Exotic Neutron-rich Nuclei Research Articles

Fragment detection system for studies of exotic neutron-rich nuclei

We have developed a fragment detection system for use in studies of exotic, neutron-rich nuclei. Using a C-shaped dipole magnet, the system sweeps charged fragments and un-reacted beam particles through an angle before stopping them in an array of plastic scintillator detectors, recording time-of-flight and total energy. The system also includes a pair of silicon strip detectors to measure fragment angle of emergence from the target and energy loss for particle identification.

Shell, Shape and Spin/Isospin Structures of Exotic Nuclei

The magic numbers and shell structure of exotic nuclei are discussed, and their novel origin is shown to be the spin-isospin dependent part of the nucleon-nucleon interaction in nuclei. The importance and robustness of this mechanism is shown in terms of meson exchange, G-matrix and QCD theories. The analytic expression of the monopole components of the τ.τ σ.σ interaction is presented for the first time, showing a strong attraction between spin-flip partners of a proton and a neutron. In neutron-rich exotic nuclei, magic numbers such as N=8, 20, etc., can disappear, while N=6, 16, etc., arise, affecting the structure of lightest exotic nuclei to nucleosynthesis of heavy elements. Effects due to the variation of the shell gap are discussed, in particular on the excitation energies and B(E2) values of 3 2 , 3 4 Mg exotic nuclei. It is suggested that various anomalous properties of exotic nuclei can be studied with the paradigm of shell evolution.

In-beam gamma-ray spectroscopy with exotic beams at the NSCL

Projectile fragmentation provides radioactive beams at intermediate velocities (v/c = 0.3-0.5) by physical means of fragment separation. With the development of position-sensitive photon detectors it has become possible to measure the energies and directions of photons emitted in-flight from such fast-moving exotic beams. This allows the reconstruction of the photons' energies emitted from an exotic projectile with high accuracy. It can be advantageous to employ photon detection in experiments with exotic beams since photons can traverse matter easily and their attenuation can be calculated. Experiments with standard luminosities can be carried out at intermediate beam energies with thick secondary targets (order of g/cm2) and very low incident beam rates (order of particle/s or less). Experimental success in this field is strongly correlated with the development of photon detectors such as position-sensitive scintillation detectors or segmented germanium detectors. In-beam gamma-ray spectroscopy of fast exotic beams has been successfully used at all projectile fragmentation facilities in intermediate-energy heavy-ion inelastic scattering experiments, knockout reactions and fragmentation reactions. Here, we focus on experimental results for neutron-rich exotic nuclei in the π(sd )-shell. Measurements and detector developments carried out at the NSCL at Michigan State University during the last four years are discussed.

Future prospects for secondary-beam production

This contribution discusses the characteristics of different types of nuclear reactions and the influence of the beam energy in view of future prospects for secondary-beam production. First, electronic interactions in the target are considered because they define the usable target thickness. Rather high beam energies are advantageous. Secondly, the nuclear-reaction aspects are discussed. Three reaction mechanisms provide the most promising prospects for the production of secondary beams. Fusion is best suited for the production of nuclei near the proton drip line and for the heaviest elements. Fission specifically populates mid-mass neutron-rich isotopes. Fragmentation and spallation reactions represent rather universal production mechanisms for both neutron-deficient and neutron-rich exotic nuclei, since the fluctuations in the N-over- Z ratio are very important. Due to these large fluctuations, this is the most promising reaction mechanism to reach extremely exotic nuclei over the whole mass range, if sufficiently high primary-beam intensities are available. In particular, it seems to be a unique process for the production of extremely neutron-rich isotopes of elements above the region of fission fragments. These considerations are verified with experimental data. In particular, a systematic overview on the production of residual nuclei in reactions at relativistic energies has been obtained in several experiments recently performed at GSI in inverse kinematics. They allow to give a rather realistic estimate on the prospects for secondary-beam production in next-generation facilities.

Isospin dependence of mechanical and chemical instabilities in neutron-rich matter

The rapid advance in experiments with rare isotopes has opened up several new frontiers in nuclear science @1#. The planned Rare Isotope Accelerator ~RIA! will further enhance the exploration of these forefronts dramatically @2#. Besides detailed information on the structure of exotic nuclei in many unexplored regions of the periodic chart, it is now possible to study novel properties of isospin-asymmetric nuclear matter with extreme neutron to proton ratios. Prospects for discovering new physics in warm neutron-rich matter of various densities that can be created transiently in nuclear reactions induced by exotic neutron-rich nuclei have generated much interest in the nuclear science community @3‐9#. In particular, the density dependence of the symmetry term in the equation of state ~EOS! has recently received much attention since it is among the most important but very poorly known properties of neutron-rich matter @10‐15#. This term is very important to the mechanisms of type II supernova explosions and neutron-star mergers. It also determines the proton fraction and electron chemical potential in neutron stars at b equilibrium. These quantities consequently influence the cooling rate of protoneutron stars and the possibility of kaon condensation in dense stellar matter @16 ‐20#. Based on the EOS obtained from various microscopic many-body theories and phenomenological models @21‐26#, it has long been predicted that isospin-asymmetric nuclear matter under certain conditions can be mechanically or chemically unstable, i.e.,

Magic numbers in exotic nuclei and spin-isospin properties of the NN interaction.

The magic numbers in exotic nuclei are discussed, and their novel origin is shown to be the spin-isospin dependent part of the nucleon-nucleon interaction in nuclei. The importance and robustness of this mechanism is shown in terms of meson exchange, G-matrix, and QCD theories. In neutron-rich exotic nuclei, magic numbers such as N = 8, 20, etc. can disappear, while N = 6, 16, etc. arise, affecting the structure of the lightest exotic nuclei to nucleosynthesis of heavy elements.

Estimate of the anapole moment of an exotic nucleus

Using the information on the nuclear structure of exotic neutron-rich halo nucleus 11 Be, we evaluate the parity violating anapole moment in its ground state. The resulting value κ( 11 Be )=0.17 is fifteen times bigger than the typical value of the anapole moment of a normal nucleus of the same mass, and in fact exceeds by few times anapole moments of any known neutron-odd nuclei (e.g., κ( 11 Be) > 2|κ( 207 Pb) |. It is also few times bigger than the neutral current contribution to the lepton–nucleus interaction.

Static Moments of Neutron-Rich Ground States and Isomers Produced by Projectile Fragmentation

New achievements in nuclear moment measurements on nuclei produced and spin-oriented in intermediate energy (50–80 MeV/u) projectile fragmentation reactions are presented. By combining different types of spin-orientation with different experimental techniques, we show that the magnetic dipole moments and electric quadrupole moments of exotic neutron rich nuclei in their ground state as well as in their excited isomeric states can be measured. A selection of recent experiments performed at the LISE spectrometer at GANIL (Caen, France) is described.

Radiative neutron captures by neutron-rich nuclei and the r-process nucleosynthesis

The radiative neutron capture by neutron-rich nuclei is estimated with an improved description of the electric giant dipole resonance. In addition, 3 major effects affecting the capture rates by exotic neutron-rich nuclei are studied. These concern the existence of a low-energy E1 pygmy resonance, the overestimate of the statistical predictions for resonance-deficient nuclei and the direct capture mechanism. The total ( n, γ) reaction rates including these 3 effects are evaluated for 3100 neutron-rich nuclei and used in parametric r-process calculations to analyze their impact on the r-abundance distribution.

Superasymmetric fission at intermediate energy and production of neutron-rich nuclei with A < 80

Abstract The yields of neutron-rich Ni, Cu, Zn, Ga and Ge-isotopes were measured in 25 MeV proton induced fission of 238 U using the ion guide-based isotope separator technique. The results indicate enhancement for superasymmetric mass division at intermediate excitation energy of the fissioning nucleus and show the potential of this reaction for the production of neutron-rich exotic nuclei around Z = 28.

Dipole states in the deuteron, other exotic nuclei, and “Schrödinger’s Cat”

The question whether the “soft giant dipole modes” in alo nuclei are resonances is discussed. It is argued that a resonance picture of these modes can be envisaged with a non Breit-Wigner line shape. The dipole Coulomb breakup of the deuteron is worked out as a specific example of such a picture. The “soft” and “hard” dipole modes of the more complex neutron-rich exotic nuclei are treated from this point of view. An interpretation of the coherent production of these modes in Coulomb excitation as a nuclear instance of Schrodinger cat state is advanced.

Neutron-rich B isotopes studied with antisymmetrized molecular dynamics.

Structure of odd-even B isotopes up to the neutron dripline is studied systematically with the antisymmetrized molecular dynamics (AMD). The AMD method has already proved to be a powerful theoretical approach for the systematic study of nuclear structure in extensive region including exotic neutron-rich nuclei as well as ordinary nuclei. It is owing to its flexible nature free from any model assumptions such as the existence of clusters. The energies and other observed data of B isotopes are reproduced well. Especially very good reproduction of electromagnetic properties is obtained. The systematic behavior of the electromagnetic properties is explained in relation to the drastic change between clustering structure and shell-model-like structure. This explanation gives us an important indication that clustering structure in neutron-rich B nuclei is strongly suggested by the experimental data. It is shown that the structure change with increase of the neutron number is largely governed by the shell effect of neutron orbits. Exotic structure with new type of clustering is suggested to evolve in neutron-rich nuclei near the dripline.

Rho meson coupling in the relativistic mean field theory and description of exotic nuclei

The role of the isovector rho-meson coupling on the symmetry energy in the relativistic mean-field theory has been investigated. In a multi-dimensional parameter search for an appropriate force within the framework of the non-linear σωϱ model the ϱ-meson coupling has been found to have a significant effect on the neutron skin of nuclei. It has been shown that a low value of ϱ-meson coupling strength remedies the defect of some of the existing parameter sets of providing a higher neutron skin thickness of nuclei. An extraordinarily good description of highly proton- as well as neutron-rich exotic nuclei has been found. It has been observed that the proper description of both proton-rich and neutron-rich nuclei is sensitive to the symmetry energy.

Coulomb and nuclear effects in the fragmentation of exotic nuclei

The role of the nuclear and the Coulomb interactions in the dissociation of the neutron-rich exotic nuclei 11Li, 11Be and 14Be incident on a number of targets has been studied. The two-neutron removal cross sections of 11Li and 14Be and one-neutron removal cross section of 11Be have been calculated. The theory of diffraction dissociation and diffraction stripping has been used to account for the nuclear effects. The Coulomb-dissociation cross sections have been calculated within the framework of a direct fragmentation model formulated earlier. The Coulomb dissociation along with the nuclear cross sections explain satisfactorily the target-mass dependence of the experimental total two-neutron removal cross sections of 11Li and 14Be around 30 MeV/ A incident energy. The beam-energy dependence of this cross section is also well reproduced. The stripping contributions dominate the nuclear cross sections. The dependence of the reaction mechanism underlying the two-neutron removal process on the separation energy of the two-neutron system in the projectile is discussed. The role of the “post-acceleration effect” on the energy spectra of the produced fragments is investigated.

Structure of exotic neutron-rich nuclei.

A new framework, the variational shell model, is proposed to describe the structure of neutron-rich unstable nuclei. An application to $^{11}\mathrm{Be}$ is presented. Contrary to the failure of the spherical Hartree-Fock model, the anomalous 1/${2}^{+}$ ground state and its neutron halo are reproduced with the Skyrme (SIII) interaction. This state is bound due to dynamical coupling between the core and the loosely bound neutron, which oscillates between the 2${\mathit{s}}_{1/2}$ and the 1${\mathit{d}}_{5/2}$ orbits.

RHO-MESON-NUCLEON COUPLING IN RELATIVISTIC MEAN FIELD THEORY

We obtain a more accurate rho-meson-nucleon coupling constant gρ for studying nuclei in relativistic mean field theory of nucleons and mesons than used before. With this gρ we get a better description of finite nuclei. We have studied a number of isotopic chains of neutron-rich nuclei using the new gρ parameter, with important consequences for very neutron-rich exotic nuclei and the neutron-drip line. The binding energies are sensitive to the coupling constant gρ, but the rms radii and deformation parameters of nuclei are not at all sensitive to gρ.

Measurements of reaction cross sections for neutron-rich exotic nuclei by a new direct method

A new, simple, direct method for the measurement of reaction cross sections ( σ R ) is presented. The ( σ R ) for neutron-rich exotic nuclei were measured and the reduced strong absorption radii r 0 2 were deduced. A strong isospin dependence of r 0 2 was observed for measured isobars with masses from A = 10 up to A = 18. For heavier nuclei, this dependence is strongly reduced.

Evidence for dineutrons in extremely neutron-rich nuclei.

It is shown that the continuum missing-mass spectra for the (${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$,${\mathrm{\ensuremath{\pi}}}^{+}$) and (${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$,p) reactions leading to extremely neutron-rich exotic nuclei can be explained in terms of phase-space distributions by invoking the presence of dineutrons as one of the products of the breakup. It is suggested that this indicates the presence of the dineutron as a cluster in these neutron-rich systems during their breakup. It is noted that these observations in weakly unbound systems may be analogs of the dineutron halos for which evidence has been found in weakly bound nuclei near the neutron drip line.

Fast and nonselective on-line mass separation of neutron-rich isotopes produced in proton-induced fission

On-line mass separation of short-lived radionuclides, based on the use of thermalized primary ions from nuclear reactions, termed as an ion guide method, has recently been widely applied in the spectroscopy of both proton-rich and neutron-rich exotic nuclei. The technique has been found to be applicable to all elements and it has allowed the detection of the mass-analyzed activities with half-lives as short as 0.1 ms. The ion guide isotope separator on-line, IGISOL, has recently been used to discover over ten new short-lived isotopes or isomers produced in the nearly symmetric fission of uranium induced by 20 MeV protons. In addition to applications in nuclear-structure physics and astrophysics the present experiments can also be utilized in studies of basic collision processes between ions, atoms and electrons in weakly ionized plasmas.

Search for the exotic nucleus 10He.

A search has been conducted for the exotic neutron rich nucleus /sup 10/He among projectile fragments produced by an E/A = 30 MeV /sup 18/O beam. No /sup 10/He/sup 2+/ ions were observed. The upper limit on the production rate of /sup 10/He is determined to be less than 3 x 10/sup -5/ that of /sup 8/He.