V.H. Gillette

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.

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.

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.

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.

Application of a synthetic model to the description of neutron scattering properties of liquid H2 and D2

Abstract The interaction of slow neutrons with liquid H 2 and D 2 is described in a simple way with the use of a synthetic scattering function. The analytical expressions derived from this model allow a very fast evaluation of the total cross section as well as the isotropic and the anisotropic energy-transfer kernels. From these, neutron scattering and transport properties are calculated and compared with available experimental results for both liquids.

Thermal neutron cross sections and diffusion parameters of diphenyl and Dowtherm-A using a synthetic scattering function

Abstract We have used a synthetic scattering function to evaluate thermal neutron cross sections and diffusion parameters for diphenyl and Dowtherm-A, over a range of temperatures of practical interest. The present formalism is based on our benzene model, which has been already validated in a previous application. Our calculated results for diphenyl are in excellent agreement with several measured quantities, whereas those for Dowtherm-A are at variance with the scarce available information.

Evaluation of neutron thermalization parameters and benchmark reactor calculations using a synthetic scattering function for molecular gases

Abstract Using a synthetic incoherent scattering function which describes the interaction of neutrons with molecular gases we provide analytical expressions for zero- and first-order scattering kernels, σ0(E0 → E), σ1(E0 → E), and total cross section σ0(E0). Based on these quantities, we have performed calculations of thermalization parameters and transport coefficients for H2O, D2O, C6H6 and (CH2)n at room temperature. Comparison of such values with available experimental data and other calculations is satisfactory. We also generated nuclear data libraries for H2O with 47 thermal groups at 300 K and performed some benchmark calculations (235U, 239Pu, PWR cell and typical APWR cell); the resulting reactivities are compared with experimental data and ENDF/B-IV calculations.

Inelasticity corrections in thermal neutron scattering by molecules using a synthetic (incoherent) scattering function

Abstract Recently a synthetic scattering law has been developed to describe the interaction of thermal neutrons with molecular gases. Although this function does not contain a detailed description of the atomic motions in the molecular unit (nor pair correlations), the main features of the molecular dynamics are retained through the introduction of effective quantities which are directly related to the basic properties of the system. In this work we present evaluations of inelasticity corrections using the synthetic model for both reactor and pulsed neutron diffraction experiments. Predictions are made of differential cross sections (self component) for light and heavy water and these are compared with available experimental results.

Corrections in thermal-neutron scattering experiments on molecular systems

Abstract The traditional procedures employed to correct an observed scattering spectrum in order to isolate the structure factor, involve approximations that are not valid in many real cases. We present here Monte Carlo simulations based on a Synthetic Model to describe the (incoherent) neutron—molecule interaction, which allow a simultaneous description of multiple, inelastic and beam-attenuation processes into the sample, as well as the contributions due to the presence of a container. The procedure is applied to experiments performed on H 2 O and D 2 O at a reactor two-axes instrument. We show that our unified scheme produces very good agreement with the measurements and discuss some of its merits as compared with the traditional method.

Neutron detection system for extremely low count rate calculation, construction and employment in search for “cold fusion”

Abstract A 22% efficiency thermal neutron detection system was designed for the investigation of neutron emission from pulsed D2O electrolysis. Reasons are discussed for the choice of 10 atm 3He proportional counters. Optimization calculations carried out through standard reactor code system (AMPX-II) are presented along with construction details and characteristics of the associated electronics. Experimental verification of calculated efficiency and examples of measurements performed with the detector are included.