J.R. Granada

Thermal neutron cross section and transport properties of polyethylene

Abstract We report a new measurement of the total cross section of polyethylene at room temperature, for neutrons with energies between 10 −3 and 10 eV. It is shown that a synthetic scattering function is able to produce good agreement with the measured values, as well as with other quantities of interest in moderator design problems. On this basis, thermal neutron diffusion parameters are predicted for polyethylene over a range of useful temperatures.

On the analysis of deep inelastic neutron scattering experiments for light nuclei

We analyze different procedures for data processing in deep inelastic neutron scattering experiments focussing our attention on experiments performed on light nuclei. We first examine experiments in which the desired information is obtained from the observed peak areas in the particular case of mixtures of light and heavy water. In the second part we examine the procedure to obtain an effective temperature from the experimental peaks. As a consequence of the results emerging from both cases we trace the limits of validity of the convolution formalism usually employed, and propose a different treatment of the experimental data for this kind of measurements.

Thermal neutron cross sections and diffusion parameters of Plexiglass

Abstract The neutron total cross section of Plexiglass has been measured for energies between 10 −3 and 10 3 eV by the transmission method with pulsed-neutron time-of-flight techniques. A calculation based on a synthetic scattering function shows a very good agreement with the measured values over the entire energy range. This model has been used to evaluate other quantities of interest in moderator design problems, including energy-transfer kernels and thermal neutron diffusion parameters. These experimental and theoretical results are compared with available data for Plexiglass.

Analysis of peak areas in deep inelastic neutron scattering on a H2O/D2O mixture

Abstract In this paper we examine the standard procedure employed to obtain peak intensities from the experimental data of deep inelastic neutron scattering experiments. Our analysis is focussed on the hydrogen, deuterium and oxygen peaks observed in experiments on heavy water and the D2O/H2O equimolar mixture. As a result of our analysis we conclude that the usual procedure (based on a convolution formalism) introduces errors in the determination of the peak areas, and that the analysis must be based on an exact formalism, that was previously published. The present paper brings consequences on the quantitative results of recent publications.

Multiple scattering and inelasticity corrections in thermal neutron scattering experiments on molecular systems

Abstract In this work we address the problem of multiple scattering and inelasticity corrections in neutron diffraction measurements for structural studies of molecular systems. A variety of pulsed neutron and reactor experiments was performed on hydrogenous samples under different experimental conditions. Monte Carlo simulations were carried out on the basis of a Synthetic Model to describe the neutron-molecule interaction, and they allowed a simultaneous evaluation of multiple, inelastic and beam attenuation processes into the samples. A very good agreement between measurements and simulations was obtained in all cases, for our demanding choice of samples and experimental conditions.

Formalism for obtaining nuclear momentum distributions by the deep inelastic neutron scattering technique

We present a formalism to obtain momentum distributions in condensed matter from neutron Compton profiles measured by the deep inelastic neutron scattering technique. The formalism describes exactly the neutron Compton profiles as an integral in the momentum variable y. As a result we obtain a Volterra equation of the first kind that relates the experimentally measured magnitude with the momentum distributions of the nuclei in the sample. The integration kernel is related with the incident neutron spectrum, the total cross section of the filter analyzer, and the detectors efficiency function. A comparison of the present formalism with the customarily employed approximation based on a convolution of the momentum distribution with a resolution function is presented. We describe the inaccuracies that the use of this approximation produces, and propose a data treatment procedure based on the present formalism.

Non-destructive determination of very low hydrogen content in metals with the use of neutron techniques

Abstract In this work we present two methods, based on neutronic techniques, for the non-destructive determination of low hydrogen content ( ⩽ 20 wt ppm) in metals. The first method is based on neutron transmission measurements over the thermal and subthermal energy ranges, and exploits the very large cross-section of hydrogen at low neutron energies. The second method is based on scattering measurements, using epithermal neutrons in combination with time-of-flight techniques and exploiting the large difference in energy-transfer in the neutron—metal and neutron—H interaction processes. We will discuss the conceptual aspects of both methods, the models developed to describe those processes, the experiments performed, and the results obtained for samples of steel and Zircaloy-IV.

Total neutron cross section of Teflon between 0.00038 and 590 eV

Abstract The neutron total cross section of Teflon has been measured for energies between 3.8 × 10 −4 eV and 5.9 × 10 2 eV by the transmission method with pulsed neutron time-of-flight techniques. In order to cover that wide energy range, two independent measurements were performed on samples with different thicknesses and employing as neutron moderator a conventional polyethylene slab for the epithermal region and a liquid nitrogen cooled paraffin block for the cold/thermal region. Due to some inhomogeneities in the sample material, the transmission data were normalized using the best available values of the scattering lengths and incoherent cross sections for carbon and fluorine.

Multiple scattering and attenuation corrections in Deep Inelastic Neutron Scattering experiments

Multiple scattering and attenuation corrections in Deep Inelastic Neutron Scattering experiments are analyzed. The theoretical basis of the method is stated, and a Monte Carlo procedure to perform the calculation is presented. The results are compared with experimental data. The importance of the accuracy in the description of the experimental parameters is tested, and the implications of the present results on the data analysis procedures is examined.

High efficiency moderator for pulsed neutron diffraction

Abstract Moderators consisting of clusters of smaller slow-neutron-decoupled moderating elements were tested through experiments of pulsed neutron diffraction, leakage spectrum measurement, and neutron pulse decay. A simple slab and a thin “sandwich” moderator were also measured for comparison purposes. For a given time width of the neutron pulse in the usual wavelength range for neutron diffraction, the proposed assembly produced a much higher neutron yield. Clues to the implementation of the desired time-response in moderator design and optimization are suggested by the present results.