Measurement of the Anisotropic Response of the ZnWO4 Crystal for Developing the Direction-Sensitive Dark Matter Detector

We measured the scintillation light yield of ZnWO₄ crystals using a quasi-monochromatic neutron beam energy of 865 keV. Irradiating the neutron beam on two surfaces, which are almost perpendicular to the crystal axis, the light yield of oxygen recoil was measured. The obtained light yield ratio of a neutron recoil to gamma-ray-induced electron recoil for two surfaces was 0.235 ± 0.026 and 0.199 ± 0.020, respectively, which corresponds to 15.3% of anisotropy for ~200-keV nuclear recoils of this crystal. This property can be applied to a direction-sensitive dark matter (DM) detector.