Development of the X-ray polarimeter using CMOS imager: Polarization sensitivity of a 1.5μm pixel CMOS sensor

We are developing an imaging polarimeter by combining a fine-pixel CMOS image sensor with a coded aperture mask as part of the cipher project, aiming to achieve X-ray polarimetry in the energy range of 10-30keV. A successful proof-of-concept experiment was conducted using a fine-pixel CMOS sensor with a 2.5μm pixel size. In this study, we conducted beam experiments to assess the modulation factor (MF) of the CMOS sensor with a 1.5μm pixel size manufactured by Canon and to determine if there was any improvement in the MF. The measured MF was 8.32%±0.34% at 10keV and 16.10%±0.68% at 22keV, exceeding those of the 2.5μm sensor in the 6-22keV range. We also evaluated the quantum efficiency of the sensor, inferring a detection layer thickness of 2.67±0.48μm. To develop a more sensitive polarimeter, a sensor with a thicker detection layer, smaller pixel size, and reduced thermal diffusion effect is desirable.