Application of a neutron-polarizing device based on a quadrupole magnet to a focusing SANS instrument with a magnetic neutron lens

A magnetic neutron lens based on a sextupole magnet is considered an ideal neutron-focusing device for focusing-geometry small-angle neutron scattering (FSANS) instruments using polarized neutrons. However, the magnetic neutron lens functions both as focusing and defocusing lens for positive and negative polarity neutrons, respectively. Negative polarity neutrons are defocused by the lens and spread over the detector of the instrument, resulting in an increase of the background level. Therefore, a neutron-polarizing device with a very high polarizing efficiency P greater than or similar to 0.99 is required to be employed together with the magnetic neutron lens. A neutron-polarizing device based on a quadrupole magnet has a very high polarizing efficiency P greater than or similar to 0.99, which is sufficient for the application to the FSANS instrument. In this study, we installed the quadrupole magnet in the FSANS instrument SANS-J-II and investigated its performance with respect to the conventional magnetic super-mirror polarizers of the FSANS instrument.