Lifetime measurement for the21+state inSm140and the onset of collectivity in neutron-deficient Sm isotopes

Abstract

Background: The chain of Sm isotopes exhibits a wide range of nuclear shapes and collective behavior. While the onset of deformation for N > 82 has been well studied both experimentally and theoretically, fundamental data is lacking for some Sm isotopes with N < 82. Purpose: Electromagnetic transition rates represent a sensitive test of theoretical nuclear structure models. Lifetime measurements are furthermore complementary to Coulomb excitation experiments, and the two methods together can give access to spectroscopic quadrupole moments. Method: The lifetime of the 2 + 1 state in 140 Sm was measured with the recoil-distance Doppler shift technique using the reaction 124 Te(20 Ne, 4n) 140 Sm at 82 MeV. Theoretical calculations were performed based on a mapped collective Hamiltonian in five quadrupole coordinates (5DCH) and the Gogny D1S interaction. Results: The lifetime of the 2 + 1 state in 140 Sm was found to be 9.1(6) ps, corresponding to a B(E2; 2 + 1 → 0 + 1) value of 51(4) Weisskopf units. The theoretical calculations are in very good agreement with the experimental result. Conclusions: The B(E2; 2 + 1 → 0 + 1) value for 140 Sm fits smoothly into the systematic trend for the chain of Sm isotopes. The new beyond-mean field calculations are able to correctly describe the onset of collectivity in the Sm isotopes below the N = 82 shell closure for the first time.

Revised spin values of the 991 keV and 1599 keV levels inSm140

“…Relativistic mean-field calculations predict small oblate deformation for the N = 78 isotones 138 Nd, 140 Sm, 142 Gd, and 144 Dy, and rapidly increasing prolate deformation for the more neutron-deficient nuclei with N < 78 [1]. Woods-Saxon [2], relativistic Hartree-Fock-Bogoliubov (HFB) [3], and HFB calculations with the Gogny D1S interaction [4], which consider also the triaxial degree of freedom, predict 138 Sm and 140 Sm to be γ -soft. The observation of two almost degenerate isomeric 10 + states with the (πh 11/2 ) 2 and (νh 11/2 ) −2 configuration and the properties of the rotational bands on top of these isomers suggest prolateoblate shape coexistence in 140 Sm above 3 MeV excitation energy [5].…”

Structure of low-lying states inSm140studied by Coulomb excitation

“…We have calculated the energies of the low-lying states and the transition strengths between them using microscopic calculations based on constrained Hartree-Fock-Bogoliubov (CHFB) theory using the Gogny D1S interaction [49,50] and mapping to the five-dimensional collective Hamiltonian (5DCH) for quadrupole excitations at low energy. The method has been described in detail elsewhere [2], and results for the B(E2; 2 + 1 → 0 + 1 ) values for the chain of neutron-deficient even-even samarium isotopes were presented in the context of the recent lifetime measurement in 140 Sm [15], where it was demonstrated that the calculations are able to correctly describe the onset of quadrupole collectivity for the samarium isotopes below the N = 82 shell closure. It should be noted that the CHFB+5DCH calculations contain no free parameters except for those specifying the phenomenological D1S interaction, which is globally used across the entire nuclear chart.…”

“…The theoretical calculations explain the onset of quadrupole collectivity in the samarium isotopes below N = 82 by a gradual shape transition from large prolate deformation with axial symmetry for the lightest isotopes to spherical shape in 144 Sm, with the triaxial degree of freedom becoming more important as N = 82 is approached. The Gogny 5DCH calculations find average quadrupole parameters of β = 0.17 and γ = 29 • for the ground state of 140 Sm [15].…”

“…The collectivity of the 2 + 1 state in 140 Sm was studied in a recent recoil distance lifetime measurement [15]. The resulting B(E2; 2 + 1 → 0 + 1 ) value and its comparison to the neigboring samarium isotopes indicates a gradual onset of deformation when removing neutrons from the closed-shell nucleus 144 Sm 82 .…”

Lifetime measurements in Dy162

“…The question revolves around the possible degrees of freedom in deformed nuclei [1][2][3][4]. Rotational motion is an expected feature of deformed nuclei, the open challenge is whether the "granularity" of nuclei [1] allows single or multiple quanta of vibrational oscillations or excitations superimposed on the equilibrium deformed shape of the nucleus.…”