Z. Phys. A 356, 125—132 (1996)

High spin states in 121Te

Jagbir Singh1, Harjeet Kaur1,|, A. Sharma1, J. Goswamy1,||, D. Mehta2, Nirmal Singh1, P.N. Trehan1, E.S. Paul3,R.K. Bhowmik4

1Department of Physics, Panjab University, Chandigarh 160014, India2Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India3Oliver Lodge Laboratory, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK4Nuclear Science Centre, JNU, New Delhi 110067, India

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

Abstract. High spin states of 121Te, populated in the114Cd (11B, p3n) reaction, have been studied through c-rayspectroscopy. The level scheme has been established up toJn "51/2~. Three-quasiparticle states, based on theng

7@22 ? lh

11@2and ng

7@2d5@2

? lh11@2

configurations,have been identified. A favoured 39/2~ state is suggestedto be the fully aligned [ng

7@22]

6`? [lh

11@23]

27@2~yrast

non-collective oblate configuration. This assignment issupported by Total Routhian Surface (TRS) calculationswhich also suggest a similar oblate assignment to thestates at Jn"21/2~ and 23/2~. A higher 47/2~ state isalso found and is suggested to be the fully aligned[ng

7@22]

6`? [lh

11@25]

35@2~configuration.

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

Introduction

Investigations of collective rotational structures in nucleiin the region of the spherical Z"50 closed shell havereceived added impetus in recent years. The collectivity inthese nuclei is postulated as due to the quadrupole defor-mation induced by the particle-hole excitations involvingthe promotion of g

9@2protons across the closed shell to

either of the d5@2

, g7@2

or h11@2

orbitals [1, 2]. In the even-A106~118Sn nuclei [3—5], collective rotational bands basedon the 2p-2h ng

7@22?ng

9@2~2 configuration have been

observed. This 2p-2h deformed Sn-core, coupled toa h

11@2neutron in 111Sn [6] and to a low-X valence

proton (nd5@2

, ng7@2

, nh11@2

) in odd-A Sb nuclei [7, 8], isfound to give rise to DI"2 decoupled rotational bands.The single ng

9@2excitations also lead to collective DI"1

strongly coupled bands based on the ng7@2

2? ng9@2

~1(2p-1h) configuration in the odd-A Sb [7, 8], 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, 14] nuclei. In Te nuclei, a collective rotationalband based on the well deformed 4p-2h(nh

11@22? ng

7@22 ?ng

9@2~2) configuration has been re-

cently reported in the even-A 112~116Te nuclei [15—18].However, as yet nothing is known regarding the existenceof well developed rotational bands in the odd-A115~119Te nuclei [19—22]. In addition to the rotationalbands based on p-h configurations, collective prolatebands based on a low-X nh

11@2intruder orbital in Sn [4],

Sb [8, 23] and I [9—12] nuclei and also collective oblatebands based on a high-X nh

11@2orbital in I nuclei [10—12]

have been observed. Several yrast aligned non-collectiveoblate states have also been identified and characterizedin these near spherical nuclei with Z552. Investigationshave revealed such states in the 118,121,122Xe [24—26]nuclei at spins 20—30+. In the odd-A 115~121I nuclei[9—12], the yrast h

11@2rotational bands are crossed by

non-collective oblate states at spins around 20+. In recentinvestigations of the 114~119Te [16—22, 27] isotopes, sev-eral non-collective oblate (c"60°) states based on then[(g

7@2)2]

6`? l[(d

5@2)x (h

11@2)y] 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, anexperiment has been performed to investigate high spinstates using heavy-ion fusion-evaporation reaction andgamma ray spectroscopic techniques. The level schemehas been established up to Jn"51/2~. Several non-collec-tive oblate states have been identified and interpreted interms of TRS calculations.

Experimental details and data analysis

The excited states of 121Te were populated in the 114Cd(11B, p3n) fusion-evaporation reaction at a beam energy of64 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/cm2 thick en-riched 114Cd foil, onto which 20 mg/cm2 Pb had been

Fig. 1. The partial level scheme of 121Te deduced from the present work

evaporated in order to stop the recoiling nuclei. Thegamma-rays emitted by the evaporation residues weredetected using the Gamma Detector Array (GDA). Thearray consisted of eight Ge detectors (efficiency&23%relative to 7.6 cm]7.6 cm NaI(Tl) crystal at 1.33 MeV)and a 14-element BGO (Bismuth Germanate) multiplicityfilter. Each Ge detector was operated in conjunction witha symmetrical BGO Compton suppression shield. The Gedetectors, located at 18 cm from the target position, weremounted in two groups, making angles of 99° and 153°with the beam direction and tilted at $23° with respectto the horizontal plane. The multiplicity filter consisted oftwo sets of seven closely packed hexagonal BGO elements(3.8 cm]7.5 cm long), mounted above and below the tar-get chamber at a distance of 4 cm from the target. Thecoincidence events, in which two or more Ge detectorsfired within a 200 ns time window, were written on mag-netic tape in the LIST mode. Approximately 100 millionevents were collected in this experiment with a hardwarecondition of BGO multiplicity, K52. The Compton sup-pressed Ge detectors were calibrated for energy and effici-ency using the standard energy calibration c-lines fromthe radioactive decay of 133Ba, 134Cs and 152Eu.

In the off-line analysis, the recorded c-c coincidencedata were sorted event by event into two dimensionalEc—Ec matrices. The gains of the Ge detectors were soft-ware matched to 0.7 keV/channel before incrementing thematrices. 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 inthe vicinity of the photopeaks to remove the contributionsdue to the background underlying the photopeaks of thegated transitions.

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

sity balance arguments. The representative coincidencespectra depicting the various newly placed transitions areshown in Fig. 2. Gamma-ray intensities for the assignedtransitions were determined from different spectra in coin-cidence with the transitions deexciting lower levels andfrom the total projection spectrum, deduced from theEc—Ec matrix. The energies and relative intensities of thec-transitions assigned to 121Te are given in Table 1.

In order to obtain information on the gamma raymultipolarities, Directional Correlation (DCO) ratios[29] were extracted from the coincidence data. The coin-cidence events were sorted into an asymmetric matrixwith 153° detectors on one axis and 99° detectors on theother axis. By setting gates on the E2 transitions along thetwo axes of this matrix, the peak areas Acp (153°) and Acp(99°) were obtained from the projected spectra. The DCOratios for transitions (cp) in the projected spectra werededuced using the relation

DCOcp"Acp(153°)

ecp(153°)

ecp (99°)

Acp(99°)

ecg(153°)

ecg (99°)

e(99°) and e(153°) are the detection efficiencies of the set ofdetectors at 99° and 153° respectively and the subscriptscg and cp correspond to the gated and the projectedtransition 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. TheDCO ratios and the assigned multipolarities fortransitions in 121Te, along with their placements in thelevel scheme are also given in Table 1.

Experimental results and discussion

The low lying levels in 121Te were earlier investigated inthe b/EC decay studies of 121I (¹

1@2"2.12 h) by various

workers [31, 32] and most recently by Mantica et al. [33]

126

ZPHYA 717

Fig. 2a—e. The gamma ray coincidence spectra with gates on a the265keV doublet (27/2P25/2~ and depopulating the 33/2~ state)b the 533keV (29/2P27/2) transition c the 1120 keV doublet(25/2~P21/2~ and populating the 27/2 state) d the 719 keV(27/2(`)P23/2(`)) transition and e the 217 keV (39/2P37/2)transition

127

ZPHYA 717

Table 1. Gamma ray energies, intensities,DCO ratios and multipolarities fortransitions assigned to 121Te

Ec (keV)! Ic" DCO Ratio Multipolarity Assignment

144.5 2.5 M1# 9/2~P11/2~217.4 9.9 0.67(14) Dipole 39/2P37/2224.9 2.2 deexciting 39/2~264.9 4.2 deexciting 33/2~265.4 13.6 0.74(8)$ Dipole 27/2P 25/2~287.7 6.2 0.90(14) E2 (47/2~)P (43/2~)315.9 26.6 0.57(5) M1 23/2~P21/2~354.1 17.2 1.09(12) E2 39/2~P35/2~361.6 68.2 0.50(5) M1 21/2~P19/2~373.7 7.7 0.54(11) (E1) 33/2~P31/2(`)384.4 4.3 23/2~P23/2~393.6 20.4 0.53(7) M1 35/2~P33/2~416.2 13.0 1.12(16) E2 21/2~P17/2~419.4 6.0 0.50(8) M1 25/2~P23/2~487.2 4.4 0.56(11) M1 33/2(`)P 31/2(`)509.8 12.7 0.57(10) Dipole 37/2P 35/2~514.5 5.5 0.62(13) Dipole 43/2P 41/2532.7 6.4 0.56(12) Dipole 29/2P27/2536.3 2.4 E2# 13/2~P9/2~547.4 16.2 1.12(13) E2 33/2~P29/2~561.8 7.8 above 41/2564.3 3.5 above 41/2576.6 3.4 above 41/2609.3 7.2 1.09(19) E2 35/2(`)P 31/2(`)611.0 2.7 feeding 43/2624.3 9.0 E2# 17/2~P13/2~631.5 100.0 1.04(8) E2 15/2~P11/2~638.7 12.8 0.50(8) Dipole 41/2P39/2668.5 4.1 37/2P35/2`

673.7 13.5 0.28(4) M1 17/2~P15/2~677.8 10.2 0.90(13) E2 23/2~P19/2~681.1 7.0 M1# 13/2~P11/2~719.0 26.0 1.06(10) E2 27/2(`)P 23/2(`)728.5 86.3 1.00(7) E2 19/2~P15/2~740.1 19.0 0.89(10) E2 31/2(`)P 27/2(`)804.1 8.7 0.52(13) 25/2~P23/2~807.2 10.8 1.02(16) E2 35/2~P31/2~848.9 5.0 feeding 27/2`912.0 4.5 0.69(23) Dipole (45/2)P 43/2936.2 3.0 feeding 31/2~936.7 26.1 0.53(4) (E1) 23/2(`)P 21/2~971.0 15.9 0.99(11) E2 31/2~P27/2~

1062.2 7.3 0.98(23) E2 23/2~P19/2~1069.4 19.0 1.03(12) E2 27/2~P23/2~1102.2 17.9 0.79(10) E2 29/2~P25/2~1120.3 26.5 1.03(11)d E2 25/2~P21/2~1120 1.5 feeding 27/21230.7 4.4 1.2(3) E2 (51/2~)P (47/2~)1240.7 11.5 0.87(14) (E2) (43/2~)P (39/2~)

! Energies are accurate to 0.3 keV for strong transitions. The errors increase to 0.7 keV forweaker transitions (relative intensity Ic(3)"Errors in c- ray intensities are 5—20%# Assigned multipolarity adopted from the earlier work [30]$Value given for the doublet

through gamma-ray and conversion electron spectro-scopy. From these studies, the ground state (1/2`), firstexcited state at 212 keV (3/2`) and an isomeric state at294keV (11/2~, ¹

1@2"154d) have been identified as

single particle states corresponding to the occupancy ofs1@2

, d3@2

and h11@2

neutron orbitals, respectively. Previousin-beam studies of 121Te by Hagemann et al. [30], usingthe 119Sn(a, 2n) and 121Sb(d, 2n) reactions, revealed two

positive parity DI"1 coupled bands based on thed5@2

and g7@2

neutron orbitals. Also, the low lying statesup to Jn"23/2~ above the 11/2~ isomer were estab-lished.

The level scheme of 121Te, shown in Fig. 1, has beenestablished up to Jn"(51/2~) in the present investiga-tions. The level structure above the lh

11@2isomer has been

extended substantially by the addition of 35 transitions to

128

ZPHYA 717

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

The present level scheme preserves most of the featuresreported earlier by Hagemann et al [30]. The earlierreported positive parity DI"1 level sequence [30] basedon a lg

7@2orbital along with the crossover transitions has

been confirmed (not shown in Fig. 1). The other smallsequences of gamma transitions shown earlier to feed theground state and the 3/2` first excited state could not beestablished conclusively in the present work owing totheir weak population. The two level sequences built onthe 11/2~ isomer, known previously up to Jn"(23/2~)and (21/2~) are confirmed in the present work. The earlierreported 724-320keV and 202-675keV cascades [30],shown to feed the 9/2~ and 13/2~ states, respectively, arenot observed. The placement of the 265 keV transition,shown earlier [30] to feed the (21/2~) state is found to bedifferent. In the present level scheme, two mutually coinci-dent 265keV transitions have been placed (Fig. 2a). The265.4keV (27/2P25/2~) transition is placed on the basisof its strong coincidence with the 533 (Fig. 2b) and 1120keV (Fig. 2c) transitions and its intensity value. The place-ment of the 264.9 keV weak transition, depopulating the33/2~ level, is well supported from the facts (i) the 265 keVtransition is seen in self-coincidence (Fig. 2a) (ii) it is foundto be in weak coincidence with the 937keV(23/2(`)P21/2~), 719 keV (27/2(`)P23/2(`)) (Fig. 2d),849keV (feeding 27/2(`)) transitions and also with the394keV (35/2~P33/2~) and higher transitions (iii) theobservation of the 1120keV transition as a self-coincidentdoublet (Fig. 2c) and not in coincidence with thetransitions of sequence labelled 1 (Fig. 2c, d) and the849keV transition. The 739keV transition shown earlier[30] to feed the 9/2~ level is not seen in the present work;however a rather strong 740 keV transition has beenplaced as 31/2(`)P27/2(`) (Fig. 2d).

During the final stages of this work, a short note byBlasi et al [35], reporting the experimental data on 121Teappeared in the literature. The results seem to be ingeneral agreement with those presented here. The maindisagreement noticed with this work is regarding the partof the level scheme between the 25/2~ and 33/2~ levelsand parallel to the 1102-547keV cascade. In the levelscheme proposed by Blasi et al. [35], this part is shown asconsisting of a 536 keV (27/2(`)P25/2~) transition anda cascade of 849-265keV transitions from the 33/2~ stateto the 27/2(`) state. However, the present work does notsupport this placement of the 536keV transition as thetransitions of the sequence labelled 1 are not seen incoincidence with the 1120 keV transition (Fig. 2c). Also,no 536keV transition is seen above the 25/2~ level, rathera weak 536keV transition is placed as 13/2~P9/2~,which is well confirmed from the energy sum relationshipsand intensity flow in the gates above. This placement isalso 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~P9/2~ , seems to beerroneous as it does not follow the energy sum relation-ships. The placements of the 265 and 849keV transitionsin the present level scheme have been discussed earlier inthis section. The placement of the 533keV (29/2P27/2)

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 tohave any links with the upper part of the level structure(Fig. 2b). Other disagreements noticed are (i) The place-ment of the second 265keV transition by Blasi et al [35]as 27/2~P25/2~ is ruled out because 265 and 1120keVtransitions are not found to be in coincidence with the971keV transition (Fig. 2a,c) (ii) The ordering of the 1241and 288 keV transitions in the present work is found to bedifferent. However, the present ordering is well supportedby the intensity values (iii) The multipolarity of the217keV (39/2P37/2) transition is found to be dipole. The562, 564, 611, 577 and 912keV transitions have beenplaced above the 41/2 level as suggested by Blasi et al[35]. In the present work, these transitions are seen in thespectra with gates on the lower transitions (Fig. 2e), how-ever, their placement was difficult due to the presence ofintense close-lying transitions in the 120,122Te [36] and120,121I [37, 11] nuclei, also populated in this reaction.The ordering of the 217 and 639keV transitions could notbe ascertained from the present work due to their similarintensity values. The tentative placement has been donekeeping in view the observation of the 354 keV peak in thespectrum gated by the 639keV transition.

As in the case of other Te nuclei [15—22, 27, 36], thelevel structure of 121Te is complex due to the mixing ofcollective quadrupole excitations with the members of thebands based on quasiparticle configurations. The levelscheme, 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 theexcitation energy and spin value of the 11/2~ isomericstate to be zero and comparing it with that of the neigh-bouring even-A 120Te isotope [36]. The lower states withJn"11/2~, 15/2~, 19/2~, 23/2~ have been observed in allthe odd-A Te nuclei and are interpreted [30] as thealigned coupling of an h

11@2neutron to the 0`, 2`, 4` and

6` vibrational states of the neighbouring even-even Tecore [36]. The levels with Jn"9/2~, 13/2~, 17/2~ and21/2~ can be attributed to the J

.!9!1 coupling states

where J.!9

is the maximum aligned angular momentum ofthe h

11@2?R (core spin value) multiplets [30]. Following

the interpretation of quasiparticle states on the basis ofone broken-pair model (BPM) in 118,120Te [36], the yrast23/2~ state can also be interpreted as ng

7@22 ? lh

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)

114~120Te nuclei [15—22, 27, 36], is based on theng

7@2d5@2

? lh11@2

quasiparticle configuration. In thelighter 115,117,119Te isotopes [19—22], a strong sequenceof E2 transitions built on the non-yrast 23/2~ state hasbeen seen, however, no such sequence is observed in121Te. By comparing the members of the sequence label-led 3 with the positive parity yrast states in 120Te, the27/2~ state is the fully aligned (lh

11@2)3 state correspond-

ing to the (lh11@2

2)10`

yrast state in 118,120Te [36]. Theyrast 31/2~, 35/2~ and 39/2~ states are suggested to resultfrom the (pg

7@2)2? (lh

11@2)3 configuration with ng2

7@2con-

tributing the even spins (2, 4, 6).Most of the intensity is found to flow through the

21/2~ (J.!9

!1) coupling state, which indicates it to be

129

ZPHYA 717

Fig. 3. Systematics of the energy levels in odd-A 117~121Te nuclei. The data for 117,119Te are taken from [20—22]

more favourable as compared to the yrast 23/2~ state. Thegamma ray intensity collection by this 21/2~ state in-creases and that by the yrast 23/2~ state decreases con-siderably in going from 117Te (N"65) to 121Te (N"69)[20—22]. It can be seen from Fig. 3 that the excitationenergy of the 23/2~ state remains nearly constant and thatof the 21/2~ state decreases with increasing neutron num-ber. An intense E2 sequence labelled 1, extending from23/2(`) to 35/2(`), has been observed to feed the 21/2~state in 121Te. The 23/2(`) state is possibly l(h2

11@2d3@2

),corresponding to the 7~ state interpreted as l(h

11@2d3@2

) in118,120Te [36], and the higher positive parity states aregenerated with the pg

7@2pair contributing the even spins,

2, 4, 6. No positive parity sequence has been observed in119Te [21, 22], while such a sequence extending from25/2` to 37/2` is found to feed the yrast 23/2~ state in115,117Te and has been interpreted as the(ng

7@22)

0~6? l(h

11@22d

5@2~1)

25@2`[19, 20].

The negative parity states in sequences 2 and 3 arepresented in the form of a rigid-rotor plot in Fig. 4, wherea rotating liquid drop energy reference has been subtrac-ted. The decreased energy of the 39/2~P35/2~ transitionas seen in 121Te indicates a loss of collectivity. TotalRouthian Surface calculations [38—40] for the 121Te nu-cleus reveal favoured (yrast) oblate states, involving thealignment of ng

7@2and lh

11@2quasiparticles, as listed below.

In b2

Configuration21/2~, 23/2~ 0.123 n[g

7@22]

6`? l[h

11@2]9@2~,11@2~

37/2~, 39/2~ 0.135 n[g7@2

2]6`

? l[h11@2

3]25@2~,27@2~

Fig. 4. Energies of the negative-parity states in 121Te, relative toa rigid-rotor reference, shown as a function of spin. The dotted linesconnect states where spin assignments are only tentative. Statespredicted to be oblate are labelled

The yrast 21/2~, 23/2~ and 39/2~ states observed in121Te correspond to the mentioned TRS predictions.These non-collective oblate states have also been pre-dicted and observed in the lighter odd-A 117,119Te [20—22,28]. Similarly, in even-A 116,118Te, the yrast 16` state hasbeen interpreted as the non-collective oblate state corres-ponding to the ng

7@22? lh

11@22 configuration [27, 28].

The lighter 115Te nucleus does not show such a low-lying39/2~ fully aligned state [19], which is possibly due to the

130

ZPHYA 717

low lying neutron Fermi surface, where the occupation ofthree lh

11@2orbitals is energetically expensive. In addition

to the 37/2~ state shown in Table 2, a yrast 37/2` non-collective state based on the ng

7@22? l(h

11@22d

5@2) config-

uration has also been predicted in the odd-A Te nucleiwith A4121 [28]. A 37/2 state is indeed seen in 121Tewhich could be either of the predicted ones. Further, thepresence of the 288keV ((47/2~)P(43/2~)) low energyE2 transition also indicates loss of collectivity and the(47/2~) state possibly refers to a fully aligned ng

7@22 ?

l(h11@2

5)35@2~

quasiparticle configuration. For theupper part of the level scheme, protons from the g

7@2and d5@2

orbitals, neutrons from the s1@2

, d3@2

and h11@2orbitals and neutron holes from the g

7@2and d

5@2orbitals

could be responsible for the single particle character ofthis region.

The authors wish to acknowledge the support of GDA scientific staffand accelerator crew at NSC. The authors are indebted to Drs.W.Nazarewicz and R.Wyss for providing the TRS codes. Thanks arealso due Dr. A.Gizon, ISN, Grenoble, France for providing the114Cd target. Financial support from UGC, DAE, CSIR (India) andEPSRC (UK) is duly acknowledged.

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