Browsing by Author "Bentley, PM"

Now showing 1 - 6 of 6
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    (3)He polarization for ISIS TS2 phase I instruments
    (Elsevier, 2011-06-01) Beecham, CJ; Boag, S; Frost, CD; McKetterick, TJ; Stewart, JR; Andersen, KH; Bentley, PM; Jullien, D
    Here we report on the development of polarization analysis (PA) techniques to be employed at the ISIS pulsed neutron source second target station. Both spin exchange optical pumping and metastability exchange optical pumping techniques are being developed at ISIS to produce polarized neutron spin filters for use as neutron polarizers and analysers. We focus on the developments of a polarization solution on the LET spectrometer, including the updated design of the PASTIS XYZ coil set and single crystal silicon analyser cell. We also report on the construction of a combined polarizer/analyser solution for the WISH diffractometer. (C) 2010 Elsevier B.V.
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    The ABC of powder diffractometer detector coverage
    (Wiley-Blackwell, 2011-04-01) Andersen, KH; Bentley, PM; Cussen, LD
    This article addresses the question of the most effective detector configuration for neutron spectrometers using a discussion of a particular case - constant-wavelength powder diffractometers at continuous neutron sources. A first variation uses an essentially one-dimensional `banana' detector coupled with out-of-scattering-plane beam divergence before and after the sample. A second variation uses an incident beam tightly defined both in- and out-of-plane coupled to a `4 pi' detector covering all possible scattering angles after the sample. It is widely believed that the 4 pi arrangement is superior for most varieties of neutron spectrometer but is more difficult and more expensive to implement. Starting from a commonly used overall instrument figure-of-merit, this article presents simple arguments leading to the surprising conclusion that this is untrue for these constant-wavelength powder diffractometers, provided only that the cylindrical samples used with the banana detector have a height greater than 2.4 times their diameter. © 2011, Wiley-Blackwell.
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    Correction of optical aberrations in elliptic neutron guides
    (Elsevier Science BV, 2012-11-21) Bentley, PM; Kennedy, SJ; Andersen, KH; Rodriguez, DM; Mildner, DFR
    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, a 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 neutron scattering instruments that this new shape can bring. © 2012, Elsevier Ltd.
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    Global optimization of an entire neutron guide hall
    (Wiley-Blackwell, 2011-06-01) Bentley, PM; Fouquet, P; Bohm, M; Sutton, I; Dewhurst, CD; Andersen, KH
    This paper describes the optimization of an entire neutron guide system, from the moderator to the sample position for several instruments simultaneously, using no more than a desktop computer and a few days of CPU time. This is made possible by merging two relatively advanced computational techniques. Neutron acceptance diagram shading is a fast new method for modelling neutron beams, using an approach based on polygons similar to those featuring in computer games. Optimization algorithms based on swarm intelligence are efficient and reliable ways to maximize numerically calculable figures of merit with many strongly coupled geometry parameters. Recent developments in these methods are described, as well as their combination to optimize the geometry of the H5 beamlines at the Institut Laue-Langevin. The optimization was such that all instruments simultaneously gain as much on-sample flux as possible by taking unused phase space from their neighbours, whilst no instrument suffers any losses in its useful flux. © 2011, Wiley-Blackwell.
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    Magnetic fluctuations and correlations in MnSi: Evidence for a chiral skyrmion spin liquid phase
    (American Physical Society, 2011-06-20) Pappas, C; Lelièvre-Berna, E; Bentley, PM; Falus, P; Fouquet, P; Farago, B
    We present a comprehensive analysis of high-resolution neutron scattering data involving neutron spin echo spectroscopy and spherical polarimetry, which confirm the first-order nature of the helical transition in MnSi. The experiments reveal the existence of a totally chiral dynamic phase in a very narrow temperature range above T-C. This unconventional magnetic short-range order has a topology similar to that of a skyrmion liquid or the blue phases of liquid crystals. © 2011, American Chemical Society.
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    Properties of elliptical guides for neutron beam transport and applications for new instrumentation concepts
    (Wiley-Blackwell, 2011-08-01) Rodriguez, DM; Kennedy, SJ; Bentley, PM
    The use of nonlinear tapered guides is becoming more common in advanced neutron scattering facilities around the world. Elliptical guides offer the promise of high performance not only as focusing devices but as an efficient way to transport neutrons over long distances. Here, the analytical expressions to determine their performance are derived and discussed. Under certain conditions, an increase in flux delivery is observed with increasing guide length, due to an increase in the angular spread of the neutrons reflected in the guide. The performance is only limited by the distance between the source and the guide entrance, the dimensions of the instrument placed after it, and the supermirror coating. As an example of the potential of elliptical geometry in instrumentation, a new small-angle neutron scattering (SANS) instrument concept is proposed, in which the neutron source is directly coupled to a half-ellipse, and the instrumental performance is evaluated by means of analytical expressions. The results show that such an instrument may provide a viable alternative to conventional pinhole SANS for high-resolution measurements and small samples, being substantially more compact and simpler to operate. The main limitation comes from the coma aberration which is inevitable on extended sources. The extent of the coma problem is also analysed. © 2011, Wiley-Blackwell.