Ad A. van Well

Phase-object approximation in small-angle neutron scattering experiments on silicon gratings

The phase-object approximation for neutron scattering based on a one-dimensional dynamic forward scattering theory is discussed and used to calculate the differential cross section of an object. It is shown that this approximation is valid in ultra-small-angle neutron scattering (USANS) and spin-echo small-angle neutron scattering (SESANS) experiments on silicon gratings. In the weak scattering limit, the phase-object approximation reduces to the kinematic or first Born approximation. The spatial coherence function is used to describe instrumental resolution effects. Measurements on three different instruments are in good agreement with calculation results. In the experiment with a time-of-flight SESANS instrument, the effect of Pendellosung with object size is observed.

Calculation of the dynamic structure factor from thermal neutron time-of-flight spectra

Abstract A description is given of a system of computer programs to calculate the dynamic structure factor from thermal neutron scattering time-of-flight measurements. A high degree of flexibility is obtained, in order to suit different demands of various experiments. Each program, as well as the data organisation, is described separately. The general applicability will be illustrated with two case histories: an experiment on liquid argon to obtain accurate data on the dynamic structure factor and an experiment on solid NH4MnCl3 to obtain information on the rotational motions of the NH4-group.

Real-space form factor of spherical particles in kinematic and dynamic scattering

The phase-object approximation for neutron scattering is applied to determine the real-space sample correlation function for a sphere for kinematic and dynamic scattering. The sample correlation function determined in this way corresponds to the kinematic and refraction limits.

Transmission of thermal and fast neutrons and gamma rays in a novel filter

Abstract The properties of a filter consisting of a 3 m long parallel stacking of curved glass plates covered with 58Ni are calculated. It is demonstrated that for thermal and cold neutrons (λ > 0.15 nm) this filter is a considerable improvement on conventional materials such as beryllium, silicon, sapphire or pyrolytic graphite.

Performance of an area scintillator detector

A two-dimensional position-sensitive neutron detector (PSD) was constructed using a scintillator attached to a position-sensitive photo-multiplier with 100 mm diameter. Position determination is based on charge division. Spatial resolution, gamma sensitivity, linearity, temperature dependence and radiation shielding have been investigated. The shaping amplifiers used in the detection electronics were optimized. Two kinds of scintillators were examined: LiI(Eu) crystal and Li-glass. Concerning the gamma sensitivity and resolution, the best performance was obtained using the LiI. A spatial resolution with full width half maximum of 1.3(1) mm was measured. A good shielding against gamma radiation was found to consist out of three layers: 5 mm of boron rubber, 5 cm of lead and 5 mm of boron rubber. The influence of the temperature on the PSD signal output was measured and indicates that a stabilization is necessary. The PSD will be used in a time-of-flight neutron reflectometer.A two-dimensional position-sensitive neutron detector (PSD) was constructed using a scintillator attached to a position-sensitive photo-multiplier with 100 mm diameter. Position determination is based on charge division. Spatial resolution, gamma sensitivity, linearity, temperature dependence and radiation shielding have been investigated. The shaping amplifiers used in the detection electronics were optimized. Two kinds of scintillators were examined: LiI(Eu) crystal and Li-glass. Concerning the gamma sensitivity and resolution, the best performance was obtained using the LiI. A spatial resolution with full width half maximum of 1.3(1) mm was measured. A good shielding against gamma radiation was found to consist out of three layers: 5 mm of boron rubber, 5 cm of lead and 5 mm of boron rubber. The influence of the temperature on the PSD signal output was measured and indicates that a stabilization is necessary. The PSD will be used in a time-of-flight neutron reflectometer.

Short wavelength sound dispersion in liquid argon close to solidification

Interuniversitair Reactor Instituut, 2629 JB, Delft, The Netherlands In addition to previously published neutron scattering experiments on liquid argon at 120 K and four pressures up to 400 bar, more data are obtained at 120 K and 19.5, 266.6 and 844 bar. The data at the two lower pressures have been used to check the consistency of the two sets of measurements. The data at 844 bar have been analyzed in terms of three extended hydrodynamic modes for wave numbers 4.2 ≤ k ≤ 22.2 nm-1. Within the experimental error no anomalous dispersion is visible. The sound propagation gap, previously found in liquid argon, neon and computer simulations at liquid densities, decreases with density and disappears completely close to solidification. This has been confirmed by a computer simulation of a Lennard-Jones liquid.

Development of the Neutron Reflectometer OffSpec at the Delft University of Technology

The fact that a neutron possesses a spin can be used to develop devices to manipulate the polarization of the neutron beam in such a way that much higher resolution (in energy and/or wave vector) can be obtained, without sacrificing intensity. An example of this application is the neutron-spin echo technique. It has been used for several decades for inelastic neutron scattering, where a resolution can be obtained down to the neV range (or times up to several tens of ns) [1]. Since the end of last century, a new application has matured, i.e., the use of spin-echo techniques to label small scattering angles, down to several microradians; see [2,3]. In the preceding paper [4], the application of the spin-echo technique in small-angle scattering is described for the Delft setup, where a monochromatic beam is used (SESANS). Here we will describe a technical solution for a polychromatic beam that will be used in the neutron reflectometer OffSpec for the second target-station at ISIS, which is planned to be operational at the end of 2008. The complicated magnet system to perform the angle labeling is tested in Delft. We describe this system and compare it with the monochromatic version. The different modes for OffSpec are discussed and a few first experimental results are presented.