Neutron Physics Division

Keywords: Zr alloys, Neutron Diffraction, Metastable phases, Lattice constants
Participants: J.R. Granada, G.J. Cuello, G.B. Grad, J. Pieres, A. Fernández Guillermet (Metal Physics Div.), R.E. Mayer.
Description: Pure Zr is stable at room temperature with an hcp structure, but other allotropic forms become more stable with an increase in either temperature or pressure. Above 1139K and normal pressure, the stable form is the bcc structure (Beta), whereas at lower temperatures and pressures beyond 2GPa the stable form is the hexagonal phase Omega. The addition of Nb has a stabilizing effect over both structures. Furthermore, through the addition of more than 7 % Nb and a fast cooling process, it is posible to obtain the Omega structure. Finally, by fast cooling of Nb rich alloys the bcc form can be retained at room temperature. It is known that the Omega phase which is formed in these alloys strongly affects the physical properties of the material, and it is therefore important - from the point of view of applications - to have experimental tools with the capacity to determine the relative proportions of Beta and Omega formed in an alloy produced by a fast cooling process. Also, thermal treatments of those alloys promote changes which suggest variations in the chemical composition of Omega, and the onset of a metastable equilibrium between Omega and Beta. The general purpose of this project is the study, by means of neutron diffraction techniques, of the structure of the Omega phase in quenched Zr-Nb alloys, and the Omega - Beta mixtures in metastable equilibrium which are present in thermally treated alloys.

Intra and Inter-Molecular Order in Water

Keywords: D2O, H2O; Liquids; Correlation functions; Hydrogen bonding, Intermolecular order
Participants: V. H. Gillette, J. R. Granada, R. E. Mayer, J. Dawidowski. M. C. Bellissent-Funel (C.E.A-C.N.R.S, Lab. Léon Brillouin, C.E.Saclay, Francia)
Description: Water is a system that presents singularities in its behaviour as compared to other liquids, being the most known its density maximum close to 4C. To understand these abnormalities,the determination of the intermolecular potential is required. Several potentials have been proposed, and to test them molecular dynamic simulations may be performed to obtain the pair correlation functions and check the results against experimental data. So, good determinations of the pair correlation functions are required. These potentials are needed not only from the basic research point of view, but for biological and electrochemical research among others as well. Neutron scattering experiments have been performed on mixtures of D2O and H2O using the Orphée Reactor (Lab. Léon Brillouin, C.E.Saclay) and the Bariloche LINAC. Two ways of data processing are being explored to reach optimal results. The former is a semiempirical approach, and has been previously applied to a single sample of pure D2O; it is now being optimized to analyse consistently and simultaneously the mixtures. The latter is a Montecarlo simulation based on the Synthetic Model.

Neutron Scattering Studies of Hydrogen in Metals

Keywords: Hydrogen in metals, Neutron Scattering, Content, Structure
Participants: J. R. Granada, J. R. Santisteban, R. E. Mayer, J. Dawidowski
Description: Two well-separated aspects concerning the effect of H in metals are being studied with the use of thermal neutron techniques:

INITIAL CONTENT OF H: Bearing in mind that the expected concentration of H is usually very low ( 10 - 20 wt ppm), its accurate determination poses a serious experimental problem. We performed neutron transmission experiments, together with the development and application of high precision models to describe the neutron-atom interaction in hydrogenated systems. A new method has been developed, based on time-of-flight neutron scattering, which exploits the big difference in the energy transferred to the system when hydrogen or metal atoms are involved in the scattering process.
VOLUME AND STRUCTURE OF HYDRURATED PHASES: Based on the large coherent cross section of Deuterium, diffraction experiments on deuterated samples of Zr are able to directly reveal the evolution of relative volume and structure of hydrurated (deuterated) phases, as a function of the thermodynamical parameters which define its location in the phase diagram.

Moderators for Pulsed Neutron Diffraction

Keywords: Moderators, Heterogeneous poisoning, Pulsed sources, Neutron diffraction
Participants: R.E.Mayer, J.Dawidowski, V.H.Gillette, J.R.Granada
Description: Neutron moderation in media heterogeneously poisoned with a suitable neutron absorber is studied through extensive experimental research, and by means of numerical simulation. The final phase of this work aims at the definition of optimisation criteria for the design and construction of slow neutron pulsed sources, of adequate time resolution, with higher neutron yields than those hitherto employed throughout the world. Attention is now given to liquid H₂ and solid CH₄.

Inelasticity and Multiple Scattering Corrections in Neutron Scattering Experiments

Keywords: Neutron Scattering, Multiple Scattering, Monte Carlo, Numeric simulations, Inelasticity Corrections.
Participants: J. R. Granada, J. Dawidowski, G. J. Cuello, R. E. Mayer
Description: To obtain structural information from Neutron Scattering data of liquid and amorphous systems, it is imperative the evaluation of corrections due to multiple scattering and inelasticity, in order to extract the elastic coherent component in the observed spectra. Other effects, such as the influence of the sample holder, and detector efficiencies, must also be adequately evaluated. Notorious deficiencies are observed among corrections procedures employed in the main centres dedicated to neutron scattering experiments. For this reason, numerical simulation codes employing Monte Carlo method have been developed in our Division. In these analyses, the model for inelasticity employed is essential. In our simulations we included models previously developed in this Division.

Generation of Nuclear Data for Molecular Systems

Keywords: Nuclear Data, Neutron Moderation, Neutron Thermalization, Bound Atom Model
Participants: V. H. Gillette, J. R. Granada
Description:: Representing the neutron incoherent scattering function by means of the Synthetic Model developed within our group, it is possible to describe the double differential cross section, total cross section, and energy transfer kernels, through simple expressions. With these elements, nuclear data can be generated to feed suitable reactor codes, through which, neutron moderation and thermalization can be studied under different conditions. The Synthetic Model's versatility allows nuclear data to be generated for atypical molecular systems such as Polymethylmetacrylate, Tri-butylphosphate, Dowtherm, Polyethylene, Teflon, H2, D2, among others. The capacity of generating and handling nuclear data libraries, constitutes a strategic element from the point of view of the nuclear industry.

Portable Neutron Prompt Gamma Analysis

Keywords: R.E. Mayer, V.H. Gillette, S.A. Korochinski
Description: At an early stage, this project is concerned with the conceptual design of Portable Neutron Prompt Gamma set-ups, through extensive experimental in-the-lab testing of possible combinations and geometries.

Neutron Scattering in Solids

Keywords: Neutrons, Solids, Scattering Law
Participants: J. R. Granada, G. J. Cuello
Description: In order to obtain a dynamic structure factor to describe the nuclear interaction between neutrons and solid systems, we are developing a synthetic model. Using this scattering law, it will be posible a fast and precise evaluation of the double differential cross section, which is a fundamental magnitude for the knowledge of the neutron-nucleus interaction and for the description of the scattering process effects. To describe the solid system, we will use Eintein's and Debye's spectra, exploring the convergence of phonon and mass expansions. We will try to obtain an approximated analytic model to the scattering law, in order to obtain the cross section in a simple and compact form, without the difficulties involved in the phonon and mass expansions. With that model, we will specifically tackle the vanadium case, which is a material largely used as normalizer in neutron diffraction experiments, and we will make Monte Carlo simulations of such experiments. This model will also be applied to describe the Doppler broadening of nuclear resonances in solids containing nuclear fuel. This is an important feature to control the neutron production in nuclear reactors. This project includes the realization of a PhD Thesis.

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