Focusing cold neutrons with multiple biconcave lenses for small-angle neutron scattering

The focusing of a cold neutron beam by multiple biconcave lenses has recently been proposed as a practical means of extending the lower limit of Q in conventional, long flight-path small-angle neutron scattering (SANS) instruments. To test the feasibility of this approach, we have carried out extensive measurements on one of the 30 m SANS instruments at NIST of the focusing characteristics of a set of 28 biconcave MgF₂ lenses. The focused beam profile has been measured over several orders of magnitude using high resolution neutron auto-radiography. The focusing lens configuration outperforms the pinhole collimation at Q_min lower than 0.004 Å^-1. At Q_min = 0.001 Å^-1, the intensity gain of the lens configuration over the pinhole collimation is greater than one order of magnitude.