Collective multipole excitations of exotic nuclei in relativistic continuum random phase approximation

The isoscalar and isovector collective multipole excitations in exotic nuclei are studied in the framework of a fully self-consistent relativistic continuum random phase approximation （RCRPA）. In this method the contri- bution of the continuum spectrum to nuclear excitations is treated exactly by the single particle Green＇s function. Different from the cases in stable nuclei, there are strong low-energy excitations in neutron-rich nuclei and proton-rich nuclei. The neutron or proton excess pushes the centroid of the strength function to lower energies and increases the fragmentation of the strength distribution. The effect of treating the contribution of continuum exactly is also discussed.     

Collective excitations of nuclei in a relativistic mean-field theory

Collective excitations of finite nuclei are discussed in a model relativistic baryon-meson field theory. Eigenvalue equations for small amplitude motion about the static mean-field solutions assuming irrotational flow are derived. Variational estimates of the lowest modes are obtained.     

A unitary correlation operator method

The short range repulsion between nucleons is treated by a unitary correlation operator which shifts the nucleons away from each other whenever their uncorrelated positions are within the repulsive core. By formulating the correlation as a transformation of the relative distance between particle pairs, general analytic expressions for the correlated wave functions and correlated operators are given. The decomposition of correlated operators into irreducible n-body operators is discussed. The one- and two-body-irreducible parts are worked out explicitly and the contribution of three-body correlations is estimated to check convergence. Ground state energies of nuclei up to mass number A = 48 are calculated with a spin-isospin-dependent potential and single Slater determinants as uncorrelated states. They show that the deduced energy- and mass-number-independent correlated two-body Hamiltonian reproduces all “exact” many-body calculations surprisingly well.     

Present and future experiments with stored exotic nuclei at relativistic energies

Pioneering experiments with stored and cooled exotic nuclei at relativistic energies have been performed using the combination of the fragment separator FRS with the storage-cooler ring ESR. Exotic nuclei created in peripheral collisions are spatially separated in-flight and injected into the storage ring for high-precision mass and unique lifetime measurements. Lifetimes of stored bare and few-electron nuclei have been measured to study the influence of the electron density on the β decay. This condition, relevant for stellar plasma, can now be systematically investigated in the laboratory for the first time. Characteristic experiments of the present FRS-ESR system are presented and perspectives for a next-generation facility are briefly outlined. Received: 1 May 2001 / Accepted: 4 December 2001     

Collective excitations in the 123, 125I nuclei

The odd-proton nuclei ¹²³I and ¹²⁵I have been studied in the reactions ¹²¹Sb(α, 2n)¹²³I and ¹²³Sb(α, 2n)¹²⁵I, respectively. The level schemes, spin and parity assignments are based on results obtained from singles γ-ray spectra (E_α = 27 MeV) and excitation functions, from measurements of delayed γ-rays, γ-γ coincidences, internal conversion electrons and γ-ray angular distributions. High-spin positive- and negative-parity bands with energies up to 2948 keV and spins up to $\text{23}\text{2}$ in ¹²³I and with energies up to 3775 keV and spins up to $\text{27}\text{2}\text{in}{}^{125}\text{I}$ have been established. In the decay schemes of both nuclei two separated structures of levels have been observed. One group of levels shows a strong ΔJ = 2 quasirotational pattern predicted by the particle-vibration coupling model. The ΔJ = 1 sequence on a $\text{9}\text{2}$⁺ state is assigned as a rotational structure built on the axially symmetric deformed state $\text{9}\text{2}{}^{\mathrm{+}}\text{[404]}$. In ¹²³I a 28 ns isomer at 2660.0 keV has been found.     

Collective excitations in neutron-rich nuclei within the model of a Fermi liquid drop

We discuss a new mechanism of splitting of giant multipole resonances (GMR) in spherical neutron-rich nuclei. This mechanism is associated with the basic properties of an asymmetric drop of nuclear Fermi liquid. In addition to well-known isospin shell-model predictions, our approach can be used to describe the GMR splitting phenomenon in the wide nuclear-mass region A ～ 40–240. For the dipole isovector modes, the splitting energy, the relative strength of resonance peaks, and the contribution to the energy-weighted sum rules are in agreement with experimental data for the integrated cross sections for photonuclear (γ, n) and (γ, p) reactions.     

Matrix elements for 0νββ decay calculated with the operator expansion method and QRPA wave functions

Matrix elements for 0νββ decay of⁷⁶Ge,⁸²Se,¹¹⁶Cd,^128,130Te,¹³⁶Xe and¹⁵⁰Nd are calculated combining proton-neutron quasiparticle RPA wave functions with the operator expansion method (OEM). The differences between OEM and earlier QRPA calculations, using the closure approximation, are investigated in detail. Special emphasis is put on the discussion on the differences between the 2νββ and the 0νββ mode, since currently 0ν matrix elements can only indirectly be tested. By a comparison with experimental data upper limits on the effective neutrino mass are derived.     

Collective excitations of the polaron-gas

The dispersion of the collective excitations of a polaron-gas is calculated. It is found that the interaction between the L.O. phonons and the plasmons induces a relatively strong wave vector dependence in the ω⁺, ω^- modes. This dispersion is primarily a consequence of the interaction of the L.O. phonons with the resonant plasmon branch in the pair excitation spectrum.     

Collective excitations in the anharmonic crystal

We consider a lattice model of an anharmonic crystal and compute the isothermal (χ_T) and adiabatics (χ_S) susceptibilities for the lattice displacement below T_c. A detailed analysis is given in terms of the dimension of the lattice and a decay parameter of the harmonic force. It turns out that there is a phase-space region for which 0 < χ_T/χ_S < 1, indicating breaking of ergodicity, and a region for which χ_T is finite, indicating the presence of a plasmon frequency due to the anharmonicity.     

SSNTD and radiochemical studies on the transmutation of nuclei using relativistic ions

Extended targets were irradiated for transmutation studies with relativistic heavy ions. For this, a metal core was surrounded by a paraffin moderator. The metal is either copper or lead and it was irradiated with deuterium, alpha, or carbon beams of 1.5 or 3.7 GeV/u at the SYNCHROPHASOTRON, LHE, JINR, Dubna, Russia. During this irradiation copious amounts of secondary neutrons are produced and studied with SSNTD detectors and radiochemical sensors, for example ¹³⁹La (n, γ) ¹⁴⁰La→β⁻. The yield of reaction products allows an estimation of secondary neutron fluxes. The yields of all kinds of reactions produced with deuterium and alpha beams obey to some extent the law of “limiting fragmentation”, i.e. they show little influence on the energy and the kind of incoming particles. However, one observes with 44 GeV ¹²C ions always enhanced nuclear cross-sections induced by secondary particles. This behavior could not be confirmed with theoretical estimations based on the Dubna Cascade Model in its Cascade Evaporation Model version (DCM-CEM). Finally, some results for transmutation studies on ¹²⁷I and Cu will be presented.     

Finite particle number description of neutron matter using the unitary correlation operator and high-momentum pair methods

Using bare Argonne V4' (AV4'), V6' (AV6'), and V8' (AV8') nucleon–nucleon (begin{document}$NN$end{document}) interactions, the nuclear equations of state (EOSs) for neutron matter are calculated with the unitary correlation operator and high-momentum pair methods. Neutron matter is described using a finite particle number approach with magic number begin{document}$N=66$end{document} under a periodic boundary condition. The central short-range correlation originating from the short-range repulsion in the begin{document}$NN$end{document} interaction is treated by the unitary correlation operator method (UCOM), and the tensor correlation and spin-orbit effects are described by the two-particle two-hole (2p2h) excitations of nucleon pairs, where the two nucleons with a large relative momentum are regarded as a high-momentum (HM) pair. With increasing 2p2h configurations, the total energy per particle of the neutron matter is well-converged under this UCOM+HM framework. Comparing the results calculated with AV4', AV6', and AV8' begin{document}$NN$end{document} interactions, we demonstrate the effects of the short-range correlation, tensor correlation, and spin-orbit coupling on the density dependence of the total energy per particle of neutron matter. Moreover, the contribution of each Hamiltonian component to the total energy per particle is discussed. The EOSs of neutron matter calculated within the present UCOM+HM framework agree with the calculations of six microscopic many-body theories, especially the auxiliary field-diffusion Monte Carlo calculations.     

Compression modes in nuclei: RPA and QRPA predictions with Skyrme interactions

Isoscalar monopole and dipole resonances in ⁹⁰Zr, ¹¹⁶Sn, ¹⁴⁴Sm, and ²⁰⁸Pb are studied within the framework of self-consistent HF-RPA or HF-BCS plus quasiparticle RPA. A comparison with recent experimental data obtained at Texas A&;M University, as well as the problems related to the determination of the nuclear incompressibility K _∞, is discussed.     

Cluster and symplectic excitations in nuclei

To incorporate two successful microscopic theories, the cluster and the symplectic models, into a single theory we have developed a recursion formula method of calculating overlaps between cluster and symplectic states. This has facilitated the calculation of cluster widths and isoscalar E2 transition rates in a basis including both types of state. A method of calculating interaction matrix elements is briefly discussed. We have calculated the overlaps between the cluster and the symplectic states for the ¹⁶O, ²⁰Ne, and ²⁴Mg systems. Comparison of the E2 transition rates between the cluster and the symplectic states has clearly indicated the complementary role of these excitations. The feasibility of unifying the cluster and the symplectic excitations is expected to be very useful for a number of nuclear structure problems, particularly those associated with both cluster correlation and quadrupole collectivity.     

Pairing and quartetting in exotic nuclei

A review is given of pair correlations in nuclei with an emphasis on the symmetry character of the superfluid solution which depends on (i) the isospin of the nucleus and (ii) the relative strength of the T=0 and T=1 pairing forces. The most general SO(8) model which accommodates neutrons and protons as well as T=0 and T=1 pairing, is solvable in three limits: only T=0 pairing, only T=1 pairing and equal strengths in the two channels. In these limits, the superfluid ground-state solution of N=Z nuclei exhibits a quartet structure. The competition between superfluidity and magicity is discussed with reference to integrable models. To cite this article: P. Van Isacker, C. R. Physique 4 (2003).     

Collective and two-particle excitations in78Se

Excited states in⁷⁸Se have been studied up to spin (12)? at about 5.8 MeV in the⁷⁶Ge(α, 2n) reaction using in-beamγ-ray spectroscopy. Mean lifetimes could be determined for 27 of the 33 levels observed by applying Doppler shift and pulsed-beam timing methods. According to theB(E2) values most of the levels have been grouped into collective bands. Irregularities in the level spacings of the yrast band above spin 6? are interpreted to be due to the interaction of the ground state band withg _9/2 two-proton and two-neutron excitations. The mutual mixing of these configurations is reflected by strongM 1 transitions between the mixed states. The interaction strengths between the configurations involved have been estimated from three-band mixing calculations.