白云石中Mn~(2+)及其占位的电子顺磁共振研究

本文用EPR法研究了白云鄂博矿床白云石中Mn~(2+)在Ca、Mg两个位置上的分配。通过比较白云石与菱镁矿中Mn~(2+)的精细结构线间的强度比,确定了几乎所有Mn~(2+)都处于Mg位置。其自旋哈密顿参数为:g=2.0022,A_∥=93.0×10~(-4)T,A_⊥=91.4×10~(-4)T,D=153.3×10~(-4)T。实验结果表明,白云石中Mn~(2+)的位置分配不取决于Mn~(2+)的浓度。离子半径和配位氧八面体畸变程度是控制位置分配的两个主要因素。     

Resonant motion in the restricted three body problem

The resonant structure of the restricted three body problem for the Sun- Jupiter asteroid system in the plane is studied, both for a circular and an elliptic orbit of Jupiter. Three typical resonances are studied, the 2 : 1, 3 : 1 and 4 : 1 mean motion resonance of the asteroid with Jupiter. The structure of the phase space is topologically different in these cases. These are typical for all other resonances in the asteroid problem. In each case we start with the unperturbed two-body system Sun-asteroid and we study the continuation of the periodic orbits when the perturbation due to a circular orbit of Jupiter is introduced. Families of periodic orbits of the first and of the second kind are presented. The structure of the phase space on a surface of section is also given. Next, we study the families of periodic orbits of the asteroid in the elliptic restricted problem with the eccentricity of Jupiter as a parameter. These orbits bifurcate from the families of the circular problem. Finally, we compare the above families of periodic orbits with the corresponding families of fixed points of the averaged problem. Different averaged Hamiltonians are considered in each resonance and the range of validity of each model is discussed.     

A fourth-order solution of the ideal resonance problem

The second-order solution of the Ideal Resonance Problem, obtained by Henrard and Wauthier (1988), is developed further to fourth order applying the same method. The solutions for the critical argument and the momentum are expressed in terms of elementary functions depending on the time variable of the pendulum as independent variable. This variable is related to the original time variable through a Kepler-equation. An explicit solution is given for this equation in terms of elliptic integrals and functions. The fourth-order formal solution is compared with numerical solutions obtained from direct numerical integrations of the equations of motion for two specific Hamiltonians.     

Geochronology of Ailuropoda–Stegodon fauna and Gigantopithecus in Guangxi Province, southern China

Pleistocene faunas from south China are difficult to subdivide based on the long temporal ranges of many taxa and a reduced number of genera in comparison to faunas from temperate north China. In south China, the Ailuropoda–Stegodon fauna is a very general one and includes a relatively stable suite of genera that apparently persisted for long periods of time. These attributes have made constraining its time range difficult. Application of electron spin resonance (ESR) dating of tooth enamel constrains the ages well where uranium uptake was minor. Where uranium uptake into teeth was significant, an approach combining ESR and ²³⁰Th/²³⁴U isotopic analysis also yields excellent ages. Previous estimates of early, middle and late Pleistocene time ranges previously determined by biostratigraphic seriation for the Ailuropoda–Stegodon fauna are confirmed in all cases but are made more precise with our approach, including specific time ranges for certain archaic taxa. Absolute dating also yields an extended time range for Gigantopithecus blacki of 1200 to 310 ka.     

三维动态对比增强磁共振血管成像诊断下肢动脉硬化闭塞症的应用价值

目的:探讨三维动态对比增强磁共振血管成像(3D CE-MRA)在诊断下肢动脉硬化闭塞症(LEASD)中的应用价值。方法:收集临床诊断为LEASD的64例患者行下肢动脉3D CE-MRA及数字减影血管造影(DSA)检查,分析两者结果是否有差异。结果:以DSA金标准作为对照组,3D CE-MRA对下肢动脉狭窄及闭塞程度的分级与DSA比较,Kappa值为0.84≥0.75,一致性较好,差异无统计学意义(P>0.05);但3D CE-MRA对下肢动脉狭窄及闭塞有一定程度的高估及低估,对轻度狭窄、中度狭窄、重度狭窄的高估率分别为3.3%(11/332)、4.0%(5/125)、4.5%(5/112)。对中度狭窄、重度狭窄及闭塞的低估率分别为9.6%(12/125)、4.0%(5/112)、15%(20/132)。以DSA为标准,3D CE-MRA诊断下肢动脉狭窄及闭塞的敏感性、特异性、阳性预测值及阴性预测值分别为93.7%(701/753)、93.1%(563/605)、95.1%(701/737)、94.9%(563/599)。结论:3D CE-MRA下肢动脉成像是一种无创、无辐射、有效的血管检查方法,可作为LEASD的优选检查方法。      

不同初始损伤下大理岩卸荷的核磁共振试验研究

为研究不同初始损伤下大理岩的卸荷特性,开展相同初始围压、不同初始轴压的常规三轴加-卸载试验,并进行加-卸载后岩样的核磁共振（NMR）特性试验研究,得到岩样孔隙度、应变、核磁共振图像及T2谱分布等参量。结果表明：（1）随着卸荷的进行,岩样孔隙度增大,且当初始轴压为三轴抗压强度（TCS）的90%时,其卸荷过程比80%与70%时更加平稳;（2）初始损伤可以有效促进应变的增大,尤其对环向扩容效应影响显著,同时应变的增长能促进裂隙的发育;（3）初始损伤较小时,岩样经历弹性变形到塑性变形的转化,岩石的T2图谱中小孔隙谱峰先向左移,后向右移,大孔隙谱峰不断向右扩展,反映了卸荷初期产生新的小孔隙,卸荷后期主要是大、小裂隙的扩展、贯通。当初始损伤较大时,T2图谱只向右扩展,反映岩样以扩展大孔隙为主,岩样只经历塑性变形;（4）初始损伤越大,相同卸荷围压比的核磁共振图像白斑亮度和面积越大,岩样的孔隙度就越大。     

Analyzing the effect of moving resonance on seismic response of structures using wavelet transforms

There is a complex interaction between the seismic response (i.e., peak displacements) of a nonlinear structure and the characteristics of a ground motion. One ground motion characteristic that contributes to record‐to‐record variability is spectral nonstationarity, or the variation of signal's frequency content with time. When the predominant natural periods of a nonlinear structure elongate in such a way as to match with the predominant frequency content in the ground motion, a phenomenon called moving resonance occurs. The effect of moving resonance on the response of nonlinear structures is investigated. Continuous complex wavelet transforms are used to examine the spectral nonstationarity of ground motion acceleration histories and associated structural displacement histories to identify the occurrences of moving resonance. A three‐dimensional displacement response spectrum is used to determine which combinations of initial period and strength create the largest displacements and thus are candidate configurations for experiencing moving resonance. A method is then proposed for quantifying the effect of moving resonance on structural response. The method utilizes discrete wavelet transforms to decompose a ground motion into component signals with limited frequency band and examines the structural response due to each individual component. A discussion is provided as to how these tools can be used to identify ground motion characteristics that may be conducive to moving resonance. Copyright © 2013 John Wiley & Sons, Ltd.     

The Dynamics of Low-Perihelion Meteoroid Streams

The Canadian Meteor Orbit Radar (CMOR) has collected information on a number of weak meteor showers that have not been well characterized in the literature. A subsample of these showers (1) do not show a strong orbital resemblance to any known comets or asteroids, (2) have highly inclined orbits, (3) are at low perihelion distances ( AU) and (4) are at small semimajor axes (<2 AU). Though one might conclude that the absence of a parent object could be the result of its disruption, it is unclear how this relatively inaccessible (dynamically speaking) region of phase space might have been populated by parents in the first place. It will be shown that the Kozai secular resonance and/or Poynting–Robertson drag can modify meteor stream orbits rapidly (on time scales comparable to a precession cycle) and may be responsible for placing some of these streams into their current locations. These same effects are also argued to act on these streams so as to contribute to the high-ecliptic latitude north and south toroidal sporadic meteor sources. There remain some differences between the simple model results presented here and observations, but there may be no need to invoke a substantial population of high-inclination parents for the observed high-inclination meteoroid streams with small perihelion distances.     

Periodic Orbits in Rotating Second Degree and Order Gravity Fields

Periodic orbits in an arbitrary 2nd degree and order uniformly rotating gravity field are studied. We investigate the four equilibrium points in this gravity field. We see that close relation exists between the stability of these equilibria and the existence and stability of their nearby periodic orbits. We check the periodic orbits with non-zero periods. In our searching procedure for these periodic orbits, we remove the two unity eigenvalues from the state transition matrix to find a robust, non-singular linear map to solve for the periodic orbits. The algorithm converges well, especially for stable periodic orbits. Using the searching procedure, which is relatively automatic, we find five basic families of periodic orbits in the rotating second degree and order gravity field for planar motion, and discuss their existence and stability at different central body rotation rates.     

胼胝体非出血性损伤的CT/MRI和临床表现

目的提高对胼胝体非出血性损伤的影像诊断水平和临床表现认识.材料与方法回顾性分析8例胼胝体非出血性损伤的临床和MRI表现.全部病例均进行了CT和MRI非增强检查,检查时间外伤后半小时至3天不等.结果所有病例的胼胝体损伤灶在CT图像上均未见明显密度异常.MRI上表现为T1WI低或稍低信号,T2WI上高信号,FLAIR序列上呈明显高信号.病灶位于胼胝体膝部4例,胼胝体压部2例,胼胝体嘴部1例,胼胝体广泛性损伤1例.8例中都合并有1处以上的胼胝体外其他部位的颅内损伤,包括多发性脑挫伤、脑内血肿、硬膜下或硬膜外血肿、蛛网膜下腔出血等.2例有MRI复查资料.结论 MRI是胼胝体非出血性损伤最佳的影像学检查手段,明显优于CT,不仅对较小和轻微的挫伤敏感,而且能多方位地显示病变,为临床制定治疗方案提供依据.