Design of a wavelength frame multiplication system using acceptance diagrams

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 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.Comment: Published in Nuclear Instruments and Methods in Physics Research, Section

An improved elliptic guide concept for a homogeneous neutron beam without direct line of sight

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 article, 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

Final Report German Work package K7 Simulation Code Development, Help Desk ESS ID SD015DE

The work package K7 Simulationscode Entwicklung, Helpdesk comprised the further development of the Monte Carlo simulation package VITESS for the simulation of neutron scattering experiments, its use to study general questions related to ESS instrumentation in collaboration with the ESS Data Management and Software Center DMSC in Copenhagen and support for users. The package contains now a visualization of instrument and trajectories, the possibility for numerical optimization of instrument parameters and options for parallelization of the simulation for multi core PCs and for computer clusters. Among other improvements, modules to simulate lenses and true elliptical guides have been written, and the monochromator and the detector module have been improved significantly. The graphical user interface and the treatment of events are more user friendly now. For the ESS project, the performance of different guide geometries were compared, different options for extraction systems and guides without direct line of sight developed and optimized, and the effect of gravity and waviness investigated. The work was executed in collaboration with the DMSC. Most of the results have been published already. Apart from direct support for some users, user support was realized by completing and improving the documentation, updating the internet site, setting up a Facebook account and a new email lis

Computer simulation tests of optimized neutron powder diffractometer configurations

Recent work has developed a new mathematical approach to optimally choose beam elements for constant wavelength neutron powder diffractometers. This article compares Monte Carlo computer simulations of existing instruments with simulations of instruments using configurations chosen using the new approach. The simulations show that large performance improvements over current best practice are possible. The tests here are performed on instruments optimized for samples with a cubic structure which differs from the optimization for triclinic structure samples. A novel primary spectrometer design is discussed and simulation tests show that it performs as expected and allows a single instrument to operate flexibly over a wide range of measurement resolutio

India\u27s Strategic Defense Transformation: Expanding Global Relationships

India’s transformation to modernize its military, obtain “strategic partnerships” with the United States and other nations, and expand its influence in the Indian Ocean and beyond includes a shift from an emphasis on the former Soviet Union as the primary supplier of defense articles to a western base of supply and an increasing emphasis on bilateral exercises and training with many of the global powers. The author explores the nature of this transformation, offers insights into the history of Indian defense relations, and suggests implications to U.S. foreign and defense policy. Much has been written regarding India’s relations with its neighbors, especially Pakistan and China. The author adds a new perspective by taking a global view of India’s rise as a regional and future global military power through its bilateral defense relations and the potential conflict this creates with India’s legacy as a leader of the Non-Aligned Movement.https://press.armywarcollege.edu/monographs/1350/thumbnail.jp

Bi-spectral beam extraction in combination with a focusing feeder

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 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.Comment: Published in Nuclear Instruments and Methods in Physics Research, Section

The dependence of the gravity effect in elliptic neutron guides on the source size

Elliptic neutron guides are expected to be widely used for construction of long neutron beamlines at the future European Spallation Source and other facilities due to their superiour transmission properties compared to conventional straight guides. At the same time, neutrons traveling long distances are subject to the action of gravity that can significantly modify their flight paths. In this work, the influence of gravity on a neutron beam propagating through elliptic guides is studied for the first time in a systematic way with Monte Carlo simulations. It is shown that gravity leads to significant distortions of the phase space during propagation through long elliptic guides, but this effect can be recovered by a sufficiently large source size. The results of this analysis should be taken into account during design of long neutron instruments at the ESS and other facilities

Design of a horizontal neutron reflectometer for the European Spallation Source

A design study of a horizontal neutron reflectometer adapted to the general baseline of the long pulse European Spallation Source (ESS) is presented. The instrument layout comprises solutions for the neutron guide, high-resolution pulse shaping and beam bending onto a sample surface being so far unique in the field of reflectometry. The length of this instrument is roughly 55 m, enabling $\delta \lambda / \lambda$ resolutions from 0.5% to 10%. The incident beam is focussed in horizontal plane to boost measurements of sample sizes of 1*1 cm{^2} and smaller with potential beam deflection in both downward and upward direction. The range of neutron wavelengths untilized by the instrument is 2 to 7.1 (12.2, ...) {\AA}, if every (second, ...) neutron source ulse is used. Angles of incidence can be set between 0{\deg} and 9{\deg} with a total accessible q-range from 4*10^{-3} {\AA}^{-1} up to 1 {\AA}^{-1}. The instrument operates both in {\theta}/{\theta} (free liquid surfaces) and {\theta}/2{\theta} (solid/liquid, air/solid interfaces) geometry. The experimental setup will in particular enable direct studies on ultrathin films (d ~ 10 {\AA}) and buried monolayers to multilayered structures of up to 3000 {\AA} total thickness. The horizontal reflectometer will further foster investigations of hierarchical systems from nanometer to micrometer length scale, as well as their kinetics and dynamical properties, in particular under load (shear, pressure, external fields). Polarization and polarization analysis as well as the GISANS option are designed as potential modules to be implemented separately in the generic instrument layout. The instrument is highly flexible and offers a variety of different measurement modes. With respect to its mechanical components the instrument is exclusively based on current technology. Risks of failure for the chosen setup are minimum.Comment: Matched to the version submitted to Nuclear Instruments and Methods

Bi spectral extraction through elliptic neutron guides

In this paper we present the results of investigating a suggested guide extraction system utilizing both a thermal and a cold moderator at the same time, the so called bi spectral extraction. Here, the thermal moderator has line of sight to the sample position, and the neutrons from the cold source are reflected by a supermirror towards the sample. The work is motivated by the construction of the European Spallation Source ESS but the results are general and can be used at any neutron source. Due to the long pulse structure, most instruments at ESS will be long, often exceeding 50 m from moderator to detector. We therefore investigate the performance of bi spectral extraction for instrument lengths of 30 m, 56 m, 81 m and 156 m. In all these cases, our results show that we can utilize both moderators and thus high intensity in a wide wavelength band in the same instrument at a cost of flux of 5 30 for neutrons with wavelength larger than 1 . In general, the divergence distribution is smooth at the sample position for all wavelength

K7 2013 Q1 Work Package Report ESS

The main activity in this quarter was the development of VITESS version 3.1 D.K.7.2.4 . Most changes are included parallelization, numerical optimization, extension of visualization, new and improved modules e.g. detector, reflectometer sample, monitors, i o of trajectories , and improvements in GUI and moderator characteristics. Testing has started; the release is planned for May 2013. A publication about the effect of gravity on a neutron beam transported in elliptic guides D.K.7.1.7 has been written. The results will be presented on NOP amp;D in July. The comparison of brilliances D.K.7.1.6 has been continued. Most ISIS moderators are now correctly described and McStas and VITESS output are in agreement. Two moderators still need to be checked