Small-angle neutron scattering spectrometer Suanni equipped with ultra-thin biconcave focusing lenses

The small-angle neutron scattering (SANS) spectrometer Suanni at the liquid hydrogen cold neutron source of the 20 MW China Mianyang Research Reactor has recently been upgraded. Ultra-thin biconcave MgF₂ lenses with a central thickness down to 0.2 mm have been installed between the collimator chamber and the sample stage. The lenses are able to improve the flux without too excessive an increase in the neutron beam size on the detector. A smaller minimum Q (Q_min) can be obtained by decreasing the beam size without changing the total length of the spectrometer. By testing the central beam profiles under different neutron wavelengths (∼0.56–1 nm) with both traditional pinhole SANS (PSANS) and focusing SANS (FSANS) geometries, the gain factor thanks to the neutron lenses is about one order of magnitude. Given the loss of intensity due to the absorption of neutrons by the lenses, the benefits of the focusing can only be realized if it is possible to increase the aperture size. With an identical source aperture, FSANS can minimize the nominal Q_min from 7.20 × 10⁻³ nm⁻¹ (for PSANS) to 5.55 × 10⁻³ nm⁻¹ at a neutron wavelength of 1 nm. The practical benefit provided by the lenses is verified with a solution of poly(methyl methacrylate) nanospheres, which yields a scattering intensity one order of magnitude higher and a better resolution with the FSANS geometry than with that of PSANS.