Bi-spectral beam extraction combines neutrons from two different kind of moderators into one beamline, expanding the spectral range and thereby the utilization of an instrument. This idea can be realized by a mirror that reflects long wavelength neut
rons from an off-axis colder moderator into a neutron guide aligned with another moderator emitting neutrons with shorter wavelengths which will be transmitted through the mirror. The mirror used in such systems is typically several meters long, which is a severe disadvantage because it reduces the possible length of a focusing device in design concepts requiring a narrow beam at a short distance from the source, as used in many instruments under development for the planned European Spallation Source (ESS). We propose a shortened extraction system consisting of several mirrors, and show that such an extraction system is better suited for combination with a feeder in an eye of the needle design, illustrated here in the context of a possible ESS imaging beamline.
The concept of Wavelength Frame Multiplication (WFM) was developed to extend the usable wavelength range on long pulse neutron sources for instruments using pulse shaping choppers. For some instruments, it is combined with a pulse shaping double chop
per, which defines a constant wavelength resolution, and a set of frame overlap choppers that prevent spurious neutrons from reaching the detector thus avoiding systematic errors in the calculation of wavelength from time of flight. Due to its complexity, the design of such a system is challenging and there are several criteria that need to be accounted for. In this work, the design of the WFM chopper system for the potential future liquids reflectometer at the European Spallation Source (ESS) is presented, which makes use of acceptance diagrams. They prove to be a powerful tool for understanding the work principle of the system and recognizing potential problems. The authors assume that the presented study can be useful for design or upgrade of further instruments, in particular the ones planned for the ESS.
A search for a very-high-energy (VHE; >= 100 GeV) gamma-ray signal from self-annihilating particle Dark Matter (DM) is performed towards a region of projected distance r ~ 45-150 pc from the Galactic Center. The background-subtracted gamma-ray spectr
um measured with the High Energy Stereoscopic System (H.E.S.S.) gamma-ray instrument in the energy range between 300 GeV and 30 TeV shows no hint of a residual gamma-ray flux. Assuming conventional Navarro-Frenk-White (NFW) and Einasto density profiles, limits are derived on the velocity-weighted annihilation cross section < sigma v> as a function of the DM particle mass. These are among the best reported so far for this energy range. In particular, for the DM particle mass of ~1 TeV, values for <sigma v> above 3 * 10^(-25) cm^3 s^(-1) are excluded for the Einasto density profile. The limits derived here differ much less for the chosen density profile parametrizations, as opposed to limits from gamma-ray observations of dwarf galaxies or the very center of the Milky Way, where the discrepancy is significantly larger.
