Multi-mirror imaging optics for low-loss transport of divergent neutron beams and tailored wavelength spectra

Abstract

A neutron optical transport system is proposed which comprises nested short elliptical mirrors located halfway between two common focal points M and M'. It images cold neutrons from a diverging beam or a source with finite size at M by single reflections onto a spot of similar size at M'. Direct view onto the neutron source is blocked by a central absorber with little impact on the transported solid angle. Geometric neutron losses due to source size can be kept small using modern supermirrors and distances M-M' of a few tens of metres. Very short flat mirrors can be used in practical implementations. Transport with a minimum of reflections remedies losses due to multiple reflections that are common in long elliptical neutron guides. Moreover, well-defined reflection angles lead to new possibilities for enhancing the spectral quality of primary beams, such as clear-cut discrimination of short neutron wavelengths or beam monochromation using bandpass supermirrors. Multi-mirror imaging systems may thus complement or even replace ordinary neutron guides, in particular at the European Spallation Source.

In-beam superfluid-helium ultracold neutron source for the ESS

“…There, in order to avoid obstructing a direct view at neighboring beamlines of the moderator surface, which is only 3 cm tall, neutron optical elements are restricted from being placed closer than 2 m from the moderator. This difficulty provided the original impetus for the development of elliptic nested mirror optics (NMO) [53], which can be placed far from both the moderator and target and still accept neutrons within a broad angular range. However, the advantage of a larger emission surface was eventually recognized, leading to the conception of the large LD 2 moderator, currently being studied within the HighNESS project.…”

“…The mirror surfaces are formed by truncating the ellipses to the common length l. See Ref. [53] for details of the mathematical construction. The two types of symmetry about the optical axis of the NMO are indicated in the upper corners; the views show the NMO surfaces projected onto a surface normal to the optical axis z.…”

The Development of the NNBAR Experiment

“…Both the reflector in its baseline configuration and the "differential reflector" will be large, mechanically complicated, logistically difficult to assemble and install, and therefore expensive. Different options are therefore being considered, in particular, "nested reflectors", as proposed in [25,26] and depicted in Figure 12. These reflectors are easier to assemble, they can be much shorter in 𝛥𝑧 along the beam axis and, after optimization, may represent a more economical solution for the NNBAR experiment.…”