Identification of excited states in 107,52Te55

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Identification of excited states in 107,52Te55 © Authors, 2021 Published version Zhang, W.; Cederwall, B.; Qi, C.; Ertoprak, A.; Aktas, Ö.; Liu, X.; Andgren, K.; Auranen, K.; Bäck, T.; Barber, L.; Beeton, G.; Cullen, D. M.; Darby, I. G.; Dimmock, M. R.; Eeckhaudt, S.; Ganioğlu, E.; Górska, M.; Grahn, T.; Greenlees, P. T.; Hadinia, B.; Ideguchi, E.; Illana, A.; Jones, P. M.; Joss, D. T.; Julin, R.; Juutinen, S.; Keatings, J. M.; Khaplanov, A.; Kulali, F.; Leino, M.; Luoma, M.; Lv, B.; Nara Singh, B. S.; Nelson, L.; Niikura, M.; Nyman, M.; Ojala, J.; Page, R. D.; Pakarinen, J.; Paul, E. S.; Petrache, C.; Petri, M.; Rahkila, P.; Ruotsalainen, P.; Sandzelius, M.; Sarén, J.; Scholey, C.; Smith, J. F.; Sorri, J.; Tann, H.; Zimba, G.; Uusitalo, J.; Wadsworth, R.; Wyss, R. Zhang, W., Cederwall, B., Qi, C., Ertoprak, A., Aktas, Ö., Liu, X., Andgren, K., Auranen, K., Bäck, T., Barber, L., Beeton, G., Cullen, D. M., Darby, I. G., Dimmock, M. R., Eeckhaudt, S., Ganioğlu, E., Górska, M., Grahn, T., Greenlees, P. T., . . . Wyss, R. (2021). Identification of excited states in 107,52Te55. Physical Review C, 104(6), Article 064305. https://doi.org/10.1103/PhysRevC.104.064305 2021 PHYSICAL REVIEW C 104, 064305 (2021) Identification of excited states in 107 52 Te55 W. Zhang ,1,*B. Cederwall,1C. Qi,1A. Ertoprak,1Ö. Aktas,1X. Liu,1K. Andgren,1K. Auranen,2T. Bäck,1L. Barber,3 G. Beeton,4D. M. Cullen,3I. G. Darby,5M. R. Dimmock,6S. Eeckhaudt,2E. Ganio˘ glu,7M. Górska,8T. Grahn,2 P. T. Greenlees,2B. Hadinia,1E. Ideguchi,9A. Illana,2P. M. Jones,2D. T. Joss,6R. Julin,2S. Juutinen,2J. M. Keatings,4 A. Khaplanov,1F. Kulali,10 M. Leino,2M. Luoma,2B. Lv,11 B. S. Nara Singh,4L. Nelson,6M. Niikura,12 M. Nyman,2 J. Ojala,2R. D. Page,6J. Pakarinen,2E. S. Paul,6C. Petrache,11 M. Petri,13 P. Rahkila,2P. Ruotsalainen,2M. Sandzelius,2 J. Sarén,2C. Scholey,2J. F. Smith,4J. Sorri,2H. Tann,2,6G. Zimba,2J. Uusitalo,2R. Wadsworth,13 and R. Wyss1 1KTH Royal Institute of Technology, 10691 Stockholm, Sweden 2Department of Physics, University of Jyväskylä, FIN-40014 Jyväskylä, Finland 3Department of Physics and Astronomy, Schuster Building, The University of Manchester, Manchester M13 9PL, United Kingdom 4School of Computing Engineering and Physical Sciences, University of the West of Scotland, Paisley PA1 2BE, United Kingdom 5SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom 6Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE, United Kingdom 7Department of Physics, Istanbul University, 34134 Istanbul, Turkey 8GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany 9Research Center for Nuclear Physics, Osaka University, JP-567-0047 Osaka, Japan 10Nuclear Technology and Radiation Safety Department, Üsküdar University, Istanbul, Turkey 11Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France 12Center for Nuclear Study, University of Tokyo, Wako, Saitama 351-0198, Japan 13Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom (Received 31 August 2021; accepted 22 November 2021; published 8 December 2021) Excited states in the extremely neutron-deficient nucleus 107Te have been identified from two separate experiments using the recoil-decay tagging technique. Two connected structures were observed on the basis of γγ-coincidence relations and tentatively assigned as built on the mixed-parentage νg7/2d5/2and νh11/2 intruder configurations. The observed structures were compared with large-scale shell-model calculations and total Routhian surface calculations. Collective behavior was discovered to persist in the νh11/2band of 107Te which highlights the shape-polarizing effect of a single valence neutron occupying the h11/2intruder orbit as the N=50 shell closure is approached. DOI: 10.1103/PhysRevC.104.064305 I. INTRODUCTION The structures of nuclei close to the presumed doubly magic nucleus 100Sn have been the subject of numerous experimental and theoretical studies during the past two decades [1]. The region of nuclei with a few particles outside the 100Sn core, in particular, provides a special “laboratory” for the observation of the competition between single-particle and collective degrees of freedom. In this region, a small change in the number of valence particles can introduce dramatic changes in the nuclear structure. *Corresponding author: [email protected] Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by Bibsam. The nearly equidistant energy spacings in the ground-state bands of even-mass Te isotopes and the νh11/2bands of odd-mass Te isotopes between N=56 and 70 [2,3] indicate that tellurium isotopes may be among the best examples of collective quadrupole vibrations in nuclei. However, the most neutron-deficient tellurium isotope for which excited states have been identified to date, 106Te, shows a sudden decrease of energy in the 6+→4+transition, which is explained to be a senioritylike ground-state band structure [2]. This is quite different from the vibrational-like patterns observed in the low-lying excited states of the heavier tellurium isotopes [4–6]. In the neighboring xenon isotopes [7,8], an enhanced collectivity was seen and interpreted as partially an effect of the enhanced neutron-proton interactions [9,10]. The underlying mechanism behind the abrupt change of structure between 106Te [2] and 108Te [5] is not easily explained by theoretical models to date. Experimental information on the structure of 107Te may shed light on this effect and elucidate the mechanisms behind the development of quadrupolecollective structures close to the N=Z=50 shell closures. 2469-9985/2021/104(6)/064305(8) 064305-1 Published by the American Physical Society W. ZHANG et al. PHYSICAL REVIEW C 104, 064305 (2021) In addition to the quadrupole-collective degrees of freedom, octupole correlations have been predicted to maximize in 112Ba and could also play an important role in neighboring Xe and Te nuclei [11], due to l=j=3 residual interactions between 1d5/2and 0h11/2subshells. Experimental evidence for octupole collectivity has been identified from the intermediate-spin states of the xenon isotopes 112Xe [12] and 114Xe [13] and the tellurium isotopes 108Te [5], 109Te [14], and 110Te [15]. Its limit of existence in this region still remains an open question to be explored experimentally. Additional interest in the nucleus 107Te stems from astrophysical considerations since it has been predicted to constitute the endpoint of the rapid-proton-capture process leading to a closed Sn-SbTe cycle [16,17]. It is still extremely challenging to directly measure the 106Sb(p,γ) reaction rate, and experimental information on the level structure of the 107Te above the proton threshold might contribute to constraining this rate [17]. In this paper, we report a new spectroscopic study of 107Te, which significantly extends the limited information on the excited states of 107Te reported previously [18]. II. EXPERIMENTAL DETAILS Excited states in 107Te were populated in two separate experiments performed at the Accelerator Laboratory of the University of Jyväskylä, Finland. Both experiments employed fusion-evaporation reactions and the recoil-decay tagging (RDT) technique [19,20] in order to select events associated with the population of excited states in the rare 107Te products. The first experiment was primarily aimed at identifying excited states of 110Xe [7]. This experiment used a 54Fe beam at 195 MeV to bombard an isotopically enriched (99.8%) 58Ni target consisting of a 1.0 mg/cm2self-supporting foil. The average beam intensity was 5 pnA during 5 days of irradiation time. Prompt γrays were detected at the target position by the Jurogam γ-ray spectrometer consisting of 43 Eurogam Phase I [21] and GASP [22] type Compton-suppressed highpurity germanium detectors. The recoiling fusion evaporation products were separated in-flight from the beam particles by the gas-filled recoil separator RITU [23,24] and implanted into two double-sided silicon strip detectors (DSSDs) of the GREAT spectrometer [25]. Each DSSD has an effective area of 60 ×40 mm2and a strip pitch of 1 mm in both directions, thus yielding 4800 independent pixels in total. The second experiment was carried out using the inverse reaction 54Fe(58Ni, 2p3n)107Te. The 58Ni ions were accelerated to an energy of 215 MeV and then used to bombard the target foils consisting of 99.9% isotopically enriched 54Fe with an areal density of 0.77 mg/cm2. The beam intensity varied between 3 and 4 pnA during 12 days of irradiation time. The upgraded Jurogam III γ-ray spectrometer [26], consisting of 15 Eurogam Phase I-type [21] and 24 Euroball clover [27] escape-suppressed detectors, was used to detect prompt γrays at the target position. The fusion-evaporation products were separated by the new vacuum-mode recoil separator MARA (mass analyzing recoil apparatus) [28,29], which is a complementary device to RITU and can separate the reaction products from the primary beam in symmetric and inverse kinematics more effectively [29]. At the focal plane of 0 5×106 1×107 0 60 120 0 5×106 1×107 0 200 400 600 800 1000 1200 1400 0 60 120 E γ (keV) Counts / keV 1324 1131 1023 980.2 960.3 895.4 888 935 791.8 805.1 750.4 722.5 729.8 669.3 677.2 689/ 695.3 58Ni (54Fe) 632 573.4 90.1 468.7 437.8 506.6 598/ 608.5 (b) (c) MARA 54Fe (58Ni) (d) (a) RITU FIG. 1. (a) Recoil-correlated γ-ray energy spectrum from the RITU experiment, dominated by transitions in the strongest threeproton evaporation reaction channel, 109Sb [35]. (b) As in panel (a), with additional selection on the characteristic α-decay energy of 107Te. Contamination lines arising from 109Sb are indicated by filled squares. (c) Recoil-correlated γ-ray energy spectrum from the MARA experiment. (d) RDT γ-ray spectrum for 107Te. The energy calibration of the germanium detectors was performed offline using in-beam γrays of 109Sb for both experiments, indicated by solid circles. The black (and red) dashed lines are drawn to guide the eye and illustrate the energy consistency in the calibration (and γrays for 107Te) from the two experiments. MARA, the fusion residues were passed through a multiwire proportional counter and were then implanted into the DSSD of model BB20, which has 72 strips on the yplane and 192 strips on the xplane, each with a width of 0.67 mm [29]. In both experiments, the signals from all detectors were time-stamped using a 100-MHz clock and recorded independently by the triggerless total data readout acquisition system [30]. The data were sorted online and offline using the GRAIN software package [31]. The αdecay of 107Te has been reported with Eα=3.862(10) MeV [32], bα=70(30)% [33], and T1/2=3.6(2) ms [34], which is short enough to allow clean selections with the highly selective RDT technique [19,20]. The search time between a recoil implant and its αdecay was limited to 11 ms, and in addition, a gate on the 3.862-MeV α energy was set to select prompt γrays associated with 107Te. III. RESULTS Energy spectra for the prompt γrays recorded at the target position in delayed coincidence with detected recoils in the DSSDs from both experiments are shown in Figs. 1(a) and 1(c), respectively. Figures 1(b) and 1(d) show the RDT γ spectra for weakly populated (σ≈1μb) 107Te from both 064305-2 IDENTIFICATION OF EXCITED STATES IN … PHYSICAL REVIEW C 104, 064305 (2021) 0 10 20 0 15 30 0 20 0 15 0 15 0 20 200 400 600 800 1000 1200 1400 0 20 40 980 750 723 669/677 689/695 90.1 632 573 1324 Eγ (keV) Counts / 2 keV 980 1324 723 750 1324 632 1324 573 960 90.1 1324 980 960 772 792 750 689/695 723 669 632 573 90.1 90.1 573 669 689/695 723 750 980 960 598/ 609 469 438 895 90.1 (a) Gate: 632 keV (e) Gate: 723 keV (c) Gate: 573 keV (d) Gate: 960 keV 723 750 960 792 (f ) Gate: 677 keV 90.1 573 573 895 960 960 598/609 689/695 632 632 980 669 (b) Gate: 669 keV 669 689 723 750 792 980 895 669/677 689/ 695 (g) Gates: 980+750+669 keV 895 895 FIG. 2. Coincidence γ-rayspectraobtainedbygatingonthe (a) 632-keV, (b) 669-keV, (c) 573-keV, (d) 960-keV, (e) 723-keV, and (f) 677-keV transitions. Panel (g) shows a sum of gates on some transitions in the γγ matrix, specifically the 980-, 750-, or 669-keV transitions. experiments, illustrating the selective power of the RDT technique. It is worth noting that the contamination from stronger reaction channels has been more efficiently suppressed in Fig. 1(d) from the MARA separator compared with Fig. 1(b) from the RITU separator. However, in the offline analysis for the MARA experiment, in order to maintain statistics, the mass spectrum (mass/charge ratio, A/q) was not used for the selection and subsequent analysis. In order to construct a level scheme for 107Te, a recoildecay tagged γγ-coincidence matrix was produced using the combined data from both experiments. Examples of gated coincidence spectra are presented in Fig. 2. Based on coincidence relationships as well as intensity arguments, the deduced level scheme of 107Te is shown in Fig. 3. The tentative spin and parity assignments for the shown levels rely on systematic trends in neighboring odd-mass Te isotopes. The measured energies and relative intensities of the γrays are listed in Table I. Due to the limited statistics and the presence of energy doublets, only about half of the observed γrays listed in Table Icould be placed in the level scheme. The low statistics also precluded firm multipolarity assignment by means of angular distribution measurements. The ground state of 107Te has previously been assigned to have spin-parity (5/2+) as being built on a νd5/2configFIG. 3. A tentative level scheme deduced for 107Te from this work. The transition energies are given in keV. uration, and the 90-keV transition connects the first excited (7/2+) state with the ground state [18]. This continues the decreasing trend of the first excited states in the odd-mass tellurium isotopes from 117 keV in 111Te [36]to98keVin 109Te [14]. Hadinia et al. [18] also tentatively proposed that the 632-keV transition deexcites a (9/2+) state to the first excited (7/2+) state and that the 723-keV transition deexcites the same (9/2+) state to the ground state. The coincidence relationship between the 632and 90-keV transitions is confirmed in the present work, as shown in the spectrum in Fig. 2(a). However, the previously suggested placement of the 723-keV transition could not be confirmed. Spectra in Figs. 2(b)–2(f) show the γrays that are in coincidence with the 669-, 573-, 960-, 723-, and 677-keV γrays, respectively. Figure 2(g) shows a coincidence spectrum gated on the 980-, 669-, or 750-keV transitions in the RDT-gated γγ matrix. Strong selfcoincident doublets occur for both the 573and 669-keV γ 064305-3 W. ZHANG et al. PHYSICAL REVIEW C 104, 064305 (2021) TABLE I. Measured energies (Eγ) and relative intensities (Irel, without consideration of internal conversion) for γ-ray transitions in 107Te observed in this work. The initial and final spins and parities (Jπi iand Jπf f), where given in the third column, are tentatively assigned from systematics. Eγ(keV) Irel Jπ i→Jπ f 90.1(1)a100(8) (7/2+)→(5/2+) 437.8(8) 17(5) 468.7(4)*34(5) 506.6(8) 16(6) 573.4(3)a,*119(8) (15/2−)→(11/2−) 598.0(7) 24(5) 608.5(4) 31(5) 632.0(3)a76(6) (9/2+)→(7/2+) 669.3(3)a,*124(8) (19/2−)→(15/2−) (17/2+)→(13/2+) 677.2(3)a96(8) (11/2+)→(7/2+) 689.0(3)a72(7) (13/2+)→(9/2+) 695.3(4)a,*80(8) (15/2−)→(17/2+) 722.5(5)a,*98(3) (23/2−)→(19/2−) (15/2+)→(11/2+) 729.8(8) 24(5) 750.4(6) 47(6) (31/2−)→(27/2−) 772(2) 15(10) 778(2) 18(11) 791.8(5) 37(5) (11/2−)→(13/2+) 805.1(9) 16(5) 888(1) 19(5) 895.4(7) 32(6) (29/2+)→(27/2−) 935(1) 11(4) 960.3(4) 45(5) (35/2−)→(31/2−) 980.2(4) 74(6) (27/2−)→(23/2−) 1023.1(9) 16(4) 1131(1) 17(5) 1324(1) 17(4) (39/2−)→(35/2−) aTransitions also observed in Ref. [18]. *The stars denote doublet transitions. rays, which cannot be resolved experimentally. With reference to the coincidence relationships shown in Fig. 2,asetof mutually coincident transitions with energies 632, 689, 573, 669, 723, 980, 750, and 960 keV is observed. Based on the fact that the 689-keV γ-ray transition is strongly correlated with the 632-keV transition but relatively more weakly correlated with the 573and 960keV transitions, the 689-keV line has been assigned as depopulating the (13/2+) state into the (9/2+) state. In the nearby heavier odd-ATe isotopes, the level schemes are dominated by a yrast sequence which is built on the νh11/2 single-particle configuration. The systematic behavior in the yrast sequences shown in Fig. 4agrees with the placement of the 573-keV transition as depopulating the (15/2−) state to the (11/2−) state and the 669-keV transition deexciting the (19/2−) state to the (15/2−) state. The ordering of the other four transitions built on the νh11/2yrast band in Fig. 3is based on their relative γ-ray intensities and is less certain due to the presence of doublet transitions. Two other γrays, i.e., the 895and 1324-keV transitions shown in Fig. 2(g), were found to be FIG. 4. Excitation energies of states in the yrast νh11/2bands in the odd-Atellurium isotopes, relative to the respective 11/2−bandheads. The data for heavier Te isotopes are taken from Refs. [3,14,36] and the data for 107Te are derived from the present work. The dashed lines are drawn to show the energy evolution as the number of neutrons changes. weakly coincident with the lower-spin members of the h11/2 band. Their placements are tentatively assigned and indicated by the dashed lines in Fig. 3. As the neutron number decreases towards the N=50 shell gap, the νh11/2state increases to a higher energy relative to the ground state, from an excitation energy of 280 keV in 115Te [37] to 839 keV in 111Te [36] and then to 1089 keV in 109Te [14]. In 109,111Te, this νh11/2state decays through the 9/2+and 7/2+states to the 5/2+ground state. In particular, in the case of 109Te, several decay paths have been observed [14]. Such decay paths in 107Te could not be firmly determined in the present work. Linking transitions between the negative-parity structure and the low-lying states are tentatively assigned and indicated by the dashed lines in Fig. 3. The 677-keV transition has a relatively high intensity but is only observed in strong coincidence with the 723and 90-keV transitions, as shown in Fig. 2(f). This 723-keV transition is not the same transition as that one in the νh11/2yrast band, indicating that the 723-keV transition is also a doublet. This could be confirmed from the spectrum in Fig. 2(d) where the 960-keV transition is in coincidence with the 723-keV transition much more strongly than with the 677-keV transition. Considering the systematic trends of the positive-parity states in 109,111Te [14,36], the 677and 723-keV transitions are tentatively assigned to a positive-parity structure in 107Te. IV. DISCUSSION The systematics of the yrast νh11/2bands of the neutrondeficient odd-Atellurium isotopes with N<68 are shown in Fig. 4, in which the excitation energies of states are given relative to the respective 11/2−bandheads. It can be seen that the excitation energies of states within the νh11/2bands vary smoothly as a function of the neutron number N.Asthe 064305-4 IDENTIFICATION OF EXCITED STATES IN … PHYSICAL REVIEW C 104, 064305 (2021) 50 55 60 65 70 2 3 4 + 2 / E+ 6 E ) - 11/2 - E - 15/2 ) / ( E - 11/2 - E - 23/2 (E + 2 / E + 4 E ) - 11/2 - E - 15/2 ) / ( E - 11/2 - E - 19/2 (E )+ 7/2 - E + 11/2 ) / ( E+ 7/2 - E + 15/2 (E Neutron Number Energy ratio FIG. 5. Energy ratios plotted versus Nfor evenand odd-A52Te isotopes in the mass region A⩽119. The data are taken from the present work and Refs. [2,3,5,14,15,18,36]. neutron number is reduced from around the midshell at N= 65, the energy of the 15/2−→11/2−transition decreases until N=57 (109Te), where it reaches a minimum, and then its expected increasing trend is eventually restored at 107Te. This tendency follows the behavior analogous to that in the more extensively studied even-Atellurium isotopes [2], of which the increasing trend of the E(2+) energy is restored at 106Te. To elucidate the structural evolution in the tellurium isotopic chain, excitation energy ratios, E4+ 1/E2+ 1and E6+ 1/E2+ 1for evenATe isotopes, and (E19/2−−E11/2−)/(E15/2−−E11/2−) and (E23/2−−E11/2−)/(E15/2−−E11/2−) energy difference ratios for odd-ATe isotopes with N<68, are shown in Fig. 5. These energy ratios are valuable indicators of nuclear deformation and collectivity, and compared with the predictions for a harmonic vibrator, E4+ 1/E2+ 1=2 and E6+ 1/E2+ 1=3 as indicated by dashed lines. Even-ATe isotopes are usually considered to be vibrational at low spin inferred from the energy ratios E4+ 1/E2+ 1≈2. The energy ratios of odd-mass Te isotopes appear to follow the same trends as the even-mass Te isotopes. As the neutron number decreases from the midshell (N=66) towards the N=50 shell closure, these ratios for both even-Aand odd-A Te isotopes show a noticeable deviation from the harmonic limits, in contrast to the smoothly decreasing behavior of even-mass tellurium isotopes when approaching N=82 [2]. This distinctive deviation is firstly reminiscent of enhanced collectivity. However, from lifetime measurements of the first 2+excited states in the neighboring even-ATe isotopes (108Te [38], 110Te [39], 112Te [40]), it is found that the measured B(E2; 2+ gs →0+) values show an apparent decrease as N decreases, indicating that these isotopes do not show any enhanced transition probabilities. In the most neutron-deficient tellurium isotopes, neutrons and protons are predicted to occupy the same sets of orbitals, i.e., the near-degenerate 1d5/2 and 0g7/2subshells, and the 0h11/2intruder state at relatively higher excitation energy, and thus enhanced neutron-proton correlations are expected to come into play when approaching the N=Zline. This uncoordinated relationship between excitation energies and B(E2) values can be associated with the effect of enhanced neutron-proton pairing. This intriguing phenomenon has been investigated in several theoretical calculations using the quasiparticle random-phase approximation model [10] and the shell model [41,42], revealing that excitation energies are more sensitive to pairing than the B(E2) values [41]. Figure 5also shows the energy ratios (E15/2+− E7/2+)/(E11/2+−E7/2+) in the positive-parity bands of 107Te, 109Te, and 111Te. These ratios are well interpolated into the ratio curve of even-ATe isotopes. However, the energy ratios of the νh11/2bands in odd-ATe isotopes are generally higher than those of the ground-state bands in neighboring even-ATe isotopes, implying that the negative-parity νh11/2bands have an appreciably stronger collectivity and larger deformations. In addition, the measured B(E2; 15/2−→11/2−) value in the νh11/2band of 109Te is larger than the B(E2; 0+ gs →2+) value in the ground band of 108Te by almost a factor of 2.5 [43], and the measured B(E2) value in the positive-parity band of 109Te is approximately equal to the B(E2; 0+ gs →2+) value of 108Te [43]. Such a significant enhancement of collectivity in the negative-parity bands was attributed to a deformation driving force by the polarization of the valence h11/2neutron [36,43,44]. This polarization seems to persist in the νh11/2 band of 107Te, but the presence of such a relatively strong collectivity is unexpected as the N=50 shell closure is approached. This unusual effect has been previously suggested to arise from increased octupole correlations [12]. However, it is excluded as a possible cause here since the onset of octupole correlations were observed at the intermediate and high spins of 7−of 108Te [5] and 33/2+of 109Te [14], respectively. An alternative explanation may be related to the enhanced neutron-proton correlations taking place in the presence of neutrons and protons occupying near-identical orbits. This might suggest that in 107Te the polarization of the odd h11/2neutron will enlarge the overlap of the neutron-proton wave functions, leading to an increase in the neutron-proton quadrupole-quadrupole interaction strength. One of the consequences is that a single-particle effect like the compression of the 6+→4+transition energy [2]in106Te does not appear to be present in the νh11/2band of 107Te (see the most striking discrepancy of energy ratios between oddand even-ATe isotopes in Fig. 5). To gain further insight into the structures of 107Te, largescale shell-model (LSSM) calculations have been performed using a realistic CD-Bonn potential [45], renormalized using the perturbative G-matrix approach [46]. The model space included single-particle orbitals 0g7/2,1d5/2,1d3/2,2s1/2, and 0h11/2between the N,Z=50 to 82 shell closures (see details in Refs. [42,47]). Calculated positive-parity levels are shown 064305-5 W. ZHANG et al. PHYSICAL REVIEW C 104, 064305 (2021) FIG. 6. Experimental positive-parity levels in 107Te derived from this work in comparison with LSSM calculations. See text for details. in Fig. 6in comparison with the experimental results. The calculations are in good agreement with the observed excitation energies. In Fig. 7, the experimental energy levels of the yrast νh11/2band of 107Te deduced from the present work are shown in comparison with the ground-state band level energies for the neighboring even-even 106Te [2] and 108Te [5] isotopes and the results from the LSSM calculations. The νh11/2band of 107Te reflects a structure more similar to that of the ground-state band of 108Te rather than that of 106Te for which the compression of the 6+→4+transition energy signals an emerging dominance of single-particle structure. Incidentally, this transition to a less collective ground-state band in 106Te is not well reproduced by the shell-model calculation. In the νh11/2band of 107Te, it appears that enhanced neutron-proton correlations might suppress the emergence of seniority coupling effectively. Lifetime measurements in the νh11/2band of 107Te would be of benefit to examine this effect more conclusively. To investigate theoretical predictions for the shape-driving effects of the odd h11/2neutron, total Routhian surface (TRS) calculations [48,49] were carried out for 106,107,108,109Te in this FIG. 7. Excitation energies of states in the yrast νh11/2band of 107Te and the ground-state bands of the neighboring even-even 106Te [2]and108Te [5] isotopes in comparison with the shell-model calculations. See text for details. (a) (b) (c) (d) (e) (f) FIG. 8. Calculated TRSs for ground-state bands of 106,108Te and for positive-parity and negative-parity bands of 107,109Te at a rotational frequency of ¯hω=0.2 MeV. The energy difference between contour lines is 200 keV. The minimum points are indicated by the red dots. Calculated deformation parameters (β2,γ) are (a) (0.12, −1◦)for106Te; (b) (0.13, −5◦)for(+,+1/2) and (c) (0.15, 10◦)for (−,−1/2) of 107Te; (d) (0.14, −2◦)for108Te; and (e) (0.14, −5◦)for (+,+1/2) and (f) (0.16, 9◦)for(−,−1/2) of 109Te. work. As shown in Fig. 8, TRS calculations demonstrate that a neutron occupying the negative-parity h11/2orbital can indeed polarize the quadrupole deformation to a more pronounced value than occupying the positive-parity orbital for both 107Te and 109Te (from β2=0.14 to β2=0.16 for 109Te and from β2=0.13 to β2=0.15 for 107Te). There is also a visible effect of the triaxial-shape driving force of the h11/2valence neutron in the potential energy surfaces for negative parity, negative signature in Fig. 8. Moreover, the difference in the β2value between the positive-parity configuration of 107Te and the ground state of 106Te is very small, and the same is also true for 109Te and 108Te. It is therefore concluded that the odd neutron occupying the positive-parity orbital has little effect on either the vibrational collectivity or the nuclear deformation. In addition, it would be of considerable interest to extend the positive-parity bands in 107Te to identify whether the expected seniority coupling will come into play at a higher spin and to build the connections between the negative-parity and positive-parity bands. V. CONCLUSIONS In summary, excited states in the very neutron-deficient nuclide 107Te were observed for the first time in the present work. Two distinct structures tentatively assigned to the νh11/2 and νg7/2quasiparticle states were identified. The structures have been discussed within the context of LSSM and total Routhian surface calculations. It is suggested that a neutron occupying the h11/2orbit has a more significant effect on the collectivity than in the positive-parity d5/2and g7/2orbitals. It is concluded that enhanced neutron-proton correlations might 064305-6 IDENTIFICATION OF EXCITED STATES IN … PHYSICAL REVIEW C 104, 064305 (2021) play a defining role in the balance between single-particle and collective structures in the Te isotopes when approaching N=50. ACKNOWLEDGMENTS This work was supported by the Swedish Research Council under Grant No. 2019-04880; the United Kingdom Science and Technology Facilities Council (STFC) under Grants No. ST/L005727/1 and No. ST/P003885/1; the EU 7th Framework Programme, Integrating Activities Transnational Access, Project No. 262010 ENSAR; and the Academy of Finland under the Finnish Centre of Excellence Programme (Nuclear and Accelerator Based Physics Programme at JYFL). W.Z. acknowledges support from the China Scholarship Council under Grant No. 201700260239. The authors acknowledge the support of the GAMMAPOOL for the JUROGAM detectors. [1] T. Faestermann, M. Górska, and H. Grawe, Prog. Part. Nucl. Phys. 69, 85 (2013). [2] B. 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