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Methoden und Instrumentierung Poster: Mi., 14:00–16:30 M-P53
Advances in small-angle neutron scattering on D22: Biology, Soft Matter,
Physics and Materials
Roland May 1 , Charles Dewhurst 1 , Stefan Egelhaaf 2 , Isabelle Grillo 1
1 Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France – 2 Institut für
Physik der Kondensierten Materie, Universitätsstraße 1, 40225 Düsseldorf, Germany
Small-angle neutron scattering (SANS) has been one of the most successful techniques
provided by the ILL. Based on the success of D11, another dedicated SANS instrument,
D22, was designed and implemented in the second guide hall of the ILL. It started full
operation in 1996. The project of a third instrument, D33, is on its way.
The sample-to-detector distance range of D22 is 1.05 to 18 m. The detector has an
area of roughly 1 m 2 and can be moved laterally by 50 cm, considerably increasing the
simultaneous momentum-transfer range.
D22 is probably the SANS instrument with the highest flux at the sample position.
It reached its full performance only about two years ago, when a detector consisting
of a vertical array of 128 linear sensitive Reuter-Stokes tubes of 8 mm diameter was
installed; each of those has a dead-time of 2 µs. The whole detector can therefore
accept a uniform load of more than 6 MHz at 10 % dead-time loss, a performance
improvement of a factor of 60.
The high flux (> 10 8 cm −2 s −1 ) at optimal conditions for the standard ∆λ/λ of
10 % can now be fully used, while formerly the limited detector count rate often
forced one to over-collimate the incoming beam. This is of interest when the signal
is small compared to a huge (often incoherent) background. The gain is particularly
important for time-resolved experiments, which are becoming more and more popular
in Soft Matter and Biology. The number of total counts that is required for a given
statistics in every time frame (that can be ≤ 50 ms) is accumulated with much less
repetitions if the count rate is not limited by the detector, thus wasting less precious
sample material. Another example is the observation of inelastic SANS, like the
temperature-dependent scattering from silicon (Cheung et al.), where the reduction
of counts due to chopping the beam needs to be counterbalanced by a high flux.
The high flux is also advantageous in the case of weakly scattering samples. One
prominent example is the measurement of the scattering from the vortex lattice of the
topical superconductor MgB2, where Cubitt et al. were able to observe scattering
from a crystal of less than 100 µg.
The treatment of SANS data has made a considerable step forward at the ILL with
the development of the Grasp suite, available at http://www.ill.fr/lss/grasp.
After successful tests that employed an improvised polariser and a Mezei spin flipper,
e.g. on ferrofluids and fusion-reactor steels, we are about to install a transmission
polariser in the selector bunker, a permanent-magnet guide field along the collimation,
and a radio-frequency spin flipper.