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.     

原子核的中子俘获截面半经验公式研究

根据最近发表的稳定核及有较长半衰期非稳定核的中子俘获截而数据［１］，考虑了原子核的壳层改正与幻数效应［２］，重新拟合了Ｗｏｏｓｌｅｙ于１９８６年提出的关于重原子核的中子俘获平均热截面半经验公式中的经验系数，并且把新结果推广应用到非稳定核上．我们发现，对于幻数附近以外区域的核素，这种简易的半经验方法给出的σｎｙ（３０Ｋｅｒ）结果大大改进。     

Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes

A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements, owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes--40Mg and 42Al--that are predicted to be drip-line nuclei. In the past, several attempts to observe 40Mg were unsuccessful; moreover, the observation of 42Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neutrons. In contrast, the present work shows that nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons.     

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.     

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.     

原子核幻数与群子结构之间界定

原子核的幻数已被大量实验所证明是一系列的正整数,如有2.8,2.0,2.8,5.0,8.2,12.6等,但从理论上推导这些整数的存在并不是轻易的事情。为此,曾提出过单粒子壳层模型,进一步发展成为自旋轨道耦合模型,成功地推导出上述所有幻数系列并预测了各种核素的自旋角动量。大量的实验也表明实测的核素的自旋角动量与理论预测值之间有相当大的差距。在这种情况下,人们开始倾向于原子核的集团运动的看法,始终未能出现突破性的进展。文中运用作者创立的第四统计力学———JRG群子统计理论的观点,提出了原子核内存在亚结构———核群子的观点,用k、l、n、m等群子参数标定每一个核素,并与幻数比较,凡是具有幻数的核素具有简单的核群子结构,有较明显的界定。     

U同位素壳效应的理论分析

在考虑了BCS近似的相对论平均场模型的框架内,通过系统研究U同位素费米面附近的单粒子能级、单粒子能级占有概率和BCS近似下的粒子数偏差,首次发现U同位素中的大多数元素均有比一般元素更稳定的壳结构,即具有和闭壳核相似的壳效应特性．同时,给出了一种判断幻数存在与否的理论方法．     

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.     

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.     

Na变质对Zn—27Al—Si合金组织性能的影响

通过试验和分析，发现Na对Zn-Al-Si合金中硅晶体的变质机理是Na吸附在硅晶体的生长方式，改变了硅晶产生大量分枝，而不影响硅的形核，试验结果还表明硅相由板块状变为球状后，合金的机械性能有显著提高。     

稳定核^91Zr高自旋态实验研究

利用能量为44MeV的13C束流,通过重离子熔合蒸发反应82Se（13C,4n）91Zr布居了稳定核91Zr的高自旋态.基于γ-γ符合关系扩展了91Zr21/2＋同核异能态上的能级纲图,根据实验提取的γ跃迁的角分布系数对部分能级的自旋进行了指认.前人对21/2＋同核异能态下正负宇称能级已指认为质子在π（p1/2,g9/2）轨道的激发耦合价中子vd5/2;在π（f5/2,p3/2,p1/2,g9/2）和v（d5/2,s1/2,d3/2,g7/2,h11/2）空间内,经过系统性的分析和比较91Zr邻近核高自旋态能级结构,指出同核异能态上正宇称态激发机制依次为质子跨越38子壳π（f5/2,p3/2）→πp1/2激发和中子跨越50闭壳vg9/2→vd5/2激发,同核异能态上负宇称态可能涉及质子跨越38子壳π（f5/2,p3/2）→πg9/2激发与价中子vd5/2→vh11/2激发的竞争.     

SrF2颗粒涂覆增强Si/C负极电化学性能

The silicon-based material exhibits a high theoretical specific capacity and is one of the best anode for the next generation of advanced lithium-ion batteries (LIBs). However, it is difficult for the silicon-based anode to form a stable solid-state interphase (SEI) during Li alloy/de-alloy process due to the large volume change (up to 300%) between silicon and Li_4.4Si, which seriously limits the cycle life of the LIBs. Herein, we use strontium fluoride (SrF₂) particle to coat the silicon?carbon (Si/C) electrode (SrF₂@Si/C) to help forming a stable and high mechanical strength SEI by spontaneously embedding the SrF₂ particle into SEI. Meanwhile the formed SEI can inhibit the volume expansion of the silicon?carbon anode during the cycle. The electrochemical test results show that the cycle performance and the ionic conductivity of the SrF₂@Si/C anode has been significantly improved. The X-ray photoelectron spectroscopy (XPS) analysis reveals that there are fewer electrolyte decomposition products formed on the surface of the SrF₂@Si/C anode. This study provides a facile approach to overcome the problems of Si/C electrode during the electrochemical cycling, which will be beneficial to the industrial application of silicon-based anode materials.     

核素稳定区范围常数44与核分布的一条递变线

在引入新参量ＳＨ 的正方形核素图中,由同类核素的范围规律,综合得出核素稳定范围常数４４ 可分解为２ ,４ ,８ ,１６ ,８ ,４ ,２ ,其相邻数之间以２ 倍增长与简并,似能以核子由三个夸克组成解释。文中还说明了核素分布的三条坐标△Ｈ＝ １ ,２ ,４ ,８ 递变的折线在坐标Ｈ＝ １４ 与２９ 处联系后,形成从稳定区下界坐标Ｈ＝ － １ 递变至上界坐标Ｈ＝ ４４ 。     

核素的自旋角动量量子数和群子参数间关系

详细考察了核群子结构的参数 (m ,n ,k ,l )与核素角动量量子数之间的关系, 提出了核素群子模型理论及其四项原则。分别讨论了核素角动量量子数为1/2, 3/2, 5/2, 7/2, 9/2及0, 1, 2, 3, 4, 5, 6, 7, 8的情形, 较圆满地解释了各种核素所具有的总角动量量子数与核素群子结构之间关系。从中进一步看出单粒子壳层模型理论难以全面地预测核素的总角动量量子数。     

Proton-proton correlations observed in two-proton radioactivity of 94Ag

The stability and spontaneous decay of naturally occurring atomic nuclei have been much studied ever since Becquerel discovered natural radioactivity in 1896. In 1960, proton-rich nuclei with an odd or an even atomic number Z were predicted to decay through one- and two-proton radioactivity, respectively. The experimental observation of one-proton radioactivity was first reported in 1982, and two-proton radioactivity has now also been detected by experimentally studying the decay properties of 45Fe (refs 3, 4) and 54Zn (ref. 5). Here we report proton-proton correlations observed during the radioactive decay of a spinning long-lived state of the lightest known isotope of silver, 94Ag, which is known to undergo one-proton decay. We infer from these correlations that the long-lived state must also decay through simultaneous two-proton emission, making 94Ag the first nucleus to exhibit one- as well as two-proton radioactivity. We attribute the two-proton emission behaviour and the unexpectedly large probability for this decay mechanism to a very large deformation of the parent nucleus into a prolate (cigar-like) shape, which facilitates emission of protons either from the same or from opposite ends of the 'cigar'.     

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.     

A stable silicon-based allene analogue with a formally sp-hybridized silicon atom

Carbon chemistry exhibits a rich variety in bonding patterns, with homo- or heteronuclear multiple bonds involving sp-hybridized carbon atoms as found in molecules such as acetylenes, nitriles, allenes and carbon dioxide. Carbon's heavier homologues in group 14 of the periodic table--including silicon, germanium and tin--were long thought incapable of forming multiple bonds because of the less effective p(pi)-p(pi) orbital overlap involved in the multiple bonds. However, bulky substituents can protect unsaturated bonds and stabilize compounds with formally sp-hybridized heavy group-14 atoms: stable germanium, tin and lead analogues of acetylene derivatives and a marginally stable tristannaallene have now been reported. However, no stable silicon compounds with formal sp-silicon atoms have been isolated. Evidence for the existence of a persistent disilaacetylene and trapping of transient 2-silaallenes and other X = Si = X' type compounds (X, X' = O, CR2, NR, and so on) are also known, but stable silicon compounds with formally sp-hybridized silicon atoms have not yet been isolated. Here we report the synthesis of a thermally stable, crystalline trisilaallene derivative containing a formally sp-hybridized silicon atom. We find that, in contrast to linear carbon allenes, the trisilaallene is significantly bent. The central silicon in the molecule is dynamically disordered, which we ascribe to ready rotation of the central silicon atom around the molecular axis.     

稳定核素线性7核素的分布

在坐标单位是氘氚结团的正方形核素图中分析了稳定核素区线性7核分布.稳定区的下、中、上区域中各有7列线性7核,这些核素列的分布具有系统性.跨区域的3类参量S-2H,3S-2H,3S-H的核素列各有3列的始点联系在坐标S轴上,这些点在S轴上排列成(10,0),(12,0),…,(26,0)的9个坐标.通过坐标变换可看出下区的系列7核素列随参量逐层分布.     

预硬型贝氏体钢中合金元素的作用

研究了新型预硬型贝氏体钢（Ｙ８２）的合金设计及性能,分析了合金元素Ｓｉ, Ｓ, Ｃａ 的作用。结果表明： 新型预硬型贝氏体钢（Ｙ８２）成分简单,价格低廉,空冷淬透性好; 合金元素Ｓｉ改变了奥氏体连续冷却转变曲线（ＣＣＴ）的位置,使ＣＣＴ曲线中贝氏体转变区向右下方移动。使贝氏体转变发生在较低的温度,可以形成细小的针状下贝氏体,从而提高了钢的机械性能; 合金元素Ｓ提高了钢的易切削性,元素Ｃａ 对夹杂物有变质作用,使钢中ＭｎＳ形成一种特殊的形态,因而使Ｙ８２ 在预硬状态时（硬度约为３５～４０ ＨＲＣ）仍具有良好的切削加工性能。     

Another continental pool in the terrestrial silicon cycle

Silicon is the second most abundant element on Earth. It is an important nutrient for phytoplankton and is readily absorbed by terrestrial vegetation; it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates. But the continental cycle of silicon is not well known, and only a few studies have attempted to use silicon stable isotopes (28Si, 29Si and 30Si) to quantify the continental silicon reservoirs. Dissolved silicon in sea and river waters forms a reservoir of mean isotopic value +1.1 per thousand (refs 7, 10). It is enriched in 30Si with respect to the igneous rocks reservoir, which has a mean isotopic value of -0.3 per thousand (refs 4, 9). This enrichment can only be produced by a major fractionation during weathering, and should result in the formation of a continental 30Si-depleted reservoir. Such a reservoir, however, has not been identified to date. Here we analyse silicon isotopes of in situ quartz from a sandstone series in France, using a new-generation secondary ion mass spectrometry apparatus. We show that quartz that precipitates as siliceous cements forms a strongly 30Si-depleted reservoir with isotopic values down to -5.7 per thousand, a more negative value than any previously published for terrestrial samples. Our findings suggest that quartz re-precipitation plays an important role in the biogeochemical cycle of silicon.