Volume 45, Number 56, 2000
General Physics  Relativity
The Thermodynamic Equilibrium of (2+1) Dimensional Black Holes and Radiation C. Wolf Romanian Journal of Physics 45,
383388 (2000)
By considering the equilibrium of (2+1) dimensional BTZ black holes and radiation we demonstrate that if the angular momentum is zero radiation will preferentially generate a multiblack hole configuration at low temperatures thus giving rise to a foam like structure of space time. When the angular momentum of a black hole is nonzero conservation of angular momentum prevents the transformation of radiation into black holes.
General Physics
Asymptotic Expansions for Relativistic Celestial Mechanics M. Arminjon Romanian Journal of Physics 45,
389414 (2000)
The method of asymptotic expansions is used to build an approximation scheme relevant to celestial mechanics in relativistic theories of gravitation. A scalar theory is considered, both as a simple example and for its own sake. This theory is summarized, then the relevant boundary problem is seen to be the full initialvalue problem. It is shown that, with any given system of gravitating bodies, one may associate a oneparameter family of similar systems, the parameter measuring the gravitational fieldstrength. After a speciﬁc change of units, the derivation of asymptotic expansions becomes straightforward. Two hypotheses could be made as to which time variable has to be used in the expansion. The first one leads to an "asymptotic" postNewtonian approximation (PNA) with instantaneous propagation, differing from the standard PNA by the fact that in the asymptotic PNA all fields are expanded. The second hypothesis could lead to an “asymptotic" postMinkowskian approximation (PMA) allowing to describe propagation effects, but it is not compatible with the Newtonian limit. It is shown that the standard PNA is not compatible with the application of the usual method of asymptotic expansions as envisaged here.
General Physics  Theory
PP Gravitational Waves with Cylindrical Symmetries Gh. Maftei, Iordana Aştefănoaei Romanian Journal of Physics 45,
415422 (2000)
The metric given by (1) presents the special (cylindrical and planar) symmetries. In this cosmological Universe the gravitational field propagates as pp gravitational waves. Using the NewmanPenrose formalism, we calculate in the nulltetradic base $\{e_a\} = \{m, \bar{m}, l, k\}$, the spin coefﬁcients, the directional derivates, the tetradic components of Ricci and Weyl tensor and we verify the commutation relations. From the Einstein field equations we obtain the solutions for $a(R,T)$, $b(R,T)$ and $c(R,T)$ functions. When the functions $a(R,T)$ and $b(R,T)$ describe the pp cylindrical gravitational waves the function $\phi^2$ deﬁnes the gravitational potential. The nonzero component of Weyl tensor is $\Psi_4 \ne 0$. The metric has the $N$ Petrov algebrical type. The gravitational field is radiative and propagates as transversal gravitational wave in $k$ direction. For $b(R,T) = 0$, the spacetime defined by the metric (1) has a planar symmetry and it is conformplat [3]. In this case $a(R,T)$ solution describes the evolution of a electromagnetic plane wave in the conformplat spacetime.
\begin{equation} \mathrm{d}s^2 = a^2 \cdot (e^{2b} \cdot \mathrm{d}\zeta^2 + e^{2b}\cdot \mathrm{d}\eta^2) + e^{2c}\cdot\mathrm{d}R^2  e^{2d}\cdot\mathrm{d}T^2 \qquad (1) \end{equation}
[3] H. Kramer, H. Stephani, Exact solutions of Einstein's Field Equations (Cambridge University Press, 1980).
General Physics  Mechanics
HamiltonJacobi Formalism for Constrained Systems Ovidiu Ţîntăreanu, Cătălin Florian Popa Romanian Journal of Physics 45,
423426 (2000)
The Hamiltonian treatment of constrained systems in Güler's formalism leads us to the total differential equations in many variables. These equations are integrable if the corresponding system of partial differential equations is a Jacobi system. The main aim of this paper is to investigate the systems with constraints using HamiltonianJacobi formalism. This approach is applied for a twodimensional system with constraints and the results are in agreement with those obtained by Dirac's canonical method.
Mathematical Physics
The Evolution of Time Periodic Hamiltonian Systems in the Floquet Space Radu Paul Lungu Romanian Journal of Physics 45,
427449 (2000)
A general formalism that describes the solution of a time periodic Hamiltonian quantum system is presented, using the Floquet theorem. Introducing an auxiliary parameter analogous to the time and using the extended Hilbert space in which the time is an additional coordinate, the quantum system evolution is simulated by a method which is similar to that usually employed in the standard quantum mechanics. The obtained general results allow to develop a perturbation method, founded on the switching on the perturbation adiabatically, which is similar to the one that is used in the theory of conservative manyparticle systems.
$Sp(3)$ Symmetry for Abelian Gauge ﬁelds C. Bizdadea, R. Constantinescu, S. O. Saliu Romanian Journal of Physics 45,
451457 (2000)
The $Sp(3)$ symmetry for abelian gauge fields is derived by triplicating the original constraints and by approaching the resulting system according to the standard Hamiltonian BRST formalism. The relation with the gaugefixing procedure in the $Sp(3)$ formalism is also established.
Nuclear Physics
PressurizedGas Spherical Ionization Chamber M. R. Călin Romanian Journal of Physics 45,
459467 (2000)
A pressurized ionization chamber detector able to measure radioactive sources in internal $2\pi$ or $4\pi$ geometry was built in order to characterize alpha and beta radioactive sources, i.e., to calibrate these sources by a relative method, and to test the behavior of gas mixtures in pressurizedgas radiation detectors. The detector we made is of spherical shape and works by collecting in a uniform electric field the ionization charges resulting from the interaction of ionizing radiation with gas in the sensitive volume of the chamber. An ionizing current proportional to the activity of the radioactive source to be measured is obtained. Pressure inside the detector in standard conditions is maximum $6\times 1.013\cdot10^5$ Pa and the maximum allowed voltage is 3000 V.
Atomic Physics
Na$^+$ Neutralization in Low Energy Scattering from Ni/(111)w Surface I. Inţa, M. Assunção, O. Teodoro, M. Volmer, A.M.C. Moutinho Romanian Journal of Physics 45,
469478 (2000)
By the neutralization of Na^{+} scattering from Ni/(111)W we have demonstrated that the assumption of loss memory in the incoming trajectory is right. From these measurements we have determined the work extraction of electron from Ni surface.
The Application of the Ionic Nitriding Process on Steels for Tools and Cold Deformation Devices Manufacturing Aurel Arabagiu, Gh. Vlaicu, Cristina Măcriş, Anca Arabagiu Romanian Journal of Physics 45,
479486 (2000)
This work is a study, on experimental basis, concerning the way of application of the ion nitriding process for tools used for cold deformation of metals, with special referring on 12% Cr steels. It takes VMoC 120 steel like representative steel grade (that means an alloyed steel with V = 0.3%, Mo = 0.5%, Cr = 12% and C = 1.6%). The technological parameters used in the secondary heat treatment applied on these kinds of steels are presented in the ﬁrst part of the work. The experimental studies are made to find the best technological parameters of the ionic nitriding process for VMoC 120 steel and are presented in the second part of the paper. The fatigue strength tests are made for durability testing of the layers obtained. The experimental results and the tests made with fatigue strength tests show that the ionic nitriding application on these tools, in the best conditions experimentally stated set out the obtaining of superficial layers conﬁgurations that increase with almost twice the life time in their exploitation.
Condensed Matter
Van Hove Singularity and $T_c$  Peak Effect in Irradiated YBa$_2$Cu$_3$O$_{7X}$ V. Sandu Romanian Journal of Physics 45,
487491 (2000)
The increase of the critical temperature in neutron irradiated YBa_{3}Cu_{3}O_{7x} at ﬂuences below 10^{17} cm^{2} is depicted in the framework of the Van Hove singularity model for superconductivity. In the kinetics of defect generation we distinguish a regime where the irradiation stimulated defect recombination prevails over defect generation. The most sensitive to radiation effects is the chain oxygen that is also responsible for the charge carrier concentration in the CuO_{2} planes. So, depending on the oxygen defects concentration, the position of Fermi energy shifts relative to the Van Hove energy, increasing or decreasing the critical temperature. The experimental data fit rather well with this model.
Studies of TwoPowders Mixtures in Bi(Pb)Sr(Ba)CaCuOxide/Nitrate System Obtained by Freeze Drying Used for Synthesis of 2223 Superconducting Ceramic G. Aldică, P. Bădică Romanian Journal of Physics 45,
493500 (2000)
Nitrate freeze dried raw powders Bi:Pb:Sr:Ba:Ca:Cu=1.8:0.4:1.8:0.2:2.2:3.0 ("2223nitrate"). Bi:Pb:Sr:Ba:Ca:Cu = 1.8:0.4:1.8:0.2:1.2:2.0 ("2212nitrate") and Ca:Cu=1:1 ("11nitrate") have been decomposed at 840°C for 30 min. to obtain "2223oxide", "2212oxide" and "11oxide" powders. From the above powders the following mixtures (mechanically mixed) were prepared: "2212oxide" + "11nitrate", "2212nitrate" + "11oxide" and "2212nitrate" + "11nitrate" and thermally decomposed in the same conditions. The decomposed powders were investigated by XRD analysis. From the indicated decomposed mixtures, "2223oxide" powder and mixture "2212oxide + 11oxide" superconducting pellets with composition 2223 were sintered. Electrical resistance $R(T)$ measurements on pellets have been performed. The obtained data have shown different phase compositions in the decomposed samples indicating different phase formation reactions. Because of this fact superconducting parameters in sintered pellets are changing. The optimum precursor powder considering $R(T)$ results for the synthesis of 2223 phase is the decomposed sample "2212nitrate + 11nitrate", although the best level of mixing is in the "2223oxide" powder. The most unfavorable mixture for 2212 and 2223phase formation is the decomposed sample "2212nitrate + 11oxide" due to the lack of (Sr,Ca)CuO_{2} phase.
Optics
Zero Order Filtering of the Gravity Map in a Fourier Optical System Constantin Dănciulescu Romanian Journal of Physics 45,
501507 (2000)
The processing of data by zero order optical filtering in monochromatic and coherent light is relied on the possibility to implement a spatial filtering operator similar to those deﬁned in the known optical data processing. By zero order optical filtering is defined a new linear optical operation in order to improve the signal/noise ratio, to emphasize the signal characteristics and to extract and recognize some speciﬁc features of the signal.
Applications of the Optical Filtering in the Frequency Domain to the Wave Table Processing Constantin Dănciulescu Romanian Journal of Physics 45,
509515 (2000)
The processing of data by zero order optical filtering in monochromatic and coherent light relies on the possibility to implement a spatial filtering operator similar to those defined in the known optical data processing. By zero order optical filtering is defined a new linear optical operation in order to improve the signal/noise ratio, to emphasize the signal characteristics and to extract and recognize some specific features of the signal.
