Structure of low-lying states inSm140studied by Coulomb excitation

Abstract

The electromagnetic structure of 140 Sm was studied in a low-energy Coulomb excitation experiment with a radioactive ion beam from the REX-ISOLDE facility at CERN. The 2 + and 4 + states of the ground-state band and a second 2 + state were populated by multi-step excitation. The analysis of the differential Coulomb excitation cross sections yielded reduced transition probabilities between all observed states and the spectroscopic quadrupole moment for the 2 + 1 state. The experimental results are compared to large-scale shell model calculations and beyond-mean-field calculations based on the Gogny D1S interaction with a five-dimensional collective Hamiltonian formalism. Simpler geometric and algebraic models are also employed to interpret the experimental data. The results indicate that 140 Sm shows considerable γ softness, but in contrast to earlier speculation no signs of shape coexistence at low excitation energy. This work sheds more light on the onset of deformation and collectivity in this mass region.

Lifetime measurements in Nd138

“…The Gogny calculations reproduce the energy ratios for the states in the ground-state band, but the absolute energies are overestimated by about 20%. Such an overestimation of energies of the Gogny calculations has been observed already earlier for transitional, γ -soft nuclei [16]. It should be noted that the 5DCH approach contains no free parameters other that those specifying the Gogny D1S interaction, which is used globally for all nuclei.…”

“…Lifetime measurements above the 10 + states in 140 Sm support the notion of shape coexistence at intermediate spins [15]. Electromagnetic matrix elements obtained from Coulomb excitation [16] and lifetime measurements [17] for 140 Sm and their comparison to theoretical calculations suggest weak deformation and softness to γ deformation with an equilibrium shape of γ ≈ 30 • near the ground state. For 138 Nd, indication of γ softness comes from the low excitation energy of the γ -vibrational band [18] and the observation of a two-phonon γ band [19].…”

“…The method used in the calculations is described in more detail elsewhere [36]. The calculations predict a smooth onset of collectivity with decreasing neutron number and generally reproduce the trend of the experimental values for the 2 [16]. A comparison of the experimental excitation energies and B(E2) transition probabilities with theoretical calculations using the 5DCH approach and with shell model calculations for selected states is presented in Table III.…”

“…It should be noted that the 5DCH approach contains no free parameters other that those specifying the Gogny D1S interaction, which is used globally for all nuclei. Evidence for triaxiality was found in neighboring 140 Sm [16]. The potential energy surface (Fig.…”

“…To gain further insight into the structure of the low-lying states in 138 Nd, large-scale shell model calculations were carried out using a 100 Sn core with a valence space including the g 7/2 , d 3/2 , s 1/2 , d 5/2 , and h 11/2 orbitals for both protons and neutrons, and the effective GCN5082 interaction [42], based on a realistic renormalized G matrix with phenomenological monopole constraints. Similar to previous shell model calculations for 140 Sm [16], a seniority truncation scheme was introduced to limit the otherwise prohibitively large basis dimensions. Effective charges of π = 1.65 for protons and ν = 0.65 for neutrons were used to determine the transition probabilities.…”