Small Angle Neutron Scattering (SANS)

Instrument Description

The NBL₁-SANS instrument is the first of 4 neutron scattering instruments to be operational at LENS. It utilizes a pin-hole collimation and covers a Q-range of 0.008 – 0.5 Å^-1 with a neutron flux of ~10⁴ n/cm²/sec. This instrument allows one to probe scattering length density correlations over length scales from 20 -500Å. The instrument consists of 4 instrument sections (Primary Flight Path, Sample Area, Secondary Flight Path, and Beam Dump), where the vault wall constitutes the boundary between the TMR (Target Moderator Reflector) facility and the SANS Instrument. Figure 2 and 3 below show a layout and a picture of the SANS Instrument, respectively.

**Figure 1:** Layout diagram of the SANS Instrument and TMR (Target Moderator Reflector) showing the primary and secondary flight paths with labels pointing to and identifying the target/moderator, be-filter, beam shutter, shielding wall, beam monitor, evacuated AI housings with tapered collimation system, sample area, detector & stop assembly, evacuated SANS tank, and beam dump.

**Figure 2:** Layout diagram of the SANS Instrument (top view, horizontally cut along the beam line) outside of the vault wall showing the PFP, sample area, SFP, and beam dump with labels pointing to and identifying to the vault wall, beam monitor, concrete shielding support & shielding arches, collimators #5-7, SANS tank, and ordella detector.

The Primary Flight Path (PFP) is the neutron transport system from the source (moderator) to the sample position. This is achieved by a series of evacuated Al-housings (square Al-tubes with Si-windows on each end) and a tapered collimation system (series of BN/Stainless Steel/BN sandwiches) placed inside. The source aperture views the entire moderator and has a diameter of 10 cm, whereas the sample aperture can be changed to various diameter sizes (1.5 cm, 2.0 cm, and 2.5 cm). A LN₂ cooled Be-filter (4”x4”x4” Be block, beryllium assay 99.4%) is located inside of the TMR vault close to the caramel corn shielding with the purpose to minimize the flux of fast and epithermal neutrons. A beam monitor (low efficiency 3He detector) is located app. 20 cm from the vault wall at a break in the evacuated Al-housings. It is used for calibration and normalization of the scattering data.

**Figure 3:** Close-up of the Small Angle Neutron Scattering (SANS) Instrument area.

The Sample Area (SA) consists of a 71 cm diameter sample rotation table, on which a variety of sample environment equipment can be mounted. Available equipment includes a closecycle refrigerator, sample changer with temperature control over a range from 10 to 60 C, and a pressure cell.

The Secondary Flight Path (SFP) incorporates the detection system for the scattered neutrons. It consists of an evacuated stainless steel tank (Si entrance window), the 2D detector (manufactured by ORDELA, model 2651N, serial# 002), and a beam stop assembly. The latter allows BN-beam stops of various sizes (3cm, 5cm, 8cm, and 10cm diameter, depending on sample-to-detector distance) to be driven into the incident beam in order to prevent damage of the area detector. The whole beam stop assembly can also be moved vertically for alignment purposes. The 2D-detector can be translated inside of the evacuated SANS tank, in order to change sample-to-detector distance, i.e. Q-range and Q-resolution. At the door in the back of the SANS tank, a 12” diameter ISO cross with a 20” long extension piece is attached and also evacuated. The Beam Dump (BD) functions as the beam stop for higher energy neutrons transmitted through the 2D detector and exiting the SFP.

SANS Instrument Characteristics
Instrumentation Feature	Characteristics
Source Frequency	10 – 30 Hz
Moderator	Coupled < 20 K solid methane
Wavelength Range	4 Å - 20 Å
Wavelength Resolution (Δλ/λ)	≤ 6% (FWHM) (wavelength dependent)
Source-to-Sample Distance	8.55 m (fixed)
Sample-to-Detector Distance	1.1 m to 4.2 m (variable)
θ-Resolution (Δθ/θ)	3 - 48% (FWHM)
Collimation	Circular pinhole collimation
Beam Diameter at Moderator	10 cm
Sample Aperture Diameter	1.5 cm, 2.0 cm, 2.5 cm (selectable)
Sample Size	0 to 25 mm diameter
Area Detector (³He 2D ORDELA): Active Volume	64 x 64 cm², 4.4 cm thick
Area Detector (³He 2D ORDELA): Pixel Size	1 x 1 cm²
Area Detector (³He 2D ORDELA): Detector Efficiency	71% for 5 Å neutrons, 52% for 3 Å neutrons
Count-Rate Capability	10⁴ n/s for 10% coincidence losses (10⁵ n/s max.)
Background	~ 0.5 Hz on whole 2D detector
Q-Range	0.008 Å^-1 to 0.5 Å^-1
Integrated Flux on Sample	10⁴ n/cm²/s

SANS Typical Results

To date this instrument has been used in the study of confined and complex fluids, including a recently published study of the temperature and pH induced changes in the size and shape of highly concentrated micelles. Representative data from this work are shown below (Das et al., Langmuir, 42, 11962 (2012).