'Magic' nucleus 42Si

Nuclear shell structures--the distribution of the quantum states of individual protons and neutrons--provide one of our most important guides for understanding the stability of atomic nuclei. Nuclei with 'magic numbers' of protons and/or neutrons (corresponding to closed shells of strongly bound nucleons) are particularly stable. Whether the major shell closures and magic numbers change in very neutron-rich nuclei (potentially causing shape deformations) is a fundamental, and at present open, question. A unique opportunity to study these shell effects is offered by the 42Si nucleus, which has 28 neutrons--a magic number in stable nuclei--and 14 protons. This nucleus has a 12-neutron excess over the heaviest stable silicon nuclide, and has only one neutron fewer than the heaviest silicon nuclide observed so far. Here we report measurements of 42Si and two neighbouring nuclei using a technique involving one- and two-nucleon knockout from beams of exotic nuclei. We present strong evidence for a well-developed proton subshell closure at Z = 14 (14 protons), the near degeneracy of two different (s(1/2) and d(3/2)) proton orbits in the vicinity of 42Si, and a nearly spherical shape for 42Si.     

The limits of the nuclear landscape

In 2011, 100 new nuclides were discovered. They joined the approximately 3,000 stable and radioactive nuclides that either occur naturally on Earth or are synthesized in the laboratory. Every atomic nucleus, characterized by a specific number of protons and neutrons, occupies a spot on the chart of nuclides, which is bounded by 'drip lines' indicating the values of neutron and proton number at which nuclear binding ends. The placement of the neutron drip line for the heavier elements is based on theoretical predictions using extreme extrapolations, and so is uncertain. However, it is not known how uncertain it is or how many protons and neutrons can be bound in a nucleus. Here we estimate these limits of the nuclear 'landscape' and provide statistical and systematic uncertainties for our predictions. We use nuclear density functional theory, several Skyrme interactions and high-performance computing, and find that the number of bound nuclides with between 2 and 120 protons is around 7,000. We find that extrapolations for drip-line positions and selected nuclear properties, including neutron separation energies relevant to astrophysical processes, are very consistent between the models used.     

7核素列的三角形与4线联系

在坐标单位是氘氚结团的正方形核素图中,描述一个线性7核素组成的三角形,其顶点与3边中点都是7核素列的端点;另外,说明开始点核素在坐标S轴上的4列偶奇相间的7核素列统一于关系式:S=a+n H(n=1,2,3,4).可以以上结论为突破点,探索核素体系的本质.     

Superallowed Gamow-Teller decay of the doubly magic nucleus 100Sn

The shell structure of atomic nuclei is associated with 'magic numbers' and originates in the nearly independent motion of neutrons and protons in a mean potential generated by all nucleons. During β(+)-decay, a proton transforms into a neutron in a previously not fully occupied orbital, emitting a positron-neutrino pair with either parallel or antiparallel spins, in a Gamow-Teller or Fermi transition, respectively. The transition probability, or strength, of a Gamow-Teller transition depends sensitively on the underlying shell structure and is usually distributed among many states in the neighbouring nucleus. Here we report measurements of the half-life and decay energy for the decay of (100)Sn, the heaviest doubly magic nucleus with equal numbers of protons and neutrons. In the β-decay of (100)Sn, a large fraction of the strength is observable because of the large decay energy. We determine the largest Gamow-Teller strength so far measured in allowed nuclear β-decay, establishing the 'superallowed' nature of this Gamow-Teller transition. The large strength and the low-energy states in the daughter nucleus, (100)In, are well reproduced by modern, large-scale shell model calculations.     

14 MeV中子反应和远离核研究

指出了中子源的重要性,扼要地说明了加速器中子源的特点。简单地介绍了14MeV中子引起的核反应,发现了14MeV中子可以引起重核的奇异(n,2p)反应,并以此为基础,形成了合成和研究重丰中子新核素的一条物理思想和生成、分离鉴别技术路线;先后合成和研究了^185Hf、^237Th、^175Er和^197Os等四种重丰中子新核素。     

A triplet of differently shaped spin-zero states in the atomic nucleus 186Pb

Understanding the fundamental excitations of many-fermion systems is of significant current interest. In atomic nuclei with even numbers of neutrons and protons, the low-lying excitation spectrum is generally formed by nucleon pair breaking and nuclear vibrations or rotations. However, for certain numbers of protons and neutrons, a subtle rearrangement of only a few nucleons among the orbitals at the Fermi surface can result in a different elementary mode: a macroscopic shape change. The first experimental evidence for this phenomenon came from the observation of shape coexistence in 16O (ref. 4). Other unexpected examples came with the discovery of fission isomers and super-deformed nuclei. Here we find experimentally that the lowest three states in the energy spectrum of the neutron deficient nucleus 186Pb are spherical, oblate and prolate. The states are populated by the alpha-decay of a parent nucleus; to identify them, we combine knowledge of the particular features of this decay with sensitive measurement techniques (a highly efficient velocity filters with strong background reduction, and an extremely selective recoil-alpha-electron coincidence tagging methods). The existence of this apparently unique shape triplet is permitted only by the specific conditions that are met around this particular nucleus.     

Evidence for a spin-aligned neutron-proton paired phase from the level structure of (92)Pd

Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work that introduced a strong spin-orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron-proton pairing, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus (92)Pd. Gamma rays emitted following the (58)Ni((36)Ar,2n)(92)Pd fusion-evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron-proton coupling scheme, different from the previous prediction. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron-proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.     

科学:永无止境的迷宫--谈人类对物质结构的认识

通过人类对物质结构认识的发展简史以及粒子物理的重大进展，说明科学是类似于永无止境的迷宫，不可能走向终极。     

核素稳定区的外围线

在参量S=2Z-N、H=N-Z的正方形核素图中,分析说明稳定区左界的以坐标△H=8、△H=8,4、△H=2,4,8的三部分递变.稳定区上区与右侧边界,是从核素坐标(30,30)至上界坐标H44,再至坐标H16,再向下递变至坐标H2的核素225042Cr226的边界,显示出以坐标△H=2,4,8、△H=2,2,4,4,8,8、△H=1,1,2,2,4,4的系统递变.根据各类核素分布以2递变的规律,把稳定范围常数44分解成2的幂数列,综合得到稳定核素分布的新基本规律是:1,2,4,8,16,8,4,2,1.核素图中坐标的物理意义是,氘氚结团是能发现新规律的原因,因为它自动地包含了核内质子与中子间的主要相互作用.     

MONTE-CARLO CALCULATIONS OF NUCLEON EMISSION AND ENERGY DEPOSITION OF SPALLATION NEUTRON SOURCES INDUCED BY INTERMEDIATE ENERGY PROTONS

ＭＯＮＴＥ┐ＣＡＲＬＯＣＡＬＣＵＬＡＴＩＯＮＳＯＦＮＵＣＬＥＯＮＥＭＩＳＳＩＯＮＡＮＤＥＮＥＲＧＹＤＥＰＯＳＩＴＩＯＮＯＦＳＰＡＬＬＡＴＩＯＮＮＥＵＴＲＯＮＳＯＵＲＣＥＳＩＮＤＵＣＥＤＢＹＩＮＴＥＲＭＥＤＩＡＴＥＥＮＥＲＧＹＰＲＯＴＯＮＳＳｈｅｎＱｉ...     

Nuclear isomers in superheavy elements as stepping stones towards the island of stability

A long-standing prediction of nuclear models is the emergence of a region of long-lived, or even stable, superheavy elements beyond the actinides. These nuclei owe their enhanced stability to closed shells in the structure of both protons and neutrons. However, theoretical approaches to date do not yield consistent predictions of the precise limits of the 'island of stability'; experimental studies are therefore crucial. The bulk of experimental effort so far has been focused on the direct creation of superheavy elements in heavy ion fusion reactions, leading to the production of elements up to proton number Z = 118 (refs 4, 5). Recently, it has become possible to make detailed spectroscopic studies of nuclei beyond fermium (Z = 100), with the aim of understanding the underlying single-particle structure of superheavy elements. Here we report such a study of the nobelium isotope 254No, with 102 protons and 152 neutrons--the heaviest nucleus studied in this manner to date. We find three excited structures, two of which are isomeric (metastable). One of these structures is firmly assigned to a two-proton excitation. These states are highly significant as their location is sensitive to single-particle levels above the gap in shell energies predicted at Z = 114, and thus provide a microscopic benchmark for nuclear models of the superheavy elements.     

Z=40～43同位素链上原子核β-衰变寿命的研究

介绍了远离稳定线原子核β-衰变寿命与母核核子数之间的指数规律和理论计算公式,具体对Z=40～43同位素链上原子核的β-衰变寿命进行了计算,并将计算结果与实验数据和宏观微观模型的结果进行了比较.发现计算结果与实验数据符合较好,也与其他模型符合较好.此外还对这些同位素链上原子核的未知β-衰变寿命进行了理论预言.     

用相对论平均场理论研究氧同位素及其镜核的基态性质

用相对论平均场理论计算了氧同位素及其镜核的基态性质，并和实验比较，发现当质子（中子）是幻数时，核的性质有规律地随中子（质子）数的变化而变化，由此间接证明了幻数核子对另一类核子无直接影响；同时，也检验了相对论平均场理论在该区域的适用性．     

14.7MeV中子在^9Be核上散射微分截面的测量

用伴随粒子飞行时间谱仪，测量了１４．７ＭｅＶ中子在９Ｂｅ核上的弹性及２．４３ＭｅＶ能级的非弹性散射微分截面．给出了从１５～１３５°（实验室系）范围内散射截面的数据，以及散射角分布的勒让德系数．对中子通量衰减、电子学死时间和初级束形状等作了修正．实验数据的总误差为４．０％～７．０％，其中统计误差为０．５％～３．５％．     

基于Monte Carlo方法的中子标识输运计算

为了考察中子与介质相互作用的细致物理过程,得到不同种类中子对所关心物理量的贡献,利用Monte Carlo方法的模拟能力,由中子标识记录中子及其次级粒子的输运历史,在程序中赋予中子一个整型标识变量,记录中子的出身、碰撞过程等历史,在记数时根据该变量的值进行分门别类的统计以达到求解众多物理量的目的。结果表明,该方法能得到其它方法不易得到的物理量,如不同种类中子出壳流等。     

Neutron-rich 104,106,108Mo Isotopes in the Generalized Collective Model

Introduction　　Theneutron-rich104,106,108MoisotopeswithZ=42,N=6064,liewithintheA=100deformedregion.The104,106Moisotopesareontheedgeoftheregionofsuperdeformedgroundstatesonthereinforcingprotonandneutronshellgapsatthesamesuperdeformation,forZ=38,40andN=60,62.Studyontheseneutron-richnucleicanprovideimportantinsightsintothechangeinnucleideformationandshapeandtestsofnuclearmodelsinthisimportantregion[15].ItwassuggestedthattheheavierMoisotopeswithN≥60aredeformedinthegroundstates[48].However,th…     

提高14MeV中子热化效率的研究

为了提高14 MeV中子的热化效率,采用MCNP模拟,用多种材料、多层结构的组合对其慢化.用铅反射中子、慢化快中子,用镁聚集热中子,用水慢化中子.MCNP模拟结果显示,快中子反射层、快中子慢化层、热中子聚集层以及中子反射层可以使热化效率分别提高185％,37％,80％和538％.     

14 MeV中子照相中CCD芯片的屏蔽计算

快中子照相实验中,电荷耦合装置(CCD)是重要的成像器件。快中子辐射不仅会减少CCD的使用寿命,而且会对快中子图像带来影响,因此必须对CCD芯片进行有效的屏蔽,减少快中子辐射对芯片的损伤。该文利用MCNP/4B程序计算了14MeV中子照相中不同屏蔽材料组合条件下CCD芯片的吸收剂量。计算结果表明,在对CCD进行有效的屏蔽后,芯片的吸收剂量是屏蔽前的3%,按源中子数归一后仅为1.29 aGy,已经达到屏蔽要求。计算结果还表明,环境散射中子辐射对芯片吸收剂量贡献较小,可以忽略。     

初生中子星星风中正反中微子的吸收反应截面

正反中微子的吸收反应截面是影响初生中子星演化的重要参量.改进了在初生中子星高温环境中任意简并条件下的中微子和反中微子被重子吸收截面的计算方法,同时综合考虑了能量动量守恒、磁场效应和重子（中子或质子）的热运动效应,得到更精确的中微子和反中微子吸收反应截面.结果对进一步研究星风中的中微子加热率、中微子输运过程中的不透明度、合理解释中子星的相关观测现象有重要作用.     

No enhancement of fusion probability by the neutron halo of 6He

Quantum tunnelling through a potential barrier (such as occurs in nuclear fusion) is very sensitive to the detailed structure of the system and its intrinsic degrees of freedom. A strong increase of the fusion probability has been observed for heavy deformed nuclei. In light exotic nuclei such as 6He, 11Li and 11Be (termed 'halo' nuclei), the neutron matter extends much further than the usual nuclear interaction scale. However, understanding the effect of the neutron halo on fusion has been controversial--it could induce a large enhancement of fusion, but alternatively the weak binding energy of the nuclei could inhibit the process. Other reaction channels known as direct processes (usually negligible for ordinary nuclei) are also important: for example, a fragment of the halo nucleus could transfer to the target nucleus through a diminished potential barrier. Here we study the reactions of the halo nucleus 6He with a 238U target, at energies near the fusion barrier. Most of these reactions lead to fission of the system, which we use as an experimental signature to identify the contribution of the fusion and transfer channels to the total cross-section. At energies below the fusion barrier, we find no evidence for a substantial enhancement of fusion. Rather, the (large) fission yield is due to a two-neutron transfer reaction, with other direct processes possibly also involved.