KWS-3 :: Very-High Resolution Focusing SANS Diffractometer
KWS-3 is a very small angle neutron scattering (VSANS) instrument running on the focusing mirror principle at the Research Neutron Source Heinz-Maier Leibnitz (FRM-II) in Garching. Standard configuration of the instrument with 9.5 m sample-to-detector distances allows performing scattering experiments with a wave vector transfer resolution between 10-4 and 3 · 10-3 Å-1, bridging a gap between Bonse-Hart and pinhole cameras. Second sample position at 1.3 m distances has extended Q-range of the instrument to 2 · 10-2 Å-1 and reached more than one-decade overlapping with the classical pinehole SANS instruments. The principle of this instrument is a one-to-one image of an entrance aperture onto a 2D position sensitive detector by neutron reflection from a double-focusing toroidal mirror. Small-angle scattering is used for the analysis of structures with sizes just above the atomic scale, between 1 and about 100 nm, which cannot be assessed or sufficiently characterized by microscopic techniques. KWS-3 is an important instrument, which extends the accessible range of scattering angles to very small angles with a superior neutron flux when compared with a conventional instrumental setup with pinhole geometry. Thus the length scale that can be analyzed is extended beyond 10 µm for numerous materials from physics, chemistry, materials science and life science, such as alloys, diluted chemical solutions and membrane systems.
KWS-3 :: Current configuration, flux, and Q-range
Instrument Configurations (01.06.2013)
|#0:||1)12.8Å||2)9.50m||3)0.5×0.5||4)100×20||5)1200n/s||Future Option||[in test]|
|#1:||12.8Å||9.50m||2.0x2.0||100×20||18000n/s||Standard||[in user operation]|
|#2:||12.8Å||9.50m||5.0x5.0||100×20||105000n/s||Standard||[in user operation]|
|#3:||12.8Å||1.30m||5.0x5.0||15×7||105000n/s||Standard||[in user operation]|
|#4:||12.8Å||0.15m||5.0x5.0||5×5||105000n/s||Standard||[Detector sensitivity and selector calibration ]|
Q-range of the KWS-3 diffractometer for all available configurations (#0..#5) is plotted.
For comparison, the Q-range of a classical pine-hole SANS instrument is plotted too.
|2010:||4400n/s||x7||Entrance Beam Size:||2mmx2mm|
|2012:||11500n/s||x20||Beam Size at Sample Position:||100mmx20mm|
A number of improvements that have been carried out during the last years and a high brilliance of the FRM-II reactor in Garching allowed for the neutron flux gain by more than twenty times in comparison to the FRJ-2 reactor in Jülich, that however is much lower than expected factor of 60-100, predicted by the simulations. Thorough studies allowed us to predict a failure in the S-neutron guide providing KWS-3 with neutrons with wavelength 12.8Å. In January 2012, the visual inspection of the neutron guide using an endoscope (carried out by the neutron optic group of FRM-II) has confirmed this prediction and showed that the coating of the first 40cm of the curved guide is completely destroyed. In close future the damaged guide will be changed and KWS-3 should get the expected flux (2013).
Flux boosting to factor 3 was reached by significant upgrade of the instrument entrance:
- too broad trumpet was exchanged by the normal neutron guide;
- vacuum tube was extended to 60 cm;
- new 4-blades entrance aperture allows to find maximal flux;
- flux monitor was moved out from used beam.
The “main sample chamber” is located at 9.5m distances from the detector. In this chamber by selecting the “standard mode” (12.8Å neutrons, (2×2)mm2 entrance aperture) one can reach the minimum wave vector Q[min]=10-4Å with the flux better than 11000 n/s for the sample size of (10×2)cm2. In the “high intensity mode” - (5×5)mm2 entrance aperture- the flux is increased up to 65000n/s by the price of the reduction of the instrument resolution to Q[min]=2.5·10-4Å. In the “high resolution mode” - 1.5×1.5mm2 entrance aperture and 20Å neutrons – the resolution reaches Q[min]=4·10-5Å with the flux more than 1000 n/s.
The maximum Q achievable in this case is about Q[max]=3·10-3Å. As the minimum Q of a classical pinhole SANS diffractometers KWS-1 and KWS-2 is about 2·10-3Å, the Q-overlap between these instruments and KWS-3 was not large enough. To have more massive overlap we have built the second sample chamber at the 1.2m sample-to-detector distances: it is adapted to the flight tubes by additional vacuum valves and is equipped with an additional sample stage. The implementation of the second sample position has allowed extending the Q-range of KWS-3 by an order of magnitude.
Polystyrene spheres of 7600 Å diameter measured at 9.5 and 1.2 m.
The red curve is the theoretical scattering curve (form factor of 7600 Å spheres).
The instrumental resolution and sample polydispersity are not taken into account.
Mismatching of the measured curve and model at low Q is due to the presence of the structure factor.