Z. Phys. A 356, 125 132 (1996) ZEffSCHRIFT FOR PH IK A @ Springer-Verlag 1996

High spin states in Jagbir Singh ~, Harjeet Kaur 1'*, A. Sharma 1, J. Goswamy 1'**, D. Mehta 2, Nirmal Singh l, P.N. Trehan *, E.S. PauP, R.K. Bhowmik 4

~Department of Physics, Panjab University, Chandigarh 160014, India 2Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India 3Oliver Lodge Laboratory, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK 4Nuclear.Science Centre, JNU, New Delhi 110067, India

Received: 9 April 1996/Revised version: 1 July 1996 Communicated by B. Herskind

Abstract. High spin states of 121Te, populated in the 14Cd (I~B, p3n) reaction, have been studied through y-ray

spectroscopy. The level scheme has been established up to J~ = 51/2-. Three-quasiparticle states, based on the gg7/22 ® Vhll/2 and 7r, g7/2d5/2 ® Vhll/Z configurations, have been identified. A favoured 39/2- state is suggested to be the fully aligned [7~g7/2216+ ® [vh11123127/2- yrast non-collective oblate configuration. This assignment is supported by Total Routhian Surface (TRS) calculations which also suggest a similar oblate assignment to the states at J= = 21/2- and 23/2-. A higher 47/2- state is also found and is suggested to be the fully aligned [ng7/2216+ ® [vh11125135/2_ configuration.

PACS: 21.10.-k; 21.60.-n; 25.70.-z; 27.60.+j

Introduction

Investigations of collective rotational structures in nuclei in the region of the spherical Z = 50 closed shell have received added impetus in recent years. The collectivity in these nuclei is postulated as due to the quadrupole defor- mation induced by the particle-hole excitations involving the promotion of g9/2 protons across the closed shell to either of the d5/2, g7/2 or ha 1/z orbitals [1, 2]. In the even-A lO6-t tasn nuclei [3-5], collective rotational bands based on the 2p-2h 7zg7/22 ® 7Zg9/z -2 configuration have been observed. This 2p-2h deformed Sn-core, coupled to a h1~/2 neutron in ~ lSn [6] and to a low-f2 valence proton (nd572, 7197/2, nh11/2) in odd-A Sb nuclei [7, 8], is found tO give rise to AI = 2 decoupled rotational bands. The single 7Zg9/2 excitations also lead to collective A ! = 1 strongly coupled bands based on the 7~g7/22 ® g99/2-1 (2p-lh) configuration in the odd-A Sb [7,81, I [9-12]

*Present address: Govt. College, S.A.S. Nagar, Punjab, India **Present address: Govt. College for Women, Ludhiana, Punjab, India

and Cs [13, I4] nuclei. In Te nuclei, a collective rotational band based on the well deformed 4p-2h (nh11/22 ® ngT/22 ® ngg/2 -2) configuration has been re- cently reported in the even-A 1,2 - 116Te nuclei [15-18]. However, as yet nothing is known regarding the existence of well developed rotational bands in the odd-A

15-119Te nuclei [19-22]. In addition to the rotational bands based on p-h configurations, collective prolate bands based on a low-O nh11/2 intruder orbital in Sn [4], Sb [8, 23] and I [9-12] nuclei and also collective oblate bands based on a high-f2 nhl 1/2 orbital in I nuclei [10-12] have been observed. Several yrast aligned non-collective oblate states have also been identified and characterized in these near spherical nuclei with Z > 52. Investigations have revealed such states in the 118"121'122Xe [24-261 nuclei at spins 20-30h. In the odd-A 115-1211 nuclei [9-12], the yrast h11/2 rotational bands are crossed by non-collective oblate states at spins around 20h. In recent investigations of the t ~4-1 t9Te [16-22, 27] isotopes, sev- eral non-collective oblate (y = 60 °) states based on the 7r[-(g7/2)216+ ® v[(d5/2)X(hll/2)Yl configuration (x = 0,1, y = 1-3), have been identified and interpreted [20, 21, 27, 28] in the framework of Total Routhian Surface (TRS) calculations.

In order to explore further the above mentioned struc- tural features in the heavier odd-A 121Te isotope, an experiment has been performed to investigate high spin states using heavy-ion fusion-evaporation reaction and gamma ray spectroscopic techniques. The level scheme has been established u[o to J= = 51/2-. Several non-collec- tive oblate states have been identified and interpreted in terms of TRS calculations.

Experimental details and data analysis

The excited states of 121Te were populated ifl the t>~Cd (11B, p3n) fusion-evaporation reaction at a beam energy of 64 MeV. The 11B ion-beam was provided by the 15UD- pelletron accelerator at the Nuclear Science Centre, New Delhi. The target used was a 3 mg/cm 2 thick en- riched lX4Cd foil, onto which 20mg/cm z Pb had been

126

8s83 @ i (51/2) 8293

912.0 I 7671 121Te > 7 6 . 6 56~.3 611,0 1230,7

, 43/2 + 7060 (47/2 ~ Jr 7062 41/2 561.8 514.5~ 6545 (43/2) 287.7 t 6774

39/2 638"7 1 5907 1240.7 "" ~ 5534 @ / 6 6 8 . 5 ~/7 - 21--7 -4 -~- - - g'68"9-- . . . . ,39/2- 224.._..~

35/2 509"64. 5179 354.1 35/2 (+) 5020 I~'~ (+) i 33/2 - 609.3.11 ~ . 2 ~ _ / ~ . . . . . . 393.6/'4785 ~ 936.2 37/2(+) ~ 'I' ~ 3 1 - , . J . l ~ 4 . r 9 29/2- 547.4 4238 ~ 31/2- 807.2

740.1 8 4 8 . 9 / , ,l..,^ 29__.9/2__ 1" t 4372~ ..11.. 3671 ../ I izu I . . . . = 27/2(+1 ;J[ ~'27/2 J,,, 3401 J,.aoz. t_ 110_2.2 2712- 971.0

23/2 (+1 7 ~1~0 2952 26 5"4""~25/2 ~ 3136 N. \419.4 t 3401 I o~, ,N,. ~ 3 8 4 4 936.7 1120.3 ou~. i ~ . "~a-~-~ . ~ ~ "23/2" 1069.4

21/2 # 20,5 ~ / 6 2 2 " ~, 2332 17/2-416.2 4. 1599 36_1.6~19/2- .4, - 677.8 1654 .%. 13/2- 6 2 4 ' 3 1 975 673.7 728.5

3 ~ ~t 15/2- 926 9/2- 536. i 438 681 1 631.5 11/2- 144.5.~,, "~ Tla =154d ~ 294

Fig. 1. The partial level scheme of ~2~Te deduced from the present work

evaporated in order to stop the recoiling nuclei. The gamma-rays emitted by the evaporation residues were detected using the Gamma Detector Array (GDA). The array consisted of eight Ge detectors (efficiency ~ 23% relative to 7.6 cm x 7.6 cm NaI(T1) crystal at 1.33 MeV) and a 14-element BGO (Bismuth Germanate) multiplicity filter. Each Ge detector was operated in conjunction with a symmetrical BGO Compton suppression shield. The Ge detectors, located at 18 cm from the target position, were mounted in two groups, making angles of 99 ° and 153 ° with the beam direction and tilted at _+ 23 ° with respect to the horizontal plane. The multiplicity filter consisted of two sets of seven closely packed hexagonal BGO elements (3.8 cm x 7.5 cm long), mounted above and below the tar- get chamber at a distance of 4 cm from the target. The coincidence events, in which two or more Ge detectors fired within a 200 ns time window, were written on mag- netic tape in the LIST mode. Approximately 100 million events were collected in this experiment with a hardware condition of BGO multiplicity, K ___ 2. The Compton sup- pressed Ge detectors were calibrated for energy and effici- ency using the standard energy calibration 7-lines from the radioactive decay of t33Ba, 134Cs and ~52Eu.

In the off-line analysis, the recorded 7-7 coincidence data were sorted event by event into two dimensional E~-E~ matrices. The gains of the Ge detectors were soft- ware matched to 0.7 keV/channel before incrementing the matrices. The gamma-ray coincidence relations were es- tablished by setting gates on the photopeaks of the indi- vidual transitions and projecting the corresponding coin- cidence spectra. Gates were also set on the background in the vicinity of the photopeaks to remove the contributions due to the background underlying the photopeaks of the gated transitions.

The level scheme of 121Te from the present work is shown in Fig. 1. The ordering of the transitions in the construction of the level scheme is based on the coincid- ence relationship between them and on energy and inten-

sity balance arguments. The representative coincidence spectra depicting the various newly placed transitions are shown in Fig. 2. Gamma-ray intensities for the assigned transitions were determined from different spectra in coin- cidence with the transitions deexciting lower levels and from the total projection spectrum, deduced from the E~-Ey matrix. The energies and relative intensities of the 7-transitions assigned to 12tTe are given in Table 1.

In order to obtain information on the gamma ray multipolarities, Directional Correlation (DCO) ratios [29] were extracted from the coincidence data. The coin- cidence events were sorted into an asymmetric matrix with 153 ° detectors on one axis and 99 ° detectors on the other axis. By setting gates on the E2 transitions along the two axes of this matrix, the peak areas A.zp (153 °) and A~p (99 °) were obtained from the projected spectra. The DCO ratios for transitions (),p) in the projected spectra were deduced using the relation

A~p(153 °) 07p(99 °) e~y(153 °) D C O ; , p = - -

e,;p(153 °) A,~p(99 °) ~r~(99 °)

e(99 °) and e(153 °) are the detection efficiencies of the set of detectors at 99 ° and 153 ° respectively and the subscripts yg and yp correspond to the gated and the projected transition energies. The DCO ratios of the stretched E2- stretched E2 and the stretched dipole-stretched E2 cor- relations are expected to be 1.0 and 0.55, respectively. The DCO ratios and the assigned multipolarities for transitions in t21Te, along with their placements in the level scheme are also given in Table 1.

Experimental results and discussion

The low lying levels in t21Te were earlier investigated in the fl/EC decay studies of t 2 1 I (Wl/2 = 2.12 h) by various workers [31, 32] and most recently by Mantica et al. [33]

. - , 6

x

= o 2

U

6

× 4

g 2 U

Gate : 265 keV (a)

I I I /

200 400 600

~, ~ Gate : 265 keV • ~ (a)

i , , ,

700 900 1100 Energy (keV)

y

" ~ 4 ~ r/l

2O0

~, Gate : 719 keV r (d)

V 1 2 0 ]

I ~ - r - . ~ e a , . ¢ a • ¢ g . ~ , , q 'g , . -g

400 600

Gate : 71 9 keV I ~ (d)

o 2 N$ ~ m

700 900 11 O0 Energy (keV)

127

3 -~ Gate : 533 keV (b)

2 f ~ . ~ v v v ' v ~ v

U

300 600 900 1200 Energy (keV)

X

~ 1 ~ Gate : 1120 keV , (c)

Y - - contamination

o 7 ~ g ,q .,,. r, ,

k

i

200 400 600

4

~ 2

o

2 Gate : 1120 keV , ~ (c) ?

P ~e2 ~ , m ~ e-.

5" T

700 900 1100

Energy (keV)

W " O

o r,.)

o r,.)

400 600

Gate : 217 keV 3

2 o: I

I o ~ _ ~ e q

, ~ I i i

800 1000 1200

Energy (keV)

Fig. 2a e. The gamma ray coincidence spectra with gates on a the 265 keV doublet (27/2 ---, 25/2- and depopulating the 33/2- state) b the 533keV (29/2--+27/2) transition e the l l20keV doublet (25/2---- ,21/2- and populating the 27/2 state) d the 719keV (27/2 C+)--+23/2 ~+~) transition and e the 217 keV (39/2 ~ 37/2) transition

128

Table 1. Gamma ray energies, intensities, DCO ratios and multipolarities for transitions assigned to ~ 2~Te

E~ (keV)" I~ b DCO Ratio Multipolarity Assignment

144.5 2.5 M1 c 9/2- ~ 11/2- 217.4 9.9 0.67(14) Dipole 39/2 ---, 37/2 224.9 2.2 deexciting 39/2- 264.9 4.2 deexeiting 33/2- 265.4 13.6 0.74(8) d Dipole 27/2 ---, 25/2- 287.7 6.2 0.90(14) E2 (47/2-) ---, (43/2-) 315.9 26.6 0.57(5) M1 23/2- ~ 21/2- 354.1 17.2 1.09(12) E2 39/2- ---* 35/2- 361.6 68.2 0.50(5) M1 21/2- --, 19/2- 373.7 7.7 0.54(11) (El) 33/2- --* 31/2 ~+) 384.4 4.3 23/2- ~ 23/2- 393.6 20.4 0.53(7) M1 35/2- ---, 33/2- 416.2 13.0 1.12(16) E2 21/2- ~ 17/2- 419.4 6.0 0.50(8) ?vii 25/2- ---, 23/2- 487.2 4.4 0.56(11) M1 33/2 ~+) --* 31/2 t+~ 509.8 12.7 0.57(10) Dipole 37/2 ~ 35/2- 514.5 5.5 0.62(13) Dipole 43/2 ---, 41/2 532.7 6.4 0.56(12) Dipole 29/2 ---, 27/2 536.3 2.4 E2 c 13/2- ~ 9/2- 547.4 16.2 1.12(13) E2 33/2- ---, 29/2- 561.8 7.8 above 41/2 564.3 3.5 above 41/2 576.6 3.4 above 41/2 609.3 7.2 1.09(19) E2 35/2 ~+) ~ 31/2 I+~ 611.0 2.7 feeding 43/2 624.3 9.0 E2 ~ 17/2- --* 13/2- 631.5 100.0 1.04(8) E2 15/2- ---, 11/2- 638.7 12.8 0.50(8) Dipole 41/2 --, 39/2 668.5 4.1 37/2 --, 35/2 + 673.7 13 5 0.28(4) M1 17/2- ~ 15/2- 677.8 10.2 0.90(13) E2 23/2- ---, 19/2- 681.1 7.0 M1 ~ 13/2- ---, 11/2- 719.0 26.0 1.06(10) E2 27/2 (+) ~ 23/2 (+) 728.5 86.3 1.00(7) E2 19/2- ---, 15/2- 740.1 19.0 0.89(10) E2 31/2 ~+) ---, 27/2 ~+) 804.1 8.7 0.52(13) 25/2- --,, 23/2- 807.2 10.8 1.02(16) E2 35/2- ---, 31/2- 848.9 5.0 feeding 27/2 + 912.0 4.5 0.69(23) Dipole (45/2) ~ 43/2 936.2 3.0 feeding 31/2- 936.7 26.1 0.53(4) (El) 23/2~+) --, 21/2- 971.0 15.9 0.99(11) E2 31/2- ~ 27/2-

1062.2 7.3 0.98(23) E2 23/2- --, 19/2- 1069.4 19.0 1.03(12) E2 27/2- ---, 23/2- 1102.2 17.9 0.79(10) E2 29/2- ~ 25/2- 1120.3 26.5 1.03(11) ~ E2 25/2- ~ 21/2- 1120 1.5 feeding 27/2 1230.7 4.4 1.2(3) E2 (51/2-) ~ (47/2-) 1240.7 11.5 0.87(14) (E2) (43/2-) ---, (39/2-)

"Energies are accurate to 0.3 keV for strong transitions. The errors increase to 0.7 keV for weaker transitions (relative intensity I.~ < 3) u Errors in 7- ray intensities are 5-20% c Assigned multipolarity adopted from the earlier work [30] d Value given for the doublet

t h rough g a m m a - r a y and conve r s ion e lec t ron spect ro- scopy. F r o m these studies, the g r o u n d state (1/2+), first exci ted state at 2 1 2 k e V (3/2 +) and an i somer ic s tate at 294keV (11/2- , T1/2= 154d) have been identif ied as single part ic le states co r r e spond ing to the o c c u p a n c y of sl/2, da/2 and hi 1/2 neu t ron orbi tals , respectively. P rev ious in -beam studies of 121Te by H a g e m a n n et al. [30], using the a 198n(~ ' 2n) and lZlSb(d, 2n) react ions, revealed two

posi t ive par i ty A I = 1 coup led bands based on the ds/2 and 97/2 neu t ron orbitals. Also, the low lying states up to J ' ~ = 2 3 / 2 - above the 11 /2 - i somer were es tab- lished.

The level scheme of 121Te, s h o w n in Fig. 1, has been establ ished up to j r = (51 /2- ) in the present inves t iga- tions. The level s t ructure above the vhl 1/2 i s o m e r has been ex tended substant ia l ly by the add i t ion of 35 t rans i t ions to

129

that reported earlier by Hagemann et al [30]. The prelimi- nary results for this nucleus from the present analysis have been reported in [34].

The present level scheme preserves most of the features reported earlier by Hagemann et al [30]. The earlier reported positive parity AI = 1 level sequence [30] based on a vgT/2 orbital along with the crossover transitions has been confirmed (not shown in Fig. 1). The other small sequences of gamma transitions shown earlier to feed the ground state and the 3/2 + first excited state could not be established conclusively in the present work owing to their weak population. The two level sequences built on the 11/2- isomer, known previously up to J== (23/2-) and (21/2-) are confirmed in the present work. The earlier reported 724-320keV and 202-675keV cascades [30], shown to feed the 9/2- and 13/2- states, respectively, are not observed. The placement of the 265 keV transition, shown earlier [30] to feed the (21/2-) state is found to be different. In the present level scheme, two mutually coinci- dent 265 keV transitions have been placed (Fig. 2a). The 265.4keV (27/2 --+ 25/2-) transition is placed on the basis of its strong coincidence with the 533 (Fig. 2b) and 1120 keV (Fig. 2c) transitions and its intensity value. The place- ment of the 264.9 keV weak transition, depopulating the 33/2- level, is well supported from the facts (i) the 265 keV transition is seen in self-coincidence (Fig. 2a) (ii) it is found to be in weak coincidence with the 937keV (23/2(+)-+ 21/2-), 719keY (27/2(+)-+ 23/2 (+)) (Fig. 2d), 849keV (feeding 27/2 I+)) transitions and also with the 394keV (35/2- + 33/2-) and higher transitions (iii) the observation of the 1120 keV transition as a self-coincident doublet (Fig. 2c) and not in coincidence with the transitions of sequence labelled 1 (Fig. 2c, d) and the 849 keV transition. The 739 keV transition shown earlier [30] to feed the 9/2- level is not seen in the present work; however a rather strong 740keV transition has been placed as 31/21+)--+ 27/2 I+) (Fig. 2d).

During the final stages of this work, a short note by Blasi et al [35], reporting the experimental data on 'Z'Te appeared in the literature. The results seem to be in general agreement with those presented here. The main disagreement noticed with this work is regarding the part of the level scheme between the 25/2- and 33/2- levels and parallel to the l102-547keV cascade. In the level scheme proposed by Blasi et al. [35], this part is shown as consisting of a 536keV (27/2(+)-+ 25/2-) transition and a cascade of 849-265 keV transitions from the 33/2- state to the 27/2 (+~ state. However, the present work does not support this placement of the 536keV transition as the transitions of the sequence labelled 1 are not seen in coincidence with the 1120keV transition (Fig. 2c). Also, no 536 keV transition is seen above the 25/2- level, rather a weak 536keV transition is placed as 13/2- -+9/2- , which is well confirmed from the energy sum relationships and intensity flow in the gates above. This placement is also consistent with the one given by Hagemann et al [30]. Further, the placement of the 533 keV transition, shown by Blasi et al [35] as 13/2- ~ 9/2-, seems to be erroneous as it does not follow the energy sum relation- ships. The placements of the 265 and 849 keV transitions in the present level scheme have been discussed earlier in this section. The placement of the 533 keV (29/2 ~ 27/2)

transition is well supported by its coincidence relation- ships (Fig. 2a-c) and intensity value in the present work. This transition, despite its good intensity, is not found to have any links with the upper part of the level structure (Fig. 2b). Other disagreements noticed are (i) The place- ment of the second 265 keV transition by Blasi et al [351 as 27/2- -+ 25/2- is ruled out because 265 and 1120 keV transitions are not found to be in coincidence with the 971 keY transition (Fig. 2a,c) (ii) The ordering of the 1241 and 288 keV transitions in the present work is found to be different. However, the present ordering is well supported by the intensity values (ii i)The multipolarity of the 217 keY (39/2 --+ 37/2) transition is found to be dipole. The 562, 564, 611, 577 and 912keV transitions have been placed above the 41/2 level as suggested by Blasi et al [35]. In the present work, these transitions are seen in the spectra with gates on the lower transitions (Fig. 2e), how- ever, their placement was difficult due to the presence of intense close-lying transitions in the 12°'122Te [36] and 120'1211 [37, 11] nuclei, also populated in this reaction. The ordering of the 217 and 639 keV transitions could not be ascertained from the present work due to their similar intensity values. The tentative placement has been done keeping in view the observation of the 354 keV peak in the spectrum gated by the 639 keV transition.

As in the case of other Te nuclei [15-22, 27, 361, the level structure of tZ'Te is complex due to the mixing of collective quadrupole excitations with the members of the bands based on quasiparticle configurations. The level scheme, shown in Fig. 1, mainly consists of three se- quences of E2 transitions labelled 1, 2, 3. The interpreta- tion of the level scheme becomes easier by taking the excitation energy and spin value of the 11/2- isomeric state to be zero and comparing it with that of the neigh- bouring even-A 12°Te isotope [361. The lower states with J= = 11/2-, 15/2-, 19/2-, 23/2- have been observed in all the odd-A Te nuclei and are interpreted [30] as the aligned coupling of an hi 1/2 neutron to the 0 +, 2 +, 4 + and 6 + vibrational states of the neighbouring even-even Te core [36]. The levels with J = = 9/2-, 13/2-, 17/2- and 21/2- can be attributed to the Jma~ -- 1 coupling states where J~ax is the maximum aligned angular momentum of the hi 1/2 ® R (core spin value) multiplets [30]. Following the interpretation of quasiparticle states on the basis of one broken-pair model (BPM) in 11s.~2OTe [361, the yrast 23/2- state can also be interpreted as/z97/22 (~) vh 11/2. The non-yrast 23/2-(62 +) state, which is about 350 keV higher than the yrast 23/2- (61 + ) state in the odd(even) 11~-lZ°Te nuclei [15-22,27,36], is based on the ny7.zds/12@vh1112 quasiparticle configuration. In the lighter 5"117'119Te isotopes [19-221, a strong sequence of E2 transitions built on the non-yrast 23/2- state has been seen, however, no such sequence is observed in 121Te. By comparing the members of the sequence label- led 3 with the positive parity yrast states in 12°Te, the 27/2- state is the fully aligned (vhl 1/2) 3 state correspond- ing to the (vhtl/22)lo+ yrast state in l'8'lZ°Te [36]. The yrast 31/2-, 35/2- and 39/2- states are suggested to result from-the (71:97/2) 2 @ (vh t 1/2) 3 configuration with/zg2/2 con- tr ibuting the even spins (2, 4, 6).

Most of the intensity is found to flow through the 21/2- (Jmax -- 1) coupling state, which indicates it to be

130

47/2- 7528

4312- 6552

47/2- 7393

(4_7/2 - ) 6768 4312- 6567 (43/2-) 6480

41/2 + 5823

39/2- 5584

37•2 + 4920 35/2- 5085

33/2 + 4062

29/2 + 3532

25/2 + 3020

3112- 4239

27/2- 3323 25/2- 3054-

23/2- 2314 23/2- 2008 2112- 21350

19/2- 1384 17/2- 1432

13/2 799 15/2- 671

11/2- 0 912 :78

31t2- 4311

27/2- 3363

23/2- ~368

39•2- 5186 41/2- 5188 39/2- 5240

35•2- 4993 37/2- 4771 35/2- 4885

33•2- 4491

31/2- 4117 33/2- 4188 31/2- 4078 29/2- 3944

29/2- 3407

27/2- 3087 27/2- 3107 25/2- 2745 25/2- ~84~

23/2- 242!

23/2- 2011 2312- 2038 21/2- 1841 21/2- 1721

19/2- 1358 17/2- 1338 19/2- 1360 17/2- 1305

15/2- 640 13/2- 719 15/2- 632 13/2- 6~1

9/2- 207 9/2- 14~ 11/2- 0 11/2- 0

II7Te l l9Te 121Te

Fig. 3. Systematics ofthe energy levels in odd-A t17-121Te nuclei. The data for l17"l19Te are taken ~om[20-22]

35/2 + 4726

31~ + 411__7

27/2 + 3377

23/2 + 2658

more favourable as compared to the yrast 23/2- state. The gamma ray intensity collection by this 21/2- state in- creases and that by the yrast 23 /2- state decreases con- siderably in going from l:TTe (N = 65) to 121Te (N = 69) [20-22]. It can be seen from Fig. 3 that the excitation energy of the 23/2- state remains nearly constant and that of the 21/2- state decreases with increasing neutron num- ber. An intense E2 sequence labelled 1, extending f r o m 23/2 ~+) to 35/2 (+), has been observed to feed the 21/2- state in :ZlTe. The 23/2 (+) state is possibly v(h21 x12d312), corresponding to the 7 - state interpreted as v(ht ,12d312) in l~S'~2°Te [36], and the higher positive parity states are generated with the rc9712 pair contributing the even spins, 2, 4, 6. No positive parity sequence has been observed in l l9Te [21, 22], while such a sequence extending from 25/2 + to 37/2 + is found to feed the yrast 23/2- state in

,5,1 ~ 7Te and has been interpreted as the (7997/22)0-6 @ •(hll/22d5/2-1)25/2+ [19, 20].

The negative parity states in sequences 2 and 3 are presented in the form of a rigid-rotor plot in Fig. 4, where a rotating liquid drop energy reference has been subtrac- ted. The decreased energy of the 39/2- ~ 35/2- transition as seen in lZlTe indicates a loss of collectivity. Total Routhian Surface calculations [38-40] for the :21Te nu- cleus reveal favoured (yrast) oblate states, involving the alignment of ~97/2 and vh~,lZ quasiparticles, as listed below.

21/22, 23/2- 37/2-, 39/2-

f12 Configuration 0.123 ~[.q712216+ @ F[hll/2]9/2_,ll/2_ 0.135 7c[g7/22]6+ (~) 3 V[hlll'2 ]25/2-,27/2-

. 0 • ~ • , . , . ,

> 1.5 i i Ii I

7 1.C 1/2-// _ 39/2 ,

0.5 ¢ / ~ 23/2 47/2 o, : /

0.0

; 1'0'1'5 20

Fig. 4. Energies of the negative-parity states in X21Te, relative to a rigid-rotor reference, shown as a function of spin. The dotted lines connect states where spin assignments are only tentative. States predicted to be oblate are labelled

The yrast 21/2- , 23/2- and 39/2- states observed in 12:Te correspond to the mentioned TRS predictions. These non-collective oblate states have also been pre- dicted and observed in the lighter odd-A ' 17,119Te [20-22, 28]. Similarly, in even-A :: 6.~ ~ STe ' the yrast 16 + state has been interpreted as the non-collective oblate state corres-

2 2 ponding to the ~g712 @ vhll/2 configuration [27,28]. t15 The lighter Te, nucleus does not show such a low-lying

39/2- fully aligned state [19], which is possibly due to the

131

low lying neutron Fermi surface, where the occupat ion of three vhl 1/2 orbitals is energetically expensive. In addi t ion to the 37/2- state shown in Table 2, a yrast 37/2 + non- collective state based on the 2 //:(:]7/2 @ V(hl 1/22d5/2) config- ura t ion has also been predicted in the odd-A Te nuclei with A < 121 [28]. A 37/2 state is indeed seen in 12~Te which could be either of the predicted ones. Further , the presence of the 288keV ((47/2-)---, (43/2-)) low energy E2 transit ion also indicates loss of collectivity and the (47/2-) state possibly refers to a fully aligned ~97/2z® v(h~l/25)35/2_ quasipart icle configurat ion. Fo r the upper part of the level scheme, pro tons from the 97/2 and ds/2 orbitals, neutrons from the sl/2, da/2 and th 1/2 orbi tals and neutron holes from the 9v/2 and d5/2 orbitals could be responsible for the single particle character of this region.

The authors wish to acknowledge the support of GDA scientific staff and accelerator crew at NSC. The authors are indebted to Drs. W.Nazarewicz and R.Wyss for providing the TRS codes. Thanks are also due Dr. A.Gizon, ISN, Grenoble, France for providing the 114Cd target. Financial support from UGC, DAE, CS1R (India) and EPSRC (UK) is duly acknowledged.

References

1. Heyde, K., Isacker, P., van, Waroquier, M., Wood, J.L., Mayer, R.A.: Phys, Reports 102, 291 (1983)

2. Wood, J.L., Heyde, K., Nazarewicz, W., Huyse, M., Duppen, P. van: Phys. Reports 215, 101 (1992)

3. Wadsworth, R., Andrews, H.R., Beausang, C.W., Clark, R.M., DeGraaf, J., Fossan, D.B., Galindo-Uribarri, A., Hibbert, I.M., Hauschitd, K., Hughes, J.R., Janzen, V.P., LaFosse, D.R., Mul- lins, S.M., Paul, E.S., Persson, L., Pilotte, S., Radford, D.C., Schnare, H., Vaska, P., Ward, D., Wilson, J.N., Ragnarsson, I.: Phys. Rev. C 50, 483 (1994)

4. Wadsworth, R., Andrews, H.R., Clark, R.M., Fossan, D.B., Galindo-Uribarri, A., Janzen, V.P., Hughes, J.R., LaFosse, D.R., Mullins, S.M., Paul, E.S., Radford, D.C., Vaska, P., Waring, M.P., War.d, D., Wilson, J.N., Wyss, R.: Nucl. Phys. A 559, 461 (1993)

5. Bron, J., Hesselink, W.H.A., Poelgeest, A.Van, Zalmstre, J.J.A., Uitzinger, M.J., Verheul, H.: Nucl. Phys A 318, 335 (1979)

6. LaFosse, D.R., Fossan, D.B., Hughes, J.R., Liang, Y., Vaska, P., Waring, M.P., Zhang, J.-y., Clark, R.M., Wadsworth, R., Forbes, S.A., Paul, E.S.: Phys. Rev. C 51, R2876 (1995)

7. LaFosse, D.R., Fossan, D.B., Hughes, J.R., Liang, Y., Vaska, P., Waring, M.P., Zhang, J.-y.: Phys. Rev. Lett. 69, 1332 (1992)

8. Janzen, V.P., Andrews, H.R., Haas, B., Radford, D.C., Ward, D., Omar, A., Prevost, D., Sawiski, M., Unrau, P., Waddington, J.C., Drake, T.E., Galindo-Uribarri, A., Wyss, R.: Phys. Rev. Lett 70, 1065 (1994)

9. Paul, E.S., Clark, R.M., Forbes, S.A., Fossan, D.B., Hughes, J.R., LaFosse, D.R., Liang, Y., Ma, R., Nolan, P.J., Regan, P.H., Vaska, P., Wadsworth, R., Waring, M.P.: J.Phys. G 18, 837 (1992)

10. Paul, E.S., Waring, M.P., Clark, R.M., Forbes, S.A., Fossan, D.B., Hughes, J.R., LaFosse, D.R., Liang, Y., Ma, R., Vaska, P., Wadsworth, R.: Phys Rev C 45, R2531 (1992)

11. Liang, Y., Fossan, D.B., Hughes, J.R., LaFosse, D.R., Lauritsen, T., Ma, R., Paul, E.S., Vaska, P., Waring, M.P., Xu, N.: Phys. Rev. C 45, 1041 (1992)

12. Paul, E.S., Simpson, J., Timmers, H., Ali, I., Bentley, M.A., Bruce, A.M., Cullen, D.M., Fallon, P., Hanna, F.: J.Phys G 18, 971 (i992)

13. Hughes, J.R., Fossan. D.B., LaFosse, D.R., Vaska, P., Waring, M.P., Zhang, J.-Y.: Phys. Rev. C 45, 2177 (1992)

14. Hughes, J.R., Fossan. DB., LaFosse, D.R., Liang, Y. Vaska, P., Waring, M.P.: Phys. Rev. C 44, 2390 (I991)

15. Paul, E.S., Beausang, C.W., Forbes, S.A., Gale, S.J., James, A.N., Jones, P.M., Joyce, M.J., Andrews, H.R., Janzen, V.P., Radford, D.C., Ward, D., Clark, R.M., Hauschild, K., Hibbert, H.M., Wadsworth, R., Cunningham, R.A., Simpson, J., Davinson, T., Page, R.D., Sellin, P.J., Woods, P.J. , Fossan, D.B., LaFosse, D.R., Schnare, H., Waring, M.P., Gizon, A., Gizon, J., Drake, T.E., DeGraaf, J., Pilotte, S.: Phys. Rev. C 50, 698 (1994)

16. Moon, C.-B., Kwon. J.U., Chae, SJ., Kim, J.C., Bhatti, S.H., Lee, C.S., Komatsubara, T., Mukai, J., Hayakawa, T., Kimura., H., Lu, J., Matsuda, M., Watanabe, T., Furuno, K.: Phys. Rev. C 51, 2222 (1995)

17. Thorslund, I., Fossan, D.B., LaFosse, D.R., Schnare, H., Haus- child, K., Hibbert., 1.M., Mullins, S.M., Paul, E.S., Ragnarsson, I., Sears, J.M., Varka, P., Wadsworth, R.: Phys. Rev. C 52, R2839 (1995)

18. Sharma, A., Goswamy, J., Mehta, D., Singh, J., Kaur, H., Chand, B., Singh, N., Bhowmik, R.K., Trehan, P.N.: Z.Phys. A 346, 321 (1993)

19. Moon, C.-B., Chae, S.J., Kim, J.C., Kim Y.K., Kwon, J.U., Lee, C.S., Furune, K., Komatsubara, T., Hayakawa, T., Kimura, H., Lu, J., Matsuda, M., Mukai, J., Watanabe, T.: Z. Phys. A 349, 1 (1994)

20. Duyar, C., Draper, J.E., Rubel, E.C., Deleplanque, M.A., Dia- mond, R.M., Stephens, F.S., Beck, E.M., Stoyer, M.A.: Z.Phys. A 348, 63 (1994)

21. Singh, J., Kaur, H., Sharma, A., Goswamy, J., Mehta, D., Singh, N., Trehan, P.N., Paul, E.S., Bhowmik, R.K.: Z.Phys. A 353, 235 (1995), ibid Z. Phys. A 351, 3 (1995)

22. Papadopoulos, C.T., Vlastou, R., Serris, M., Hartas, H.G., Fotiades, N., Kalfas, C.A., Harrissopoulos, S., Kossionides, S., Simpson, J., Paul, E.S., Araddad, S., Beausang, C.W., Bentley, M.A., Joyce, M.J., Sharpey-Schafer, J.F.: Z. Phys. A 352, 243 (1995)

23. Janzen, V.P., LaFosse, D.R., Schnare, H., Fossan, D.B., Galindo-Uribarri, A., Mullins, S.M., Paul, E.S., Persson, L., Pilotte, S., Radford, D.C., Timmers, H., Waddington, J.C., Wad- sworth, R., Ward, D., Wilson, J.N., Wyss, R.: Phys. Rev. Lett 72, 1160 (1994)

24. Juutinen, S., Tormanen, S., Cederwall, B., Johnson, A., Julin, R., Mitarai, S., Mukai, J., Ahonen, P., Fant, B., Liden, F., Nyberg, J., Ragnarsson, I.:'Z. Phys. A 338, 365 (1991)

25. Timar, J., Simpson, J., Paul, E.S., Araddad, S., Beausang, C.W., Bentley, M.A., Joyce, M., J., Sharpey-Schafer, J., F.: J. Phys. G 21,783 (1995)

26. Simpson, J., Timmers, H., Riley, M.A., Bengtsson, T., Bentley, M.A., Hanna, F., Mullins, S.M., Sharpey-Schafer, J.F., Wyss, R.: Phys. Lett. B 262, 388 (1991)

27. Sharma, A., Singh, J., Kaur, H., Goswamy, J., Mehta, D., Singh, N., Trehan, P.N., Paul, E.S., Bhowmik, R.K.: Z.Phys. A (in press I996)

28. Paul, E.S., Fossan, D.B., Sears, J.M., Thorslund, I.,: Phys. Rev. C 52, 2984 (1995)

29. Krane, K.S., Steffen, R.M., Wheeler, R.M.: Nucl. Data Tables A 11,351 (1973)

30. Hagemann, U., Keller, H.-J., Protochristow, Ch., Stray, F.: Nucl. Phys. A 329, 157 (1979)

31. Bonch-Osmolovskaya, N.A., An, N.F., Adyev, S., Norsyeyev, U.V., Milanov, M.: Akad. Nauk SSSR Izv. Ser. Fiz. 44, 2286 (1980)

32. Spejewski, E.H., Hopke, P.K., Loeser, F.W.: Nucl. Phys. A 146, 182 (1970)

33. Mantica, P.F., Jr., Zimmerman, B.E., Waiters, W.B., Rupnik, D., Zganjar, E.F., Kormicki, J., Carter, H.K., Schmitt-Ott, W.-D.: Phys. Rev. C 42, 1312 (1990)

34. Singh, J., Kaur, H., Sharma, A., Goswamy, J., Mehta, D., Singh, N., Trehan, P.N., Bhowmik, R.K.: Z. Phys. A 353, 125 (1995)

132

35. Blasi, N., Falconi. G., Bianco, G.Lo, Protochristov, Ch., Bazzacco, D., Angelis, G.De, Napoli, D.R.: Z. Phys. A 352, 359 (1995)

36. Ruyven, J.J.van, Hesselink, W.H.A., Akkermans, J., Nes, van, P., Verheul, H.: Nucl. Phys. A 380, 125 (1982)

37. Kaur, H., Singh, J., Sharma, A., Goswamy, J., Mehta, D., Singh~ N., Trehan, P.N., Bhowmik, R.K.: Z. Phys. A 353, 11 (1995)

38. Nazarewicz, W., Leander, G.A., Dudek, J.: Nucl. Phys. A 467, 437 (1987)

39. Wyss, R., Nyberg, J., Johnson, A., Bengtsson, R., Nazarewicz, W.: Phys. Lett. B 215, 211 (1988)

40. Nazarewicz, W., Wyss, R., Johnson, A.: Nucl. Phys. A 503, 285 (1989)