Matrix Elements in Fermion Dynamical Symmetry Model

In a neutron-proton system, the matrix elements of the generators for SO(8)×SO(8) symmetry are constructed explicitly, and with these matrix elements the low-lying excitation spectra obtained by diagonalization are presented. The excitation spectra for SO(7) nuclei Pd and Ru isotopes and SO(6) r-soft rotational nuclei Xe, Ba, and Ce isotopes are calculated, and comparison with the experimental results is carried out.     

稀土区奇A核晕带存在从转动到振动形状演化的直接证据(英文)

原子核的形状演化效应是核结构研究的重要基础问题之一。通常认为,A=160质量区的奇A核位于大形变核区域,它们的激发态能谱将呈现出典型的转动激发特征。然而,基于E-GOS曲线方法,发现随着角动量的增加,该质量区奇A核的晕带具有显著地从转动激发演化成为振动激发的形状演化现象。此外,为深入理解原子核形状演化的微观机制,采用Total-Routhian-Surface（TRS）方法针对稀土区的奇A核进行了理论计算,结果表明,165Yb和157Dy同位素在低激发态时具有稳定的长椭形变,当角动量大于0.50 MeV后,核芯的四极形变显著减小并开始产生三轴形变。The phase transition of nuclei with increasing angular momentum (or spin) and excitation energy is one of the most fundamental topics of nuclear structure research. The odd-N nuclei with A ≈160 are widely considered belonging to the well-deformed region, and their excitation spectra are energetically favored to exhibit the rotational characteristics. In this work, however, the evidence suggesting that the nuclei changes from rotation to vibration along the yrast lines as a function of spin was found. The simple method, named as E-Gamma Over Spin (E-GOS) curves, would be used to discern the evolution from rotational to vibrational structure in nuclei for various spin ranges. Meanwhile, in order to understand the band structure properties of nuclei, theoretical calculations have been performed for the yrast bands of the odd-A rare-earth nuclei within the framework of the total routhian surface (TRS) model. The TRS plots predict that the 165Yb and 157Dy isotopes have large quadrupole shapes at low spin states. At higher rotational frequency (hω~ >0.50 MeV), a clear reduction of the quadrupole deformation is indicated by the present results, and the isotopes become rigid in the γ deformation.     

A gamma-ray signature of energetic sources of cosmic-ray nuclei

Astrophysical sources of nuclei are expected to produce a broad spectrum of isotopes, many of which are unstable. An unstable nucleus can beta-decay outside the source into a single-electron ion. Heavy one-electron ions, thus formed, can be excited in their interactions with cosmic microwave background photons, in which case they relax to the ground state with the emission of a gamma ray. Repetitive cycles of excitation and gamma-ray emission can produce an observable feature in the gamma-ray spectrum with a maximum around 8 GeV (for iron). We find that the observed spectrum of Centaurus A is consistent with a substantial flux of nuclei accelerated to 0.1 EeV. A characteristic 5-10 GeV (iron) shoulder in the gamma-ray spectra of various sources can help identify astrophysical accelerators of nuclei or set upper limits on nuclear acceleration.     

Search for light neutron-rich isotopes in stopped pion absorption

The results based on the spectroscopy of superheavy hydrogen isotopes (^4?7H), heavy helium isotopes (^6,7He), and heavy lithium isotopes (^7?12Li) produced in stopped pion absorption by light nuclei were analyzed. Search for nuclear states was performed in inclusive and correlation measurements of missing mass spectra. A broad range of excitation energies studied in correlation measurements provided the possibility of search for isobaric analog states and cluster resonances. A comparison with experimental and theoretical results of other authors was conducted.     

Three-body forces in sd-shell nuclei

The influence of three-body forces on the excitation spectra of nuclei with 3 valence nucleons in the sd-shell is investigated. Three-body forces are considered, which arise from an intermediate excitation of the interacting nucleons to the Δ(3, 3) resonance. Besides these real three-nucleon forces, effective three-body interactions are taken into account which are due to the restriction of the nuclear structure calculation to sd-shell configurations. Significant cancellations are observed between the different contributions to the effective three-nucleon force. The resulting three-body matrix elements yield only a small influence on the spectrum of the A = 19 systems. The typical size of the matrix elements, however, is large enough to expect a serious influence on the results of shell-model calculations with more than three valence nucleons.     

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.     

Matrix elements of Yale potential and level properties of light nuclei

Shell model calculations using bare and renormalized matrix elements of the Yale potential are reported for the normal-parity states ofA=6–9 nuclei. Renormalization of the two-body matrix elements using second-order perturbation theory is not found to improve the agreements with the experimental data. Inclusion of the energy shifts of ground state rotational bands in⁸Be and⁹Be are, however, found to improve the agreements with the excitation energies of nuclear levels. The need for carrying out more calculations of these nuclei with realistic forces is pointed out.     

Compound nucleus contribution and even-odd effect for (3He,p) reactions in the Ni region

The compound nucleus contributions to the proton spectra from 8 MeV and 10 MeV ³He induced (³He, p) reactions on even-A Ni isotopes were obtained. The relative cross sections for ⁵⁸Ni/⁶⁰Ni/⁶²Ni in the high excitation region are in fair agreement with predictions of statistical theory, but the absolute cross sections in the same region are smaller than the prediction by a factor of 3 to 8, and the shapes of the measured spectra for heavier isotopes do not agree with the prediction. These discrepancies between experiment and theory are in sharp contrast to the situation in (p, p′), (p, α), (α, p) and (α, α′), where good agreement was found.The proton spectra from (³He, p) reactions on nuclei in the A = 54–68 mass range have a systematic difference in slope between even-A targets and odd-A targets; it is similar to the systematic difference found previously in (p, p′) and (α, p) reactions, but none of these is readily explainable by theory.     

MICROSCOPIC EXTENSION OF IBM BEYOND VALENCE SHELLS

A way of microscopic study of shape coexistence in nuclei near closed shell is presented, in which one needs making an extension of IBM to include 2p-2h excitation besides the excitations of the valence nucleons.The calculations of energy spectra and electronic transition rates for Tin isotopes from A=112 to A=116 were performed and compared with experimental data.A good agreement with experiment has been achieved.     

0(+)(gs) --> 2(+)(1) excitations in the mirror nuclei 32Ar and 32Si

We measured the strength of the 0(+)(gs)-->2(+)(1) excitations in the radioactive mirror nuclei 32Ar and 32Si using the techniques of intermediate-energy Coulomb excitation for 32Ar and inelastic proton scattering in inverse kinematics for 32Si. The 32Ar measurement, taken together with previously existing Coulomb excitation data for 32Si, yields the isoscalar and isovector multipole matrix elements for the 0(+)(1)-->2(+)(1) transition between T = 2 states in the A = 32 system. The proton scattering measurement for 32Si, when combined with the Coulomb excitation data for this nucleus, yields a ratio of neutron and proton matrix elements, M(n)/M(p), for 32Si.     

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.     

Neutrinoless double beta decay studied with configuration mixing methods

We study neutrinoless double beta decay of several isotopes with state-of-the-art beyond self-consistent mean field methods to compute the nuclear matrix elements (NME). The generating coordinate method with particle number and angular momentum projection (GCM +PNAMP) is used for finding mother and granddaughter states and evaluating transition operators between different nuclei. We analyze explicitly the role of the deformation, pairing and configuration mixing in the evaluation of the NME.     

奇质量核的超越平均场玻色子费米子模型描述(英文)

对近年发展起来的一个基于核密度泛函理论和粒子核心耦合方案来计算中重质量奇A核谱性质的理论方法进行了评述。该方法首先在平均场层面通过选择合适的能量密度泛函和对力结构来自洽求解偶偶核心的势能曲面、球单粒子能级和奇粒子占有率,进一步将得到的结果作为微观输入来建立相互作用玻色子费米子模型哈密顿量,其中三个与粒子核心耦合强度相关的参数需要通过拟合一些特定奇质量核低激发谱数据来最终确定。通过对轴形变奇质量Eu同位素的低激发能谱和电磁跃迁几率的系统研究来说明该模型方法的有效性。另外,还讨论了该方法在描述轴形变奇质量核形状相变以及描述丰中子奇质量Ba同位素中八极关联方面的应用。A recently developed method for calculating spectroscopic properties of medium-mass and heavy atomic nuclei with an odd number of nucleons is reviewed, that is based on the framework of nuclear energy density functional theory and the particle-core coupling scheme. The deformation energy surface of the eveneven core, as well as the spherical single-particle energies and occupation probabilities of the odd particle(s), are obtained by a self-consistent mean-field calculation with the choice of the energy density functional and pairing properties. These quantities are then used as a microscopic input to build the interacting bosonfermion Hamiltonian. Only three strength parameters for the particle-core coupling are specifically adjusted to selected data for the low-lying states of a particular odd-mass nucleus. The method is illustrated in a systematic study of low-energy excitation spectra and electromagnetic transition rates of axially-deformed odd-mass Eu isotopes. Recent applications of the method, to the calculations of the signatures of shapes phase transitions in axially-deformed odd-mass nuclei, octupole correlations in neutron-rich odd-mass Ba isotopes, are discussed.     

Shape coexistence and evolution in neutron-def icient krypton isotopes

Total Routhian Surface(TRS) calculations have been performed to investigate shape coexistence and evolution in neutron-deficient krypton isotopes72,74,76 Kr. The ground-state shape is found to change from oblate in72 Kr to prolate in74,76 Kr, in agreement with experimental data. Quadrupole deformations of the ground states and coexisting 0+2states as well as excitation energies of the latter are also well reproduced. While the general agreement between calculated moments of inertia and those deduced from observed spectra confirms the prolate nature of the low-lying yrast states of all three isotopes(except the ground state of72Kr), the deviation at low spins suggests significant shape mixing. The role of triaxiality in describing shape coexistence and evolution in these nuclei is finally discussed.     

Some regularities in the production of isotopes with 4 ≦ Z ≦ 9 in the interaction of 22Ne with 232Th

In the bombardment of ²³²Th with ²²Ne ions with an energy of 172 MeV the energy spectra and production cross sectionslfor isotopes of elements ranging ifrom Be to F have been measured at an emission angle of 12°. It is shown that all of the isotopes detected have been produced by deep inelastic collisions of the initial nuclei, i.e. the kinetic energies of the reaction products are close to the exit Coulomb barriers. It is found that the energy spectra widths (FWHM), relative yields at 12° and 40° and the Q_gg dependences of isotopic production cross sections differ considerably for stable, neutron-deficient and neutron-rich isotopes. This difference can be interpreted as being due to a contribution from secondary processes such as α-particle and nucléon evaporation from the excited ²²Ne and light transfer reaction products. The data obtained can be employed to choose optimal conditions for the detection of the extremely neutron-rich isotopes of light elements produced in multinucleon transfer reactions.     

Neutron strength functions in rare earth nuclei

The coupling of the single particle motion of the neutron to other degrees of freedom is studied by the (d, p) reaction leading to highly excited states in rare earth nuclei. Experimental spectra have been obtained with a telescope counter system for 13 rare earth isotopes of Gd, Dy, Er and Yb. The results are compared with calculations based upon the single particle model in a deformed Saxon-Woods potential. The spreading of the single particle strength resulting from the coupling to other degrees of freedom is taken into account in a simple phenomenological way. A qualitative similarity between the experimental and calculated spectra is observed, and the total integrated (d, p) cross section up to the neutron binding energy is reproduced quite closely by the calculations. For a given excitation energy, the amount of structure in the experimental spectrum seems to decrease with neutron number for each element investigated.     

Dy3+激活无水芒硝在真空紫外光激发下的发光性质

在空气中900℃温度下,将纯天然无水芒硝( Na2 SO4)和DyF3的混合粉末加热25 min,制备了Na2SO4:Dy3+新型发光材料.通过同步辐射研究了NaSO4:Dy3+的发光性质.并测量了在室温中真空紫外-紫外光下的发射和激发光谱.根据发射光谱得到了不同Dy3+掺杂浓度和不同激发下发光的黄蓝比(Y/B)是不同的.通过监测黄色发光得到的激发光谱,分别由Dy3+,4f9→4f85d跃迁(172 nm)、O2--Tm3+之间的电荷转移带(165 nm)引起的强激发谱和基质吸收(138,245 nm)、对应Dy3+,6 H15/2→4 D7/2,6H15/2→6 P3/2,6 H15/2→6P7/2跃迁(299,325,351nm)引起的弱激发谱组成.     

A Note on the Pair Excitation Across Major Shell in Deformed Nuclei

Valence nucleons usually play a dominant role in low energy nuclear spectra. The original version of the Interacting Boson Model[1](IBM) with the assumption that all degrees of freedom arise from the valence shell reproduces well large amount of experimental data. However this assumption does not work well in some cases, for instance, in the even isotopes of Sn, Cd, and Hg, Pb rather well developed intruder bands121 lie outside the IBA model space. K. Heyde et al. attributed these low lying intruder states to the pair excitation (PE) across major shell131. In the well deformed nuclei one can hardly think of the closed shell core as being spherical and inert. The rapid change of Nilsson levels with deformation seems to indicate that there should be excitations of nucleons across major shells in the well deformed nuclei.