Systematic studies of fission fragment kinetic energy distributions by Langevin simulations

Fission fluctuation-dissipation dynamics of heavy nuclei has been studied using Langevin Monte Carlo simulations. The covariant form of the fission transport equation and the coefficients related to it are investigated. To learn about the influence of the dynamics from the ground state to the saddle point on the kinetic energy distributions we have studied various systems and compared the calculations both starting from the ground state and from the saddle point. Both the mean total kinetic energy of the fission fragments and its variances can fit with the experimental values in terms of a finite neck radius as scission condition.This work was supported by the National Natural Science Foundation of China.     

Low-energy fission investigated in reactions of 750 AMeV 238U-ions on 208Pb. II: Isotopic distributions

Projectile fission of 750 AMeV ²³⁸U-ions interacting with a Pb target was studied by means of the spectrometer FRS, GSI-Darmstadt. One of the two fission fragments was detected with a transmission of few percent and identified in mass and charge. Low-energy fission (E^* < 25 MeV) events were selected by their magnetic rigidity. Whereas the production of asymmetric fission events is dominated by the GDR excitation, very asymmetric fission and symmetric fission take place after a GQR or DGDR excitation or after a nuclear interaction. Cross sections of more than 250 isotopes were measured. Isotopic distributions of low-energy fission were reconstructed for elements from Se to Te. The fission modes SI, SII and SL were clearly shown in these distributions and in the mass and TKE distributions. Charge polarization and mass dispersion were deduced for each fission mode. Finally, the characteristics of the low-energy fission process explain the production rates of neutron-rich species. Received: 14 July 1997 / Revised version: 6 October 1997     

In-situ observation of MOVPE epitaxial growth

Metal organic vapour phase epitaxy (MOVPE) is nowadays one of the leading techniques for epitaxial growth. While the processes in the gas phase of MOVPE are reasonably well understood, the processes on the growing surface are not. This situation is in contrast to molecular beam epitaxy (MBE), where considerable knowledge about growth processes on the surface could be gained. The main reason is that all the UHV-based classical surface-science tools (using electrons and ions), especially reflection high-energy electron diffraction, can be applied in the vacuum-based MBE but not under the gas-phase conditions of MOVPE. This situation has changed in the last decade since optical surface-science tools have been developed. Especially, with the linear optical techniques like reflectance anisotropy spectroscopy and spectroscopic ellipsometry, there is now a quasi-standard tool at hand which allows for the study of all kinds of pregrowth and growth situations in MOVPE (in MBE of course, as well). These optical methods give, moreover, chemical information also. In this article we will describe shortly the features of these optical techniques and then concentrate on III-V-semiconductor growth. The spectral definition of surface reconstructions and time-resolved studies of phase transitions between them (adsorption/desorption kinetics of group-III and group-V elements) are discussed next. Under growth, the surfaces can be classified and defined according to their optical surface response into a pressure versus temperature phase diagram. The regions of such a phase diagram correspond to different geometrical and chemical surface structures and consequently lead to different growth modes. Finally, as an example of modern nanogrowth, monitoring of the growth of quantum-dot structures is presented. Received: 2 August 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

乳液体系中影响分散相粒子大小及分布的因素——cmcO及cmcW的协同作用

用马尔文MastersizerMicroplus型激光粒度仪对液体石蜡及环氧树脂E-20与甲苯等有机溶剂以不同比例所形成的体系经乳化后的粒子大小及分布进行研究,结果表明,随着油溶性表面活性剂在油相中溶解性的改善,分散相的粒子大小及分布也明显得到改善.cmcO及cmcW的协同作用对获得理想的分散相粒子大小及分布起到了至关重要的作用     

Fabrication and Properties of Double-Side Tl2Ba2CaCu2O8 Thin Film on CeO2 Buffered Sapphire Substrate

The double-side Tl2Ba2 CaCu2O8 （Tl-2212） superconducting thin films were fabricated on CeO2 buffered sapphire substrates. The reactive magnetron sputtering technique was used to grow CeO2 buffer thin films on sapphire substrates. Making use of the metal cerium as a sputtering source, the depositing rate is much higher compared with the CeO2 target. The Ti-2212 thin films on CeO2 buffered sapphire substrates were fabricated by adc magnetron sputtering and post-annealing process. The x-ray diffraction indicates that the thin film is pure Tl-2212 phase with the e-axis perpendicular to the substrate surfaces, and epitaxially grown on the CeO2 buffered sapphire. The critical transition temperature Tc is around 106K, the critical current density Jc is around 3.5 MA/cm^2 at 77K, and the microwave surface resistance R8 at 77K and 10 CHz of the film is as low as 390μ Ω.     

Application of optical and luminescent techniques to the characterization of oxide thin films

The interaction between light and electrons in oxide compounds forms the basis for many interesting and practical effects, which are related to microstructure, energy band, traps, carrier transport and others. Thin films of oxides like WO₃, Ga₂O₃, Y₂O₃ and SrTiO₃ were investigated using various improved optical and luminescent techniques. The home-made systems for optical and luminescent measurements were described in detail. The facilities of photo-Hall and photoconductivity transients have been proven to be powerful tools in the studies, which allow us to perform photoinduced process and relaxation measurements over a wide time range from 10⁻⁸ to 10⁴ s. Furthermore, we extended the measurement capabilities of the commercial luminoscope by using an interferometer system with optical fiber and illuminance meter instead of an optical microscope. The cathodoluminescent measurements can be performed at a relative high pressure (20-60 mTorr) compared to ultra-high-vacuum condition of most commercial products. Luminescent characterization was employed as a probe to study doping ions, oxygen vacancies, trap and/or exciton levels in oxide thin films. Our results suggest that various traps and/or excitons in thin films of WO₃, Ga₂O₃ and SrTiO₃ involve in the process of photoconductivity relaxation and emission.     

Self-cleaning characteristics on a thin-film surface with nanotube arrays of anodic titanium oxide

Self-cleaning of a surface of nanotube arrays of anodic titanium oxide (ATO) is demonstrated. The ATO was prepared in fluoride ion containing sulfate electrolytes with a structure of 0.4 μm length, 100 nm pores diameter, 120 nm interpore distance, 25 nm pore wall thickness, a 8×10⁹ pores cm⁻² pore density, and 68.2% porosity. Prepared as thin films either directly from a Ti foil or on a glass substrate, these arrays have the property that water drops spread quickly over the surface of the films without irradiation. In contrast, a flat anatase TiO₂ film requires irradiation with UV light for several minutes before the contact angle decreases to zero. The observed self-cleaning behavior of the ATO thin films is due to the capillary effect of the nanochannel structure and the superhydrophilic property of the anatase TiO₂ surface inside the tube.     

Determination of avrami parameters of hydroxyapatite and β-wollastonite in bioactive phosphor—Silica glasses

The properties of biologically active glasses in the system SiO₂?P₂O₅?MgO?CaO were studied. Crystalline hydroxyapatite (HA) and β-wollastonite (β-W) were used after heat treatment (1100°C). The influence of the glass particle size (0.071–2.5 mm) and of glass powder (d ₅₀=15.1 μm) on the behaviour of the products during differential thermal analysis was followed. These analyses indicated that the β-W probably originates from surface nucleation, and HA from bulk nucleation. The differentiation was confirmed by calculation of the Avrami parameters (n) with the Pi-loyan-Borchardt analytical method. For HA and β-W, the calculated values ofn were 2.96 and 1.91. The surface-nucleated glasses exhibited predominant bidimensional crystal growth.     

Existence of different intermediate Hamiltonians in type A -fold supersymmetry

Type A -fold supercharge admits a one-parameter family of factorizations into product of first-order linear differential operators due to an underlying symmetry. As a consequence, a type A -fold supersymmetric system can have different intermediate Hamiltonians corresponding to different factorizations. We derive the necessary and sufficient conditions for the latter system to possess intermediate Hamiltonians for the case. We then show that whenever it has (at least) one intermediate Hamiltonian, it can admit second-order parasupersymmetry and a generalized 2-fold superalgebra. As an illustration, we construct a set of generalized Pöschl–Teller potentials of this kind.     

Size dependence of non-linear optical properties of SiO2 thin films containing InP nanocrystals

SiO₂ composite thin films containing InP nanocrystals were fabricated by radio-frequency magnetron co-sputtering technique. The microstructure of the composite thin films was characterized by X-ray diffraction and Raman spectrum. The optical absorption band edges exhibit marked blueshift with respect to bulk InP due to strong quantum confinement effect. Non-linear optical absorption and non-linear optical refraction were studied by a Z-scan technique using a single Gaussian beam of a He-Ne laser (632.8 nm). We observed the saturation absorption and two-photon absorption in the composite films. An enhanced third-order non-linear optical absorption coefficient and non-linear optical refractive index were achieved in the composite films. The nonlinear optical properties of the films display the dependence on InP nanocrystals size. Received: 27 June 2000 / Accepted: 27 June 2000 / Published online: 13 September 2000