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CHRNS VSANS - Very Small-Angle Neutron Scattering
Sponsored by the National Science Foundation as part of the Center for High Resolution Neutron Scattering (CHRNS), this brand new 45 meter long Small-Angle Neutron Scattering (VSANS) instrument on neutron guide NG3 was put into operation in 2017.
The instrument has a number of design features to make it more versatile than the 30 m instruments:
- High Resolution detector
- Converging beam collimation
- Multiple detector arrays
- Narrow or broad wavelength resolution
Specifications/Capabilities
A high resolution (0.2 mm fwhm) 2D detector along with a longer flight path (45 m) allows a factor of four smaller q to be measured, qmin = 0.002 nm-1, compared to the 30 m instruments. To enhance count rate at the smaller q-range, larger samples or relaxed resolution is employed. Converging beam collimation allows utilization of larger samples with a gain of a factor of 200 over standard pinhole collimation. Relaxed resolution using slit collimation with small samples produces a gain of 25,000 over pinhole collimation. The instrument has three separate detectors that can be placed independently at different distances from the sample allowing the full q-range, 0.002 nm-1 ≤ q ≤ 10 nm-1 to be measured in one setting. The incident wavelength, and wavelength resolution, are controlled over a wide range with either a standard resolution mechanical velocity selector (Δλ/λ = 12%), high resolution graphite monochromator (Δλ/λ = 1%), or low resolution filtered beam covering 0.4 nm ≤ λ ≤ 0.8 nm with Be filter and guide deflector. The instrument has a large 2 m sample area allowing larger ancillary sample environments to be used. Full beam polarization using a 3He analyzer is also available. All instrument configurations are controlled by computer.
| Q-range | 0.002 nm-1 to 10 nm-1 | ||
| Size Regime | 1 nm to 2,000 nm | ||
| Source | Neutron Guide (NG-3), cross-section: 60 mm x 150 mm | ||
| Power Outlets | 220V, 30A, 1-phase - Qty. 3 208V, 30A, 3-phase - Qty. 2 208V, 60A, 3-phase - Qty. 2 110V, 30A, 1-phase - Numerous UPS and non-UPS | ||
Three interchangeable devices are used to monochromate the source:
- Mechanical Velocity Selector: Helical path selects triangular wavelength distribution having fwhm of 12%. Changing rotation speed allows range in mean wavelength: 4.5 Å < λ < 12.0 Å
- Double reflection Pyrolytic Graphite monochromator provides higher resolution: 1% fwhm, range: 4.0 Å < λ < 6.0 Å
- Beryllium filter cuts λ < 4.0 Å, mirror cuts λ > 8.0 Å, providing lower resolution but factor of five flux gain.
| Velocity selector | Mirror | HOPG | |
| Wavelength Range | 4.5 Å to 12 Å | 5.3 Å | 4 Å to 6 Å |
| Wavelength Resolution (fwhm) | 12 % | ~ 40% | 1% |
Three types of collimation are provided:
- Standard circular pinholes, up to 60 mm diameter source and 30 mm for sample.
- Narrow slits, providing high resolution in width but low resolution vertically for high flux. Slit width continuously variable.
- Converging Narrow Beams, using lenses for focussing and prisms to correct for gravity smearing. Requires large (36 mm wide x 72 mm tall) sample for maximum signal.
| Source-to-Sample Distance | 4 m to 22 m in 2 m steps | ||
| Collimation Types | Circular Pinhole, Multiple converging beams or Narrow slits | ||
| Sample Size | 5 mm diameter to 36 mm x 72 mm | ||
| Sample-to-Detector Distance | Front | Middle | Rear Carriage |
| 0.6 m to 10 m | 2.5 m to 18 m | 10 m to 22 m | |
A number of new capabilities are provided with the new detectors:
- By using a high resolution (0.2 mm) detector on the rear carriage, much higher q-resolution is achievable.
- By using nine panels on three carriages the entire q-range can be covered in one shot, particularly useful for kinetic measurements.
Higher data rate capability. Each tube has 10% deadtime at 100 kHz. With 48 x 8 = 384 tubes, count rates of 38 Mhz can be handled.
Credit: John Barker
| Front | Middle | Rear Carriage | |
| Type | He(3) tubes | He(3) tubes | Scintillator + CCD |
| Resolution | 8 mm | 8 mm | 0.2 mm |
| #Panels | 4 | 4 | 1 |
| Size | 2 x 380 mm wide x 1000 mm tall 2 x 500 mm wide x 380 mm tall | 220 mm wide 500 mm ta | |