Constraint Molecular Dynamics and results on isotopic distributions produced in heavy ion collision — The Sn + Ni systems

The Constraint Molecular Dynamics approach is illustrated together with calculation on different heavy ion collisions. In particular the charge-mass distribution produced in the collision ¹²⁴Sn + ⁶⁴Ni and ¹¹²Sn + ⁵⁸Ni at 35 MeV/nucleon is discussed. By comparing the Y = (N ? Z)/A distributions for fragments produced in central collisions it results that this observable can be sensitive to the dynamic effects induced on the two systems by the different charge/mass ratio.     

Reception of a Signal with Unknown Duration

The maximum likely and optimal (Bayesian) algorithms for detecting an arbitrary-shaped signal observed against the background of Gaussian white noise and for measuring the duration are synthesized. Exact expressions for the characteristics of the maximum likely algorithms are found. The characteristics of the Bayesian algorithms are obtained using computer simulations.     

Spontaneous formation of a system of highly ordered germanium nanoislands upon epitaxial deposition on profiled (111) silicon substrates under electromigration conditions

Atomic-force microscopy was used to study the surface topography of SiGe structures grown by epitaxial deposition of Ge on profiled Si(111) substrates under electromigration conditions. Systems of highly ordered germanium nanosized islands with dimensions of 10–20 nm and a density of 6×10¹⁰ cm^?2 were obtained. It is shown that the geometrical parameters of self-organizing nanoislands can be controlled by a proper choice of the growth and postgrowth annealing conditions for these structures.     

Localized bifurcations and defect instabilities in the convection of a nematic liquid crystal

The stationary and the time-dependent homogeneous ordered states in convection may both become unstable against localized perturbations. Defects are then created and they may contribute to the disorganization of the homogeneous state. We present an experimental study of defects in some homogeneous stationary structures as well as in the traveling-wave states of convection of a nematic liquid crystal. We show that the core of the defects is a germ of the unstable state and it can become unstable under the external stress. Then, either fully homogeneous states with the symmetry of the core, or complex disordered states can develop from the local instability of defects in processes quite similar to displacive transitions in solids. Some of the main features are qualitatively similar to numerical simulations of an appropriate Landau-Ginzburg equation.     

The design of the main accelerator for a pulsed positron beam

The pulsed positron beam at the Helsinki University of Technology is designed for the end energy of 3-30 keV and grounded target. This is achieved with a constant voltage acceleration followed by an adjustable deceleration. In the design of this accelerator-decelerator the possibility for electrical breakdowns and partial discharges must be eliminated.For designing the electrode and insulator structures for accelerator-decelerator configuration electric field simulations were carried out with the finite element analysis program. In this paper we present the design of the accelerator-decelerator and the results of the electric field simulations. The results of high voltage tests will also be presented and compared with the simulations.     

Fermi surface and thermoelectric power of (Bi1−x Sbx)2Te3<Ag, Sn> mixed crystals

The Fermi surface anisotropy of (Bi_1?x Sb_x)₂Te₃ single crystals (0.25 ≤ x ≤ 1) was studied by analyzing the angular dependence of the frequency of Shubnikov-de Haas oscillations and the effect of tin and silver doping on the thermoelectric power in these crystals in the temperature range 77 ≤ T ≤ 300 K. It was shown that silver doping of (Bi_1?x Sb_x)₂Te₃ mixed crystals produces acceptors, while silver in Bi₂Te₃ acts as a donor. Tin also exhibits acceptor properties. Both tin and silver doping of p-(Bi_1?x Sb_x)₂Te₃ mixed crystals decrease the thermoelectric power due to an increase in the hole concentration.     

Prognostication of aging of polymeric composite materials

Experimental data demonstrate that a study of accelerated aging can enable prognostication of the working capacity and storage life of polymeric formulations used in aircraft components on the basis of changes in thermogravimetric constants of these polymeric formulations.     

Novel methods and universal software for HRXRD, XRR and GISAXS data interpretation

Several novel methods for evaluation and interpretation of X-ray data from modern nanostructures are presented along with their applications. The background of methods and their relations to fundamental problems of X-ray analysis is shortly described. The key features of LEPTOS software, which is designed for the analysis of X-ray data measured with various geometries and setups and implements all discussed techniques, are discussed.     

A Symmetric-Impulse Approximation Applied to the Faddeev Theory on the Charge Transfer Amplitude

Off-the-shell anomalous factors of the two-body Coulomb transition matrices appear in the integral form of the Faddeev second-order nuclear-electronic amplitude, for proton-hydrogen charge transfer scattering in a typical nlm → n′l′m′ transition. A symmetric-impulse approximation (SIA) is applied to eliminate these factors and an induction method is proposed to analytically calculate the remaining integrals. The nuclear-electronic amplitude is derived for the general case, and for totally symmetric collisions, in terms of generalized hypergeometric functions of two variables, F ₄, and of one variable, ₃ F ₂, respectively. The angular distribution of the second-order nuclear-electronic charge transfer amplitude shows the Thomas mechanism as a peak or a hump for symmetric and asymmetric collisions. There also exists a peak in the forward angular distribution of the second-order nuclear-electronic amplitude, which partly cancels the kinematic peak in the angular distribution of the charge transfer differential cross sections.     

Effect of hardness and composition gradients on Barkhausen emission in case hardened steel

The effect of gradients in hardness, structure and composition in the surface layers on magnetic Barkhausen noise (MBN) was investigated in two widely used low alloy steels. One material was case hardened by induction hardening. The other was case carburised. Similar hardness profiles were produced by the two treatments. It was found that the variation in structure and hardness through the case in the induction-hardened steel had a minor effect on the MBN profile. In contrast, the inhomogeneity in the case-carburised material showed up clearly. This took the form of a broadened or two-peak MBN profile. When the surface layer containing the case was removed by etching, the broadened profile was replaced by a profile with a single peak. It is concluded that the shape and position of the MBN profile is significantly affected when a gradient in microstructure is induced by a gradient in carbon content. On the other hand, a gradient in microstructure induced by heat treatment with a constant carbon level has much less affect on the MBN profile for the induction-hardened steel.