

Volume 47, Number 78, 2002


General Physics  Thermodynamics 

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Title & Author(s) 
701 
A Study of Thermodynamic Stability of Stress Relaxation Processes in Viscoelastic Fluids within the Framework of Comprehensive Thermodynamic Theory of Stability of Irreversible Processes (CITSIP)
Chandrakant S. Burande, Anil A. Bhalekar
In the present paper the comprehensive thermodynamic theory of stability of irreversible processes (CTTSIP) has been applied to the stress relaxation processes in viscoelastic fluids. CTTSIP is based on Lyapunov's direct method and Malkin's theorem of stability of motion in which the second law of thermodynamics helps in identifying the corresponding Lyapunov function. Accordingly, the stability of stress relaxation in viscoelastic fluids obeying Maxwell's Lower Convected and Upper Convected Models has been investigated. Taking the thermodynamic space of extended irreversible thermodynamics (EIT) the thermodynamic Lyapunov function, $L_S$, of CTTSIP has been worked out. This investigation establishes that the considered processes are asymptotically stable and are stable under constantly acting small disturbances.


Nuclear Physics 

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Title & Author(s) 
709 
Analysis of Quasiresonant Reactions in a Hybrid Angular Momentum Scheme (I)
M. S. Ata, H. Comişel, C. Haţegan, N. A. Sliakhov
A hybrid angular momentum coupling scheme for analysis of quasiresonant reactions interfering with stripping reactions on zerospin target nucleus is proposed. The method is based on the similarity between the transition amplitude elements derived in the total as well as in the transferred angular momentum schemes.


725 
Modelling a Neutron Rich Source Target Based on PhotoFission
O. Bajeat, M. Mirea
A survey of the radiative electron energy loss theory is reported in order to estimate numerically the bremsstrahlung production of thick targets. A neutron rich nuclei source based on photofission can be conceived using the bremsstrahlung radiation. A combination given by W converter associated with an uranium carbide target source is analyzed in order to optimize the characteristics of this neutron rich nuclei source.


Nuclear Physics  Instrumentation 

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Title & Author(s) 
741 
Radon Metrology with a Radiation Detector of Ionization Chamber Type in Impulse Mode
Marian Romeo Calin, Nicolae Valcov
Measuring atmospheric radon is a highly interesting area of environmental radiation protection. As is well known, radon penetrates the human organism along with atmospheric air through breathing. The element is emitted from the ground due to processes involved in chain nuclear disintegration and can also be produced by other sources such as building materials, which are part of the buildings, underground stations, coal pits, salt works, quarries, etc. In the circumstances, it is quite important that the volume activity of radon and its progenies be checked, which accounts for the broad diversity of methods that have been developed for this purpose. One of the most widespread such methods is based on measuring the average ionization current generated by radon circulation through an ionizationchamber detector. This paper presents the development of a lab device for monitoring radon concentration using a radiation detector of ionization chamber type (in 3 different variants). It is a pulsed mode device, in which the measurement of the average value of the ionization current is complemented with, or replaced by a recording of the ionization current pulses generated by alpha disintegration within the useful volume of the detector. The ionization chamber thus operates as a $4\pi$ counter, in which radon and its progenies are the alpha source. The paper provides a mathematical evaluation of the efficiency of the method and of the measuring device, based on a data acquisition system and a special calculation program, which make it possible to study the time and amplitude distributions of the pulses produced by alpha radiation in the ionization chamber. Early results have indicated the volume activity of radon and its descendents can be measured with a 5%  6% uncertainty margin.


Nuclear Physics  Applied 

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Title & Author(s) 
761 
Theory of in Situ GammaRay Spectrometry
G. Balea
The technique of in situ spectrometry had its origins during the time of atmospheric nuclear weapons testing where it was found to provide quick, reliable information on the components of the outdoor environment. It provided a means to separate natural background from manmade sources and gave quantitative results. Over the years, it has been employed by various groups for assessing radiation sources in the environment not only via ground based detectors, but with aircraft systems as well. It proved particularly useful following the Chernobyl accident and was employed by a number of European laboratories. It should prove adaptable to site assessments in the current era of environmental rescue.


Accelerator Physics 

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Title & Author(s) 
771 
Proposal of a Modified Betatron Accelerator
F. Scarlat, Fl. Scarlat
At present, there is the possibility of powering a new class of free electron lasers with intense cyclic electron beams generated by racetrack induction accelerators or modified betatrons. The most striking advantage of cyclic devices over their linear counterparts is their small size ranging between 1/10 and 1/100. In this paper, we present a proposal of modification of the 30 MeV NILPRP Betatron into a 280 MeV Modified Betatron with two points of operation. The first point is 50 MeV energy, including an azimuthal magnetic field for focusing, in order to be used for a free electron laser. The second operation point is 280 MeV energy, without azimuthal magnetic field, in order to be used for a laser synchrotron source. We give a short description of its basic components. The existing 30 MeV NILPRP Betatron building with other associated installations represents about 70% of the necessary investment.


Condensed Matter 

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Title & Author(s) 
777 
ParaFerroelectric Phase Transition in $^{57}$Fe Doped Ammonium Sulphate Investigated by Mössbauer Effect
Ş. Constantinescu, M. N. Grecu
The Mössbauer spectroscopy of the $^{57}$Fe center in the two chemically distinct ammonium sites in (NH$_4$)$_2$SO$_4$ was investigated in the temperature range 77 ÷ 300 K. Discontinuous changes in the values of the experimental parameters, integrated and normalized experimental area $a_{exp}$, central shift $\delta_C$, and quadrupole splitting $\Delta E_Q$ were observed at $T_{tr} = 229$ K. Numerical calculations of the electric field gradients at $^{57}$Fe isotope in the two ammonium sites, of the temperature dependencies of recoilless fraction $f$, and second order Doppler shift $\delta_{SODS}$ were performed by using specialized computing programs. The results obtained in both phases are consistent with a ferrielectric model in (NH$_4$)$_2$SO$_4$ and are discussed in terms of the existence of a vibration mode whose softening occurs due to local anharmonicity. The critical exponent $\gamma \simeq 1.23$ in paraelectric phase fits well with the early reported value by EPR.


Atmospheric Physics 

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Title & Author(s) 
795 
The Structure of Stationary Planetary Waves in Winter
C. Stan, S. Ştefan, M. Caian
The general features of the atmospheric planetaryscale Rossby waves, their propagation and their interaction with the mean zonal flow during wintertime are investigated, using a linear, timedependent, semispectral primitive equation model.
The basic equations for excited perturbation waves are linearized primitive equations in spherical coordinates with a logpressure system in the vertical.
A thermal mechanism parametrized as a Newtonian cooling besides a friction mechanism parameterized as a Rayleigh friction are involved.
In a basic state corresponding to wintertime conditions, model runs show the structures of simulated stationary waves similar to the observations for the both waves of zonal wavenumber 1 and 2.
The results show that the planetary waves in the troposphere can be traced into the stratosphere but, they cannot always propagate into the stratosphere, because the tropopause acts on the propagation of planetary waves. One of the key parameters controlling the propagation is the vertical gradient of buoyancy frequency.


Astrophysics 

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Title & Author(s) 
809 
Nucleosynthesis of the Elements in ClS Region, During Helium and Carbon Burning in 25 M$_{\bigodot}$ Stars
P. Szalanski, I. Pădureanu, M. Stepinski, Yu. M. Gledenov, P. V. Sedyshev, J. Marganiec, D. Aranghel, R. Machrafi
This paper presents the results of the calculations of the element overproduction in ClS region during the helium and carbon burning in massive stars (25 M$_{\bigodot}$). The calculations for the given set of differential equations were performed using integrated mathematical systems. The authors consider that such a method may be successfully adopted for other physical problems, in particular for the problem of transmutation of heavy elements and chemical kinetics.


Earth Physics  Seismology 

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Title & Author(s) 
821 
Nonlinear Modelling of FrequencyMagnitude Distribution for Vrancea Strong Earthquakes
Alexandru Mărmureanu, Carmen Cioflan, Bogdan Felix Apostol
Given a set of observations like frequencymagnitude distributions for Vrancea earthquakes, one often wants to condense and summarize the data by fitting it to a "model" that depends on adjustable parameters. We consider fitting when the model depends nonlinearly on the set of $M$ unknown parameters $a_k$, $k = 1, 2, \ldots , M$. With nonlinear dependences, the minimization process proceeds iteratively. Finally, it is developed a procedure that improves the trial solution and the LevenbergMarquandt method works very well in practice to find their frequencymagnitude distribution for Vrancea strong earthquakes.


833 
Inversion of Short Period Records of Local Seismic Events  A Tool for the Study of Source Parameters of Small Vrancea Earthquakes
Luminiţa Ardeleanu
The short period records of a low magnitude crustal earthquake from Vrancea seismic area are inverted for the seismic moment tensor. Although the structure of the region is quite complex, simple 1D layered models are used in the Green's function synthesis. Azimuthal dependence was simulated by specifying different 1D structures for different stations. The waveform inversion was performed by processing the available records grouped into data sets, corresponding to individual subsets of 4 stations. The most uncertain inverted parameter is the source time function; the mechanism is more confident, especially the orientation of its deviatoric part; the most robust source parameter is the scalar moment. An averaging algorithm is tested to constrain better the time history and to obtain representative estimates of the source parameters of the seismic event under study.




