In this appendix, we present an updated table of neutron scattering lengths and cross sections for most of the known nuclides, based on preexisting works. We include the coherent and incoherent scattering lengths, as well as the spin-dependent scattering lengths b + and b − , in cases where this information is available. Also presented are the scattering coherent, incoherent, total, and absorption cross sections. We present an overview of the fundamental theory of the magnitudes to which the table is referred, and also a summary of the main experimental techniques used in determining these magnitudes.

Neutron Scattering Lengths and Cross Sections

Recent advances in sources, optics, and detectors for soft x rays have led to rapid developments in applications to microscopy. The history of the field and the advantages of soft x rays for the examination of biological specimens are briefly described. Limitations in resolution and radiation induced damage are mentioned before embarking on a survey of the various approaches in use. Contact microscopy is the highest resolution technique available today. It is particularly powerful when used with flash sources for stop‐action imaging. Imaging with Fresnel zone plates provides direct magnification, high resolution, and short exposure times. Scanning microscopy leads naturally to digitally recorded images, with minimal radiation exposure to the specimen. The focused beam for scanning can be generated with multilayer mirrors, or with zone plates. This last approach is treated in greater detail. Extensions of soft x‐ray imaging to three dimensions, and to elemental mapping, are considered as important areas under active development.

Soft x‐ray microscopes

REVIEW ARTICLE: Neutron optics

Fundamental aspects of neutron optics

Atom optics and interferometry based on laser manipulation of atoms and a use of microfabricated structures are reviewed.

Optics and Interferometry with Atoms

The use of refractive lenses for long-wavelength neutrons,although possible1, is severely limited by the fact that the refractive index differs only very slightly from unity (forexample, n − 1 = 2.3 × 10−4for quartz at λ = 20 A). However, the fact thatonly a relatively thin layer of material is needed to produce an optical path difference of half a wavelength (for example, 2.4 µm of Cu at λ = 20 A) led us to propose2 previously that neutrons could be focused by diffraction froma phase grating of variable spacing; this is , in fact, a type of zone plate. Zone plates, first described by Soret3, have been used satisfactorily over a large part of the electromagneticspectrum4–6. We now report the successful focusing and imaging of a slow neutron beam by means of a Fresnel zone plate8.

Imaging and focusing of neutrons by a zone plate

A novel type of neutron interferometer was constructed and tested employing a split cylindrical zone plate with neutrons of 20 A wavelength Its performance and relative merits are discussed

P. D. Kearney

Papers

Imaging and focusing of neutrons by a zone plate

Neutron Interference by Division of Wavefront

Focussing of slow neutrons with cylindrical zone plates