Light Rare-earth Region Research Articles

The use of EXOGAM for in-beam spectroscopy of proton drip-line nuclei with radioactive ion beams

One of the first fusion–evaporation experiments using radioactive ion beams was performed at GANIL in order to study proton-rich nuclei of the light rare-earth region. The low production cross-section of the exotic species of interest, in combination with the low intensity of the beam and its induced background, demanded the use of a highly efficient experimental setup. This consisted of the EXOGAM γ -ray spectrometer coupled for the first time with both the DIAMANT charged-particle array and the VAMOS recoil spectrometer. In this report the experimental challenges of such studies will be discussed and the experimental achievements of the in-beam spectroscopy of proton drip-line nuclei using EXOGAM will be presented.

Probing the maximally deformed light rare-earth region around the drip-line nucleus 130Sm

The neutron deficient rare-earth nuclei of the A∼130 region are of particular interest since highly deformed prolate ground states are expected. Indeed these nuclei are predicted to show maximal ground-state deformations of β2 ∼ 0.40 (axis ratio of 3:2), comparable to the deformation deduced for superdeformed cerium isotopes at high spin. A fusion–evaporation experiment was performed with radioactive ion beams at GANIL in October 2004 which had the goal to reach very proton-rich exotic nuclei located near the proton drip-line. A radioactive 76Kr beam, delivered by the SPIRAL facility, was used to bombard a thin 58Ni target. Emitted γ-rays were detected by the EXOGAM γ-ray spectrometer which was, for the first time, coupled with both the DIAMANT charged-particle array and the VAMOS spectrometer.

Quadrupole moments of highly deformed structures in the A approximately 135 region: probing the single-particle motion in a rotating potential.

The latest generation gamma-ray detection system, GAMMASPHERE, coupled with the Microball charged-particle detector, has made possible a new class of nuclear lifetime measurement. For the first time differential lifetime measurements free from common systematic errors for over 15 different nuclei ( >30 rotational bands in various isotopes of Ce, Pr, Nd, Pm, and Sm) have been extracted at high spin within a single experiment. This comprehensive study establishes the effective single-particle transition quadrupole moments in the A approximately 135 light rare-earth region. Detailed comparisons are made with theoretical calculations using the self-consistent cranked mean-field theory which convincingly demonstrates the validity of the additivity of single-particle quadrupole moments in this mass region.

Stretched alignment due to pairing correlation between the normal and abnormal parity orbits for the gamma -soft nuclei in the light rare-earth region.

For the \ensuremath{\gamma}-soft rotational motion, a two-parameter model is constructed considering the pair correlation between the normal and abnormal parity states based on the fermion dynamical symmetry model. The numerical calculation of this model shows that the data of ground bands of ten typical \ensuremath{\gamma}-soft rotational nuclei in the light rare-earth region are fitted in a very high quality. \textcopyright{} 1996 The American Physical Society.

Quantum rotational band formulas from a two-parameter potential and the microscopic explanation from the fermion dynamical symmetry model.

In this paper, we first demonstrate the applicability of a phenomenological two-parameter formula, as introduced by Holmberg and Lipas from the Bohr Hamiltonian in a way that is different from Wu and Zeng. Second, for the first time, we show microscopically that Holmberg's two-parameter formula can be applied to diatomic molecules and that it can fit the experimental data of rotational spectra of HCl, HBr, and HF very well when the parameters are determined by two arbitrary experimental levels. Third, we derive a two-parameter formula describing \ensuremath{\gamma}-soft rotational spectra which is similar to the Holmberg formula, called Holmberg-like formula in this paper. The experimental yrast lines of nine nuclei in the light rare-earth region are fitted by this formula. For the nuclear \ensuremath{\gamma} stiffness, \ensuremath{\gamma} softness, and for molecular rotational spectra, all the two-parameter formulas are obtained by making use of a single potential function. It is demonstrated that the reason why one can give a unified description for those three systems is the common rotational features like the widely used harmonic oscillator approximation. More importantly, from the more microscopic nuclear fermion dynamical symmetry model (FDSM), we may derive the variable moment of inertia (VMI) model, and further obtain the Holmberg formula and Holmberg-like formula under a certain approximation, as from the nuclear geometric description within the Bohr-Mottelson model (BM). It is shown that the bridge between the descriptions of the FDSM and of the BM is the effect of stretched alignment (stretching effect). According to another interpretation of the FDSM for the nuclear stretching effect, we also give a simple formula to explain the \ensuremath{\gamma}-soft rotational spectra and compare the formula with the above one. Finally, we give a phenomenological generalization to the combination of the Holmberg and Holmberg-like formulas, which may describe a transition from \ensuremath{\gamma}-stiff rotations to \ensuremath{\gamma}-soft rotations.

Study on the γ-stability of nuclei in the light rare-earth region

The polarization effect of the SO(6) symmetry in the fermion dynamical symmetry model (FDSM) was applied to studying the γ-stability of nuclei in the light rare-earth region. The potential energy in the Bohr hamiltonian was calculated microscopically. The γ-stable points of the potential were compared with another work and some experimental results were discussed. In addition, a new approach to determining more exactly the distribution of the particle numbers in the normal and abnormal parity states was given.

Connection of a He-jet to the medium-current source of the SARA on-line separator

The performances of a Bernas-Nier ion source coupled to a helium-jet transport system are reported. The results of coupling efficiency measurements using various cluster materials are given for a number of elements, especially of the light rare-earth region reached through heavy-ion (HI) fusion-evaporation reactions at SARA. The identification and study of several new neutron-deficient isotopes investigated using this system are presented.

Rotation-alignment of three particles in high- j orbits

We investigate the problem of whether rotation-alignment of nucleons in high- j orbits, which causes the first backbend of even nuclei in the light rare-earth region, manifests itself or is blocked in the adjacent odd- A nuclei when the last odd like nucleon is present in the same j-orbits. The problem is formulated in an extended particle-rotor model where pairing correlation is adequately taken into account. The result shows that rotation-alignment of three nucleons in the j-orbits should be observed in the odd- A nuclei. It suggests a relation between the values of spins around which band-crossings take place in neighboring even-even and odd- A nuclei. The relation is found to be consistent with available experimental data.

Particle-hole structure of nuclear isomers at high angular momenta

Yrast states are calculated for a number of nuclei with 62 ≦ Z ≦ 72 and 80 ≦ N ≦ 90 on the basis of the deformed Woods-Saxon potential. Particular emphasis is put on the search for the yrast traps. Some new traps, mainly around the neutron number N = 86 nuclei are predicted in the light rare earth region. In particular, five high spin isomers are predicted in the nucleus 152 Dy and their magnetic moments are calculated. Calculations are limited to the axially symmetric nuclear deformations.

New Neutron-Deficient Radioactive Isotopes of the Light Rare-Earth Region

In the light rare-earth region, seven new neutron-deficient radioactive isotopes have been produced by proton bombardment in the 184-in. cyclotron and the linear accelerator; their radiations have been studied by absorption curve and magnetic counter methods. The new isotopes and their half-lives are ${\mathrm{La}}^{134}$, 6.5\ifmmode\pm\else\textpm\fi{}0.25 min; ${\mathrm{Ce}}^{133}$, 6.30\ifmmode\pm\else\textpm\fi{}0.1 hr; ${\mathrm{Ce}}^{134}$, 72.0\ifmmode\pm\else\textpm\fi{}0.5 hr; ${\mathrm{Pr}}^{138}$, 120\ifmmode\pm\else\textpm\fi{}5 min; ${\mathrm{Pr}}^{139}$, 4.50\ifmmode\pm\else\textpm\fi{}0.2 hr; ${\mathrm{Nd}}^{138}$, 22\ifmmode\pm\else\textpm\fi{}2 min; ${\mathrm{Nd}}^{139}$, 5.50\ifmmode\pm\else\textpm\fi{}0.2 hr. Additional data have been obtained for the isotopes, ${\mathrm{Ce}}^{135}$ and ${\mathrm{Ce}}^{137}$.