Dynamic nuclear polarization and nuclear orientation in terbium vanadate

Dynamic nuclear polarization is a well established technique, which has been used to produce polarized targets for experiments in nuclear physics. This paper suggests new experiments, involving the nuclear magnetic resonance of two isotopes, one stable, the other radioactive, in an antiferromagnet, terbium vanadate. At temperatures well below the Néel point, the line widths should be comparatively small. Resonance may be detected through changes in the rate of gamma-ray emission observed by a nuclear orientation experiment.     

Cold nuclear fusion

Recent accelerator experiments on fusion of various elements have clearly demonstrated that the effective cross-sections of these reactions depend on what material the target particle is placed in. In these experiments, there was a significant increase in the probability of interaction when target nuclei are imbedded in a conducting crystal or are a part of it. These experiments open a new perspective on the problem of so-called cold nuclear fusion.     

锕系核靶制备与应用北大核心CSCD

基础核物理、核能开发、超重元素研制等领域对精确、值得信赖的核参数的需求日益增大,锕系核靶作为核数据测量实验的核心部件,其产品种类及关键质量参数的优劣直接制约核数据测量的发展水平。系统介绍了国内外锕系核靶制备及检测方法以及国内外核靶研究团队技术储备及发展方向。     

Dynamic nuclear polarisation in metallic silver

Dynamic nuclear polarisation is a well established technique, which has been used to produce polarised targets for experiments in nuclear physics. This paper suggests that the nuclei spins in metallic silver may be polarised by inclusion of erbium as a small impurity with an electronic spin.     

Nucleon mean free path in nuclear matter

In calculations of nuclear reaction yields at incident energies of some tens of MeV consistently better agreement with experiments is obtained by assuming a nucleon mean free path in nuclear matter longer than that deduced from the Fermi gas model and free nucleon-nucleon cross sections.     

Implications of the nuclear EMC effect

The discovery more than twenty years ago, by the EMC Collaboration, that the deep-inelastic-scattering DIS structure functions are influenced by the nuclear environment stunned the nuclear physics community and brought quarks and gluons into the field with great impact. A great length of time has passed, but despite a semi-infinite number of papers on the subject, there is no explanation that is universally accepted. Many models (related in one way or another to QCD) have been successful in reproducing data for deep inelastic scattering on nuclear targets, but fewer have described both the DIS and nuclear Drell-Yan experiments. Although there are some positive indications, no model has been used to predict correctly and unambiguously new independent phenomena. We review the history and discuss the best experimental prospects for future discovery.     

Chemically amplified 19F-1H nuclear Overhauser effects

Chemically induced dynamic nuclear polarisation (CIDNP) is explored as a source of nuclear hyperpolarisation in heteronuclear Overhauser effect experiments. A photochemical reaction proceeding through a radical pair intermediate is used to enhance (19)F nuclear magnetisation in 3-fluorotyrosine by more than an order of magnitude with a corresponding increase in the amplitudes of (19)F-(1)H cross-relaxation and cross-correlation effects. The reactions employed are cyclic and leave the sample chemically unchanged. The potential for enhancing the sensitivity of heteronuclear NOEs in (19)F-labelled proteins is discussed.     

Time reversal invariance in nuclear gamma-decay

Test experiments to study of T-and PT-reversal symmetry in nuclear γ-decay, using linear polarization of γ-rays from nuclei oriented by hyperfine interaction at low temperatures are discussed. The necessary formalism is reviewed and details of new proposals for future experiments are given.     

Direct photons,dimuons and nuclear effects

Largep _T direct photon and dimuon production in experiments using nuclear targets have been studied in detail. We show that, by a careful choice of kinematic configurations in these experiments, very good estimates of nuclear quark and gluon densities can be obtained. Information on the cumulative region of the nuclear structure functions i.e. the regionx>1 can also be obtained by considering semi-integrated cross sections.     

New directions in low temperature nuclear orientation

The basis of the low temperature nuclear orientation technique is summarized. The present limitations on accessible temperatures and the orders of magnitude of hyperfine interactions in the metallic systems currently studied are discussed briefly. The broad applicability to many elements and the high sensitivity of this singles counting method are emphasized. Specific recent developments are discussed in more detail. The use of a dilution refrigerator to cool to ≈ 10mK nuclei of isotopes far from stability ‘on-line’, after production in an accelerator and electromagnetic selection, is a major extension of the method. The minimum half-life is now limited by the nuclear spinlattice relaxation time, typically of order 10–100 s at 10 mK. Aspects of these experiments are considered and recent results given for Cs and I isotopes. Secondly, the extension of the related technique of nuclear magnetic resonance or oriented nuclei (NMR/ON) to antiferromagnetic insulators is described. A new cooling mechanism involving nuclear-magnon coupling gives access to much lower temperatures than previously reached in these systems. Recent precision work on MnCl₂, 4H₂O is discussed, along with its possible extension to nuclei of lanthanide elements. Finally, the use of nuclear orientation to study ordering below 10 mK of enhanced nuclear moment systems is briefly surveyed, with HoVO₄ as the test case. NMR/ON experiments at high pressure are proposed.     

Two-body correlations in nuclear systems

Correlations in the nuclear wave-function beyond the mean-field or Hartree-Fock approximation are very important to describe basic properties of nuclear structure. Various approaches to account for such correlations are described and compared to each other. This includes the holeline expansion, the coupled cluster or “exponential S” approach, the self-consistent evaluation of Greens functions, variational approaches using correlated basis functions and recent developments employing quantum Monte-Carlo techniques. Details of these correlations are explored and their sensitivity to the underlying nucleon-nucleon interaction. Special attention is paid to the attempts to investigate these correlations in exclusive nucleon knock-out experiments induced by electron scattering. Another important issue of nuclear structure physics is the role of relativistic effects as contained in phenomenological mean field models. The sensitivity of various nuclear structure observables on these relativistic features are investigated. The report includes the discussion of nuclear matter as well as finite nuclei.     

Enhancement and speedup in nuclear resonant diffraction

The interaction of Mössbauer radiation with the nuclei in a single crystal provides the unique possibility to enhance the coherent channel in nuclear resonance scattering by means of a properly phased excitation of the scattering centers. When a primary beam is incident in the exact Bragg direction, all nuclei are excited in phase. The resonance parameters of such a collective nuclear excitation of a perfect single crystal (γ-exciton) are entirely different from those of an individual nuclear excitation. In Bragg geometry diffraction, the resonance lines are shifted and broadened (enhancement effect), the lifetime of the collective excited state is shortened (speedup effect) and the reflectivity becomes total (suppression effect). Recent experiments arc reviewed, where these effects were studied in the resonant diffraction of Mössbauer and of synchrotron radiation.

     

Dephasing of atomic tunneling by nuclear quadrupoles

Recent experiments revealed a most surprising magnetic-field dependence of coherent echoes in amorphous solids. We show that a novel dephasing mechanism involving nuclear quadrupole moments is the origin of the observed phenomena.     

Generating entanglement and squeezed states of nuclear spins in quantum dots

We present a scheme for achieving coherent spin squeezing of nuclear spin states in semiconductor quantum dots. The nuclear polarization dependence of the electron spin resonance generates a unitary evolution that drives nuclear spins into a collective entangled state. The polarization dependence of the resonance generates an area-preserving, twisting dynamics that squeezes and stretches the nuclear spin Wigner distribution without the need for nuclear spin flips. Our estimates of squeezing times indicate that the entanglement threshold can be reached in current experiments.     

The impact of nuclear magnetism on superconductivity

The influence of nuclear magnetism on the superconducting critical field B_c has been measured in the nuclear magnets Al, AuIn₂, and Sn, at ultralow temperatures, 17 μKT1 K. The materials have been chosen in respect to their distinctive Korringa constants κ(Al, AuIn₂, Sn)=(1.8, 0.1, 0.05) Ks. Both in the weakly and in the strongly coupled nuclear magnets, Al and Sn, a reduction of B_c is observed being proportional to the nuclear magnetization at T4 mK. However, using the high resolution of the experiment performed on Sn, the decrease of B_c at cooling down to 70 μK is found to be nonmonotonic. The features of the recently measured B_c(T) curve of Sn which are not understood yet as well as the formerly observed results of the interplay of nuclear ferromagnetism and superconductivity in AuIn₂ have motivated ongoing experiments which try to clear up the pair breaking contribution of nuclear magnetism.     

Magnetic structure determined by nuclear orientation

Conclusion In conclusion we suggest that nuclear orientation is a useful technique for determining nuclear spin structures. For a known hyperfine interaction atomic magnetic structures may also be deduced. Measurements on antiferromagnets and the rare earth magnet holmium demonstrate the method, although in the latter case the turn angle cannot be determined because of the limitation of L2 radiation. These experiments show that nuclear orientation can at least complement neutron diffraction and at best furnish information about magnetic structure when the latter technique is not applicable. We intend to study other rare earth magnets and more complicated antiferromagnetic structures.     

Time structure and atomic excltation spectra in heavy-ion collisions with nuclear contact

From a quantum mechanical model for quasielastic nuclear scattering, employing a pocket in the internuclear potential at close distances, a distribution of nuclear delay times is derived. The influence of this time structure on atomic excitation spectra is demonstrated using positron emission from supercritical collisions as an example. In a narrow regime of beam energies close to the Coulomb barrier, a considerable probability for collisions with long nuclear delay times is found, associated with a sharp peak in the positron spectrum which is due to enhanced spontaneous positron production. It is pointed out that quasielastic nuclear scattering alone cannot account for the absolute numbers of spontaneous positrons as extracted from recent experiments. A possible generalization of the theory to include inelastic nuclear processes is briefly discussed.     

Surface physics with nuclear probes

A series of typical experiments using nuclear moments as microscopic probes for hyperfine fields on surfaces are discussed: Mössbauer spectroscopy, perturbed angular correlation studies, nuclear magnetic resonance, nuclear spin relaxation due to spin-lattice relaxation, and low-dimensional diffusion. Experiments are selected which deal mainly with well-characterized surfaces. Out of a wealth of data, particularly those that contribute to currently discussed topics in surface science are selected.Supported partly by the Bundesministerium für Forschung und Technologie (contract 06 MR 853 I) and the Deutsche Forschungsgemeinschaft (contract Fi 311-1 and Fi 311-1)     

关于原子核电荷半径的研究

结合原子核电荷半径实验数据, 对885个中子数N≥8和质子数Z≥8的核电荷半径做了系统的研究. 对于单参数核电荷半径公式, Z^1/3律公式计算的结果优于A^1/3律的结果, 而对于两参数和三参数公式, Z^1/3律和A^1/3律的结果基本相当. 考虑到壳效应及奇偶摆动现象, 在原有的三参数公式基础上提出了加入Casten因子项和δ项的核电荷半径新公式. 利用该公式计算得到的核电荷半径理论值和实验值符合得非常好, 均方根偏差仅为σ=0.0266 fm, 此值比常用的三参数公式的结果下降了近50%, 理论计算值能更好地反映出壳效应及核电荷半径奇偶摆动的变化趋势.     

The NSW interaction with nuclear local modes in antiferromagnets

The nuclear spin wave (NSW) relaxation rates due to the interaction with nuclear impurity local modes (NILM) in antiferromagnets are considered in the framework of the Keldysh formalism based on the spin operator diagram technique. It is shown that the NSW relaxation frequency due to the scattering on the fluctuations of impurity nuclear spins is of resonance character. The NSW damping due to the resonance absorption by NILM depends strongly on the NSW amplitude N, which accounts for the “hard” excitation of NSW's in parallel pumping experiments /6/. The NSW relaxation rates due to the processes involving two NSW's and one impurity nuclear excitation are also calculated.