A time-of-flight small-angle diffractometer employing seven tapered collimator elements and a two-dimensional gas proportional counter was successfully utilized to collect small-angle scattering data from a solution sample of the lipid salt cetylpyridinium chloride, C₂₁H₃₈N⁺.Cl^-, at the Argonne National Laboratory prototype pulsed spallation neutron source, ZING-P'. Comparison of the small-angle scattering observed from the same compound at the University of Missouri Research Reactor corroborated the ZING-P' results. The results are used to compare the neutron flux available from the ZING-P' source relative to the well characterized University of Missouri source. Calculations based on experimentally determined parameters indicated the time-averaged rate of detected neutrons at the ZING-P' pulsed spallation source to have been at least 33% higher than the steady-state count rate from the same sample. Differences between time-of-flight techniques and conventional steady-state techniques are discussed.

A pulse-shortening device is described for use on pulsed thermal-neutron sources. The device employs rotating single crystals and has applications in the design of high-resolution cold-neutron spectrometers.

We report calculations describing the Bonse-Hart ultra-small-angle neutron scattering (USANS) instrument with triple-bounce Si channel-cut crystals, which show that significant gains in Q-resolution and neutron flux can be achieved using multiple high-order Bragg reflections. These reflections become usable only after combining the Bonse-Hart and time-of-flight (t-o-f) techniques, thus this variant of the USANS camera needs a pulsed neutron source. We demonstrate that t-o-f USANS instruments installed, for example, at the SNS water moderator will improve the current state of the art. Through the use of multiple wavelengths, these will provide means to detect multiple scattering effects.

Small-angle neutron scattering instruments can be built to use either steady-state or time-of-flight techniques, although only the latter are practical at pulsed neutron sources. The techniques used to provide beams of suitable quality, wavelength range and angular collimation are considered in detail for steady-state and time-of-flight instruments at reactor neutron sources, and for time-of-flight instruments at pulsed neutron sources. For both instrument types a cold neutron source provides a definite advantage. Most, but not all, steady-state instruments use long flight paths, which can be shown to provide conditions which are optimum in many ways. However, frame overlap considerations force the use of a short flight path for time-of-flight instruments, and this in turn forces these instruments to use different collimation and beam-quality techniques from those that are usually used for steady-state instruments. Although adequate techniques now exist for building short-flight-path small-angle neutron scattering instruments, some of these short-path techniques are still developing, and can be expected to improve in the future. At present the time-of-flight instruments are more difficult to build and use, but for many experiments this difficulty is more than compensated by the large wave-vector range covered in a single measurement with such instruments.

Two time-of-flight powder diffractometers have operated at the Intense Pulsed Neutron Source (IPNS) since August 1981. These instruments use dedicated microcomputers to focus time-of-flight events so that data from different detectors can be summed into a single histogram. Thus, large multidetector arrays can be employed at any scattering angle from 12 to 157°. This design permits data to be collected over a uniquely wide range of d spacings while maintaining high resolution and count rates. The performance of the two instruments is evaluated by analyzing data from a standard Al₂O₃ sample by the Rietveld method. These instruments provide the capability for moderate- to high-resolution measurements with the duration of a typical run being a few hours.

The effect of Soller collimation on the resolution characteristics of small-angle scattering spectrometers for which there is a limitation to the total length, as in the case of a pulsed source instrument, is examined. Any finite distance between the exit of the collimation and the sample position causes the resolution of the instrument to deteriorate. This results from a correlation in angle and position for points at the sample producing an extra (cross-correlation) term in the resolution function. Expressions for this general case are derived. They are similar to those when the incident beam is defined at the sample position, except that both the source and the sample contributions have an extra term which is proportional to the finite distance between the end of the incident collimation and the sample.

The resolution of a time-of-flight small-angle scattering spectrometer is complicated because many wavelengths contribute to the intensity for a particular scattering vector. The resolution function varies according to the wavelength distribution as well as with collimation. A rigorous calculation of the instrumental resolution requires averaging the resolution function weighted by the wavelength spectrum within the band of wavelengths used in a given measurement. For each scattering vector an effective wavelength is defined which, after substitution into the resolution expression developed for fixed wavelength, gives the resolution of the time-of-flight measurement. Alternatively, the resolution may be established by Monte Carlo methods. In order to check the calculated resolution function and the overall instrument resolution, measurements have been made on biological samples (myelin and collagen) which give sharp Bragg peaks at small values of scattering vector. The analysis of these results provides a direct comparison with calculations and with a similar measurement on a steady-source instrument.

The instrumental optimization conditions for most small-angle scattering experiments in which the data are azimuthally symmetric require that the scattered flight path be equal to the incident flight path. This is in contrast to a recent analysis which shows that under some conditions the incident and scattered flight paths are in a ratio of two to one. The equal flight-path condition is also valid for experiments measuring sharp (Bragg-like) peaks, or where the intensity is required at specific scattering vectors, as in low-angle diffraction of ordered or semiordered systems. The implications of the optimization conditions on the resolution and count rates at the detector are discussed for both types of experiment, and the dependence of the resolution on the spectrometer geometry is considered.

The accuracy of the Chebyshev expansion coefficients used for the calculation of attenuation correction factors for cylindrical samples has been improved. An increased order of expansion allows the method to be useful over a greater range of attenuation. It is shown that many of these coefficients are exactly zero, others are rational numbers, and others are rational fractions of π⁻¹ The assumptions of Sears [J. Appl. Cryst. (1984), 17, 226–230] in his asymptotic expression of the attenuation correction factor are also examined.

The method of determining corrections to neutron time-of-flight diffraction data by Monte Carlo techniques is costly since a large range of neutron wavelengths must be considered. An optimized simulation is presented in which many wavelengths are considered simultaneously, while paths are traced only once for all wavelengths. Collision positions and scattering angles are selected from cumulative distribution functions representing all neutron wavelengths simulated. The scattered intensity is computed by weighting the simulated paths to account for the probability of their occurrence for each wavelength. The results of a calculation for a vanadium slab are given as an example.

A Monte Carlo computer simulation of a neutron diffraction experiment is presented which is of particular use for systems where the structure factor modulates considerably. The program has been optimized by forcing successive scattering events to occur within the specimen under study, and by calculating the score into the detector at each scattering point to allow each simulated path to contribute to the detected scattered intensity. A method of determining the multiple scattering correction for time-of-flight diffraction is shown, and the results of such an analysis are presented for a specimen with intense forward angle scattering and strongly varying S(Q).