A compact TOF-SANS using focusing lens and very cold neutrons

Abstract

We are developing a high-resolution small angle neutron scattering instrument for very cold neutrons (VCN). Our concept includes a magnetic lens for focusing of the beam at the detector plane. The lens consists of one permanent-magnet sextupole array rotating outside another stationary sextupole array, to focus a pulsed white beam of neutrons. Thus the instrument operates in time of flight mode. The prototype magnetic lens has a bore of 15mm diameter and length of 66mm, producing a magnetic field gradient oscillating from 1.5×104 to 5.9×104T/m2, with frequency ≤25Hz. A torque-canceling magnet around the lens suppresses the torque of rotation from the outer array to 1/3.We have demonstrated the performance of the lens, over wavelength range from 30 to 48Å, on the PF2-VCN beam line at the Institut Laue-Langevin, France. The focused beam image was the same size as the source, without chromatic aberration, with focal length of 1.14m. We also studied the performance of this configuration for high-resolution SANS, in a compact geometry (just 5m long). The measurable q range of this system was 0.009Å−1≤q≤0.3Å−1 or 0.004Å−1≤q≤0.08Å−1 for sample to detector distances of 100 and 465mm, respectively. Here, we present the results of our lens characterization study along with the SANS results on a tri-block copolymer (F127 Pluronic) and on a stretched polymer blend (with the Shish-Kebab structure).