A tiny neutron spin filter

We have designed and demonstrated a very compact neutron spin filter (NSF) based on polarized gaseous 3He. It is 5 cm in diameter and 20 cm in length; the total volume is only 400 cc. It also features a fast spin flip capability. The 3He spin, corresponding to that of neutrons, can be reversed by adiabatic fast passage (AFP) in less than a second. The AFP spin flip provides NMR signals for polarization monitoring and leaves no magnetic field change before and after the spin flip, which is essential in polarized neutron scattering experiments to cancel systematic uncertainties. In the article, we describe the design and performance of this tiny NSF.