Neutron-rich Rare-earth Nuclei Research Articles

Three-quasiparticle isomers in odd-even Pm159,161 : Calling for modified spin-orbit interaction for the neutron-rich region

Neutron-rich Pm ( Z = 61 ) isotopes were studied by delayed γ -ray spectroscopy at RIBF, RIKEN Nishina Center using the in-flight fission of a 345 MeV/nucleon U 238 beam. A cluster-type Ge detector array, EURICA, was used to measure the delayed γ rays from stopped ions. Isomeric γ decays were observed in Pm 159 and Pm 161 with half-lives of 4.97(12) μ s and 0.79(4) μ s , respectively. Level schemes for Pm 159 and Pm 161 were constructed in this study. The isomeric states of Pm 159 and Pm 161 could be interpreted as two quasiparticle excitations of neutrons with the configurations of ν ( 7 / 2 [ 633 ] ⊗ 5 / 2 [ 523 ] ) and ν ( 7 / 2 [ 633 ] ⊗ 1 / 2 [ 521 ] ) , respectively. They are analogous to the isomers that have been observed systematically in other even-mass N = 98 and N = 100 isotones in this region. A projected shell model calculation was performed and it reproduced the order of three-quasiparticle states only if new Nilsson parameters with an N -dependent spin-orbit interaction were used. This work demonstrates that the strength of spin-orbit interactions in standard Nilsson parameters needs to be modified to study the properties of neutron-rich rare-earth nuclei around A = 165 , and provides new evidence supporting the existence of the deformed N = 98 subshell gap in odd-mass nuclei for the first time.

Systematic study of high-K isomers in the midshell Gd and Dy nuclei

High-K isomers are well known in the rare-earth region and provide unique access to the high spin structures of the nuclei. With the current interest in the study of neutron-rich rare-earth nuclei at Radioactive Ion Beam (RIB) facilities, we present here theoretical results of the band structures of neutron-rich Gd and Dy nuclei, including the high K-isomers. Apart from the already known K-isomers, we predict some more K-isomers and these are suggested for future studies at RIB facilities. Self-consistent Deformed Hartree-Fock and Angular Momentum Projection theories are used to get the intrinsic structures, band-spectra and electromagnetic transitions probabilities of the ground band as well as bands based on isomers.

Enhanced collectivity of $\gamma$ vibration in neutron-rich Dy isotopes with $N=108$–$110$

Background: The $\gamma$ vibrational mode of excitation is an acknowledged collective mode in deformed nuclei. The collectivity depends on the details of the shell structure around the Fermi levels, in particular the presence of the orbitals that have the enhanced transition matrix elements of the non-axial quadrupole excitation. Quite recently, a sudden decrease in the excitation energy of the $\gamma$ vibration was observed at RIKEN RIBF for the neutron-rich Dy isotopes at $N=106$. Purpose: In the present work, by studying systematically the microscopic structure of the $\gamma$ vibration in the neutron-rich Dy isotopes with $N=98-114$, we try to understand the mechanism of the observed softening. Methods: The low-frequency modes of excitation in the neutron-rich rare-earth nuclei are described based on nuclear density-functional theory. We employ the Skyrme energy-density functionals (EDF) in the Hartree-Fock-Bogoliubov calculation for the ground states and in the Quasiparticle Random-Phase Approximation (QRPA) for the excitations. Results: The lowering of the excitation energy around $N=106$ is reproduced well by employing the SkM* and SLy4 functionals. We find the similar isotopic dependence of the excitation energy in the neutron-rich Er and Yb isotopes as well. Conclusions: The microscopic framework of the Skyrme-EDF based QRPA describes well the isotopic dependence of the energy of the $\gamma$ vibration in the well-deformed neutron-rich rare-earth nuclei. The strong collectivity at $N=108-110$ is expected as the Fermi level of neutrons lies just among the orbitals that play an important role in generating the collectivity around $N=106$.

Nuclear deformations in the region of the A=160 r-process abundance peak

In the abundance spectrum of r-process nuclei the most prominent features are the peaks that form when the r-process flow passes through the closed neutron shells. However, there are also other features in the abundance spectrum that cannot be explained by shell effects; for example, the peak in the region of the rare-earth nuclei around mass A=160. It has been argued that this peak is related to the deformation maximum of the neutron-rich isotopes. In recent years, both experimental and theoretical work has been carried out to study the deformation of neutron-rich rare-earth nuclei and to search for the point of maximum deformation. This work has focused on the nuclei around 170Dy with the aim of understanding the evolution of collectivity in the neutron shell with 82<N<126. These investigations will be discussed in terms of the Harris parameters of the variable moment of inertia model. Finally, we will discuss the possibility of reaching further into the neutron-rich rare-earth region in the future at the new experimental facilities using radioactive beams.

Practical Qβ analysis method based on the Fermi–Kurie plot for spectra measured with total absorption BGO detector

A practical method based on Fermi–Kurie plots was newly proposed in order to analyze β-decay energy ( Q β) measured with a total absorption BGO detector. The detector has large efficiencies; all β-rays and subsequent γ-rays can be absorbed simultaneously, and the endpoint energy of the spectrum shows the Q β. In the spectrum, different β-rays having maximum endpoint energies and forbiddenness are superimposed. To apply this method to the measured spectra, in addition to a simplified decay scheme that has a one-component β-ray fed to a pseudo-level E γ, a mixed transition of the allowed and the unique-type first-forbidden transitions with a ratio of α was newly taken into account. Using the theoretical β-ray spectra, we verified that the Q β can be deduced without information about the decay scheme, and described the dependences of the derived Q β on E γ and α. We also checked the reliability of this method by analyzing the spectra of fission products of 91 − 94Rb, 139 − 143Cs, 142Ba and 142,144La, which had well-determined Q β up to 11 MeV. Consequently, we proposed that this method was applicable for analyzing spectra with a systematic uncertainty of 60 keV, when the analyzing regions were limited to about 1 MeV below the Q β. Then, this method was also applied for re-analysis of the Q β of neutron-rich rare earth nuclei.

NUCLEAR SHAPE PROPERTIES OF THE EVEN–EVEN NEUTRON-RICH RARE-EARTH NUCLEI

The physical properties of the nuclear shape have been investigated through the charge square radius (&lt;r2&gt;) and the quadrupole (Q2) and hexadecapole (Q4) moments of the even–even neutron-rich rare-earth nuclei. The single-particle energies used are those of a deformed Woods–Saxon mean-field. The pairing effects have been included by means of an exact projection method. The model has been tested for the "ordinary" nuclei near the shell closure N = 82 and has correctly reproduced the experimental data and particularly the "Kink" effect. The study has then been extended to the neutron-rich nuclei and has shown a stability of the &lt;r2&gt; and Q2 results for N≃100 which may be attributed to the existence of a new magic number. On the other hand, a saturation of the prolate shape appears around N = 108 for the elements Nd , Sm and Gd and near N = 102 for the Dy , Er and Yb . These observations could not be confirmed by the investigation of the hexadecapole moment.

CERN n_TOF facility and its possible impact on producing nuclei with large neutron excess

The n_TOF facility at CERN is a source of neutrons produced by spallation of 20 GeV/ c protons onto a lead target. The outstanding characteristics of this source (very high intensity, 200 m flight path, wide spectral function) make it an extremely useful tool for studying neutron induced reactions. The neutrons in the energy region from 1 to 10 MeV are particularly interesting for producing nuclei with large neutron excess by fission. The onset of an asymmetric fission mode at these energies seems to considerably enhance the yield of neutron rich species. Therefore, a method could emerge for producing neutron rich rare earth nuclei in a region up to now inaccessible.

Investigation of neutron-rich rare-earth nuclei including the new isotopes 177Tm and 184Lu

Decays of neutron-rich isotopes in the rare-earth region were studied by means of on-line mass separation and β− γ spectroscopy using multinucleon-transfer reactions between beams of 136Xe (9 and 11.7 MeV/u), 186W (11.7 and 15 MeV/u) and 238U (11.4 MeV/u) and targets of natW and Ta. The higher beam energies appear to be advantageous for the production of such isotopes. Two new isotopes were identified: 177Tm with a half-life T 1 2 = 85 −15 +10 s , and 185Lu with T 1 2 ≈ 18 s . A new 47 s activity found at A = 171 is tentatively assigned to the decay of the new isotope 171Ho. The properties of the ground and excited states of neutron-rich lanthanide isotopes are interpreted within the shell model using the deformed Woods-Saxon potential. A change of the ground-state configuration for odd-mass neutron-rich lutetium isotopes from π 7 2 +[404] to π 9 2 −[514] is suggested, this change being due to the influence of a large hexadecapole deformation. The role of the possible isomeric state in 180Lu for the nucleosynthesis of 180mTa is discussed.