We propose to develop focusing devices using non-linear tapering. The goal of the research is the manufacturing of a) elliptic devices to image a point-to-point source and b) parabolic devices for focusing neutron beams as delivered by neutron guides. We envisage designing large devices using technology similar as for the transport of neutrons with glassy neutron guides. In particular ballistic guides with elliptically and/or parabolically shaped walls shall allow i) the use of smaller cold sources thus reducing the flux depression, ii) increasing the S/N ratio by more than a factor of two, and ii) to homogenize the phase space at the neutron scattering instrument, iii) decreasing the flux of fast neutrons and -rays. Therefore, an overall increase of performance of an order of magnitude is expected. In parallel we shall develop the simulation program McStas further in order to simulate and optimise guide components and neutron devices with arbitrary shapes of guide walls. In a preliminary simulation/experiment we have already received indications that parabolic guides can focus neutron beams down to an area of 0.7 mm, yielding flux gains of the order of 50. The new technology of non-linearly tapered guides will markedly increase the performance of neutron guides and make the expensive and fragile Kumakhov lenses obsolete.



Spatial intensity distribution of an elliptic guide for a wavelength l = 4 Ǻ at a distance of a) 0 m, b) 0.5 m and c) 1.3 m from the guide exit (intensity is color-coded). The intensity is highest away from the exit of the guide. The typical gains compared to a rectangular guide with m = 2 are of the order of 6.

Results
Elliptic guides have been developed for the transport of neutrons. First prototypes demonstrate that the expected flux gains of more than a factor of five can be realised. An elliptic neutron guide with a length of 90 m was recently installed at the HRPD diffractometer at ISIS and it performs as specified. Recent neutron scattering experiments with thermal neutrons (PUMA at FRM II) demonstrated the possibility of measuring excitations in samples as small as 1 mm3 ( mg) opening the way of performing inelastic neutron scattering under extreme conditions.


The elliptic guides installed at the High Resolution Powder Diffractometer at ISIS.

• Focusing report (PDF 0.172 MB)
• Upgrade of HRPD with 100 m Long Elliptic Neutron Guide - A report on the installation of the very first (fully) elliptic neutron guide worldwide at ISIS.
(PDF 1,2MB)