Anisotropic Response Measurements of ZnWO$_4$ Scintillators to Neutrons
  for Developing the Direction-Sensitive Dark Matter Detector
  The scintillation yields of ZnWO$_4$ crystals change depending on the
incident direction of particles. This property can be used as a
direction-sensitive dark matter detector. So, we investigated the ZnWO$_4$
light yields ratio of neutron-induced nuclear recoils to gamma-ray induced
electron recoils(quenching factor). Two surfaces almost perpendicular to the
crystal axis of ZnWO$_4$ were irradiated with a quasi-monochromatic neutron
beam of 0.885MeV, and the quenching factors of both surfaces for the
oxygen-nucleus recoil in the ZnWO$_4$ crystal were measured. The obtained
quenching factors of the two surfaces were 0.235 $\pm$ 0.026 and 0.199 $\pm$
0.020, respectively confirming 15.3$\%$ of anisotropy.
