Spectroscopic factors within the dinuclear-system model

A model of the cluster radioactivity of even—even nuclei is presented. In this model, zero-point vibrations in the charge-asymmetry coordinate determine spectroscopic factors, while tunneling in the coordinate of the relative separation of the centers of mass of the cluster and the daughter nucleus determine the penetrability of the barrier of the nucleus-nucleus potential. The conservation of the total spin and energy in the decay process is taken into account. The potential of the model for describing experimental half-lives is demonstrated. A number of predictions for possible cluster-emission reactions in the regions of lead and tin radioactivities are made.     

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.     

Alpha-decay fine structures of U isotopes and systematics for isotopic chains of Po and Rn

A model of the cluster radioactivity of even-even nuclei is presented. In this model, the zero-point vibrations in the charge-asymmetry coordinate determine the cluster formation (spectroscopic factor), while the tunneling in the coordinate of the relative separation of the centers of mass of the cluster and the daughter nucleus determines the penetrability of the barrier of the nucleus-nucleus potential. The quality of the model is demonstrated for describing cluster decay half-lives and the fine structure in alpha decays of even-even U isotopes. The model is applied to describe the alpha decays in the isotopic chains of Po, Rn, and U. The correspondence of the calculated half-lives to the Geiger-Nuttall law is discussed.     

Generalization of α-Decay Cluster-Model to Nuclei Near Spherical and Deformed Shell Closures

The cluster model of a-decay is extended to the regions around doubly magic spherical nucleus ^208Pb and around deformed shell closure ^270Hs, respectively. The effects of spherical shell closures (N=126 and Z=82) on α-decay are investigated by introducing an N-dependent α-preformation factor and a Z-dependent one inspired by a microscopic model. Good agreement between the theoretical a-decay half-lives and the measured ones is obtained for the spherical nuclei near the doubly magic nucleus ^208Pb, where the nuclear shell effect is included in the expression of α-preformation factor. The cluster model is also generalized for the decay of deformed nuclei. The branching ratios of α-decays from the ground state of a parent nucleus to the ground state (0^ ) of its deformed daughter nucleus and to the first excited state (2^ ) are calculated in the framework of the cluster model. The results indicate that a measurement of α spectroscopy is a feasible method to extract the information of nuclear deformation of superheavy nuclei around the deformed nucleus ^270Hs.     

α结团模型在形变核区域的推广

将α结团模型推广至形变核,计算偶偶形变母核α衰变基态到子核基态和子核第一激发态的分支比,显示出α衰变精细结构的测量是提取核形变信息的有效手段.The cluster model of α-decay is extended to deformed nuclei. The branching ratios of α-decays from the ground state of a parent nucleus to the ground state 0~(+) of its deformed daughter nucleus and to the first excited state 2~(+) are calculated in the framework of the cluster model. The results indicate that a measurement of α spectroscopy is a feasible method to extract the information of nuclear deformation.     

密度依赖的结团模型研究原子核的α衰变和结团放射性

讨论了原子核α衰变和结团放射性的一个新的结团模型:密度依赖的结团模型（DDCM）。DDCM基于微观真实的核势和库仑势,可以在统一的框架下计算原子核的各种衰变模式的半衰期。通过对原子核α衰变和结团放射性的系统计算,发现DDCM能够精确给出原子核α衰变和结团放射性的半衰期,理论值和实验值的偏差一般在3倍以内。这表明密度依赖的结团模型具有很好的适用性并可以为将来的实验提供较准确的预言。We have discussed a new cluster model of α-decay and cluster radioactivity: the density-dependent cluster model （DDCM）, which is based on the microscopic nuclear potential and Coulomb potential. DDCM can give accurate theoretical half-lives for various decays of nuclei in a unified framework. Through a systematical calculation of half-lives of α-decay and cluster radioactivity, we find that the deviation between DDCM and experiment is usually less than a factor of 3. This good agreement shows that the density-dependent cluster model can further provide reliable predictions for future experiments.     

Spectroscopic factors and barrier penetrabilities in cluster radioactivity

The cold cluster decay model is presented in the framework of a dinuclear system concept. Spectroscopic factors are extracted from barrier penetrabilities and measured half-lives. The deformation of the light cluster and residual nucleus is shown to affect the nucleus-nucleus potential and decay characteristics. Half-lives are predicted for neutron-deficient actinides and intermediate-mass nuclei. The connection between spontaneous fission and cluster radioactivity is discussed.     

Cubic potential models for cluster radioactivity

Cluster radioactivity is a process by which nuclei equal and heavier than the α-particle is emitted spontaneously. The clusters usually emitted in this process are the α-particle, carbon, oxygen, neon, magnesium, silicon etc. When the mass of the cluster becomes comparable with the mass of the daughter, symmetric fission takes place. Thus the cluster radioactivity is an intermediate process between the well known α-decay and the spontaneous fission. In earlier years such cluster radioactivity was found mostly in actinide nuclei like radium, uranium etc. Very recently it has been predicted that such decays are possible in a new region around ¹¹¹Ba. There has been an exciting experimental detection of the emission of ¹²C from ¹¹¹Ba leading to ¹⁰²Sn, which is attracting a lot of attention recently. To study the phenomenon of cluster radioactivity there are various theoretical models in vogue. The existing models generally fall under two categories: the unified fission model (UFM) and the preformed cluster model (PCM). The physics of the UFM and the PCM are completely different. The UFM considers cluster radioactivity simply as a barrier penetration phenomenon in between the fission and the α-decay without worrying about the cluster being or not being preformed in the parent nucleus. In the PCM clusters are assumed to be preborn in a parent nucleus before they could penetrate the potential barrier with a given Q-value. The basic assumption of the UFM is that heavy clusters as well as the α-particle have equal probability of being preformed. In PCM, clusters of different sizes have different probabilities of their being preformed in the parent nucleus. We have developed three fission models during the last decade using the cubic potential for the pre-scission region. The use of these models in the study of cluster radioactivity in both the actinide and barium regions will be discussed in this talk in comparison with the other existing theories.     

The synthesis and properties of superheavy nuclei produced in the 48Ca+244Pu, 248Cm reactions

We present the results of the experiments aimed at producing hypothetical long-lived superheavy elements located near the spherical shell closures with Z>-114 and N>-172. For the synthesis of superheavy nuclei a combination of neutron-rich reaction partners, such as ²⁴⁴Pu and ²⁴⁸Cm targets and a ⁴⁸Ca projectile have been used. The sensitivity of the present experiment exceeded by more than two orders of magnitude previous attempts to synthesize superheavy nuclides in reactions of ⁴⁸Ca projectiles with actinide targets. We observed new decay sequences of genetically linked α-decays terminated by spontaneous fission. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides ^288,289114, produced in the 3n and 4n-evaporation channels, and ²⁹²116 — in the 4n-evaporation channel with cross sections of about a picobarn. The present observations can be considered an experimental evidence of the existence of the “island of stability” of superheavy elements and are discussed in terms of modern theoretical approaches.     

Structure of 6Li from high-energy proton scattering

The elastic and inelastic (2.189 MeV level) scattering of 1 GeV protons from ⁶Li is predicted using Glauber multiple-scattering theory in conjunction with an α-d cluster model capable of fitting quantitatively a large number of measured form factors, in particular the most recent high-momentum-transfer electron scattering data. The predictions, especially in the inelastic channel, are quite sensitive to the parameters of the model. A valuable opportunity is thus provided via experiments well within the capabilities of LAMPF for comparison of structure information obtained from proton scattering with that from other sources in the case of a “test” nucleus whose properties are relatively well determined and unambiguous. Moreover, proton scattering goes beyond electron scattering in possibly providing information about the effective real part of the proton-deuteron cluster amplitude, and thus indirectly about D-state effects in the deuteron cluster.     

Role of 208Pb daughter nucleus and nuclear shell effects in trans‐lead cluster radioactivity by emission of neon clusters from 218–236U isotopes

In the present work, the cluster radioactivity of even A uranium isotopes (^218–236U) with the emission of both alpha‐like and non‐alpha‐like neon clusters (^{20,22,24,26,28}Ne) was studied. The decimal logarithm of half‐lives (expressed in seconds) were calculated by three different approaches based on (i) the single line of Universal curve (UNIV) for alpha and cluster radioactive decay, (ii) the Universal Decay law (UDL) and (iii) by considering a fission‐like model in which the interacting nuclear potential barrier was taken to be the sum of Coulomb and proximity potentials (CPPM) respectively. Based on the half‐lives calculated by using the three different approaches mentioned above, significance of the role of ²⁰⁸Pb nucleus (doubly magic nucleus) and nuclear shell effects in trans‐lead cluster radioactivity were investigated. The calculated half‐lives have also been compared with available experimental results. It was observed that cluster decay modes leading to the formation of ²⁰⁸Pb daughter nucleus have the lowest half‐lives. This implies that there is a shell closure at proton number (Z) = 82 and neutron number (N) = 126. Hence, it confirms the existence of nuclear shell effect and stresses the significance of role of ²⁰⁸Pb daughter nucleus in the trans‐lead cluster radioactivity. It can be noticed that the calculated half‐lives for several cluster decay modes are well within the present experimental upper limit for measurements (T_1/2 < 10³⁰S). These results may be useful for future experiments.     

Recoil corrections for single-nucleon transfer reactions

A formalism is developed for approximately treating the recoil of the projectile in single-nucleon transfer reactions. The “no-recoil” term is evaluated exactly by rotating the nuclear matrix element to a coordinate system where the lighter projectile lies along the z-axis relative to the target nucleus. Recoil terms are constructed by expanding the “recoil phase factor” in terms of spherical harmonic and Bessel functions, and they are again evaluated exactly. Numerical results are then obtained for some representative light-ion reactions of high incident energies. For these reactions, we find that although the first-order recoil term may be very large, higher order terms are correspondingly less important, and convergence is always easily obtained. However, absolute magnitudes of cross sections at forward angles predicted by the present theory and by the local energy approximation differ by up to 50% for the reactions we consider.     

8He Nuclei Stopped in Nuclear Track Emulsion

A nuclear track emulsion (NTE) is exposed to 60 MeV ⁸He nuclei. Measurements of decays of ⁸He nuclei stopped in NTE allow one to evaluate possibilities of α-spectrometry. Thermal drift of ⁸He atoms is observed. Knowledge of the energy and emission angles of α-particles allows one to derive the energy distribution of α-decays Q_2α . The presence of a “tail” of large values Q_2α is established.     

Resonating group method and frequency transformation

A new technique for calculating the resonating-group-method (RGM) kernels is proposed by use of frequency transformations in order to treat cluster systems involving unequal oscillator frequencies. The present technique is useful for both analytical and numerical deviations of matrix elements of the RGM kernels. The eigenvalue problem of the norm kernel of the RGM is solved for a wide range of systems consisting of an α-particle and a closed-shell nucleus (¹⁶O, ⁴⁰Ca, ⁴⁸Ca, ⁵⁶Ni, ⁸⁰Zr, ⁹⁰Zr, ¹²⁰Sn, ¹⁴⁰Yb or ²⁰⁸Pb) with realistic oscillator frequencies. These eigenvalues and eigenfunctions are shown to have characteristic features. With these eigenvalues, α-particle reducedwidth amplitudes and spectroscopic factors of the ground states of ²⁰Ne, ⁴⁴Ti, ⁹⁴Mo, ¹²⁴Te and ²¹²Po are calculated by employing simple configurations of the harmonic-oscillator shell model. In heavy nuclei the results with realistic oscillator frequencies are shown to be very different from those with a common oscillator frequency. The effect of the spurious c.m. motion in the generator coordinate method is discussed.     

Large-angle quasi-free scattering in 6Li(p,pd)4He at 670 MeV

The ⁶Li(p, pd)⁴He reaction was investigated at 670 MeV by a coincidence experiment with a large-angle scattering geometry. The energy sharing and angular correlation of the reaction products were measured and the momentum distribution of the recoil nucleus was determined for transitions leading to residual nuclei in the ground and excited states. Results were analysed in terms of a simplified distorted-wave impulse approximation using cluster-model and three-body wave functions. The observed momentum distribution of the p-n pair in the p-shell of ⁶Li is in agreement with three-body calculations, while the spectroscopic factor is larger than predicted by theory. Transitions to breakup states of the α-particle also have the characteristics of quasi-free scattering on deuteron clusters.     

Inclusion of the spin-orbit force in an α-cluster model

A simplified treatment is proposed to take into account the effect of the spin-orbit force within the framework of the conventional α-cluster model for light nuclei. It is very important for a study of the stability of α-cluster structure in light nuclei to include the effect of the spin-orbit force, but it has been considered that its inclusion is not so easy because (0s)⁴ internal states are usually assumed for α-clusters in the model. The proposed method introduces states with [44…431] spatial symmetry by means of an operator which is just equal to a one-body spin-orbit potential. The method is applied to a 3α cluster model for ¹²C.     

Statistical characteristics of the motion of a pair of heavy ions in non-convex cavities of complicated geometry with fixed crosspieces and charges

Using the method of classical trajectories, the motion of a pair of ions Cs⁺ and Cl^– in three non-convex closed cavities of complicated geometry containing neutral cylindrical “crosspieces” and charged spherical “nuclei” is simulated. The collisions of the ions with the cavity boundary, crosspieces, and nuclei are supposed to be inelastic. The signs of the charges of the nuclei are varied. In each cavity, in one of the calculation series, the nuclei are assumed to be shielded (to carry charges smaller than 1 a.u. in absolute value), whereas in another calculation series, the possibility of the mutual neutralization of an ion and an oppositely charged nucleus at their encounter is taken into account. The statistics of various events, such as the “stickings” of ions to nuclei, neutralizations of ions and nuclei, collisions of ions with obstacles, recombinations of ions, and dissociations of the CsCl ionic-bond molecule are analyzed.     

Alpha clusters in nuclei with 41 ≦ A ≦ 45

The effect of pairing and surface α-clustering structure was studied by means of a Hamiltonian with pairing and four-body interaction terms in a simple two-level model in odd nuclei with 41 ≦ A ≦ 45. It was shown that the dynamical α-structures can be induced by the pairing correlation alone, but with an unusually large value of the strength G of the pairing interaction. The addition of a small four-body interaction term allowed us to diminish the G-value to a physically reasonable one and, at the same time, to get meaningful α-clustering. The four-body term added to the Hamiltonian is a simplified way to improve, in this respect, the two-body pairing interaction (itself a simplified model interaction). For example, in the ground state of ⁴⁵Ti the “α-clusters” appeared with larger probability than that for separate nucleons, while in ⁴⁵Ca both probabilities are approximately equal. The calculated binding energy was also applied to reproduce the experimental difference of the masses of neighbouring nuclei. With the eigenstates of the Hamiltonian, schematic spectroscopic factors for two-nucleon transfer, α-transfer, and elastic scattering of α-particles were also calculated. Spectroscopic factors for the transitions from ground state to ground state and from ground state to excited state were then used to distinguish between superfluid or normal behaviour of these nuclei. In addition to the superfluidity induced by pairing forces, a superfluidity due to four-body correlations was also predicted.     

基于折叠势的两势方法系统研究质子放射性(英文)

基于两势方法系统地研究了质子数51 ≤ Z ≤ 83质子放射性核素的衰变半衰期。总的质子-子核相互作用势包括:通过单折叠子核密度和DDM3Y有效相互作用得到的微观核势,通过单折叠子核电荷密度和质子-质子库仑相互作用得到的真实库仑势以及离心势。同时,预测了同一区域16个核的质子放射性半衰期,并且预测的质子放射性半衰期在4.11倍的范围内。此外,还研究了质子放射性的Geiger-Nuttall定律。结果表明,Geiger-Nuttall定律可以用来描述角动量相同的同位素的质子放射性。In the present work, we systematically study the half-lives of proton radioactivity for 51 ≤ Z ≤ 83 nuclei within the two-potential approach. The total emitted proton-daughter nucleus interaction potential is composed of the microscopic nuclear potential obtained by single folding the density of the daughter nucleus with the DDM3Y effective interaction, the realistic Coulomb potential obtained by single folding the charge density of the daughter nucleus with the proton-proton Coulomb interaction and the centrifugal potential. We extend our study to predict proton radioactivity half-lives of 16 nuclei in the same region within a factor of 4.11. In addition, the Geiger-Nuttall law for proton radioactivity is researched. The results indicate that the Geiger-Nuttall law can be used to describe the proton radioactivity isotopes with same angular momentum.     

Studying of the neutron halo structure in quasi-free proton scattering from 6He Halo nucleus

An experimental method based on an analysis of the neutron-neutron correlations in reactions induced by halo nuclei in nuclear photoemulsion is proposed for studying the two-neutron halo structure. Photoemulsion stacks were exposed to a beam of radioactive ⁶He nuclei. Preliminary data on interaction of the ⁶He halo nucleus to hydrogen are compared with the results of the kinematic calculation for the reaction of quasi-free proton scattering from clusters making up the ⁶He halo nucleus. A fraction of quasi-free scattering events with “survival” of the dineutron is evaluated for scattering of protons by the dineutron configuration of the ⁶He nucleus.