Mass Nuclei Research Articles

Gravitational loss-cone instability

We study physical corollaries of the existing analogy between the simplest plasma traps (mirror traps) and star clusters surrounding massive black holes or dense galactic nuclei. There is a loss cone in the system through which plasma particles with low velocities transverse to the trap axis or, similarly, stars with low angular momenta (destroyed or absorbed by the central body) escape. The consequences of the “beam-like” deformation of the plasma distribution function in a trap are well known: a peculiar loss-cone instability producing a plasma flow into the loss cone develops as a result. We show that a similar gravitational loss-cone instability can also arise under certain conditions in the galactic case of interest to us. This instability is related to the slow precessional motions of highly elongated (nearly radial) stellar orbits and the main condition for its growth is that the precession of such orbits be retrograde (in the direction opposite to the orbital rotation of stars). Only under this condition do oscillations that can become unstable in the presence of a loss cone arise instead of the radial orbit instability (a variety of the Jeans instability in systems with highly elongated orbits) that takes place in the case of prograde precession. The instability produces a stream of stars onto the galactic center, i.e., serves as a mechanism of fueling the nuclear activity of galaxies. For a mathematical analysis, we have obtained relatively simple characteristic equations that describe small perturbations in a sphere of radially highly elongated stellar orbits. These characteristic equations are derived through a number of successive simplifications from a general linearized system of equations, including the collisionless Boltzmann kinetic equation and the Poisson equation (in action-angle variables). The central point of our analysis of the characteristic equations is preliminary detection of neutral modes (or proof of their absence in the case of stability).

Formation of heavy compound nuclei, their survival, and correlation with longtime-scale fission

Fusion of two massive nuclei with formation of a superheavy compound nucleus is driven by the potential energy gradient, as follows from the analysis of nuclear reaction cross sections. The conservative energy of the system is deduced in a simple approximation using regularized nuclear mass and interaction barrier values. Different reactions for the synthesis of Z = 110−118 nuclei are compared and favorable conditions are found for fusion of the stable W-Pt isotopes with radioactive fission fragment projectiles, like 94Kr or 100Sr. Thus, the cold-fusion method can be extended for a synthesis of elements with Z > 113. Survival of the evaporation residue is defined by the neutron-to-fission probability ratio and by the successful emission of gammas at the final step of the reaction. Numerical estimates are presented. Fixation of evaporation residue products must correlate with longtime-scale fission, and available experimental results are discussed.

Gamow-Teller transitions in pf-shell nuclei studied in ( 3He, t) reactions

The Gamow-Teller transition strengths in pf-shell nuclei are important parameters in a presupernova model. A high resolution (He-3,t) reaction was performed from T-z = +1 nuclei (Ca-42, Ti-46, Cr-50 and Fe-14) to T-z = 0 nuclei ((SC)-S-42, V-46, Mn-50 and Co-54) at an incident energy of 140 MeV/nucleon and at 0 degrees for the precise study of GT transition strengths B(GT). To derive B(GT) values for these pf-shell nuclei, the R-2 value, the ratio between the GT and Fermi unit cross sections, was introduced. The R-2 values were studied for various mass nuclei by using the B(GT) values from beta-decay studies as standard, and the mass dependence was derived. It was found that GT strengths in these four pf-shell nuclei move to higher excitation energies as the mass number A increases. The obtained GT strength distributions were compared with shell model calculations.

Quenching of the Giant Dipole Resonance Strength at High Excitation Energy

The evolution with excitation energy of the Giant Dipole Resonance features in nuclei of mass A ≈ 108 − 136 is reviewed. We first discuss the results of the experiments performed with MEDEA studying the GDR gamma decay from hot nuclei populated at excitation energies above 300 MeV. The focus of the paper is on the excitation energy region between 160 and 290 MeV. This region has been investigated through the study of the reactions 116 Sn + 12 C at 17 and 23 A MeV, and 116 Sn + 24 Mg at 17 A MeV. Gamma-rays were detected using MEDEA in coincidence with evaporation residues detected in MACISTE. The analysis of the gamma-ray spectra and their comparison with statistical calculations are presented. The comparison with γ -ray spectra from the reaction 36 Ar + 98 Mo at higher excitation energies shows a coherent scenario where a progressive reduction of γ multiplicity relative to predictions for 100% of the Energy Weighted Sum Rule is observed above 200 MeV excitation energy. Finally, the existence of a link between disappearance of collective motion and the liquid-gas phase transitions is discussed.

HASSIUM-270 IS LONG-LIVED

A NEWLY DISCOVERED radioactive isotope of element 108 hangs around for a half-minute before falling apart. Compared with the fleeting lifetimes of their superheavy atomic neighbors, the roughly 30-second period that transpired from the creation to disintegration of four atoms of hassium-270 qualifies those atoms as rock solid. Theoretical physicists predicted years ago that superheavy nuclei with 114 protons and 184 neutrons should survive for a relatively long time if they could be synthesized. The nuclear longevity, according to the models, is due to the closing of proton and neutron shells, which renders the particles stable against spontaneous fission. Experimentalists, though, haven't yet found a route to prepare such massive nuclei, which are described as lying at the center of an island of nuclear stability. Other theoreticians calculated the effects of sub-shell closings and concluded that an isotope of hassium containing 162 neutrons (270 Hs) should also be fairly stable—surviving much longer tha...

Capture cross sections for the near symmetricSn124+Zr96reaction

Capture-fission cross sections were measured for the near symmetric reaction between the massive nuclei {sup 124}Sn and {sup 96}Zr for center of mass energies from 195 to 265 MeV. Coincident fission fragments were detected and separated from elastic and deep inelastic scattering products by angle/energy/mass conditions. The measured capture cross sections agree quite well with calculations using the dinuclear system (DNS) model. The DNS model also predicts the fusion cross section for this reaction with a fusion barrier height of 208.0 MeV. The deduced extra push energy, corresponding to this barrier height, differs from that deduced from evaporation residue measurements.

Report on the dynamical evolution of an axially symmetric quasar model

The role of the angular momentum in the regular or chaotic character of motion in an axially symmetric quasar model is examined. It is found that, for a given value of the critical angular momentumLzc, there are two values of the mass of the nucleusMn for which transition from regular to chaotic motion occurs. The [Lzc– Mn] relationship shows a linear dependence for the time independent model and an exponential dependence for the evolving model. Both cases are explained using theoretical arguments together with some numerical evidence. The evolution of the orbits is studied, as mass is transported from the disk to the nucleus. The results are compared with the outcomes derived for galactic models with massive nuclei.

NECK FORMATION IN FUSION DYNAMICS OF MASSIVE NUCLEI

Dynamics of fusion of massive nuclei is described by a simple diffusion model in terms of the two-center shape parameterizations {z, ε} and the fusion probability is calculated by a Brownian particle passing over the top of a parabolic potential along the elongation coordinate z. The neck variable ε at initial shape is supposed to be a Gaussian distribution after a transitive process, and the average initial kinetic energy of the fusing system is determined by energy conservation. The fusion probabilities of several symmetric or nearly symmetric systems in central collisions are calculated as functions of the center-of-mass energy and compared with the experimental data. Effects of energy loss and neck folding on the fusion hindrance are discussed.

Constraints on the accretion history of massive black holes from faint X-ray counts

We investigate how hierarchical models for the co-evolution of the massive black hole (MBH) and active galactic nucleus (AGN) population can reproduce the observed faint X-ray counts. We find that the main variable influencing the theoretical predictions is the Eddington ratio of accreting sources. We compare three different models proposed for the evolution of an AGN Eddington ratio, f Edd : constant/f Edd = 1, f Edd decreasing with redshift and/f Edd depending on the AGN luminosity, as suggested by simulations of galactic mergers including MBHs and AGN feedback. We follow the full assembly of MBHs and host haloes from early times to the present in a A cold dark matter (ACDM) cosmology. An AGN activity is triggered by halo major mergers and MBH accrete mass until they satisfy the observed correlation with velocity dispersion. We find that all the three models can reproduce fairly well the total faint X-ray counts. The redshift distribution is, however, poorly matched in the first two models. The Eddington ratios suggested by merger simulations predict no turn-off of the faint end of the AGN optical luminosity function at redshifts z? 1, down to very low luminosity.

The Granada workshop on High Redshift Radio Galaxies: An overview

AbstractThe Granada workshop on High Redshift Radio Galaxies (HzRGs) gave an excellent overview of the progress that has been made in this field during the last 3 years. Here we briefly review some of the results, with an emphasis on what studies of HzRGs can teach us about the formation and evolution of massive galaxies, clusters and active galactic nuclei (AGN). Of great relevance for this workshop are scenarios that describe certain aspects of the evolution of radio galaxies, including (i) the sequence of events after merging of galaxies that ultimately lead to extended powerful radio sources and (ii) the mass assembly and virialization of the hosting massive galaxies and their associated (proto‐)clusters. Furthermore, I briefly discuss two projects that are important for a further understanding of AGN and high redshift radio galaxies. First, using the MIDI instrument mounted on the VLT Interferometer, the dusty tori of nearby AGN can be studied in the range of 8–13 micron at high angular resolution. The first result on the nearby AGN NGC 1068 as presented by Jaffe et al. (2004) indicated the presence of a hot (T > 800 K), compact (≲1 pc) component, possible identified with the base of the jet and a warm (270 K), well‐resolved (3 × 4 pc) component associated with the alleged torus. Second, LOFAR is a new low frequency radio telescope that is currently being build in the Netherlands and is expected to be operational in 2008. With 50 stations spread over an area of 100 km in diameter, its resolution and sensitivity will be unprecedented in the frequency range 10–240 MHz. LOFAR will be a unique instrument that will impact a broad range of astrophysical topics varying from the epoch of reionisation, to gamma ray bursts and cosmic rays. Surveys with LOFAR will be of paramount importance for studies of HzRGs: It will enable (i) defining samples of radio galaxies with redshifts higher than 6, (ii) observations of starbursting galaxies in proto‐clusters, and (iii) mapping out the low‐frequency radio emission of virtually all northern radio‐loud AGN in revolutionary detail. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Shape coexistence in the very neutron-rich odd-oddRb96

Microsecond isomers of neutron-rich nuclei in masses $A=96$ and 98 were reinvestigated at the Institut Laue-Langevin reactor (Grenoble). These nuclei were produced by thermal-neutron induced fission of $^{241}\mathrm{Pu}$. The detection is based on the time correlation between fission fragments selected by the Lohengrin mass spectrometer and the \ensuremath{\gamma} rays and conversion electrons from the isomers. A new level scheme of $^{96}\mathrm{Rb}$ is proposed. We have found that the ground state and low-lying levels of this nucleus are rather spherical, and a rotational band develops at 461-keV energy. This band has properties consistent with a $\ensuremath{\pi}[431\phantom{\rule{0.3em}{0ex}}3/2]\ifmmode\times\else\texttimes\fi{}\ensuremath{\nu}[541\phantom{\rule{0.3em}{0ex}}3/2]K={3}^{\ensuremath{-}}$ Nilsson assignment and a deformation ${\ensuremath{\beta}}_{2}>0.28$. It is fed by a ${I}^{\ensuremath{\pi}}={10}^{\ensuremath{-}}$ microsecond isomer consistent with a $\ensuremath{\pi}({g}_{9/2})\ensuremath{\nu}({h}_{11/2})$ spherical configuration. It is interesting to note that the same unique-parity states $\ensuremath{\pi}({g}_{9/2})$ and $\ensuremath{\nu}({h}_{11/2})$ are present in the same nucleus in a deformed and in a spherical configuration. The neighboring odd-odd nucleus $^{98}\mathrm{Y}$ presents a strong analogy with $^{96}\mathrm{Rb}$ and is also discussed.

Orbits in disc galaxies with rapidly rotating massive nuclei: comparison with data from observations

An axially symmetric galactic dynamic model is used, with a disc, a massive nucleus and halo components, in order to reproduce recently obtained observation data in disc galaxies. The model reproduces very well two basic physical quantities, i.e. the high-velocity rotation and the high surface mass density in the central regions of some galaxies such as the galaxy NGC 3079. Going a step further, we investigate the properties of orbits in the model, the behaviour of the velocities of the system and their connections with the value of the conserved L z component of the angular momentum. The regular or chaotic nature of motion is also studied. All the above studies are made in two different cases: firstly, when the model is time independent and, secondly, when the model evolves with time.

Anomalous quantum diffusion over a saddle point and application to fusion of massive nuclei

Abstract The quantal diffusion of a particle with an initial velocity coupled to a non-Ohmic environment passing over a potential top is considered. It is shown that sub- and super-diffusions hinder the directional motion of the particle in two different ways. The former makes the particle have a strong memory to its initial position rather than initial velocity and the latter increases the randomness of motion. The quantum fluctuation helps the particle with a small initial velocity to cross the saddle point, however, it is harmful to the directional motion of the particle with a large initial velocity. The present model is applied to calculate the fusion probability of massive nuclei and the fusion probability curve increases slowly with the center-of-mass energy. This is in agreement with the experimental fact.

Chaotic orbits of distant stars

A disk galaxy model with a dense nucleus and additional perturbing terms is used for the study of properties of orbits of distant stars. Our numerical experiments show that the majority of distant stars are in chaotic orbits. There are distant stars displaying regular orbits as well. A number of distant stars are ejected to the halo on approaching the dense and massive nucleus. A polynomial relationship exists between the mass of the nucleus and the critical angular momentum of the distant stars. Numerical calculations suggest that the majority of distant stars spend their orbital time in the halo where it is easy to be observed. We present evidence that the dominant term for driving stars to distant orbits is the presence of the dense nucleus combined with the perturbation caused by nearby galaxies. The origin of young OB stars observed in the halo is also discussed.

Chaos in a quasar model with a disk and a massive nucleus

AbstractWe present a quasar model with a rotating disk and a massive nucleus. We use this model in order to characterize the motion in the model (regular or chaotic) and to connect the extent of the chaotic regions to the physical parameters of the model. Numerical experiments suggest that, there are connections between the extent of the chaotic areas and the parameters of the system. Furthermore, it is shown that the form of numerically found relationships can be expressed analytically. Comparison to previous work is also made. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of the Entrance Channel on the Fission of the Compound Nucleus

The effects of entrance channel with different projectile and target nuclei combinations on the fission of the 220 90 Th * , 254 102 No * and 256 104 Rf * compound nuclei are investigated. The dependence of the de-excitation of the compound nucleus on the dynamics of the entrance channel is analyzed for the first time. We find that the fission branching ratio Γ f / Γ tot of the compound nucleus formed in the two different reactions with massive nuclei is different due to the dynamical effects in the entrance channel even at the same excitation energy E * . It is caused by the different partial fusion cross section σ ℓ fus ( E ) for those reactions. Consequently, the excitation function of evaporation residues measured in such reactions is strongly related to the effect of the entrance channel. This phenomenon is explained by the dependence of the capture and fusion cross sections on the orbital angular momentum of collision.

Laser-induced phase separation in the nickel–fullerene nano-composite

Metal–fullerene (Ni–C 60) thin film has been synthesized by co-deposition of Ni and C 60 on the Ni/Si backing (i.e. on the Ni-coated Si wafer). The hybrid Ni+C 60 mixture (with C 60 affluent component) was grown as a nano-structured composite which consists of the dispersed Ni clusters and the complex C 60/a-C-based matrix (a-C—amorphous carbon). The laser beam irradiation (Nd:YAG, λ=1064 nm, τ=8 ns) led to an unusual restructuring of the hybrid matter. In an area restricted by a small aperture, the single-pulse irradiation resulted in formation of a two-stage protrusion encircled with a-C ring and C 60-depleted halo. On the other hand, a broad defocused beam irradiation resulted in extensive Ni/C 60(a-C) phase separation with interspersed Ni- and C 60(a-C)-rich zones. Surprisingly in the C 60(a-C)-rich areas, a number of micrometer-sized periodic surface patterns (so-called LIPSS—laser-induced periodic surface structure) were observed. They consist of a massive central nucleus (made of a-C) surrounded with an array of periodic domains (made of C 60(a-C)–Ni). We suggest that the formation of the LIPSS patterns of second kind is due to the coordinated relaxation of the stressed structure during the rapid dissipation of the laser beam energy.

Nuclear collisions induced by single-cycle laser pulses: Molecular approach to fusion

We study the possibility for energetic nuclear collisions in a gas of molecules driven by ultra-intense and ultra-short laser pulses. The idea is to use the fact that nuclei in a molecule can be pre-aligned in front of each other and can be driven into each other by a very strong (field intensities approaching 1023 W/cm2) and ultra-short (few-femtosecond, single-cycle) laser pulse, since the nuclei of different masses will acquire different velocities when driven by the same electric field. The nuclei may collide with high kinetic energy needed to overcome the Coulomb barrier, and these collisions may lead to fusion. We have performed classical simulation of nuclear motion and a simple statistical ensemble calculation. Our results show that nuclear collisions may occur on a sub-attosecond time scale, during which the nuclei will experience large acceleration and emit zeptosecond bursts of light.

Perturbation Analysis and Simulation Study of the Effects of Phase on the Classical Hydrogen Atom Interacting with Circularly Polarized Electromagnetic Radiation

The classical hydrogen atom is examined for the situation where a circularly polarized electromagnetic plane wave acts on a classical charged point particle in a near-circular orbit about an infinitely massive nucleus, with the plane wave normally incident to the plane of the orbit. The effect of the phase α of the polarized wave in relation to the velocity vector of the classical electron is examined in detail by carrying out a perturbation analysis and then comparing results using simulation methods. By expanding the variational parts of the radius and angular velocity about their average values, simpler nonlinear differential equations of motion are obtained that still retain the key features of the oscillating amplitude, namely, the gradual increase of the envelope of the oscillating amplitude and the point of rapid orbital decay. Also, as shown here, these key features carry over nicely to conventional quantities of interest such as energy and angular momentum. The phase α is shown here to have both subtle yet very significant effects on the quasistability of the orbital motion. A far wider range of phase conditions are found to provide stability than might intuitively be expected, with the time to orbital decay, td, varying by orders of magnitude for any plane wave with an amplitude A above a critical value, Ac.

Surface Hydrogen-burning Modeling of Supersoft X-Ray Binaries: Are They Type Ia Supernova Progenitors?

Nova explosions occur on the white dwarf (WD) component of a cataclysmic variable stellar system that is accreting matter lost by a companion. A Type Ia supernova (SN Ia) explosion is thought to result when a WD, in a similar binary configuration, grows in mass to the Chandrasekhar limit. Here, we present calculations of accretion of solar matter, at a variety of mass accretion rates, onto hot (2.3 × 105 K), luminous (30 L☉), massive (1.25, 1.35 M☉) carbon-oxygen WDs. In contrast to our nova simulations, where the WD has a low initial luminosity and a thermonuclear runaway (TNR) occurs and ejects material, these simulations do not eject material (or only a small fraction of the accreted material), and the WD grows in mass. A hydrogen TNR does not occur because hydrogen fuses to helium in the surface layers, and we call this process surface hydrogen burning (SHB). As the helium layer grows in mass, it gradually fuses either to carbon and oxygen or to more massive nuclei, depending on the WD mass and mass accretion rate. If such a WD were to explode in a SN Ia event, therefore, it would show neither hydrogen nor helium in its spectrum as is observed. Moreover, the luminosities and effective temperatures of our simulations agree with the observations of some of the supersoft X-ray binary sources, and therefore, our results strengthen previous speculation that some of them (CAL 83 and CAL 87, for example) are probably progenitors of SN Ia explosions. Finally, we have achieved SHB for values of the mass accretion rate that almost span the observed values of the cataclysmic variables.