超重核性质与合成机制的理论研究

探索原子核的电荷与质量极限,合成长寿命超重核是当前原子核物理研究的重要前沿问题之一。本文综述了我们近几年在超重原子核结构性质与合成机制方面取得的理论研究进展。在结构性质方面,利用处理对关联的粒子数守恒方法,基于推转壳模型,系统研究了锕系核与超镄核低激发谱,发展了多维形状约束的协变密度泛函理论并用于研究锕系核势能面和裂变位垒以及N=150同中子素中的非轴对称八极关联等。在超重核合成机制方面,系统研究了利用重离子熔合反应合成超重核的三步过程,包括俘获过程——提出了一个位垒穿透概率新公式、熔合过程——提出了一个基于动力学形变势能面的双核模型、存活过程——系统研究了激发态超重复合核存活概率等。系统研究了合成超重核的热熔合反应,得到的熔合蒸发截面与实验符合,并预言了合成119和120号超重元素的生成截面。     

超重原子核与新元素研究

当前,原子核物理研究的一个重要前沿是探索原子核的电荷与质量极限,研究超重原子核与超重元素的性质,以及合成超重原子核。20世纪60年代,基于量子壳效应,理论预言质子数为114、中子数为184的原子核及其相邻核具有较长的寿命,甚至可能是稳定的,形成一个超重稳定岛。这个理论预言促进了重离子加速器及相关探测设备的建造,推动了重离子物理的发展。到目前,已经合成到了118号元素,填满了元素周期表的第7行。然而,合成更重的超重元素或包含更多中子的超重原子核面临着很多挑战,需要理论与实验密切结合,探索超重原子核的性质与合成机制,以登上超重稳定岛。文章概要评述超重原子核与新元素研究。首先介绍超重原子核与超重元素研究的背景及理论预言,包括超重核存在的根源、理论预言的概况等。之后简要给出实验合成超重核取得的主要进展和新元素命名情况。关于合成更重的超重元素面临的挑战,文章将针对利用重离子熔合蒸发反应合成超重核的截面低、所合成的超重核缺中子等情况展开讨论。最后评述近年来超重原子核结构性质、衰变、裂变与合成机制等方面的理论研究进展,包括超重核区的幻数和超重岛的位置,超重核的稳定性,利用重离子熔合蒸发反应合成超重核的三步过程及其复杂性,利用多核子转移合成超重核的探索,等等。The exploration of charge and mass limits of atomic nuclei and the synthesis of long-lived or stable superheavy nuclei (SHN) are at the frontier of modern nuclear physics. In the 1960s, based on the stability originating from quantum shell effects, the possible existence of an island of stability around 298114 was predicted. This prediction advanced the construction of heavy ion accelerators and detectors and the development of heavy ion physics. So far, superheavy elements (SHE) with Z up to 118 have been synthesized via heavy ion fusion reactions in laboratories. Recently the IUPAC/IUPAP Joint Working Party (JWP) concluded that criteria for the discovery of new elements have been met for those with Z=113, 115, 117 and 118. Therefore the seventh period of the periodic table of elements is completed. To synthesize even heavier elements or more neutron-rich SHN by using heavy ion fusion reactions, one confronts many challenges. More efforts should be made to study the properties of SHN both experimentally and theoretically. In this short review on the study on SHN and SHE, we will first introduce the background and theoretical predictions of SHN, including the origin of the possible existence of SHN and the predicted island of stability of SHN, etc. Then we will present progresses made up to now concerning the synthesis of SHN and the naming of the four new elements. As for the challenges nuclear physicists confront in synthesizing even heavier SHEs, we will detail those connected with heavy ion fusion-evaporation reactions, namely, the tiny cross sections to produce SHN and the fact that only neutron-deficient SHNs can be synthesized. Finally we will discuss some theoretical progresses on the study of SHN, including the structure of SHN and proton and neutron magic numbers after 208Pb, the stability and the synthesis mechanism of SHN as well as what we should focus on in the future.     

On the Possibility of Delayed Fission of Nuclei in the Region of Superheavy Transuranium Elements

Delayed fission of atomic nuclei was discovered in 1966. It is observed primarily in odd–odd nuclei for which the energy released in beta decay (K capture) is commensurate with the fission barrier in the nucleus formed after this process. Delayed fission was found in four nuclide regions: neutrondeficient isotopes in the Pb region, neutron-deficient isotopes in the Ac and Pa regions, and neutrondeficient and neutron-rich isotopes of transuranium elements. In the wake of investigations into the properties of isotopes of superheavy transuranium elements, numerous calculations were performed in order to determine the masses of new nuclei and to predict their decay properties. Explored and predicted properties of superheavy-element nuclides, where, for some odd–odd nuclei of transuranium elements, the K-capture energy is commensurate with the fission barriers in the corresponding daughter nuclei formed after K capture, are analyzed. Estimates of the delayed-fission probability are presented for some isotopes of elements whose charge number Z ranges from 103 to 107.     

Production mechanism of superheavy nuclei in massive fusion reactions

Within the concept of the dinuclear system (DNS), a dynamical model is proposed for describing the formation of superheavy nuclei in complete fusion reactions by incorporating the coupling of the relative motion to the nucleon transfer process. The capture of two heavy colliding nuclei, the formation of the compound nucleus and the de-excitation process are calculated by using an empirical coupled channel model, solving a set of microscopically derived master equations numerically and applying statistical theory, respectively. Fusion-fission reactions and evaporation residue excitation functions of synthesizing superheavy nuclei (SHN) are investigated systematically and compared them with available experimental data. The possible factors that affecting the production cross sections of SHN are discussed in this workshop.     

Production of neutron-deficient nuclei around N = 126 by proton-induced spallation

Many isotopes of Np, Pu, Am, and Cm around the N = 126 shell still have not been produced in the laboratory. This study aims to investigate the cross sections and yields of the neutron-deficient nuclei of Np, Pu, Am, and Cm produced in the proton-induced spallations of transuranium elements. The isospin-dependent quantum molecular dynamics (IQMD) model is applied to study the dynamical process of reaction, and the subsequent decay process is simulated by the GEMINI++ model. The IQMD-GEMINI++ model is applied to calculate the cross section, kinetic energy, and angular distribution of the isotopic productions around N = 126. The Lindhand, Scharff, and Schiott theory is applied to calculate the energy loss of different heavy nuclei in the target material. A comparison between the data and the calculations shows that the IQMD-GEMINI++ model can reproduce the production cross sections of the neutron-deficient nuclei in spallation within approximately 1.5 orders of magnitude. The maximum cross section of the undiscovered isotopes of Np, Pu, Am, and Cm is about 10^?5 mb, while the kinetic energies of the productions are all less than 16 MeV. The angular distribution shows that the emission direction of production is mostly at a backward angle. The range of production in the target is within the range of 10^?7 to 10^?5 cm. This range is the effective target thickness for the online identification of undiscovered isotopes. Based on the effective thickness of the target and assuming an intensity of 120 μA for the proton beam, the yields of the undiscovered neutron-deficient nuclei are calculated. Productions of the undiscovered isotopes of Np, Pu, Am, and Cm by the proton-induced spallations of transuranium elements are feasible. However, experimental techniques for online identification of neutron-deficient nuclei produced in proton-induced spallation should be developed.     

Production mechanism of neutron-rich nuclei around N =126 in the multi-nucleon transfer reaction ~(132)Sn+~(208)Pb

The time-dependent Hartree-Fock approach in three dimensions is employed to study the multi-nucleon transfer reaction~(132) Sn +~(208) Pb at various incident energies above the Coulomb barrier. Probabilities for different transfer channels are calculated by using the particle-number projection method. The results indicate that neutron stripping(transfer from the projectile to the target) and proton pick-up(transfer from the target to the projectile)are favored. De-excitation of the primary fragments is treated by using the state-of-art statistical code GEMINI++.Primary and final production cross sections of the target-like fragments(with Z =77 to Z =87) are investigated. The results reveal that fission decay of heavy nuclei plays an important role in the de-excitation process of nuclei with Z 82. It is also found that the final production cross sections of neutron-rich nuclei depend only slightly on the incident energy, while those of neutron-deficient nuclei depend strongly on the incident energy.     

Discovery of doubly magic 48Ni

In an experiment at the SISSI/LISE3 facility of GANIL, we used the projectile fragmentation of a primary 58Ni26+ beam at 74.5 MeV/nucleon with an average current of 3 &mgr;A on a natural nickel target to produce very neutron-deficient isotopes. In a 10-day experiment, 287 42Cr isotopes, 53 45Fe isotopes, 106 49Ni isotopes, and 4 48Ni isotopes were unambiguously identified. The doubly magic nucleus 48Ni, observed for the first time, is the most proton-rich isotope ever identified with an isospin projection T(z) = -4. It is probably the last doubly magic nucleus with "classical" shell closures accessible for present-day facilities. Its observation allows us to deduce a lower limit for the half-life of 48Ni of 0.5 &mgr;s.     

Ground State Properties of Ds Isotopes Within the Relativistic Mean Field Theory

The ground state properties of Ds (Z=110) isotopes (N=151-195) are studied in the framework of the relativistic mean field (RMF) theory with the effective interaction NL-Z2.The pairing correlation is treated within the conventional BCS approximation.The calculated binding energies are consistent with the results from finite-range droplet model (FRDM) and Macroscopic-microscopic method (MMM).The quadrupole deformation,α-decay energy,α-decay half-live,charge radius,two-neutron separation energy and single-particle spectra are analyzed for Ds isotopes to find new characteristics of superheavy nuclei (SHN).Among the calculated results it is rather distinct that the isotopic shift appears evidently at neutron number N=184.     

Possibilities of producing superheavy nuclei in multinucleon transfer reactions based on radioactive targets

The multinucleon transfer(MNT) process has been proposed as a promising approach to produce neutronrich superheavy nuclei(SHN).MNT reactions based on the radioactive targets ~(249)Cf,~(254)Es,and ~(257)Fm are investigated within the framework of the improved version of a dinuclear system(DNS-sysu) model.The MNT reaction~(238)U+~(238)U was studied extensively as a promising candidate for producing SHN.However,based on the calculated cross-sections,it was found that there is little possibility to produce SHN in the reaction ~(238)U+~(238)U.In turn,the production of SHN in reactions with radioactive targets is likely.     

Systematic study of survival probability of excited superheavy nuclei

The stability of excited superheavy nuclei (SHN) with 100 Z 134 against neutron emission and fission is investigated by using a statistical model. In particular, a systematic study of the survival probability against fission in the 1n-channel of these SHN is made. The present calculations consistently take the neutron separation energies and shell correction energies from the calculated results of the finite range droplet model which predicts an island of stability of SHN around Z = 115 and N = 179. It turn...     

Self-Consistent Study of Nuclear Charge Radii in Ar–Ti Region

Physics of Atomic Nuclei - Fully self-consistent study of the charge radii in the long chains of isotopes from Ar to Ti is presented. The neutron-deficient and neutron-rich nuclei with pairing in...     

Systematic study of survival probability of excited superheavy nuclei

The stability of excited superheavy nuclei (SHN) with 100 ⩽ Z ⩽ 134 against neutron emission and fission is investigated by using a statistical model. In particular, a systematic study of the survival probability against fission in the 1n-channel of these SHN is made. The present calculations consistently take the neutron separation energies and shell correction energies from the calculated results of the finite range droplet model which predicts an island of stability of SHN around Z = 115 and N = 179. It turns out that this island of stability persists for excited SHN in the sense that the calculated survival probabilities in the 1n-channel of excited SHN at the optimal excitation energy are maximized around Z = 115 and N = 179. This indicates that the survival probability in the 1n-channel is mainly determined by the nuclear shell effects.

     

Near-Barrier Proton Transfer in Reactions with <Superscript>3</Superscript>He Nucleus

Calculations of production cross sections for isotopes ¹⁹⁴Au in the ³He + ¹⁹⁴Pt reaction and ⁴⁵Ti in the ³He + ⁴⁵Sc reaction are performed, based on the solution to a time-dependent Schrödinger equation in combination with calculations in the statistical model using the computational code of the NRV data base. The experimental differences in the near-barrier energy dependences of the isotope production cross sections in these reactions are explained by the difference between the proton and neutron shells of the target nuclei, and by the different evolutions of the probability density for protons of the projectile nucleus and neutrons of the target nuclei in collisions.     

Isotopic production cross sections of fragmentation residues produced by 18O ions on a carbon target near 260 MeV/nucleon

The isotopic cross sections of residual nuclei produced in fragmentation reactions of ¹⁸O projectiles impinging on a carbon target at energies near 260 MeV/nucleon were measured at the HIRFL facility in Lanzhou (China). A full identification of atomic and mass numbers of fragments was achieved from the determination of their magnetic rigidity, energy loss, and time of flight. The production cross sections for a dozen of nitrogen, carbon, and boron isotopes were determined with uncertainties below 30% for most of the cases. The obtained cross sections for N and B isotopes show a rather good agreement with previous experimental data obtained with different projectile energies. The cross sections for some C isotopes seem to exhibit a dependence on the projectile energy. A comparison of the data and several theoretical model calculations are presented.     

Topology of “white stars” in the relativistic fragmentation of light nuclei

Experimental observations of the multifragmentation of relativistic light nuclei by means of emulsions are surveyed. Events that belong to the type of “white stars” and in which the dissociation of relativistic nuclei is not accompanied by the production of mesons and target-nucleus fragments are considered. An almost complete suppression of the binary splitting of nuclei to fragments of charge in excess of two, Z > 2, is a feature peculiar to charge topology in the dissociation of Ne, Mg, Si, and S nuclei. An increase in the degree of nuclear fragmentation manifests itself in the growth of the multiplicity of singly and doubly charged fragments (Z = 1, 2) as the charge of the unexcited fragmenting-nucleus part (which is the main part) decreases. Features of the production of systems formed by extremely light nuclei α, d, and t are studied in the dissociation of the stable isotopes of Li, Be, B, C, N, and O to charged fragments. Manifestations of ³He clustering can be observed in “white stars” in the dissociation of neutron-deficient isotopes of Be, B, C, and N.     

Spontaneous fission of neutron-deficient fermium isotopes and the new nucleus 241Fm

Spontaneous fission of neutron-deficient fermium isotopes (N ≤ 144) was investigated. The nuclei were produced in fusion-evaporation reactions of ⁴⁰Ar beams with ^{204,206–208}Pb targets. Excitation functions of the two-and three-neutron evaporation channels were measured. The new isotope ²⁴¹Fm was identified, and its decay properties were determined. A previously made assignment of a 0.8 ms spontaneous-fission activity to the isotope ²⁴²Fm could not be confirmed. Isomeric states were not observed in any of the investigated isotopes. The experiments were performed at the velocity filter SHIP of GSI in Darmstadt.     

Sub and near barrier fusion of neutron rich heavy nuclei – studies with radioactive ion beams

The availability of accelerated fission fragments at HRIBF allows us to study fusion reactions where one of the reactants is a short-lived exotic nucleus. Most interesting in this respect are entrance channels involving neutron-rich target and projectile – where enhanced survival probability of the compound system may allow the synthesis of heavier system. Much depends though on the dynamic evolution of the captured nuclei into a compound nucleus and the ensuing competition between fission and evaporation residue decay modes. Our studies of fusion between heavy neutron-rich nuclei are aimed at acquiring data that will lead to the understanding and eventually the ability to predict the probabilities for these different processes.     

Systematic Study on Alpha Decay Half-Lives of Superheavy Nuclei

The α-decay half-lives of a set of superheavy nuclear isotope chain from Z = 105 to 120 have been analyzed systematically within the WKB method, and some nuclear structure features are found. The decay barriers have been determined in the quasi-molecular shape path within the Generalized Liquid Drop Model （GLDM） including the proximity effects between nucleons in a neck and the mass and charge asymmetry. The results are in reasonable agreement with the published experimental data for the alpha decay half-llves of isotopes of charge 112, 114, and 116, of the element 294118 and of some decay products. A comparison of present calculations with the results by the DDM3Y effective interaction and by the Viola-Seaborg-Sobiczewski （VSS） formulae is also made. The experimental a decay half lives all stand in between the GLDM calculations and VSS formula results. This demonstrates the possibility of these models to provide reasonable estimates for the half-lives of nuclear decays by a emissions for the domain of SHN. The half-lives of these new nuclei are thus well tested from the reasonable consistence of the macroscopic, the microscopic, the empirical formulae and the experimental data. This also shows that the present data of SHN themselves are consistent. It could suggest that the present experimental claims on the existence of new elements Z =110 - 118 are reliable. It is expected that greater deviations of a few SHN between the data and the model may be eliminated by further improvements on the precision of the measurements.     

Nuclear radius systematics of Kr isotopes studied via their interaction cross-sections at relativistic energies

The interaction cross-sections of neutron-deficient ^72,76,80Kr isotopes have been measured by use of the transmission method at relativistic energies. Based on the Glauber model analysis effective root-mean-square matter radii of these nuclei have been determined for the first time. A systematic evolution of the matter radii in the mass A ∼70 region is discussed and compared with that of light nuclei.