On Line Nuclear Orientation at the Nuclear Structure Facility,Daresbury

Performance figures are given for the elements of this combined accelerator, separator, dilution refrigerator facility which will give its first fully on-line results in the near future.     

Nuclear Orientation Studies of Magnetic Materials

Low temperature nuclear orientation (LTNO) and nuclear magnetic resonance on oriented nuclei (NMRON) can be utilized to investigate the magnetic properties of solids, and are especially useful when high sensitivity is required, for example in the study of small or dilute systems. Spins of stable isotopes can also be studied using NMR thermally detected by NO (NMR-TDNO) of the radioactive nuclei. An effect of nuclear spin coupling in ordered magnets is frequency pulling of the abundant spins, and this has been investigated in the quasi-2-dimensional ferromagnet ⁵⁴Mn–Mn(COOCH₃)₂4H₂O by comparing the NMRON of the dilute radioactive nuclei with the NMR-TDNO of the abundant host nuclei. A structure in the spectra is observed that is yet to be explained. Recent LTNO experiments on magnetic multilayers are described. Experiments implanting radioactive ions into insulating magnets show that the implanted nuclei experience sizeable hyperfine fields, and this may be useful for probing magnetic materials that are difficult to dope by conventional means.     

Recent Trends in Low-Temperature Nuclear Orientation and Nuclear Magnetic Resonance on Oriented Nuclei

Recent trends in Low-Temperature Nuclear Orientation (LTNO) and its site-selective derivative, Nuclear Magnetic Resonance on Oriented Nuclei (NMRON), are reviewed. Traditional areas of endeavour using elemental ferromagnetic hosts remain strong with significant improvements in methodology since last reviewed [1]; but there are new emphases emerging in solid-state physics and especially magnetism. Exotic single crystals hosts and MBE-grown multi-layers are gaining increased prominence as the principal focus of LTNO study. Increasingly, in off-line work and very recent on-line implantation into insulators [2], the radioactive probes are often chosen to be isoelectronic with an abundant chemical species within the host, rather than representing an extremely dilute, electronic impurity spy. Recent NMRON on a heavy rare earth, isoelectronic probe in an ordered rare earth halide opens up new dimensions. This revised version was published online in September 2006 with corrections to the Cover Date.     

典型磁性材料价电子结构研究面临的机遇与挑战

目前在磁性材料磁有序现象研究中广泛使用的交换作用、超交换作用和双交换作用模型形成于1950年代及其以前,这些模型都涉及材料中的价电子状态,但那时还没有充分的价电子状态实验依据.1970年代以来,有关价电子结构实验结果的报道越来越多,这些实验结果表明传统的磁有序模型需要改进.首先,大量电子谱实验表明,在氧化物中除存在负二价氧离子之外,还存在负一价氧离子,并且负一价氧离子的含量可达30%或更多.这说明以所有氧离子都是负二价离子为基本假设的超交换和双交换作用模型需要改进.其次,一些实验证明,铁、钴、镍自由原子的一部分4s电子在形成铁磁性金属的过程中变成了3d电子,这为探讨金属磁性与电输运性质的关系提供了依据.此外,即使在现代的密度泛函计算中,仍不能给出磁性交换作用能的函数表达式,只能采取各种不同模型进行模拟计算,从而使磁性材料的模拟计算遇到严重困难.寻求一个磁有序能的函数表达式可能是解决这个困难的途径.这些研究表明磁性材料价电子结构研究面临着重大的机遇与挑战.本文首先介绍一些典型的实验例证,然后介绍了基于这些实验结果的一套典型磁性材料的磁有序新模型,随后介绍了基于新模型的磁性材料价电子结构与旧模型的主要区别,最后指出了未来研究工作面临的挑战.     

Impurities in Magnetic Insulators Studied by Low-Temperature Nuclear Orientation

Although the effects of impurities in metallic magnets have been extensively studied there have been far fewer experiments on impurities in insulating magnets, and, in an effort to understand this problem, we have investigated the effect of impurities in various systems. In MnCl₂⋅4H₂O samples doped with 1.6, 4.0 and 6.2% Co, the ⁵⁴Mn NMRON line is significantly broadened compared to the line in the pure crystal and shows structure on the low frequency side. The nuclear spin-lattice relaxation time is also lower by an order of magnitude. ⁵⁴Mn has also been doped into FeCl₂⋅4H₂O for which a 4-sublattice structure has been proposed and some details of this structure have been obtained. This revised version was published online in September 2006 with corrections to the Cover Date.     

Nuclear Power System Based on Fission and Fusion Reactors is the Strategic Line of Nuclear Power Industry Development

Physics of Atomic Nuclei - The strategic line of development of a nuclear power system based on fission and fusion reactors which ensures electricity generation on a specified scale, solves the...     

一种新的灵敏核探针——慢正电子束流装置

简要介绍了广泛应用于表面科学的灵敏核探针——慢正电子束设备的原理、构造和应用 ,阐述了北京慢正电子束流装置的设计原理和性能 ,讨论了北京慢正电子束流装置今后的研究前景. A novel and compact slow positron beam line——Beijing Slow Positron Beam Line was described. The basic physical concepts of the slow positron beam and its applications were reviewed. Recent experimental results showed the converting effcicency for the total positrons from the radioisotope to slow positrons was 8×10 -5 and the moderation efficiency was 1.7×10 -4 . The future development of Beijing Intense Slow Positron Beam, based on the BEPC LINAC, was also briefly discussed.     

原子核的巨形变态研究

简要叙述了该课题研究的现状,并从核裂变和巨形变结合的观点上,对中重核和重核中的巨形变态研究提出一些看法,以推动对这个课题的探讨 . The recent development on investigation of hyperdeformation is outlined. From point of view of combining hyperdeformation with nuclear fission, a discussion on approaches for studying hyperdeformation is given for pushing ahead with this research.     

On Nuclear Reaction Rate Theory

A fundamental problem in the theory of cosmological and stellar nucleosynthesis is the determination of reaction rates. The current theories of nuclear kinetics are equilibrium theories with a strict Maxwell-Boltzmannian approach to the evaluation of reaction rates. This paper investigates in some detail the relation between the macroscopic nuclear reaction probability and the microscopic nuclear reaction cross section on the basis of equilibrium-thermodynamic arguments. A general form of the nuclear reaction probability is derived, and the most general collision probability integral is evaluated in closed form for all convenient values of the parameters by using random variables.     

原子核状态方程

介绍了在核子自由度占优势时原子核状态方程的一些知识,并对在饱和状态下直接研究和在重离子碰撞中间接研究原子核状态方程的情况做了评述． The knowledge of nuclear equation of state for which nucleonic effects are dominant is presented. The direct studies which close to the saturation state and the indirect studies involving heavy ion collisions on nuclear equation of state are reviewed.     

Nuclear Orientation Studies of Magnetism in Single Crystal Pr0.5Nd0.5Ni

The angular distributions of -rays from ¹⁴²Pr, ¹⁴⁷Nd and ¹⁴⁴Pm in Pr_0.5Nd_0.5Ni were investigated by means of low-temperature nuclear orientation. The results show that the Pr and Nd electronic moments order along the c- and a-axis, respectively. Pm ions introduced as a very dilute magnetic impurity, preferentially align with the Nd ions.     

On Eta-Meson Interaction with Nuclear Matter

Physics of Atomic Nuclei - The transparency of the carbon nucleus with respect to eta mesons of energy 25 to 95 MeV was measured. Mesons of this energy originated from the photoproduction on...     

On the Intermittency in Nuclear Fragmentations

A simple Monte Carlo simulation and even a partition method can be used to reproduce quite well the power law behavior between the factorial moment and the charge interval of fragment charge distributions from the multifragmentation of ¹⁹⁷Au nucleus bombarding emulsion at～1 GeV/nucleon. This indicates that the above power law seems not to be a unique precursor of the intermittent behavior in nuclear multifragmentations.     

重离子碰撞中的核温度

介绍了基于不同近似下的核温度的测量方法。不同测量方法给出的核温度不尽相同,这是因为它们使用的基本假设不一样。讨论了原子核系统的特殊性,这些特殊性使得核温度的提取特别困难。重点比较了动力学近似的几种温度计:使用经典Maxwell-Boltzmann 近似的,和使用量子Fermi-Dirac近似的。使用氦锂温度计对照考虑费米子属性后的斜率温度计和涨落温度计,发现考虑费米属性后提取的核温度更接近于热力学温度。仍需更多的工作,从核体系的孤立性、有限性和费米子属性等方面来研究核系统热力学温度的提取方法。Nuclear thermometers based on different approaches are reviewed. Due to the different basic assumptions of these methods, the nuclear temperatures extracted by different thermometer approaches are not consistent. Moreover,several problems which make the extraction of the nuclear temperatures even more difficult are discussed. The nuclear thermometers based on different kinetic approaches are compared. The nuclear temperatures extracted by methodsbased on quantum (Fermi-Dirac) kinetic approaches are compared to the double ratio temperatures THe; Li. Perspectives for future investigations of the thermal temperature are given.     

关于我国核能发展技术路线之我见

简要地阐述了核能在当前世界和我国能源发展中的地位和作用,提出了在近１０～２０年内我国核能发展应遵循的原则和技术路线的建议.The position and role of nuclear energy in china’s energy sources development is briefly outlined and a proposal about strategic principles and technology line of china’s nuclear energy development in recent 10～20 years is put forward.