Browsing by Author "Rodriguez, DM"

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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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    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.
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    Upgrade of the neutron guide system at the OPAL neutron source
    (Insitute of Physics, 2010-12-16) Rodriguez, DM; Kennedy, SJ; Klose, F
    The new research reactor at ANSTO (OPAL) is operating with seven neutron beam instruments in the user programme and three more under construction. The reactor design provides for expansion of the facility to eighteen instruments, and much of the basic infrastructure is already in place. However, an expansion of the neutron guide system is needed for further beam instruments. For this purpose, several possibilities are under consideration, such as insertion of multi-channel neutron benders in the existing cold guides or the construction of a new elliptic cold guide. In this work Monte Carlo (MC) simulations have been used to evaluate performance of these guide configurations. Results show that these configurations can be competitive with the best instruments in the world. © 2010, Insitute of Physics