Clusters in light nuclei

EPJ Web of Conferences, 2016

Due to their high selectivity, transfer and sequential decay reactions are powerful tools for studies of both single particle (nucleon) and cluster states in light nuclei. Their use is particularly simple for investigations of α-particle clustering (because α-particle has J π =0 + , which simplifies spin and parity assignments to observed cluster states), but they are also easily applicable to other types of clustering. Recent results on clustering in neutron-rich isotopes of beryllium, boron and carbon obtained measuring the 10 B+ 10 B reactions (at 50 and 72 MeV) are presented. The highly efficient and segmented detector systems used, built from 4 Double Sided Silicon Strip Detectors (DSSSD) allowed detection of double and multiple coincidences and, in that way, studies of states populated in transfer reactions, as well as their sequential decay. Clustering was recognized [1] as an essential ingredient in description of light nuclei already in the early days of nuclear physics, even before the actual formulation of nuclear shell model. A systematic picture of the phenomenon for the α-conjugate nuclei (with N =Z) emerged in the late sixties, 201 , 0 0 EPJ Web of Conferences

The European Physical Journal Special Topics, 2008

Spin-projected two-body density ρ (2) S=1,M S ,T=0 (r) (isodensity plot) "Donut" "Dumbbell" density at small distances suppressed ½ central correlations density depends strongly on spin orientation ½ tensor correlations these short-range Correlations can not be described with product states (Slater determinants)

Nuclear Physics A, 2004

A cluster model is applied to the description of the ground state alternating parity bands in actinides and some Ba and Ce isotopes and to the description of the superdeformed band in 'jOZn. The model is based on the assumption that cluster-type shapes are produced by a collective motion of the nuclear system in the mass asymmetry coordinate.

EPJ Web of Conferences, 2015

In this contribution, I discuss an algebraic treatment of alpha-cluster nuclei based on the introduction of a spectrum generating algebra for the relative motion of the alpha-clusters. Particular attention is paid to the discrete symmetry of the geometric arrangement of the α-particles, and the consequences for the structure of the rotational bands in the 12 C and 16 O nuclei.

The European Physical Journal A

Recent progress to treat clustering in nuclear systems has an impact to different branches of nuclear theory. Numerical methods as well as many-body theory (Green-function methods) have been worked out to describe few-body clusters in a nuclear environment where medium effects, in particular the antisymmetrization of the wave function (Pauli blocking) are of relevance. Nuclear structure (Hoyle-like states), nuclear reactions, and consequences for astrophysics (neutron stars, supernova explosions) are emerging applications. The main topics were (1) Cluster models, structure of light nuclei, cluster quantum phase transition; (2) Reaction theory, α-decay of heavy and superheavy nuclei; (3) Clustering in nuclear matter and consequences for thermodynamic properties (4) Heavy-ion collisions and clustering in nonequilibrium systems, transport codes; (5) Astrophysical consequences of clustering. The workshop "Light Clusters in Nuclei and Nuclear Matter: Nuclear Structure and Decay, Heavy Ion Collisions, and Astrophysics" took place at the European Centre for Theoretical Nuclear Physics and Related Areas (ECT*) in Trento, from 2 to 6 September 2019. A very broad spectrum of different fields in research has been presented [1] where cluster formation is essential in nuclear systems, see the Contribution list shown in Table 1. The great interest and activities in this

EPJ Web of Conferences, 2015

International Journal of Modern Physics E, 2011

Alpha clustering in nuclei at present is a well studied and reasonably well accepted property of the nucleus. Less well appreciated and more ambiguous is the role of A = 3 clustering, i.e. helion and triton, in nuclei. Here we try to place A = 3 clustering in nuclei into its proper perspective, first by pointing out strong experimental evidences which indicate its clear presence in nuclei and second showing as to how to include these A = 3 clusters in a proper and consistent theoretical understanding of the nuclear phenomenon.

Physics of Atomic Nuclei, 2007

NUCLEI Theory Cluster Approach to the Structure of Nuclei with Z ≥ 96 Z ≥ 96 Z ≥ 96 * Abstract-The properties of alternating parity bands in heavy nuclei 234 Th, 239−242 U, 241−245 Pu, 243−248 Cm, 245−250 Cf, 248−251 Fm, 249−254 No, 253−256 Rf, and 258

Physical Review C, 2020

It is shown, for the first time, how the exotic shapes due to cluster formation at high excitation energy and angular momentum are manifested through giant dipole resonance (GDR) strength function under the framework of the extended quantum molecular dynamics (EQMD) model. The results of EQMD calculation are compared with the existing experimental data of 32 S and 28 Si formed in the reactions 20 Ne + 12 C and 16 O + 12 C, respectively, at high angular momenta. It is found that the EQMD predicts the general trend of the experimental GDR strength functions for 32 S and 28 Si by considering the ring or toroidal configuration, whereas the linear chain configurations with α clusters can reproduce the higher-energy peak in 32 S and 28 Si. Thus, the direct signature of the cluster formation at high temperature and angular momentum is the observation of a GDR peak ≈ 25 MeV which cannot be predicted within the mean-field calculations. The present result highlights the role of α cluster states above the decay threshold, which is still an open field of investigation.

Journal of Physics: Conference Series, 2012

The fermionic molecular dynamics approach uses Gaussian wave packets as singleparticle basis states. Many-body basis states are Slater determinants projected on parity, angular momentum and total linear momentum. The wave-packet basis is very flexible-FMD contains harmonic oscillator shell model and Brink-type cluster states as special cases. The parameters of the wave packets are obtained by variation. A realistic effective interaction derived from the Argonne V18 interaction by means of the unitary correlation operator method is employed. We discuss the fully microscopic calculation of the 3 He(α,γ) 7 Be capture reaction within the FMD approach. The model space contains frozen cluster configurations at large distances and polarized configurations in the interaction region. The polarized configurations are essential for a successful description of the 7 Be bound state properties and for the Sand D-wave scattering states. The calculated cross section agrees well with recent measurements regarding both the absolute normalization and the energy dependence. We also discuss the structure of the cluster states, including the famous Hoyle state, in 12 C. From the two-body densities we conclude that the Hoyle state has a spatially extended triangular α-cluster structure, whereas the third 0 + state features a chain-like obtuse triangle structure. We also calculate the NhΩ decomposition of our wave functions to illuminate the challenges of no-core shell model calculations for these cluster states.

2004

We investigate resonances in light halo nuclei using a fully microscopic cluster model and the complex scaling method. We make use of the hermitian representation of the complex scaling method. The general structure of the cluster model is that of a correlation operator acting on a starting function that describes a number of neutrons relative to an alpha-particle. The correlation operator is expanded in terms of a small non-orthogonal set of Gaussian basis states and we make use of a simplified, central but state-dependent, interaction. The many-body integrals required for the computation are evaluated by a variational Monte-Carlo algorithm. We show how to obtain resonant states for both 5 He and 6 He.

AIP Conference Proceedings, 2004

It is shown that the static and dynamic cc-cluster models of nuclei, which describe an elastic electron scattering, photodisintegration reactions and pion double charge exchange reactions on a-cluster nuclei are in favor of the a-capture and a process of the formation of these nuclei.

Physical Review C, 1991

A semiclassical model for a clustering in heavy nuclei is developed where the nucleus is viewed as a composite system of pairwise correlated nucleons interacting with a particles. The nucleons are treated microscopically by the use of a spherical shell-model basis while the n particles are considered as elementary bosons carrying angular momentum zero and one. The description of such a system is based on a time-dependent variational formalism, which yields BCS-like equations for the ground and some lowlying excited states. It is shown that the system may undergo transitions from a nucleon superfluid phase to intermediate phases characterized by the coexistence of few a clusters with a nucleon pair condensate. The model allows also to study in a random-phase-approximation context the fluctuations of the system around the BCS-like configurations. Two peculiar aspects emerge, the occurrence of collective states describing a wobbling motion of the a particles and the unusually large number of collective states describing a coherent motion of nucleons oscillating in phase with a particles. The numerical illustrative applications refer to Ra.

Progress of Theoretical Physics, 1973

A new nuclear model is proposed that describes the cluster and the shell structures unifiedly. By the present model one can investigate characteristics of one-particle orbitals in clustering states and also polarizations of constituent clusters. The model has its base on the Hartree-Fock (H.F.) approximation for nuclear intrinsic states. The basis wave functions used in the present H.F. calculations are constructed from single-particle wave functions around each "cluster center", which can well describe possible large eccentric nuclear deformations in light nuclei. The model is analogous to the LCAO-MO-SCF method in the molecular physics. Numerical calculations are performed using an effective twonucleon force (Volkov No. 1) for the ground states of 8B e and 12 C nuclei, and interesting results are obtained about the stability of a-cluster structures. Polarizations are smallest at the equilibrium distance between a-clusters, determined by Brink's a-particle model. An artificial collapse or an extreme decomposition of the a-cluster structures brings about large polarizations. By those polarizations of constituent a-clusters themselves, the a-cluster structure recovers their density distributions. Comparisons of the results are made with those of two-center shell model, usual H. F. calculation and a-particle model. § I. Introduction Molecular viewpoints in nuclear structure were first introduced by Wheeler m 1937. 8 l He studied the usefulness and the limitations of the concept of molecular structure in the atomic nuclei; the division of the constituent particles into more or less well-defined groups. Furthermore he made qualitative discussions on the excitation modes associated with molecular structure and also proposed the method of the resonating groups as the mathematical description. Following him, simple analyses with a-particle model were applied to self-conjugated 4n nuclei. 4 l After about two decades Wildermuth and Kanellopoulos 5 l proposed a cluster model, making a careful reference to the oscillator shell model and performed a number of variational calculations. Smirnov et al. 6 l investigated physical properties of cluster wave functions, the extent of isolation of a-clusters, etc. By the resonating group method a-a scattering phase shifts were very well reproduced and the validity of effective a-a potentials was shown. 7 l Based on this result, an a-particle model of 9 Be nucleus was studied dynamically by Shimodaya *> The preliminary results were reported at the International Conference on Clustering Phenomena in Nuclei,ll Bochum, Germany, 1969 and also published in the Progress of Theoretical Physics.s>

Nuclear Particle Correlations and Cluster Physics, 2017

Clustering plays an important role in the structure of nuclei, especially for light nuclei in the p-shell. In nuclear cluster models these degrees of freedom are introduced explicitly. In the Resonating Group Method or in the Generator Coordinate Method the clusters are built from individual nucleons interacting via an effective nucleon-nucleon interaction; the total wave function is antisymmetrized. Fermionic Molecular Dynamics (FMD) goes beyond pure cluster models. It is a microscopic many-body approach using a Gaussian wave packet basis that includes the harmonic oscillator shell model and Brink-type cluster model wave functions as special cases. Clustering is not imposed but appears dynamically in the calculations. The importance of clustering for the understanding of bound states, resonances and scattering states is illustrated with examples discussing the charge radii of the Neon isotopes, the 3 He(α,γ) 7 Be capture reaction and the cluster states in the 12 C continuum.