使用α粒子轰击天然镱靶制备~(178m2)Hf同质异能素及样品γ能谱分析

178m2Hf是一种长半衰期(T1/2=31 a)、高自旋态(Iπ=16+)的Hf同质异能素.利用四川大学CS-30回旋加速器产生的30 MeV,100μA的α束流轰击天然Yb靶,通过176Yb(α,2n)反应制备178m2Hf.采用高纯Ge探测器测量未提纯样品和放化分离后的样品的γ能谱,选取178m2Hf的426,495和574 keV三个特征峰,使用高斯双峰拟合方法解谱,确定了未提纯样品中178m2Hf的含量达到1012量级,提纯后的样品中178m2Hf含量为1011量级.还对靶中其他放射性杂质核素(175Hf,172Lu,172Hf,177Lu,173Lu)的含量进行了分析.     

Fabrication and characterization of Ge20Sb15S65 chalcogenide glass for photonic crystal fibers

Chalcogenide glasses are known for their high transparency in the mid-infrared (IR) range, which includes two atmospheric windows that lie from 3 to 5 μm and 8 to 12 μm, respectively. Chalcogenide photonic crystal fibers have numerous potential applications in the field of IR, such as spectroscopy, microscopy, astronomy, biology, and sensing. In this paper, Ge₂₀Sb₁₅S₆₅ chalcogenide glass was fabricated and systematically studied. Chalcogenide glass has high transmission property (>70 %), good thermal stability, and good mechanical stability. The glass transition temperature T _g is 296 °C, and no exothermic peak was associated with crystallization up to 500 °C, which indicates its suitability for fiber drawing. As a result of its excellent mechanical properties, preforms with a variety of geometrical patterns were fabricated by using mechanical drilling. The near-field intensity distribution image of the drawn fiber shows a strong light propagation confinement.     

Random Logistic Maps. I

Let {C _i} ₀ be a sequence of independent and identically distributed random variables with vales in [0, 4]. Let {X _n} ₀ be a sequence of random variables with values in [0, 1] defined recursively by X _n+1=C _n+1 X _n(1–X _n). It is shown here that: (i) E ln C ₁<0X _n0 w.p.1. (ii) E ln C ₁=0X _n0 in probability (iii) E ln C ₁>0, E |ln(4–C ₁)| such that (0, 1)=1 and is invariant for {X _n}. (iv) If there exits an invariant probability measure such that {0}=0, then E ln C ₁>0 and – ln(1–x) (dx)=E ln C ₁. (v) E ln C ₁>0, E |ln(4–C ₁)|< and {X _n} is Harris irreducible implies that the probability distribution of X _n converges in the Cesaro sense to a unique probability distribution on (0, 1) for all X ₀0.     

Current-induced colossal electroresistance in La0.8Ba0.2MnO3 films fabricated by sol–gel method

Transport properties of La_0.8Ba_0.2MnO₃ thin films deposited by the sol–gel method were investigated. It has been found that resistivity plateaus occurred in the ρ–T curves after application of a dc bias current over a critical value. A current of 200 μA could induce a huge resistance variation ∼1200% in these La_0.8Ba_0.2MnO₃ films near room temperature, demonstrating a colossal electroresistance effect. Such strange transport behavior suggests the formation of conductive filaments and that the multi-phase coexistence is sensitive to external stimuli. This phenomenon may find applications in sensing and logic devices.     

Effect of SnI2 on the thermal and optical properties of Ge–Se–Te glasses

A systematic series of (Ge₂₀Se₁₅Te₆₅)_1?x–(SnI₂)_x (x = 0, 0.05, 0.1, 0.15) chalcogenide glasses have been prepared. The amorphous nature can be confirmed by XRD and SEM. With the SnI₂ content increasing, the indirect optical band gaps are decreased from 0.662 to 0.622 eV according to Tauc laws. The introduction of SnI₂ makes the glasses much easier to prepare and more stable against crystallization, making them drawable as optical fibers. The highest ΔT (130 °C) value for (Ge₂₀Se₁₅Te₆₅)_0.9–(SnI₂)_0.1 glass composition can be obtained. A slight red-shifting of the long-wavelength cutting-off edge from 18.4 to 19.4 μm was shown and it seems that SnI₂ in these glasses offers the improvement in the far-infrared properties.     

Hopf bifurcation in doubly fed induction generator under vector control

This paper first presents the Hopf bifurcation phenomena of a vector-controlled doubly fed induction generator (DFIG) which is a competitive choice in wind power industry. Using three-phase back-to-back pulse-width-modulated (PWM) converters, DFIG can keep stator frequency constant under variable rotor speed and provide independent control of active and reactive power output. Main results are illustrated by “exact” cycle-by-cycle simulations. The detailed mathematical model of the closed-loop system is derived and used to analyze the observed bifurcation phenomena. The loci of the Jacobian’s eigenvalues are computed and the analysis shows that the system loses stability via a Hopf bifurcation. Moreover, the boundaries of Hopf bifurcation are also given to facilitate the selection of practical parameters for guaranteeing stable operation.     

Influence of interface roughness on reflectivity of tungsten/boron-carbide multilayers with variable hi-layer number by X-ray reflection and diffuse scattering

from layer to layer during multilayer growing. The variation of the reflectivity and interface roughness with bi-layer number is accurately explained by the presented real-structure model.     

A New Gradient Method with an Optimal Stepsize Property

The gradient method for the symmetric positive definite linear system is as follows

Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles

CoFe₂O₄ ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe₂O₄. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co²⁺ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe₂O₄ samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state.