Multiple reflections in elliptic neutron guide tubes Cussen, L.D.; Nekrassov, D.; Zendler, C. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2013, Letnik:
705
Journal Article
Recenzirano
Neutron guide tubes are widely used to transport neutron beams over long distances. The neutron mirrors used to line the guide tubes have imperfect reflectivity and, in long conventional guides, the ...average number of reflections for neutron rays becomes large thus reducing the transmission. This issue is extremely important for modern spallation sources, especially for the proposed long pulse European Spallation Source to be constructed in Lund, Sweden, where technical constraints require many instruments to be far from the source. Several solutions to the problem of transporting neutrons over long distances have been proposed and currently the most favored model is that of guides with elliptic shapes. It is widely believed that elliptic guides transport neutron rays from source to sample with a single bounce, a near perfect solution for long neutron guides, and a view which is true in ideal circumstances. This article uses computed Monte Carlo ray tracing simulations (VITESS) and other techniques to demonstrate that transport of neutrons by realistic elliptic guides usually involves many reflections, contrary to the usual expectations. These multiple reflections explain the irregular divergence distributions observed in computer simulations of transmission by some elliptic guides.
Ballistic neutron guides are efficient for neutron transport over long distances, and in particular elliptically shaped guides have received much attention lately. However, elliptic neutron guides ...generally deliver an inhomogeneous divergence distribution when used with a small source, and do not allow kinks or curvature to avoid a direct view from source to sample. In this paper, a kinked double-elliptic solution is found for neutron transport to a small sample from a small (virtual) source, as given e.g. for instruments using a pinhole beam extraction with a focusing feeder. A guide consisting of two elliptical parts connected by a linear kinked section is shown by VITESS simulations to deliver a high brilliance transfer as well as a homogeneous divergence distribution while avoiding direct line of sight to the source. It performs better than a recently proposed ellipse–parabola hybrid when used in a ballistic context with a kinked or curved central part. Another recently proposed solution, an analytically determined non-linear focusing guide shape, is applied here for the first time in a kinked and curved ballistic context. The latter is shown to yield comparable results for long wavelength neutrons as the guide design found here, with a larger inhomogeneity in the divergence but higher transmission of thermal neutrons. It needs however a larger (virtual) source and might be more difficult to build in a real instrument.
VITESS is a software widely used for simulation of neutron scattering experiments. Although originally motivated by instrument design for the European Spallation Source, all major neutron sources are ...available. Existing as well as future instruments on reactor or spallation sources can be designed and optimized, or simulated in a virtual experiment to prepare a measurement, including basic data evaluation. This note gives an overview of the VITESS software concept and usage. New developments are presented, including a 3D visualization of instruments and neutron trajectories, a numerical optimization routine and a parallelization tool allowing to split VITESS simulations on a computer cluster.
A neutron transport system for the planned imaging instrument ODIN at the future European Spallation Source (ESS) based on neutron optical components was designed and optimized. Different ways of ...prompt pulse suppression were studied. The spectral performance of the optimal neutron guide configuration is presented. In addition, the influence of the gaps in the guide system needed for the required chopper configuration was investigated. Given that the requirements for an imaging instrument located on a long guide system and hosting a complex chopper system are extremely demanding in terms of spectral and divergence needs, this study can be beneficial for a wide range of instruments in various ways.
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 chopper, 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 a 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.
•Design of a wavelength frame multiplication system for a long pulse source beamline.•First application of the acceptance diagrams method to WFM systems.•Confirmation of analytical considerations by neutronic MC simulations.•Implications of this work for instrument design at the European Spallation Source.
A description of how the Monte Carlo simulation of a neutron scattering instrument can be combined with an optimization routine in order to design an instrument with the best performance is given. ...The parameters optimized here describe the converging guide exit of an instrument. A least-square fitting routine and a Metropolis algorithm (as used in the reverse Monte Carlo method) were used to reach this goal. The advantages and disadvantages of the use of these routines are discussed. A combination of both methods is proposed to find the optimal values with high accuracy. In general, the numerical optimization yields results that can sometimes not be obtained by varying the parameters by hand.
The instrument DREAM, in construction at the long pulse European Spallation Source (ESS), is a new type of neutron time-of-flight powder diffractometer, which utilizes additional choppers to meet the ...typical high resolution requests. Pulses will be of symmetric shape and their width can be varied from 10 μs to 1 ms, providing an unprecedented flexibility from highest to low resolution with optimized intensities at the superior brightness of the 5 MW source. The design is driven particularly by the needs and challenges for small and complex samples, large unit cell materials, thermoelectric cage structures or metal-organic framework structures, multiphase battery materials and complex magnetic structures. Therefore, the chosen wavelength bandwidth of 3.7 Å may cover well the peak intensities of the thermal and cold moderator used simultaneously and provides a sufficient Q (and d) range for obtaining diffraction patterns in a single setting. VITESS simulations show a performance that is about two orders of magnitude higher than current best instruments.
At Helmholtz-Zentrum Berlin (HZB) the end-station PEAXIS (Photo Electron Analysis and X-ray resonant Inelastic Spectroscopy) combining Angle-dependent X-ray Photoelectron Spectroscopy (AdXPS) and ...Resonant Inelastic X-ray Scattering (RIXS) is currently built. The latter method uses a spherical variable line space (VLS) grating to focus the beam onto the detector. Working in first-order diffraction allows resolving photon energy by transferring the energy-dependent signal to a position-dependent focal spot on the detector. Focusing requires a precise combination of various parameters of the VLS grating and the geometry of the RIXS spectrometer. The VLS grating was optimized by calculating the geometry parameters for different photon energies, simulating the instrument and evaluating the pattern on the detector. As figure of merit we chose the intensity times the square of the resolving power averaged over the photon energies.
Bi-spectral beam extraction in combination with a focusing feeder Zendler, C.; Lieutenant, K.; Nekrassov, D. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2013, Letnik:
704
Journal Article
Recenzirano
Odprti dostop
Bi-spectral beam extraction combines neutrons from two different kinds 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 neutrons 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.
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