KWS-2

This instrument is focussed on cold neutrons. Therefore, please carefully check the “Technical data WITHOUT cold source” section. Deviating parameters are in bold. The instrument team is happy to answer any further questions!

Scheme KWS-2 - 2024

KWS-2 [1] is a classic small-angle scattering diffractometer at which a large Q range between 2.0 × 10^-4 and 2.0 Å^-1 can be explored with high neutron intensity and a tunable resolution [2, 3]. The broad Q range is currently covered by combining in a versatile, user-friendly approach the SANS (conventional pinhole), USANS (focussing lenses) and WSANS modes with corresponding detectors [4-6]. In the conventional pinhole mode, the wavelength λ and the sample to main detector distance L_D can be varied between 2.8 and 20 Å and from 1.25 to 20 m, respectively, to maximise the Q-range. The main SANS detection system of KWS-2 consists of an array of ³He tubes (Ø = 8 mm) and rapid read-out electronics [3].

In focussing mode, parabolic MgF₂ lenses are used in combination with a small aperture at the entrance of the collimation system and a secondary scintillation-type high-resolution detector (HRD) that can automatically be brought in beam on demand [1, 4]. The WANS detector, which is also an array of ³He tubes (Ø = 6 mm), is placed in an inclined position below the beam axis at a distance L_D = 1.25 m downstream of the sample. It provides the possibility to record scattered neutrons in a wide angular range (up to 50° scattering angle [5, 6]) and to discard the inelastic component from the measured data by TOF analysis to improve the signal-to-noise ratio on demand [7].

The KWS-2 provides a very high neutron flux on the sample. This is the result of the combination of a neutron guide system that was specially designed to transport high intensity to the instrument [8, 9] and a versatile velocity selector (Airbus, Germany). It enables an easy selection of the wavelength λ and wavelength spread Δλ/λ, depending on whether the specific scientific experiment demands an improved resolution, thus Δλ/λ = 10 %, or high intensity, hence Δλ/λ = 20 %. The resolution can be further improved down to Δλ/λ = 2 %. This is a range where no velocity selector can compete: by using the TOF mode with a versatile chopper providing a variable slit opening and a variable frequency to match the optimal TOF conditions depending on the wavelength λ and the sample-to-detector L_D used.

The high-flux option enables scientific opportunities in the structural investigation of small soft-matter and biological systems aiming at time-resolved SANS (TR-SANS) investigations due to fast kinetical processes with a time resolution of a few tens of ms. It also provides the conditions for optimal utilisation of beam time by acquiring data in a shorter time than other SANS diffractometers. The installation of robotic elements and an automated sample changer at the sample position allows a continuous supply of samples to the instrument and the merging of experiments from different user teams when similar experimental conditions are required, contributing even more to the efficient use of the beam time.

Finally, using the mechanical monochromator in an inclined position to the beam axis offers the availability of thermal neutrons (λ = 2.8 Å) at the instrument.

[1] A. Radulescu et al., J. Phys. Conf. Series 351, 012026 (2012).
[2] A. Radulescu et al., J. Appl. Cryst. 48, 1860 (2015).
[3] J. Houston et al., J. Appl. Cryst. 51, 323 (2018).
[4] A. Radulescu et al., J. Vis. Exp. 118, e54639 (2016), KWS-2 video.
[5] A. Radulescu et al., EPJ Web Conf. 286, 03006 (2023).
[6] A. Radulescu, J. Appl. Cryst. 57, 1040 (2024).
[7] L. Balacescu et al., J. Appl. Cryst. 54, 1217 (2021).
[8] A. Radulescu, Ioffe, A., NIM-A 586, 55 (2008).
[9] A. Radulescu et al., NIM-A 689, 1 (2012).