窄缝通道中液氮的沸腾传热研究:第二部分:窄缝通道尺寸对液氮临界热流密度(CHF)的影响

在实验的基础上对窄缝通道中液氮的临界热流密度进行了实验研究.实验针对3个不同长度和间距尺寸的窄缝通道在多方位倾角的情况下进行.系统研究了窄缝的方位倾角和窄缝的几何尺寸对临界热流密度的影响,同时对液氮的膜态沸腾进行了研究.     

窄缝通道中液氮的沸腾传热研究第一部分:窄缝通道尺寸对传热特性的影响

窄缝通道中的液氮沸腾传热特性有别于开放空间中的情况.文中对常压下液氮在矩形窄缝通道内的沸腾传热进行了实验研究,系统地研究了窄缝几何结构参数和倾角对液氮的沸腾传热特性的影响.     

Investigation of the anomaly in $\eta$-photoproduction off the neutron

Quasi-free photoproduction of wmesons off the neutron and off the proton has been studied using a deuterium target and bremsstrahlung photons produced by MAMI-C with incident energies up to 1.5 GeV. The wmesons were detected in coincidence with the recoil nucleons thus a fully exclusive measurement was performed. Preliminary results show a bump-like structure in the excitation function for the neutron close to W ≈ 1675 MeV which is not seen for the proton. Considering the experimental resolution and using a Breit-Wigner fit the width of this structure was approximated below 50 MeV.     

Fine structure in the proton magnetic resonance spectrum of solid benzene and solid benzene-cyclohexane mixtures

The PMR spectrum of solid benzene in the vicinity of the melting point consists of broad and narrow lines. New narrow lines in the spectrum of benzene containing some impurity appear at the top of the broad line. Intensities of narrow lines increase with rising of temperature. The temperature dependence of line width and relaxation timesT ₁ andT ₂ have been measured in pure solid benzene an benzene-cyclohexane mixtures. It is assumed that narrow lines in spectra of pure benzene and its mixtures are caused by appearance of a phase with mobile molecules located in the crystallite joint or on its surface. The spectrum of imperfect monocrystalline benzene have been studied also. The narrow line of this sample is split into several narrow components. This is explained by demagnetization fields the distribution of which has a discrete character,     

On the conventional QED interpretation of the GSI ee narrow resonances

A possible mechanism for the GSI e⁺e^- narrow resonances as being due to non-perturbative effects of conventional QED is proposed. An application of the quasi-potential approach discloses a set of new resonances in a system of two charged particles. Numerical calculations agree with data on e⁺e^- and pp narrow resonances. Additional new resonances are predicted for e⁺e^-, pp, e^-e^- systems.     

On the use of divergent wall images in the fresnel mode of Lorentz microscopy for the measurement of the widths of very narrow domain walls

In the Fresnel mode of Lorentz microscopy, domain wall widths are often estimated using the linear extrapolation method in which the widths of divergent domain wall images recorded with different defocus distances are extrapolated to zero defocus. This paper examines the validity of the technique for very narrow walls by analysing images synthesized on a computer. The results show that the method is only of limited reliability for the estimation of very narrow wall widths. Methods of improving the accuracy are suggested.     

狭长CsI(Tl)闪烁体发光效率的研究

提出了基于狭长CsI（Tl）闪烁体和面阵CCD器件,采用光纤和光纤面板进行光耦合及传输,以扇形束线阵扫描方式实现对X射线探测与成像的工业X-CT系统探测器方案.基此,通过物理分析及数学建模,利用Matlab模拟研究了X光能量小于450 keV时狭长CsI（Tl）闪烁体的发光效率等性能指标.研究结果表明：当光电吸收截面μph和康普顿吸收截面μc分别为0.000313和0.0000295、反射层反射系数R和衬底反射系数Rs分别取0.95和0.8、荧光线性吸收系数σ取0.000222 μm－1时,得到狭长CsI(Tl)闪烁晶体的长度l、高度h和宽度w取值范围分别是926～4512 μm、242～5000 μm和242～5000 μm的结论.在此范围内,既可使闪烁晶体有较好的空间分辨率又可获得最高的发光效率.     

Effect of field of view on the accuracy of camera calibration

A long focal length lens can improve the spatial resolution and capture more detailed information, so it has been considered for three-dimensional (3D) vision reconstruction. However, the field of view (FOV) will narrow with a long focal length lens. It is a general concept that it is extremely difficult to achieve high-accuracy calibration of a narrow FOV camera because of paraxial imaging. In this paper an in-depth study on this issue is conducted from the aspect of perspective deformation. First, the perspective deformation of a point is divided into three parts. Then, the noise immunity of each part under different FOVs is discussed to reveal the root cause of the difficulty in the calibration of narrow FOV cameras. It is found that the calibration accuracy could be generally maintained at the same level if the image noise is inversely proportional to the focal length. Simulations verify the correctness of the inferences of this study that are hoped to be helpful in overcoming this disadvantage of narrow FOV cameras.     

A search for a narrow radial excitation of the $\mathrm{D}^{*\pm}$ Meson

A sample of 3.73 million hadronic Z decays, recorded with the OPAL detector at LEP in the years 1991–95, has been used to search for a narrow resonance corresponding to the decay of the meson into . The mesons are reconstructed in the decay channel with . No evidence for a narrow resonance is found. A limit on the production of narrow in hadronic Z decays is derived: Received: 16 January 2001 / Published online: 28 May 2001     

Breakdown of the atomic picture for the interband dipole matrix element and charge oscillation under interband optical excitation

Numerical results for a one dimensional model are used to illustrate the nature of charge oscillation under interband optical excitation in a quantum well with a narrow band gap. The charge oscillation shows molecular rather than atomic character ie charge is transported over the entire quantum well and not just over atomic distances. The associated dipole moment, which corresponds to the interband dipole matrix element, for narrow gap semiconductor quantum well structures can be in the order of nanometres rather than Ångstroms.     

Simultaneous narrow linewidth and broadband operation in the master oscillator power amplifier of a self-seeded Ti:sapphire laser

A narrow linewidth pulse and a broadband pulse were simultaneously operated in the master oscillator power amplifier (MOPA) of a self-seeded Ti:sapphire laser, and their intensity ratio was varied by increasing the incident pump energy. The broadband pulse from the independent free-running oscillation was preferentially amplified in the MOPA chain and replaced the narrow linewidth pulse. This explanation was confirmed by comparing the behavior with that of a bandpass filter inserted at the exit of the oscillator. The behaviors were well simulated by numerical calculations. From the results, a method to produce both narrow linewidth and broadband pulses was proposed with their ratio depending on the pump energy.     

Results of the experimental study of narrow continuum features in stimulated ionospheric emission spectra

Spectral and dynamical characteristics of the narrow continuum feature observed in the stimulated ionospheric emission spectra for small detunings from the pumping wave frequency are presented. It is found that the generation of the narrow continuum in the initial stage of interaction of a strong high-frequency o-mode pumping wave with an F-region ionospheric plasma is determined by the parametric decay instability whereas the radiation observed in the stationary stage of artificial turbulence results from the thermal parametric instability. Generation models of narrow continumm in different stages of its evolution are discussed. Radiophysical Research Institute, Nizhny Novgorod, Russia; Swedish Institute of Space Physics, Uppsala Division, Sweden. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 9, pp. 1091–1112, September, 1997.     

Pressure effect on the electron mobility in AlAs/GaAs quantum wells

By taking the influence of optical phonon modes into account, this paper adopts the dielectric continuum phonon model and force balance equation to investigate the electronic mobility parallel to the interfaces for AlAs/GaAs semiconductor quantum wells (QWs) under hydrostatic pressure. The scattering from confined phonon modes, interface phonon modes and half-space phonon modes are analysed and the dominant scattering mechanisms in wide and narrow QWs are presented. The temperature dependence of the electronic mobility is also studied in the temperature range of optical phonon scattering being available. It is shown that the electronic mobility reduces obviously as pressure increases from 0 to 4GPa, the confined longitudinal optical (LO) phonon modes play an important role in wide QWs, whereas the interface optical phonon modes are dominant in narrow QWs, the half-space LO phonon modes hardly influence the electronic mobility expect for very narrow QWs.     

Solid-state NMR studies of the crystalline and amorphous domains within PEO and PEO: LiTf systems

Solid polymer electrolytes (SPEs) contain amorphous and crystalline regions, each of which have unique contributions to the ¹³C NMR spectrum. Understanding and assigning the ¹³C NMR signals are vital to interpreting the NMR data collected for each phase. The ¹³C CPMAS solid-state NMR spectrum of poly(ethylene oxide), a common polymer electrolyte host material, has superimposed broad and narrow components. Previously, the narrow component has been assigned to the amorphous region and the broad component to the crystalline PEO fraction. These assignments for pure PEO have been applied to various PEO:salt systems. Using lithium triflate salt dissolved in PEO, we revisit the spectral assignments and discover that the narrow component is due to crystalline PEO:LiTf component, which is reversed from the previous pure PEO assignment. This paradigm shift is based on data collected from a 100% crystalline PEO:LiTf with a 3:1 oxygen:lithium ratio sample, which exhibited only the narrow peak. For dilute electrolytes, such as 20:1 PEO:LiTf, the ¹³C CPMAS spectra contain the narrow peak superimposed on a broad peak as seen with pure PEO. As dilute electrolytes are heterogeneous with crystalline and amorphous regions of both pure PEO and PEO:LiTf complex, peak assignments for pure PEO and PEO:LiTf are important. Thus, we reexamine the previous assignment for pure PEO using samples of pure powdered PEO, thermally treated pure powdered PEO, and a thin film PEO cast from an acetonitrile solution. With these different samples, we observed the growth of the narrow peak under conditions that favor crystallization. Therefore, for pure PEO, we have reassigned the narrow peak to the crystalline region and the broad peak to the amorphous region. In light of our observations, previous NMR studies of pure PEO and PEO SPEs should be reinvestigated. We also use rotational echo double resonance (REDOR) to study the 20:1 PEO:LiTf created from 2 and 100 kDa PEO. We find that the lithium environment is similar in the respective microcrystalline domains. However, the 100 kDa samples have a larger fraction of pure crystalline PEO.     

Doppler cooling to the quantum limit

Doppler cooling on a narrow transition is limited by the noise of single scattering events. It shows novel features, which are in sharp contrast with cooling on a broad transition, such as a non-gaussian momentum distribution, and divergence of its mean square value close to the resonance. We have observed those features using 1D cooling on an intercombination transition in strontium, and compared the measurements with theoretical predictions and Monte Carlo simulations. We also find that for very a narrow transition, cooling can be improved using a dipole trap, where the clock shift is canceled.     

Orbital dynamics of the 4f shell in DyB2C2

For the first time, fluctuations in the Dy quadrupole (orbital) moment have been observed in DyB(2)C(2) using inelastic neutron scattering. The observed quasielastic response is decomposed into two components, one reflecting transitions within the doublets (narrow) and the other transitions between the doublets (broad) of the effective Dy quartet ground state. The widths of the narrow and broad components are shown to arise from fluctuations in the magnetic dipole and the electric quadrupole moments, respectively.     

Remote sensing of high temperature H2O-CO2-CO mixture with a correlated k-distribution fictitious gas method and the single-mixture gas assumption

Infrared spectra of high temperature H₂O-CO₂-CO mixtures are calculated using narrow band models in order to simulate hot jet signature at long distance. The correlated k-distribution with fictitious gas (CKFG) approach generally gives accurate data in such situations (especially for long atmospheric paths) but results in long computation time in cases involving mixtures of gases. This time may be reduced if the mixture is treated as a single gas (single-mixture gas assumption, SMG). Thus the lines of the single-mixture gas are assigned to the fictitious gases. In this study, the accuracy of two narrow band models is evaluated. The first narrow band model considers one single-mixture gas and no fictitious gas (CK-SMG) whereas the second model accounts for one single-mixture gas and three fictitious gases (CKFG-SMG). Both narrow band models are compared with reference spectra calculated with a line-by-line (LBL) approach. As expected, the narrow band accuracy is improved by the fictitious gas (FG) assumption particularly when long atmospheric paths are involved. Concerning the SMG assumption, it may lead to an underestimation of about 10% depending on the variation of the gas mixture composition ratio. Nevertheless, in most of realistic situations the SMG assumption results in negligible errors and may be used for remote sensing of plume signature.     

Interaction of surface electromagnetic waves with an electron beam moving along the interface between metamaterials

Excitation of surface polaritons by an infinitely narrow electron beam propagating in the vacuum gap between a metal-like medium and an artificial dielectric with a negative permeability is studied theoretically. A dispersion relation is derived for the waves excited by the beam for an arbitrary thickness of the gap. The possibility of the emergence of absolute instability is demonstrated for an infinitely narrow vacuum gap, and the corresponding increments are calculated with allowance for small dissipative losses.     

Mapping of the roughness exponent for the fuse model for fracture

The roughness exponent for fracture surfaces in the fuse model has been thought to be universal for narrow threshold distributions and has been important in the numerical studies of fracture roughness. We show that the fuse model gives a disorder dependent roughness exponent for narrow disorders when the lattice is influencing the fracture growth. When the influence of the lattice disappears, the local roughness exponent approaches zeta(local)=0.65+/-0.03 for distribution with a tail toward small thresholds, but with large jumps in the profiles giving corrections to scaling on small scales. For very broad disorders the distribution of jumps becomes a Lévy distribution and the Lévy characteristics contribute to the local roughness exponent.     

半导体物理效应及其应用讲座第四讲光吸收跃迁效应与半导体红外探测器的应用发展

光吸收跃迁效应是半导体光电探测器的基本物理过程.文章主要介绍光吸收跃迁效应在窄禁带半导体红外探测器应用方面的研究进展.讨论窄禁带半导体带间光吸收跃迁的理论和实验.文章还介绍了本征光吸收系数的表达式及其在材料表征和确定器件截止波长方面的应用,以及它在解释近年来发现的HgCdTe光电二极管电致负荧光现象方面的应用.