以Fe作为弹核研究合成超重核的黏连截面

超重元素的研究是目前核物理和核化学领域的前沿课题之一,随着实验设备的不断完善和各种理论模型的不断发展,人们在合成超重核的研究上取得了非常大的进展。Fe作为弹核探讨形成超重核的黏连截面,主要采用两步模型来研究超重核的形成过程。主要核反应过程具体分为2步:第1步是指弹靶的黏连阶段,第2步是指弹靶形成复合核阶段。这2个阶段的动力学过程都可以用带随机力的朗之万方程来描述,并且这2个阶段是各自独立的,即可以分别来讨论这2个阶段。主要讨论黏连阶段,根据给出的58Fe+248Cm与58Fe+208Pb激发能与黏连截面的实验值,重新拟合出以Fe作为弹核与锕系中系列核形成超重核的黏连截面的最佳C与ΔB值,及C与ΔB值对拟合出的E*-σ曲线的影响、角动量和激发能等因素对黏连截面的影响。     

Shape coexistence and triaxiality in the superheavy nuclei

Superheavy nuclei represent the limit of nuclear mass and charge; they inhabit the remote corner of the nuclear landscape, whose extent is unknown. The discovery of new elements with atomic numbers Z > or = 110 has brought much excitement to the atomic and nuclear physics communities. The existence of such heavy nuclei hangs on a subtle balance between the attractive nuclear force and the disruptive Coulomb repulsion between protons that favours fission. Here we model the interplay between these forces using self-consistent energy density functional theory; our approach accounts for spontaneous breaking of spherical symmetry through the nuclear Jahn-Teller effect. We predict that the long-lived superheavy elements can exist in a variety of shapes, including spherical, axial and triaxial configurations. In some cases, we anticipate the existence of metastable states and shape isomers that can affect decay properties and hence nuclear half-lives.     

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.     

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.     

超重新核素合成研究进展

随着重离子加速器和放射性核束装置的发展,人们对于核物理的研究不再仅仅局限于稳定核,更是将研究对象扩展到一些极限条件,比如探索原子核高温高密极限、自旋极限、同位旋极限,以及电荷和质量极限等．对于探索电荷和质量极限而言,如何合成长寿命超重核、是否存在“超重稳定岛”以及如何研究超重原子核的性质,成为当前核物理学研究的前沿领域．本文回顾了超重核的产生机制、定义和衰变模式,详细介绍了国内外关于超重核合成的现状和研究进展．其中包括理论上和实验上合成超重核素最新研究进展,重点针对重离子熔合反应和多核子转移反应研究情况展开讨论,并简介了国际上核科学大装置未来开展超重核研究的计划．同时,对北京师范大学参与93号超铀新核素223,224Np的合成情况进行了评述,并针对近年来合成Z=119和120超重核素的最新研究进行了总结展望．      

利用宏观-微观模型研究超重核区原子核的α衰变能和衰变寿命

应用不同参数的宏观-微观模型对超重核区（Z≥104）70个原子核的结合能进行计算,利用所得结果计算α衰变能,并对结果进行比较,发现新参数的结果更为理想.然后将新参数的α衰变能代入新参数的Viola-Seaborg公式,计算超重核区原子核的衰变寿命,通过对所得结果和实验值进行比较,得出研究结果与实验值符合较好的结论.     

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.     

Involvement of targeted proteolysis in plant genetic transformation by Agrobacterium

Genetic transformation of plant cells by Agrobacterium represents a unique case of trans-kingdom DNA transfer. During this process, Agrobacterium exports its transferred (T) DNA and several virulence (Vir) proteins into the host cell, within which T-DNA nuclear import is mediated by VirD2 (ref. 3) and VirE2 (ref. 4) and their host cell interactors AtKAP-alpha and VIP1 (ref. 6), whereas its integration is mediated mainly by host cell proteins. The factors involved in the uncoating of T-DNA from its cognate proteins, which occurs before integration into the host genome, are still unknown. Here, we report that VirF-one of the few known exported Vir proteins whose function in the host cell remains unknown-is involved in targeted proteolysis of VIP1 and VirE2. We show that VirF localizes to the plant cell nucleus and interacts with VIP1, a nuclear protein. VirF, which contains an F-box motif, significantly destabilizes both VIP1 and VirE2 in yeast cells. Destabilization of VIP1 in the presence of VirF was then confirmed in planta. These results suggest that VIP1 and its cognate VirE2 are specifically targeted by the VirF-containing Skp1-Cdc53-cullin-F-box complex for proteolysis. The critical role of proteasomal degradation in Agrobacterium-mediated genetic transformation was also evident from inhibition of T-DNA expression by a proteasomal inhibitor.     

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'.     

rp过程核基态性质的理论研究

采用形变的相对论平均场模型对rp(快质子俘获)过程核的基态性质进行了系统研究,计算了质量数A=60～100,N=Z的偶偶核的平均结合能,均方根半径,单中子和单质子分离能.并与已知的实验数据进行比较,发现二者能够较好地符合,肯定了相对论平均场理论对rp过程核研究的可靠性.我们的计算结果可为未来rp过程的实验研究提供一些理...     

用1／N展开技术研究汞核的能谱和B(E2)

用１／Ｎ展开法加上自洽Ｑ框架的ｓｄＩＢＭ１研究比Ｈｆ更重一些的中等质量核的Ｈｇ的同位素的能谱,计算了＾１８６Ｈｇ基带的几个Ｂ（Ｅ２）值,有一定程度的符合,高激发态符合得较好,特别是能正确反映出Ｂ（Ｅ２）值随能级谱化的趋势,而且只用了很少的可调参数去拟合大量数据,注意到值随能级变化的趋势,而且只用了很少的可调参数去拟合大量数据。注意到Ｈｇ核的质子数Ｚ＝８０,很接满壳层,可见１／Ｎ展开展理论适用于很大     

Z<38稳定核素的分布特点

核素分类是核素体系基本性质规律的统一.当以氘氚结团数分析天然有丰度核素分布时,在正方形核素图中得到支持核素体系新规律是1,2,4,8,16,8,4,2,1存在的证据.具体说明了Z<38核素的三类1,2,4特点:偶S18～6连续核素列的1,2,4分布;Z20～8偶偶核素的1,2,4分布;坐标轴S≤16下界的偶4,奇4,2,1分布.偶Z36～24区域,其上、下界及中心区各有七核素,各以Z30核素703020Zn4010,673026Zn344,673023Zn377为中点对称分布.     

Optical pumping of a single hole spin in a quantum dot

The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state. For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation. However, this advantage is offset by the hyperfine interaction between the electron spin and the 10(4) to 10(6) spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds. Spin-echo techniques have been used to mitigate the hyperfine interaction, but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this but is very difficult to realize in practice. Exploring materials with zero-spin nuclei is another option, and carbon nanotubes, graphene quantum dots and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak. We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks that can intra-convert the spin state with the polarization of a photon.     

Effects of different nuclear recipients on developmental potential of mouse somatic nuclear transfer embryos

Somatic cell nuclear transfer has been succeeded in procedures of nuclear transfer. One is single nucleartransfer, the other is serial nuclear transfer. Viable animals have been cloned in different species using both me-thods[1—6]. Different nuclear recipients and donors wereused in serial nuclear transfer, namely, transferring thenuclear of reconstructed embryo into enucleated MⅡoocytes[7], transferring the nuclear of reconstructed em-bryos at one cell stage into enucleated zygote[4] and t…     

Observation of the antimatter helium-4 nucleus

High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang; in both cases, matter and antimatter are formed with comparable abundance. However, the relatively short-lived expansion in nuclear collisions allows antimatter to decouple quickly from matter, and avoid annihilation. Thus, a high-energy accelerator of heavy nuclei provides an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus (4He), also known as the anti-α (α), consists of two antiprotons and two antineutrons (baryon number B = -4). It has not been observed previously, although the α-particle was identified a century ago by Rutherford and is present in cosmic radiation at the ten per cent level. Antimatter nuclei with B?相似文献   

Glycoprotein organization of Chikungunya virus particles revealed by X-ray crystallography

Chikungunya virus (CHIKV) is an emerging mosquito-borne alphavirus that has caused widespread outbreaks of debilitating human disease in the past five years. CHIKV invasion of susceptible cells is mediated by two viral glycoproteins, E1 and E2, which carry the main antigenic determinants and form an icosahedral shell at the virion surface. Glycoprotein E2, derived from furin cleavage of the p62 precursor into E3 and E2, is responsible for receptor binding, and E1 for membrane fusion. In the context of a concerted multidisciplinary effort to understand the biology of CHIKV, here we report the crystal structures of the precursor p62-E1 heterodimer and of the mature E3-E2-E1 glycoprotein complexes. The resulting atomic models allow the synthesis of a wealth of genetic, biochemical, immunological and electron microscopy data accumulated over the years on alphaviruses in general. This combination yields a detailed picture of the functional architecture of the 25?MDa alphavirus surface glycoprotein shell. Together with the accompanying report on the structure of the Sindbis virus E2-E1 heterodimer at acidic pH (ref. 3), this work also provides new insight into the acid-triggered conformational change on the virus particle and its inbuilt inhibition mechanism in the immature complex.     

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。     

形成原子核群子结构的四项原理及其等腰三角形核素周期律

根据第四统计力学——JRG群子统计理论,首次提出了形成原子核结构的四项原理及相应的原子核周期律。这四项原理为:一是原子核有群子结构单元;二是形成原子核时群子通过热核聚合反应过程使核素结构有严格的排列顺序;三是原子核内群子结构间有动态共振作用;四是偶数群子稳定,而非偶数群子是引起总角动量和β⁺,β^-衰变的根源。基于上述四项原理,提出了核群子结构基本单元有(PB),(PB₂),(P₂B₃),并随着质子数Z的增加,核群子结构由(PB)_k过渡到(PB)_k(P₂B₃)_l;由(PB)_n(PB₂)_m过渡到(P₂B₃)_s(PB₂)_t。从而导出了k(-t)=2n(s)-Z,k(-t)=Z-2l(m)的关系式。基于此,可以画出等腰三角形原子核群子周期律。还发现,不管A,N,Z如何变化,有下列严格关系式:Z/N=(n+m)/(n+2m)或Z/N=(k+l)/(k+3l)并且Z=n+l。此公式高度地反映了所有原子核内质子和中子分布的整数规律。还可以通过等腰三角形周期律得知:k,l,m,n,s,t均与核素群子(PB),(PB₂), (P₂B₃),(P₃B₄)的2,3,5,7整数倍有关。     

'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.