Sohrab Abbas

The new V12 ultra-small-angle neutron scattering and tomography instrument at the Hahn–Meitner Institut

The new V12 instrument at the Hahn–Meitner Institute in Berlin is a multiple setup combining several techniques to investigate the internal structure of bulk samples. It consists of two double-crystal diffractometers (DCDs) and an attenuation tomography device operating with monochromatic neutrons. The three instrument parts can be used independently at the same time. The DCDs are mainly used in the ultra-small-angle neutron scattering (USANS) regime, where they overlap the accessible range of small-angle neutron scattering instruments, while tomographic methods collect real-space information on a macroscopic scale. Together they enable structural investigations over six orders of magnitude (50 nm to 5 cm). Scattering and tomographic methods can even be combined by means of diffraction and scattering-enhanced imaging. The sample environment can be varied over a large range of temperatures and pressures for USANS measurements and a polarized USANS option is available.

On the design and experimental realization of a multislit-based very small angle neutron scattering instrument at the European Spallation Source

This article reports the design of a versatile multislit-based very small angle neutron scattering (VSANS) instrument working either as a dedicated instrument or as an add-on for any small-angle neutron scattering machine like the proposed SANS instrument, SKADI, at the future European Spallation Source. The use of multiple slits as a VSANS collimator for the time-of-flight techniques has been validated using McStas simulations. Various instrument configurations to achieve different minimum wavevector transfers in scattering experiments are proposed. The flexibility of the multislit VSANS instrument concept is demonstrated by showing the possibility of instrument length scaling for the first time, allowing access to varying minimum wavevector transfers with the same multislit setup. These options can provide smooth access to minimum wavevector transfers lower than ∼4 × 10−5 A−1 and an overlapping of wavevector coverage with normal SANS mode, e.g. with the SKADI wavevector range of 10−3–1.1 A−1. Such an angularly well defined and intense neutron beam will allow faster SANS studies of objects larger than 1 µm. Calculations have also been carried out for a radial collimator as an alternative to the multislit collimator setup. This extends the SANS Q range by an order of magnitude to 1 × 10−4 A−1 with much simpler alignment. The multislit idea has been realized experimentally by building a prototype at Laboratoire Leon Brillouin, Saclay, with cross-talk-free geometry. Feasibility studies were carried out by making VSANS measurements with single- and multislit collimators, and the results are compared with multiple-pinhole geometry using classical SANS analysis tools.

Surfactant induced stabilization of nano liquid crystalline (dodecane-phytantriol) droplet

The study of formation and stabilization of dodecane–phytantriol (DPT) microemulsions using ionic and nonionic surfactants are investigated. Small Angle Neutron Scattering (SANS) and Dynamic Light Scattering (DLS) techniques have been employed to study the resulting structures of the micro emulsion droplets. We show the formation of stable microemulsion droplets with absence of lyotropic liquid crystalline phase on addition of nonionic surfactant C12E10. The oil to surfactant ratio plays the crucial role in formation of stable droplet and its size. The dense presence of C12E10 molecules between microemulsion droplets protect them from coalescence while less number of C12E10 between the surface of droplets easily triggers the coalescence process. The interaction with both anionic (SDS) as well as cationic (DTAB) surfactants with DPT phase leads to formation of microemulsion droplets with lyotropic liquid crystalline phase.The study of formation and stabilization of dodecane–phytantriol (DPT) microemulsions using ionic and nonionic surfactants are investigated. Small Angle Neutron Scattering (SANS) and Dynamic Light Scattering (DLS) techniques have been employed to study the resulting structures of the micro emulsion droplets. We show the formation of stable microemulsion droplets with absence of lyotropic liquid crystalline phase on addition of nonionic surfactant C12E10. The oil to surfactant ratio plays the crucial role in formation of stable droplet and its size. The dense presence of C12E10 molecules between microemulsion droplets protect them from coalescence while less number of C12E10 between the surface of droplets easily triggers the coalescence process. The interaction with both anionic (SDS) as well as cationic (DTAB) surfactants with DPT phase leads to formation of microemulsion droplets with lyotropic liquid crystalline phase.

Interactions in reentrant phase behavior of a charged nanoparticle solution by multivalent ions

The interactions following a reentrant phase transition of charged silica nanoparticles from one phase to two phases and back to one phase by varying the concentration of multivalent counterions have been examined. The observations are far beyond the framework of Debye-Hückel or even nonlinear Poisson-Boltzmann equations and demonstrate the universal behavior of multivalent counterion-driven charge inversion. We show that the interplay of multivalent counterion-induced short-range attraction and long-range electrostatic repulsion between nanoparticles results in reentrant phase behavior.

Kinetics of aggregation in charged nanoparticle solutions driven by different mechanisms

The structure and kinetics during aggregation of anionic silica nanoparticles as induced through different mechanisms have been studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Three different additives, namely an electrolyte (NaCl), cationic protein (lysozyme) and non-ionic surfactant (C12E10) were used to initiate nanoparticle aggregation. Electrolyte induced aggregation can be explained by DLVO interaction, whereas depletion interaction (non-DLVO interaction) is found responsible for nanoparticle aggregation in case of non-ionic surfactant. Unlike these two cases, strong electrostatic attraction between nanoparticle and oppositely charged protein results into protein-mediated nanoparticle aggregation. The electrolyte induced aggregation show quite slow aggregation rate whereas protein mediated as well as surfactant induced aggregation takes place almost instantaneously. The significant differences observed in the kinetics are explained based on range of interactions ...

Aggregation in charged nanoparticles solutions induced by different interactions

Small-angle neutron scattering (SANS) has been used to study the aggregation of anionic silica nanoparticles as induced through different interactions. The nanoparticle aggregation is induced by addition of salt (NaCl), cationic protein (lysozyme) and non-ionic surfactant (C12E10) employing different kind of interactions. The results show that the interaction in presence of salt can be explained using DLVO theory whereas non-DLVO forces play important role for interaction of nanoparticles with protein and surfactant. The presence of salt screens the repulsion between charged nanoparticles giving rise to a net attraction in the DLVO potential. On the other hand, strong electrostatic attraction between nanoparticle and oppositely charged protein leads to protein-mediated nanoparticle aggregation. In case of non-ionic surfactant, the relatively long-range attractive depletion interaction is found to be responsible for the particle aggregation. Interestingly, the completely different interactions lead to simi...

On experimental realisation of a plane wave of neutrons

We report here numerical computations based on dynamical diffraction theory, leading to the optimised collimator configuration of a silicon {333} Bragg prism for 1.75A neutrons. We propose to attain the first less than 0.07 arcsec collimation of a monochromatic neutron beam by diffracting neutrons from a Bragg prism, viz. a single crystal prism operating in the vicinity of Bragg incidence.

High-precision determination of Si-neutron coherent scattering length with a dual non-dispersive sample

We report here our new interferometric measurements of neutron coherent scattering length bC for silicon using a better polished dual non-dispersive sample. We have measured a large and exactly non-dispersive phase by this method to within 1 part in 106 and determined the silicon bC to within a few parts in 105.

Neutron Coherent Scattering Length Determination With a Dual Non-Dispersive Sample

We report here a preliminary interferometric determination of neutron coherent scattering length bC for silicon using a dual non‐dispersive sample. A large and exactly non‐dispersive phase measured by this method can afford an order of magnitude improvement in the precision of bC determination to within a few parts in 106.

First Sub-arcsecond Collimation of Monochromatic Neutrons

We have achieved the tightest collimation to date of a monochromatic neutron beam by diffracting neutrons from a Bragg prism, viz. a single crystal prism operating in the vicinity of Bragg incidence. An optimised silicon {111} Bragg prism has collimated 5.26A neutrons down to 0.58 arcsecond. In conjunction with a similarly optimised Bragg prism analyser of opposite asymmetry, this ultra-parallel beam yielded a 0.62 arcsecond wide rocking curve. This beam has produced the first SUSANS spectrum in Q ~ 10−6 A−1 range with a hydroxyapatite casein protein sample and demonstrated the instrument capability of characterising agglomerates upto 150 μm in size. The super-collimation has also enabled recording of the first neutron diffraction pattern from a macroscopic grating of 200 μm period. An analysis of this pattern yielded the beam transverse coherence length of 175 μm (FWHM), the greatest achieved to date for A wavelength neutrons.