197Au Nuclei Research Articles

Pre-Equilibrium Clustering in Production of Spectator Fragments in Collisions of Relativistic Nuclei

An algorithm of pre-equilibrium clustering of spectator matter based on the construction of the minimum spanning tree (MST) is presented. The algorithm was implemented in the Abrasion-Ablation Monte Carlo for Colliders (AAMCC) model designed to study the characteristics of spectator matter in collisions of relativistic nuclei. Due to accounting for the pre-equilibrium clusters in modelling 208Pb–208Pb collisions at the LHC, the agreement of simulation results with experimental data on the average multiplicities of spectator nucleons was improved. The results of the AAMCC-MST were compared with experimental data on the interactions of 197Au nuclei in nuclear photoemulsion. Comparison of the yields of spectator nuclei calculated for 16O–16O collisions with the yields measured in interactions of 16O with light nuclei of photoemulsion made it possible to estimate the effect of MST-clustering in small nuclear systems.

Impact of Nuclear Deformation on Relativistic Heavy-Ion Collisions: Assessing Consistency in Nuclear Physics across Energy Scales.

In the hydrodynamic framework of heavy-ion collisions, elliptic flow v_{2} is sensitive to the quadrupole deformation β of the colliding ions. This enables one to test whether the established knowledge on the low-energy structure of nuclei is consistent with collider data from high-energy experiments. We derive a formula based on generic scaling laws of hydrodynamics to relate the difference in v_{2} measured between collision systems that are close in size to the value of β of the respective species. We validate our formula in simulations of ^{238}U+^{238}U and ^{197}Au+^{197}Au collisions at top Relativistic Heavy Ion Collider (RHIC) energy, and subsequently apply it to experimental data. Using the deformation of ^{238}U from low-energy experiments, we find that RHIC v_{2} data implies 0.16≲|β|≲0.20 for ^{197}Au nuclei, i.e., significantly more deformed than reported in the literature, posing an interesting issue in nuclear phenomenology.

Correlation in formation of 8Be nuclei and α-particles in fragmentation of relativistic nuclei

In the events of peripheral dissociation of relativistic nuclei in the nuclear track emulsion, it is possible to study the emerging ensembles of He and H nuclei, including those from decays of unstable 8Be and 9B nuclei, as well as the Hoyle state. These extremely short-lived states are identified by invariant masses calculated from the angles in 2α-pairs, 2αp- and 3α-triplets in the approximation of conservation of momentum per nucleon of the primary nucleus. In the same approach, it is possible to search for more complex states. This paper explores the correlation between the formation of 8Be nuclei and the multiplicity of accompanying α-particles in the dissociation of relativistic 16O, 22Ne, 28Si, and 197Au nuclei. On the above basis, estimates of this correlation are presented for the unstable 9B nucleus and the Hoyle state. The enhancement in the 8Be contribution to dissociation with the α-particle multiplicity has been found. Decays of 9B nuclei and Hoyle states follow the same trend.

Near-barrier neutron transfer in reactions 6He + 45Sc, 64Zn, and 197Au

Experimental cross sections of formation of isotopes 46Sc (in reaction 6He + 45Sc), 196,198Au (in reaction 6He + 197Au), and 65Zn (in reaction 6He + 64Zn) are analyzed. The time-dependent Schrodinger equation for the outer neutrons of 6He and 197Au nuclei is solved numerically to calculate the probability of neutron transfer and transfer cross sections. In reaction 6He + 197Au, the contribution of fusion and subsequent evaporation to experimental data can be neglected, while the corresponding contributions to reactions 6He + 45Sc and 6He + 64Zn are considerable. Fusion–evaporation is taken into account using the computational code of the NRV knowledge base. The results of calculations are in satisfactory agreement with the experimental data.

Near-barrier neutron transfer in reactions 3,6He + 45Sc and 3,6He + 197Au

Experimental cross sections for formation of 196,198Au isotopes in reactions 3,6He + 197Au and cross sections for formation of 44,46Sc isotopes in reactions 3,6He + 45Sc have been analyzed. To calculate neutron transfer probabilities and cross sections the time- dependent Schrödinger equation for external neutrons of 3He, 6He, 45Sc and 197Au nuclei has been solved numerically. It is shown that the contribution of fusion and subsequent evaporation is significant in the case of reactions 3,6He + 45Sc, whereas in the case of reactions 3,6He + 197Au, it is negligible. Fusion-evaporation was taken into account using NRV evaporation code. Results of calculations demonstrate overall satisfactory agreement with experimental data.

Near-barrier neutron transfer in reactions with 3He nucleus

Experimental cross sections for the formation of 196,198Au isotopes in the 3He + 197Au reaction and 44,46Sc isotopes in the 3He + 45Sc reaction are analyzed. To calculate transfer probabilities and cross sections, the time-dependent Schrodinger equation is numerically solved for the external neutrons of the 3He, 45Sc, and 197Au nuclei. It is shown that the contribution from the fusion channel with subsequent evaporation is important for the 3He + 45Sc reaction and negligibly small for the 3He + 197Au reaction. Fusion–evaporation is taken into account using the NRV and PACE codes. Calculation results demonstrate overall satisfactory agreement with experimental data.

Measuring diffraction angular distributions on 59Co, 197Au, and 209Bi nuclei at an alpha-particle energy of 29 MeV

Differential cross sections for the elastic scattering of alpha-particles from 59Co, 197Au, and 209Bi nuclei are measured at an incident energy of 29 MeV. The experimental data are described using different methods of diffraction in the illuminated region (Fresnel nuclear diffraction) and shadow region (Fraunhofer nuclear diffraction). Spatial characteristics of the nuclei obtained from these two types of nuclear diffraction are compared.

Cross sections of fusion and transfer reactions in interactions between 3He and Pt and 197Au nuclei in the subbarrier energy region

The reactions of complete and incomplete fusion, along with nucleon transfer reactions in the range of 3He energies from 10 to 24.5 MeV, are investigated by irradiating gold and platinum targets with an accelerated 3He ion beam on the U-120M cyclotron at the Institute of Nuclear Physics, Czech Academy of Sciences, Řež. Activation is used to determine the yield of nuclides resulting from nuclear reactions. The γ-activity induced in the targets is measured using a with high-resolution HPGe detector. Despite the low binding energy of 3He and the reactions with positive Q values, the measured cross sections of fusion reactions exhibit no specific features, compared to reactions conducted using beams of other light stable particles. Transfer neutron reactions in the subbarrier energy region have relatively high cross sections. These cross sections continue to grow as the energy of 3He rises. When a neutron is captured by a nucleus of 3He, the cross sections of these reactions reach their maximum values in the Coulomb barrier region.

Calculations of double-differential triton emission cross sections at 62-MeV proton-induced reactions

Double-differential cross sections (d2σ/dΩdɛ) have been calculated and analyzed for triton production in proton-induced reactions on 27Al, 54,56Fe, 197Au, and 208Pb target nuclei at incident energy of 62 MeV. Calculations of double-differential cross sections have been performed using nuclear models implemented in the TALYS 1.2 code. The calculated results of the double-differential cross sections for triton emission have been compared with the existing experimental data.

Investigating the nuclear fusion and nucleon transfer channels in the 197Au + 6He reaction at 6He energies up to 20 MeV/A

New data on the cross sections of reactions that proceed during the interaction of 6He and 197Au nuclei in the 6He energy range of 40 to 120 MeV are reported. The experiments were performed using the secondary beam of the ACCULINNA separator at the Flerov Laboratory of Nuclear Reactions (FLNR), JINR. Reaction products are identified by means of activation method to measure the gamma activity of a thin-foil target assembly. Excitation functions for 6He fusion reactions with subsequent emission of up to 10 neutrons from the compound nucleus are measured. Cross sections for reactions with emission of charged particles and nucleon transfer were also measured. The experimental cross sections for the (6He, xn) reactions that proceed via the formation of a compound nucleus agree in general with calculations using models that involve the statistical approach. It is shown that the complete fusion reaction cross section drops slightly up to an energy of 114 MeV. The experimental excitation functions for the reactions resulting in the formation of mercury and gold isotopes indicate that the main contribution to their formation comes from direct processes, while the evaporation reactions (6He, pxn) and (6He, αxn) are of minor importance.

New approach to analyzing and evaluating cross sections for partial photoneutron reactions

The presence of substantial systematic discrepancies between the results of different experiments devoted to determining cross sections for partial photoneutron reactions—first of all, (γ, n), (γ, 2n), and (γ, 3n) reactions—is a strong motivation for studying the reliability and authenticity of these data and for developing methods for taking into account and removing the discrepancies in question. In order to solve the first problem, we introduce objective absolute criteria involving transitional photoneutron-multiplicity functions F 1, F 2, F 3, …; by definition, their values cannot exceed 1.0, 0.5, 0.33, …, respectively. With the aim of solving the second problem, we propose a new experimental-theoretical approach. In this approach, reaction cross sections are evaluated by simultaneously employing experimental data on the cross section for the total photoneutron yield, σ expt(γ, xn) = σ expt(γ, n) + 2σ expt(γ, 2n) + 3σ expt(γ, 3n) + …, which are free from drawbacks plaguing experimental methods for sorting neutrons in multiplicity, and the results obtained by calculating the functions F theor 1 , F theor 2 , F theor 3 , … on the basis of the modern model of photonuclear reactions. The reliability and authenticity of data on the cross sections for (γ, n), (γ, 2n), and (γ, 3n) partial reactions—σ eval(γ, in) = F theor σ expt(γ, xn)—were evaluated for the 90Zr, 115In, 112,114,116,117,118,119,120,122,124Sn, 159Tb, and 197Au nuclei.

Calculations of Double Differential Deuteron Emission Cross Sections at 62 MeV Proton Induced Reactions

In this study, double differential deuteron emission cross sections for 27Al, 54,56Fe, 197Au, 208Pb and 209Bi target nuclei have been calculated by the TALYS code at 62 MeV proton energy. The use of TALYS involved calculations by the pre-equilibrium exciton model and the Hauser–Feshbach model. The calculated double differential cross sections have been compared with the experimental data taken from the literature.

Yields of photoneutron reactions on 197Au nuclei in the giant-dipole-resonance region

Yields of photonuclear reactions on 197Au nuclei were measured in a beam of bremsstrahlung photons, the endpoint energy of the bremsstrahlung spectrum being 29.1 MeV. These measurements were performed by means of the gamma-activation procedure. The results obtained in this way were compared with the results of earlier experiments and theoretical calculations.

Effect of the orientation degree of freedom on the rate and time of fission of highly excited nuclei

The effect of the dimensionality of the dynamical model used on the fission rate and mean time is studied within a multidimensional stochastic approach to fission dynamics. These features of fission of excited compound nuclei are calculated within four-dimensional Langevin dynamics, where the coordinate K, which is the projection of the total angular momentum onto the symmetry axis of the nucleus being considered, is taken into account in addition to three collective shape coordinates introduced on the basis of the {c, h, a} parametrization. The evolution of this orientation degree of freedom (K mode) is described in terms of the Langevin equation in the overdamped regime. The effect of the orientation degree of freedom on the rate and mean time of fission of compound nuclei is studied. The introduction of the orientation degree of freedom is shown to lead to a substantial decrease in the fission rate and, accordingly, to an increase in the mean fission time upon going over from the three- to the four-dimensional model. The reactions induced by the interaction of 14N and 16O projectile ions with 197Au, 208Pb, 232Th, and 238U nuclei at energies above the Coulomb fusion barrier are considered. The effect of the increase in the fission time because of the introduction of the K mode is so strong that it compensates almost completely for an opposite effect from introducing, in the one-dimensional model, the second and third collective coordinates that take into account, respectively, the evolution of the neck in the nuclear shape and the mass asymmetry. Ultimately, the difference between the results in the four- and one dimensional problems is not more than 5 to 25% for the reactions considered here.

New approximations to the energy dependences of the total cross sections for the proton-induced fission of 197Au, 203Tl, natPb, and 209Bi nuclei

The total cross sections for 197Au and 203Tl fission induced by protons of energy varied from about 200 to 1000 MeV with a step of about 100 MeV are measured. New approximations to the energy dependences of the cross sections for the proton-induced fission of 197Au, 203Tl, natPb, and 209Bi nuclei are presented and discussed. For all of these nuclei, exponential functions are used as approximations.

Calculation of Equilibrium and Pre-equilibrium 4He-Emission Spectra at 62 MeV Proton Incident Energy

In this study, the 4He-emission cross sections for 27Al, 54,56Fe, 197Au, 208Pb and 209Bi target nuclei have been calculated at 62 MeV proton energy. In these calculations, the pre-equilibrium effects have been investigated. The pre-equilibrium calculations involve the geometry dependent hybrid model and hybrid model. A comparison with theoretical calculations (ALICE/ASH) has been performed and the empirical formulas for the 4He-emission cross sections produced at 62 MeV proton energy have been derived.

Pre-Equilibrium 3He-Emission Spectra at 62 MeV Proton Incident Energy

In this study, the 3He-emission cross sections for 27Al, 54,56Fe, 197Au, 208Pb and 209Bi target nuclei have been calculated at 62 MeV proton energy. In these calculations, the pre-equilibrium effects have been investigated. The pre-equilibrium calculations involve the geometry dependent hybrid model and hybrid model. A comparison with theoretical calculations (ALICE/ASH) has been performed and the empirical formulas for the 3He- emission cross sections produced at 62 MeV proton energy have been derived.

Photodisintegration of heavy nuclei in the energy region above the giant dipole resonance

Experimental data on yields of multiparticle photonuclear reactions (involving the emission of up to seven neutrons from the nucleus involved) on 197Au, 203,205Tl, and 209Bi nuclei in the region extending from the giant dipole resonance to an energy of 67.7 MeV are presented. These data are compared with the results of modern theoretical calculations that take into account both the excitation of a giant dipole resonance (GDR) in a nucleus and the photodisintegration of quasideutrons (QD) in it. By and large, experimental data confirm the results of theoretical calculations—that is, only upon taking simultaneously into account both alternative photodisintegration mechanisms (GDR excitation and QD photodisintegration) can one describe these experimental data. The contribution of QD photodisintegration grows with increasing photon energy and neutron multiplicity and becomes dominant for reactions involving the emission of not less than five neutrons from the nucleus being considered. The integrated cross sections for the processes in question were estimated on the basis of simultaneously employing experimental yields of multinucleon photonuclear reactions and the respective cross-section shapes calculated theoretically.

The symmetry energy in nuclear reactions

The complementary aspects of the strength of the symmetry energy at nuclear densities below and above saturation are illustrated with recent results from experiments performed at the GSI laboratory with relativistic heavy-ion beams. A systematic study of isotopic effects in spectator fragmentation was performed at the ALADIN spectrometer with 124Sn primary and 107Sn and 124La secondary beams of 600 MeV/nucleon incident energy. The analysis within the Statistical Fragmentation Model reveals that the symmetry-term coefficient entering the liquid-drop description of the emerging fragments decreases significantly as the disintegration of the spectator system into fragments and light particles increases. Higher densities were probed in more central collisions of 197Au + 197Au nuclei at 400 MeV/nucleon. The results obtained from the analysis of the FOPI/LAND experimental data on neutron and hydrogen squeeze-out in comparison with UrQMD model predictions favor a moderately soft symmetry term with a density dependence of the potential term proportional to ( ρ / ρ 0 ) γ with γ = 0.9 ± 0.3 .

Investigation of Deuteron Emission Spectra at 62 MeV Proton Incident Energy

In this study, deuteron emission cross sections for 27Al, 54,56Fe, 89Y, 120Sn, 197Au, 208Pb and 209Bi target nuclei have been calculated at 62 MeV proton incident energy. In these calculations, the pre-equilibrium and equilibrium effects have been investigated. The pre-equilibrium calculations involve the geometry dependent hybrid model and hybrid model. Equilibrium effects are calculated according to the Weisskopf–Ewing model. The calculated results are compared with the experimental data taken from the literature and found to be in good agreement.