New sideband partners of the yrast bands built on the pi(h11/2)nu(h11/2) configuration were identified in 55Cs, 57La, and 61Pm N = 75 isotones of 134Pr. These bands form with 134Pr unique doublet-band systematics suggesting a common basis. Aplanar solutions of 3D tilted axis cranking calculations for triaxial shapes define left- and right-handed chiral systems out of the three angular momenta provided by the valence particles and the core rotation, which leads to spontaneous chiral symmetry breaking and the doublet bands. Small energy differences between the doublet bands suggest collective chiral vibrations.

A model for a special configuration in triaxial odd-odd nuclei is constructed which exhibits degenerate chiral bands with a sizable rotation, a manifestation of dynamical spontaneous symmetry breaking. A quantum number obtained from the invariance of the model Hamiltonian, which characterizes observable states, is given and selection rules for electromagnetic transition probabilities in chiral bands is derived in terms of this quantum number. The degeneracy of the lowest two bands is indeed obtained in the numerical diagonalization of the Hamiltonian at an intermediate spin range, over which electromagnetic transitions follow exactly the selection rule expected for the chiral geometry.

Three odd-odd Nϭ73 isotones, namely 128 Cs, 130 La, and 132 Pr, have been studied via ͑HI, xn␥) reactions. In all three cases, ⌬Iϭ1 side bands of the h 11/2 h 11/2 yrast bands were discovered. This extends the systematic observation of nearly degenerate h 11/2 h 11/2 doublet bands in the neighboring Nϭ75 isotones; these were interpreted as resulting from chiral symmetry breaking in the intrinsic body-fixed frame.

The experimental information on the observed nearly degenerate bands in the N = 75 isotones, in particular 134Pr and 136Pm, which are often considered as the best candidates for chiral bands, is critically analyzed. Most properties of the bands, in particular, the recently measured branching ratios and lifetimes, are in clear disagreement with the interpretation of the two bands as chiral bands. For I =14-18 in 134Pr, where the observed energies are almost degenerate, we have obtained a value of 2.0(4) for the ratio of the transition quadrupole moments of the two bands, which implies a considerable difference in the nuclear shape associated with the two bands. The insufficiency of the near-degeneracy criterion to trace nuclear chirality is emphasized.

Three sets of chiral doublet band structures have been identified in the 103 Rh nucleus. The properties of the observed chiral doublet bands are in good agreement with theoretical results obtained using constrained covariant density functional theory and particle rotor model calculations. Two of them belong to an identical configuration, and provide the first experimental evidence for a novel type of multiple chiral doublets, where an "excited" chiral doublet of a configuration is seen together with the "yrast" one. This observation shows that the chiral geometry in nuclei can be robust against the increase of the intrinsic excitation energy.PACS numbers: 21.10. Hw,21.10.Re,23.20.Lv A novel form of spontaneous symmetry breaking, the chiral rotation of triaxial nuclei, was suggested in 1997 [1]. It was shown that in special circumstances, referred to as chiral geometry, in the intrinsic frame of the rotating triaxial nucleus the total angular momentum vector lies outside the three principal planes. Thus, its components along the principal axes can be oriented in leftand right-handed ways. In the laboratory frame the chiral symmetry is restored, which manifests itself as a pair of ∆I = 1 nearly degenerate bands with the same parity. Such chiral doublet bands were first identified in four N = 75 isotones in 2001 [2]. So far, many chiral candidate nuclei have been reported experimentally in the A ∼ 80, 100, 130, and 190 mass regions [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Besides the simplest chiral configurations composed of one unpaired proton and neutron, composite chiral configurations, containing more than one unpaired protons and/or neutrons, have also been observed in the odd-mass or even-even neighbors of the odd-odd chiral nuclei [10,18]. These observations show that chirality is not restricted to a certain configuration in a mass region, i.e. the chiral geometry can be robust against the change of configuration. It was even demonstrated recently by Meng et al. [22][23][24][25], based on adiabatic and configuration-fixed constrained triaxial covariant density functional theory (CDFT) calculations, that it is possible to have multiple pairs of chiral doublet bands in a single nucleus, and the acronym MχD was introduced for this phenomenon. The first experimental evidence for the predicted MχD has been reported in 133 Ce [26], and also possibly in 107 Ag [27].It is also interesting to study the robustness of chiral geometry against the increase of the intrinsic excitation energy, i.e. if the chiral geometry is sustained in the higher-lying bands of a certain chiral configuration. In all the known cases the chiral doublet corresponds to the two lowest-lying bands of a configuration. Even for MχD in133 Ce [26] and 107 Ag [27], each chiral doublet structure corresponds to two lowest-lying bands with a distinct configuration. Therefore, study of the third and forth bands of the same chiral configuration is needed to answer the question of the investigated robustness. Very recent ...

No abstract

The transition rates for the 2(1)+ states in (62,64,66)Fe were studied using the recoil distance Doppler-shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N = 40. The results are interpreted using the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N = 40 as governed by the number of valence protons with respect to Z ≈ 30. The trend of collectivity suggested by the experimental data is described by state-of-the-art shell-model calculations with a new effective interaction developed for the fpgd valence space.

No abstract