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。     

丰中子区幻数N=28消失的相对论平均场证明

提出了一种从理论上检验壳效应的方法. 即在考虑了BCS近似的相对论平均场理论框架内, 通过计算粒子数的涨落, 发现涨落的大小和核的壳结构有紧密的关系, 由此从理论上证明了在滴线附近的丰中子区, 传统幻数N=28已经消失.     

原子核质量的高精度测量

原子核的质量直接反映了核内强相互作用、电磁相互作用和弱相互作用的结果.文章简要阐述了原子核质量测量的意义、现状和主要方法,介绍了基于兰州重离子冷却储存环的原子核质量测量实验,比较了首次得到的63Ge,65As,67Se和71Kr核质量测量值与理论计算结果,探讨了65As质量对天体物理快质子俘获过程的影响,文章最后给出了今后的研究内容.中国科学院近代物理研究所在轻质量丰中子区,系统测量了从Ne到Ca核素的质量,研究了N=20和28幻数随中子数和质子数变化的演化;在丰质子区,精确测量了快质子俘获路径上关键核素的质量,为解释X射线暴等爆发性天体过程提供重要的质量数据;在中重丰中子区,系统地测量丰中子核质量,通过天体网络计算模拟超新星爆发中的快中子俘获过程.     

Gammasphere上裂变丰中子核核结构若干前沿领域的新进展(Ⅰ:第一和第二部分)(英文)

(第一和第二部分):使用Gammasphere多探测器系统对252Cf裂变源瞬发γ射线进行γ-γ-γ和γ-γ(θ)符合测量,裂变丰中子原子核核结构若干前沿领域的深入研究获得了新的进展。高达5.7×1011以上的三重和更高重符合事件的数据统计,以及更少压缩的三维数据为宽广未知丰中子核区的寻找和研究提供了有利的条件。在具有重要物理意义的若干丰中子核区首次建立,或显著扩展了一批包括转晕态和转晕附近能态的高自旋能级纲图。在偶-偶丰中子核110,112Ru和108Mo中鉴别出了手征对称破缺结构。丰中子110,112Ru附近核的三轴形变基态具有最低的能量,在它们之中已确认了接近最大值的三轴形变。在这些Ru和Mo同位素中观察到的手征双线能带展示出手征破缺的一切特征,特别是其理想的能量简并,表明它们在迄今已报道的手征破缺结构中,具有最好的手征特性。研究了手征结构从具有γ软度的108Ru到具有大三轴形变的110,112Ru的过渡。斜轴推转(TAC)和随机相近似(RPA)理论计算成功地拟合了在这些偶-偶丰中子核中观察到的手征双线能带的特性,并指定其为软手征振动态。在这些偶-偶核中观察到的手征破缺不可能归纳为奇-奇核中那样的简化的几何图像。前者来自闭壳外所有中子的相互作用。对双幻核132Sn附近N=83同中素链的系统研究为这个极富吸引力的核区提供了大量新的谱学信息。N=83同中素135Te(Z=52),136I(Z=53),137Xe(Z=54),138Cs(Z=55)和139Ba(Z=56)的最新能级信息,特别是首次建立的138Cs高自旋能级纲图和壳模型理论计算表明,Z=50质子闭壳外少数g7/2价质子激发同N=82中子闭壳外之唯一f7/2价中子的耦合对该核区能级结构具有关键作用。观察到了132Sn和208Pb附近核区谱学信息的相似性和相对应的三粒子和五粒子态。在135Te中观察到了磁转动,这是在双幻核132Sn附近观察到的首例磁转动。     

原子核质量模型的检验

基于AME2016发布的基态原子核质量数据,分别从模型的精度及实验预言的中子新幻数两方面系统比较分析了八个普适核质量模型的可靠性及预言能力.分区系统的计算了八个核质量模型预言的核质量均方根偏差,分析发现对现有实验数据精确度较好的是Bhagwat和WS4两个模型.通过分析中子壳能隙随中子数的变化趋势发现KTUY, WS3和WS4三个模型可以较好地再现中子新幻数N=32引起的突变行为,预言了在Cl和Ar同位素链中N=32极有可能是新的幻数.通过分析超重区域a衰变能随中子数的变化趋势发现FRDM12, WS3和WS4三个模型均可以较好地再现N=152, 162的子壳现象,且预言了对于质子数Z=108—114同位素链在N=184处原子核的寿命相对较长.     

相对论平均场理论框架下对能的系统研究

在相对论平均场模型的框架内,沿原子核的稳定线,以每隔4个质子或中子提取样本的方法,计算了核谱图上数十个原子核的对能,特别是研究了氧同位素偶-偶核的对能随核子数的变化规律,发现在固定能隙Δ的条件下,对能的大小和核的壳结构有关,由此提出了一种检验闭壳效应的简便方法,进而发现中子数N=6不仅在轻核的丰中子区是一个新幻数,而且在丰质子区也是一个可能的新幻数. 关键词： 相对论平均场模型 对能 能隙 幻数     

传统中子幻数稳定性的研究

在考虑了BCS理论的相对论平均场模型框架内,通过系统研究N=8,20,28,50,82和126六条同中子素链中每个元素费米面附近的单粒子能级间隔、粒子数占有概率比以及原子核体系的粒子数偏离随质子数的变化规律,讨论了传统中子幻数的壳结构在从中子滴线区到质子滴线区整个核谱上的稳定性,预言只有在轻核的丰中子区域,传统的中子幻数效应才可能消失,并把计算结果和最近的文献报道作了比较. 关键词： 相对论平均场模型 能级间隔 占有概率 粒子数偏离     

放射性核束的库仑激发

综述了利用中、高能放射性核束的库仑激发方法研究位于N＝20和28主壳隙附近的丰中子核结构所取得的进展．系统的实验结果清晰地表明,在离开β稳定线区域时N＝20兰壳隙突然消失和N＝28主壳隙的减弱过程．提出了利用兰州放射性束流线开展双幻核Ni50附近核素的低位能级核结构研究的构想． The study of coulomb excitation of the neutron-rich nuclei around N=20 and 28 shell closure with radioactive ion beam at intermediate energy is reviewed. The systematics of the measured energy of the 2+1 state shows that the N=20 shell closure in neutron-rich isotopes with Z≤12 disappears suddenly and N=28 shell elosure appears to be weak for 44S.The coulomb excitation studies of the exotic nuclei around the double magic 7828Ni50 at RIBLL are proposed.     

适用于轻核区的相对论平均场理论新参数

考虑对关联和微观质心修正效应, 适用于轻核区的两组相对论平均场理论新参数被提出:包含σ, ω介子非线性自耦合项的NLLN和包含密度依赖的介子-核子耦合项的DDLN. 对于Z≤8轻核区β稳定线附近以及远离β稳定线的原子核, 新参数对结合能等基态性质给出了比已有参数更好的描述. 同时, 新参数给出O同位素的中子滴线核为²⁶O, ²⁴O中新幻数N=16的出现也能够被合理解释.     

丰中子氮同位素β￣衰变的理论研究

使用壳模型系统地研究了丰中子氮同位素的β￣衰变,并对其衰变子核18,19,20,210的能级结构性质给出理论计算,所得结果与实验符合较好.研究了18,19,20N核的Gamow-Teller允许型β￣衰变的特性,对与近期实验观测不一致之处进行了讨论和分析.预言了21N的β￣衰变及其子核高激发态的能级结构性质.这些计算将对氮同位素的实验研究很有帮助.     

基于不稳定核基本性质测量的原子核结构研究

精密激光谱学是通过测量核素原子光谱的超精细结构和同位素移位来研究原子核的基本性质,为原子核自旋、磁矩、电四极矩及电荷均方根半径的确定提供了一种模型独立的测量方式。这些原子核基本性质的测量,能够比较精确地描述原子核微观结构的演化。近年来,随着放射性束流装置的发展,产生远离β-稳定线的丰中子/丰质子核素成为可能,也进一步促进了高分辨和高灵敏度的激光谱技术更加广泛的应用。简单介绍了基于放射性核素超精细结构的激光谱学测量原理,并通过几个经典实例来回顾近年来激光谱学在原子核奇特结构研究领域的独特贡献。主要通过分析几个重要核区原子核的基本性质,结合大尺度壳模型、ab initio理论、密度泛函理论等,来探索丰中子核中展现出来的一些新的奇特现象,如晕结构、幻数演化、形状共存等。High-precision laser spectroscopy technique is used to determine the ground state properties of exotic nuclei by probing its electronic hyperfine structure and isotope shift. It provides a model-independent measurement of nuclear spin, magnetic moment, electric quadrupole moment and charge radii. These nuclear parameters can be used to investigate the nuclear structure evolution and the nuclear shapes. With the development of accelerators and isotope separators, exotic isotopes far from β stability became accessible experimentally, which enhanced the capability of the laser spectroscopy technique being applied in the field of nuclear physics. A brief introduction to experimental principle is given, followed by a review of several typical examples for the experimental investigations in the different regions of nuclear chart. This aims to demonstrate the contributions of ground state properties measurement by using laser spectroscopy technique to the nuclear structure study of exotic isotopes. This discussion involves several different nuclear theory models in order to interpret the exotic phenomena observed in the neutron-rich isotopes, such as halo structure, shell evolution, shape coexistence and so on.     

原子核基态性质的系统研究

在Skyrme-Hartree-Fock-Bogoliubov（SHFB）理论框架下,利用SkOP1,SkOP2,SKC和SKD 4套新的Skyrme相互作用参数系统地研究了Ca,Ni,Sn和Pb同位素链上原子核的结合能、电荷半径等基态性质,并重点讨论了丰中子Ca核的新中子幻数以及Pb的同位素位移现象。通过与实验数据和SLy5相互作用参数的结果对比,发现这4套相互作用参数都能很好地再现结合能的实验数据,其预言精度比SLy5要高。对于丰中子Ca核,只有SKC和SKD相互作用参数能够再现N=28处的壳效应,而对于实验上发现的新幻数N=32和34,所有的相互作用参数均不能再现这一结果。对于电荷半径,发现所有的相互作用参数均不能很好地预言Ca同位素链电荷半径的演化规律以及Pb的同位素位移现象。另外,还将这些相互作用参数推广至远离β稳定线原子核的单粒子能级结构研究,发现其不适用于描述其随同位旋的演化行为。因此,为了更好地描述远离β稳定线原子核的宏观性质及单粒子能级,建议在拟合Skyrme相互作用参数时,除自旋-轨道耦合项包括合理的同位旋依赖外,还要考虑张量力成分。The nuclear ground state properties of Ca, Ni, Sn and Pb isotopes, such as the binding energies, the charge radii, are studied systematically by 4 sets of new Skyrme parametrizations SKC, SKD, SkOP1 and SkOP2 in the framework of the Skyrme-Hartree-Fock-Bogoliubov (SHFB) method. The new magic numbers of neutronrich Ca isotopes and the isotopic shift of Pb isotopes are discussed emphatically. By the comparisons between the calculations and the experimental data and results from the SLy5 interaction parametrization, it is found that the experimental binding energies can be reproduced accurately by all parametrizations. The calculated accuracies of SKC, SKD SkOP1 and SkOP2 parametrizations are higher than the ones of SLy5 parametrization. For the neutron-rich Ca nuclei, the shell effect of N=28 can be reproduced by the SKC and SKD parametrizations, but the magic numbers at N=32 and 34 are not found by the calculations of all the parametrizations. For the charge radii, the experimental evolution tendency of Ca isotopes and isotopic shift of Pb isotopes can not be reproduced by all the parametrizations. In addition, all Skyrme parametrizations are extended to study the structure of the nuclei far from the β stability line, it is shown that the single-particle energy evolutions with the isospin are not suitable for being studied by these parametrizations. Thus the tensor force component should be considered besides the isospin dependence in spin-orbit coupling term when the Skyrme interaction parametrizations are fitted.     

Rearrangements in neutron shell closures far from stability

A survey of experimental results obtained at GANIL (Caen, Prance) on the study of the properties of very neutron-rich nuclei (Z = 6–20, A = 20–60) near the neutron drip line and resulting in an appearance of further evidence for the new magic number N = 16 is presented. Very recent data on mass measurements of neutron-rich nuclei at GANIL and some characteristics of binding energies in this region are discussed. Nuclear binding energies are very sensitive to the existence of nuclear shells and together with the measurements of instability of doubly magic nuclei ¹⁰He and ²⁸0 they provide information on changes in neutron shell closures of very neutron-rich isotopes. The behaviour of the two-neutron separation energies S_2n derived from mass measurements gives a very clear evidence for the existence of the new shell closure N = 16 for Z = 9 and 10 appearing between 2s_1/2 and ld_3/2 orbitals. This fact, strongly supported by the instability of C, N and O isotopes with N > 16, confirms the magic character of N = 16 for the region from carbon up to neon while the shell closure at N = 20 tends to disappear for Z ≤ 13. Decay studies of these hardly accessible short-lived neutron-rich nuclei from oxygen to silicon using the in-beam γ-ray spectroscopy are also reported.     

香港大学在束伽玛谱学研究与新一代伽玛探测器阵列(英文)

探索原子核的壳层演化,验证奇特核的幻数结构是香港大学核物理研究的重要方向。目前,科研团队利用在束伽玛谱学技术已经研究了30Ne的N=20幻数消失和78Ni（Z=28,N=50）附近原子核的双幻数结构,而即将开展的53,56Ca在束伽玛谱学实验会对新幻数N=34的定量研究,以及到N=40核的壳层演化提供重要的数据。下一步的研究目标是探索100Sn（N=Z=50）的奇特结构,特别是研究它的第一个2+激发态与其邻近原子核的低激发态性质。100Sn处于质子滴线以及核天体快质子俘获路径上,因此,它的幻数结构及其临近原子核单粒子性能研究将会极大增强对核力和核合成机制的认识。为了进一步提高物理实验统计,香港大学在数量上增加了30% NaI（Tl）晶体从而全面升级了DALI2伽玛探测阵列。此外,为了探索远离稳定线核区的新物理,开展更高精度在束伽玛谱学实验,香港大学与中国科学院近代物理研究所、中国原子能科学研究院计划合作研制基于溴化镧晶体的新一代伽玛探测器阵列。这套阵列主要在兰州重离子加速器（HIRFL）和将来建成的强流重离子加速器（HIAF）等大科学装置上开展实验,从而在奇特核研究方面取得大量重要的成果,促进科研人员全面认识、理解核力以及天体核合成过程。Exploring the evolution of shell closures and examining the magicity of extremely exotic nuclei are the main research interests of HKU (University of Hong Kong) experimental nuclear physics group. The group has employed in-beam gamma-ray spectroscopy technique to investigate the vanishing of N=20 magicity in 30Ne (N=20) and the strong magicity in nuclei around 78Ni (Z=28, N=50). The approved future's experiment on spectroscopy of 53,56Ca, proposed by HKU, will give quantitative information for the "magic index" of N=34 and shell evolution toward N=40. The next goal is to investigate the structure of 100Sn (N=Z=50), particularly the energy of the first 2+ state, and the low-lying states in the neighboring nuclei. 100Sn lies on the proton drip-line and on the astrophysical rp-process path. Characterizing the magicity of 100Sn and the nature of single-particle states in its neighboring nuclei is therefore essential to the fundamental understanding of nuclear forces and nucleo-synthesis. To significantly increase the data statistics for our physics goals, HKU group has prepared the upgrade of gamma-ray spectrometer DALI2 with 30% more NaI(Tl) detectors integrated into a new array configuration. On the other hand, next significant insights into the structure of nuclei would require new gamma-ray detection array capable for higher precision gamma-ray spectroscopy. HKU group in collaboration with IMP and CIAE therefore proposes to construct a new-generation gamma-ray detection array based on the novel scintillator LaBr₃(Ce) to explore the new physics in nuclei far from the valley of stability. Utilizing the radioactive beams at the Chinese large-scale facilities such as the Heavy Ion Research Facility in Lanzhou (HIRFL) in IMP and the future's High Intensity heavy-ion Accelerator Facility (HIAF), this novel LaBr₃(Ce) array would lead to a significant boost to the frontiers of exotic-nuclei research, which will guide scientists towards the comprehensive and even beyond-traditional understanding of nuclear forces and nucleosynthesis.     

Magnetic dipole moment of the doubly-closed-shell plus one proton nucleus ^{49}sc

The nucleus ^{49}Sc, having a single f_{7/2} proton outside doubly magic ^{48}Ca (Z=20, N=28), is one of the very few isotopes which makes possible testing of the fundamental theory of nuclear magnetism. The magnetic moment has been measured by online β NMR of nuclei oriented at milli-Kelvin temperatures to be (+)5.616(25) μ_{N}. The result is discussed in terms of a detailed theory of the structure of the magnetic moment operator, showing excellent agreement with calculated departure from the f_{7/2} Schmidt limit extreme single-particle value. The measurement completes the sequence of moments of Sc isotopes with even numbers of f_{7/2} neutrons: the first such isotopic chain between two major shells for which a full set of moment measurements exists. The result further completes the isotonic sequence of ground-state moments of nuclei with an odd number of f_{7/2} protons coupled to a closed subshell of f_{7/2} neutrons. Comparison with a recent shell-model calculation of the latter sequence is made.     

Coulomb excitation of neutron-rich Zn isotopes: first observation of the 2(1)+ state in 80Zn

Neutron-rich, radioactive Zn isotopes were investigated at the Radioactive Ion Beam facility REX-ISOLDE (CERN) using low-energy Coulomb excitation. The energy of the 2(1)+ state in 78Zn could be firmly established and for the first time the 2+ --> 0(1)+ transition in 80Zn was observed at 1492(1) keV. B(E2,2(1)+ --> 0(1)+) values were extracted for (74,76,78,80)Zn and compared to large scale shell model calculations. With only two protons outside the Z=28 proton core, 80Zn is the lightest N=50 isotone for which spectroscopic information has been obtained to date. Two sets of advanced shell model calculations reproduce the observed B(E2) systematics. The results for N=50 isotones indicate a good N=50 shell closure and a strong Z=28 proton core polarization. The new results serve as benchmarks to establish theoretical models, predicting the nuclear properties of the doubly magic nucleus 78Ni.     

用于核物理研究的精密激光谱技术的发展和展望

原子核基本性质（自旋、质量、寿命、磁矩、电四极矩和电荷半径等）与原子核的内在结构密切相关,是检验和发展原子核理论模型的重要依据。实验上可以通过多学科交叉的精密激光谱技术测量原子核外电子的超精细结构和同位素移位,来模型独立地提取原子核的自旋、磁矩、电四极矩和电荷均方根半径等多个核物理参量。这些基本性质的系统测量可以用于探索不稳定原子核中展现出来的新奇的物理现象与规律。近年来,为了测量产额更低的丰中子核的基本性质,激光谱技术不断更新和发展,以实现高分辨、高效率测量。本文详细介绍了激光谱测量的基本原理以及由此发展起来的用于不稳定原子核结构研究的各类互补的激光谱学技术,如共线激光谱（高分辨率低灵敏度）、在源激光谱（高灵敏度低分辨率）、共线共振电离谱（高分辨率高灵敏度）等激光谱技术,以及在不同核区的测量优势和局限。最后结合我国正在发展和规划中的新一代放射性核束装置,讨论精密激光谱技术在国内的发展以及在核物理研究中的应用。     

丰中子锶同位素的投影壳模型研究

利用考虑跨壳激发的投影壳模型（PSM）方法,研究了质子数Z=38、中子数N=63和64大形变丰中子101,102Sr同位素的结构性质。主要计算了转动谱、转动惯量和电磁跃迁性质等（如B（E2）和g因子）,并与相应的实验数据进行系统比较和相关的理论预言。结果表明,PSM可以利用理论计算的能带图解释101,102Sr同位素的转动惯量、电磁跃迁随自旋的变化,分析晕带的结构。PSM理论可以很好地再现实验结果,说明PSM方法及其采用的有效相互作用可以外推研究丰中子核区101,102Sr同位素的原子核结构。对于101,102Sr同位素,核子开始填布质子g_9/2和中子h_11/2轨道,通过更为仔细地分析能带图中来自质子g_9/2和中子h_11/2轨道对各转动带的组态成分的贡献,清晰地表明丰中子核结构性质对不同核子填布的依赖。Recently, we have carried out systematically studies on the structural properties of proton number Z=38, neutron number N=63 and 64 neutron-rich isotopes 101,102Sr by using the projected shell model (PSM) with consideration of cross shell excitation. The rotation spectra, the moment of inertia and the electromagnetic transition properties (such as B(E2) and g-factor) are calculated and compared with the corresponding experimental data in this paper. Furthermore, more high spin states are predicted in the calculation and expected to be confirmed experimentally. The results show that the PSM can not only well analyze the structural properties of yrast bands in 101,102Sr but also interpret the variation of the moment of inertia, electromagnetic transition with spins in terms with the theoretical band diagram. The good agreement with the experimental data suggests that the PSM with the adopted effective interactions can be generalized to study the nuclear structure of 101,102Sr isotopes in neutron-rich mass region. For 101,102Sr isotopes, the nucleons begin to fill proton g_9/2 and neutron h_11/2 orbital, the dependence of nuclear structure and properties on the different orbital occupies is described by carefully analyzing the contribution from proton g_9/2 and neutron h_11/2 orbital to the configuration of rotational bands in band diagram.