Coexistence of different cluster configurations in light neutron-rich nuclei

On the basis of the algebraic version of the resonating-group method (RGM) and within the framework of the discrete representation in the Fock-Bargmann space, a microscopic theory of nuclear reactions with due regard for a coexistence of different cluster configurations in a compound nucleus is realized. Fundamental tenets of the algebraic version of the RGM are stated both for a single binary cluster configuration and for a compound system, where several cluster configurations coexist. Several examples of norm kernels, their eigenvalues, phase shifts, and effective cross sections are given for a number of binary cluster systems. The text was submitted by the authors in English.     

Rotational bands in light neutron-rich nuclei

A simple cluster model is used to describe rotational bands in light neutron-rich nuclei. It is concluded that bands observed earlier had a cluster structure.     

Haloes and clustering in light, neutron-rich nuclei

Clustering is a relatively widespread phenomenom which takes on many guises across the nuclear landscape. Selected topics concerning the study of halo systems and clustering in light, neutron-rich nuclei are discussed here through illustrative examples taken from the Be isotopic chain. The production and detection of multineutron clusters is also briefly presented. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: orr@caelav.in2p3.fr     

Isospin dependence of kinetic energies in light neutron-rich nuclei

Energy spectra andelectric dipole transitions ofN=7 isotones are studied by shell model calculations with isospin dependent kinetic energies for s-d shell orbits. The ground states of¹⁰Li and⁹He are predicted. Electric dipole transitions in¹³C and¹¹Be are studied by using the realistic single-particle wave functions in Woods-Saxon potential.JSPS Fellow for Japanese Junior Scientists.     

Coulomb breakup of neutron-rich isotopes of light nuclei

The effects of higher order multipole transitions, in particular, E2 and E1–E2 interference, in the Coulomb dissociation of neutron-rich nuclei ¹¹Be, ¹⁴B, ¹⁵C, and ¹⁹C on Pb targets at energies of 72, 86, 550, and 77 A MeV, respectively, within the framework of the eikonal-approximation approach are studied. The main steps involved in the derivation of the explicit expressions corresponding to dipole, quadrupole, and dipole-quadrupole-interference terms are outlined. The calculations reveal that the contribution of E2 transitions to the total cross section is finite but small, while that ofE1–E2 interference is nil.Nevertheless, the E1–E2 interference term introduces small distortions in the peak of the relative-energy spectrum. The calculated results are compared with the corresponding data and the comparison favors a value of 0.530 MeV as the ground-state binding energy of ¹⁹C. The text was submitted by the authors in English.     

Structure of light neutron-rich nuclei through coulomb dissociation

Coulomb breakup of neutron-rich nuclei around mass A ∼ 20 has been studied experimentally using secondary beams (∼ 500–600 MeV/u) of unstable nuclei produced at GSI. The spectroscopic factor deduced for the neutron occupying s _1/2 level in ¹⁵C ground state is consistent with the earlier reported value. The data analysis for Coulomb breakup of ¹⁷C shows that most of the cross section yields the ¹⁶C core in its excited state. For ^17–22O, the low-lying E1 strength amounts up to about 12% of the energy weighted dipole sum rule strength depending on neutron excess. The cluster sum rule limit with ¹⁶O as a core is almost exhausted for ^17,18O, while for more neutron rich isotopes the strength with respect to that limit decreases.     

The structure and spectroscopy of neutron-rich nuclei

Our knowledge of the structure and spectroscopy of neutron-rich nuclei has greatly increased due to two important developments in nuclear physics: the construction of large γ-ray arrays to investigate prompt γ-rays from fission and deep-inelastic reactions; and the availability of radioactive nuclei from fragmentation and spallation reactions. In this review examples will be given of the advances that have been made in our understanding of the properties of neutron-rich nuclei. The examples are necessarily selective, given the limitations of space and time. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: nsd@mags.ph.man.ac.uk     

Alpha decay and cluster decay of some neutron-rich actinide nuclei

Nuclei in the actinide region are good in exhibiting cluster radioactivity. In the present work, the half-lives of α-decay and heavy cluster emission from certain actinide nuclei have been calculated using cubic plus Yukawa plus exponential model (CYEM). Our model has a cubic potential for the overlapping region which is smoothly connected by a Yukawa plus exponential potential for the region after separation. The computed half-lives are compared with those of other theoretical models and are found to be in good agreement with each other. In this work, we have also studied the deformation effects on half-lives of cluster decay. These deformation effects lower the half-life values and it is also found that the neutron-rich parent nuclei slow down the cluster decay process. Geiger–Nuttal plots for various clusters are found to be linear and most of the emitted clusters are α-like nuclei.     

Clustering in neutron-rich nuclei

Clustering in nuclei is discussed putting emphasis on the investigation of the role of nuclear clustering in neutron-rich nuclei. The subjects we discuss include clustering in neutron-rich Be, B and C isotopes, clustering in the island of inversion around N = 20, and clustering in the region with A ≈ 40. Be isotopes present us typical examples of clustering in neutron-rich nuclei not only in their ground band states but also in their excited band states, for which we show the analyses based on antisymmetrized molecular dynamics (AMD). Clustering in Be isotopes near neutron dripline is intimately related to the breaking of the neutron magic number N = 8. In this connection we report our study about the possible relation of the clustering with the breaking of the neutron magic number N = 20 in the island of inversion including ³²Mg and ³⁰Ne. Our discussion is not only about the positive parity states but also about negative parity states. Recently in the latter half of sd shell and in the pf shell many excited rotational bands with large deformation have been found to exist. Since the first excited K ^π = 0⁺ and K ^π = 0^- bands in ⁴⁰Ca have been regarded as constituting inversion doublet bands having the ³⁶Ar + α structure, and since the first excited K ^π = 0^- band in ⁴⁴Ti has been concluded to have ⁴⁰Ca + α structure through the α transfer reaction and by using the unique α optical potential on ⁴⁰Ca, it is important to investigate the role of α clustering in these newly-found rotational bands with large deformation. We will report the AMD study about this problem.     

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.     

Cluster states in neutron-rich nuclei

The structure of light neutron-rich nuclei is investigated within the cluster model. Moments of inertia are systematized for low-lying bands of positive and negative parity. It is shown that the experimentally observed rotational bands can be described in the scope of a simple alpha particle model.     

Two-quasiparticle bands in neutron-rich nuclei

The large multi-detector arrays such as EUROBALL and GAMMASPHERE have made possible the study of the spectroscopy of neutron-rich nuclei through the observation of discrete, prompt -rays emitted following fission. Most of the information obtained has concerned yrast states and collective excitations. In the present work, a search has been made for excited bands based upon two-quasiparticle intrinsic structures. Such bands have been found in several even-even nuclei from ⁹⁶Sr to ¹¹²Pd. Careful analysis of triple-coincidence spectra has been performed in order to determine branching ratios within the bands. These branching ratios are then used to establish the magnetic properties of the intrinsic structure, permitting, in most cases, the determination of which Nilsson orbits (and whether they are neutron or proton states) are contributing to the excitation. Some example results from this search are presented.Received: 1 November 2002, Published online: 2 March 2004PACS: 23.20.Lv transitions and level energies - 21.60.Ev Collective models - 27.60. + j      

Structure of light neutron-rich nuclei and mechanism of elastic proton scattering

Differential cross sections for elastic p ⁶He, p ⁸Li, and p ⁹Li scattering at two energies of 70 and 700 MeV per nucleon were calculated within the Glauber theory of multiple diffractive scattering. Threeparticle wave functions (α-n-n for ⁶He, α-t-n for ⁸Li, and ⁷Li-n-n for ⁹Li) were used for realistic potentials of intercluster interactions. The sensitivity of elastic scattering to proton-nucleus interaction and to the structure of nuclei was explored. In particular, the dependence of the differential cross section on the contribution of higher order collisions, on scattering on the core and peripheral nucleons, and on the contribution of small wave-function components and their asymptotic behavior was determined. A comparison with available experimental data and with the results of calculations within different formalisms was performed.     

Very neutron-rich exotic nuclei

A brief summary is done of the various types of experiment used in studies of the very neutron rich nuclei. Some highlights are given for the two-neutron halo and¹¹Li nucleus and for the one-neutron halo and¹¹Be nucleus.     

Electric dipole transitions in neutron-rich nuclei

The structure of neutron-rich nuclei in several isotopes is investigated by shell model calculations. We study the electric dipole (E1) transitions in C isotopes focusing on the interplay between the low-energy Pigmy strength and the giant dipole resonance (GDR). Reasonable agreement is obtained with available experimental data for the photoreaction cross sections in ¹²C, ¹³C, and ¹⁴C with the inclusion of the quenching effects. A low-energy peak in the dipole strength in ¹⁵C is associated with a single-particle motion of the 1s_1/2 valence neutron relative to the ¹⁴C core. The calculated transition strength below the GDR in C isotopes heavier than ¹⁵C is found to exhaust about 50–80% of the cluster sum rule value and 12–16% of the classical Thomas-Reiche-Kuhn sum rule value. Next, we point out that the quadrupole and magnetic moments in the odd C isotopes strongly depend on configuration, which will be useful to determine the spin parities and the deformations of the ground states of these nuclei. The electric quadrupole (E2) transitions in even C isotopes are also studied. The isotopic dependence of the E2 transition strength is found to be reasonably well explained, although the calculated strength largely overestimates the unexpectedly small strength observed in ¹⁶C. The E1 strength in ¹⁸N and ¹⁹N as well as in Ne isotopes is also investigated.     

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.     

Modeling a neutron-rich nuclei source

The deuteron break-up process in a suitable converter gives rise to intense neutron beams. A source of neutron-rich nuclei based on the neutron-induced fission can be realised using these beams. A theoretical optimization of such a facility as a function of the incident deuteron energy is reported. The model used to determine the fission products takes into account the excitation energy of the target nucleus and the evaporation of prompt neutrons. Results are presented in connection with a converter-target specific geometry. Received: 1 December 2000 / Accepted: 12 May 2001