Continuous Angular Multiple Energy Analysis Spectrometer Report on Instrument Concept and Scientific Case

Abstract This work unit proposes to construct a cold indirect geometry time of flight spectrometer for performing inelastic neutron scattering experiments optimized for optimal efficiency in the horizontal scattering plane at the European Spallation Source. A horizontal geometry is chosen for compatibility with performing neutron scattering experiments under extreme conditions. The instrument concept is called the Continuous Angular Multiple Energy Analysis Spectrometer, CAMEA. In this report we will outline the science case for CAMEA, highlighting the science that could be performed on CAMEA, the demand for an instrument of this type, aswell as identifying the current and future technology in neutron scattering that can be utilized. The basic concept of CAMEA is to maximize neutron count rates for scattering in the horizontal plane, with a quasi-continuous angular coverage of the scattered neutrons. High neutron detection efficiency will be obtained by using banks of concentric analysers placed behind each other, analysing different neutron energies of the scattered neutrons. Optimization of a horizontal scattering geometry has been chosen to be compatible with extreme sample environments and the ability to perform inelastic neutrons scattering studies. We will highlight how this provides a generation of advancement in inelastic neutron scattering under extreme environments, and what new possibilities for scientific studies CAMEA enables in inelastic neutron scattering.

Energy-selective neutron imaging for materials science

Steven Luc X Peetermans

Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample.

EPFL2015

The instrument suite of the European Spallation Source

Paul Gregory Freeman, Ursula Bengaard Hansen, Michael Müller, Henrik Moodysson Rønnow, Markus Strobl

An overview is provided of the 15 neutron beam instruments making up the initial instrument suite of the European Spallation Source (ESS), and being made available to the neutron user community. The ESS neutron source consists of a high-power accelerator and target station, providing a unique long-pulse time structure of slow neutrons. The design considerations behind the time structure, moderator geometry and instrument layout are presented. The 15-instrument suite consists of two small-angle instruments, two reflectometers, an imaging beamline, two single-crystal diffractometers; one for macromolecular crystallography and one for magnetism, two powder diffractometers, and an engineering diffractometer, as well as an array of five inelastic instruments comprising two chopper spectrometers, an inverse-geometry single-crystal excitations spectrometer, an instrument for vibrational spectroscopy and a high-resolution backscattering spectrometer. The conceptual design, performance and scientific drivers of each of these instruments are described. All of the instruments are designed to provide breakthrough new scientific capability, not currently available at existing facilities, building on the inherent strengths of the ESS long-pulse neutron source of high flux, flexible resolution and large bandwidth. Each of them is predicted to provide world-leading performance at an accelerator power of 2 MW. This technical capability translates into a very broad range of scientific capabilities. The composition of the instrument suite has been chosen to maximise the breadth and depth of the scientific impact of the early years of the ESS, and provide a solid base for completion and further expansion of the facility.

2020

Energy-selective neutron imaging for materials science

Steven Luc X Peetermans

EPFL2015