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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.