The investigation of multiple particles production can be possible by using a 4 pi detection consisting of a position sensitive detector, placed in the immediately vicinity of the target, and thus allowing the determination by coincidence measurements of the initial impact point. The construction, testing as well as installation procedures of such a kind of detector designed and built at the National Institute of Physics and Nuclear Engineering - Bucharest are presented and discussed.

Processing techniques were applied to seismic data acquired by reflection methods. The seismic methods are efficient research methods for civil engineering and environmental geology, which invite to develop specific methodologies. Therefore, soft-programs for processing data collected with refraction seismic techniques (based on head and transmitted waves) and by transmission tomography for velocity were made. The visual programming medium Borland Delphi was utilized to create the program MEDCONT, whose abilities, by menus and dialog windows, are both commanded and controlled. The accuracy and the adaptability of the program to field cases are validated by data resulted from forward models and also collected by applications on field objectives.

To address the complexity of the phenomena that occur in a nuclear fuel element, a multi-scale method was developed. The method incorporates theory-based atomistic and continuum models into finite element simulations to predict heat transport phenomena. By relating micro and nano-scale models to the macroscopic equilibrium and non-equilibrium simulations, the predictive character of the method is improved. The multi-scale approach was applied to calculations of point defect concentration, helium bubbles formation, oxygen diffusivity, and simulations of heat and mass transport in UO2+x.

We present thermodynamic models of point defects formation in fluorite-type oxides with important technological applications, such as hypostoichiometric ceria CeO2-x and plutonia PuO2-x. The concentrations of defect species were calculated at different temperatures and partial pressures of oxygen. To highlight some of the current capabilities of the approach, a number of example applications, including the calculation of defect configuration entropy, and the calculation of self and chemical diffusivities of oxygen as a function of nonstoichiometry and temperature are addressed. The results provided by the modeling are validated against available experimental data on non-stoichiometry and oxygen diffusivity.

We present finite-element simulations of coupled heat and oxygen atom diffusion for UO2 fuel pellets. The expressions for thermal conductivity, specific heat and oxygen diffusivity for the fuel element are obtained directly from previously published correlations, or from analysis of previously published data. We examine the temperature and non-stoichiometry distributions for a varying range of conditions. Simulations are performed for steady-state and transient regime in one-dimensional (purely radial) configurations. For steady-state conditions we perform parametric studies that determine the maximum temperature in the fuel rod as a function of non-stoichiometry and heat generation rate intensity. For transient simulations, we examine the time lag in the response of the temperature and non-stoichiometry distributions with respect to sudden changes in heat generation rate intensity and oxygen removal rate. All simulations are performed with the commercial code COMSOL Multiphysics™.

A thermochemical model of defects formation is proposed. Using this model, defect species concentrations are determined. The model allows for the calculation of the non-stoichiometry of PuO2−x, as functions of temperature and oxygen pressure. A model of oxygen diffusion is proposed and the oxygen self and chemical diffusivities are calculated for temperatures in the (900–1400°C) range and partial pressure of oxygen PO2∈(1–10−25atm). This work shows that, given appropriate parameters, the model is able to describe the non-stoichiometry and oxygen diffusivity in a reasonable way.

The single crystalline growth of the GaN x Sb 1-x system is difficult due to the miscibility gap expected for nearly the whole composition range under thermodynamic equilibrium conditions. The gap is determined by the differences of the atomic radii and of the electro negativities for N and Sb. To overcome this problem crystal growth has to be performed under non-equilibrium conditions with kinetically controlled growth, as it is observed for molecular beam epitaxy (MBE) growth. A single crystalline MBE-growth within the miscibility gap has been demonstrated already in the GaAs x N 1-x system exhibiting a similar large miscibility gap. GaN:Sb-layers were grown on Si(111)-substrates by MBE using NH 3 as a N-source and solid element sources for Ga and Sb. The parameter window for growth was limited due to side reactions like the decomposition of NH3, the desorption of (at high temperature volatile) compounds like Sb and GaSb or the reaction of Sb with NH3. The composition of the layers was analyzed by XRD and RBS. Antimony bulk concentrations of up to 1.6 % could be obtained in GaN. Optical characterization of the samples was performed by CL-measurements and indicate Sb-induced transitions in the 2.2 eV and 1.42 eV range.

The titanium incorporation in GaSb layers grown by molecular beam epitaxy (MBE) on Sl GaAs Substrates was studied in order to investigate the influence of Ti-doping on the electrical and optical properties and the interaction with the intrinsic defect [V-Ga, Ga-Sb], causing a hole concentration in the range of 10(16) cm(-3) in the epitaxial layer. The incorporation of titanium is verified by secondary ion mass spectrometry investigations. Besides the increase of the hole concentration from 10(16) cm(-3) to 10(17) cm(-3) a reduction of the intrinsic defect is indicated by photoluminescence measurements at 4K. Furthermore a decrease of hole mobility from 700 to 330 cm(2)/Vs is observed by Hall measurements. From the temperature dependence of the hole concentration an activation energy of 37.7meV is assigned to a Ti acceptor transition.