Unbound Systems Research Articles

Resonance and continuum Gamow shell model with realistic nuclear forces

Starting from realistic nuclear forces, we have developed a core Gamow shell model which can describe resonance and continuum properties of loosely-bound or unbound nuclear systems. To describe properly resonance and continuum, the Berggren representation has been employed, which treats bound, resonant and continuum states on equal footing in a complex-momentum (complex-k) plane. To derive the model-space effective interaction based on realistic forces, the full Qˆ-box folded-diagram renormalization has been, for the first time, extended to the nondegenerate complex-k space. The CD-Bonn potential is softened by using the Vlow-k method. Choosing 16O as the inert core, we have calculated sd-shell neutron-rich oxygen isotopes, giving good descriptions of both bound and resonant states. The isotopes 25,26O are calculated to be resonant even in their ground states.

Dynamics of wide binary stars: A case study for testing Newtonian dynamics in the low acceleration regime

Extremely wide binary stars represent ideal systems to probe Newtonian dynamics in the low acceleration regimes ([Formula: see text][Formula: see text]ms[Formula: see text]) typical of the external regions of galaxies. Here, we present a study of 60 alleged wide binary stars with projected separation ranging from 0.004[Formula: see text]pc to 1[Formula: see text]pc, probing gravitational accelerations well below the limit where dark matter or modified dynamics theories set in. Radial velocities with accuracy [Formula: see text][Formula: see text]m/s were obtained for each star, in order to constrain their orbital velocity, that, together with proper motion data, can distinguish bound from unbound systems. It was found that about half of the observed pairs do have velocity in the expected range for bound systems, out of the largest separations probed here. In particular, we identified five pairs with projected separation [Formula: see text][Formula: see text]pc that are useful for the proposed test. While it would be premature to draw any conclusion about the validity of Newtonian dynamics at these low accelerations, our main result is that very wide binary stars seem to exist in the harsh environment of the solar neighborhood. This could provide a tool to test Newtonian dynamics versus modified dynamics theories in the low acceleration conditions typical of galaxies. In the near future the GAIA satellite will provide data to increase significantly the number of wide pairs that, with the appropriate follow-up spectroscopic observations, will allow the implementation of this experiment with unprecedented accuracy.

Bound and unbound nuclear systems at the drip lines: a one-dimensional model

We construct a one-dimensional toy model to describe the main features of Borromean nuclei at the continuum threshold. The model consists of a core and two valence neutrons, unbound in the mean potential, that are bound by a residual point contact density-dependent interaction. Different discretization procedures are used (harmonic oscillator and transformed harmonic oscillator bases, or use of large rigid wall box). Resulting energies and wave functions, as well as inelastic transition intensities, are compared within the different discretization techniques, as well as with the exact results in the case of one particle and with the results of the di-neutron cluster model in the two particles case. Despite its simplicity, this model includes the main physical features of the structure of Borromean nuclei in an intuitive and computationally affordable framework, and will be extended to direct reaction calculations.

Structure and decay correlations of two-neutron systems beyond the dripline

The two-neutron unbound systems of 16Be, 13Li, 10He, and 26O have been measured using the Modular Neutron Array (MoNA) and 4 Tm Sweeper magnet setup. The correlations of the 3-body decay for the 6Be and 3Li were extracted and demonstrated a strong correlated enhancement between the two neutrons. The measurement of the 10He ground state resonance from a 14Be(-2p2n) reaction provided insight into previous predictions that wavefunction of the entrance channel, projectile, can influence the observed decay energy spectrum for the unbound system. Lastly, the decay-in-target (DiT) technique was utilized to extract the lifetime of the 26O ground state. The measured lifetime of 4.5+1.1−1.5(stat.)±3(sys.) ps provides the first indication of two-neutron radioactivity.

Small asteroid system evolution

AbstractRecently, the discovery of small unbound asteroid systems called asteroid pairs have revolutionized the study of small asteroid systems. Observations with radar, photometric and direct imaging techniques have discovered that multiple asteroid systems can be divided clearly into a handful of different morphologies. Simultaneously, new theoretical advances have demonstrated that solar radiation dictates the evolution of small asteroids with strong implications for asteroid internal structure. We review our current understanding of how small asteroid systems evolve and point to the future.

Masses of unbound nuclear systems

The availability of intense secondary beams in conjunction with modern efficient detection setups allows for the production and detailed study of the most extreme nuclear systems, in terms of asymmetry of proton and neutron number, in the continuum. The extraction of even fundamental quantities like masses together with other spectroscopic information is challenging, as their determination requires disentangling the production process from structures appearing in the extracted spectra due to the final state interaction of the unbound system in the continuum that is to be described. Beta-unstable nuclei that are produced close to the drip-lines, can be used as starting point for reaching out even more into the unknown by producing unbound exotic nuclear systems e.g., in transfer and knockout reactions. Already these represent open quantum systems where properties are hard to predict by nuclear structure theory as they are strongly affected by their nearby lying continuum states. Experiments allow to measure the momenta of the reaction products and the energy of the gamma radiation emerging from the reaction zone. Information on momenta and correlations are gathered, which appear e.g., in the measured momentum vectors, relative energy spectra, the spin alignment during the reaction process, and can be further distinguished by observing dependent quantities like the profile function, denoting a momentum width as function of relative energy.In order to relate them to intrinsic properties of the unbound systems, properties of the seed nuclei are to be studied and related to the observations in the continuum. In this paper examples for the above-mentioned methods are presented. The quest for masses of their ground states is exemplified for light systems like H5,7,He7−10,Li10−13, and the most neutron-rich Oxygen systems.

Halo Nuclei: Stepping Stones Across the Dripline

The availability of intense secondary beams in conjunction with efficient detection setups allows for a production and study of the most extreme nuclear systems, in terms of asymmetry of proton and neutron number, in the continuum. They can be produced via transfer and knockout reactions, depending on beam energies, with beams of nuclei close to the driplines, exhibiting exotic properties themselves, as seeds. These nuclear open quantum systems far from the valley of beta stability challenge nuclear structure theory being as well as reaction theory that tries to describe their production mechanisms. Due to their strong clustering they exhibit a rather clean few-body character. From experiments momentum distributions, relative energy spectra, and spin alignment during the reaction can be determined, which leads to the observation of energy and angular correlations as well as dependent quantities like e.g. the profile function denoting a momentum width in dependence of relative energy. They are determined from momentum vectors of fragments and gamma radiation leaving the reaction zone. The link to intrinsic properties of these unbound systems has to be explored by gathering precise knowledge of the properties of the seed nuclei and compare them to the structures observed in the continuum. In this paper I will exemplify the above-mentioned methods, and apply them particularly to light systems like 10He, 10−13Li, and neutron-rich Beryllium systems. Furthermore, perspectives for the 7H and heavy Oxygen systems are discussed.

Halo nuclei, stepping stones across the drip-lines

The availability of intense secondary beams in conjunction with modern efficient detection setups allows for the production and detailed study of the most extreme nuclear systems, in terms of asymmetry of proton and neutron number, in the continuum. Nuclei close to the drip-lines, exhibiting exotic properties themselves, can be used as a basis in order to populate these even more exotic nuclear systems, e.g. in transfer and knockout reactions. The latter challenge nuclear structure theory by being open quantum systems far from the valley of beta stability as well as reaction aiming at a description of their production mechanisms. Experiments provide data on momentum distributions, while relative energy spectra, and spin alignment during the reaction can be extracted and lead to the observation of energy and angular correlations as well as to dependent quantities such as, e.g., the profile function denoting a momentum width as a function of relative energy. They are determined from reaction products and gamma radiation emerging from the reaction zone. The link to intrinsic properties of these unbound systems has to be explored by gathering precise knowledge of the properties of the seed nuclei and compare them to structures observed in the continuum. In this paper, examples of the above-mentioned methods are presented. The current knowledge about light systems such as 5,7H, 7−10He, 10−13Li and the most neutron-rich oxygen systems is reviewed.

The embedded cluster or association Trumpler 37 in IC 1396: a search for evolutionary constraints

It is currently widely accepted that open star clusters and stellar associations result from the evolution of embedded star clusters. Parameters such star formation efficiency, time-scale of gas removal and velocity dispersion can be determinants of their future as bound or unbound systems. Finding objects at an intermediate evolution state can provide constraints to model the embedded cluster evolution. In the HII region IC1396, Trumpler 37 is an extended young cluster that presents characteristics of an association. We employed the Two Micron All Sky Survey (2MASS) photometry to analysing its structure and stellar content, and determining its astrophysical parameters. We also analysed 11 bright-rimmed clouds in IC1396 in order to search for young infrared star clusters, and the background open star cluster Teutsch 74, to verify whether it has any contribution to the observed stellar density profile of Trumpler 37. The derived parameters and comparison with template objects from other studies lead us to conclude that Trumpler 37, rather than as a star cluster, will probably emerge from its molecular cloud as an OB association.

Density Functional Resonance Theory of Unbound Electronic Systems

Density functional resonance theory (DFRT) is a complex-scaled version of ground-state density functional theory (DFT) that allows one to calculate the in-principle exact resonance energies and lifetimes of metastable anions. In this formalism, the energy and lifetime of the lowest-energy resonance of unbound systems is encoded into a complex "density" that can be obtained via complex-coordinate scaling. This complex density is used as the primary variable in a DFRT calculation, just as the ground-state density would be used as the primary variable in DFT. As in DFT, there exists a mapping of the N-electron interacting system to a Kohn-Sham system of N noninteracting particles. This mapping facilitates self-consistent calculations with an initial guess for the complex density, as illustrated with an exactly solvable model system.

Performance Related Tests on Recycled Materials for Sustainable Design of Pavement Systems

Abstract This paper provides recommendations for performance-related tests to select recycled materials to be used in pavement foundations (base/sub-base layers). The test protocol presented in this paper will help transportation agencies, pavement design engineers and construction industry professionals to mechanistically evaluate and select sources of recycled hot-mix asphalt and portland cement concrete materials and identify factors that contribute most to the longevity of layers using recycled pavement materials. Extensive literature review was conducted on the aggregate specifications in the United States and other countries to identify the protocols used to evaluate the suitability of virgin and recycled aggregates in base and sub-base layers. The test procedures, typically designed for virgin aggregates, were modified to account for the recycled materials in the aggregate mix. The test procedures were further evaluated on the basis of mechanical performance, accuracy, practicality, complexity, precision, and test cost. The performance of the developed aggregate evaluation systems was determined through testing and analysis of 12 aggregates with different lithology and known field performance history. These samples were selected from seven states with different seasonal frost cycles. The performance tests were conducted and modifications were made to the tests procedures to better understand the mechanical behavior of aggregate systems with recycled materials. This study revealed that shear strength, toughness, abrasion, durability, and frost susceptibility influence the performance of the unbound aggregate layers. Statistical analysis of the data showed that shear strength of the aggregate systems has the most impact on the performance of the unbound systems consisted of recycled materials.

Nuclear Few-Body Physics at FAIR

The FAIR facility, to be constructed at the GSI site in Darmstadt, will be addressing a wealth of outstanding questions within the realm of subatomic, atomic and plasma physics through a combination of novel accelerators, storage rings and innovative experimental set-ups. One of the key installations is the fragment separator Super-FRS that will be able to deliver an unprecedented range of radioactive ion beams (RIBs) in the energy range of 0–1.5 GeV/u to the envisaged experiments collected within the NuSTAR collaboration. This will in particular permit new experimental investigations of nuclear few-body systems at extreme isospins, also reaching beyond the drip-lines, using the NuSTAR-R3B set-up. The outcome of pilot experiments on unbound systems are reported, as well as crucial detector upgrades.

Isospin mixing and the continuum coupling in weakly bound nuclei

The isospin-breaking effects due to the Coulomb interaction in weakly bound nuclei are studied using the Gamow shell model, a complex-energy configuration-interaction approach which simultaneously takes into account many-body correlations between valence nucleons and continuum effects. We investigate the near-threshold behavior of one-nucleon spectroscopic factors and the structure of wave functions along an isomultiplet. Illustrative calculations are carried out for the $T=1$ isobaric triplet. By using a shell-model Hamiltonian consisting of an isoscalar nuclear interaction and the Coulomb term, we demonstrate that for weakly bound or unbound systems the structure of isobaric analog states varies within the isotriplet and impacts the energy dependence of spectroscopic factors. We discuss the partial dynamical isospin symmetry present in isospin-stretched systems, despite the Coulomb interaction that gives rise to large mirror symmetry-breaking effects.

Statistical mechanics of unbound two-dimensional self-gravitating systems

We study, using both theory and molecular dynamics simulations, therelaxation dynamics of a microcanonical two-dimensional self-gravitatingsystem. After a sufficiently large time, a gravitational cluster ofN particles relaxes to the Maxwell–Boltzmann distribution. The time taken to reach thethermodynamic equilibrium, however, scales with the number of particles. In thethermodynamic limit, at fixed total mass, an equilibrium state is never reached and the system becomes trappedin a non-ergodic stationary state. An analytical theory is presented which allowsus to quantitatively describe this final stationary state, without any adjustableparameters.

Absorbing boundary conditions for dynamical many-body quantum systems

In numerical studies of the dynamics of unbound quantum mechanical systems, absorbing boundary conditions are frequently applied. Although this certainly provides a useful tool in facilitating the description of the system, its applications to systems consisting of more than one particle are problematic. This is due to the fact that all information about the system is lost upon the absorption of one particle; a formalism based solely on the Schrödinger equation is not able to describe the remainder of the system as particles are lost. Here we demonstrate how the dynamics of a quantum system with a given number of identical fermions may be described in a manner which allows for particle loss. A consistent formalism which incorporates the evolution of sub-systems with a reduced number of particles is constructed through the Lindblad equation. Specifically, the transition from an N-particle system to an (N − 1)-particle system due to a complex absorbing potential is achieved by relating the Lindblad operators to annihilation operators. The method allows for a straight forward interpretation of how many constituent particles have left the system after interaction. We illustrate the formalism using one-dimensional two-particle model problems.

Reaching the boundary between stellar kinematic groups and very wide binaries

Aims. I look for and characterise very wide binaries and multiple systems with projected physical separations larger than s = 0.1 pc, which is generally believed to be a sharp upper limit to the distribution of wide binay semimajor axes. Methods. I investigated in detail 30 Washington Double Stars with angular separations rho > 1000 arcsec. I discarded 23 of them as probably unbound systems based on discordant astrometry, photometry, spectral types, and radial velocities. The remaining seven systems were subject to a comprehensive data compilation and derivation (multi-wavelength photometry, heliocentric distance, multiplicity, age, mass, metallicity, membership in young kinematic group). Results. Of the seven very wide systems, six have projected physical separations larger than the hypothetical cutoff at s = 0.1 pc; four of them have separations s > 0.2 pc. Although there are two systems in young kinematic groups (namely HD 136654 and BD+32 2572 in the Hyades Supercluster, and AU Mic and AT Mic AB in the beta Pictoris moving group), there is not a clear prevalence of young systems (tau < 1 Ga) among these very wide binaries. Finally, I compare the binding energies of the seven systems with those of other weakly bound systems in the field.

TWO- AND MULTI-PARTICLE CORRELATIONS: ACCESS TO DYNAMICS AND STRUCTURE OF NUCLEAR SYSTEMS

Results from particle correlation measured in heavy-ion collision experiments are shown. Information about dynamics of the reaction as well as and spectroscopy of unbound nuclear systems can be extracted, offering important opportunities for future experiments with exotic nuclear beams.

Correlation studies in break-up experiments

Experimental studies addressing the investigation of light nuclei and unbound systems close and across the neutron dripline are presented. Borromean halo nuclei exhibit momentum correlations that can be found in the two- and three-body decay channels following break-up reactions at high energies in inverse kinematics, where the projectiles’ internal degrees of freedom can be treated as frozen. It is shown that correlation measurements, performed through kinematically complete experiments, are sensitive tools and allow to extract valuable information on ground state properties and continuum excitations.

Extended Gibbs ensembles with flow

A recently proposed [Ph. Chomaz, F. Gulminelli, and O. Juillet, Ann. Phys. (Paris) 320, 135 (2005)] statistical treatment of finite unbound systems in the presence of collective motions is applied to a classical Lennard-Jones system, numerically simulated through molecular dynamics. In the ideal gas limit, the flow dynamics can be exactly recast into effective time-dependent Lagrange parameters acting on a standard Gibbs ensemble with an extra total energy conservation constraint. Using this same ansatz for the low-density freeze-out configurations of an interacting expanding system, we show that the presence of flow can have a sizable effect on the microstate distribution.

Particle-unstable and weakly-bound light nuclei with a Sturmian approach that preserves the Pauli principle

Sturmian theory for nucleon-nucleus scattering is discussed in the presence of all the phenomenological ingredients necessary for the description of weakly-bound (or particle-unstable) light nuclear systems. Currently, we use a macroscopic potential model of collective nature. The analysis shows that the couplings to low-energy collective-core excitations are fundamental but they are physically meaningful only if the constraints introduced by the Pauli principle are taken into account. The formalism leads one to discuss a new concept, Pauli hindrance, which appears to be important to understand the structure of weakly-bound and unbound systems.