Gamma bands in doubly odd rhenium and iridium nuclei

Structure of the |K ± 2| bands in doubly-odd nuclei belonging to the transitional deformation region at A∼190 is discussed. Relation of these quasi gamma-bands with the non-axial deformation of the parent twoquasiparticle configurations is studied. Using available experimental information, new tentative |K ± 2| bands are proposed in 188Re, and 192,194Ir nuclei. Coexistence of two-quasiparticle states with different deformation modes is considered in the case of 188Re and 194Ir.


Introduction
Nuclei of the transitional A∼190 region are characterized by core instability.Shape phase transitions in the case of even-even nuclei have been a theme of extensive theoretical studies.Attempts have been made to extend these studies also to odd-A nuclei and it has been shown that the phase-transitional behaviour of the even-even core is retained when one adds an additional fermion.For doublyodd nuclei, studies of nuclear shape transitional phenomena are hindered, first, by the complexity of theoretical description, and, second, by the lack of experimental data related with collective core excitations.
In the case of non-axial nuclear shape for each value of the total nuclear momentum I, one obtains 2I + 1 levels with I projection K assuming value in the range −I, −I + 1, ..., 0, ...I − 1, I. As a result, one observes a number of |K ± 2|, |K ± 4|,. . .side-bands for each twoquasiparticle configuration.Characteristic feature of these bands is intense decay to the levels of its parent K band, predominantly with E2, or M1+E2 transitions.
One should note that establishing of gamma-bands in odd-odd nuclei is a disputable theme.Because of the complexity of experimental data related with the development of odd-odd nucleus level scheme, structure of these nuclei is not so well studied as in the case of their even-even, and odd-A neighbours.As a result, one can rarely expect to obtain confident direct proof for the collective nature of the band in terms of experimental B(E2) values for depopulating transitions.
However, there are several usable indirect indications: a) proposed gamma-band depopulates most intensely to the levels of its parent configuration band, and depopulating transitions have prominent E2 multipolarity components; b) no two-quasiparticle configuration with given spin/parity values is expected in the investigated energy a e-mail: martins.balodis@latnet.lvb e-mail: jberzins@latnet.lvc e-mail: krasta@latnet.lvrange; c) gamma-bands in neighbouring even-even and odd-A nuclei provide information about expected core E 2+ energy; d) rotational parameters of the proposed gammaband are close to those of the parent band.
In the presented study, we have tried to identify lowest |K ± 2| collective excitations of the nuclear core in some doubly-odd rhenium and iridium nuclei.For theoretical structure calculations, we have used the two-quasiparticles plus asymmetric rotor model [1,2].Using available experimental data [3,6], we have reconsidered recently the 192 Ir level scheme proposed in [4].In the new level scheme, there are more than 10 levels below 530 keV which one cannot interpret as members of some already existing band, or the bandheads of expected two-quasiparticle configurations.We believe that it is a clear indication of triaxiality and three of these new levels (6 + 352.9 keV, 4 + 368.2 keV, and 4 − 380.4 keV) can be tentatively associated with the |K ± 2| core excitations of lowest two-quasiparticle configurations.All three levels have confidently established spins and parities and decay mostly to the levels of corresponding parent configuration.Their energies correspond approximately to the two-quasiparticle state bandhead energy plus energy of the core 2 + state derived from the 351.7 keV 2 + level energy.Now, let us consider the neighbouring 194 Ir nucleus.In [5], the low-lying 3 + rotational band has been established at 270.9 keV (see figure 1).However, the available proton and neutron orbits do not predict 3 + band in this energy region.The 270.9 keV level decays with intense E2 transition to the 161.5 keV 5 + level, and M1+E2 transition to the 4 + level at 147.1 keV.We suggest that the 3 + 270.9 keV band can be a |K − 2| gamma band of the K π = 5 + (p:11/2[505]+n:1/2[510]) configuration established at 161.5 keV.Though, in such a case, one must consider coexistence of different non-axial shapes in 194 Ir since energy of the 2 + core state for the 270.9 keV band is much smaller than that for the 518.6 keV 2 + |K − 2| band.
Shape coexistence is possible also in 188 Re.Our study of the 188 Re levels [7] disclosed some problems for interpretation of the level scheme above 400 keV.First, the experimental density of positive parity levels below 1 MeV is about three times higher than predicted by the performed particle plus rotor coupling model (PRC) calculations with the axially-symmetric core deformation.Second, we have not observed population of the K π = 1 + (p:9/2[514]n:7/2[503]) bandhead predicted at about 418.7 keV.
Since the low-lying proton and neutron orbits do not predict a 2 + bandhead below 700 keV in 188 Re, we have considered possibility that the 2 + band at 482.1 keV is a |K + 2| band of the K π = 0 + (p:9/2[514]-n:9/2[505]) configuration.The established decay pattern agrees with such interpretation.The 274.31 keV transition to the 207.9 keV 0 + level is a part of doublet line resolved by γγ-coincidences.Therefore, the M1+E2 multipolarity assigned in [8] does not contradict assumption that the 274.31 keV transition has pure E2 multipolarity.
We have performed the PRC model calculations of the 188 Re positive parity levels in the case of non-axial core deformation.The theoretical predictions at asymmetry angle γ = 20 • show that one obtains bandhead of the |K + 2| side-band at about 480 keV energy and the K π = 1 + bandhead ∼50 keV higher.However, the results of calculations were unsatisfactory for positive parity bands with K ≥3.For these levels, the axially-symmetric PRC model was preferable.Therefore, we assume that the non-axial deformation in 188 Re is associated only with the (p:9/2[514]n:9/2[505]) configuration when both proton and neutron occupy high j orbits.

Figure 1 .
Figure 1.Interpreted positive parity level scheme of 194 Ir.Experimental data are taken from [5].

Figure 2 .
Figure 2. Partial positive parity level scheme of 188 Re.Experimental data are taken from the manuscript in preparation.