the same in 1°8Sb, a consequence of occupation of somewhat higher-ll h11/2 neutron orbital in 112Sb.

The high-spin behaviour of 112Sb was investigated through Cranked Shell Model (CSM) calculations. The deformation parameters were taken to be the same as those found for the deformation minimum of l°sSb [ 11]. The routhians for protons and neutrons are shown in Figs. 4 and 5, respectively. The AB crossings for protons and neutrons at 0.56 and 0.37 MeV, respectively, are blocked because of the presence of

1.5 >

1.0

Z <

,~ 0 . 5 [..

0 0 . 0

a O ¢d

- 0 . 5

Z I l.O

< 29 O" 1.5

N \ \ , - , \ \ -- . /"--- " ,

/

0 . 0 0.1 0 . e 0 . 3 0 .4 0 .5 0 . 6 0 . 7 0 .8 0 . 9 .0

hc0(MeV)

Fig. 5. Quasineutron routhians for l l2Sb calculated at f12 ~-- 0 .16 , f14 ~ 0 .04 and y "" 1.0 ° . The notations are the same as in Fig. 4.


last odd proton and odd neutron. The calculations predict an allowed BC crossing for neutrons at hto _~ 0.56 MeV. However, experimentally the deformed band has been established up to ha) = 0.48. Thus, the predicted band crossing has not been observed in 112Sb"

Nes et al. [ 14] showed that a striking similarity exists between the excitation energies of the rotational band built on the above configuration in ll4Sb and a positive

parity rotational band based on 2p- lh proton configuration in 115Sb. A rotor-plus two quasiparticles model calculation carried out by them shows that the observed A J = 1 bands in 1148b and 1165b can be explained in terms of rotational alignment of the hi1/2 neutron with the deformed rotating odd-A core. However, the AJ = 1 band in ll2Sb shows no such resemblance with the 2p- lh band structures observed in l llSb and 113Sb '

- 1 - 1 involving the lg9/2 proton hole. There is one significant difference between the 7rg9/2®

(77"g7/2) 2 proton 2p- lh band observed in 111Sb and those observed in higher mass Sb nuclei. In lllSb, this band is not observed down to the expected 9/2 + band head but instead decays out at the I 'r = 21/2 + band member. LaFosse et al. [4] offered a possible explanation for this observed difference. They attributed this change in decay process to the reduced value of deformation of the 7r9/~® ('rrgT/2) 2 band head in 111Sb compared with the heavier mass nuclei. The results of their potential energy surface calculation for this configuration show that the deformation decreases steadily from 132 = 0.206 for ll9Sb (N = 68) to /32 = 0.172 for lllSb (N = 60) as the neutron number decreases.

In addition, the potential minimum is broader in the lesser deformed nuclei, which al- lows for vibrational-like levels to be mixed with the rotational ones. This may be the reason for the observed complexity in the decay modes of this band in lllSb. They also observed another strongly coupled negative parity band (Band 4 of Ref. [4] ) and sug-

- 1 gested that this band originates from the coupling of the qrg9/2 ® (~g7/2) 2 configuration

to a two-neutron negative parity state of the ~ l°Sn core. The low-spin members ( 8 - and 9 - ) of the observed negative-parity band in 112Sb

decay predominantly to several low lying negative-parity states at 1673, 1267, 825 and 796 keV, where the last state is most likely populated by a highly converted 29 keV transition which was not observed in the y-ray spectrum. These negative parity levels

most probably arise from two quasiparticle ¢rd5/2/ ~g7/2 ® hll/2 configuration. These decay modes are consistent with the intrinsic structures of the negative parity band and of lower lying two-quasiparticle negative parity levels. The possibility of admixture of vibration-like states in the lower-spin members of the band as well as the negative parity levels at 1673 and 1267 keV cannot be ruled out. The 14- member of the band decays to two very close lying 13- levels at 3379 and 3397 keV levels. The additional 13- at 3379 keV level may arise from the coupling of the "rrg9~ 1 ® ~'hll/2 configuration to

a positive parity quasi-neutron state of the l l°Sn core. As already mentioned, one such band arising from the 2p- lh proton configuration to two quasi-neutron 7 - state of the II°Sn core has been observed in 111Sb [4].

The experimental B (M 1 ) / B (E2) ratios of the y-transitions in this band are plotted in Fig. 4 as a function of spin. In obtaining these ratios we have assumed all the intraband

360 A.K. Singh et aL/Nuclear Physics A 607 (1996) 350-362

40 1-

35 ,.Q

~ \ 30

v

25

20

15

10

• E x p L 1 C o n f i g u r a t i o n : rrgg/a uh11/2

fo r K=5 - - - for K=6

\

13 10 12 14 16 1/3 2 0

S p i n (h)

Fig. 6. Experimental ly measured (scattered points) and theoretically calculated ( l ines) B ( M I : I --, 1 - 1 ) / B (E2 : I --~ I - 2) ratios.

I --~ 1 - 1 transitions to be pure M1. The experimental relative intensities have been

obtained from the gated spectra. The large uncertainties associated with the intensities

of the weak cross-over E2 transitions give rise to large error bars in the corresponding

B(M1)/B(E2) ratios. However, the experimental ratios still show a definite pattern,

i.e., in the low-spin region they increase with increasing spin, then suddenly drop to a

very low value near 1 ~ = 14- and again show a upward trend. It may be mentioned

here that these ratios tor the same band in l°8Sb [11 ] show identical behaviour near I ~r= 14-.

The corresponding theoretical B(M1)/B(E2) ratios are calculated using D6nau-

Frauendorf formula [ 17,18] for K = 5 and 6 and are shown in Fig. 6. The g-factors were

taken from Ref. [ 19] and the quadrupole moment was calculated according to the liquid

drop formula [20] to be 2.7 e.b. It is observed that there is a sizable difference between

the theoretical and experimental values at low spins and in addition the smoothly falling

behaviour of the theoretical values is not in agreement with experimental observations. - I 2 In 111Sb, it has been suggested [4] that the low-spin members of the ~g9/2~gT/2 ®

vhi 1/2 band may be mixed with particle-vibration multiplets. However, with increasing rotational frequencies, the band is expected to become more deformed and in this region

one would expect a gradual decrease of this ratio with increasing spin.

The 14 member of the band decays to two close lying 13- levels at 3379 and

3397 keV. One of these states belongs to the band based on the 7"rg~/~g~/2 ® Phil~2 configuration and other most probably arises from the coupling of this configuration to

a two-quasineutron positive parity state of the ~J°Sn core. It may be mentioned that two 10 + states have been identified in 112Sn at 4680 and 4819 keV [21] of which the level

at 4680 keV was interpreted as a spherical ( r ' h l l / 2 ) 2 s t a t e . Two similar I = 10h states have also been observed in I I0Sn of which the 5227 keV state (10 (+)) was suggested


to be the spherical (~ 'h l l /2 ) 2 s tate which is similar to the 4680 keV state of l l2Sb [21].

Thus the probable quasiparticle assignment of the other 13- state (at 3379 keV) is -1 2 ~g9/27rg7/2 ~ ~'h11/2 coupled to the 10 + state of the ll°Sn core. The sudden decrease

of the B(M1)/B(E2) ratio for the transitions depopulating the 14- level may be due

to significant mixing of these two band structures. Since the other band structure could not be established in the present work, the exact configuration of this band and the

effect of its admixture on the decay modes of the yrast 14-, 15- states cannot be

predicted. However, on the basis of the observed variation of the B ( M 1 ) / B ( E 2 ) ratios with increasing rotational frequencies, it may be concluded that the members of the

-1 2 7"rg9/277"g7/2 @ ~'hl 1/2 band contain significant admixture from states of different origin.

The other interesting feature of the level scheme of l l2Sb is the presence of four

y-rays of stretched dipole character in cascade (295, 318, 356, 414 keV) feeding the 13- level at 3397 keV. In 114Sb, Paul et al. have reported [7] two pairs of bands

(referred as bands 1/2 and bands 6/7) which shows strong AI = 1 transitions. The -1 2 bands 6/7 were associated with the 7"rg9/27rg7/2 @ uh 11/2 configuration which is the same

for the deformed band reported in the present work. A possible configuration for bands 1/2 of Ref. [7] was suggested to be a high-K 7"rgg/~qrg2/2 ® ugy/2 structure which was

consistent with the observation that the alignment was small for these bands. However, this configuration have a positive parity and the cranking calculations did not identify any other negative parity configuration which exhibits the observed feature of very low

alignment. In line with this discussion the levels de-exciting through 295, 318, 356, 414 keV y-rays, may be assigned positive parities. However, it is to be noted that in

such a situation the cascade de-excites through the low energy 295 keV El transition

and no other positive parity levels could be identified in the vicinity of the 3692 keV level. In the present situation it is difficult to ascertain the structure of this cascade due

to scarcity of data.

4. Conclusion

The level structure of ll2Sb has been extended up to Ex -~ 5 MeV and J~" = 17-.

The low energy levels are interpreted as two-quasiparticle states and a strongly coupled negative parity rotational band based on deformed 2p- lh proton state has been identified

at higher excitation energy. The decay modes of the members of this band indicate

significant admixture with a band of somewhat different origin.

Acknowledgements

The authors wish to thank Dr. S.K. Dutta and all the staff members of the Pelletron Centre, NSC for their cooperation during the experiment. G. Gangopadhyay and A.K. Singh want to thank U.G.C., New Delhi for financial assistance.


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High-spin structures in 112Sb