Creation of electron-positron pairs in the collision of heavy atomic nuclei. Systematic quantum-mechanical approach

The creation of electron-positron pairs in the collision of two heavy nuclei is considered. A systematic quantum-mechanical approach is developed for the complete electron-nuclear system. The finite size of the nuclei and the possible resonances of the superheavy compound nucleus formed from the initial nuclei are taken into account. The energetic approach known in the theory of the breakdown of quasisteady systems is generalized to the case of the collisional problem. This approach is combined with perturbation theory for the complete electron-nuclear system. The rearrangement of the negative-continuum spectrum is taken into account using the procedure of [14], which is generalized with the aim of introducing additional conditions, in particular, the condition associated with correct choice of the zero approximation. The method is the same for Z < Z_cr and Z > Z_cr, where Z_cr is the minimum nuclear charge at which spontaneous pair creation is possible. The contribution of the spontaneous channel to the amplitude of the process is automatically taken into account.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 97–107, August, 1990.     

Neutronization and Protonization of Nuclei: Two Possible Ways of the Evolution of Astrophysical Objects and the Laboratory Electron-Nucleus Collapse

We consider the peculiarities of the fundamental nuclear transformations running both in the shell of a heavy star compressed by the strong gravitational field and during the laboratory electron-nucleus collapse where the compression occurs at the expense of the electron-nucleus interaction in a volume occupied by a degenerate electron gas, define their analogs, and analyze the differences. It is shown that the account of relativistic and nonlinear corrections to the Coulomb electron-nucleus interaction gives the possibility to realize two alternative ways for the evolution of the star matter which depend on both the rate of compression upon the gravitational collapse and the initial isotope composition of a star on the stage preceding the collapse. Upon the relatively slow compression of a heavy star in the process of gravitational collapse after the attainment of the threshold electron density, there occur the stage-by-stage neutronization of nuclei and the formation of a neutron star with a great concentration of neutrons and a low concentration of protons and electrons. This process is characterized by the presence of a bounded interval of the density of a relativistic degenerate gas of electrons (“the neutronization corridor”), in the scope of which the neutronization runs with a decrease in the Fermi energy and the release of energy in the form of fast neutrinos. At a higher electron density, the process of protonization becomes energy-gained. In this case, an increase in both the charge of nuclei and the concentration of degenerate electrons causes the continuous increase in the binding energy of electrons and nuclei which turns out to be more significant than the increase in the Fermi energy of electrons. The transition of nuclei through “the neutronization corridor” into “the protonization zone”, which ranges up to the nuclear density of a substance, is possible only in the case of a very fast compression of a heavy star. Such a process leads to the possibility of the formation of proton stars with a very small residual concentration of neutrons and a great (nuclear) concentration of protons and electrons. It is shown that analogous effects can be realized during the laboratory electron-nucleus collapse. Due to a microscopic size of the collapse zone, a great velocity of its formation, and a relatively low rate of neutronization, the passage of the electron-nucleus substance through “the neutronization corridor” weakly affects its state. In this case, the main mechanism of transformations is the process of protonization with a simultaneous increase in the concentration of degenerate electrons.     

Study of heavy nuclei at FLNR (Dubna)

Experiments are described and main results presented on the synthesis and decay properties of superheavy nuclides, produced in fusion reactions induced by a ⁴⁸Ca-beam on heavy actinide targets. In such reactions neutron-rich nuclei are formed. For them, according to theory, an abrupt enhancement of stability due to nuclear shell effects is expected. The decay properties of the new nuclides are compared with calculations of theoretical models, which predict the existence of “islands of stability” in the region of hypothetical superheavy elements.     

Heavy element research at FLNR (Dubna)

The setting of the experiments is described and the results obtained on the synthesis of the heaviest nuclides in reactions induced by ⁴⁸Ca projectiles are presented. The decay properties of the new nuclei confirm the theoretical predictions about the new nuclear shells and the “islands of stability” of superheavy elements. Briefly, also the results of the first experiments aimed to study the atomic and chemical properties of element 112 are reported.     

Fusion-fission dynamics in the superheavy nucleus production

The fusion-fission reaction mechanism leading to the massive nucleus formation is studied. We investigate the superheavy nucleus formation in heavy-ion induced reactions by analysing the evaporation residue (ER) production in order to study the fusion dynamics and the decay properties of nuclei close to the stability island at Z=114. We consider the ⁶¹Ni+²⁰⁸Pb, ⁴⁸Ca+²³⁸U and ⁴⁸Ca+²⁴⁴Pu reactions that lead to the Z=110, 112 and 114 superheavy elements respectively. By using the dinuclear system (DNS) concept of the two interacting nuclei we calculate the quasifission-fusion competition in the entrance channel and the fission-evaporation competition along the de-excitation cascade of the compound nucleus. The dynamics of the entrance channel allows us to determine the beam energy window which is favorable to the fusion, while the dynamic evolution of the compound nucleus on the shell correction to the fission barrier and the dissipative effects influence the fission-evaporation competition in order to obtain the residue nuclei from the superheavy nucleus formation. We also calculate the τ _n /τ_tot ratio at each step of the de-excitation cascade of the compound nucleus and we present a systematics of τ _n /τ_tot (at first step of the cascade) for many reactions that lead to nuclei with Z=102–114.     

Anomalies in orientation interaction of slow neutrons with nuclei and the problem of neutron molecule existence

Based on the Dirac equation, the features of long-range electromagnetic orientational interaction of slow neutrons with even-even and even-odd nuclei are considered. This interaction is controlled by a narrow potential barrier arranged beyond the nucleus. The barrier height is U _tot = 20–40 eV and depends on Z, A, and the nucleus magnetic moment μ_nucl. The barrier formation is associated with the ponderomotive nonlinear interaction of the anomalous neutron moment with the nucleus electric field. The barrier transparency for thermal neutrons is D(E) ≈ 0.8–0.95. For cold neutrons, the barrier transparency and their reaction cross sections with nuclei sharply decrease and, at E → 0, their cross sections tend toward zero. It was shown that the combined effect of the magnetic dipole-dipole and ponderomotive interaction of the neutron and even-odd nucleus results in the formation of removed symmetrically positioned potential wells for neutrons beyond the nucleus. The presence of these wells results in the possible existence of short-lived or virtual nucleus-neutron molecules and the “neutron halo” effect beyond the nucleus.     

The dubna gas-filled recoil separator: Software and algorithms

Experiments carried out on the gas-filled separator of recoil nuclei at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, in the past several years have proved the hypothesis of the existence of a stability island of superheavy nuclei. The accelerator, beam diagnostics, and detector technologies underlay the success. It is the highly sensitive detecting system of the separator combined with the method of “active correlations” that allowed rare events of the decay of superheavy nuclei to be reliably detected under almost background-free conditions. In turn, to a large extent, this has been made possible owing to special algorithms and software developed for long-term experiments in the beam of ⁴⁸Ca ions.     

Bremsstrahlung spectrum for α decay and quantum tunneling

Spectra of the electromagnetic radiation arising during α decay of atomic nuclei as a consequence of the motion of the α particle through a Coulomb potential barrier and in the Coulomb field of the daughter nucleus are calculated via a quantum-mechanical approach. The contributions of the E1 and E2 multipoles are calculated. Model problems of emission during motion of a charged particle through a spherically symmetric, rectangular potential barrier and a “cut-off” Coulomb barrier are treated. Numerical calculations are performed for ^210,214Po and ²²⁶Ra nuclei. Emission spectra are derived for an α particle propagating along classical trajectories in these potentials. Zh. éksp. Teor. Fiz. 116, 390–409 (August 1999)     

Impact of nuclear structure on production and identification of new superheavy nuclei

Using the microscopic-macroscopic approach based on the modified two-center shell model, the low-lying quasiparticle spectra, ground-state shell corrections, mass excesses and Q _α -values for even Z superheavy nuclei with 108 ≤ Z ≤ 126 are calculated and compared with available experimental data. The predicted properties of superheavy nuclei show that the next doubly magic nucleus beyond ²⁰⁸Pb is at Z ≥ 120. The perspective of using the actinide-based complete fusion reactions for production of nuclei with Z = 120 is studied for supporting future experiments.     

Coulomb excitation of neutron-rich 138,140,142Xe at REX-ISOLDE

We report on “safe” Coulomb excitation of neutron-rich ^138,140,142Xe nuclei. The radioactive nuclei have been produced by ISOLDE at CERN and post-accelerated by the REX-ISOLDE facility. The γ-rays emitted by the decay of excited states have been detected by the MINIBALL array. Recent results are presented.     

奇Z超重核的禁戒α衰变(英文)

采用密度依赖的结团模型研究了奇Z 超重核的禁戒α衰变, 粒子与子核之间的微观核势通过双折叠模型对M3Y 核子-核子相互作用势以及 粒子与子核的密度积分给出。 粒子与子核之间的库仑相互作用也通过 粒子与子核的电荷密度积分给出。计算发现,由于非零角动量带来的禁戒效应和小的α粒子预形成几率,奇Z 超重核的α衰变寿命会明显变长。We investigate the α-transition of odd-Z superheavy nuclei by the density-dependent cluster model (DDCM). The microscopic nuclear potential between the -particle and the daughter nucleus is evaluated numerically from the double-folding model with the standard M3Y nucleon-nucleon interaction. The Coulomb potential is also obtained from the double-folding integral of the proton-proton Coulomb interaction with the charge density distributions of α-particle and daughter nucleus. From our calculations, enhanced stability againstα-decays is found for the odd-Z superheavy nuclei due to the hindrance effect of non-zero angular momentum and the small preformation factor of the -particle.     

Square-well nuclear-potential depths and bound energy states from neutron scattering data

Using the neutron scattering lengths b determined experimentally for a majority of isotopes in last decades, one can in principic extract systematic information on some nuclear properties of elements. A significant “scatter” of experimental values of the (related to b) nuclear radius R around the “classical” dependence R = r ₀ A ^1/3, where A is the mass number, is intriguing and requires a special attention. In this work, on extending the use of known formulas of the theory of neutron scattering on nucleus represented by a rectangular radial symmetry potential well (or barrier), we have determined the depths V ₀ of the potential well and for many isotopes the position of the bound-state energy level E _b in the well. The “scatter” mentioned above can be in part attributed to the four types of the s-type wave functions of slow neutron interacting with nucleus, which appear in this model. In several cases the bound-state energy level is close to the Fermi E _F level of the free-nucleon model of nuclear matter of the constant density, independent of A.     

Spectroscopy of neutron-rich nuclei at REX-ISOLDE with MINIBALL

We report on “safe” Coulomb excitation of neutron-rich nuclei. The radioactive nuclei have been produced by ISOLDE at CERN and postaccelerated by the REX-ISOLDE facility. The γ rays emitted by the decay of excited states have been detected by the MINIBALL array. Recent results are presented and compared to theoretical models. on behalf of the MINIBALL and REX-ISOLDE Collaborations The text was submitted by the author in English.     

An attempt to build a statistical model for a pips detector operated in a real-time mode

Basic approaches by K.-H. Schmidt and V.B. Zlokazov to estimate the probability of registered multichain event to be explained by random coincidences are considered. A specific feature of the long-term experiments aimed at the synthesis of superheavy nuclei with the Dubna Gas-Filled Recoil Separator is usage of a real-time mode for radical suppression of background products. In fact, this assumes that the first correlation group, namely, recoil-alpha correlation, stops target irradiation for a short time and the forthcoming signals are detected in more favorable background conditions. Due to the application of this detection mode the first recoil-alpha chain can be considered as a “starter” for the detection of the forthcoming alpha-particle signals with high background suppression. This fact is taken into account in the given PIPS detector statistical model.     

Synthesis of superheavy nuclei in the reactions of 244Pu and 248Cm with 48Ca

This paper presents results of the experiments aimed at producing long-lived superheavy elements located near the spherical-shell closures with Z ⩾ 114 and N ⩾ 172 in the ²⁴⁴Pu + ⁴⁸Ca and ²⁴⁸Cm + ⁴⁸Ca reactions. The large measured α-particle energies of the newly observed nuclei, together with the long decay times and spontaneous fission terminating the chains, offer evidence of the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides ^{288, 289}114 and ²⁹²116, produced in the 3n and 4n evaporation channels with cross-sections of about a picobarn. The present observations can be considered as experimental evidence of the existence of the “island of stability” of superheavy elements. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: utyonkov@sungns.jinr.ru     

Attenuation of Λ(1520) Hyperons in near-threshold <Emphasis Type="Italic">pA</Emphasis> reactions

In the framework of the nuclear spectral function approach for incoherent primary proton-nucleon and secondary pion-nucleon production processes we study the inclusive Λ(1520)-hyperon production in the interaction of 2.83-GeV protons with nuclei. In particular, the A and momentum dependences of the absolute and relative Λ(1520)-hyperon yields are investigated within the different scenarios for their in-medium width. Our model calculations show that the pion-nucleon production channel contributes distinctly to the “low-momentum” Λ(1520) creation both in light and heavy nuclei in the chosen kinematics and, hence, has to be taken into consideration on close examination of the dependences of the Λ(1520)-hyperon yields on the target mass number with the aim to get information on their width in the medium. They also demonstrate that both the A dependence of the relative Λ(1520)-hyperon production cross section and momentum dependence of the absolute Λ(1520)-hyperon yield at incident energy of interest are appreciably sensitive to the Λ(1520) in-medium width, which means that these observables may be an important tool to determine the above width.     

New nuclear charge radii database

Several large relational databases (DBs) containing various atomic nucleus parameters and nuclear reaction features were produced at the Centre for Photonuclear Experiments Data (Centr Dannyh Fotoyadernykh Eksperimentov (CDFE)) of the Skobeltsyn Institute of Nuclear Physics, Moscow State University). The sources are numerical data founds maintained by International Nuclear Data Centers Network of the International Atomic Energy Agency (IAEA) and produced by CDFE. The original CDFE product is the electronic “Chart of Quadrupole Nuclear Deformations” which includes ∼2000 sets of data on nuclei quadrupole moments Q and quadrupole deformation parameters β₂ for ∼1500 nuclei. At last time, in the frame of joint research with the Joint Institute for Nuclear Research (JINR) that electronic Chart was supplemented with the data on nuclear mean-root-square (MRS) charge radii (∼900 isotopes of 90 elements (Z = 1–96, N = 0–152)) and therefore transformed into the “Chart of Nucleus Shape and Size Parameters”—complete collection of data under discussion. New Chart allows one to investigate the isotopic and isotonic behavior of nuclei quadrupole moments, parameters of quadrupole deformation and charge radii, and study the R(Z, N) surface structure and R(A) dependence of the fine structure. Original Russian Text ? I.N. Boboshin, V.V. Varlamov, Yu.P. Gangrsky, B.S. Ishkhanov, S.Yu. Komarov, K.P. Marinova, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 6, pp. 857–862.     

Annealing of sputtered gold nano-structures

Abstract The effects of annealing on gold structures sputtered onto glass substrate were studied using AFM, UV-Vis methods and electrical measurements. The colour of the as-deposited films changes from blue to green with increasing deposition time. After 1 hour annealing at 300°C the structures acquire red colour regardless of the film thickness. The annealing results in dramatic changes of surface morphology and roughness and creation of relatively large “spherolytic and hummock-like” structures in the gold layer. For deposited structures a non-zero optical band gap E_g^optE_{\mathrm{g}}^{\mathrm{opt}} was determined from UV-Vis spectra using Tauc’s model and it indicates a semi-conducting character of the structures. The annealing leads to an increase of the band gap. Electrical resistance of the deposited unannealed structures decreases dramatically for deposition times above 50 s. For annealed structures the resistance fall comes until after 250 s deposition time.     

Detecting hidden spatial and spatio-temporal structures in glasses and complex physical systems by multiresolution network clustering

We elaborate on a general method that we recently introduced for characterizing the “natural” structures in complex physical systems via multi-scale network analysis. The method is based on “community detection” wherein interacting particles are partitioned into an “ideal gas” of optimally decoupled groups of particles. Specifically, we construct a set of network representations (“replicas”) of the physical system based on interatomic potentials and apply a multiscale clustering (“multiresolution community detection”) analysis using information-based correlations among the replicas. Replicas may i) be different representations of an identical static system, ii) embody dynamics by considering replicas to be time separated snapshots of the system (with a tunable time separation), or iii) encode general correlations when different replicas correspond to different representations of the entire history of the system as it evolves in space-time. Inputs for our method are the inter-particle potentials or experimentally measured two (or higher order) particle correlations. We apply our method to computer simulations of a binary Kob-Andersen Lennard-Jones system in a mixture ratio of A₈₀B₂₀ , a ternary model system with components “A”, “B”, and “C” in ratios of A₈₈B₇C₅ (as in Al₈₈Y₇Fe₅ , and to atomic coordinates in a Zr₈₀Pt₂₀ system as gleaned by reverse Monte Carlo analysis of experimentally determined structure factors. We identify the dominant structures (disjoint or overlapping) and general length scales by analyzing extrema of the information theory measures. We speculate on possible links between i) physical transitions or crossovers and ii) changes in structures found by this method as well as phase transitions associated with the computational complexity of the community detection problem. We also briefly consider continuum approaches and discuss rigidity and the shear penetration depth in amorphous systems; this latter length scale increases as the system becomes progressively rigid.     

Charge-exchange resonances and restoration of Wigner’s supersymmetry in heavy and superheavy nuclei

Various facets of the question of whether Wigner’s supersymmetry [SU(4) symmetry] may be restored in heavy and superheavy nuclei are analyzed on the basis of a comparison of the results of calculations with experimental data. The energy difference between the giant Gamow–Teller resonance and the analog resonance (the difference of E _G and E _A) according to calculations based on the theory of finite Fermi systems is presented for the case of 33 nuclei for which experimental data are available. The calculated difference ΔE _G–A of E _G and E _A tends to zero in heavier nuclei, showing evidence of the restoration of Wigner’s SU(4) symmetry. Also, the isotopic dependence of the Coulomb energy difference between neighboring isobaric nuclei is analyzed within the SU(4) approach for more than 400 nuclei in the mass-number range of A = 5–244. The restoration of Wigner’s SU(4) symmetry in heavy nuclei is confirmed. It is shown that the restoration of SU(4) symmetry is compatible with the possible existence of the stability island in the region of superheavy nuclei.