Development of the X-ray polarimeter using CMOS imager: polarization sensitivity of a 1.5~ μ m 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$${x2013}30~keV. A successful proof-of-concept experiment was conducted using a fine-pixel CMOS sensor with a 2.5~μ m pixel size. In this study, we conducted beam experiments to assess the modulation factor (MF) of the CMOS sensor with a 1.5~μ m pixel size manufactured by Canon and to determine if there was any improvement in the MF. The measured MF was 8.32% ± 0.34% at 10~keV and 16.10% ± 0.68% at 22~keV, exceeding those of the 2.5~μ m sensor in the $6$${x2013}22~keV range. We also evaluated the quantum efficiency of the sensor, inferring a detection layer thickness of 2.67 ± 0.48~ μ m. To develop a more sensitive polarimeter, a sensor with a thicker detection layer, smaller pixel size, and reduced thermal diffusion effect is desirable.