Rotational bands in 122 Cs

Nuclear Physics A 674 (2000) 343–356 www.elsevier.nl/locate/npe

Rotational bands in 122Cs C.-B. Moon a,∗ , T. Komatsubara b , K. Furuno b a Department of Physics, Hoseo University, Chung-Nam 336-795, South Korea b Institute of Physics and Tandem Accelerator Center, University of Tsukuba, Ibaraki 305-8577, Japan

Received 18 February 2000; revised 3 March 2000

Abstract Excited states of the 122 Cs nucleus have been studied by in-beam γ -ray spectroscopy with the 109 Ag(16 O, 3n)122 Cs fusion-evaporation reaction at a beam energy of 80 MeV. γ –γ coincidence and γ –γ angular correlation analyses were employed for determining the level scheme of 122 Cs. We have determined the spin of the πh11/2 ⊗ νh11/2 quasiparticle band with spin inversion structure and the negative-parity band with strongly coupled structure. These two rotational bands are found to be built on the same high-spin isomeric 8− state at 500 keV. In addition, three rotational bands have been newly identified in the present work.  2000 Elsevier Science B.V. All rights reserved. PACS: 21.60.Ev; 23.20.Lv; 27.60+j Keywords: N UCLEAR REACTIONS 109 Ag(16 O, 3n)122 Cs, Elab = 80 MeV; Array of Compton-suppressed HP Ge and LEPS detectors; Measured Eγ , Iγ , DCO ratios, γ γ -coincidence; 122 Cs deduced levels I , π , experimental Routhians; Total Routhian surface calculations, cranked shell model calculations

1. Introduction Nuclei in the Z = 50 and A = 120 mass have been of considerable interests since they lie in a transitional region between the primarily spherical Sn(Z = 50) nuclei and the well deformed Ce(Z = 58) nuclei. In this mass region the proton Fermi surface lies in the lower part of the h11/2 subshell, while the neutron Fermi surface lies in the h11/2 midshell. Thereby the nuclei in this mass region are soft with respect to γ , the triaxiality parameter in the polar description of quadrupole shapes. Total Routhian surface (TRS) [1,2] and cranked shell model (CSM) [3,4] calculations suggest that these high-j valence particles exert a strong and specific driving force on the γ -soft core: particles in the lower part of the h11/2 subshell favor a collectively rotating prolate shape (γ ≈ 0◦ in the Lund convention), while those in the middle part of the h11/2 subshell favor a collectively rotating triaxial shape (γ ≈ −30◦ ). In view of such driving effect, the doubly odd Cs ∗ Corresponding author: [email protected]

0375-9474/00/$ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 4 7 4 ( 0 0 ) 0 0 1 7 4 - 3

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nuclei have attracted much attention since they provide an opportunity to understand the possible interaction and competitive shape driving effects between proton and neutron quasiparticles. The collectivity in the Cs nuclei has been arisen from the well-developed quadrupole deformation built on the neighboring even–even Xe and Ba core nuclei. For the πh11/2 band in the light odd-mass Cs nuclei, the structure is very resemble to that of Xe core states and its deformation has been shown a definite prolate shape with γ ≈ 0◦ , thereby producing a large signature splitting in the yrast band. Such a large signature splitting in the πh11/2 bands of the odd-mass Cs nuclei has been led to the difficulty in finding the unfavored signature states [5]. For odd–odd Cs nuclei, the signature inversion phenomena, which the excitation energy in unfavored signature states is lower than that in favored signature states, have been reported in the πh11/2 ⊗ νh11/2 quasiparticle bands [6]. Such a signature inversion has been commonly observed in the πh11/2 ⊗ νi13/2 bands of deformed doubly odd nuclei in the mass region of A ∼ 160 [7]. Although several investigations [8–11] for the level structure of 122 Cs using in-beam spectroscopy have been made, the spin-parity for the observed rotational bands remains still uncertain. The spin of the yrast band with signature inversion structure has also been tentatively assigned on the basis of systematic study for the excitation energies [12]. In the present work, we established the spin-parity assignment to the yrast band as well as a negative-parity rotational band. Moreover, we report three new rotational bands in 122 Cs.

2. Experimental details The level structure has been studied with the 109 Ag(16 O, 3n)122Cs reaction at a beam energy of 80 MeV. The beam was provided by the 12UD tandem accelerator at the University of Tsukuba. The target was a self-supporting foil of 109 Ag 4 mg/cm2 with the Pb backing 5 mg/cm2 in thickness. The γ -ray spectra were taken with seven highpurity (HP) Ge detectors with BGO anti-Compton shields (ACS). One of them was the LEPS (low energy photon spectrometer) detector to ensure sensitivity for important lowenergy transitions at the bottom parts of γ -ray cascades. The present detector systems have been described in detail in [13]. Efficiency and energy calibration were performed with a standard γ -ray 152 Eu source. The intrinsic resolution of the HP Ge detectors was typically about 2.2 keV for a 1.33 MeV γ -ray. Data were written onto 8 mm tapes (EXABYTE) for events in which two or more HP Ge detectors registered in prompt coincidence. Approximately 98 million events were collected. In the off-line analysis, the coincidence data were recalibrated to 0.5 keV/channel and sorted into 4096 by 4096 channel triangular matrices. The gamma-ray coincidence relations were established by setting gates on photopeaks of the individual transitions and projecting the coincidence spectra. Gates were also put on the background in the vicinity of the photo-peaks to remove contributions due to the background below the photo-peaks of the gating transitions. Multipolarity information was extracted from the data using the method of directional correlation of oriented states (DCO ratios). To this end, the coincidence events were sorted

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into an asymmetric matrix with energies of γ rays detected in 5 detectors at 117◦ along one axis and energies of γ rays detected in a detector at 0◦ along the other axis. The intensities of Iγ (0◦ ) and Iγ (117◦) used to determine the DCO ratio, R = Iγ (0◦ )/Iγ (117◦ ), for the transitions of interest were extracted from spectra obtained by setting gates on the 0◦ and 117◦ axis of γ –γ asymmetric matrix. In general, the DCO ratios have been determined by gating on transitions in the band sequence preceding or following the transition of interest. When both the gating and observed transitions are stretched and have the same multipolarity, the DCO ratio is R ≈ 1; if gate is set on a stretched dipole transition and one looks at a stretched quadrupole transition, the DCO ratio is R ≈ 2.0. If the gate is on a stretched dipole transition and one observes a mixed dipole-quadrupole transition with 1I = 1, the R values depend on the δ(M1/E2) mixing parameter and range from R ≈ 0.5 for δ = −1.0, to R ≈ 2.5 for δ = 1.0. In the present work, the DCO values were mainly deduced from the gates on the dipole transitions. Measured angular correlation ratios Iγ (0◦ )/Iγ (117◦) are included in Table 1 together with energies, relative intensities, and spin-parity assignments of the transitions in 122 Cs.

3. Level scheme of 122 Cs Examples of gated coincidence spectra of γ -rays assigned to 122 Cs are shown in Fig. 1. The ordering of the γ -ray transitions has been determined from coincidence relationships and relative intensities. Information on γ -ray multipolarities was obtained from DCO ratios mentioned in previous section. The level scheme of 122 Cs deduced from the present work is shown in Fig. 2. Band 1 has been identified in previous works as a negative-parity band starting at I π = (6− ) [9], as a positive-parity band starting at I π = (11+ ) [10,12] based on the systematic transition energies, and as a positive-parity band starting at I π = (9+ ) through in-beam spectroscopy [11]. Here the starting spin-parity state indicates, in Fig. 2, an 11+ state fed by the 81.3 keV transition and the 387.5 keV transition, respectively. Band 2 has also been observed in the previous works as a negative-parity band starting at I π = (5− ) [9,10] and I π = (4− ) [11]. In the previous works [9,11], however, a linking transition between band 1 and band 2 was reported differently each other. Xu et al. [9] reported a 488 keV linking transition from the state fed by the 166 and 559 keV transitions in band 1 to the state fed by the 203 and 422 keV transitions in band 2. Meanwhile, Lu et al. [11] reported the 95 keV linking transition from the state fed by the 51 and 154 keV transitions in band 1 to the state fed by the 203 and 422 keV transitions in band 2. In the present work, however, no transitions of 488 and 95 keV between band 1 and band 2 were observed. Moreover, no evidence for indicating a doublet of the 95 keV transition reported in [11] was given in spectra gated by the 95 keV transition as shown in Fig. 1. Instead, as shown in Fig. 1(a) we could observe a 37 keV line through the LEPS detector in coincidence with the 95 keV transition and the 51–103–81 keV transitions cascading to the 9+ state. On the other hand, this 37 keV line does not appear in spectra gated by a 132 keV transition as shown in

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Table 1 Excitation energies, γ -ray energies, intensities, DCO ratios, and spin-parity assignments to 122 Cs following the 109 Ag(16 O, 3n)122 Cs reaction at a beam energy of 80 MeV Ei (keV)

Ef (keV)

Eγ (keV) a

632.3 683.5 868.1 595.3 786.8 725.6 632.3 786.8 1340.7 868.1 928.9 813.8 1147.6 725.6 2000.0 1432.5 1721.1 1174.3 928.9 2411.7 2067.3 1174.3 1733.2 2804.1 3196.6 1147.6 3623.1 2437.7 4066.8 5234.5 1718.2 3269.5 1269.4 4209.2 1340.7 1432.5 1442.7 1251.5 1269.4 1415.6 1733.2 1721.1

595.3 632.3 786.8 500.0 683.5 595.3 500.0 632.3 1174.3 683.5 725.6 595.3 928.9 500.0 1733.2 1147.6 1432.5 868.1 595.3 2067.3 1721.1 786.8 1340.7 2411.7 2804.1 725.6 3196.6 2000.0 3623.1 4786.6 1269.4 2814.3 813.8 3751.8 868.1 928.9 928.9 725.6 725.6 868.1 1174.3 1147.6

37.0 51.2 81.3 95.3 103.3 130.3 132.3 154.5 166.4 184.6 203.3 218.5∗ 218.7∗ 225.6 266.8 284.9 288.6 306.2 333.6 344.4 346.2 387.5 392.5 392.4∗ 392.5∗ 422.0 426.5 437.7 443.7 447.9 448.8 455.2∗ 455.6∗ 457.4 472.6 503.6 513.8 525.9 543.8 547.5 558.9 573.5

Iγ b

79 520 710 870 303 1000 29 295 52 245 44 100 18 51 102 59 570 31 38 47 60 350 102 41 142 25 10 36 48 43 13 732 63 16 39 41 117 138 105

Rc

0.9e 0.9f 0.7e 0.6d 0.7d 2.0e 0.8e 1.6e 0.9f 0.9g 1.0f 2.3e 0.7e 0.9f 0.8f 0.5e 2.0g 0.8f 0.8f 1.7e 0.6e 0.7f 0.7f 1.7f 0.7f 0.6e

1.6f 0.5e 0.4h 0.6e 1.5e 2.1f 0.6f 1.0f 0.9f 0.7e 1.3e 1.9f

Multipolarity

M1/E2 M1/E2 M1/E2 M1/E2 M1/E2 E1 E2 M1/E2 E2 M1/E2 dipole M1/E2 E2 M1/E2 M1/E2 M1/E2 M1/E2 E2 M1/E2 M1/E2 E2 M1/E2 M1/E2 M1/E2 E2 M1/E2 M1/E2 M1/E2 M1/E2 E2 M1/E2 dipole M1/E2 E2 E2 dipole dipole dipole M1/E2 E2 E2

Assignment 9+ −→ 9− 10+ −→ 9+ 12+ −→ 11+ 9− −→ 8− 11+ −→ 10+ 10− −→ 9− 9+ −→ 8− 11+ −→ 9+ 14+ −→ 13+ 12+ −→ 10+ 11− −→ 10− (10) −→ 9− 12− −→ 11− 10− −→ 8− 16+ −→ 15+ 13− −→ 12− 14− −→ 13− 13+ −→ 12+ 11− −→ 9− 16− −→ 15− 15− −→ 14− 13+ −→ 11+ 15+ −→ 14+ 17− −→ 16− 18− −→ 17− 12− −→ 10− 19− −→ 18− 17+ −→ 16+ 20− −→ 19− 23+ −→ 22+ (13) −→ (11) 19+ −→ 18+ (11) −→ (10) 21+ −→ 20+ 14+ −→ 12+ 13− −→ 11− (12) −→ 11+ (11) −→ 10− (11) −→ 10− (13+ ) −→ 12+ 15+ −→ 13+ 14− −→ 12−

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Table 1 — continued Ei (keV)

Ef (keV)

Eγ (keV) a

Iγ b

Rc

Multipolarity

1992.4 2297.7 2059.5 2067.3 1992.4 2000.0 2983.9 2411.7 2758.6 2437.7 2728.6 2804.1 3767.1 3196.6 3554.1 2814.3 3623.1 3269.5 3594.4 4066.8 4639.8 4647.4 4516.9 3751.8 4209.2 5012.9 5481.3 5234.5 4786.6

1415.6 1718.2 1442.7 1432.5 1340.7 1340.7 2297.7 1721.1 2059.5 1733.2 1992.4 2067.3 2983.9 2411.7 2758.6 2000.0 2804.1 2437.7 2728.6 3196.6 3767.1 3767.1 3623.1 2814.3 3269.5 4066.8 4516.9 4209.2 3751.8

576.8 579.5 616.8 634.8 651.7 659.3 686.2 690.6 699.1 704.5 736.2 736.8 783.2 784.9 795.5 814.3 819.0 831.8 865.8 870.2 872.7 880.3 893.8 937.5 939.7 946.1 964.4 1025.3 1034.8

50 58 19 76 111 572 58 81 27 114 47 98 48 63 8 241 59 69 17 48 8 6 25 105 56 16 12 6 22

1.6e 1.8f 3.0f 2.2f 0.8f 1.5e 1.8f 2.2f 2.7f 1.5e 1.7e 1.7f 1.7f 2.2f

E2 E2 E2 E2 M1/E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2

1.8e 1.7f 2.8e 1.6e 1.6f 0.9h 0.8h 2.0f 2.6e 1.9e 1.8f 1.3f 1.5e 1.7e

Assignment (15+ ) −→ (13+ ) (15) −→ (13) (14) −→ (12) 15− −→ 13− (15+ ) −→ 14+ 16+ −→ 14+ (15) −→ (13) 16− −→ 14− (16) −→ (14) 17+ −→ 15+ (17+ ) −→ (15+ ) 17− −→ 15− (19) −→ (17) 18− −→ 16− (18) −→ (16) 18+ −→ 16+ 19− −→ 17− 19+ −→ 17+ (19+ ) −→ (17+ ) 20− −→ 18− (21) −→ (19) (21) −→ (19) 21− −→ 19− 20+ −→ 18+ 21+ −→ 19+ 22− −→ 20− 23− −→ 21− 23+ −→ 21+ 22+ −→ 20+

a γ -ray energies are accurate to ±0.2 keV for the transitions with I > 100 rising to ±0.4 keV γ for the weaker ones. b Errors are estimated to be less than 5% of the quoted values for the transitions with I > 100 γ and less than 15% for the weaker ones. The values are normalized to 1000% for the 132.3 keV transition in band 1. c Errors are estimated to be less than 25% of the quoted values for the transitions with I > 100 γ and less than 40% for the weaker ones. d DCO ratios from the total γ –γ asymmetric (0◦ by 117◦ ) matrix. e DCO ratios from the gate on 132.3 E1 transition. Note that DCO ratio of the 132.3 transition from the total γ –γ asymmetric matrix is 0.73. f DCO ratios from the gate on 130.3 M1/E2 dipole transition. Note that DCO ratio of the 130.3 transition from the total γ –γ asymmetric matrix is 0.59. g DCO ratios from the gate on 95.3 M1/E2 dipole transition. h DCO ratios from the gate on 579.5 E2 quadrupole transition. ∗ γ ray is a doublet.

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Fig. 1. Examples of gated coincidence spectra with transition energies labeled in keV. Contaminants from other nuclei are labeled by asterisks. (a) Spectrum gated by a 95 keV transition; (b) spectrum gated by a 132 keV transition; (c) spectrum gated by a 51 keV transition; (d) spectrum gated by a 166 keV transition. Note that spectra in (a) and (b) are obtained by the LEPS detector.

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Fig. 2. The level scheme built on the isomeric 8− state at 500 keV of 122 Cs deduced from the 109 Ag(16 O, 3n)122 Cs reaction at a beam energy of 80 MeV. The width of arrows corresponds roughly to the intensity of γ -ray transition. Transition and excitation energies are given in keV.

Fig. 1(b). Note that sum energy of a 37 keV and a 95.3 keV equals to the 132.3 keV. This means that band 1 and band 2 should be built on the same state. The ground state of 122 Cs has been known to have the spin and parity of 1+ with a magnetic dipole moment, µ = 0.133 nm, an electric quadrupole moment, Q = −0.19 b, and a half-life, t1/2 = 21 s [14]. Meanwhile, a high-spin isomer with I π = 8− at 500 keV has been reported to be µ = +4.77 nm, Q = +3.29 b and t1/2 = 4.5 m [14,15]. Many quasiparticle states built on the ground state have been observed below 500 keV excitation. No transitions, however, were observed connecting the ground 1+ state to the high-spin isomeric 8− state. We suggest that bands 1 and 2 should built on the isomeric 8− state at an excitation of 500 keV. The reasons are as follows: first, any transitions built on the ground 1+ state could not be observed in coincidence with the transitions in bands 1 and 2. This means that bands 1 and 2 should have an isomeric state with a long lifetime.

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Second, we emphasize that bands 1 and 2 show clearly the deformed rotational structure. As mentioned above, the isomeric 8− state has a large positive electric quadrupole moment, namely Q = +3.29 b that corresponds to the well-developed prolate shape with a large β2 deformation. So one easily expect that the deformed rotational bands 1 and 2 could be built on such a deformed isomeric state. Finally, our result is also supported by TRS calculations for the deformation of two-quasiparticle configurations in 122 Cs as described in the next section. In order to determine the spin-parity assignment for the 132.3 keV transition, Komatsubara et al. [16] performed the angular distribution measurement at the angles of 20, 30, 45, 60, 75, and 90 degrees and the linear polarization measurement using a Compton polarimeter that consisted of two germanium detectors surrounded by lead shield. From these angular distribution and linear polarization measurements, the 132.3 keV transition turned out to be an electric dipole character. Thereby the spin-parity of the band head for band 1 has to be 9+ . Surprisingly, the present result for the spin-parity assignment of band 1 does agree well with that obtained from the study of systematic transition energies by Liu et al. [12]. In addition, we emphasize that the spin-parity assignment of band 2 could be definitely determined by this work. This is straightforward for this band built directly on the 8− isomer since all possible I to I − 1 and I to I − 2 transitions were observed. In the present work, we could establish three new rotational bands, bands 3–5 in Fig. 2. Band 4 shows a very regular sequence of E2 transitions above the 13 state. DCO ratios for 218.5, 455.6, 525.9 and 543.8 keV transitions showed dipole characters. In order to assign the parity of this band, it is necessary to perform the linear polarization measurement for these dipole transitions. Band 5 is a weakly populated band connected by quadrupole transitions finally decaying to the 11− state in band 2 through the 513.8 keV dipole transition. It is not certain that bands 4 and 5 are signature partners each other since no linking transitions could be observed. Band 3 also consists of quadrupole transitions. This band is quite strongly connected to band 1 by the 547.5 and the 651.7 keV M1/E2 transitions.

4. Discussion In order to investigate the quasiparticle configuration and the deformation for the observed bands in 122 Cs, we have performed the CSM [3,4] and the TRS [1,2] calculations. For the odd–odd 122 Cs, with Z = 55 and N = 67, the available spherical shell model orbitals near the Fermi surface are d5/2 , g7/2 , and h11/2 for protons and d3/2 , d5/2 , g7/2 , and h11/2 for neutrons. However, in the deformed nucleus, these orbitals are strongly mixed and are no longer treated as good quantum numbers for describing shell structure. Instead, only parity (π ) and signature component (α), namely (π, α) are the conserved quantum numbers. Here signature component, α, is the quantum number associated with the r = exp(−iπα) symmetry, i.e., a rotation of 180◦ about the axis of nuclear rotation. The relation between angular momentum and signature component can be expressed as I = α mod 2. See Refs. [3,4] for more details about the cranked shell model.

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4.1. Band 1 The signature inversion feature in the πh11/2 ⊗ νh11/2 quasiparticle bands of doubly odd Cs nuclei has been commonly observed [6,12]. We suggest that band 1 should be associated with the νh11/2 orbitals coupled by the favored states in the proton h11/2 orbital, namely the πh11/2 ⊗ νh11/2 quasiparticle configuration. Our result is well consistent with those obtained in the previous works [10,12] and also supported by the similar structure in the πh11/2 ⊗ νh11/2 bands observed in the neighboring odd–odd nuclei with A ∼ 130. In order to compare the experimental data with the cranked shell model (CSM) calculations, the experimental level energies are transformed into rotating frame of reference following outlined in [3,4]. For subtracting a rotational reference, we used the Harris parameters J0 = 17.0 h¯ 2 /MeV and J1 = 25.8 h¯ 4 /MeV3 , which are the same as used for several neighboring nuclei [5]. Figs. 3 (a) and (b) show plots of the extracted experimental Routhian e0 that is the quasiparticle energy in the rotating frame and the gain in aligned angular momentum as a function of the angular frequency h¯ ω for bands in 122 Cs. One can see that band 1 is the lowest in energy, thereby confirming the yrast band. Figs. 4 (a)–(d) show the theoretical quasiparticle Routhians predicted by the CSM calculations based on the TRS formalism for the quasiproton and quasineutron configurations in 122 Cs. In addition, in Table 1 we present predicted deformations (β2 , γ , β4 ) and Routhian e0 for various proton– neutron configurations at a given angular frequency of h¯ ω = 0 and 0.18 MeV, respectively. Following the cranked shell model nomenclature [4], the nucleus 122 Cs is represented via two-quasiparticle configuration with the quasiproton and the quasineutron orbitals labeled A and B for positive parity and E and F for negative parity. The A and B configurations correspond to the d5/2[420]1/2 or g7/2 [422]3/2 orbitals for the proton shell

Fig. 3. Plots of the experimental quasiparticle Routhian (a) and gain in aligned angular momentum (b) as a function of rotational frequency. A rotational reference with the Harris parameters J0 = 17.0 h¯ 2 /MeV and J1 = 25.8 h¯ 4 /MeV3 has been subtracted.

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Fig. 4. Plots of theoretical quasiparticle Routhian for various proton–neutron configurations in 122 Cs by the TRS calculations.

and the d5/2[402]5/2 orbital for the neutron shell. It is noted that the positive-parity proton orbitals πd5/2[420]1/2 and πg7/2 [422]3/2 are strongly mixed. Meanwhile, the E and F trajectories originate from the h11/2 shell with Nilsson [550]1/2 orbital for the quasiproton and [532]5/2 for the quasineutron, respectively. Here E and F orbitals are called favored and unfavored states that correspond to parity and signature of (−, −1/2) and (−, +1/2), respectively while B and A correspond to (+, −1/2) and (+, +1/2), respectively. For the πh11/2 ⊗ νh11/2 quasiparticle band, α = −1 states, namely odd-spin members should lie lower in energy than those of even-spin members. It has, however, been observed that the even-spin members lie lower in energy up to a certain spin Ic and the normal signature

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dependence is restored afterwards. This anomalous effect is known as signature inversion. Such a signature inversion has been clearly seen in band 1 as shown in Fig. 3(a). In this paper, however, signature inversion is not discussed since detailed discussions have been already done in previous works [10,12]. It is noted that the TRS calculations could not explain the signature inversion feature as shown in Fig. 4. 4.2. Band 2 As mentioned in previous section, band 2 was tentatively assigned as to have the spin-parity of 5− of its band-head and interpreted as resulting from the πg7/2 ⊗ νh11/2 configuration without excluding the πh11/2 ⊗ νg7/2 configuration [10]. First of all, we have to point out that band 2 should be associated with the coupling of a quasiparticle to the strongly coupled bands observed in the neighboring odd-mass Xe or Cs nuclei since there is no signature splitting as shown in Fig. 3(a). In fact, such a strongly coupled band built on the ground 5/2+ state based on the d5/2 [402]5/2 orbital has been found in 121 Xe [17,18] and must be involved in the spectrum of bands in 122 Cs. In other words, its coupling with the expected proton h11/2 orbital can produce a band with small signature splitting in 122 Cs. Meanwhile, the extracted gain of aligned angular momentum of about 6–7 h¯ is suggested to be caused by spin contributions from the low- h11/2 [550]1/2 orbital. We thus propose that band 2 should be associated with the favored h11/2 proton coupled to the two signature states of the strongly coupled neutron [402]5/2+ orbital. Furthermore, the ratios of the reduced transition probabilities extracted for this band 2 is in reasonable agreement with the predictions of the semiclassical B(M1)/B(E2) calculations [19] for the assigned πh11/2 1/2 ⊗ νd5/25/2 configuration. It is thus natural to assume that the isomeric 8− state at 500 keV should be attributed to the coupling of the 11/2− state in 121 Cs with the ground 5/2+ state in 121 Xe. The gradual increase in aligned angular momentum at h¯ ω = 0.35–0.45 MeV as shown in Fig. 3(b) is indicative of an alignment effect due to a very strong interaction between proton and neutron orbitals as seen in the odd-mass Cs isotopes [5]. Such a feature has been known as a delayed alignment phenomenon when a pair neutron h11/2 alignment occurs in the yrast proton h11/2 band. Therefore, this feature in band 2 is most likely caused by the alignment of a pair neutron in the h11/2 orbital. The present assignment of configuration for band 2 is also consistent with the result assigned to a similar band in 120 Cs by Cederwall et al. [20]. As shown in Table 2, it should be emphasized that deformation for the negative-parity configurations is the largest for the πEνB and πEνA configurations, namely the πh11/2 ⊗ νd5/2 configuration. This is also accordance with the experimental large quadrupole value of Q = 3.29b. Meanwhile, the band caused by the coupling between the ground 3/2+ state based on the πg7/2 orbital in 121 Cs [5] and the 11/2− state built on the νh11/2 orbital in 121 Xe [17,18] is supposed to find in 122 Cs. In fact, the predicted Routhian for the πg7/2 ⊗ νh11/2 configuration is almost all equivalent to that of the πh11/2 ⊗ νd5/2 configuration as shown in Figs. 4 (b) and (c). The rotational band built on the g7/2 orbital in 121 Cs, however, was found to be not a strongly coupled band but a decoupled band [5]. On the other hand, the strongly coupled band without splitting in 121 Cs has been built on the proton excitation in the hole

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Table 2 Predicted deformation and Routhian e0 by the TRS calculations for various proton–neutron quasiparticle configurations at hω ¯ = 0 and 0.18 MeV in 122 Cs Configuration

πEνE πEνF πF νE πF νF πEνB πEνA πBνE πBνF πAνE πAνF

h¯ ω = 0.18 MeV

(π, α)

(+, −1) (+, 0) (+, 0) (+, +1) (−, −1) (−, 0) (−, −1) (−, 0) (−, 0) (−, +1)

β2

γ

β4

0.247 0.248 0.243 0.250 0.261 0.261 0.224 0.230 0.228 0.233

3.6 5.5 −6.1 −2.4 2.7 2.6 −4.8 −2.2 -3.2 −0.2

0.006 0.007 0.005 0.005 0.007 0.011 0.005 0.002 −0.001 0.001

hω ¯ = 0.0 MeV e0 (MeV) −0.57 −0.57 0.07 0.12 −0.16 0.03 −0.08 0.00 0.03 0.09

β2

γ

β4

0.254 0.254 0.254 0.254 0.258 0.258 0.237 0.237 0.237 0.237

0.2 0.2 0.2 0.2 0.8 0.8 −0.5 −0.5 −0.5 −0.5

0.008 0.008 0.008 0.008 0.008 0.008 0.003 0.003 0.003 0.003

g9/2 orbital. In our semiclassical B(M1)/B(E2) calculation and in a previous work [10], the πg9/2 ⊗ νh11/2 configuration could not reproduce the structure of band 2. 4.3. Band 3 Band 3 has a quite similar pattern in Routhian plot compared with those of bands 4 and 5 while its aligned angular momentum with an almost constant of 8.5 h¯ is very similar to that of unfavored states in band 1. First of all, one can suppose that this band may be attributed to the unfavored states of proton h11/2 orbital. If band 3 might be associated with the πF νF , namely πh11/2 (α = +1/2) ⊗ νh11/2 (α = +1/2) configuration, it is also again faced to the signature inversion problem since πF νF configuration is most unfavorable in energy among four combinations of the πh11/2 ⊗ νh11/2 configuration. The intensities of the 651.7 and 547.5 keV interband transitions are quite strong compared with those of intraband transitions in band 3. This decay pattern is very similar to that of unfavored states built on the proton h11/2 orbital in 121 Cs as obtained in our experiment [21]. Our interpretation indicates that a neutron in h11/2 orbital simply adds a constant amount of aligned angular momentum to the unfavored rotational band of the odd-mass 121 Cs core, in other words, a neutron in h11/2 orbital is merely a spectator. On the other hand, if an h11/2 proton is a spectator, one could expect a behavior similar to the yrare bands in 121 Xe where the yrare bands are associated with the coupling of a neutron in h11/2 orbital to the γ vibration. So alternative descriptions can involve the favored states due to the proton h11/2 orbital coupled to the favored yrare band in 121 Xe or the unfavored states due to the proton h11/2 orbital coupled to the unfavored yrare band in 121 Xe. The fact that band 3 decays only toward the unfavored band 1, thereby decaying to the unfavored states in the neutron h11/2 orbital, is that the structure of band 3 may be attributed to the favored yrare band in 121 Xe. One can see that states decaying towards the unfavored yrast band consist of a favored yrare band in 121 Xe [17]. It is thus suggested that band 3 could be associated with

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the (π = +, α = −1) configuration based on the πh11/2 (α = −1/2) orbital coupled to the νh11/2 (α = −1/2) ⊗ γ vibration. In the present experiment, we have failed to establish the signature partner with α = 0 of band 3 in spite of observing some transitions decaying to the favored states in band 1. 4.4. Bands 4 and 5 Bands 4 and 5 are characterized by the 1I = 2 quadrupole sequences without 1I = 1 dipole transitions. As far as the excitation energy and the spin of the band head are concerned, it is difficult to interpret these bands as a two-quasiparticle or a fourquasiparticle structure. A possible configuration for the structure of these bands is the coupling between the two quasiparticle excitation and the γ -vibration. As already mentioned, the yrare bands in 121 Xe were found and interpreted as to be caused by the excitation of an h11/2 neutron coupled to the γ -vibration of the even–even Xe core nucleus. It is also observed such a yrare band in 121 Cs [21]. It is thus proposed that bands 4 and 5 be attributed to the πh11/2 ⊗ νd5/2 configuration coupled to the quasi-γ rotational band in the even–even Xe core nucleus. It is necessary to appear a more sophisticated treatment that can take into account the interactions between the two neutron–proton quasiparticle states and the different deformed structures.

5. Conclusions We have determined the excitation energies and the spin-parity assignments for the positive-parity yrast band with signature inversion structure and the negative-parity strongly coupled band. In addition, we newly identified three decoupled rotational bands. We confirmed the spin-parity assignments based on systematic of the transition energies for the πh11/2 ⊗ νh11/2 quasiparticle bands by Liu et al. [12] and thus signature inversion for these bands is a general phenomenon in the odd–odd Cs nuclei.

Acknowledgements The authors are indebted to Dr. Y.-R. Shimizu for providing the CSM computer code and Dr. R. Wyss and Dr. W. Nazarewicz for providing the TRS computer code. This work was supported by Korea Research Foundation Grant (KRF-99-0084).

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