Journal of Neutron Research

We consider recent developments on nano-diamond reflectors for slow neutrons, which dramatically improve their efficiency.

A mirror system for neutron transport with high brilliance transfer from a source or a divergent beam to an instrument is presented. The assembly of nested short elliptical (or very short flat) mirrors located halfway between two common focal points M and M′ images cold neutrons by single reflections from an area around M onto an area of similar size at M′. An absorber on the straight line MM′ blocks the direct view onto the source, with little impact on the transported solid angle. The simple geometry with well-defined, non-grazing angles of reflection off the individual mirrors opens up versatile possibilities to tailor beam size, divergence, wavelength spectrum and polarization to experimental needs. A common small-wavelength cut-off of the transported spectrum can be set by proper choice of the m values of supermirrors. Monochromatic beams can be generated using bandpass supermirrors. Adjustable apertures far away from the instrument define the size and the divergence of the beam at the sample, thus keeping background radiations low. The absence of mirrors in the harsh radiation environment close to an intense source simplifies the maintenance of beam tubes and increases mirror lifetimes.

As the world’s leading centre for neutron science, the ILL provides scientists with a very high flux of neutrons. The transport of neutrons to the different instruments is continually being adapted to the latest technologies in order to maximise the useful neutron flux available. Dedicated neutron guides can now be created with a geometry and wavelength spectrum that are optimised for individual instruments and this paper gives an overview on the recent achievements as well as some insight on guides projects foreseen for the ILL future.

Overview of the future guide systems at the ILL.

This paper presents the results of a study of the oxidation of thin titanium films and their use in neutron spin optics. Kinetics of Ti nanofilm oxidation (at T = 200 ° C ) was found to be described by the logarithmic law. The asymptotic law of the oxide layer growth and the long-term stability of the Ti/TiO₂ bilayer demonstrate the possibility of using the Ti nanofilms in neutron spin optics. It is experimentally proven that a protective Ti layer reliably prevents the oxidation of the functional magnetic layer of the mirror spin-flipper.

We present the results of simulations to optimize a tapered neutron guide, with an initially rectangular cross-section that smoothly transitions to an irregular octagon. This guide serves as an adapter section between an existing cold-neutron guide (rectangular cross-section 120 × 60  mm², with m = 1.2 coating), and a 3 m long circular guide (76 mm inner diameter, with m ⩾ 2.5 ). The circular guide is filled with superfluid ⁴He to convert neutrons with wavelengths around 8.9 Å into ultracold neutrons (UCN) by inelastic scattering; it serves as the centerpiece of the ILL’s new UCN source SuperSUN. The octagonal guide exit provides a better match than a rectangle to the circular aperture, and the beam divergence is transformed so that a larger fraction of the 8.9 Å neutrons can be guided through the converter. The required edge-reflectivity for the circular guide is also reduced: simulations show that, with a converter coating of m = 2.5 , 85% of the 8.9 Å neutrons from the primary, rectangular beam can be made available for UCN production. Due to the aspect ratio of the initial rectangle, an irregular octagonal cross section is found to perform better than a regular one. The corresponding guide has been fabricated and delivered, with first neutron measurements expected in 2019.

The replacement of the H16-IN5 guide has been decided in the frame of the ILL Endurance program. This guide is made of heteroclite sections and the upgrade aims at improving the performances with a modern homogeneous ballistic guide. Several constraints applied to the project: (i) the instrument will remain at the same location, (ii) considering (i), in terms of gain relative to cost, it was judged more reasonable to keep the same chopper system, (iii) many technical elements under control of the nuclear safety authorities cannot be changed within the lifetime of the project. We present the results of the McStas optimization of the replacing guide performed under these constraints.

We report on calculated probabilities for neutron capture upon specular reflection from Ni/Ti supermirror coatings. The note represents a summary of the talk given at the 5th workshop on Neutron Delivery systems, 25–27 June, Grenoble, France.

Fundamental optical concepts are utilized across all fields of scientific instrumentation. In the case of beam-like systems in neutron scattering instrumentation (low-divergence trajectories taken over long distances), quantities describing a system can be scaled into dimensionless figures of merit that convey performance characteristics and geometric limits. Furthermore, this first-principles approach facilitates a straight forward means by which to tailor an optical system to a defined source-sample configuration, as well as further inform guide surface shapes and reflectivity requirements. These principles will be presented and applied incorporated to examples showing their relevance to practical applications and future instrumentation.

The construction price is an important parameter when designing neutron guides, but it has so far difficult to take into account during the overall optimiation. We therefore introduce the CoatingWriter add-on for the guide design tool guide_bot under McStas. This add-on allows for optimization of guide coating (m-value), taking both performance and price into account, in addition to the geometry optimization provided by guide_bot. We find that by using CoatingWriter we are able to generate good guides at significantly lower overall price levels than was possible before.