CHRNS: vSANS Converging Beam Collimation
The converging beam option on vSANS provides 18 circular beams which converge at the detector. Fourteen biconcave MgF2 lenses focus the beam for neutrons having 6.7 Å mean wavelength. Nine prisms counteract the wavelength dependent distortion of the beam due to gravity. A sequence of 13 masks placed along the incident flightpath eliminates crosstalk between the 18 individual beams .
The new liquid D2 cold source will shift the spectrum to longer wavelengths. To take full advantage of this upgrade, we have changed the number of lenses per beam from 14 to 9, increasing the focusing wavelength from 6.7 Å to 8.36 Å. This modification not only reduces the minimum Q from 3.2x10-4 Å-1 to 2.6x10-4 Å-1, it also increases the data rate by about 50%, after accounting for the transmission losses and the new cold source. In addition to removing some lenses and prisms, this change required realigning the 13 masks as the longer wavelength neutrons follow a higher parabolic trajectory due to the slower mean neutron velocity.
 J.G. Barker, J J. Moyer, S.R. Kline, G.V. Jensen, J.C. Cook, C.V. Gagnon, E.G. Kelley, J.P. Chabot, N.C. Maliszewskyj, C. Parikh, W.C. Chen, R.P. Murphy & C.J. Glinka (2022). “The Very Small-Angle Neutron Scattering Instrument at the National Institute of Standards and Technology” J. Appl. Cryst., 55, ###-###.