Wolter Optics for Neutron Focusing

Focusing optics based on Wolter optical geometries developed for x-ray grazing incidence beams can be designed for neutron beams. Wolter optics are formed by grazing incidence reflections from two concentric conic sections (for example, a paraboloid and a hyperboloid). This has transformed observational X-ray astronomy by increasing the sensitivity by many orders of magnitude for research in astrophysics and cosmology. To increase the collection area, many reflecting mirrors of different diameters are nested with a common focal plane. These mirrors are fabricated using nickel-electroformed replication techniques. We apply these ideas to neutron focusing using nickel mirrors. We show an initial test of a conical mirror using a beam of cold neutrons. key words: electroformed nickel replication, focusing optics, grazing angle incidence, mirror reflection, neutron focusing, Wolter optic

A time lens for high resolution neutron time of flight spectrometers

We examine in analytic and numeric ways the imaging effects of temporal neutron lenses created by traveling magnetic fields. For fields of parabolic shape we derive the imaging equations, investigate the time-magnification, the evolution of the phase space element, the gain factor and the effect of finite beam size. The main aberration effects are calculated numerically. The system is technologically feasible and should convert neutron time of flight instruments from pinhole- to imaging configuration in time, thus enhancing intensity and/or time resolution. New fields of application for high resolution spectrometry may be opened.Comment: 8 pages, 11 figure

Background study for the pn-CCD detector of CERN Axion Solar Telescope

The CERN Axion Solar Telescope (CAST) experiment searches for axions from the Sun converted into photons with energies up to around 10 keV via the inverse Primakoff effect in the high magnetic field of a superconducting Large Hadron Collider (LHC) prototype magnet. A backside illuminated pn-CCD detector in conjunction with an X-ray mirror optics is one of the three detectors used in CAST to register the expected photon signal. Since this signal is very rare and different background components (environmental gamma radiation, cosmic rays, intrinsic radioactive impurities in the set-up, ...) entangle it, a detailed study of the detector background has been undertaken with the aim to understand and further reduce the background level of the detector. The analysis is based on measured data taken during the Phase I of CAST and on Monte Carlo simulations of different background components. This study will show that the observed background level (at a rate of (8.00+-0.07)10^-5 counts/cm^2/s/keV between 1 and 7 keV) seems to be dominated by the external gamma background due to usual activities at the experimental site, while radioactive impurities in the detector itself and cosmic neutrons could make just smaller contribution.Comment: Comments: 10 pages, 9 figures and images, submitted to Astroparticle Physic

Mirrors for slow neutrons from holographic nanoparticle-polymer free-standing film-gratings

We report on successful tests of holographically arranged grating-structures in nanoparticle-polymer composites in the form of 100 microns thin free-standing films, i.e. without sample containers or covers that could cause unwanted absorption/incoherent scattering of very-cold neutrons. Despite their large diameter of 2 cm, the flexible materials are of high optical quality and yield mirror-like reflectivity of about 90% for neutrons of 4.1 nm wavelength

Axisymmetric Grazing-Incidence Focusing Optics for Small-Angle Neutron Scattering

We propose and design novel axisymmetric focusing mirrors, known as Wolter optics, for small-angle neutron scattering instruments. Ray-tracing simulations show that using the mirrors can result in more than an order-of-magnitude increase in the neutron flux reaching detectors, while decreasing the minimum wave vector transfer. Such mirrors are made of Ni using a mature technology. They can be coated with neutron supermirror multilayers, and multiple mirrors can be nested to improve their flux-collection ability. Thus, these mirrors offer simple and flexible means of significantly improving existing and future SANS instruments. In addition, short SANS instruments might become possible, especially at compact neutron sources, when high-resolution detectors are combined with Wolter optics

Generic guide concepts for the European Spallation Source

The construction of the European Spallation Source (ESS) faces many challenges from the neutron beam transport point of view: The spallation source is specified as being driven by a 5 MW beam of protons, each with 2 GeV energy, and yet the requirements in instrument background suppression relative to measured signal vary between 10$^{-6}$ and 10$^{-8}$. The energetic particles, particularly above 20 MeV, which are expected to be produced in abundance in the target, have to be filtered in order to make the beamlines safe, operational and provide good quality measurements with low background. We present generic neutron guides of short and medium length instruments which are optimized for good performance at minimal cost. Direct line of sight to the source is avoided twice, with either the first point out of line of sight or both being inside the bunker (20\,m) to minimize shielding costs. These guide geometries are regarded as a baseline to define standards for instruments to be constructed at ESS. They are used to find commonalities and develop principles and solutions for common problems. Lastly, we report the impact of employing the over-illumination concept to mitigate losses from random misalignment passively, and that over-illumination should be used sparingly in key locations to be effective. For more widespread alignment issues, a more direct, active approach is likely to be needed

Correction of Optical Aberrations in Elliptic Neutron Guides

Modern, nonlinear ballistic neutron guides are an attractive concept in neutron beam delivery and instrumentation, because they offer increased performance over straight or linearly tapered guides. However, like other ballistic geometries they have the potential to create significantly non-trivial instrumental resolution functions. We address the source of the most prominent optical aberration, namely coma, and we show that for extended sources the off-axis rays have a different focal length from on-axis rays, leading to multiple reflections in the guide system. We illustrate how the interplay between coma, sources of finite size, and mirrors with non-perfect reflectivity can therefore conspire to produce uneven distributions in the neutron beam divergence, the source of complicated resolution functions. To solve these problems, we propose a hybrid elliptic-parabolic guide geometry. Using this new kind of neutron guide shape, it is possible to condition the neutron beam and remove almost all of the aberrations, whilst providing the same performance in beam current as a standard elliptic neutron guide. We highlight the positive implications for a number of neutron scattering instrument types that this new shape can bring.Comment: Presented at NOP2010 Conference in Alpe d'Huez, France, in March 201