Deep inelastic reactions and isomers in neutron-rich nuclei across the perimeter of the A = 180-190 deformed region

Recent results on high-spin isomers populated in deep-inelastic reactions in the transitional tungsten-osmium region are outlined with a focus on 190Os, 192Os and 194Os. As well as the characterization of several two-quasinutron isomers, the 12 and 20 isomers in 192Os are interpreted as manifestations of maximal rotation alignment within the neutron i13/2 and possibly proton h11/2 shells at oblate deformation.


Introduction
The region of deformed nuclei near Z = 72 and N = 104 is prolific in multi-quasiparticle high-K isomers, formed by combining high-Ω orbitals near the proton and neutron Fermi surfaces.More are expected to occur in the more neutron-rich isotopes but few of these are accessible by conventional fusion-evaporation reactions.As reviewed recently [1], multi-nucleon transfer or "deep-inelastic" reactions with heavy energetic beams offer an alternative, although non-selective, means of production, complementing the broader reach of fragmentation reactions.We have carried out a series of deepinelastic studies that extend into the transitional region of neutron-rich W, Ir, Os and Au isotopes where static and dynamic triaxial-, oblate-and prolate-deformed structures are expected to compete.Some of our results obtained for these isotopes have been reported recently [2][3][4][5][6].

Experiments with deep-inelastic and transfer reactions
Figure 1 is a snapshot of the nuclei in this region where new isomers have been identified and characterized.With the current experimental capabilities, in general it is possible to extend to about four neutrons beyond stability.Measurements were made using 6 MeV per nucleon, pulsed and chopped a e-mail: george.dracoulis@anu.edu.au 136Xe beams from the ATLAS facility at Argonne National Laboratory, incident on a range of enriched targets including 186 W, 187 Re and 192 Os.Gamma-rays were detected with Gammasphere with about 100 detectors in operation.Experimental details and analysis techniques including time-correlations to construct the level schemes and γ-γ-correlations to aid in spin assignments can be found in [2][3][4][5][6][7].

Os isotopes
Some representative γ-ray coincidence spectra produced with double gates and different time regions with respect to the beam pulses, are given in Fig. 2.  not observed in gates on decays from the lower-spin yrast states [3]. Figure 2(b) shows part of the evidence for a high-lying isomer in 194 Os [4], while double gates on the lower-lying yrast transitions during the beam pulse give the spectrum of Fig. 2(c).This clearly shows the transitions in the main complementary partner, 134 Xe, produced from the removal of two neutrons from the 136 Xe beam.(Space considerations preclude reproducing detailed level schemes here.See, instead, Refs.[3,4]).

Interpretation
The net alignments of the band structures in the even-even isotopes are illustrated in figure 3.These have been evaluated using a reference with a lower moment-of-inertia than would be used for a prolate deformation, as is appropriate for oblate rotation.Because of this, the low-frequency trajectories of the ground and γ bands in each case have a small (artificial) upward slope.This is also the case for the other bands, such as the K π = 4 + band in 192 Os.The newly identified 10 − band shows about 3 more alignment, consistent with the presence of the 11/2 + [615], i 13/2 neutron orbital in its configuration.As discussed in Ref. [3] there are two sharp increments in the yrast sequence of 192 Os, beginning near the 12 + and 20 + states, both of which are isomeric, corresponding to alignment gains of ∼12 and ∼8 respectively.(The results for 190 Os are similar but at this stage, less well defined.) The first alignment in 192 Os was attributed in Ref. [3] to the AB alignment expected for the i 13/2 neutron shell when the Fermi level is close to the low-Ω orbitals, as is the case for oblate deformation and predicted for the 192 Os case in Ref. [8].The second corresponds to either alignment of the BC neutrons, or possibly the ab protons1 .(Although various intrinsic states are predicted by the calculations in the energy region of the 20 + isomer, the structures observed do not have the associated rotational bands that would be expected if they were high-K states.)The alignment gains are in fact, similar to those observed in the N = 116 isotones , 194 Pt and 196 Hg [3].At the first alignment, shortlived 12 + isomers resulting from low-energy E2 transitions with enhanced strengths (∼ 2-10 W.u.) occur in both 194 Pt and 196 Hg [10], comparable to the strength of decay from the isomer in 192 Os Therefore, the current conclusion is that the observed 20 + , 295-ns and 12 + , 2-ns isomers in 192 Os are products of alignment gains at oblate deformation.The dynamical effects and deformation changes supporting this conclusion are contained implicitly in the Total Routhian Surface (TRS) results given in Fig. 4 (from [3]).The ground-state configuration is associated with a soft prolate deformation at  low frequency (left panel) while the change towards oblate deformation (minimum at γ = -76 o ) is associated with about 12 of alignment from neutrons (middle panel).At higher frequencies, the deformation is similar but more localised (right-hand panel).However, from the wave functions, the total aligned angular momentum now has a component of about 10 from the protons, indicating that the second alignment could be from the ab protons rather than the next pair of i 13/2 neutrons (CD).

Figure 3 .
Figure 3. Net alignments for bands in 190 Os, 192 Os 194 Os obtained with a common reference.

Figure 4 .
Figure 4. Total Routhian surfaces for 192 Os at the frequencies indicated with a contour separation of 200 keV.
[1]roximate reach (dashed line) of deep-inelastic reactions with current detector sensitivities in the well-deformed region[1].Nuclei in which new isomers have been identified or significant information on existing isomers obtained are indicated by the hatching.Question marks indicate cases currently under evaluation.