Eur. Phys. J. A (2015) 51: 60 DOI 10.1140/epja/i2015-15060-9
THE EUROPEAN PHYSICAL JOURNAL A
Regular Article – Experimental Physics
Reinvestigation of the collective band structures in odd-odd 138 Pm nucleus H.J. Li1,2 , Z.G. Xiao1,3,a , S.J. Zhu1 , C. Qi2 , E.Y. Yeoh1 , Z. Zhang1 , R.S. Wang1 , H. Yi1 , W.H. Yan1 , Q. Xu1 , X.G. Wu4 , C.Y. He4 , Y. Zheng4 , G.S. Li4 , C.B. Li4 , H.W. Li4 , J.J. Liu4 , S.P. Hu4 , J.L. Wang4 , and S.H. Yao4 1 2 3 4
Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China Department of Physics, Royal Institute of Technology, Stockholm 10691, Sweden Collaborative Innovation Center of Quantum Matter, Beijing, China China Institute of Atomic Energy, Beijing, 102413, China Received: 21 August 2014 / Revised: 25 April 2015 c Societ` Published online: 25 May 2015 – a Italiana di Fisica / Springer-Verlag 2015 Communicated by H. Miyatake Abstract. The high-spin states in the odd-odd 138 Pm nucleus have been reinvestigated via the 124 Te(19 F, 5n) reaction at the beam energy of 103 MeV. Most of the known transitions and levels are confirmed. A number of bands are revised and one new band has been established. For the yrast πh11/2 ⊗νh11/2 band based on 8+ state, no evidence supporting the occurence of signature inversion is found. The experimental and theoretical B(M 1)/B(E2) ratios have been calculated for band (2), which support the πg7/2 [413]5/2+ ⊗ νh11/2 [514]9/2− Nilsson configuration assignment. Four bands with I = 2 transitions are tentatively assigned as doubly decoupled bands. The other three bands are proposed as oblate-triaxial bands. The possible configuration assignments for these bands are also discussed under the calculations of total Routhian surface and particle-rotor model.
1 Introduction The studies of deformed odd-odd nuclei in the mass region A = 130–140 reveal many interesting phenomena in relation to the interplay between collective rotation and single particle motion of the unpaired nucleons. For nuclei in this mass region, the proton Fermi surface lies around the lower part of h11/2 subshell, while the neutron Fermi surface lies at the upper part of h11/2 subshell. The yrast bands originating from the πh11/2 ⊗ νh11/2 configuration in the oddodd nuclei like 134,136,140 Pm [1–3] are expected to exhibit signature inversion based on systematics [4]. Shape coexistence is also expected since the h11/2 neutron alignment will drive the nucleus to the oblate shape with γ ∼ −60◦ , whereas the h11/2 proton alignment will drive the nucleus to the prolate shape with γ ∼ 0◦ [5]. Collective bands with different shapes have been observed, including the oblate bands (γ ∼ −60◦ ) in 134 La [6], 136 Pr [7] and 136,140 Pm [2, 3], the oblate-triaxial bands (γ ∼ −90◦ ) in 140,141 Pm [3,8] and 136,139 Pr [7, 9], and the prolate bands (γ ∼ 0◦ ) in 132 Ba [10, 11] and 134,135 Ce [12, 13]. Particularly for the odd-odd 138 Pm nucleus, the level scheme was first established using the 116 Cd(27 Al, 5n) reaction [14] and later expanded using the 115 In(28 Si, 2p3n) [15] reaction. However a shift of the spin of the yrast band was proposed in a
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ref. [4] to account for the systematics of signature inversion among the neighboring nuclei. This requires further experimental investigation. Since the high spin data of 138 Pm are limited compared with the neighboring nuclei, it is of interest to reinvestigate this nucleus using a different reaction channel. In this work, the 138 Pm level scheme is restudied using the 124 Te(19 F, 5n) reaction at 103 MeV. The paper is structured as follows: The experimental setup and the level scheme construction is presented in sect. 2. Section 3 discusses the band properties. Section 4 is the summary.
2 Experiment and results The level structures of 138 Pm have been reinvestigated via the 124 Te(19 F, 5n) fusion-evaporation reaction at the beam energy of 103 MeV. The excitation functions were done with the beam energy from 99 MeV to 105 MeV at a step of 2 MeV, where the 5n-channel is found most populated at 103 MeV. The target with 3 mg/cm2 thick enriched 124 Te was prepared by evaporating tellurium metal powder on a 4 mg/cm2 gold backing foil. The 19 F beam was provided by the HI-13 tandem accelerator at the China Institute of Atomic Energy (CIAE). The detector array consists of nine Compton-suppressed HPGe detectors, two planar HPGe detectors and one clover detector. However, the addback was not applied for the
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Eur. Phys. J. A (2015) 51: 60
(6) 22
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327.5 177
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704.9 584.3 410.8
260.8 150.0
150.0
(7)
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3063.6 (13 )
(2)
984.0 1858.0
474.6 1383.4
765.2
1044.4
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3050.8
518.9 (12 )
2531.9
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138
398.4 (13 ) 2784.2
2096.7
396.2 (10 ) 1700.5 - 236.5 (9 ) 1464.0 - 227.1 (8 ) 1236.91 4 131.9 8.4 1088.5 7 1105.0 (7-) 955.0
938.5
0.0
138Pm Fig. 1. The level scheme of
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Pm deduced from the present work.
clover detector in the analysis. The detectors were placed at forward (40◦ ), 90◦ , and backward (140◦ ) directions with respect to the beam direction. The energy and efficiency calibrations were done with a 133 Ba and an 152 Eu standard source before and after the beam time. The typical energy resolution of Compton-suppressed HPGe detectors and the clover detector is 2.0 to 2.5 keV for the 1332.5 keV γ-ray of 60 Co, while it is around 0.6 keV at 152 Eu 121.8 keV γ-ray for the planar HPGe detectors. The total photo peak efficiency of the detector array at 1 MeV γ-ray is about 0.3%. Coincidences were recorded event-by-event onto the disk requiring γ multiplicity being greater than 1. The time resolution of the detectors is about 15 ns. The offline
data were sorted into 4k×4k matrices. A γ −γ coincidence symmetric matrix was constructed with the subtraction of the background spectrum, where a total of 6.8 × 107 coincidence events were obtained. Also an asymmetric twodimensional angular-correlation matrix with 90◦ detectors in one axis and non-90◦ detectors in the other axis was constructed to obtain the directional correlation of oriented state (DCO) ratios, from which the multipolarities of the observed γ transitions can be obtained [16]. The RADWARE software package was applied for the γ − γ coincidence data analysis [17]. The proposed level scheme of 138 Pm deduced from the present work is shown in fig. 1. The 0.0 keV level in the
Eur. Phys. J. A (2015) 51: 60
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level scheme is set as a reference. In this figure, the collective bands are labeled with numbers (1)–(9) on the top of each band. The γ-transition energies, the relative transition intensities, the DCO ratios, the multipolarities, and the spin and parity (I π ) assignments are shown in table 1. The I π assignments are based on the previous results [14, 15] and the DCO ratios of the γ transitions obtained in the present work. The DCO ratios are obtained with the form RDCO =
Iγθ11 (gate on γ2 at θ2 ) Iγθ12 (gate on γ2 at θ1 )
,
(1)
where θ1 stands for the 90◦ detectors while θ2 represents the non-90◦ detectors. By gating on stretched quadrupole transitions assigned in refs. [14, 15], RDCO ratio is around 1.00 for a known stretched quadrupole γ transition and RDCO value is about 1.70 for a known pure stretched ΔI = 1 γ transition. The DCO ratios of some γ rays could not be determined due to poor statistics. Band (1) up to 17+ state has been confirmed in this work compared to the results in refs. [14, 15]. However, the transitions above the 16+ level seen in ref. [15] are not observed in the present work. In refs. [14, 15], the levels in bands (2) and (8) in fig. 1 were assigned as one band structure. Based on the different structural patterns at the low spin and high spin parts, we reassign these levels as two independent bands based on 618.2 and 3063.6 keV levels, respectively. The reason will be discussed below. A few high-spin transitions and one level in band (8), such as the 517 and 1013 keV transitions from the 5368(20− ) level and the 895 keV transition from the (19− ) level reported in ref. [15] are not observed in the present work. For the linking transitions between bands (1) and (8), except for the 783 keV transition, the 196, 240, 555, 435, 1013 and 1010 keV transitions and the corresponding levels reported in ref. [15] are not observed in this work either. Some levels and transitions in bands (3), (4) and (5) were firstly reported in ref. [14] and updated in ref. [15]. Here we correct these bands as follows: As the 435.4 keV γ transition between 762.9 and 327.5 keV levels belongs to a ΔI = 1 M 1 transition, band (3) with the ΔI = 2 transitions inside the band should be based on the 762.9 keV (7− ) level instead of the 327 keV (6− ) level assigned in ref. [15]. Band (5) is updated at the highspin states with the 643.5 and 764.8 keV transitions instead of the 684 and 850 keV ones reported in ref. [15], respectively. According to the transition intensities and level spacings as indicated in table 1 and fig. 3(b), we tentatively rearranged band (6) with 726.9 and 861.9 keV transitions inside the band and the 554.7 keV transition as a linking one between bands (6) and (1). The transitions in band (7) have been reported in ref. [15], and here we reconstruct this band tentatively based on the 7− state (1105.0 keV) with I π up to (13− ). The 1105.0 and 1088.5 keV states can possibly be doublet and either of them can fit band (7). Band (9) built on the 2784.2 keV (13− ) level is newly identified in this work. This band is observed with I π up to (16− ). Three linking transitions of 687.5, 650.7 and 599.9 keV between bands (8) and (9) are also identified in this work.
As examples, figs. 2 and 3 show part of the coincident γ-ray spectra in 138 Pm. In fig. 2, a γ-ray spectrum is obtained by gating on 150.0 keV γ transition, from which one can see most of the γ transitions above the 150.0 keV level shown in fig. 1. Figure 3(a) is generated by summing gating on 698.8, 596.5 and 852.1 keV γ transitions. In this figure, the contamination transitions from other reaction channels have also been marked. From fig. 3(a), one can see the 598.5, 698.4, 852.1 and 1075.0 keV γ peaks in band (4), and the 544.9, 643.5 and 764.8 keV γ peaks in band (5). Figure 3(b) shows a spectrum obtained by gating on 1026.3 keV γ transition, in which one can see the 726.9 and 861.9 keV γ peaks in band (6), as well as the linking 554.7 keV γ peak between bands (1) and (6). In this figure, one can see clearly that the linking 554.7 keV γ-transition is much stronger than the transitions in band (6). In fig. 3(c), a spectrum is obtained by gating on 396.2 keV γ transition, from which one can see the transitions in bands (7) and (9), as well as the 687.5, 650.7 and 599.5 keV intraband transitions.
3 Discussion The configuration of the yrast band (1) built on the 8+ state in 138 Pm has been assigned as the πh11/2 [541]3/2− ⊗ νh11/2 [514]9/2− configuration. The 8+ state is an isomeric state with the half-life 21 ns [14]. The detailed characteristics for this band have been discussed in refs. [14,15]. The systematic signature inversion of yrast πh11/2 ⊗ νh11/2 bands in odd-odd nuclei in the A = 130–140 region is studied in ref. [4]. Figures 4(a) and (b) show the staggering functions of the level energy [E(I) − E(I − 1)]/2I vs. spin I for the πh11/2 ⊗ νh11/2 bands in the N = 77 isotones, 134 La [6], 136 Pr [4], 138 Pm (present work) and 140 Eu [18] and the Z = 61 isotopes, 134 Pm [1], 136 Pm [2], 138 Pm (present work) and 140 Pm [3], respectively. One can see from fig. 4 that the signature inversion indeed occurs in most nuclei in this region. However, no signature inversion will happen in 138 Pm based on the assignments of the yrast band in refs. [14, 15]. The total Routhian surface (TRS) calculations [19, 20] for different intrinsic configurations in 138 Pm at h ¯ω = 0.0 MeV have been performed, which are shown in fig. 5. The TRS calculations predict that all the three configurations exhibit the triaxial characteristics at the initial rotational frequency. The corresponding deformation parameters of β2 and γ are 0.18 and −93◦ for the (proton: π=−; neutron: π=−) configuration, 0.19 and 27◦ for the (proton: π = −; neutron: π = +) configuration, and 0.19 and −23◦ for the (proton: π = +; neutron: π = −) configuration. Triaxial quasiparticle-rotor model (PRM) calculations were carried out by Tajima in ref. [21] to demonstrate the effect of γ deformation and residual proton-neutron interaction on the signature splitting in odd-odd nuclei in the A = 130 region. While the calculations of most of the cesium isotopes reproduce the experimental trends and reveal the necessity of including the quasiparticle residual interaction, the consistence is still missing for N = 75 isotonic chain and calls for further correct spin-parity data
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Eur. Phys. J. A (2015) 51: 60
Table 1. The energies, relative intensities, DCO ratios and multipolarities of the γ transitions, and spin and parity (I π ) assignments of the levels in 138 Pm. The multipolarities with superscripts a and b in the last column are taken from refs. [14] and [15], respectively. Eγ (keV) 120.6 131.9 148.4 150.0 173.5 177.5 227.1 231.1 236.5 241.3 260.8 287.8 349.6 352.1 356.9 382.6 392.2 396.2 398.4 399.1 402.0 410.8 426.2 435.2 435.4 449.5 452.3 460.0 465.1 468.1 468.2 474.6 477.0 494.6 518.9 544.9 545.0 546.1 554.7 576.8 580.6 586.1 596.5 598.5 599.9 610.8 618.7 633.6 643.5 650.7
Intensity (%) 38.7(28) 6.3(12) 4.1(8) 125.3(30) 100 7.3(23) 17.5(16) 6.1(10) 11.5(15) 5.9(7) 87.7(40) 5.6(6) 12.7(9) 7.7(8) 25.1(8) 2.6(2) 8.7(8) 6.0(3) 1.4(2) 1.5(2) 14.6(14) 27.9(20) 4.9(5) 1.7(2) 7.1(5)
