PHYSICAL REVIEW C 80, 014320 (2009)

High spin states in 139Pm

A. Dhal,* R. K. Sinha, and L. Chaturvedi†

Department of Physics, Banaras Hindu University, Varanasi-221 005, India

P. Agarwal, S. Kumar, and A. K. JainDepartment of Physics, IIT Roorkee, Roorkee-247 667, India

R. Kumar and I. M. GovilDepartment of Physics, Panjab University, Chandigarh-160 014, India

S. Mukhopadhyay,‡ A. Chakraborty,§ Krishichayan , S. Ray, S. S. Ghugre, and A. K. SinhaUGC-DAE Consortium for Scientific Research, Kolkata Centre, Kolkata-700 098, India

R. Kumar, R. P. Singh, S. Muralithar, R. K. Bhowmik, and S. C. Pancholi‖Inter University Accelerator Centre, New Delhi-110 067, India

J. B. GuptaRamjas College, Delhi University, Delhi-110 007, India

(Received 6 October 2008; revised manuscript received 25 February 2009; published 27 July 2009)

The odd mass nucleus 139Pm has been studied to high spins through the 116Cd(27Al,4n)139Pm reaction atan incident beam energy of 120 MeV. The de-exciting γ -rays were detected using an array of 12 Comptonsuppressed Ge detectors. A total of 46 new levels have been proposed in the present work as a result ofthe observation of 60 new γ -rays. Four new bands including a �J = 1 sequence have been identified and all theearlier reported bands, other than the yrast band, have been extended to higher spins and excitation energy. Thespin assignments for most of the newly reported levels have been made using the observed coincidence angularanisotropy. Tilted axis cranking calculations support the interpretation of two of the observed magnetic dipolesequences as examples of magnetic rotational bands.

DOI: 10.1103/PhysRevC.80.014320 PACS number(s): 21.10.Re, 21.60.Ev, 23.20.−g, 27.60.+j

I. INTRODUCTION

The nuclei in the mass A ∼ 135–140 region are known tobe soft with respect to the triaxial deformation parameter γ andare therefore sensitive to the γ -polarizing effects of specificquasiparticle configurations. This can lead to the coexistenceof quadrupole structures of different spectroscopic character,which are determined by the configuration of the valencequasiparticle. The polarizing effect is most pronounced forintruder high-j quasiparticles in the favored signature orbitals.When occupied, these orbitals tend to stabilize the nuclearshape around a specific γ value that is strongly dependenton the position of the Fermi surface within high-j shell. Innuclei in this mass region, for Z = 57–63 [1–5], the protonFermi surface lies in the lower h11/2 midshell where theoccupied quasiparticle orbitals favor prolate axial symmetry.

*anukul uu@rediffmail.com†Present address: Vice Chancellor, Guru Ghasidas University,

Bilaspur, Chhattisgarh-495 009, India.‡Present address: Department of Physics, Mississippi State Univer-

sity, MS 39762, USA.§Present address: Department of Physics, Krishnath College,

Berhampore, West Bengal-742 101, India.‖Formerly at: Department of Physics and Astrophysics, Delhi

University, Delhi-110 007, India.

Odd-proton nuclei therefore exhibit rotational bands builton these low-Kπh11/2 intruder levels. Such bands show amaximal signature splitting [6]; only the favored signaturecomponent (α = − 1

2 ) of the band, consisting of stretched�J = 2 transitions, is observed experimentally. Conversely,the neutron Fermi surface for N = 74–78 lies in the upperh11/2 midshell where the predicted deformation driving force istoward a collectively rotating oblate shape. Thus the alignmentof a pair of h11/2 protons tend to drive the nuclear shapetoward γ � 0◦ (prolate shape), while the alignment of a pair ofh11/2 neutrons tends to drive the nucleus toward a collectivelyrotating oblate shape (Lund convention [7]), i.e., the nuclei inthis mass region exhibit relatively flat potential energy surfaces[8,9] with respect to γ . If the nucleus is sufficiently ‘stiff’ withrespect to shape change in the γ -plane, the neutron alignmentwill be hindered or suppressed. If the nucleus is ‘soft’ withrespect to γ -plane, i.e., if the prolate-oblate potential energydifference Vpo between the prolate and oblate shapes issufficiently small (i.e., �300 kV), a neutron alignment willdrive the nucleus to an oblate shape. With Vpo small, twostructure features occur in odd-Z nuclei: (i) the h11/2 neutronalignment competes with h11/2 proton alignment and (ii) theco-existence of proton aligned collective prolate and neutronaligned collective oblate bands is possible in the same nucleus.

The existence of regular rotational bands consisting ofstrong magnetic dipole transitions, in nearly spherical nuclei, is

0556-2813/2009/80(1)/014320(11) 014320-1 ©2009 The American Physical Society

A. DHAL et al. PHYSICAL REVIEW C 80, 014320 (2009)

well established [10]. These sequences are known as magneticrotational bands (MR bands) and they are expected in regionsclose to magic numbers if high-j nucleons are available andthe deformation is small. In the A ∼ 135–140 region, thelow-�h11/2 proton and high-� h11/2 neutron orbitals lie nearthe Fermi surface and hence favour the occurrence of MRbands. Indeed several �J = 1 bands have been reported in141Eu [11], 137Pr [12].

The 139Pm nucleus has been studied earlier by Xu et al. [13].They have reported the observation of a �J = 1 sequence. Inthe present work, the level structure and properties of two ofthe �J = 1 bands are investigated.

II. EXPERIMENTAL DETAILS

Excited states in the 139Pm nucleus were populated uti-lizing the 116Cd(27Al,4n)139Pm reaction at a beam energy of120 MeV. The 27Al beam was provided by the 15-UD Pelletronaccelerator [14,15] at the Inter University Accelerator Centre(IUAC), New Delhi. An isotopically enriched, ∼1.75mg/cm2

thick 116Cd target on a ∼90 mg/cm2 of Pb backing was used.The γ -rays were detected using Gamma Detector Array (GDA)[16], an array consisting of twelve 23% Compton-suppressedn-type HPGe detectors fixed on two horizontal rings at ±25◦to the median plane and a multiplicity filter consisting of 14hexagonal (38 mm × 75 mm) BGO detectors, placed aboveand below the median plane. The distance between the targetand the HPGe detectors was ∼18 cm whereas that of the BGOdetectors was ∼4 cm. The BGO elements cover a total of35% of the 4π solid angle. The HPGe detectors were arrangedin three groups, of four detectors each, at 45◦, 99◦, and 153◦,respectively, with respect to the beam direction, covering about5% of the total solid angle. The data were acquired in listmode using a CAMAC-based multiparameter data acquisitionsystem developed under PC-LINUX platform. A software pro-gram package CANDLE-ONLINE was used for this purpose.

A total of ∼6.3 × 108 two- and higher-fold coincidenceevents were recorded. These were analyzed off-line usingthe analysis programs CANDLE [17], INGASORT [18], andRADWARE (ESCL8R) [19]. The coincidence events were sortedinto the conventional γ -γ symmetric as well as asymmetricmatrices. The 4k × 4k matrices had an energy dispersion of0.5 keV/channel.

The multipolarity of the γ -transitions were assigned usingthe observed coincidence angular correlations. The details ofthis procedure are provided by Stephens et al. [20]. For thisanalysis the data were sorted into the angle dependent matrices.One axis of this matrix contained the γ -energy recorded bythe detectors at backward angles viz. 153◦ while the otheraxis contained the coincidence γ -energy as recorded in thedetectors at 99◦. Then a gate is set say on the x-axis and thegated spectrum is generated along the y-axis and vice versa.Assuming the commonly encountered situation of stretchedtransitions, which is a valid assumption for high spin yraststates, it is observed that the intensity of the γ -transition (purequadrupole or dipole) having the same multipolarity as thegated transition is almost the same in both the spectra, whereas the intensity differs by a factor of 2 for those transitions

whose multipolarity differs from that of the gated transition.Hence we could define a quantity RDCO, which is the intensityanisotropy as

RDCO (γ1) = I (γ1) at 153◦, gated by γG at 99◦

I (γ1) at 99◦, gatedby γG at 153◦ . (1)

The results of this procedure are depicted in Fig. 1. As seenfrom the figure, when gated by the �J = 2, 466 keV transitionin 139Pm, the intensity of the �J = 1, 138 and 212 keVtransitions differs by a factor of 2, whereas the intensities forthe �J = 2, 751 and 946 keV transitions remains the same,since it involves a change in angular momentum by two units.

The value of intensity anisotropy, RDCO as a function of theγ -transition energy is plotted in Fig. 2. The 120-, 477-, and869 keV transitions in 138Pm [21] are of known multipolarities.The former two being pure dipoles and the latter purequadrupole. The RDCO values for these transitions are alsoshown in the figure (Fig. 2) for comparison. The quoted errorsinclude the statistical and efficiency errors [22].

III. RESULTS

More than 60 new transitions belonging to 139Pm, have beenobserved and placed in the level scheme, using coincidencerelationships and intensity arguments. The level schemefrom the present work is depicted in Fig. 3. The details ofthe observed γ -ray energies, intensities, RDCO values, andmultipolarities for the transitions belonging to 139Pm aresummarized in Table I. The uncertainties in the intensities andRDCO comprises of the errors due to background subtraction,peak fitting, statistical fluctuations and efficiency correction.The level sequences shown in Fig. 3 have been labeled(Bands 1–9) to facilitate discussion.

A. �J = 1 band 1

This sequence was known [13] up to the J = 252 level.

A 225-keV transition was tentatively placed above thislevel in [13]. In the present work, we did not observe the225 keV transition in coincidence with the other transitionsof this sequence. The crossover 453 keV transition (reportedin [13]), decaying from the same J = 25

2 level to J = 232

level, was also not observed in the present study. We haveplaced six new transitions viz. 274, 320, 360, 369, 401, and433 keV in this sequence. The placement of the 590 and693 keV transition was based on the observed coincidencerelations. These transitions are in coincidence with eachother and they decay to the 25

2 level of sequence 1. Allother transitions of this sequence reported earlier were alsoconfirmed. From the intensity anisotropy, RDCO, calculation itis found that the in-band 274-, 320-, 360-, 369-, and 433-keVtransitions are �J = 1 in nature, hence have been assignedas M1 transitions. The decay out 1298 keV transition fromband 1 has been assigned E1 on the basis of a similar transitionin 141Eu [11] which is an isotone of 139Pm. Representativepartial γ -γ coincidence spectra, gated sum of the of 212-,138-, and 268-keV transitions (upper panel) and gated bythe 1298 keV transition (lower panel) are shown in Fig. 4.

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HIGH SPIN STATES IN 139Pm PHYSICAL REVIEW C 80, 014320 (2009)

TABLE I. Transition energy (Eγ ), relative intensity (Iγ ), DCO ratios (RDCO), nature of the gating transition, natureof the transition, and the decay from an initial state (J π

i ) to the final state (J πf ) for the transitions placed in the level

scheme of 139Pm are listed.

Eγ (keV) Iγ (Rel.) RDCO Gating transition Nature of the transition J πi → J π

f

103.8(0.1) 3.1(0.4) 0.92(0.28) D D 272

− → 252

−

138.5(0.1) 8.9(0.2) 0.55(0.10) Q D 212

(+) → 192

(+)

211.8(0.1) 13.2(0.6) 0.61(0.11) Q D 192

(+) → 172

(+)

224.1(0.1) 4.2(0.3) 1.25(0.21) D D 432

(−) → 412

(−)

227.8(0.1) 4.9(0.3) 0.70(0.26) Q D 252

(+) → 232

(+)

237.8(0.1) 1.9(0.3) 0.70(0.41) Q D 172

(+) → 152

(+)

261.9(0.1) 4.7(0.6) 1.16(0.28) D D 412

(−) → 392

(−)

268.4(0.1) 14.1(0.9) 0.62(0.09) Q D 232

(+) → 212

(+)

274.4(0.1) 3.7(0.4) 0.45(0.25) Q D 292

(+) → 272

(+)

313.4(0.1) 1.4(0.2) 0.55(0.29) Q D 472

(−) → 452

(−)

316.5(0.1) 12.7(0.8) 1.10(0.09) D D 312

− → 292

−

320.1(0.1) 4.8(0.3) 0.60(0.28) Q D 272

(+) → 252

(+)

329.4(0.1) 18.5(0.9) 0.45(0.05) Q D 292

− → 272

−

338.5(0.2) 1.9(0.7) D 232

+ → 232

−

360.2(0.2) 2.9(0.5) 0.41(0.28) Q D 332

(+) → 312

(+)

369.4(0.2) 3.5(0.6) 0.53(0.42) Q D 312

(+) → 292

(+)

388.7(0.1) 2.4(0.3) 232

+ → 212

(+)

393.7(0.1) 5.3(0.4) 0.74(0.33) Q D 252

+ → 232

(+)

394.2(0.1) 1.6(0.3) 0.67(0.21) Q D 452

(−) → 432

(−)

400.7(0.2) 1.0(0.6) 0.65(0.40) Q D 372

(+) → 352

(+)

402.8(0.3) 0.9(0.5)

432.7(0.2) 2.5(0.9) 0.45(0.30) Q D 352

(+) → 332

(+)

446.2(0.1) 1.7(0.6) 0.96(0.55) Q Q453.7(0.1) 4.8(0.6) 1.05(0.30) D D 35

2

− → 332

−

465.9(0.1) ≡100 0.96(0.06) Q Q 152

− → 112

−

472.7(0.1) 8.3(0.3) 0.85(0.12) D D 332

− → 312

−

490.0 <1.0

497.1(0.2) 2.8(0.5) Q 252

(+) → 212

(+)

526.7(0.2) 3.2(0.8)532.9(0.2) 2.9(0.7) 0.81(0.53) Q D ( 39

2

−) → ( 37

2

−)

563.7(0.2) 2.7(0.5)570.0 <1.0583.7(0.1) 7.3(0.4) 1.10(0.35) Q Q 23

2

+ → 192

+

590.1(0.1) 8.0(0.3) 1.06(0.26) Q Q 272

+ → 232

+

590.5(0.2) 1.6(0.3) D 132

− → 112

−

590.5(0.2) 1.6(0.3)593.9(0.1) 11.4(0.4) 1.17(0.12) Q Q 29

2

+ → 252

+

596.8(0.2) 1.4(0.3) Q 172

− → 132

−

601.3(0.3) 2.2(0.9)612.2(0.1) 8.6(0.8) 0.60(0.28) D D 25

2

+ → 232

−

622.3(0.2) 2.0(0.4)633.9(0.1) 2.5(0.6)646.2(0.2) 1.9(0.5) Q 31

2

− → 272

−

658.0(0.3) 0.9(0.4)662.8(0.1) 7.1(0.5) 0.81(0.32) Q Q 43

2

− → 392

−

669.0(0.1) 4.4(0.6) 0.89(0.53) Q Q

014320-3

A. DHAL et al. PHYSICAL REVIEW C 80, 014320 (2009)

TABLE I. (Continued.)

Eγ (keV) Iγ (Rel.) RDCO Gating transition Nature of the transition J πi → J π

f

672.6(0.3) 1.9(0.8)687.9(0.2) 0.8(0.1)692.7(0.1) 3.4(0.4)696.4(0.1) 2.7(0.5)701.6(0.1) 7.4(0.6) 0.90(0.40) Q Q 19

2

+ → 192

−

721.7(0.1) 13.8(0.9) 0.45(0.12) Q D 172

− → 152

−

732.1(0.1) 2.2(0.1) 0.81(0.53) Q Q744.5(0.1) 2.3(0.4)751.3(0.1) 65.4(0.8) 1.02(0.06) Q Q 19

2

− → 152

−

775.1(0.1) 3.6(0.5) Q 312

+ → 272

+

781.3(0.1) 7.8(0.6) 0.91(0.35) Q Q 472

− → 432

−

784.7(0.1) 7.4(0.5) 0.68(0.24) Q D 212

− → 192

−

789.0 <1.0 Q 332

− → 292

−

792.3(0.2) 3.0(0.7)800.7(0.1) 11.5(0.8) 1.21(0.38) Q Q 39

2

− → 352

−

805.8(0.2) 1.8(0.6) 252

− → 232

−

814.8(0.1) 7.0(0.9) 0.94(0.31) Q Q 212

− → 172

−

830.9(0.3) 0.6(0.3)

847.2(0.1) 3.7(0.5) 1.06(0.35) Q Q 392

(−) → 352

−

850.4(0.2) 1.7(0.4)859.3(0.1) 7.3(0.3) 1.30(0.24) Q Q 33

2

+ → 292

+

865.8(0.2) 3.8(0.8)875.6(0.1) 8.7(0.4) 0.97(0.44) Q Q 35

2

− → 312

−

875.6(0.1) 2.4(0.2) Q 352

+ → 312

+

883.0 <1.0

896.9(0.1) 5.6(0.9) D 212

(+) → 192

−

898.9(0.3) 0.9(0.6)903.2(0.2) 1.4(0.6)909.8(0.1) 16.7(0.9) 1.04(0.14) Q Q 27

2

− → 232

−

916.4(0.3) 1.6(0.8)922.9(0.3) 2.2(0.8)928.0(0.3) 1.9(0.7)942.8(0.2) 1.7(0.4) Q ( 39

2

+) → 35

2

+

942.8(0.2) 6.7(0.3) 0.90(0.14) Q Q 512

− → 472

−

946.5(0.1) 45.9(0.7) 1.07(0.05) Q Q 232

− → 192

−

955.1 <1.0966.8(0.1) 11.3(0.7) 1.09(0.39) Q Q 31

2

− → 272

−

967.0(0.1) 4.3(0.3) 1.80(0.41) D Q 252

− → 212

−

981.0 <1.0997.2(0.2) 1.0(0.4)

1009.3(0.1) 3.9(0.5) 0.70(0.33) Q D ( 372

−) → 35

2

−

1014.9(0.1) 4.2(0.5) 1.02(0.45) Q Q1035.8(0.2) 1.5(0.6) Q 55

2

− → 512

−

1050.5(0.3) 1.8(0.8)

1059.7(0.2) 2.9(0.5) D 152

(+) → 152

−

1064.2(0.3) 0.8(0.3) Q ( 432

+) → ( 39

2

+)

1064.5(0.1) 12.6(0.9) 1.12(0.11) Q Q 272

− → 232

−

1087.7(0.2) 2.8(0.4) Q 372

+ → 332

+

1112.3(0.2) 2.5(0.4) 212

+ → 192

−

1122.0(0.3) 3.6(1.0) 312

− → 272

−

1199.0(0.2) 1.0(0.6) Q ( 412

+) → 37

2

+

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HIGH SPIN STATES IN 139Pm PHYSICAL REVIEW C 80, 014320 (2009)

TABLE I. (Continued.)

Eγ (keV) Iγ (Rel.) RDCO Gating transition Nature of the transition J πi → J π

f

1214.0 <1.0 Q ( 452

+) → ( 41

2

+)

1254.0 <1.01274.8(0.3) 1.1(0.5)

1297.7(0.1) 14.5(0.5) 0.58(0.10) Q D 172

(+) → 152

−

1327.0(0.3) 0.8(0.3)1608.0(0.3) 2.5(0.9)

100 150 200 250

0

200

400

600

800700 800 900 1000

0

200

400

600

800

100 150 200 2500

200

400

600

800

700 800 900 10000

200

400

600

800

Energy (keV)

Cou

nts

138

212 75

1

946

138 21

2

751

946

θ

θ

θ

θ

= 98 = 98

= 45 = 45

o

o o

o(1a)

(1b)

(2a)

(2b)

∆

∆

∆

∆

J = 1

J = 1

J = 2

J = 2

FIG. 1. (Color online) Angle gated DCOcorrelation spectra showing the differencebetween dipole and quadrupole transitions(when gated on a �J = 2466 keV transition)in 139Pm.

400 600 800 1000

0

0.4

0.8

1.2

1.6

2

100 200 300 400 500Transition Energy (keV)

0

0.4

0.8

1.2

RD

CO

Gate on: Quadrupole

Gate on: Quadrupole

J = 2

J = 1

∆

∆

466

663

751

781

801

869

876 94

294

696

7

1064

120

138

212

23822

8

268

274

320

360

369 40

1

433

477

FIG. 2. (Color online) RDCO for �J =2 and �J = 1 transitions in 139Pm, Thepoints shown by circles are known quadrupoleand dipole transitions of neighboring 138Pmnucleus [21], for reference.

014320-5

A. DHAL et al. PHYSICAL REVIEW C 80, 014320 (2009)

FIG

.3.

The

leve

lsch

eme

of13

9Pm

dedu

ced

from

the

pres

entw

ork.

014320-6

HIGH SPIN STATES IN 139Pm PHYSICAL REVIEW C 80, 014320 (2009)

FIG. 4. (Color online) Representative par-tial γ -γ coincidence spectra, gated sum of theof 212-, 138-, and 268-keV transitions (upperpanel) and gated by the 1298-keV transition(lower panel) belonging to band 1 in 139Pm.The peaks marked with an asterisk ‘∗’ arecontaminants from the 138Nd nucleus.

B. Bands 2, 3, and 4

Band 2 is newly established in the present work and consistsof 622-, 696-, 883- (tentative), and 997-keV transitions. This isconnected to band 3 via the 850 keV interband transition whichconnects this band to the 29

2+

level in band 3. The observationof 1199- and 1214-keV transitions have extended band 3 up toa tentative spin of 45

2

+. In addition to these topmost transitions,

the 446- and 1112-keV transition were found to connect thisband to the 19

2+

level of the yrast band (band 5). The 403-,831-, and 928-keV transitions were found to be side-feedinginto this band. From the intensity anisotropy, RDCO, calculationit is found that the the transitions 446, 594, and 859 keVwhich involve a �J = 2 change in angular momentum areassigned an E2 multipolarity. The new in-band members ofthis sequence viz. 1199 and 1214 are tentatively assignedas E2. The level de-exciting via the 1112-keV transition isassigned as J = 21

2+

due to the observation of a 446 keV,�J = 2 transition which decays to this level from the Jπ =252

+level. The 612-keV transition connecting band 3 to band

5 at 232

−level is found to be dipole in nature. In band 4,

three new γ -transitions, 775, 943, and 1064 keV were placed.The observed intensity has necessitated a reordering of theearlier reported levels. The position of earlier reported 876 keVtransition is replaced by the newly observed 775-keV( 31

2+ → 27

2+

) transition and the 876-keV ( 352

+ → 312

+) tran-

sition is placed on the top of 775 keV. The newly observed389-keV ( 23

2+ → 21

2 ) transition is an interconnecting transitionbetween band 4 and band 1. The intensity anisotropy, RDCO,value suggests that the 584, 590, and 702 keV γ -transitions arequadrupole in nature. The present statistics did not permit us toobtain the RDCO values for the other members of this sequenceviz. 338-, 775-, 876-, 943-, and 1064-keV transitions. How-ever, the in-band transitions have been assigned tentativelyas E2, based on the similar transitions in the neighboringnuclei.

C. Band 5

All the reported transitions belonging to this yrast bandhave been confirmed. In the earlier work [13] a 1112-keVtransition was tentatively placed as the topmost member ofthis sequence. The present coincidence measurements indicatethat this transition connects band 3 to this band at the 19

2−

level. Two parallel transitions, 732 keV and 916 keV, feedthe Jπ = 51

2

−level. A new band comprising of the 564-, 669-,

792-, and 899-keV transitions was found to side-feed this bandat Jπ = 27

2−

. The RDCO for the 669-keV transition indicates aquadrupole nature for this transition. On the basis of the γ -rayintensity considerations of 967- and 669-keV transitions, itappears that this new band decays out to the Jπ = 27

2−

levelof the band 5 by unobserved transition. Besides these we haveobserved several side-feeding transitions, i.e., 490, 570, 673,923, 955, 1050, and 1327 keV.

D. Bands 6 and 7

In band 6 all the γ -transitions reported earlier werereproduced and their placement confirmed, according tocoincidence relationships and intensity arguments. Band 7 is anewly identified band from the present work. This band feedsto the 17

2−

level of band 6, and consists of 527-, 634-, 866-, and981-keV γ -transitions. It is found that this sequence is fed fromthe 31

2−

level of yrast band (band 5). The intensity anisotropy,RDCO, values show that the 785 and 722 keV transitionsdecaying from 21

2−

and 172

−levels in band 6, respectively,

are dipole in nature whereas the 815 and 967 keV in-bandtransitions in band 6 are quadrupole in nature.

E. Bands 8 and 9

Along with the four earlier reported γ -transitions of band 8,four new γ -transitions 533, 646, 789, and 1009 keV wereobserved. 533- and 1009-keV transitions were placed on

014320-7

A. DHAL et al. PHYSICAL REVIEW C 80, 014320 (2009)

FIG. 5. (Color online) Representative par-tial γ -γ coincidence spectrum gated by 473 keV( 33

2

− → 312

−) transitions belonging to band 8.

The peaks marked with an asterisk ‘∗’ arecontaminants from the 138Nd nucleus.

the top of the earlier observed transitions of the band and646( 31

2− → 27

2−

) and 789( 332

− → 292

−) keV transitions are

found to be crossover transitions. The RDCO values suggestthat the 533- and 1009-keV transitions are dipole and the910 keV decay out transition at 27

2−

level is quadrupole innature. In addition, nine new γ -transitions 601, 658, 688,744, 903, 1015, 1254, 1275, and 1608 keV are observed tofeed in to this band at different levels. The 903 and 688 keVγ -transitions are in coincidence with each other as wellas with the transitions in band 8. These feed band 8 at332

−level. The 806 keV transition de-excites from the 25

2

−

level of this band and feeds the 232

−level of band 5. The

1122 keV transition populates the 272

−level from the 31

2−

level of band 5. The newly observed band 9 consists of 224-,262-, 313-, and 394-keV γ -transitions. These transitions wereobserved to involve a �J = 1 change in angular momentum,as from the measured RDCO all are dipole in nature and hence,have been assigned as M1 transitions. The 847-keV transitionis an interconnecting transition between band 9 and band8 and is found to be quadrupole in nature. Representativepartial coincidence spectrum gated by the 473-keV transitionbelonging to band 8 is shown in Fig. 5. The new γ -transitionsof bands 8 and 9 along with some of the strong transitions ofband 6 are observed. Upon checking the intensity balance at the352

−level of band 8 we notice missing decay out intensity. It is

not possible to resolve this problem from other coincidencegates in this band as there are many contaminant γ -rayssuch as 454 and 533 keV in band 8 and 224, 262, 847 inband 9.

IV. DISCUSSION

In the odd-proton nuclei of the mass A ≈ 135–140 regionthe alignment of the first pair of the h11/2 protons is blocked.However, the alignments of the second and third proton tend todrive the nucleus toward a prolate shape, while the alignment of

the neutrons has an opposite shape-driving effect, i.e., it drivesthe nucleus toward an oblate shape. The yrast sequence in139Pm (band 5) is based on the negative parity signature of the[541] 3

2−

Nilsson orbital [13]. A large gain in the alignment hasbeen observed. This is indicative of the involvement of morethan one pair of high-j nucleons. This has been attributed tothe alignment of the protons and a pair of neutrons [13]. CSMcalculations do indicate the alignment of both a pair of h11/2

protons and h11/2 neutrons simultaneously with a gain of ∼14hat hω ≈ 0.4 MeV, which is indeed observed experimentally asreported by the earlier workers and confirmed by the presentinvestigations.

The positive parity �J = 1 band (band 1) is an irregularband. No definite configuration could be assigned to it.However, as its intensity flows through the proton h11/2 band,and on the basis of similar but regular dipole band observedin 141Eu [11], the likely configuration for this band could beπh11/2 ⊗ ν[s1/2 ⊗ h11/2] or πh11/2 ⊗ ν[d3/2 ⊗ h11/2].

In this mass region, the πd5/2 and πg7/2 orbitals are nearlydegenerate. Hence one expects to observe positive parity bandsbased on the three quasiparticle configuration, i.e., πg7/2 ⊗(πh11/2)2 or πd5/2 ⊗ (πh11/2)2 [23]. Bands 3 and 4 could beattributed to originate from these configurations.

For the newly observed band 7 our CSM calculationspredict a proton h11/2 band-crossing frequency of ∼0.3 MeVin agreement with the experimental value. We are howeverunable to predict any nucleon configuration for this band.

The focus of the present investigation was to identify �J =1 sequences which resemble magnetic rotational (MR) bands.We have extended the earlier reported �J = 1 sequences(bands 1 and 8) and established a new �J = 1 sequence(band 9).

The tilted axis cranking (TAC) [24] model naturallydescribes the observation of MR bands. In addition a semi-classical approach as proposed in Ref. [25] also providesa qualitative understanding of the phenomenon of magneticrotation. We have attempted to understand the observed �J =1 bands by using the TAC model.

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HIGH SPIN STATES IN 139Pm PHYSICAL REVIEW C 80, 014320 (2009)

Spin [I(h)]

2

3

4

5

6

7

8

9

10

Exc

itat

ion

ener

gy

Band 8 Expt

TAC 3qp

Band 9 Expt

TAC 5qp

TAC 5qp

TAC 5qp

π(h11/2

)

25/2 29/2 33/2

O

37/2

O

41/221/2 45/2 49/2

O

53/2

ν(h11/2

)-2

π(h11/2

(d5/2

)2)

ν(h11/2

)-4

π(h11/2

)3 ν(h

11/2)-2

ν(h11/2

)-2

π(h11/2

)

O

x

x

x

x

FIG. 6. (Color online) Excitation energy asa function of spin, for bands 8 and 9 (bothexperimental and theoretical plots included).

Tilted axis cranking (TAC) [24] calculations have beenperformed for the dipole bands 8 and 9. Pairing is takeninto account by using gap energies �p = 1.16 MeV and�n = 0.707 MeV as deduced from 80% of the odd-even massdifferences.

On the basis of the earlier studies [11,12] in this massregion for such bands, it is expected that the bands wouldbe based on three-quasiparticle configurations. Indeed theπ (h11/2) ⊗ ν(h11/2)−2 configuration resulted in a minimum inthe total energy at a deformation of ε2 = 0.124, γ = 52◦ witha tilt angle θ = 25.7◦. The possibility of a contribution froma five-quasiparticle configuration cannot be ruled out. Suchconfigurations have been reported in the neighboring 137Pr[12] and 141Eu [11]. For the five-quasiparticle [π (h11/2)1 ⊗ν(h11/2)−4] configuration a minimization in the energy yieldeda deformation of ε2 = 0.11, γ = 60◦, and θ = 26◦. Further

the π (h11/2)(d5/2)2 ⊗ ν(h11/2)−2 configuration also predicts aminimum for ε2 = 0.12, γ = 50◦, and θ = 26◦. As mentionedpreviously, the d5/2 and g7/2 are nearly degenerate in this massregion and the low � orbital could well originate from g7/2

instead of d5/2.Figure 6 compares the Ex vs I plots for the 3qp (band 8)

and 5qp (band 9) configurations. As seen from the figure,a crossing is seen in both the experimental as well asthe calculated values, and individually neither the 3qp and northe 5qp results reproduce the data. These result suggest that thelower part of the band corresponds to a 3qp configuration whileafter the crossing (at I ∼= 37

2 ) the 5qp configuration dominatesthe band.

Figure 7 gives the plot of I vs hω for bands 8 and9. The TAC calculations individually for 3qp and 5qpconfigurations although in agreement with the experimental

0.1 0.2 0.3 0.4 0.5

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28Expt Band 8Expt. Band 9TAC 5qpTAC 3qp

I(h

)

(h11/2

)-2

(h11/2

)1 ν

X

h (MeV)ω

3qp

5qp (h11/2

)3

X

ππ

ν(h

11/2)-2

(ε2 = 0.130

(ε2 = 0.124, γ = 52ο)

, γ = 60ο)

FIG. 7. (Color online) Variation of spin I (h)vs hω: A comparison between experimental andtheoretical (TAC) result for bands 8 and 9.

014320-9

A. DHAL et al. PHYSICAL REVIEW C 80, 014320 (2009)

0 0.1 0.2 0.3 0.4 0.50

1

2

3

4

5

6

7

8

9

10TAC 5qp [ (h

11/2)3 (h

11/2)-2

] : Band 9

TAC 3qp [ (h11/2

)1 (h

11/2)-2

] : Band 8

15 20 250

100

200

300

400Expt Band 8

I(h)

h (MeV)ω

B(M

1)/B

(E2)

[ (

/

eb)2 ]

B(M

1)[

(

)2 ]

µ Νµ Ν

π Ο

Οπ

ν

ν

FIG. 8. (Color online) Upper panel shows the variation of B(M1)vs hω where as the lower panel shows the variation of B(M1)/B(E2)vs spin I (h) from the TAC calculation for bands 8 and 9.

results do not provide information about band crossings of thebands.

The interpretation in terms of a band crossing can beverified from the lifetime measurements, which are beyondthe scope of the present work. If a band crossing is present,an abrupt change in B(M1) values at the band crossing wouldoccur. The predicted B(M1) and B(M1)/B(E2) ratios arepresented in Fig. 8 for configurations π (h11/2) ⊗ ν(h11/2)−2

(band 8) and π (h11/2)3 ⊗ ν(h11/2)−2 (band 9). The agreementof the experimental B(M1)/B(E2) ratios with calculations aresuggestive of a 3qp configuration for band 8. The theoreticaltransition probabilities B(M1) indicate a decreasing trend withincreasing frequency for both 3qp and 5qp configurations, acharacteristic for MR [26].

The agreement between the experimental results and theTAC calculations are suggestive of a MR nature for bands 8and 9. The characteristic decrease in the B(M1) values fromlifetime measurements in these bands and their comparisonwith the TAC calculations may be able to provide evidencesfor the magnetic rotational nature of these bands.

In summary, the information on the level structure of139Pm has been considerably extended up to high spins andexcitation energy of Jπ = 55

2

−and Ex

∼= 9MeV, respectively.The present investigation has necessitated some reorderingof the earlier reported levels, besides extending the earlierestablished sequences. Three �J = 1 bands (bands 1, 8, and9) have been observed. Bands 1 and 8 have been extendedto higher spins and band 9 is a newly found band. Thepositive parity band (band 1) is an irregular band. On thebasis of its intensity flow through the proton h11/2 band andthe observation of a similar band in the isotone 141Eu, theprobable configurations for this band are πh11/2 ⊗ ν[s1/2 ⊗h11/2] or πh11/2 ⊗ ν[d3/2 ⊗ h11/2]. We have made an attemptto understand the nature of bands 8 and 9 on the basis of TACcalculations and systematics of similar bands in neighboringnuclei. The likely configuration of band 8 is π (h11/2) ⊗ν(h11/2)−2 and that of band 9 is π (h11/2)3 ⊗ ν(h11/2)−2. Acomparison of the TAC calculations with the experimentaldata indicate MR nature for these two bands (bands 8 and9). However, detailed calculation supported by additionalinformation such as lifetime measurements would elucidatethe underlying microscopic configuration.

ACKNOWLEDGMENTS

We thank the operational staff of the 15-UD Pelletronaccelerator at Inter University Accelerator Centre (IUAC),New Delhi, for smooth operation of the machine during theexperiment. The help of the target laboratory personnel ofIUAC is highly acknowledged. One of the authors (A.D.)would like to thank the UGC-DAE CSR, Kolkata Centre, forproviding financial assistance to carry out the work.

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