High spin structures in 194 Hg

Z. Phys. A 354, 169–175 (1996) ZEITSCHRIFT FÜR PHYSIK A c Springer-Verlag 1996 High spin structures in 194 Hg N. Fotiades1 , S. Harissopulos1 , C.A. Kalfas1 , S. Kossionides1 , C.T. Papadopoulos2 , R. Vlastou2 , M. Serris2 , J.F. Sharpey-Schafer3 , M.J. Joyce3 , C.W. Beausang3 , P.J. Dagnall3 , P.D. Forsyth3 , S.J. Gale3 , P.M. Jones3 , E.S. Paul3 , P.J. Twin3 , J. Simpson4 , D.M. Cullen5? , P. Fallon6 , M.A. Riley7 , R.M. Clark8 , K. Hauschild8 , R. Wadsworth8 1 Institute of Nuclear Physics, NCSR Demokritos, GR-15310 Athens, Greece Technical University of Athens, GR-15773 Athens, Greece 3 Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 3BX, U.K. 4 CCL, Daresbury Laboratory, Warrington WA4 4AD, U.K. 5 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 6 Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA 7 Florida State University, Tallahassee, Florida, FL 32306, USA 8 Department of Physics, University of York, York YO1 5DD, U.K. 2 National Received: 24 July 1995 / Revised version: 11 October 1995 Communicated by B. Herskind Abstract. High spin states in the isotope 194 Hg were populated using the 150 Nd (48 Ca,4n) reaction at a beam energy of 213 MeV. The analysis of γ-γ coincidences has revealed two new structures at excitation energies above 6 MeV and at moderate spin. The two structures are a manifestation of the deviation of nucleus from the collective rotation which dominates its lower excitation behaviour. A comparison with similar structures in the neighbouring Hg isotopes is also attempted. PACS: 27.80.+w;21.10.Hw;21.60.Ev 1. Introduction The mercury nuclei are known to exhibit small oblate deformations in low excitation energies resulting from a collectively rotating core and a number of neutron excitations out of it [1]. At higher excitation energies proton excitations become possible and departures from the original oblate shape are theoretically expected [2]. Theoretical calculations for 194 Hg [3] predict prolate non-collective, triaxial weakly collective and superdeformed shapes. Proton excitations have been already reported in the neighbouring Hg isotopes [4-7] involving the h9/2 , h11/2 and i13/2 proton orbitals which are located near the proton Fermi-level. In the case of 191 Hg [4] these excitations were connected to an irregular structure of γ-rays observed in this isotope. Similar irregular structures are also reported in 193 Hg [6]. In 192 Hg [5], 193 Hg [6] and 196 Hg [6] the proton excitations are invoked for the interpretation of bands of dipole transitions similar to those reported recently in several ? Present address: NSRL, University of Rochester, Rochester, NY 14627, USA neighbouring Pb [8-19] nuclei and in 202 Bi [20] as well as in nuclei of the A = 130 atomic mass region [21-23]. In most of the cases the excitation energy of these bands remains unknown because their levels have not been connected to the lower levels. These bands are characterized by large B(M1)/B(E2) ratios which are quenched in the Hg isotopes in comparison with the ratios in Pb isotopes. The high-K two-proton excitations π(h9/2 i13/2 )K=11 and π(h29/2 )K=8 have been successfully used to account for the large B(M1)/B(E2) values, while the quenching observed in Hg isotopes is accounted for by further inclusion of two proton holes from the h11/2 orbital. In 194 Hg the low-lying level scheme is well established with the presence of several rotational bands [1]. Traces of an irregular sequence has also been reported [1]. In the present paper we report experimental evidence in 194 Hg for two new structures of γ-rays. The first is the continuation at higher energies of the previously reported irregular sequence [1] while the second is observed for the first time. In both structures their lower part exhibits a complicated level pattern in contrast to their upper part where the level pattern is reminiscent of the sequence of γ-rays inside a dipole band. Experimentally deduced B(M1)/B(E2) ratios for the levels in the upper part of the structures are compared to theoretical calculations for various possible configurations. The structures are also compared to the similar cases in the neighbouring isotopes. 2. Experiment The reaction 150 Nd (48 Ca,4n) 194 Hg was used to populate high spin states of 194 Hg at beam energy E(48 Ca) = 213 MeV. The beam was provided by the 20 MV tandem Van de Graaff accelerator at the Nuclear Structure Facility at Daresbury, U.K.. The 150 Nd target was 2 mg/cm2 thick. 170 Fig. 1. Partial level scheme obtained in the present work showing the two new structures and the previously known bands [1] towards which these structures deexcite. The γ-ray transitions are labeled by their energy in units of keV. The width of the arrows represent the intensity of the transitions relative to the 427.9 keV (2+ 7→ 0+ ) transition [1]. The uncertainty on the γ-ray energies varies from 0.2 keV for the strong transitions to 0.5 keV for the weakest ones. Spins and parities in brackets are not firmly established In order to stop the recoiling nuclei the target was evaporated on a 7 mg/cm2 thick gold foil. Gamma-rays were detected using the EUROGAM detector array which, in our measurement, consisted of 35 large volume n-type hyperpure Ge detectors, each one surrounded by a BGO escape suppression shield [24]. Approximately 4 × 109 coincident events, of unsuppressed fold five or higher, were collected and stored on magnetic tapes for subsequent off-line analysis. Two two-dimensional 4096×4096 channel matrices were produced. The first matrix was used to investigate the coincidence relationships of the γ-rays while the second one was especially built to establish the directional correlations of the γ-rays I(90◦ ,158◦ )/I(158◦ ,90◦ ) (DCO-ratios). From the analysis of the former matrix the level scheme, reported previously by Hübel et al. [1], has been confirmed. This work has extended the level scheme with the addition of two new structures. These are the subject of this communication. 3. Results The two new structures are shown in the partial level scheme of Fig. 1 labelled as “1” and “2” and extend up to 11013 keV and 9933 keV excitation energy, respectively. They deexcite towards previously known rotational bands of the lower level scheme (bands AB, ABCD, ABCE and T [1]). Excitation energies, intensities and DCO values for the transitions of the two structures as well as for some transitions of band T and its deexcitation are given in Table 1. Structure “1” is the direct continuation of the previously known band T [1]. The spins in this band, which were uncertain in the previous work, were established in our experiment. The parity of the band still remains uncertain. Tentative is also the assignment of the parity as well as the spin of the higher levels of structure “1”. Figure 2 being a 171 Table 1. Excitations, energies, intensities, DCO-ratios and assignment for all new transitions in Ei (keV) Excitationa Ef (keV) Energyb (keV) Intensityc (◦ /◦◦ ) DCO Assignment ( Iiπ 7→ Ifπ ) 0.93(10) 0.82(10) 21(−) 7→ 19(−) 23(−) 7→ 21(−) 23(−) 7→ 22+ 19(−) 7→ 17(−) 21(−) 7→ 20+ 17(−) 7→ 15(−) 19(−) 7→ 18+ 17(−) 7→ 16+ (15− ) 7→ 14+ Band T 5611.0 6120.8 6120.8 5104.2 5611.0 4492.1 5104.2 4492.1 4005.4 5104.2 5611.0 5578.6 4521.7 4986.0 3820.6 4275.5 3531.8 2888.6 506.7 509.9 541.9 582.5 625.1 671.5 828.6 960.1 1116.6 100(4) 108(4) 31(3) 49(2) 19(1) 26(1) 10(1) 6(1) 15(1) Structure 1 9882.4 9882.4 10226.0 10604.0 11013.0 6816.3 10604.0 7556.1 7588.9 11013.0 8562.2 9565.3 8562.2 9592.2 9592.2 9565.3 9882.4 10226.0 10604.0 6120.8 9882.4 6816.3 6816.3 10226.0 7588.9 8562.2 7556.1 8562.2 290.4 317.0 343.6 377.9 409.1 695.5 721.7 739.8 772.6 786.8 973.6 1003.2 1006.0 1029.7 16(1) 32(1) 32(2) 22(1) 11(1) 84(1) 12(1) 65(5) 29(2) 20(1) 26(2) 37(3) 53(4) 18(1) Structure 2 7582.1 6790.6 8287.3 7941.3 8664.8 9068.3 6455.2 6032.8 7262.6 6032.8 6032.8 7941.3 8664.8 6790.6 6777.2 6032.8 9068.3 7582.1 7582.1 7262.6 9500.4 6032.8 5493.5 6013.6 6032.8 5392.0 7262.6 6455.2 7941.3 7582.1 8287.2 8664.8 6013.6 5578.6 6790.6 5493.5 5392.0 7262.6 7941.3 6032.8 6013.6 5266.4 8287.2 6790.6 6777.2 6455.2 8664.8 5163.7 4497.7 4986.0 4986.0 4275.5 319.1 335.3 345.4 359.8 377.6 403.5 441.5 454.7 472.3 538.9 640.8 678.7 723.0 757.8 763.6 766.4 781.0 791.5 804.8 807.4 835.7 869.1 995.8 1027.6 1047.0 1116.5 34(1) 12(1) 122(4) 115(3) 115(3) 45(3) 12(1) 39(1) 27(1) 24(1) 35(1) 55(2) 49(2) 93(4) 11(1) <5 28(1) 103(4) 13(1) 7(1) 22(2) 74(3) 49(2) 30(1) 24(1) 20(1) a The 194 Hg 1.07(10) 0.45(7) 1.30(30) 0.56(8) 0.51(10) 0.51(4)† 0.95(10) 0.80(10) 1.27(40) 0.59(10)∗ (32− ) (32− ) (33− ) (34− ) 7→ (31− ) 7 (31− ) → 7 (32− ) → 7 (33− ) → 7→ (34− ) 25(−) 7→ 23(−) (34− ) 7→ (32− ) 27(−) 7→ 25(−) 27(−) 7→ 25(−) 7→ (33− ) 29(−) 7→ 27(−) (31− ) 7→ 29(−) 29(−) 7→ 27(−) (31− ) 7→ 29(−) 0.53(9) 0.31(9) 0.51(4)† 22+ 7→ 22+ 22+ 7→ 20+ 0.48(7) 22+ 7→ 21− 7→ 19− 7→ 20+ 22+ 7→ 20+ 7→ 18+ uncertainty on the excitation varies from 0.2 keV to 0.8 keV uncertainty on the γ-ray energies varies from 0.2 keV to 0.4 keV for the strong transitions and from 0.8 keV to 1.0 keV for the weakest ones c Intensities derived from coincidence gates ∗ DCO values marked with an asterisk were obtained by gating on a ∆I = 1 transition † DCO values for energetically close γ-rays which were impossible to separate b The 172 Fig. 2. Gamma-ray spectrum in coincidence with the 695.5 keV (25(−) 7→ 23(−) ) transition illustrating the quality of the data. The new transitions belonging to structure “1” can be seen and are marked with “1”. All transitions belonging to band T are marked with “T” and finally the label “t” is used for all connections of band T to bands AB and ABCD. Finally, some lines from the normal level scheme belonging mainly to bands AB, AE and ABCD are explicity labeled by their energy. All transitions are quoted in keV units spectrum gated on the 695.5 keV γ-ray shows all transitions of structure “1”. The level pattern in the lower part of structure “1” is rather unusual. A common level at 6816 keV is fed by two branches which deexcite one single level at 8562 keV. The sum of the intensities of the two branches (∼9%) is almost equal to the intensity of the 695.5 keV transition (∼8%) indicating that there is no significant connection from 6816 keV level towards another band. The intensities of all the transitions throughout this communication are normalized to the intensity of the 427.9 keV transition (2+ 7→ 0+ [1]). The 8562 keV level is fed by two decay paths. Above this level the spin assignment is still uncertain. The two decay paths join up to a single level at 9882 keV. The DCO value for the 317.0, 343.6 and 377.9 keV γ-rays suggest that these are dipole transitions. Moreover, these dipole transitions are stronger than the crossover 721.7 and 786.8 keV transitions. Thus, the pattern above level 9882 keV is similar to the ∆I = 1 bands in the neighbouring Hg and Pb nuclei. The bulk of intensity of structure “2” is gathered at the 6033 keV 22+ state. The decay out of this level is fragmented into several branches with summed intensity of ∼25% feeding bands AB, ABCD and ABCE. The intensity of 757.8 keV transition is ∼10%, hence there must be more transitions feeding the 6033 keV level which still remain unobserved. There is also a minor (intensity ∼3%) deexcitation branch feeding the bandhead of band ABCD and going through the 6014 keV level. Above 6014 and 6033 keV levels the level pattern appears complicated but in the upper part of the structure the pattern is similar to that of a ∆I = 1 band. This is also supported by the DCO values measured for the 345.4 and 377.6 keV transitions indicating that they are dipole. Figure 3 shows all transitions of structure “2”. Finally, there is some evidence of a direct connection between the two new structures (for example, the presence of the 343.6 keV transition in the gate of 345.4 keV transition – see Fig. 3) but this should be very weak and remains still unclarified. Overviewing the level pattern in Fig. 1 we can clearly identify three different regions: a) a lower region governed by bands of E2 transitions and extending up to roughly 6.0 MeV b) an intermediate region of complicated level pattern extending up to roughly 9.9 and 8.0 MeV for structures “1” and “2” respectively and c) an upper region more regular than the intermediate one governed by sequences similar to the ∆I = 1 bands recently observed in the neighbouring Hg and Pb nuclei and extending up to roughly 11.0 MeV. The limits of these regions can not be strictly defined since there is a certain overlapping between them. 4. Discussion The lower region has been extensively discussed in previous works [1]. The intermediate and upper regions are the aim of the discussion in this report. As mentioned above the level pattern in the upper region exhibits similar features to the dipole bands observed in the neighbouring Pb [8-19] isotopes as well as in 192 Hg [5], 193 Hg [6] and 196 Hg [7]. Moreover, the B(M1)/B(E2) ratios for the levels of this region are large (see Fig. 4) as it is the case in the dipole bands of Pb and Hg isotopes. Based on these similarities we can suggest that the upper part of the two new structures is a manifestation of a similar phenomenon as the one producing the dipole bands in the neighbouring Pb and Hg isotopes. The interpretation of the dipole bands has been based in all cases on high-K proton configurations which can produce large B(M1)/B(E2) ratios. The proton configura- 173 Fig. 3. Gamma-ray spectrum in coincidence with the 345.4 keV transition. New transitions belonging to structure “2” are marked with “2”. The presence of the 343.6 keV transition of structure “1” (marked with “1”) is an evidence of a weak connection existing between the two structures. Transitions from the normal level scheme belonging mainly to bands AB, AE, AF, ABCD and ABCE are explicity labeled by their energy. All transitions are quoted in keV units tions consist of two quasiprotons from the h9/2 and i13/2 high-Ω orbitals. The high spin of the levels is then reproduced by coupling these configurations to high-J neutron configurations. In order to search for the configurations responsible for the new structures we compared the experimental B(M1)/B(E2) values with theoretical calculations of the B(M1)/B(E2) ratios based on the model introduced by Dönau and Frauendorf [25]. Both experimental points and theoretical curves are plotted in Fig. 4. The configurations used in the calculations have been chosen following the suggestions of ref. [5] and [6] for the 192 Hg and 193 Hg isotopes, respectively. Thus, we used the π(h9/2 i13/2 )K=11 and π(h29/2 )K=8 high-K proton combinations coupled to high-J (h−2 11/2 )J=10 proton holes and i13/2 neutrons. Combinations of up to four and five i13/2 neutrons were used in 192 Hg and 193 Hg, respectively. It is therefore reasonable to choose up to six neutrons (i613/2 )J=24 in the case of 194 Hg. The values for the quadrupole moment and the g-factors used in the calculations were the ones described in ref. [6]. It can be 4 seen from Fig. 4 that the π h29/2 ⊗ π h−2 11/2 ⊗ ν i13/2 and 6 π h29/2 ⊗ π h−2 11/2 ⊗ ν i13/2 configurations best reproduce the experimental B(M1)/B(E2) values. From these configurations the second one, which produces higher spin (∼ 35~ in full alignment), could be proposed for the interpretation of the upper part of structure “1”, while the first (spin ∼ 31~ in full alignment) seems more probable for the upper part of structure “2”. The configuration π h29/2 ⊗ ν i613/2 produces increased values of B(M1)/B(E2) (see Fig. 4 ). However, due to the uncertainties of the model and the parameters used for the calculations, this configuration can not be totally excluded from the interpretation of these structures. The same uncertainty about the contribution of h11/2 proton excitation in the corresponding configuration has also been reported in ref. [5] for 192 Hg. Tackling the intermediate region we note first that such complicated level patterns have been already observed in neighbouring Hg isotopes [4,6]. Common features in such sequences, beside the complexity of the level pattern, are the presence of a level which gathers the decay out of the sequences and then fragments towards several rotational bands of the lower level scheme as well as the unbroadened line shapes in experiments with backed targets suggesting lifetimes expected in non-collective behaviour. The complexity of the intermediate region in 194 Hg can be deduced from Fig. 1. In the case of structure “2” the role of the level gathering the bulk of intensity is played by the 22+ level at 6033 keV excitation energy which fragments towards bands AB, ABCD and ABCE. In the case of structure “1” the 25(−) level at 6816 keV excitation energy is a possible candidate for the same role but in this structure the fragmentation towards bands AB and ABCD is more gradual and does not occur only from one level. Finally, in our spectra no Doppler broadening for the transitions in the intermediate region has been observed. All these facts lead us to suggest that the intermediate region is one of single-particle character. The complexity of the level pattern in the intermediate region makes it difficult to assign configurations in this region. Here we discuss only the possible configurations of the 22+ and 25(−) levels. The configurations proposed for the 41/2− levels which gather the bulk of intensity below the irregular structures in 191 Hg [4] and in 193 Hg [6] are based in the coupling of two h11/2 proton holes to the neutron excitations in the bands in which these levels fragments. These configurations drive a nucleus towards prolate noncollective deformation [4]. In 194 Hg, the spin of the 22+ level can be easily reproduced by the coupling of the neutron 174 in the upper part of the level scheme in 194 Hg is a trace of a subsequent shape change towards triaxality since the dipole bands in 192 Hg [5], 193 Hg [6] and 196 Hg [7] have been connected to such a shape change. Once more, this is in accordance with the theoretical calculations of ref. [3]. 5. Summary In conclusion, two new structures precisely located in excitation energy have been observed in 194 Hg. They become yrast at moderate spins and extend up to ∼11 MeV excitation energy. The overall level pattern allows us to identify three regions. The lower region with the rotational bands, the intermediate region with irregular sequences of γ-rays and finally, the upper region with sequences of dipole transitions. This splitting of the level scheme in regions is in accordance with the theoretical expectations on the shape changes for this isotope. Comparison of theoretical calculations of the B(M1)/B(E2) ratio to the experimentally deduced values favour the π(h29/2 )K=8 high-K proton combination coupled Fig. 4. Experimental B(M1)/B(E2) values for some of the levels in the upper part of structure 1 (filled squares) and structure 2 (filled circles). Solid lines represent theoretical calculations for various configurations. “Mixed” and “pure” stand for the π(h9/2 i13/2 ) and π(h9/2 )2 proton combinations proton hole combination and ν x for ix respectively. π −2 stands for h−2 13/2 11/2 neutron combinations configuration for the bandhead of band AB ν(i213/2 )J=12 to the two fully aligned proton holes π(h−2 11/2 )J=10 . However, the situation in 194 Hg needs more attention because the 22+ level is also connected directly to the negative parity ABCE band which has an i313/2 p3/2 configuration [1]. Hence, an (i413/2 p23/2 )J=22 configuration is also possible for this level. A similar situation, with a level gathering a large amount of intensity and then deexciting towards bands of different parity, exists also in 193 Hg [6]. It is the case of the 47/2+ level at 5407 keV excitation energy to which no certain configuration has been so far assigned. For the 25(−) level an (i513/2 f5/2 )J=25 configuration is possible but, apart from the spin reproduction, no other justification can be given for such a suggestion. Theoretical calculations reported in ref. [3] for 194 Hg predict a successive shape change of the nucleus from oblate collective (γ=−65◦ ) towards prolate non-collective (γ=−120◦ ) and triaxial weakly collective (γ=−80◦ ) before the prolate (γ=0◦ ) superdeformed minimum becomes yrast. The observation of a single-particle region (the intermediate one) in the present work is in accordance with these calculations since this region could be associated with a similar shape change. Indeed, TRS calculations performed for the irregular structure in 191 Hg [4] support such a shape change. Furthermore, the observation of dipole sequences to i13/2 neutrons and most probably to high-J (h−2 11/2 )J=10 proton holes for the interpretation of the sequences in the upper region of the level scheme. This configuration is similar to the configurations used in the dipole bands of 192 Hg [5], 193 Hg [6] and 196 Hg [7] but different to the ones used in the Pb nuclei [8-19] which do not involve rotation aligned protons holes. We express our gratitude to the crew and technical staff at the now defunct NSF at Daresbury for their collaboration. The EUROGAM project is supported jointly by SERC (U.K.) and IN2P3 (France). 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