Neutron guides are widely used for improving the angular aperture of neutron scattering instruments in a broad band of wavelength. However, the usual guides are not effective enough in the short wavelength range. This is especially critical for time-of-flight instruments, which cannot take advantage of focusing techniques designed for steady-state monochromatic instruments. We discuss alternative ways to shape the reflecting surfaces in order to obtain a maximum angular aperture at the sample position at the expense of beam cross-section reduction. An optimal piecewise solution is proposed and Monte Carlo simulations with the IDEAS package are presented. Simulations for General Purpose Powder Diffractometer (GPPD) at Argonne National Laboratory are presented and the impact of a vertically focusing multiple-stage tapered guide is discussed. The results obtained by simulating the guide system options of engineering diffractometer VULCAN at the Spallation Neutron Source (SNS) are also presented, including vertically and horizontally tapered guide sections. The optimal multi-stage tapered guide design is discussed in terms of instrument figure of merit corresponding to different experimental needs ranging from high Q resolution to high intensity and/or high spatial resolution.
Many topics of interest for neutron scattering demand small sampling volumes. Then the scattering instruments should include focusing devices in order to deliver a sharp spatially shaped neutron beam at the sample position. Moreover the wavelength bandwidth should be sufficiently large if time-of-flight method is used. In this contribution a new compact focusing device is proposed. The device is made of a stack of bent silicon wafers, each having a glancing reflective layer deposited on one side and a neutron absorbing layer on the other side. This device acts as a lens by superposing the images delivered by individual mirrors. The aberrations are minimized due to the short length of the device. From this point of view this type of superposition lens is equivalent to a long elliptic or parabolic mirror. Consequently a two dimensional focusing could be obtained by combining two devices in a Kirkpatrik-Baez set-up. Basic design principles are described and Monte-Carlo simulation results are presented. Possible applications in neutron instrumentation are reviewed.
The feasibility of Guinier cameras for small angle neutron scattering (SANS) is analyzed theoretically and experimentally. Small angle X-ray scattering (SAXS) is commonly measured with Guinier cameras1 that use bent perfect crystals to focus to detector beams from point sources of characteristic X-rays. Neutron Guinier cameras do not exist yet, although focusing to detector has occasionally been tried. The philosophy of current SANS pinhole instruments is to gain intensity from broad wavelength bands at tight collimation. With characteristic X-rays, intensity gains can only come from broad angular divergences. Neutron focusing instruments represent a return, at a higher level, to the philosophy of characteristic X-rays. Such a return is advocated in this paper for SANS.
The resolution of Guinier cameras is defined not by the collimation (which is relaxed), but by the beam size at focus and the spatial resolution of the position sensitive detector (which should match each other). Within the recent concept of neutron imaging2 multi-wafer monochromators can provide image sizes comparable to the thickness of one wafer in the bent packet. The imaging may be non-dispersive, at broad wavelength bands, like with mirrors in conventional optics. These are the right ingredients for convergent neutron beams in Guinier cameras. The paper addresses the question whether the increased angular divergence can compensate for the reduced size of the source that is imaged into a sharp spot at detector.
A neutron Guinier camera at thermal neutron energies is evaluated. It turns to be quite feasible, providing moderate resolution at high intensity with detection systems in current use for high-resolution neutron diffraction. High-resolution SANS is also possible with detection by image plates or microchannel plate systems.
Tests were performed using a single wafer and a packet of bent silicon wafers in both Bragg and Laue (transmission) geometry in non-dispersive imaging arrangements. Experiments have confirmed expectations. SANS data obtained in neutron Guinier camera conditions on samples of collagen and lipids are presented.
A moving diffractor changes the energy of elastically diffracted neutrons by the Doppler effect. Depth-graded multilayers can diffract neutrons over a large band of energy. Using a pulsed neutron source, such a depth-graded multilayer, decelerating synchronously with the incident neutron pulse, can shift the reflected neutrons into a compressed energy window. This focusing in energy is associated with a broadening of the pulse in time, but the process does not involve a significant decrease in the neutron phase-space density. The proposed method can be used to design long pulse or quasi-continuous sources of cold, very cold or ultra cold neutrons (UCN). The analysis concentrates on enhanced production of UCN at pulsed neutron sources.
Phase-space analysis of neutron optics has revealed that neutron imaging by Bragg reflection from thick bent perfect crystals can be non-dispersive (independent of the neutron wavelength), like with an optical mirror. The corresponding devices, called Bragg mirrors (BM), can be used for neutron imaging at pulsed neutron sources. Using a position sensitive detector (PSD) and time-of-flight analysis (TOF), a BM imaging system will make it possible to collect both real space mapping data and scattering space data simultaneously. Each pixel of PSD will correspond to a point in the sample and will contain a segment of the diffraction pattern (useful for strain, texture or phase analysis), or of an inelastic spectrum. In this paper the resolution and efficiency of BM in TOF diffraction experiments are calculated and compared with the usual sequential method of mapping. Experimental tests performed at steady state neutron sources showed sub-millimeter spatial resolution in the one-dimensional case.
Multi-wafer silicon monochromators for neutron focusing instruments have been developed at MURR. A first unit, made from commercial thin silicon  wafers with manual control of horizontal curvature was designed and fabricated for the MURR stress machine. It was tested on the stress machine at the HFIR reactor at ORNL. A second similar unit but with stepper motor control of curvature was installed on the NIST stress machine. Both confirmed expectations, with significant intensity gains at equal or better resolution in comparison with the monochromators they replaced. A third unit with two back-to-back assemblies of non-standard silicon wafers custom sliced obliquely from big  ingots has been fabricated for an upgrade of the ORNL stress machine. The phase space analysis of the neutron optics of multi-wafer assemblies has revealed exciting new possibilities for applications. The correlation between the coordinates of real space and wavevector space allows a new type of focusing, the thickness focusing. The many wafers in a packet can be made to look as a single wafer when seen from a given point of a position sensitive detector (PSD). This allows high resolutions in scattering, corresponding to a bent thin single wafer, at intensities given by the whole packet, that is comparable with pyrolytic graphite crystals. One can thus have the best of two worlds - but only in PSD instruments. A whole array of new applications becomes possible, including dispersive and non-dispersive neutron imaging at the spatial resolution of a single thin wafer. Some of these applications are discussed and demo experiments are presented.
The neutron optics of three-axis spectrometers with bent perfect crystals and position sensitive detection (PSD) has been developed. Theoretical analysis in the phase space and in the scattering space shows that simultaneous PSD scans can be performed along any given direction in the scattering (h(omega) ,Q) plane, including energy transfer scans or Q- scans. For instance, to perform a simultaneous energy transfer scan the curvature of the monochromator must be set for 'monochromatic focusing' while the curvature of the analyzer must be significantly away from the 'monochromator focusing' value. A new kind of focusing was found to be possible. Under the right conditions the resolution in scattering becomes insensitive to the thickness of the analyzer crystal. Packets of commercial thin silicon wafers can then give resolutions corresponding to a single wafer at considerable gains in intensity. Control experiments with a 14-wafer assembly have confirmed this conclusion. Resolutions below 3 minutes of arc on the angular scale were obtained (corresponding to energy transfer resolutions in the range of 10 to 150 (mu) eV, depending on the neutron energy). A practical difficulty is that very high spatial resolutions of the PSD, in the submillimeter range, are needed to take full advantage of the possibilities offered by commercial thin silicon wafers.
Bent crystals are well suited to high-brilliance sources and add flexibility to the design of synchrotron radiation (SR) monochromators. A double crystal monochromator for high resolution in elastic scattering of SR is examined by methods developed for neutron optics. Computations with a neutron code adapted to the case of SR show that bending the first crystal can improve performance by strongly reducing the beam size at marginal gain in full beam intensity.