Systematic investigation of the high- K isomers and the high-spin rotational bands in the neutron-rich Nd and Sm isotopes by a particle-number conserving method

Abstract

The rotational properties of the neutron rich Nd and Sm isotopes with mass number A ≈ 150 are systematically investigated using the cranked shell model with pairing correlations treated by a particle-number conserving method, in which the Pauli blocking effects are taken into account exactly. The 2-quasiparticle states in even-even Nd and Sm isotopes with excitation energies lower than 2.5 MeV are systematically calculated. The available data can be well reproduced and some possible 2 and 4-quasiparticle isomers are also suggested for future experiments. The experimentally observed rotational frequency variations of moments of inertia for the even-even and odd-A nuclei are reproduced very well by the calculations. The effects of high-order deformation ε 6 on the 2-quasiparticle excitation energies and moments of inertia of the ground state bands in even-even Nd and Sm isotopes are analyzed in detail. By analyzing the occupation probability n µ of each cranked Nilsson orbitals near the Fermi surface and the contribution of each major shell to the angular momentum alignments, the alignment mechanism in these nuclei is understood clearly. * zhzhang@ncepu.edu.cn For the neutron rich rare-earth nuclei with mass number A ≈ 150, especially Nd (Z = 60) and Sm (Z = 62) isotopes, there are many novel phenomena, e.g., nuclear quantum phase transition from spherical to deformed shape [1,2], octupole vibration [3-5], K isomers [6], etc. From neutron number N = 92, the nuclei are well-deformed and possess prolate ground state rotational bands. In this mass region, there are several high-K orbitals around the proton and neutron Fermi surface, e.g., π5/2 + [413], π5/2 − [532], π7/2 − [523], ν5/2 − [523], ν5/2 + [642] and ν7/2 + [633]. Therefore, this may give rise to the formation of various high-K multi-quasiparticle (qp) isomers, which are particularly favorable for studying the blocking effects of the pairing correlations.Due to the high statistics, the spontaneous fission of the actinide nuclei has been used to populate the isomeric and high-spin states of neutron rich nuclei in A ≈ 150 mass region [7][8][9]. Up to now, using the spontaneous fission of 252 Cf [10-19] and in-flight fission of a 238 U beam on a 9 Be target [20][21][22], various high-K isomers and high-spin rotational bands for the neutron rich Nd and Sm isotopes, including both the even-even and the odd-A nuclei, have been established. Most recently, the lightest 4-qp high-K isomer in this mass region has been observed in 160 Sm [21]. These data can reveal detailed information on the singleparticle structure, shell structure, the high-K isomerism, etc., thus providing a benchmark for various nuclear models.Several nuclear models have been used to investigate the properties of these neutron rich nuclei, including quasiparticle rotor model [16,17], a mean-field type Hartree-Fock-Bogoliubov theory with Gogny force D1S [13], projected shell model [23,24] and potential energy surface calculations [20,21]. However, most of these models focus on the eveneven nuclei. ...

Configuration and bandhead spin assignments for the two-quasiparticle rotational bands in the neutron-rich nuclei Pm154,156

“…[13]. Note that the parameter set we adopted in the present PNC-CSM calculation can also reproduce the rotational bands in the neighboring Nd and Sm isotopes, including both even-even and odd-A nuclei [26]. Therefore, PNC-CSM is reliable to make the configuration and bandhead spin assignments for the 2-qp bands observed in odd-odd 154,156 Pm [13].…”

“…Since these Pm isotopes are the neighbors of the Nd and Sm nuclei in Ref. [26], their pairing strengths should be similar. The proton and neutron cranked Nilsson levels near the Fermi surface of 155 Pm are shown in Fig.…”

“…In the present work, the cranked shell model (CSM) with pairing correlations treated by a particle-number conserving (PNC) method [24,25] will be used for investigating the rotational bands in these Pm isotopes, and the configuration and bandhead spin assignments will be made for the three 2-qp bands observed in 154,156 Pm. Note that PNC-CSM has already been used for the systematic investigation of the high-K isomers and high-spin rotational bands in the neighboring neutron rich Nd (Z = 60), Sm (Z = 62) and Gd (Z = 64) isotopes [26,27], in which the experimental data are reproduced quite well. Therefore, the calculations for Pm isotopes by PNC-CSM should be quite reliable.…”

“…separately for each signature sequence (α = I mod 2), where I x (I) = (I + 1/2) 2 − K 2 , and K is the projection of the total angular momentum onto the symmetry z axis. In the present calculation for Pm isotopes, the parameters in the PNC-CSM are taken the same as our previous investigation for the neighboring Nd and Sm isotopes [26]. Here we show them again briefly for convenience.…”

“…Note that the paring strengths in Ref. [26] are determined by the odd-even differences in nuclear binding energies of Nd and Sm isotopes [see Fig. 1 in Ref.…”