Pb Reaction Research Articles

Investigation of neutron skin effect with density dependence by using a new calculation method for initial exciton numbers on pre-equilibrium reactions

In this study, we investigated the pre-equilibrium effect by using a new evaluated geometry-dependent hybrid model and a neutron skin effect with density dependence for the 204,206,207Pb(p, xn) reaction at 25.5 MeV incident proton energy. We calculated the initial exciton numbers obtained from the modified Skyrme force based on fitting the fission barriers of heavy deformed nuclei (SKM*) and SLy4 for proton induced reaction on target nuclei 204,206,207Pb. We calculated, by using a new calculation method, the initial neutron and proton exciton numbers for 204,206,207Pb from the neutron and proton density and we investigated the shell effect by using an effective nucleon–nucleon interaction with Skyrme force. We also compared the geometry-dependent hybrid model (GDH), newly evaluated with the initial exciton number calculations of the neutron emission spectra of the 204,206,207Pb(p, xn) reaction, with the values reported in the literature at 25.5 MeV incident proton energy. The obtained results were investigated and compared with the pre-equilibrium calculations and experimental results.

Centrality and system size dependence of (multi-strange) hyperons at40Aand158A GeV: A comparison between a binary collision model and a Boltzmann+hydrodynamic hybrid model

We present results on the centrality and system size dependence of (multi-strange)hyperons in Pb+Pb collisions at 40$A$ and 158$A$ GeV from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD-v2.3) model and a coupled Boltzmann+hydrodynamics calculation. The second approach is realized in a hybrid fashion based on UrQMD with an intermediate hydrodynamical evolution for the hot and dense stage of the collision. This implementation allows a comparison of microscopic transport calculations with hydrodynamic simulations to explore the transition from a system that is not fully equilibrated such as C+C or Si+Si collisions, to a supposedly fully equilibrated system, such as that created in Pb+Pb reactions. The results of our calculation are compared to measurements of the (anti-)hyperon yields at midrapidity ($\mid$y$\mid$ $\leq$ 0.5) and total multiplicities performed by the NA49 and NA57 Collaborations at 40$A$ and 158$A$ GeV. Furthermore, we compared our predictions to the centrality dependence of $\Lambda$, $\bar{\Lambda}$ and $\Xi^{-}$ rapidity spectra and total multiplicities at 40$A$ and 158$A$ GeV, where possible.

System-size dependence ofΛandΞproduction in nucleus-nucleus collisions at40Aand158AGeV measured at the CERN Super Proton Synchrotron

Results on Lambda, Anti-lambda, and Xi- production in centrality selected Pb+Pb reactions at 40A and 158A GeV and in near-central C+C and Si+Si reactions at 158A GeV are presented. Transverse mass spectra, rapidity spectra, and multiplicities in dependence of the system size are discussed. Comparisons to transport models (UrQMD2.3 and HSD) and to the core-corona approach are performed. While Lambda and Anti-lambda production can be described by transport models reasonably well, these models do not agree with the Xi- measurements. On the other hand, the core-corona picture fits very well the system-size dependence of Lambda and Xi-, while it agrees less well with the Anti-lambda data.

Parton–hadron–string dynamics: An off-shell transport approach for relativistic energies

The dynamics of partons, hadrons and strings in relativistic nucleus–nucleus collisions is analyzed within the novel Parton–Hadron–String Dynamics (PHSD) transport approach, which is based on a Dynamical QuasiParticle Model (DQPM) for partons matched to reproduce recent lattice-QCD results – including the partonic equation of state – in thermodynamic equilibrium. Scalar- and vector-interaction densities are extracted from the DQPM as well as effective scalar- and vector-mean fields for the partons. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark–antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energy–momentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant ‘pre-hadronic’ color-dipole states ( q q ¯ or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. The PHSD approach is applied to nucleus–nucleus collisions from 20 to 160 A GeV in order to explore the space–time regions of ‘partonic matter’. We find that even central collisions at the top-SPS energy of 158 A GeV show a large fraction of non-partonic, i.e. hadronic or string-like matter, which can be viewed as a hadronic corona. This finding implies that neither hadronic nor only partonic ‘models’ can be employed to extract physical conclusions in comparing model results with data. On the other hand – studying in detail Pb + Pb reactions from 40 to 158 A GeV – we observe that the partonic phase has a very low impact on rapidity distributions of hadrons but a sizeable influence on the transverse mass distribution of final kaons due to the repulsive partonic mean fields. Furthermore, we find a significant effect on the production of multi-strange antibaryons due to a slightly enhanced s s ¯ pair production in the partonic phase from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process.

Engineering a Unimolecular DNA-Catalytic Probe for Single Lead Ion Monitoring

The binding of proteins and small molecules by DNA is well established, but more recently, DNA molecules have been selected to catalyze biochemical reactions. These catalytic DNAs, or DNAzymes, can be activated by metal ions. In this paper, we take advantage of DNA molecular engineering to improve the properties of DNAzymes by designing a unimolecular probe for lead ion (Pb(2+))-catalyzed reaction, achieving in turn, the ability to monitor a single Pb(2+) in solution by fluorescence microscopy. Specifically, by applying a unimolecular design, a leaving substrate DNA strand labeled with a fluorophore is linked to a hairpin 8-17 DNAzyme sequence labeled with a quencher. The hairpin structure and the substrate are connected using poly T, which brings the quencher into close proximity with the fluorophore in the inactive state. The intramolecular linkage of the two strands assures efficient quenching of the fluorescence, generating almost zero background. In the presence of Pb(2+), however, the leaving substrate fragment is cleaved at the RNA site by the enzyme, releasing a fluorescent fragment for detection with repetitive cycling for signal amplification. The resulting high sensitivity with a quantifiable detection range from 2 nM to 20 microM was achieved with a high selectivity in excess of 80-fold for Pb(2+) over other metal ions. The limit of detection is about 167 times better than the previously reported similar probes (Liu, J; Lu, Y. Anal. Chem. 2003, 75, 6666-6672) and 1600 times better compared to the Pb(2+) detection limit obtained from atomic spectroscopy. Thus, this probe could provide a simple, yet rapid and sensitive measurement for Pb(2+). Furthermore, we used this probe to monitor single Pb(2+) reaction kinetics. Given this degree of sensitivity and selectivity, our new probe design may prove useful in the development of other nucleic acid-based probes for intracellular, toxicological, and environmental monitoring.

Strangeness fluctuations and MEMO production at FAIR

We apply a coupled transport-hydrodynamics model to discuss the production of multi-strange meta-stable objects in Pb + Pb reactions at the FAIR facility. In addition to making predictions for yields of these particles we are able to calculate particle dependent rapidity and momentum distributions. We argue that the FAIR energy regime is the optimal place to search for multi-strange baryonic object (due to the high baryon density, favoring a distillation of strangeness). Additionally, we show results for strangeness and baryon density fluctuations. Using the UrQMD model we calculate the strangeness separation in phase space which might lead to an enhanced production of MEMOs compared to models that assume global thermalization.

Comparison of complementary reactions in the production of Mt

Author(s): Nelson, Sarah; Gregorich, Kenneth; Dragojevic, Irena; Ellison, Paul; Garcia, Mitch Andre; Gates, Jacklyn; Stavsetra, Liv; Ali, Mazhar; Nitsche, Heino | Abstract: The new reaction 208Pb(59Co,n)266Mt was studied using the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. A cross section of 7.7+5.2-3.3 pb was measured at a compound nucleus excitation energy of 14.9 MeV. The measured decay properties of 266Mt and its daughters correspond well with existing data. We compare this experimental result to transactinide compound nucleus formation model predictions, and the previously studied 209Bi(58Fe,n)266Mt reaction.

Study of the 2n-evaporation channel in the 4,6He + 206,208Pb reactions

Excitation functions of the reaction products were measured for the reactions induced by 4,6He projectiles on 208,206Pb targets, leading to the same compound nucleus. This was accomplished by using the stacked-foil activation technique. The identification of the reaction products (accumulated in the Pb targets) was done by their radioactive α-decays. The excitation functions for the various products were obtained at energies including the sub-Coulomb barrier region. A large value of the fusion cross section was observed in the case of the reaction induced by the weakly bound 6He projectile.

Isotopic production cross section of fragments in56Fe+pand136Xe(124Xe)+Pbreactions over an energy range from300Ato1500AMeV

Using a unified description on multiplicity distributions of final-state particles, the spallation residues in {sup 56}Fe+p reactions and fragmentation products in {sup 136}Xe({sup 124}Xe)+Pb reactions at intermediate energy and at the low end of high energies are studied. The isotopic production cross sections of fragments produced in the reactions are calculated by using a multisource ideal gas model. Each source contributes multiplicity-like distribution of neutrons like a radioactive object.

Energy dependence of Λ and Ξ production in centralPb+Pbcollisions at20A,30A,40A,80A, and 158AGeV measured at the CERN Super Proton Synchrotron

Results on $\Lambda$, $\bar{\Lambda}}$, $\Xi^{-}$, and $\bar{\Xi}^{+}$ production in central Pb+Pb reactions at 20A, 30A, 40A, 80A, and 158A GeV are presented. The energy dependence of transverse mass spectra, rapidity spectra, and multiplicities is discussed. Comparisons to string hadronic models (UrQMD and HSD) and statistical hadron gas models are shown. While the latter provide a reasonable description of all particle yields, the first class of models fails to match the $\Xi^{-}$, and $\bar{\Xi}^{+}$ multiplicities.

New calculation method for initial exciton numbers on nucleon induced pre-equilibrium reactions

In this study, we investigate the pre-equilibrium effect by using new evaluated geometry dependent hybrid model for the {sup 208}Pb (p,xn) reaction at 25.5 and 62.9 MeV incident proton energies. We also suggest that the initial neutron and proton exciton numbers for the nucleon induced precompound reactions be calculated from the neutron and proton density by using an effective nucleon-nucleon interaction with Skyrme force. We calculate the initial exciton numbers obtained from SKM* and SLy4 for a proton induced reaction on target nuclei {sup 208}Pb. The obtained results have been investigated and compared with the pre-equilibrium calculations and experimental results.

Anti-nuclei and nuclei production in Pb+Pb collisions at CERN SPS energies

We present new results on production of 3He and t obtained by the NA49 experiment in 20A, 30A, 40A and 80A GeV central Pb+Pb collisions at the CERN SPS. Transverse mass spectra and rapidity distributions for clusters measured over a large phase space domain are discussed. We observe a weak dependence of the mid-rapidity t/3He ratio on collision energy at SPS. The energy dependence of the total yield for 3He is remarkably reproduced by a statistical hadron gas model. A gradual decrease of the coalescence parameters B3 for 3He with s√SNN is observed. In addition, a measurement of the yield of antideuterons in Pb+Pb reactions at SPS's top energy of s√SNN = 17.2 GeV is presented.

High transverse momentum hadron spectra atsNN=17.3GeV inPb+Pbandp+pcollisions

Transverse momentum spectra up to 4.5GeV/c around midrapidity of π±,p,p¯, and K± in Pb+Pb reactions were measured at sNN=17.3GeV by the CERN-NA49 experiment. The nuclear modification factors RAA for π± and RCP for π±,p,p¯,K± were extracted and compared with results obtained at the BNL Relativistic Heavy Ion Collider (RHIC) at sNN=200GeV. The modification factor RAA shows a rapid increase with transverse momentum in the covered region. This indicates that the Cronin effect is the dominating effect in our energy range. The modification factor RCP, in which the contribution of the Cronin effect is reduced, shows a saturation well below unity in the π± channel. The extracted RCP values follow the 200GeV RHIC results closely in the available transverse momentum range, except for π± above 2.5GeV/c transverse momentum. There the measured suppression is smaller than that observed at RHIC.5 MoreReceived 6 November 2007DOI:https://doi.org/10.1103/PhysRevC.77.034906©2008 American Physical Society

Fission fragment properties obtained in the γ - γ - γ coincidence method in the reaction 208Pb(18O, f )

Independent fission fragment yields have been obtained for the six fragment pairs ( Z L/Z H = 44/46 , 42/48, 40/50, 38/52, 36/54 and 34/56) in 208Pb(18O, f ) at the beam energy E = 85 MeV. γ -ray spectroscopy technique was applied to analyze the experimental data. With use of these data the mass distribution of fission fragments was restored. Data on average fragment spin distribution and total neutron multiplicity are presented. The agreement of the obtained results with other experimental data proves the validity of the applied method to solve this kind of problems.

Investigation of the reaction 208Pb(18O, f): Folding angular distributions of fission fragments and gamma-ray multiplicity

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for 18O + 208Pb interactions at energies of the beam of 18O ions in the range E lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of 18O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus.

Investigation of the reaction 208Pb(18O, f): Fragment spins and phenomenological analysis of the angular anisotropy of fission fragments

The average multiplicity of gamma rays emitted by fragments originating from the fission of 226Th nuclei formed via a complete fusion of 18O and 208Pb nuclei at laboratory energies of 18O projectile ions in the range E lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier (E c.m. − E B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory.

Experiment on Synthesis of an Isotope 277112 by 208Pb+70Zn Reaction

The production and decay of 277 112 have been investigated using a gas-filled recoil ion separator in irradiations of 208 Pb targets with a 70 Zn beam at 349.5 MeV. We have observed two α-decay chains that can be assigned to subsequent decays from 277 112 produced in the 208 Pb( 70 Zn,n) reaction. After emitting four consecutive α-particles, both the chains terminate by spontaneous fission decays of 261 Rf, and the decay energies and decay times of both the chains obtained in the present work agree well with those reported by a group at Gesellschaft fur Schwerionenforschung (GSI), Germany. The present result gives the first clear confirmation of the discovery of 277 112 and its α-decay product 273 Ds reported previously.

Effects of the density-dependence symmetry potential on spallation neutrons induced by 1.2 GeV and 1.6 GeV protons on Pb

We have investigated the influence of different forms of the density dependence of the symmetry potential on spallation neutrons for p + Pb reactions at 1.2 GeV and 1.6 GeV by means of the improved ultra-relativistic quantum molecular dynamics (UrQMD) model. We find that the cascade component of the neutron spectra, with kinetic energies from 22 to 50 MeV and at forward angles from θ = 50° to 60°, is quite sensitive to the symmetry potential. The evaporation component is shown to be most sensitive to the stiffness of the symmetry potential at densities 1 ⩽ ρ/ρ0 ⩽ 2. The spallation data for the reactions under study are shown to be more consistent with improved UrQMD calculations employing symmetry potential of the form USym ≈ 25–30(ρ/ρ0)0.9 at ρ ⩽ 2ρ0.

Charmonium dynamics in nucleus–nucleus collisions at SPS and FAIR energies

Charmonium production and suppression in In + In and Pb + Pb reactions at SPS energies is investigated with the HSD transport approach within the ‘hadronic comover model’ as well as the ‘QGP threshold scenario’. The results of the transport calculations for J / Ψ suppression and the Ψ ′ to J / Ψ ratio are compared with the recent data of the NA50 and NA60 Collaborations. We find that the comover absorption model—with a single parameter | M 0 | 2 for the matrix element squared for charmonium-meson dissociation—performs best with respect to all data sets. The ‘threshold scenario’—within different assumptions for the melting energy densities—yields a reasonable suppression for J / Ψ but fails in reproducing the Ψ ′ to J / Ψ ratio for Pb + Pb at 158 A GeV . Predictions for Au + Au reactions are presented for a bombarding energy of 25 A GeV in the different scenarios which will allow for a clear distinction between the models from the experimental side at the future FAIR facility.

Directed and elliptic flow in heavy-ion collisions fromEbeam=90MeV/nucleon toEc.m.=200GeV/nucleon

Recent data from the NA49 experiment on directed and elliptic flow for Pb+Pb reactions at CERN-SPS are compared to calculations with a hadron-string transport model, the ultrarelativistic quantum molecular dynamics (UrQMD) model. The rapidity and transverse momentum dependence of the directed and elliptic flow, i.e., v{sub 1} and v{sub 2}, are investigated. The flow results are compared to data at three different centrality bins. Generally, a reasonable agreement between the data and the calculations is found. Furthermore, the energy excitation functions of v{sub 1} and v{sub 2} from E{sub beam}=90A MeV to E{sub c.m.}=200A GeV are explored within the UrQMD framework and discussed in the context of the available data. It is found that, in the energy regime below E{sub beam}{<=}10A GeV, the inclusion of nuclear potentials is necessary to describe the data. Above 40A GeV beam energy, the UrQMD model starts to underestimate the elliptic flow. Around the same energy the slope of the rapidity spectra of the proton directed flow develops negative values. This effect is known as the third flow component (''antiflow'') and cannot be reproduced by the transport model. These differences can possibly be explained by assuming a phase transition from hadron gas to quark gluonmore » plasma at about 40A GeV.« less