Abstract:
Neutron sources in modern neutron facilities produce neutron beams of a large diameter and angular distribution. One of the main points of neutron optics is the minimization of beam losses during the transport to the particular neutron experiment. The investigated neutron lenses in this work are very specific neutron focusing elements. Their purpose is a gain of flux intensity in the sample area. Two new designed neutron lenses were built and tested at the Helmholtz-Centre Berlin for Materials and Energy. The neutron wavelength is much smaller than the lens dimensions, so the laws of geometrical optics could be applied. The reflective solid state lens was developed on the basis of an concept from Mildner. The lens consists of two main parts. On each side of the lens 95 silicon wafer are stacked together and bent in a circular form (R=1 m) in an aluminum frame. All wafers are coated with a neutron reflective (m=2) supermirror layer. They guide nearly every entering neutron through the lens mount to a line focus. The silicon wafers are curved in only one spatial direction. Measurements on a neutron reflectometer result in a focal distance of 31 mm and an intensity gain of 5.6 for the solid state lens construction. The lens has the ability to focus the whole beam divergence of a (m=2) coated neutron guide on a 2.5 mm wide sample. Furthermore measurements on two refractive lead prism lenses of an effective range of 1 and 2 mm showed an intensity maximum in the neutron signal of a factor of 2 and 3. The focal distance was 1.0 and 1.4 m, respectively. The lead prism lenses are made of many lead layers with some hundred prism profile lines. The concept is based on a prototype named Clessidra from the field of X-ray optics. The performed calculations indicate a possible 100% intensity rise from neutrons with a wavelength of 5 Å in the focal region. For a two dimensional focusing neutron lens both lens types, solid state and Clessidra lens, were combined. As result, a 12-times intensity gain in a (0.9x2.4) mm² focal area was measured. Finally, a third lens was investigated, a two dimensional focusing polycapillary optic. This lens is built up of many thousands of micrometer fine glass capillaries. The lens with a 77 mm focal distance was studied. In the 1 mm² small point focus area, a maximum intensity gain of factor 16 was determined. In a distance between 0 and 300 mm behind the focal spot, a near homogeneous neutron distribution was shown. In the outcome of this work there is a potential for improvements of the Clessidra lead prism lens. Another application for the solid state lens is the investigation of small samples with neutron reflectometry. In the end, the polycapillary lens should be a useful tool for neutron tomography applications.