Tuning the instrument resolution using chopper and time of flight at the small-angle neutron scattering diffractometer KWS-2

Using a double-disc chopper with a variable slit opening in concert with a velocity selector and the time-of-flight data acquisition mode, controlled variation of the wavelength spread Δλ/λ between 2 and 20% has become routinely possible at the KWS-2 SANS diffractometer of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center.


Table S1
Summary of the quantities and parameters involved in the determination of the instrumental resolution for different experimental configurations routinely used at KWS-2; neutron wavelength, / -wavelength spread, LDdetection length, LCcollimation length, Xmin, Yminbeam spot size (horizontal, vertical); all quantities and parameters are defined within the text; the calculations were done according to Eq. 2-4 and to Hammouda & Mildner (2007), for the size of the source and sample rectangular apertures 30mmx30mm and 10mmx10mm, respectively (RE=17.3mm and RS=5.78mm, after the conversion of rectangular aperture on round aperture of the same area).

The design and operation of the KWS-2 chopper
The chopper system was designed, produced and installed at KWS-2 by the Central Institute for Engineering, Electronics and Analytics -Engineering and Technology (ZEA-1) of Research Centre Jülich GmbH.
The chopper contains two identical discs which are driven independently of each other by two rotor shafts. Both discs rotate counter-clockwise when viewed from the neutron source in the direction of the sample. The rotation frequency is the same for both discs and can be adjusted within a wide range, between 5 and 200 Hz. The discs have a diameter of 620mm and are placed at 5cm from each other.
Each disc has two 90° windows with a depth of 65mm (Fig. S1). The rims of the discs out of the window area are coated over a wider range than the height of the neutron guide with 10 B containing epoxy paint disposed in a layer of a few tenth of a mm on each of the disc faces (Fig. S1). The content of 10 B is of 32 mg/cm 2 , which enables a neutron transmission as low as 10 -6 . J. Appl. Cryst. (2015). 48, doi:10.1107/S1600576715019019 Supporting information, sup-3 a) b)

Figure S1
The discs (a) and the slit geometry (b) of the KWS-2 chopper.
The chopper system is located at 0.7 m in front of the beginning of the collimation system. The chopper-to-detector distance LTOF can be thus varied between 21.7 m and 40.7m. The whole system is placed in a stainless steel housing consisting of two compartments ( When the magnet passes the pickup sensor, the disc must rotate further 45 ° for the opening of the neutron guide to begin simultaneously over the full height (Fig. S3a). At 56.12 ° the guide is fully open (Fig. S3b). The closing of the guide starts at 123.88 ° (Fig. S3c) and ends at 135 ° (Fig. S3d).

Figure S4
The same processes as in Fig. 3 presented in terms of the neutron intensity as a function of disc position (disc opening and pickup signal) in linear time sequence.

Figure S5
Schematic presentation of the relationship between the phase difference of chopper discs and neutron intensity at the phase setting 45°/90° in linear time sequence.

Figure S6
The neutron guide (blue) is fully opened twice per cycle for a short time when the two chopper discs have a phase difference of =67.76° to each other; this corresponds to an opening slit window of =22.24°.

Figure S7
The neutron intensity in linear time sequence for a phase setting corresponding to the configuration shown in Fig. 6. aim= 20%, 10%, 5% and 2.5%. The slit opening window  was 60°, 36°, 20° and 10.6°, while the chopper frequency was set to 32.0Hz, 37.9Hz, 42.13Hz and 44.6Hz, respectively. The intensity distribution in the Fig. S8b shows a rather trapezoidal profile in the case of /aim=20% and 10%, while for /aim=5% and 2.5% the profile is triangular. These two profiles are similar to those schematically shown in Figs. 5S and 7S, corresponding to a slit with either larger than the guide width ( Fig. S5) or equal to it (Fig. 7S). a) b)

Figure S8
The direct beam intensity (expressed in counts per second) on the detector (LD=1.3m) measured for =4.72Å in TOF mode for different chopper settings corresponding to aim=20%, 10%, 5% and 2.5%: intensity distribution over the TOF channels (a) and as a function of TOF, after merging the two pulses into a single one.