A transportable neutron spin filter

We report a transportable neutron spin filter based on the strong spin dependence of neutron proton scattering. The necessary high proton polarization is created by dynamic nuclear polarization (DNP) that uses short-lived optically excited triplet states instead of stable radicals, which eliminates the main path of nuclear spin lattice relaxation. The polarization can be preserved for many days under moderate experimental conditions and allows to realize a transportable spin filter which is conveniently polarized in the laboratory and then operated on a neutron beam line with practically frozen polarization maintained with a minimum of equipment. Using triplet DNP bulk proton spins in a pentacene doped single crystal of naphthalene have been polarized to a record value of 80% at a field of 0.36 T using a simple helium flow cryostat for cooling. This highly polarized sample can be transported in the compact cryostat without loss of polarization to a neutron beam line where it serves as a spin filter for polarization analysis in a neutron scattering experiment. Relaxation times of $T_1\sim 800$ h are achieved at a field of 20 mT and the polarization can be reversed by adiabatic fast passage with an efficiency of above 99%.