Thomas Hils¹, Roland Gähler², Peter Böni¹, Robert Georgii^1,3
1 Physics Department E21, Technische Universit¨at München, 85747 Garching, Germany
² Institut Laue-Langevin, F-38042 Grenoble, France
³ ZWE FRM-II, Technische Universität München, 85747 Garching, Germany

Small angle neutron scattering (SANS) is a powerful method for measuring lateral correlation lengths in the 0.01 to 0.1 μm range. The method finds applications in biology, polymer physics, material science etc. The corresponding Q-range is around 5^.10⁻³ nm⁻¹ to 5 nm⁻¹. The aim of our proposed MSANS (multiple SANS) is to increase the Q-resolution by at least an order of magnitude compared to SANS at equal intensity extending the measurement of correlations to a scale of several microns. In conventional SANS this would lead to an unacceptable loss of intensity by a factor of 10⁻⁴. As an alternative one can overlay multiple individual beams (up to 10⁴). With MSANS an incoherent superposition of multiple beams is achieved increasing usable divergence and thus intensity. Moreover, the scattering pattern is two-dimensional and not restricted to one dimension as in the commonly used USANS technique.

The proposed MSANS is a new USANS option for a standard long baseline SANS instrument. It uses the common SANS infrastructure except for the detector, which requires enhanced spatial resolution. We aim at improving the Q-resolution to about 10⁻⁵Å⁻¹ at 10 Å , so that correlations up to 60 μm can be measured. Using multi-hole apertures at the entrance (M_e) of the collimator and near the sample (M_s) with lattice constants ae, as and hole diameters d_e, d_s respectively and with the choice (Fig. 1)
G_e,s =2π/a_e,s ; G_e · L₁ = (G_s − G_e) · L₂ ; G_d = G_s − G_e
an intensity pattern of well separated peaks with lattice constant ad in the detector plane is observed (a_d = 2 /G_d).

Short range correlations in the sample may lead to significant overlap, however typical SANS intensities drop very rapidly with increasing Q and overlap will not be fatal in many cases. Sets of apertures with different relations ae,s / de,s can be used to adapt the pattern to the demand. For an ideal MSANS, the resolution is decoupled from the intensity, as long as the area of the openings of the apertures is kept constant. The increase in Q-resolution in MSANS is typically one order of magnitude, compared to SANS at equal intensity. The gain originates from the reduction in Q-range in MSANS and the increase of the cross section of the guide and its divergence.

Fig. 3 shows a preliminary MSANS pattern using an image plate detector and a cut along the y-direction. From the data we obtain a peak width at the detector of 1.5 mm. Therefore, lateral correlations of the order of 3.8 μm can be measured. This value is a lower limit because the detector was in saturation for the largest intensities. Of course, the resolution can be increased further by increasing the distance between the entrance and exit apertures from 6 m (MIRA) to 40 m (conventional SANS). In order to increase the intensity of the instrument, the apertures at the sample position may be replaced by small lenses.

This work was supported by the European Union within the Sixth Framework Program FP6 under contract no. 505925.