Light Clusters Research Articles

Tracing the evolution of temperature in near Fermi energy heavy ion collisions

The kinetic energy variation of emitted light clusters has been employed as a clock to explore the time evolution of the temperature for thermalizing composite systems produced in the reactions of 26A, 35A and 47A MeV $^{64}$Zn with $^{58}$Ni, $^{92}$Mo and $^{197}$Au. For each system investigated, the double isotope ratio temperature curve exhibits a high maximum apparent temperature, in the range of 10-25 MeV, at high ejectile velocity. These maximum values increase with increasing projectile energy and decrease with increasing target mass. The time at which the maximum in the temperature curve is reached ranges from 80 to 130 fm/c after contact. For each different target, the subsequent cooling curves for all three projectile energies are quite similar. Temperatures comparable to those of limiting temperature systematics are reached 30 to 40 fm/c after the times corresponding to the maxima, at a time when AMD-V transport model calculations predict entry into the final evaporative or fragmentation stage of de-excitation of the hot composite systems. Evidence for the establishment of thermal and chemical equilibrium is discussed.

Rapidity spectra of the secondaries produced in heavy ion collisions and the constituent picture of the particles

The present study is both a reanalysis and an extension of the approach initiated by S. Eremin and S. Voloshin [Phys. Rev. C, 67, 064905 (2003)]. We attempt to interpret here the rapidity spectra of the various particles produced in both $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Au}+\mathrm{Au}$ collisions at SPS-CERN (Super Proton Synchrotron at Conseil Europ\'een pour la Recherche Nucl\'eaire) and RHIC-BNL (Relativistic Heavy Ion Collider at Brookhaven National Laboratory). The study made here is wider in scope and is more species specific than earlier ones, and the results obtained here are compared with those found in previous works based on HIJING, VENUS, etc., at various centralities. The study reconfirms that the constituent parton picture of the particles provides a better and more unified description of the rapidity-density yields for the various secondaries, including some light cluster particles such as deuteron, even in heavy ion collisions.

Mechanism of light cluster production in nucleon induced reactions at intermediate energy

A dynamical model dedicated to the nucleon induced reaction between 30 and 150 MeV is presented. It considers different stages of the reaction: the approaching phase, the in-medium nucleon-nucleon collisions, the cluster formation, and the secondary deexcitation process. The notions of influence area and phase-space exploration during the reaction are introduced. The importance of the reaction geometry and the conservation laws is emphasized. The model is able to globally reproduce the absolute cross sections for the emission of neutron and light charged particles for proton- and neutron induced reactions on heavy and intermediate targets ({sup 56}Fe and {sup 208}Pb)

Spectroscopic factors and cluster decay half-lives of heavy nuclei

The cluster radioactivity is treated under the assumption that the light clusters are produced by a collective motion of the nuclear system in the charge asymmetry coordinate with further penetration through the Coulomb barrier. The known experimental data on cluster radioactivity are described. The half-lives of cluster emission of neutron-deficient actinides and medium-mass nuclei are predicted.

Cluster emission in superdeformed Sr isotopes in the ground state and formed in heavy-ion reaction

Cluster decay of superdeformed76, 78, 80Sr isotopes in their ground state are studied taking the Coulomb and proximity potential as the interacting barrier for the post-scission region. The predictedT 1/2 values are found to be in close agreement with those values reported by the preformed cluster model (PCM). Our calculation shows that these nuclei are stable against both light and heavy cluster emissions. We studied the decay of these nuclei produced as an excited compound system in heavy-ion reaction. It is found that inclusion of excitation energy increases the decay rate (decreasesT 1/2 value) considerably and these nuclei become unstable against decay. These findings support earlier observation of Guptaet al based on PCM.

Light clusters in nuclear matter of finite temperature

We investigate properties and the distribution of light nuclei (A≤4) in symmetric nuclear matter of finite temperature within a microscopic framework. For this purpose we have solved few-body Alt-Grassberger-Sandhas-type equations for quasi-nucleons that include self-energy corrections and Pauli blocking in a systematic way. In a statistical model we find a significant influence in the composition of nuclear matter if medium effects are included in the microscopic calculation of nuclei. If multiplicities are frozen out at a certain time (or volume), we expect significant consequences for the formation of light fragments in a heavy ion collision. As a consequence of the systematic inclusion of medium effects, the ordering of multiplicities becomes opposite to the law-of-mass action of ideal components. This is necessary to explain the large abundance of α-particles in a heavy ion collision that are otherwise largely suppressed in an ideal equilibrium scenario.

Patterns of gene divergence and VL promoter activity in immunoglobulin light chain clusters of the channel catfish.

The structure and genomic organization of V, J, and C segments in multiple gene clusters of the G class of catfish light (L) chain were determined to study evolutionary patterns of cluster divergence and regulatory function. The results showed that the organizational pattern is conserved; two VL segments reside in opposite transcriptional orientation to a pseudogene JL segment (psiJ), a functional JL segment, and a CL segment. Structures within the central V-psiJ-J-C regions indicate that cluster duplication occurred after the V-V-psiJ-J-C organization became fixed. VL divergence in gene clusters subsequently occurred by mechanisms that principally targeted complementarity-determining regions. The sequence of the VL-flanking regions, which contained regulatory octamer, TATA, and Pax-5 (BSAP) binding site motifs, was conserved during G cluster duplication and within VL-flanking regions in divergent lineages of bony fish. Reporter assays of catfish B cells transfected with a 742-bp VL-flanking fragment showed promoter activity in the absence of enhancer elements. The promoter's activity doubled when coupled with the catfish IgH enhancer. A 136-bp fragment containing the motifs conserved in bony fish phylogeny and located between the leader initiation codon and the initiation sites of sterile transcripts served as a minimal promoter and provided the highest B-cell activity. These constructs, however, did not act as promoters in catfish non-lymphoid cells or mammalian BJA-B B cells or fibroblasts. These results indicate that the structure and function of VL promoter regions in the regulation and tissue specificity of L-chain gene expression evolved early in phylogeny.

Dissipative and fluctuating dynamical description of nuclear reactions

We address the description of nuclear dynamics at intermediate energies within the DYWAN model. New fluctuation-dissipation aspects are discussed and confronted with experimental results in nucleus-nucleus and in nucleon-nucleus reactions. We show that the implemented treatment of fluctuations provides with a convenient tool to investigate the main characteristics of particle and light clusters emission in the intermediate energy range.

Infrared electron modes in light deformed clusters

Infrared quadrupole modes (IRQM) of the valence electrons in light deformed sodium clusters are studied by means of the time-dependent local-density approximation (TDLDA). IRQM are classified by angular momentum components $\lambda\mu =$20, 21 and 22 whose $\mu$ branches are separated by cluster deformation. In light clusters with a low spectral density, IRQM are unambiguously related to specific electron-hole excitations, thus giving access to the single-electron spectrum near the Fermi surface (HOMO-LUMO region). Most of IRQM are determined by cluster deformation and so can serve as a sensitive probe of the deformation effects in the mean field. The IRQM branch $\lambda\mu =$21 is coupled with the magnetic scissors mode, which gives a chance to detect the latter. We discuss two-photon processes, Raman scattering (RS), stimulated emission pumping (SEP), and stimulated adiabatic Raman passage (STIRAP), as the relevant tools to observe IRQM. A new method to detect the IRQM population in clusters is proposed.

Ground‐state geometry and electronic structure of light atom clusters, especially H isotopes, Li, B, and C

AbstractThe ground‐state geometry and electronic structure of light clusters remains an area of considerable current interest for both many‐body theory and experiment. We cite here recent experimental studies of H isotopes that may have important implications for energy production. Therefore a summary will first be given of quantum‐chemical results on the singly charged cationic clusters H(H2)m for some values of m ≤ 14, studied by Gobet et al. (2002). More recent theoretical work on doubly charged D clusters is also reported. Some attention is then given to results on small Li clusters, isolated in argon matrices by Kornath et al. (2003), in relation to many‐body theory. This is followed by a study of almost‐spherical cages of B and of C atoms: both neutral and charged clusters again being referred to in the case of boron. Finally, some mixtures of light atoms are considered, with special focus on BN cages and cationic hydrocarbons. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Spectroscopic factors of cluster decays in an algebraic cluster model

We propose a new ansatz for the cluster spectroscopic factor in an algebraic cluster model. The parameters of the ansatz are fitted to the experimental and Blendowske–Walliser values of the spectroscopic factor for light emitted clusters and then applied to predict the spectroscopic factors for heavier clusters up to the spontaneous cold fission in two equal fragments. We show that due to the cluster and daughter shell structure, the calculated spectroscopic factors are strongly deviating from the empirical law for increasing cluster number. The observed gap between the alpha-like and cluster-like fission on one side and the spontaneous cold fission on the other side for actinides is explained by the moderate increase of spectroscopic factors, as computed in the present Letter, combined with the simultaneous decrease of the barriers when approaching symmetric mass division. This conclusion is inferred by computing the half-lives of cluster emission and spontaneous cold fission for 234U.

A new model for production of fast light clusters in spallation reactions

A model is proposed to describe emission of light charged clusters during the cascade stage in nucleon-induced spallation reactions. It consists in implementing a surface percolation procedure into the Liège intranuclear cascade (INCL4) model: when a nucleon is ready to leave the nuclear surface, it is allowed to drag along a cluster of nucleons, under some conditions of closeness in phase space. This possibility relies on the instantaneous dynamical phase space occupancy in the nuclear surface. The following clusters are considered: d,t, 3 He, 4 He . Good agreement is obtained with experimental data relative to heavy and medium-heavy targets at two different energies. It is shown that the implementation of light cluster emission in the cascade stage also improves our previous results for residue mass spectra.

Effects of momentum-dependent nuclear potential on two-nucleon correlation functions and light cluster production in intermediate energy heavy-ion collisions

Using an isospin- and momentum-dependent transport model, we study the effects due to the momentum dependence of isoscalar nuclear potential as well as that of symmetry potential on two-nucleon correlation functions and light cluster production in intermediate energy heavy-ion collisions induced by neutron-rich nuclei. It is found that both observables are affected significantly by the momentum dependence of nuclear potential, leading to a reduction of their sensitivity to the stiffness of nuclear symmetry energy. However, the t/$^{3}$He ratio remains a sensitive probe of the density dependence of nuclear symmetry energy.

A new model for production of fast light clusters inBspallationBreactions

A model is proposed to describe emission of light charged clusters during the cascade stage in nucleon-induced spallation reactions. It consists in implementing a surface percolation procedure into the Liège intranuclear cascade (INCL4) model: when a nucleon is ready to leave the nuclear surface, it is allowed to drag along a cluster of nucleons, under some conditions of closeness in phase space. This possibility relies on the instantaneous dynamical phase space occupancy in the nuclear surface. The following clusters are considered: d,t, 3 He , 4 He . Good agreement is obtained with experimental data relative to heavy and medium-heavy targets at two different energies. It is shown that the implementation of light cluster emission in the cascade stage also improves our previous results for residue mass spectra.

A Review of the Mechanism of Photographic Sensitivity and Roles of Silver Clusters

Recent progress in the mechanism of latent image formation and roles of silver clusters in photographic sensitivity is reviewed with respect to classification of silver clusters: comparison between light and reduction clusters; preferential formation of reduction sensitization R centers to P centers; difference between latent sub-image centers and P centers; characterization of R and P centers by photoconductivity measurements; utility of R centers with two electron sensitization; and direct observation of latent image and fog centers.

Orbital magnetism in axially deformed sodium clusters

Low-energy orbital magnetic dipole excitations, known as scissors mode (SM), are studied in alkali metal clusters. Subsequent dynamic and static effects are explored. The treatment is based on a self-consistent microscopic approach using the jellium approximation for the ionic background and the Kohn-Sham mean field for the electrons. The microscopic origin of SM and its main features (structure of the mode in light and medium clusters, separation into low- and high-energy plasmons, coupling high-energy M1 scissors and E2 quadrupole plasmons, contributions of shape isomers, etc) are discussed. The scissors M1 strength acquires large values with increasing cluster size. The mode is responsible for the van Vleck paramagnetism of spin-saturated clusters. Quantum shell effects induce a fragile interplay between Langevin diamagnetism and van Vleck paramagnetism and lead to a remarkable dia-para anisotropy in magnetic susceptibility of particular light clusters. Finally, several routes for observing the SM experimentally are discussed.

Angular Distribution in Ternary Cold Fission

We describe the spontaneous ternary cold fission of 252Cf, accompanied by 4He, 10Be and 14C. The light cluster decays from the first resonant eigenstate in the Coulomb potential plus a harmonic oscillator potential. We have shown that the angular distribution of the emitted light particle is strongly connected with its deformation and the equatorial distance.

Light cluster production in intermediate energy heavy-ion collisions induced by neutron-rich nuclei

The coalescence model based on nucleon distribution functions from an isospin-dependent transport model is used to study the production of deuteron, triton, and 3He from heavy-ion collisions induced by neutron-rich nuclei at intermediate energies. It is found that the emission time of these light clusters depends on their masses. For clusters with same momentum per nucleon, heavier ones are emitted earlier. Both the yield and energy spectrum of light clusters are sensitive to the density dependence of nuclear symmetry energy, with more light clusters produced in the case of a stiff symmetry energy. On the other hand, effects due to the stiffness of the isoscalar part of nuclear equation of state and the medium dependence of nucleon–nucleon cross sections on light cluster production are unimportant. We have also studied the correlation functions of clusters, and they are affected by the density dependence of nuclear symmetry energy as well, with the stiff symmetry energy giving a stronger anti-correlation of light clusters, particularly for those with large kinetic energies. Dependence of light cluster production on the centrality and incident energy of heavy-ion collisions as well as the mass of the reaction system is also investigated.

Silver Clusters of Photographic Interest VIII: Electron Microscopic Observation and Analysis of Ag Clusters Formed on AgBr Grains by Light and Reduction

An electron microscopic method was applied to the quantitative analysis of silver clusters formed on fine cubic AgBr grains by light and reduction. The observation of only one light cluster, i.e., one latent image center, per grain and many reduction clusters per grain indicated a difference in the mechanism of cluster formation, i.e., the presence and absence of the concentration principle, respectively. We concluded that the quantum yield for the photochemical formation of clusters could be unity if positive holes were prevented from reacting with growing clusters. This was consistent with reported quantum yields for print-out silver on the basis of chemical analysis, and therefore proved the applicability of the present method for the quantitative analysis. It was also found that the formation of reduction clusters on treatment of AgBr grains with DMAB occurred nearly quantitatively. The analysis of size distribution of reduction clusters gave additional support for the proposal that DMAB formed dimers of silver atoms, which were stabilized to act as reduction sensitization centers at surface kink sites, but which aggregated to form large clusters when their amount was larger than that for the saturation of the kink sites.

Light clusters production as a probe to nuclear symmetry energy

Using an isospin-dependent transport model for heavy-ion collisions induced by neutron-rich nuclei at intermediate energies, we study the production of light clusters such as deuteron, triton, and $^{3}$He via coalescence of nucleons. We find that both the yields and energy spectra of these light clusters are affected significantly by the density dependence of nuclear symmetry energy, with a stiffer symmetry energy giving a larger yield.