Gamow shell model description of neutron-rich nuclei

This work presents the first continuum shell-model study of weakly bound neutron-rich nuclei involving multiconfiguration mixing. For the single-particle basis, the complex-energy Berggren ensemble representing the bound single-particle states, narrow resonances, and the nonresonant continuum background is taken. Our shell-model Hamiltonian consists of a one-body finite potential and a zero-range residual two-body interaction. It is demonstrated that the residual interaction coupling to the particle continuum is important; in some cases, it can give rise to the binding of a nucleus.     

Structure of neutron-rich s-d shell nuclei

States in neutron-rich s-d shell nuclei were populated in the reaction of a ¹⁴C beam at E _lab=22 MeV on a ¹⁴C target. Coincidences between γ rays and either other γ rays or light charged particles were measured. γ rays in coincidence with protons established levels at 66.8, 1730, 1823, and 2219 keV in ²⁷Na. The states are compared with calculations based on the s-d shell model and the cranked Nilsson-Strutinsky model. A number of levels in ²⁴Ne were observed in both α-γ and α-γ-γ coincidences and are compared with shell-model calculations.     

Study of neutron-rich Mo isotopes by the projected shell model approach

The projected shell model (PSM) calculations have been performed for the neutron-rich even–even ^102?110Mo and odd—even ^103?109Mo isotopes. The present calculation reproduces the available experimental data on the yrast bands. In case of even–even nuclei, the structure of yrast bands is analysed and electromagnetic quantities are compared with the available experimental data. The g-factors have been predicted for high spin states. For the odd-neutron nuclei, the structures of yrast positive- and negative-parity bands are analysed and found to be in reasonable agreement with the experiments for ^103?107Mo. The disagreement of the calculated and observed plots for energy staggering quantity clearly establishes the occurrence of sizable triaxiality in ^103,105Mo and also predicts a decrease in the quantum of triaxiality with increasing neutron number and angular momentum for odd mass neutron-rich Mo isotopes.     

Study of giant pairing vibrations with neutron-rich nuclei

We investigate the possible signature of the presence of giant pairing states at an excitation energy of about 10 MeV via two-particle transfer reactions induced by neutron-rich weakly bound projectiles. Performingparticle-particle RPA calculations on ²⁰⁸Pb and BCS + RPA calculations on ¹¹⁶Sn, we obtain the pairing strength distribution for two-particle addition and removal modes. Estimates of two-particle transfer cross sections can be obtained in the framework of the macroscopic model. The weak-binding nature of the projectile kinematically favors transitions to high-lying states. In the case of the (⁶He, ⁴He) reaction, we predict a population of the Giant Pairing Vibration with cross sections of the order of a millibarn, dominating over the mismatched transition to the ground state.     

Disturbance of the shell structure of neutron-rich nuclei in the oxygen-magnesium region

The effects related to nuclear deformation and their influence on the shell structure and nuclear properties have been investigated. Calculations were performed within the self-consistent theory of finite Fermi systems. The possibility of existence of strongly deformed nuclei at the neutron drip line and behind it, such as ²⁸O, ³⁹Na, and ^41–43Na, is discussed.     

Multichannel nuclear reactions involving light neutron-rich nuclei: Microscopic approach

The influence of the Pauli exclusion principle on the relative motion of light neutron-rich nuclei in their collision is investigated within the microscopicmethod using as an example ¹¹Be + n and ¹⁰Be + ² n nuclear reactions, as well as ³ n + n and ² n + ² n reactions. Antisymmetrization effects related to the kinetic and potential energy of the relative motion of colliding nuclei are analyzed. The influence of the Pauli exclusion principle on the kinetic energy of the relative motion of the ¹¹Be nucleus and a neutron is shown to result in their attraction. The same phenomenon is observed for the case of the ³ n + n cluster system. The strength of such attraction is high enough to ensure the existence of a bound state in the ¹²Be nucleus and a low-energy resonance in the tetraneutron. The conclusion is drawn that, for a resonance state in the ⁴ n system to exist, the value of the oscillator length must be large enough. It is shown also that increasing the oscillator length results in depression of the cluster-cluster potential. The text was submitted by the authors in English.     

Shell-model studies of the N=14 and 16 shell closures in neutron-rich nuclei

Shell-model studies on the N =14 and 16 shell closures in neutron-rich Be, C, O and Ne isotopes are presented. We calculate, with the WBT interaction, the excited states in these nuclei. The calculations agree with recent experiment data. Excited energies and B(E2) values are displayed to discuss the shell closures. Our results support the N =16 shell closure in these isotopes, while indicating a disappearance of N =14 shell closure in Be and C isotopes.     

Neutrino-induced fission of neutron-rich nuclei

We calculate neutrino-induced fission cross sections for selected nuclei with Z=84-92. We show that these reactions populate the daughter nucleus at excitation energies where shell effects are significantly washed out, effectively reducing the fission barrier. If the r process occurs in the presence of a strong neutrino fluence, and electron neutrino average energies are sufficiently high, perhaps as a result of matter-enhanced neutrino flavor transformation, then neutrino-induced fission could lead to significant alteration in the r-process flow in slow outflow scenarios.     

Shell-model studies of the N=14 and 16 shell closures in neutron-rich nuclei

Shell-model studies on the N=14 and 16 shell closures in neutron-rich Be, C, O and Ne isotopes are presented. We calculate, with the WBT interaction, the excited states in these nuclei. The calculations agree with recent experiment data. Excited energies and B(E2) values are displayed to discuss the shell closures. Our results support the N=16 shell closure in these isotopes, while indicating a disappearance of N=14 shell closure in Be and C isotopes.     

Spectroscopy of neutron-rich nuclei at REX-ISOLDE with MINIBALL

We report on “safe” Coulomb excitation of neutron-rich nuclei. The radioactive nuclei have been produced by ISOLDE at CERN and postaccelerated by the REX-ISOLDE facility. The γ rays emitted by the decay of excited states have been detected by the MINIBALL array. Recent results are presented and compared to theoretical models. on behalf of the MINIBALL and REX-ISOLDE Collaborations The text was submitted by the author in English.     

Reactions with fast radioactive beams of neutron-rich nuclei

The neutron dripline has presently been reached only for the lightest nuclei up to the element oxygen. In this region of light neutron-rich nuclei, scattering experiments are feasible even for dripline nuclei by utilizing high-energy secondary beams produced by fragmentation. In the present article, reactions of high-energy radioactive beams will be exemplified using recent experimental results mainly derived from measurements of breakup reactions performed at the LAND and FRS facilities at GSI and at the S800 spectrometer at the NSCL. Nuclear and electromagnetically induced reactions allow probing different aspects of nuclear structure at the limits of stability related to the neutron-proton asymmetry and the weak binding close to the dripline. Properties of the valence-neutron wave functions are studied in the one-neutron knockout reaction, revealing the changes of shell structure when going from the beta-stability line to more asymmetric loosely bound neutron-rich systems. The vanishing of the N = 8 shell gap for neutron-rich systems like ¹¹Li and ¹²Be, or the new closed N = 14, 16 shells for the oxygen isotopes are examples. The continuum of weakly bound nuclei and halo states can be studied by inelastic scattering. The dipole response, for instance, is found to change dramatically when going away from the valley of stability. A redistribution of the dipole strength towards lower excitation energies is observed for neutron-rich nuclei, which partly might be due to a new collective excitation mode related to the neutron-proton asymmetry. Halo nuclei, in particular, show strong dipole transitions to the continuum at the threshold, being directly related to the ground-state properties of the projectile. Finally, an outlook on future experimental prospects is given.     

Evolution of the N = 20 and N = 28 shell closures in neutron-rich nuclei

The isospin dependence of shell closure phenomena is studied for light neutron-rich nuclei within a microscopic self-consistent approach using the Gogny force. Introducing configuration mixing, ³²Mg is found to be dynamically deformed, although the N = 20 spherical shell closure persists at the mean-field level for all N = 20 isotones. In contrast, the N = 28 spherical shell closure is found to disappear for N - Z≥ 10 whereas deformed shell closures are preserved and lead to shape coexistence in ⁴⁴ S. Configuration mixing shows that the ground state of this nucleus is triaxially deformed. The first 2⁺ excitation energy E_x = 1.46 MeV and the reduced transition probability B(E2;0⁺ _gs→ 2⁺ ₁)= 420 e ² fm ⁴ obtained with our approach are in good agreement with experimental data. Received: 26 July 2000 / Accepted: 30 August 2000     

Isomorphic shell model for closed-shell nuclei

The classical part of the isomorphic model for closed-shell nuclei is presented based on two physical assumptions, namely (a) the nucleons of a closed shell nucleus, considered at their most probable positions, are in an instantaneous dynamic equilibrium on spherical shells, and (b) the dimensions of the shells are determined by their close packing given that a neutron and a proton are represented by hard spheres of definite sizes. The first assumption leads to the instantaneous angular structure, and the second to the instantaneous radial structure of closed-shell nuclei. Applications of the model coming from this classical part alone and presented here are structural justification of all magic numbers, neutron (proton) and charge rms radii, nuclear densities of closed-shell nuclei, and Coulomb, kinetic, and binding energies. All the predictions are in good agreement with experimental data. A characteristic novelty of the isomphic model is that assumption (a) is related to the independent particle model, and assumption (b) to the liquid-drop model. The isomorphic model may provide a link between these two basic nuclear physics models since it incorporates features of both.     

Light neutron-rich nuclei studied with antisymmetrized molecular dynamics

Structure study of neutron-rich isotopes of Li, Be, B, and C with the antisymmetrized molecular dynamics (AMD) is reported. Good reproduction of various observed data by AMD is demonstrated. AMD wave functions which reproduce data well show large change of structure when neutron number increases. Clustering structure is predicted around neutron driplines of Be and B, while large oblate deformation with neutron skin is predicted around neutron dripline of C.     

Beta-decay properties of strongly neutron-rich nuclei

Experimental β-strength functions for about 50 neutron-rich mass-separated fission products with half-lives between 0.8 s and 30 s have been obtained at the OSIRIS facility. The β-strength to excited states depopulating by delayed-neutron emission is evaluated separately. The β^? strength functions are found to increase strongly with excitation energy, which is in contrast to the approximately energy-independent β-strength found for EC decay. By using semiempirical values for the level density, the average transition rate per energy level was evaluated under the assumption that only allowed transitions contribute to the decay. This alternative way of analyzing the data gives a more uniform picture of the β-decay to highly excited states since the transition rates are found to be roughly independent of excitation energy (above the pairing energy). A model of constant transition rate to each final level is introduced and its systematic behaviour is studied. Its use for estimating half-lives of unmeasured nuclides is of value for calculations on nucleosynthesis by the “r-process”. (A listing of β-feed and β-strength functions is available on request.)     

E1 resonances in neutron-rich nuclei within the phonon damping model

The quasiparticle representation of the phonon darnping model (PDM) is developed to include the superfluid pairing correlations microscopically. The formalism is applied to calculate the photoabsorption and the electromagnetic (EM) differential cross sections of E1 excitations in neutron-rich oxygen and calcium isotopes. The calculated photoabsorption cross sections agree reasonably well with the available data for ^16,18O and ^40,48Ca. The results of calculations show that the change of the fraction of the E1 integrated strength in the region of pygmy dipole resonance (PDR) as a function of mass number A with increasing neutron number N is in agreement with the recent experimental data, and does not follow the prediction by the simple cluster model. The EM differential cross sections obtained within PDM in this work show prominent PDR peaks below 15 MeV for ^20,22O in agreement with the recent experimental observation. It is also shown that, using low-energy RI beams at around 50–60 MeV/nucleon, one can observe clean and even enhanced PDR peaks without the admixture with the GDR in the EM differential cross sections of neutron-rich nuclei.     

Gamma spectroscopy of neutron-rich nuclei with the CLARA-PRISMA setup

The CLARA-PRISMA experimental set-up combines the large acceptance magnetic spectrometer for heavy ions PRISMA with the highly efficient CLARA array of 25 Clover detectors. Selected results on the structure of neutron-rich nuclei, populated in multinucleon transfer and deep-inelastic reactions, are presented here. Some technical developments of the setup are also discussed.