Congruent band structures in 154Gd : Configuration-dependent pairing, a double vacuum and lack of $ \beta$ -vibrations

Abstract

Abstract. We have used the (α, 2n) and (α, 4n) reactions and the AFRODITE γ-ray spectrometer to make a comprehensive study of the nucleus 154 Gd below 20h. While the first excited 0 + 2 state at 681 keV is usually considered to be the head of a K π = 0 + β-vibrational band, we propose that the data are best described as two separate vacuum states, the ground state and the 681 keV level, each with its own γ and octupole vibrations, pairing and alignments. The implications of this finding, for understanding the structure of transitional rare-earth nuclei, are discussed.It has been a natural assumption since the early days of the collective model [1,2], that the first excited 0 + 2 state in deformed even-even nuclei is a β-vibration along the symmetry axis. Doubts about this assumption have been expressed [3][4][5][6][7][8][9][10][11][12] over the years and, more recently, a comprehensive review of the characteristic properties of such states was made by Garrett [13]. He found that the properties of most 0 + 2 states were not those of β-vibrations, e.g. in 166 Er [14], and he stressed the role pairing must play in the existence of the observed states.The rare-earth nuclei with neutron numbers N = 88 and 90 have particularly low-lying first excited 0 + 2 levels with excitation energies between 600 and 800 keV, well within the pairing gap of about 2.1 MeV. It has usually been assumed that these 0 + 2 levels are collective β-vibrations based on the ground state |0 + 1 vacuum in spite of the observation that they are very weakly excited in inelastic electron scattering [15,16], which is unusual for states with a collective nature. The N = 88 0 + 2 levels are strongly populated in (p,t) two-neutron pickup reactions [17][18][19][20][21] but are weakly populated in (t,p) two-neutron stripping reactions [22,23]. In contrast the N = 90 0 + 2 levels are weakly populated in the (p,t) reactions and strongly populated in the (t,p) reactions. If one assumes that the 0 + 2 levels correspond to collective β-vibrations it remains unclear why there would be a e-mail: jfss@tlabs.ac.za a substantial difference in the population patterns of these states. For nuclei with N ≥ 92 most of the two-neutron transfer strength is concentrated in the ground states [17][18][19][20][21][22]24], which is expected [25] for superfluid nuclei with a ground-state vacuum |0 + 1 well described by the BCS or Hartree-Fock-Bogolyubov theories [26].One should keep in mind that for the N = 88 and 90 nuclei even the most sophisticated models [2,4,27] fail to predict a β-vibration below about 1.5 MeV, adding further doubts about the interpretation of these states as β-vibrations. A totally different approach has been adopted by the Yale group [28] in which the 0 + 2 levels become s = 2 boson excitations [29][30][31] at a critical point X (5) symmetry.An alternative interpretation to β-vibrations for 0states was suggested for the actinide isotopes, where several 0 + 2 states that could not be described as a pairing or β-vibration had been observed by Maher et al...

βdecay spectroscopy of light Nd isotopes

“…Note that the neighboring odd-odd nuclei undergo β decay from ground and isomeric states with a wide range of J π values. 134 Pm decays via a (2 + ) ground state and a (5 + ) isomer [16] and 130 Pr decays via three states [17] with J = 2 + , (4,5) + , and (7,8). It is possible that 132 Pm also possesses two β-decaying states, which would be consistent with a wide range of spins in 132 Nd being populated in the decay.…”

“…Interest in this region blossomed with the concept that the N = 90 nuclei exhibited a specific type of phase transitional behavior [1][2][3][4] described by the X(5) model [5]. In subsequent works [6][7][8] alternative interpretations involving more generalized shape coexistence models have been argued. Regardless of the origin of the observed properties, it is clear that the N = 90 nuclei exhibit an intriguing and unique structure.…”

“…In subsequent works [6][7][8] alternative interpretations involving more generalized shape coexistence models have been argued. Regardless of the origin of the observed properties, it is clear that the N = 90 nuclei exhibit an intriguing and unique structure.…”

“…c 0.92 (7) 23(2) 608.60 (8) 4.0(2) 100(5) 100 1737.21 (9) (9) 100 (10) (7) 100 (13) a From the adopted γ data of Ref. [16].…”

γ-softBa146and the role of nonaxial shapes atN≈90

“…The abrupt onset of deformation has received particularly intense scrutiny with general discussions often framed in terms of a phase transition [7][8][9]; in this case, a specific type of phase transition encapsulated by the X(5) model [10][11][12][13]. However, such an approach is not fully supported by all available experimental data and more generalized shape-coexistence models have been proposed [14][15][16].…”