基于不稳定核基本性质测量的原子核结构研究

精密激光谱学是通过测量核素原子光谱的超精细结构和同位素移位来研究原子核的基本性质,为原子核自旋、磁矩、电四极矩及电荷均方根半径的确定提供了一种模型独立的测量方式。这些原子核基本性质的测量,能够比较精确地描述原子核微观结构的演化。近年来,随着放射性束流装置的发展,产生远离β-稳定线的丰中子/丰质子核素成为可能,也进一步促进了高分辨和高灵敏度的激光谱技术更加广泛的应用。简单介绍了基于放射性核素超精细结构的激光谱学测量原理,并通过几个经典实例来回顾近年来激光谱学在原子核奇特结构研究领域的独特贡献。主要通过分析几个重要核区原子核的基本性质,结合大尺度壳模型、ab initio理论、密度泛函理论等,来探索丰中子核中展现出来的一些新的奇特现象,如晕结构、幻数演化、形状共存等。High-precision laser spectroscopy technique is used to determine the ground state properties of exotic nuclei by probing its electronic hyperfine structure and isotope shift. It provides a model-independent measurement of nuclear spin, magnetic moment, electric quadrupole moment and charge radii. These nuclear parameters can be used to investigate the nuclear structure evolution and the nuclear shapes. With the development of accelerators and isotope separators, exotic isotopes far from β stability became accessible experimentally, which enhanced the capability of the laser spectroscopy technique being applied in the field of nuclear physics. A brief introduction to experimental principle is given, followed by a review of several typical examples for the experimental investigations in the different regions of nuclear chart. This aims to demonstrate the contributions of ground state properties measurement by using laser spectroscopy technique to the nuclear structure study of exotic isotopes. This discussion involves several different nuclear theory models in order to interpret the exotic phenomena observed in the neutron-rich isotopes, such as halo structure, shell evolution, shape coexistence and so on.     

Tunneling between double wells of atom in crossed electromagnetic fields

The tunneling between double wells of atom in crossed electromagnetic fields is investigated by a one-dimensional Hamiltonian model. The crossed fields induced outer well is apart from the nuclear origin and it is very difficult to access by means of spectroscopy but it will be possible if there exists the tunneling of the electron between the outer well and the Coulomb potential predominated well at the nuclear origin. A one-dimensional quantum calculation with B-spline basis has been performed for hydrogen atom in crossed fields accessible in our laboratory, at B=0.8~T and F=-220~V.cm^-1. The calculation shows that the wavefunctions of some excited states close to the Stark saddle point in the outer well extend over to the Coulomb potential well, making it possible to penetrate the quantum information of the outer well. However, the tunneling rate is very small and the spectral measurement of the transitions from the ground state should be of a high resolution and high sensitivity.     

Status of the LASER-IGISOL collaboration at the University of Jyväskylä

Recent laser spectroscopy studies at the IGISOL facility, University of Jyväskylä have focussed on the shape transition at N?=?60. A new technique of optical pumping in the ion beam cooler has given unique access to the radioactive isotopes of niobium and yttrium. Further spectroscopy from metastable states allowed nuclear moments and charge radii to be extracted for isotopes in the chains of Mo and Nb, thus exploring the extent of nuclear deformation across the entire Z?=?36–42 region.     

Laser spectroscopy — Recent results and prospects for lighter nuclei

Atomic physics methods have provided important information about the electromagnetic properties of the nuclear ground states. In recent years the experimental effort has been directed towards optical high resolution spectroscopy of very high sensitivity, giving access to short-lived nuclides very far from stability. A few of these new techniques based on collinear laser spectroscopy are presented with their latest results. They are suitable for an extension of the experiments to the region of lighter nuclei.     

Optical detection and ionization of donors in specific electronic and nuclear spin States

We resolve the remarkably sharp bound exciton transitions of highly enriched 28Si using a single-frequency laser and photoluminescence excitation spectroscopy, as well as photocurrent spectroscopy. Well-resolved doublets in the spectrum of the 31P donor reflect the hyperfine coupling of the electronic and nuclear donor spins. The optical detection of the nuclear spin state, and selective pumping and ionization of donors in specific electronic and nuclear spin states, suggests a number of new possibilities which could be useful for the realization of silicon-based quantum computers.     

Oxidation and magnetic states of chalcopyrite CuFeS2: A first principles calculation

The ground state band structure, magnetic moments, charges and population numbers of electronic shells of Cu and Fe atoms have been calculated for chalcopyrite CuFeS₂ using density functional theory. The comparison between our calculation results and experimental data (X-ray photoemission, X-ray absorption and neutron diffraction spectroscopy) has been made. Our calculations predict a formal oxidation state for chalcopyrite as Cu¹⁺Fe³⁺S ₂ ^2? . However, the assignment of formal valence state to transition metal atoms appears to be oversimplified. It is anticipated that the valence state can be confirmed experimentally by nuclear magnetic and nuclear quadrupole resonance and Mössbauer spectroscopy methods.     

Local nuclear orientation of125I in tin and graphite using defect induced electric field gradients

Defect-induced field electric field gradients, generated by ion implantation of¹²⁵I in α- and β-Sn and in graphite, were used to obtain local nuclear orientation at low temperatures. The resulting nuclear orientation at the parent¹²⁵I state was detected by¹²⁵Te Mössbaner spectroscopy. From the measured nuclear quadrupole interaction strengths at the parent state, the electric field gradient and the cooling behavior of implanted atoms were studied.     

α结团模型在形变核区域的推广

将α结团模型推广至形变核,计算偶偶形变母核α衰变基态到子核基态和子核第一激发态的分支比,显示出α衰变精细结构的测量是提取核形变信息的有效手段.The cluster model of α-decay is extended to deformed nuclei. The branching ratios of α-decays from the ground state of a parent nucleus to the ground state 0~(+) of its deformed daughter nucleus and to the first excited state 2~(+) are calculated in the framework of the cluster model. The results indicate that a measurement of α spectroscopy is a feasible method to extract the information of nuclear deformation.     

Quantum computation based on magic-angle-spinning solid state nuclear magnetic resonance spectroscopy

Magic-angle spinning (MAS) solid state nuclear magnetic resonance (NMR) spectroscopy is shown to be a promising technique for implementing quantum computing. The theory underlying the principles of quantum computing with nuclear spin systems undergoing MAS is formulated in the framework of formalized quantum Floquet theory. The procedures for realizing state labeling, state transformation and coherence selection in Floquet space are given. It suggests that by this method, the largest number of qubits can easily surpass that achievable with other techniques. Unlike other modalities proposed for quantum computing, this method enables one to adjust the dimension of the working state space, meaning the number of qubits can be readily varied. The universality of quantum computing in Floquet space with solid state NMR is discussed and a demonstrative experimental implementation of Grover's search is given. Received 19 April 2001     

Coherent storage of photoexcited triplet states using 29Si nuclear spins in silicon

Pulsed electron paramagnetic resonance spectroscopy of the photoexcited, metastable triplet state of the oxygen-vacancy center in silicon reveals that the lifetime of the m(s)=±1 sublevels differs significantly from that of the m(s)=0 state. We exploit this significant difference in decay rates to the ground singlet state to achieve nearly ~100% electron-spin polarization within the triplet. We further demonstrate the transfer of a coherent state of the triplet electron spin to, and from, a hyperfine-coupled, nearest-neighbor (29)Si nuclear spin. We measure the coherence time of the (29)Si nuclear spin employed in this operation and find it to be unaffected by the presence of the triplet electron spin and equal to the bulk value measured by nuclear magnetic resonance.     

Gamma and electron spectroscopy of transfermium isotopes at Dubna: Results and plans

Detailed spectroscopic information of excited nuclear states in deformed transfermium nuclei is scarce. Most of the information available today has been obtained from investigations of fine-structure α-decay. Although α decay gives access to hindrance factors and lifetimes which are strongly correlated to shell/subshell closures and the presence of isomers, only the combined use of γ and conversion electron spectroscopy allows the precise determination of excitation energy, spin and parity of nuclear levels.     

Measurement of nuclear moments and radii by collinear laser spectroscopy and by β-NMR spectroscopy

Experiments using laser spectroscopy and nuclear magnetic resonance techniques have been performed at ISOLDE on the unstable isotopes of several light elements. The results include nuclear ground state spins, magnetic dipole and electric quadrupole moments, and the behaviour of mean square nuclear charge radii within isotopic strings. These give important information about the nuclear structure at shell closures and close to the drip lines. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nuclear spins of ionized phosphorus donors in silicon

We demonstrate the coherent control and electrical readout of ionized phosphorus donor nuclear spins in (nat)Si. By combining time-programed optical excitation with coherent electron spin manipulation, we selectively ionize the donors depending on their nuclear spin state, exploiting a spin-dependent recombination process at the Si/SiO(2) interface, and find a nuclear spin coherence time of 18 ms for the ionized donors. The presented technique allows for spectroscopy of ionized-donor nuclear spins and enhances the sensitivity of electron nuclear double resonance to a level of 3000 nuclear spins.     

Developing VUV spectroscopy for protein folding and material luminescence on beamline 4B8 at the Beijing Synchrotron Radiation Facility

The new 4B8 beamline provides UV–VUV light in the wavelength range from 360 to 120 nm. It uniquely enables two kinds of spectroscopy measurements: synchrotron radiation circular dichroism spectroscopy and VUV excited fluorescence spectroscopy. The former is mainly used in protein secondary structure studies, and the latter in VUV excited luminescent materials research. Remote access to fluorescence measurement has been realised and users can collect data online. Besides steady‐state measurements, fluorescence lifetime measurements have been established using the time domain method, while a laser‐induced temperature jump is under development for protein folding dynamics using circular dichroism as a probe.     

Persistent narrowing of nuclear-spin fluctuations in InAs quantum dots using laser excitation

We demonstrate the suppression of nuclear-spin fluctuations in an InAs quantum dot and measure the timescales of the spin narrowing effect. By initializing for tens of milliseconds with two continuous wave diode lasers, fluctuations of the nuclear spins are suppressed via the hole-assisted dynamic nuclear polarization feedback mechanism. The fluctuation narrowed state persists in the dark (absent light illumination) for well over 1 s even in the presence of a varying electron charge and spin polarization. Enhancement of the electron spin coherence time (T2*) is directly measured using coherent dark state spectroscopy. By separating the calming of the nuclear spins in time from the spin qubit operations, this method is much simpler than the spin echo coherence recovery or dynamic decoupling schemes.     

Hyperfine-interaction studies of surfaces

Applications of hyperfine-interaction techniques, like NMR, PAC and Mößbauer spectroscopy, to well-characterized surfaces are discussed and the present knowledge of surface hyperfine fields is reviewed. Measurements of nuclear spin relaxation permit to extract the local density of electron states at the Fermi level of adsorbed alkali atoms. From the observed electric-field-gradient properties surface probe sites and diffusion processes can be inferred; the experimentally determined magnetic hyperfine fields give access to the electron-spin behaviour at magnetic surfaces.     

Nuclear quadrupole moment of 57Fe from microscopic nuclear and atomic calculations

The nuclear quadrupole moment (NQM) of the Ipi = 3/2(-) excited nuclear state of 57Fe at 14.41 keV, important in M?ssbauer spectroscopy, is determined from the large-scale nuclear shell-model calculations for 54Fe, 57Fe, and also from the electronic ab initio and density functional theory calculations including solid state and electron correlation effects for the molecules Fe(CO)(5) and Fe(C5H5)(2). Both independent methods yield very similar results. The recommended value is 0.15(2) e b. The NQM of the isomeric 10+ in 54Fe has also been calculated. The new NQM values for 54Fe and 57Fe are consistent with the perturbed angular distribution data.     

The Electronic State of Flavoproteins: Investigations with Proton Electron–Nuclear Double Resonance

Electron–nuclear double resonance (ENDOR) spectroscopy provides useful information on hyperfine interactions between nuclear magnetic moments and the magnetic moment of an unpaired electron spin. Because the hyperfine coupling constant reacts quite sensitively to polarity changes in the direct vicinity of the nucleus under consideration, ENDOR spectroscopy can be favorably used for the detection of subtle protein–cofactor interactions. A number of pulsed ENDOR studies on flavoproteins have been published during the past few years; most of them were designed to characterize the flavin cofactor by means of its protonation state, or to detect individual protein–cofactor interactions. The aim of this study is to compare the pulsed ENDOR spectra from different flavoproteins in terms of variations of characteristic proton hyperfine values. The general concept is to observe limits of possible influences on the cofactor’s electronic state by surrounding amino acids. Furthermore, we compare ENDOR data obtained from in vivo experiments with in vitro data to emphasize the potential of the method for gaining molecular information in complex media.     

山东大学(威海)开展的核物理研究

综述了山东大学威海校区原子核物理研究团队在原子核精细谱学、核天体物理、探测器研制和高能核物理等方向开展的研究工作及最新进展;尤其重点介绍了$A\sim 110$核区原子核的形状共存和带交叉延迟,“订书机”和“雨伞”模式转动带,碳氮氧循环过程中关键核反应的测量进展,中子星参数化的状态方程及双中子星并合引力波研究,带电粒子探测器的设计与制作,相对论重离子碰撞物理中量子输运理论和高阶反常输运等研究工作,并展望了下一步的工作重点。