Triaxial deformations in neutron-rich nuclei with Z = 41 – 46, A ~ 100 – 116 based on prompt fission γ spectroscopy

The paper reviews the systematic studies of triaxial deformations, new mode excitations and shape evolutions with regard to triaxial deformation in the neutron-rich nuclei with Z = 41-46, A~100-116.


Introduction
The neutron-rich nuclei in this region are intermediate between the strongly deformed Sr (Z=38)  Y (Z=39)  Zr (Z=40) nuclei and the spherical doubly magic 132 Sn [1][2][3][4].The Fermi levels relative to the high-j subshell, h 11/2 , πg 9/2 locate at bottom, middle, to upper half of the shells, favoring triaxial prolate, through large triaxial deformations to triaxial oblate, and oblate shapes providing good opportunities for studying shape transition and new excitations with regard to triaxial deformations.The studies are based on the fission  spectroscopy using spontaneous fission of 252 Cf at Gammasphere [2,4].
3 New excitations and shape evolutions found in the triaxial nuclei in the region In this neutron-rich region chiral symmetry breaking has been identified in 104,106,108 Mo, 100 Tc, 110,112 Ru, 103-106 Rh and 104,105 Ag, with disturbed chirality, by soft-and lesspronounced triaxiality, proposed in 108 Ru and 112,114,116 Pd, respectively [4][5][6].The fingerprints of chirality, that is energy degeneracy, similar electro-magnetic properties of the partner levels, and small and nearly constant signature splitting of the doublet bands have been studied in chiral nuclei.While one has seen the evolution of chiral symmetry breaking with maximum triaxiality in 110,112 Ru to disturbed chirality by γ γ γ γ -softness in 108 Ru (from N = 66, 68, to N=64), the evolution of chiral symmetry breaking along the N=66, 68 isotonic chain from Z=44 to Z=46 has also been seen in chiral 110,112 Ru with maximum triaxiality to disturbed chirality in Pd isotopes with less-pronounced triaxial deformations.The doublet bands in 104 Mo recently identified (Fig. 3) show the best energy degeneracy, that is the smallest and most stable energy differences of the partner levels (Fig. 4), and near zero level staggering in this nuclear region (The latter not shown here in the paper because of the page constraint) [7].The wobbling motion predicted in triaxial nuclei constitute a revolving motion of J about an axis of a triaxial nucleus.Wobbling motion manifest itself as a fingerprint that the excitations of the  = 0 wobbling (even-spin members of the  band) are above those of the  = 1 wobbling (odd-spin members of the  band) in an even-even nucleus.The N=68 isotones 112 Ru and 114 Pd were identified as the first and second even-even wobbler at moderate spins in the region [4].An overall shape evolution from triaxial prolate via triaxial oblate to oblate was identified in even-even 110-118 Pd (see Fig. 5), showing a more complete and complex shape transition than predicted long time ago [6].

Fig. 3
Fig. 3 Chiral symmetry breaking was recently identified in 104 Mo.Fingerprints for chirality of a nucleus were confirmed in the doublet bands 4 and 5 [7].

Fig. 4
Fig. 4 Energy degeneracy of the partner levels in the doublet bands of the chiral nuclei identified in this nuclear region.The doublet bands in 104 Mo show the smallest and most stable energy differences of the partner levels among all the chiral nuclei.

Fig. 5 
Fig. 5  values corresponding to the minima of TRS calculations, indicating the triaxial deformations changing with the rotational frequency and neutron number in Pd [6].