Space- and time-correlations in coherent scattering

The pulsed time-structure of the SwissFEL beam will permit the elegant realization of an alternative scattering method which uses space- and time-correlations to determine the anisotropic structure of one or more objects undergoing tumbling motion in solution. Kam has computed [10] the X-ray scattering from a prolate ellipsoid undergoing random rotations (see Fig. III.10). Whereas the time-averaged scattering <S(κ)> shows an isotropic pattern as a function of the D1 detector position (|κ| = |κ1|, ϕ), the correlated time-averaged instantaneous product <S(κ1)S(κ2)> of the counting rates into D1 and D2, reveals the object’s anisotropy, as a function of, for example, |κ| = |κ1| = |κ2| and ψ = angle(κ12). In his paper, Kam explains how the method can be applied, with constant signal-to-noise, to increasing numbers of identical but randomly rotating objects and how a refinement can be per formed of more complex shapes, in terms of an expansion in spherical harmonics. The pulsed time structure of the SwissFEL radiation brings an obvious advantage to the Kam method: since the pulse length is much shorter than a tumbling period, a 2d-scattering pattern represents an instantaneous measurement. It is thus possible to correlate, at the same time, the counts in any detector pixel with those in any other pixel. In this way, far more correlated information is available for the structure analysis.

The PILATUS detector

A spin-off company from the Paul Scherrer Institut produces what is currently the best 2d-imaging X-ray detector in the world [30, 31]. Due to its modular design, PILATUS is available in various sizes, from 105 to 6 × 106 pixels (see Fig. III.i6). The modules consist of a silicon sensor, with 170 × 170 μm2 pixels, bump-bonded to a custom-made readout chip. The detector offers noise-free, single-photon detection, a dynamic range of 20 bits, and a readout rate of 12 Hz. Advanced versions of PILATUS, with different analog gains for different detector regions and the ability to pipeline on-chip up to 500 frames, are being developed for XFEL applications [32].