Deep-Inelastic reactions have been used to populate high-spin states in the even-even osmium isotopes and in the iridium neighbors. New isomers have been identified in 190Os, 192Os, 194Os, 191Ir and 193Ir. These include a 2 ns 12+ state at 2865 keV and a 295 ns, 20+ state at 4580 keV in 192Os. Although a number of multi-quasiparticle states arising from prolate and triaxial deformations are expected in these nuclei, the main structures in 192Os can be interpreted as a two-stage alignment of i13/2 neutrons at oblate deformation, in close analogy with similar structures in the isotones 194Pt and 196Hg. The isomers are attributed to low-energy E2 transitions at the point of the alignment gains. The isomer observed in 191Ir is long-lived (τm ∼8s) and probably arises from coupling of the h11/2 proton to the 10 -ν/9/2- [505]11/2+ [615] prolate configuration that gives rise to long-lived isomers in 190Os and 192Os, although potential-energy-surface calculations indicate that the resultant three-quasiparticle state will be triaxial

Byrne, A.P.

Carpenter, M.P.

Chowdhury, P.

Dracoulis, G.D.

Hughes, R.O.

Janssens, R.V.F.

Kondev, F.G.

Lane, G.J.

Lauritsen, T.

Lister, C.J.

Palalani, N.

Seweryniak, D.

Shi, Y.

Watanabe, H.

Xu, F.R.

Zhu, S.

Journal of Physics : Conference Series

Carolina Digital Repository

Isomers and alignments in 191Ir and 192OsG D Dracoulis1, G J Lane1, A P Byrne1, H Watanabe1,2,R O Hughes1, N Palalani1, F G Kondev3 , M P Carpenter4,D Seweryniak4, S Zhu4 ,R V F Janssens4, C J Lister4, T Lauritsen4,P Chowdhury5, Y Shi6 and F R Xu61 Department of Nuclear Physics, R.S.P.E., Australian National University, Canberra, A.C.T.Australia 02002 RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan3 Nuclear Engineering Division, Argonne National Laboratory, Argonne IL, U.S.A.4 Physics Division, Argonne National Laboratory, Argonne IL, U.S.A.5 Department of Physics, University of Massachusetts Lowell, Lowell, MA 01854, U.S.A.6 School of Physics, Peking University, Beijing 100871, ChinaE-mail: george.dracoulis@anu.edu.auAbstract. Deep-Inelastic reactions have been used to populate high-spin states in the even-even osmium isotopes and in the iridium neighbors. New isomers have been identified in 190Os,192Os, 194Os, 191Ir and 193Ir. These include a 2 ns 12+ state at 2865 keV and a 295 ns,20+ state at 4580 keV in 192Os. Although a number of multi-quasiparticle states arising fromprolate and triaxial deformations are expected in these nuclei, the main structures in 192Os canbe interpreted as a two-stage alignment of i13/2 neutrons at oblate deformation, in close analogywith similar structures in the isotones 194Pt and 196Hg. The isomers are attributed to low-energyE2 transitions at the point of the alignment gains. The isomer observed in 191Ir is long-lived (τm∼ 8 s) and probably arises from coupling of the h11/2 proton to the 10− ν9/2−[505]11/2+[615]prolate configuration that gives rise to long-lived isomers in 190Os and 192Os, although potential-energy-surface calculations indicate that the resultant three-quasiparticle state will be triaxial.1. IntroductionThe nuclear structure of the even-even isotopes of osmium, 186Os, 188Os, 190Os and 192Os is ofconsiderable interest since they fall in the transitional region where static and dynamic effects dueto the triaxial degree of freedom are expected to be important. Their description is challengingfor theoretical models. The seminal Coulomb excitation studies of Wu et al. [1, 2] that exploitedmodel-independent sum rules, concluded that the stable osmium nuclei are indeed γ-soft, butprolate deformed. While 190Os and 192Os, for example, have average asymmetry angles close to20o and 24o respectively, these are well localized.Despite the interest, the proximity of nuclei in this region to the stability line has meantthat high-spin spectroscopic information has been limited. However, with the development ofdeep-inelastic and fragmentation reactions for spectroscopic studies, both stable and neutron-rich isotopes are becoming accessible. For example, results on the ground state bands for 188Osand 190Os have been reported [3], as well as for the neutron-rich isotope 194Os [4] and someyrast states in 198Os have been identified with fragmentation [5].Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics: Conference Series 381 (2012) 012060 doi:10.1088/1742-6596/381/1/012060Published under licence by IOP Publishing Ltd 1Recent theoretical studies include extensive mean field calculations that describe ground- andexcited-state shape evolution [6, 7, 8, 9, 10] across the range of neutron-rich Hf, W, Os and Ptnuclei. In a more specific prediction, Walker and Xu [11] have proposed that a rotation-alignedoblate structure would compete with the (largely) prolate structures in the isotones 190W and192Os since the Fermi surface is close to the low-Ω orbitals within the i13/2 neutron shell atoblate deformation (thus maximising Coriolis effects), but near the top of the shell at prolatedeformation. Both limits could give low-lying 12+ states, that could also be isomers.Isomers occur for several reasons, such as when low energy and/or high multipolarities areinvolved [12]. So-called K-isomers arise when the γ-ray multipolarity λ fails to match thedifference in K between the initial and final states, (where K is the projection of the totalangular momentum on the nuclear deformation axis) the shortfall ν = ∆K -λ being termed theforbiddenness. The resultant hindrance (F ) is given by the ratio of the partial γ-ray lifetimecompared to the Weiskopff estimate, so that F = τγ/τW . In well-deformed nuclei, the hindranceis found to scale so that the reduced hindrance fν = F1ν is approximately a constant of magnitude∼ 100. Experimental reduced hindrances fν can be taken as indicators of the “goodness” of Kalthough it should be remembered that rotational effects will broaden the K distributions in awell-defined (calculable) fashion, random state mixing may be present ( see [13] for examples ofboth), and fluctuations in shape could undermine the purity of K since the projection axis is nolonger fixed.As should be clear from the above, the presence or absence of isomers, or whether theirlifetimes are long or short, is not necessarily evidence of dilution of the K-quantum number.The qualitative expectation that K-isomerism will diminish as the perimeter of the well-deformedregion is crossed, needs to be quantified in terms of transition strengths. In 190Os and 192Os,for example, despite their γ-softness, isomers with very long lifetimes are known and attributedto the Kπ = 10− ν11/2+[615], 9/2−[505] (prolate-deformed) configuration. The long lifetimes(14 min. in 190Os and 9 s in 192Os) arise because of the low-energy, high-multipolarity decays(M2 and E3) and also because of K-hindrance, even though the reduced hindrances are allanomalously low (≤ 10).2. Experimental DetailsThe present measurements to study nuclei in this region used 6.0 MeV per nucleon 136Xe beamsprovided by the ATLAS facility at Argonne National Laboratory. Nanosecond pulses, separatedby 825 ns, were incident on enriched 186W and 192Os targets. Gamma-rays were detected withGammasphere, with 100 detectors in operation. Triple coincidences were required and the maindata analysis was carried out with γ-γ-γ cubes with various time-difference conditions, and alsowith time constraints relative to the pulsed beam to select different out-of-beam regimes.Another set of measurements was carried out using a macroscopically chopped beam withvarious (beam on)/(beam off) conditions. In these, out-of-beam dual coincidence events wererecorded in reference to a precision clock and γ-γ matrices as a function of the time wereconstructed in contiguous time regions, allowing long lifetimes to be isolated by gating oncascades of interest. Scans were made with different conditions, progressing in steps of ten,from initial values in the sub-millisecond region up the region of a few seconds.The results on a range of even-even osmium and odd-A iridium isotopes will be reported indue course, the main focus here being on preliminary results for 191Ir and 192Os. Results for theneutron-rich isotopes 188W and 190W from the same measurements were published recently [14].3. Iridium isotopes and 191IrThe lightest odd-A iridium isotope identified was 187Ir, corresponding to the removal of fiveneutrons from the 192Os target, and the heaviest was 195Ir, from the addition of three neutrons.The main new results are for 191Ir and 193Ir. In 191Ir, feeding from a long-lived, but unidentifiedRutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics: Conference Series 381 (2012) 012060 doi:10.1088/1742-6596/381/1/0120602isomer, into states in the rotational band based on the h11/2 proton (the 11/2−[505] Nilssonorbital) had been reported in early studies using the 192Os(d,3n)191Ir reaction [15]. The h11/2band is also known in 193Ir [16], but no isomers had been reported. We have also obtainedcoincidence information on delayed transitions in 195Ir, previously identified from fragmentationstudies [17]. A large number of other transitions depopulating isomers in iridium nuclei, mostprobably in the odd-odd isotopes, have been identified, but as yet not assigned to specific nuclei,largely because of the absence of information on low-lying high-spin bands in those nuclei.Selected spectra obtained with double gates on delayed transitions in 191Ir are given in Fig. 1.The lower panel (figure 1(b)) shows the delayed transitions feeding into the 19/2− state of thecountsenergy [keV]420/536 (b)  100 200 300 400 500 600 700 8000100020003000  Ir X-rays   IR191147309376395472519524420/518 (a)0200400600 IrX-rays   IR191147309446536Figure 1. Out-of-beam γ-ray spectra in 191Ir with double γ-ray coincidence gates as indicated.Known contaminants are indicated by the symbol (• ).11/2− band, the main path being through the cascade of 524-keV and 395-keV transitionsdirectly above. This was the path identified previously, with a delayed component attributedto an unobserved transition from a long-lived isomer with E∗ ≤ 2123-keV and T1/2 = 5.5(7) s,(corresponding to a meanlife of 7.9(10) s), feeding a state at 2047 keV.171.3  11/2       -591.5  15/2       -1127.6  19/2       -1652.0  23/2       -2047.4  25/2       -566.8  13/2       -1037.1  17/2       -1599.6  21/2       -1646.321/2       (+)1792.925/2       (+)2101.431/2       (+)419.9536.1524.490.6395.4480562.9      518.7146.6308.5Ir191          =  8 s385.5445.6472.0[54] [52][25]Figure 2. Partial level scheme for191Ir, showing the main decays fromthe isomer and the h11/2 band.countstime [s]0 1 2 3 4 5210310 τ  = 8.4(9) sτ  = 7.9(21) sFigure 3. Intensities as a functionof time from γ-γ coincidence spec-tra selecting the two delayed pathsin 191Ir together with fits that re-turn the meanlives indicated.Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics: Conference Series 381 (2012) 012060 doi:10.1088/1742-6596/381/1/0120603The partial level scheme deduced from the present work is given in Fig. 2, with spins andparities based on a consideration of total conversion coefficients deduced from delayed intensitybalances and γ-γ angular correlations. The 395 keV transition is a mixed dipole, and probablytherefore of E2/M1 character. This leads to a Jπ = 25/2− assignment for the 2047 keV state.Although it could be a candidate for the 25/2− member of the unfavoured sequence of the11/2−[505] band, we do not observe the expected E2 branch to the 21/2− member. As well,the angular correlation obtained by gating on stretched quadrupoles lower in the scheme is ofopposite sign to those of the 472-keV and 446-keV transitions (see Fig. 2). While the mixingratio is not consistent with the 395-keV transition being a band member, there is an alignmentin this region, and therefore a change in structure that needs to be considered.Our results confirm the main transitions, but an additional delayed path is evident from thecoincidence spectra given in figure 1. The cascade of 308.5, 146.6 and 518.7 keV transitions feedsthe 19/2−, 1128 keV level, implying a state at 2101.4 keV, 54 keV above the 2047.4-keV, 25/2−state. Our contention is that the 2101.4 keV state is the isomer, with an unobserved 54-keVtransition to the 2047.4 keV state. This is consistent with the observation in the earlier workof prompt feeding to the 2047 keV state [15], and the fact that we observe the same lifetime forboth paths, evidence for which is shown in figure 3.The 308.5 keV M3 transition strength is 1.52(15)×10−3 W.u., while the implied 54 keV E3branch has a strength of 1.62(14)×10−2 W.u., both reasonable values, although a full evaluationwould need characterisation of the final state configurations. The present calculations of theexpected intrinsic states that allow for shape variations with configuration constraints [18, 19],predict a 31/2+ state from the ν11/2+[615], 9/2−[505]⊗π11/2−[505] configuration at 2210 keV,close to the observed energy of the isomer. Unlike the core, 10− two-neutron componenthowever, the equilibrium deformation has a γ-asymmetry of about 27o. In fact, the calculationspredict a number of other intrinsic states in this energy region including a 31/2− state from theν11/2+[615], 9/2+[624]⊗π11/2−[505] configuration (calculated to lie at at 2086 keV).4. The Nuclide 192OsThe main delayed transitions in 192Os are apparent in the spectrum in figure 4 obtained bysumming selected double gates on the yrast transitions that are not fed by the long-lived 10−isomer. A partial level scheme is given in figure 5. Transitions above the 295 ns isomer placedcountsenergy [keV]  100 200 300 400 500 600 700 800 900050010001500206352375 382446498509568619 681710334Figure 4. γ-ray spectrum produced by summing double γ-ray coincidence gates in 192Os in the100-700 ns out-of-beam time region.at 4580 keV with an 85-keV decay, preceding the 382, 568, 681-keV cascade, were identifiedusing time-correlated gates on the lower transitions. Other key features of the scheme includethe 2 ns, 12+ state at 2865 keV which has several decay paths, including E2 transitions to two10+ states, but a dominant decay via an E1 transition of 498 keV to the 11− state of the 10−Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics: Conference Series 381 (2012) 012060 doi:10.1088/1742-6596/381/1/0120604band. This, and other branches, have allowed the identification of the band based on the 10−isomer, normally inaccessible because of its very long lifetime.   0.0     0       + 205.8     2       + 580.3     4       +1089.2     6       +1708.4     8       +2418.8    10       +3170.5   (11)3955.5   (13)3209.9    12       +2874.3   (11) 2864.5    12       +2753.2  (10 )     + 3246.6  (14 )     + 3814.1  (16 )     + 4495.0  (18 )     + 3389   (13)4580.3  (20 )     + 4964.6  (22 )     + 5532.5  (24 )     + 6200.7  (26 )     + 6291.6  (27 )     + 2015.4    10       -2366.9    11       -2749.8    12       - 489.1     2       + 690.4     3       + 909.6     4       +1143.5     5       +1465.3     6       +1712.9     7       +2383.0     9       +2133.9     8       +2894.2    10       +1069.5     4       +1362.0     5       +1645.1     6       +1968.0     7       +2319.9     8       +205.8374.9509.0619.3710.2751.7785.0791.1 121.1445.7111.5 114.7849.0497.8334.41044.2382.1567.5680.985.3384.3567.9668.290.9524307.0 302.5351.5382.9735489.0283.3484.5101.4704.0329.3563.3884.8376.1624.0201.3219.2233.9321.6453.1569.4670.1420.5555.8668.6760.3159.9379.2580.4292.5283.2322.7575.5606.0674.8OS (partial)192   = 2 ns m          = 8.5 s m           = 295 ns m         [47.4]Figure 5. Partial level scheme for 192Os.The net alignments of the band structures are illustrated in figure 6(a). For the discussion,these have been evaluated using a reference which is appropriate for an oblate deformation.Because of this, the low-frequency trajectories of the ground-band, γ-band and K = 4+ band,while essentially the same, have an artificial upward slope. The 10− band shows about 3h̄more alignment, consistent with the presence of the 11/2+[615], i13/2 neutron orbital in itsconfiguration and there are two sharp increments, beginning near the 12+ and 20+ states,corresponding to alignment gains of ∼12h̄ and ∼8h̄ respectively. The main sequence is comparedwith that of the isotones 194Pt and 196Hg [21]. Most of the yrast sequence for 194Pt proposedby Jones et al. [20] and interpreted as being of oblate collectivity, has been confirmed in thepresent measurements, but some modifications were necessary and are reflected in figure 6(b).The comparison between these cases seems compelling: Very similar alignment gains areobserved, consistent with the AB and CD alignments expected for the i13/2 neutron shellwhen the Fermi level is close to the low-Ω orbitals, as is the case for oblate deformation.This is the accepted interpretation for 196Hg [21]. The first alignment is just that predictedfor the 192Os case in Ref. [11]. Similar 12+ isomers occur in both 194Pt and 196Hg, withcomparable transition strengths. In the present case, the 111.5 keV E2 transition fromthe 12+ state to the 10+2 state, presumably the lower-spin member of the s-band, has astrength of 9.7(26) W.u., while the competing 445.7 keV E2 to ground state band is muchweaker, at 4.4(9)×10−2 W.u.. The 20+→18+, 85.3 keV transition is 0.99(4) W.u.. TwoRutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics: Conference Series 381 (2012) 012060 doi:10.1088/1742-6596/381/1/0120605of these can be seen as essentially collective transitions, modified by the change in intrinsicstructure caused by the alignment gain. Although various intrinsic states are predicted by the(a) 0 100 200 300 40002468101214161820222426i [ ]// h/hω   [keV] gsb γ -band  K=4     +  K=10      - (b)0 100 200 300 400/hω   [keV]196   Hg 194   Pt Figure 6. (a) Net alignments for bands in 192Os. (b) comparison of net alignment curves forthe isotones 192Os, 194Pt and 196Hg, with a common reference.calculations in the same energy region, including a prolate four-quasineutron 20+ level from the11/2+[615],13/2+[606],7/2−[503],9/2−[505] configuration, the current conclusion would be thatthe observed 20+, 295 ns and 12+, 2 ns isomers in 192Os are products of alignment gains withinthe set of i13/2 neutron orbitals at oblate deformation.AcknowledgmentsThis work was supported by the Australian Research Council Discovery Programme and the U.S.Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357 andGrant No. DE-FG02-94ER40848.References[1] C Y Wu et al., Nucl. Phys. A 607, 178 (1996).[2] C Y Wu and D Cline, Phys. Rev. C 54, 2356 (1996).[3] S Mohammadi, et al. Int. J. Mod. Phys. E 15, 1797 (2006).[4] C Wheldon et al. Phys.Rev. C63, 011304 (2001).[5] Zs Podolyak et al. Phys.Rev. C 79, 031305 (2009).[6] L M Robledo, R Rodriguez-Guzman, P Sarriguren J. Phys. G 36, 115104 (2009).[7] P Sarriguren, R Rodriguez-Guzman, L M Robledo Phys. Rev. C 77, 064322 (2008).[8] R Fossion, D Bonatsos, G A Lalazissis Phys. Rev. C 73, 044310 (2006).[9] P D Stevenson et al. Phys. Rev. C 72, 047303 (2005).[10] K Nomura et al. Phys. Rev. 83, 054303 (2011).[11] P M Walker and F R Xu, Phys. Lett. B 635, 286 (2006).[12] P M Walker and G D Dracoulis Nature 399, 35 (1999).[13] G D Dracoulis et al. Phys.Rev.Lett. 97, 122501 (2006).[14] G J Lane et al. Phys. Rev. C 82, 051304(R) (2010).[15] J Lukasiak, R Kaczarowski, J Jastrzebski, S Andre, J Treherne, Nucl.Phys. A313, 191 (1979).[16] Y D Fang et al. Phys.Rev. C 83, 054323 (2011).[17] S J Steer et al., Int. J. Mod. Phys. E 18, 1002 (2009).[18] F R Xu, P M Walker, J A Sheikh and R Wyss, Phys. Lett. B 435, 257 (1998).[19] F R Xu, Chin. Phys. Lett. 18, 750 (2001).[20] G A Jones et al., Acta Physica Pononica B 36, 1323 (2005).[21] D Mehta et al, Z. Phys. A 339, 317 (1991).Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics: Conference Series 381 (2012) 012060 doi:10.1088/1742-6596/381/1/0120606

Isomers and alignments in 191Ir and 192Os

Deep-Inelastic reactions have been used to populate high-spin states in the even-even osmium isotopes and in the iridium neighbors. New isomers have been identified in 190Os, 192Os, 194Os, 191Ir and 193Ir. These include a 2 ns 12+ state at 2865 keV and a 295 ns, 20+ state at 4580 keV in 192Os. Although a number of multi-quasiparticle states arising from prolate and triaxial deformations are expected in these nuclei, the main structures in 192Os can be interpreted as a two-stage alignment of i13/2 neutrons at oblate deformation, in close analogy with similar structures in the isotones 194Pt and 196Hg. The isomers are attributed to low-energy E2 transitions at the point of the alignment gains. The isomer observed in 191Ir is long-lived (??m ??8s) and probably arises from coupling of the h11/2 proton to the 10 -??/9/2- [505]11/2+ [615] prolate configuration that gives rise to long-lived isomers in 190Os and 192Os, although potential-energy-surface calculations indicate that the resultant three-quasiparticle state will be triaxial. ? Published under licence by IOP Publishing Ltd.EI

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Isomers and alignments in 191Ir and 192Os

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