Test of a new polarized hydrogen target (spin refrigerator) as neutron polarizer

Neutron-transmission measurements were made to determine the proton polarization in a single crystal of yttrium ethylsulfate with the aim of exploring its utilization as a spin filter for thermal and epithermal neutrons. The proton polarization is coupled to that of a paramagnetic ytterbium impurity simply by rotating the extremely anisotropic crystal in a magnetic field (spin refrigeration). A proton polarization of 41% was obtained in a helium bath near 1.2 K by rotating a large single crystal at a frequency of 150 ${\sec }^{-1\mathrm{}}$ in a field of 14.6 kOe. The extrapolation of the results indicates that a proton polarization $p_{\mathrm{H}}\simeq 65\%$ can be achieved with relatively simple technology. For this value of $p_{\mathrm{H}}$ the spin refrigerator becomes a competitive neutron spin filter.