Continuum effect in resonance spectra of neutron-rich oxygen isotopes

Starting from the CD-Bonn potential, we have performed Gamow shell-model calculations for neutronrich oxygen isotopes, investigating excitation spectra and their resonant properties. The Gamow shell model is based on the Berggren ensemble, which is capable of treating the continuum effect reasonably in weakly bound or unbound nuclei. To calculate heavier-mass oxygen isotopes, we choose ~(16)O as a frozen core in the Gamow shell-model calculations. The first 2+excitation energies of the even-even O isotopes are calculated, and compared with those obtained by the conventional shell model using the empirical USDB interaction. The continuum effect is proved to play an important role in the shell evolution near the drip line. We also discuss the effect of the Berggren contour choice. We improve the approximation in the contour choice to give more precise calculations of resonance widths.     

Fermion-boson coupling model (QCVM) applied to odd Zn isotopes

Low lying energy levels and transition probabilities of^63,65,67Zn are calculated in the Quasicluster-vibration model (QCVM). This model is an extension of the Clustervibration model (CVM) for three-particle-cluster vibration coupling. Instead of a three-particle cluster, now a one broken pair (number projected three quasiparticle) cluster is coupled to phonon states. It turns out that the sequence of isotopes may be described with one set of parameters, along the same line as previously used in the CVM. The resulting spectra and electromagnetic properties, for positive and negative parity states of odd Zn isotopes, compare with experimental data equally well as large shell-model calculations; this seems to justify the extension of the CVM to these nuclei with more than three particles or holes beyond a closed shell.     

Transition from spherical to deformed shapes of nuclei in the Monte Carlo shell model

The transition from spherical to deformed shapes is studied in terms of large-scale shell-model calculations for Ba isotopes as a function of valence nucleon number with fixed single-particle space and Hamiltonian. A new version of the Monte Carlo shell model is introduced so as to incorporate pairing correlations efficiently, by utilizing condensed pair bases. The energy levels and electromagnetic matrix elements are described in agreement with experiments throughout the transitional region. The orbital M1 sum rule is calculated as a measure of the deformation evolution, and the Q-phonon picture is shown to be reasonable from spherical to deformed nuclei.     

Nuclear moments: An effective probe of nuclear stucture

Magnetic dipole and electric quadrupole moments are discussed in nuclei near doubly-closed shell nuclei (the T1 nuclei) and in nuclei along series of single-closed shell nuclei (plus of minus a few nucleons) (the In odd-mass and odd-odd nuclei). We discuss the “additivity” rules for nuclear moments. We also address the EO moment: the liquid drop model and the shell-model are discussed and compared to measurements of nuclear radii in the Ca, Sn and Pb region. In the latter region, the importance of intruder states across the Z=82 proton closed shell is emphasized.     

一些近期发现的同核异能态的壳模型解释

近期，在101In、123,125Ag和218Pa等核中，首次观测到同核异能态。本工作通过原子核壳模型解释In、Ag同位素和$N\!=\!127$同中素中的这些同核异能态及相关的同核异能态背后的物理原因。101-109In这五个奇A核In同位素中，观测到的$1/2^{-}$同核异能态的激发能非常接近。这可以通过引入中子近期，在101In、123,125Ag和218Pa等核中，首次观测到同核异能态。本工作通过原子核壳模型解释In、Ag同位素和$N\!=\!127$同中素中的这些同核异能态及相关的同核异能态背后的物理原因。101-109In这五个奇A核In同位素中，观测到的$1/2^{-}$同核异能态的激发能非常接近。这可以通过引入中子$0g_{7/2}$和$1d_{5/2}$轨道间的很强的组态混合来解释。更进一步分析表明，这些奇A核In同位素中，从$9/2^{+}$基态到$1/2^{-}$同核异能态，一个质子从$1p_{1/2}$轨道激发到$0g_{9/2}$轨道。这一质子组态变化可能引发中子$0g_{7/2}$和$1d_{5/2}$轨道的单粒子能变化。这样一个原子核内的组态依赖的壳演化被称为第二类壳演化。与In同位素类似，123,125Ag的同核异能态被发现是$1/2^{-}$态，对应着一个质子空穴在$1p_{1/2}$轨道。但之前观测到的115,117Ag的$1/2^{-}$态是基态。这意味着质子$1p_{1/2}$轨道和$0g_{9/2}$轨道在$N\!=\!72$附近发生了反转。壳模型分析表明张量力是造成这两个轨道反转的决定性原因。之前观测到的奇奇核$N\!=\!127$同中素210Bi、212At、214Fr和216Ac中，基态是$1^{-}$态，同时存在高自旋的同核异能态。然而，基于$\alpha$衰变性质和壳模型计算，推荐218Pa中的基态和新发现的同核异能态分别为$8^{-}$态和$1^{-}$态。奇奇核$N\!=\!127$同中素基态和同核异能态的演化是由质子中子相互作用从粒子粒子形式转化为空穴粒子形式以及质子组态混合所导致。总的来说，壳模型对这些双幻核100Sn、132Sn和208Pb附近核中新发现的同核异能态有较好的描述。双幻核附近核中的同核异能态，也称为壳模型同核异能态，是核结构研究中非常重要的。因为这些同核异能态常常提供了中重质量区域极端丰中子和缺中子原子核中的第一个谱学性质，并包含了丰富的物理信息，比如质子中子相互作用及其在壳演化中的作用。     

Oblique-basis shell-model calculations

A one-dimensional harmonic oscillator in a box is used to introduce the oblique-basis concept. The method is extended to the nuclear shell model by combining traditional spherical shell model states, which yield a diagonal representation of the usual single-particle interaction, with SU(3) shell model collective configurations that track deformation. An application to ²⁴Mg, using the realistic two-body interaction of Wildenthal, is used to explore the validity of this mixed-mode shell-model scheme. The theory is also applied to lower pf-shell nuclei, ^44–48Ti and ⁴⁸Cr, using the Kuo-Brown-3 interaction. These nuclei show strong SU(3) symmetry breaking due mainly to the single-particle spin-orbit splitting. Nevertheless, the results also show that yrast band B(E2) values are insensitive to fragmentation of SU(3) symmetry. Specifically, the quadrupole collectivity as measured by B(E2) strengths remains high even though the SU(3) symmetry is rather badly broken. The results suggest that an oblique-basis mixed-mode shell-model theory may be useful in situations where competing degrees of freedom dominate the dynamics.     

Cross sections for evaporation residue production near theN=126 shell closure

In fusion reactions of⁴⁰Ar with isotopes of Ho, Tm, Yb, Lu, Hf and Ta, cross sections for the production of proton-rich evaporation-residues near the 126 neutron shell were measured. This first comprehensive study of very fissile spherical residues reveals a surprisingly low stabilizing influence of the sphericalN=126 shell on the survival probability. The experimental results are compared with evaporation calculations. Conclusions for the production of superheavy nuclei are drawn.     

丰中子Zn核素奇特核结构讨论和展望

不稳定核结构是当前核物理研究的前沿热点问题之一，尤其是针对丰中子幻数核附近的区域。中子数N=40, 50附近镍区域核素展现出丰富的结构特征，激励了众多理论和实验研究。原子核的基本性质与核的结构密切相关，这里我们选择分析丰中子Zn(Z=30)同位素的基本性质来进一步了解这一核区的核结构特征。本文回顾了在欧洲核子中心(CERN)的ISOLDE测量Zn 同位素的实验，基于62–80Zn 核素基态和长寿命同核异能态的自旋、磁矩、电四极矩以及电荷均方根半径等基本性质，并结合各种大规模壳模型计算结果，系统地讨论了这一核区的壳结构演化、幻数特征、奇特形变和形状共存，以及核子间关联激发等物理现象。最后，基于已有的实验数据和物理现象，以及理论预言的 N=50以上镍核区的能级演化特征，我们提出在ISOLDE的共线共振电离谱装置上测量更加丰中子的81,82Zn 核素基本性质的实验设想。     

High-spin structures of 86,87,88,89Y isotopes

In the present paper nuclear structure properties of ^86,87,88,89Y isotopes have been investigated using large-scale shell-model calculations within the full f _5/2pg9/2 model space. The calculations have been performed with JUN45 and jj44b effective interactions that have been proposed for use in the f _5/2, p _3/2, p _1/2, g _9/2 model space for both protons and neutrons. Reasonable agreement between experimental and calculated values are obtained. This work will add more information to the previous study by the projected shell model [Eur. Phys. J. A 48, 138 (2012)], where full-fledged shell-model calculations were proposed for these nuclei.     

缺中子Np新核素的α衰变研究

合成远离稳定线的新核素、探索原子核存在的极限是目前核物理研究的重要课题。在中子壳N=126的最丰质子一侧，极端缺中子的超铀核素处于质子滴线和中子壳的交叉位置，合成和研究该核区核素对研究N=126壳结构的演化性质具有重要意义。基于兰州重离子加速器上的充气反冲核谱仪装置(SHANS)，利用36,40Ar+185,187Re熔合蒸发反应，合成了极缺中子的219,220,223,224Np新核素，在中子壳N=126附近首次建立了Np同位素链的$ \alpha$衰变系统性，获得了N=126壳效应在Np同位素链中依然存在的实验证据。依据单质子分离能的系统性分析，确定了Np同位素链中质子滴线的位置，219Np也成为目前已知的最重的质子滴线外核素。此外，基于实验测量的反应截面，并与理论模型的计算结果相比较，讨论了进一步合成该核区其它新核素218,221,222Np的可行性。     

Electromagnetic properties of neutron-rich Ge isotopes

The electric quadrupole moment Q and the magnetic momentp(or the g factor) of low-lying states in even-even nuclei ~(72-80)Ge and odd-mass nuclei ~(75-79) Ge are studied in the framework of the nucleon pair approximation(NPA) of the shell model,assuming the monopole and quadrupole pairing plus quadrupole-quadrupole interaction.HA H.Our calculations reproduce well the experimental values of Q(2_1~+) and g(2_1~+) for ~(72,74,76) Ge,as well as the yrast energy levels of these isotopes.The structure of the 2_1~+ states and the contributions of the proton and neutron components in Q(2_1~+) and g(2_1~+) are discussed in the SD-pair truncated shell-model subspace.The overall trend of Q(2_1~+) and g(2_1~+)as a function of the mass number A,as well as their signs,are found to originate essentially from the proton contribution.The negative value of Q(2_1~+) in ~(72,74)Ge is suggested to be due to the enhanced quadrupole-quadrupole correlation and configuration mixing.     

Implications of deformation and shape coexistence for the nuclear shell model

The successes of the nuclear shell model in explaining the stability properties of magic nuclei are challenged by the observation of rotational bands for which the sequential filling of single-particle energy levels of the spherical shell model are not respected. This Letter proposes criteria for identifying the shell-model configurations appropriate for describing such bands of states.     

Shell-model studies of the N=14 and 16 shell closures in neutron-rich nuclei

Shell-model studies on the N=14 and 16 shell closures in neutron-rich Be, C, O and Ne isotopes are presented. We calculate, with the WBT interaction, the excited states in these nuclei. The calculations agree with recent experiment data. Excited energies and B(E2) values are displayed to discuss the shell closures. Our results support the N=16 shell closure in these isotopes, while indicating a disappearance of N=14 shell closure in Be and C isotopes.     

Shell-model studies of the N=14 and 16 shell closures in neutron-rich nuclei

Shell-model studies on the N =14 and 16 shell closures in neutron-rich Be, C, O and Ne isotopes are presented. We calculate, with the WBT interaction, the excited states in these nuclei. The calculations agree with recent experiment data. Excited energies and B(E2) values are displayed to discuss the shell closures. Our results support the N =16 shell closure in these isotopes, while indicating a disappearance of N =14 shell closure in Be and C isotopes.     

Charge Radii of Argon Isotopes in the f7/2 Shell and Radii Systematics in the Ca-Region

Changes in mean square (ms) nuclear charge radii of Ar isotopes across the 1f_7/2 shell are studied by fast-beam collinear laser spectroscopy using an ultra-sensitive detection method based on optical pumping and state-selective collisional ionization. The new data set on Ar, in combination with the known charge radii of K, Ca and Ti in the ν1f_7/2 shell, offers an opportunity to obtain a more complete overview of nuclear radii trends around the proton shell closure Z = 20 and between the neutron shell closures N = 20 and N = 28.     

Rotational properties of shell-model states infp-shell nuclei

The relation between the microscopic shell model and the collective rotor model is investigated. As the first step an extensive shell-model calculation is performed on about twentyfp-shell nuclei with massA=52–60. It turns out that, with the model space and the effective interaction chosen, the shell model is well able to reproduce the experimental data in this mass region. As the second step the shell-model wavefunctions are used to calculate energies, electromagnetic moments and transition rates of states with spin up toJ=16. As the third step the observables calculated with the shell model are used to investigate whether these microscopic results can be reproduced by a simple rotational model. About twenty pure axially symmetric rotor bands, generated by the shell model, could be localized. Their properties are presented and discussed.     

Proton-neutron interaction near closed shells

Odd-odd nuclei around double shell closures are a direct source of information on the proton-neutron interaction between valence nucleons. We have performed shell-model calculations for doubly odd nuclei close to ²⁰⁸Pb, ¹³²Sn, and ¹⁰⁰Sn using realistic effective interactions derived from the CD-Bonn nucleon-nucleon potential. The calculated results are compared with the available experimental data, attention being focused on particle-hole and particle-particle multiplets. While good agreement is obtained for all the nuclei considered, a detailed analysis of the matrix elements of the effective interaction shows that a stronger core-polarization contribution seems to be needed in the particle-particle case.     

The axial-vector strength in the proton-rich argon isotopes

The proton-rich isotopes³²Ar-³⁵Ar represent the first sequence for which the superallowed Gamow-Teller strength is known out toZ? N=4. The results are compared with two sets of predictions from large shell-model calculations, which give systematically different quenching factors, on the average 0.54 and 0.89. Possible reasons for this are discussed. A comparison is also made with the Gamow-Teller strength of proton-rich isotopes of neighbouring elements.