Compact Glass Capillary Optics for Neutron Beams
Among methods that benefit in particular from a local increase in flux are methods of neutron scattering for the examination of small samples, as in high pressure cells, that need a small source, as in tomography, or that require high spatial resolution, as neutron activation analysis.
Neutron beams transported in neutron guides are normally quasi-parallel and have a large cross-sectional area of several cm². The best way to focus such beams is to use focusing semi lenses, as used in X-ray optics. According to their geometric parameters those semi lenses can focus a wide neutron beam to a small focal spot < 1 mm and thus achieve an intensity gain of one to two orders of magnitude in the focal spot.
The great advantage of the capillary lenses is their short length which allows their flexible use. Different focal spot sizes and focal lengths can be obtained by changing lenses, while alternatives like focusing neutron guides are many metres long and thus quite inflexible.
Up to now, capillary lenses have been used mainly with X-ray sources. Neutrons, just as photons, show the effect of total reflection at the surface of a solid. The critical angle for total reflection Θcat grazing incidence is several mrad and is in the same range for X-ray and neutron radiation. It depends on the wavelength λ, and its value for neutrons and different types of glass is:
Θc[mrad] ≈ λ[Å]
This fact allows the use of glass capillary optics for neutrons. In contrast to X-ray optics, however, it is important that the glass contains boron, causing those neutrons that enter the glass to be absorbed so that only the focused neutrons are transmitted.