6Li and 7li solid-state NMR spectroscopy of nitrogen ceramic phases

Two nitrogen ceramic phases, the oxynitride LiSiON and the nitride LiSi2N3, have been studied by 6Li and 7Li NMR. Magic angle spinning (MAS) NMR experiments have been carried out at two magnetic field strengths (7.05 and 14.1 T). The spectra give evidence of the relative effects of the quadrupolar and chemical shift interactions. The electric field gradient tensor of both phases has been determined accurately by iterative fitting of the 6Li and 7Li MAS NMR line shapes at the two magnetic field strengths. Due to the fact that for 7Li the quadrupolar interaction is much larger than the chemical shift interaction, it is shown that neither the small chemical shift anisotropy nor the relative orientation of the two interaction tensors can be determined accurately by 7Li MAS NMR. For 6Li, the two interactions are comparable and the value of these parameters obtained from the fits of the 6Li experimental MAS line shapes are therefore much more reliable.     

Measurement of the ~2H(~7Be,~6Li)~3He reaction rate and its contribution to the primordial lithium abundance

In the standard Big Bang nucleosynthesis(SBBN) model, the lithium puzzle has attracted intense interest over the past few decades, but still has not been solved. Conventionally, the approach is to include more reactions flowing into or out of lithium, and study the potential effects of those reactions which were not previously considered.~7Be(d,~3He)~6Li is a reaction that not only produces~6Li but also destroys~7Be, which decays to~7Li, thereby affecting~7Li indirectly. Therefore, this reaction could alleviate the lithium discrepancy if its reaction rate is sufficiently high.However, there is not much information available about the~7Be(d,~3He)~6Li reaction rate. In this work, the angular distributions of the~7Be(d,~3He)~6Li reaction are measured at the center of mass energies Ecm = 4.0 Me V and 6.~7Me V with secondary~7Be beams for the first time. The excitation function of the~7Be(d,~3He)~6Li reaction is first calculated with the computer code TALYS and then normalized to the experimental data, then its reaction rate is deduced. A SBBN network calculation is performed to investigate its influence on the~6Li and~7Li abundances. The results show that the~7Be(d,~3He)~6Li reaction has a minimal effect on~6Li and~7Li because of its small reaction rate. Therefore,the~7Be(d,~3He)~6Li reaction is ruled out by this experiment as a means of alleviating the lithium discrepancy.     

Global optical-model potentials for the elastic scattering of 6, 7Li projectiles

Simultaneous fits have been made to 44 ⁶Li data sets covering the mass range 24–208 and the energy range 13–156 MeV in order to determine an average (“global”) optical-model potential for ⁶Li scattering. A similar study has been made for 25 ⁷Li data sets over the same mass range and an energy range of 28–88 MeV to find an average ⁷Li potential. With Saxon-Woods form factors, constant values may be used for all parameters except for the depth of the imaginary potential which decreases in magnitude with increasing mass. The necessity of energy dependence. Coulomb correction and (for ⁷Li) a symmetry term is investigated. The variation of the integral properties of the potentials is discussed, and also a comparison is made for the two projectiles. Application of the global potentials is made to inelastic scattering and single-nucleon transfer reactions.     

Search for solar axions emitted in an <Emphasis Type="Italic">M</Emphasis>1 transition in <Superscript>7</Superscript>Li* nuclei

The resonance absorption of solar axions by ⁷Li nuclei, which is accompanied by the excitation of the first nuclear level of lithium a + ⁷Li → ⁷Li* → ⁷Li + γ, is sought. To this end, the energy spectrum has been measured by an HPGe detector that is surrounded by a LiOH layer. A new upper limit m_a ≤ 16 keV (at 90% C.L.) has been determined for the mass of the hadron axion.     

Crystal structure of Li5B7O12.5Cl at 296 and 425 K

The structure of the boracite-type Li₅B₇O_12.5Cl has been determined at two different temparatures. It has a cubic symmetry with a cell parameter of 12.136(3) Å and space group F23. The rigid B₇O₁₂ three-dimensional framework is similar to that of other boracites. Changes in the coordination of the B(2), B(3) and Li(1) arise from the insertion of the additional O(3) atom and the ordering of Li(2). On the basis of the results relative to the temperature factors and electron density of the Li(1) atom at different temperatures a transport mechanism is proposed.     

Formation of positive cluster ions Li(n) Br (n = 2-7) and ionization energies studied by thermal ionization mass spectrometry

Clusters of the type Li(n)X (X = halides) can be considered as potential building blocks of cluster-assembly materials. In this work, Li(n)Br (n = 2-7) clusters were obtained by a thermal ionization source of modified design and selected by a magnetic sector mass spectrometer. Positive ions of the Li(n)Br (n = 4-7) cluster were detected for the first time. The order of ion intensities was Li(2)Br(+) > Li(4)Br(+) > Li(5)Br(+) > Li(6)Br(+) > Li(3)Br(+). The ionization energies (IEs) were measured and found to be 3.95 ± 0.20 eV for Li(2)Br, 3.92 ± 0.20 eV for Li(3)Br, 3.93 ± 0.20 eV for Li(4)Br, 4.08 ± 0.20 eV for Li(5)Br, 4.14 ± 0.20 eV for Li(6)Br and 4.19 ± 0.20 eV for Li(7)Br. All of these clusters have a much lower ionization potential than that of the lithium atom, so they belong to the superalkali class. The IEs of Li(n)Br (n = 2-4) are slightly lower than those in the corresponding small Li(n) or Li(n)H clusters, whereas the IEs of Li(n)Br are very similar to those of Li(n) or Li(n)H for n = 5 and 6. The thermal ionization source of modified design is an important means for simultaneously obtaining and measuring the IEs of Li(n)Br (n = 2-7) clusters (because their ions are hermodynamically stable with respect to the loss of lithium atoms in the gas phase) and increasingly contributes toward the development of clusters for practical applications.     

High-lying excited states in <Superscript>10</Superscript>Be from the <Superscript>9</Superscript>Be(<Superscript>9</Superscript>Be,<Superscript>10</Superscript>Be)<Superscript>8</Superscript>Be reaction

A transfer-reaction experiment of ⁹Be(⁹Be, ¹⁰Be)⁸Be was performed at a beam energy of 45 MeV. Excited states in ¹⁰Be up to 18.80 MeV are produced using missing mass and invariant mass methods. Most of the observed high-lying resonant states, reconstructed from the α + ⁶He and t + ⁷Li decay channels, agree with the previously reported results. In addition, two new resonances at 15.6 and 18.8 MeV are identified from the present measurement. The 18.55 MeV state is found to decay into both the t + ⁷Li_g.s. and t + ⁷Li* (0.478 MeV) channels, with a relative branching ratio of 0.93 ± 0.33. Further theoretical investigations are encouraged to interpret this new information on cluster structure in neutron-rich light nuclei.     

Spin-alignment echo NMR: probing Li+ hopping motion in the solid electrolyte Li7La3Zr2O12 with garnet-type tetragonal structure

(7)Li spin-alignment echo (SAE) nuclear magnetic resonance (NMR) spectroscopy has been used to measure single-spin hopping correlation functions of polycrystalline Li(7)La(3)Zr(2)O(12). Damping of the echo amplitude S(2)(t(m),t(p)), recorded at variable mixing time t(m) but fixed preparation time t(p), turns out to be solely controlled by slow Li jump processes taking place in the garnet-like structure. The decay rates τ(SAE)(-1) directly obtained by parametrizing the curves S(2)(t(m),t(p)) with stretched exponential functions show Arrhenius behaviour pointing to an activation energy of approximately 0.5 eV. This value, probed by employing an atomic-scale NMR method, is in very good agreement with that deduced from impedance spectroscopy used to measure macroscopic Li transport parameters. Most likely, the two methods are sensitive to the same hopping correlation function although Li dynamics are probed in a quite different manner.     

Excitation of 6 Li above the breakup threshold in the 6 Li + 208 Pb system around the Coulomb barrier

The excitation energy above the breakup threshold of the Li system, after the interaction of ⁶Li nuclei with a ²⁰⁸Pb target, was deduced from the invariant mass of the system. Data were collected with a large-solid-angle detector set-up at four beam energies around the Coulomb barrier. The excitation of the ⁶Li nucleus above the breakup threshold (1.47 MeV) has a quite similar behavior at each measured beam energy and angle; it is peaked at MeV above the threshold and shows an exponential decay on the high energy side, which is a clear signature of a direct breakup process. The experimental excitation energies are reproduced both in shape and absolute value by 1) fully quantum-mechanical Coupled-Channel calculations with coupling to discretized-continuum ⁶Li excitations, 2) semi-classical Coupled-Channel approach, where the relative motion is treated along a classical trajectory.Received: 2 June 2003, Published online: 18 November 2003PACS: 25.70.Ji Fusion and fusion-fission reactions - 25.70.Mn Projectile and target fragmentation - 24.10.Eq Coupled-channel and distorted-wave modelsThis revised version was published online in December 2003 with corrections to the communicators name.     

Predissociation induced by ungerade-gerade symmetry breaking in 6Li7Li molecule

In the recent sub-Doppler experiment on the B1Pi(u) state of the Li2 molecule by Bouloufa et al. [J. Chem. Phys. 111, 1926 (1999)], where the dissociation of vibrational levels due to the tunneling through the potential barrier was investigated, several vibrational levels with abnormally large dissociated rates were observed in the case of the 6Li7Li isotopomer. This dynamical effect cannot be explained by tunneling as in the case of 6Li2 or 7Li2. A simple model of coupling between B1Pi(u) and 1(1)Pi(g) states involving the u-g symmetry breaking for 6Li7Li is proposed. Rates of the B1Pi(u) predissociated levels due to this coupling are calculated. A good agreement with the experiment is found.     

Changes in the solar wind’s isotope component,due to interaction between solar flare particles and the photosphere

Possible mechanisms responsible for changes in the solar wind’s isotope component caused by interaction between solar flare protons and helium isotopes with photospheric nuclei are considered. Depth profiles of the rates of ⁶Li, ⁷Li, ⁷Be, ¹⁴C production in the solar atmosphere are simulated using the GEANT4 software package. It is concluded that anomalous isotopic compositions of the solar wind relative to the average values can form during the coronal mass ejections.     

Optical model and DWBA analysis of the7Li(d,d)- and7Li(d,p 0)-reactions in the energy region 1.0 to 2.6 MeV

Absolute differential cross sections for the elastic scattering of deuterons by⁷Li have been determined in the energy range of 1.0 to 2.6 MeV. Two excitation curves measured at laboratory angles of 90° and 160° vary smoothly with energy. At higher energies an extremely strong enhancement of the cross section relative to the Rutherford value was observed at large angles. It was, however, possible to find realistic optical potentials that describe these distributions fairly well over the whole energy region. For two of these potentials good fits could be obtained with DWBA calculations on the⁷Li (d, p ₀)-reaction, one of them yielding a spectroscopic factor in close agreement with shell model predictions.     

Hyperfine splitting,isotope shift,and level energy of the 3S states of (6,7)Li

We study the 2S-3S transition of (6,7)Li by high-precision laser spectroscopy using two-photon Doppler-free excitation and photoionization detection. Interferometric cross referencing to metrologic Rb 3S-5D two-photon transitions allowed measurement of the transition isotope shift and hyperfine splitting in the 3S state with precision at the 30 kHz level. The results are IS=11 453.734(30) MHz, A(3S)(6Li)=35.263(15) MHz, and A(3S)(7Li)=93.106(11) MHz. Combined with recent theoretical work, the isotope shift yields a new value for the change in squared nuclear charge radii DeltaR(2)=0.47(5) fm(2). This is compared with other work and some existing discrepancies are resolved.     

原子能院的核天体物理研究和串列升级工程进展(英文)

在北京串列实验室建立了次级束流实验装置 ,用于放射性核束物理和核天体物理研究 .先后开展了7Be(d ,n) 8B ,11C(d ,n) 12 N ,8Li(d ,p) 9Li和6 He(p ,n) 6 Li核天体物理重要反应的研究 .介绍了串列加速器升级工程的进展情况 .该工程在现有串列加速器的基础上 ,将建立 10 0MeV/ 2 0 0 μA的质子回旋加速器、在线同位素分离器和超导加速段 .在此装置上 ,将可以产生质量数最高为 12 0 ,强度最高为 10 9particles/s的放射性束流 . A secondary beam line (GIRAFFE) at the Beijing Tandem accelerator lab was constructed for yielding low energy secondary beams. The current progress on the study of nuclear astrophysics and nuclear structure is presented. Up to now, We have carried out measurement of~(7)Be(d, n)~( 8)B,~(11)C(d, n)~(12)N,~(8)Li(d, p)~(9)Li, and~(6)He(p, n)~(6)Li reactions. The proposed Beijing radioactive nuclear beam facility (BRIF ) and its current R&D progress are briefly introduced. This facility is based on...     

Nuclear charge radius of 11Li

We have determined the nuclear charge radius of ¹¹Li by high-precision laser spectroscopy. The experiment was performed at the TRIUMF-ISAC facility where the ⁷Li-¹¹Li isotope shift (IS) was measured in the 2s→3s electronic transition using Doppler-free two-photon spectroscopy with a relative accuracy better than 10⁻⁵. The accuracy for the IS of the other lithium isotopes was also improved. IS’s are mainly caused by differences in nuclear mass, but changes in proton distribution also give small contributions. Comparing experimentally measured IS with advanced atomic calculation of purely mass-based shifts, including QED and relativistic effects, allows derivation of the nuclear charge radii. The radii are found to decrease monotonically from ⁶Li to ⁹Li, and then increase with ¹¹Li about 11% larger than ⁹Li. These results are a benchmark for the open question as to whether nuclear core excitation by halo neutrons is necessary to explain the large nuclear matter radius of ¹¹Li; thus, the results are compared with a number of nuclear structure models.     

NMR study of 6Li in LiNbO3

An experimental NMR study of the ⁶Li isotope in single crystals of lithium niobate has been performed, along with a computer simulation of ⁶Li NMR spectra for a crystal of congruent composition, containing defects in the cation sublattice. It is found that the mean value of the principal component of the electric field gradient tensor at the ⁶Li nuclei is 1.48 times larger than at the ⁷Li nuclei. It is surmised that there is a substantial difference in the character of the mobility of the ⁶Li and ⁷Li nuclei in the LiO₆ octahedra at room temperature. Fiz. Tverd. Tela (St. Petersburg) 40, 122–125 (January 1998)     

A density functional theory study on size-dependent structures,stabilities,and electronic properties of bimetallic M_nAg_m(M=Na,Li;n + m≤7) clusters

The equilibrium geometries,relative stabilities,and electronic properties of Mn Agm(M=Na,Li;n + m ≤ 7) as well as pure Ag n,Na n,Li n(n ≤ 7) clusters are systematically investigated by means of the density functional theory.The optimized geometries reveal that for 2 ≤ n ≤ 7,there are significant similarities in geometry among pure Ag n,Na n,and Li n clusters,and the transitions from planar to three-dimensional configurations occur at n = 7,7,and 6,respectively.In contrast,the first three-dimensional(3D) structures are observed at n + m = 5 for both Na n Ag m and Li n Ag m clusters.When n + m ≥ 5,a striking feature is that the trigonal bipyramid becomes the main subunit of Li n Ag m.Furthermore,dramatic odd-even alternative behaviours are obtained in the fragmentation energies,secondorder difference energies,highest occupied and lowest unoccupied molecular orbital energy gaps,and chemical hardness for both pure and doped clusters.The analytic results exhibit that clusters with an even electronic configuration(2,4,6) possess the weakest chemical reactivity and more enhanced stability.     

Precision mass measurements of neutron-rich Y, Nb, Mo, Tc, Ru, Rh, and Pd isotopes

Direct mass measurements with typical uncertainties of 1-10keV have been performed for the first time for ten neutron-rich isotopes ^{102, 103}Y , ¹⁰⁸Nb , ¹¹¹Mo , ^{113, 114}Tc , ¹¹⁶Ru , ¹¹⁹Rh , and ^{121, 122}Pd . The obtained mass data compared with the 2003 atomic-mass evaluation shows systematic overestimation of binding energies far from stability. The relationship between two-neutron separation energies, nuclear structure and shape changes has been investigated by comparing the experimental data with a theoretical calculation based on the Hartree-Fock-Bogoliubov approximation using modern energy density functionals.     

The mass of 18N

States in ¹⁸N have been populated with the ¹⁸O(⁷Li, ⁷Be)¹⁸N reaction at 52 MeV. The single ground-state peak observed in previous measurements using charge-exchange reactions is shown to be at least a doublet, which leads to a substantial revision of the ground-state mass of ¹⁸N. The revised value of the mass excess is 13.116 ± 0.020 MeV. Several excited states of ¹⁸N are also observed.     

Depth of origin of sputtered particles under the oblique incidence of a primary ion beam

A physical model and mathematical calculations of the maximum partial depths of origin of sputtered particles versus mass, energy, and the angle of incidence of ions bombarding a one-component amorphous target are presented. Calculations reveal that the maximum depth of origin of secondary particles depends on the primary-ion incidence angle, which attains the highest value at angles of 30°–60° relative to the normal to the sample surface. When the primary beam and the target material have identical parameters, the maximum depths of origin of light secondary particles exceed those of heavy ones. Secondary particles exhibit the isotope effect. For light elements (Li, Be, and B), a heavier isotope has a larger maximum depth of origin than a lighter one. In the case of heavy elements (e.g., Mo), a lighter isotope has a greater maximum depth of origin than a heavier one.