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

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Accelerators, spectrometers, detectors and associated equipment, 2024-08, Vol.1065, Article 169487
• Main Authors: Iwata, Toshiya, Hagino, Kouichi, Odaka, Hirokazu, Tamba, Tsubasa, Ichihashi, Masahiro, Kato, Tatsuaki, Ishiwata, Kota, Kuramoto, Haruki, Matsuhashi, Hiroumi, Arai, Shota, Minami, Takahiro, Takashima, Satoshi, Bamba, Aya
• Format: Article
• Language: English
• Subjects: CMOS imaging sensor; Hard X-rays; Polarimetry
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2024.169487

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. •A 1.5 μm pixel CMOS image sensor successfully detected X-ray polarization.•The 1.5μm sensor showed 1.2 times higher modulation factor than the 2.5̃ μm sensor.•The 1.5μm sensor had a detection layer thickness of 2.67 ± 0.48μm.•Polarimeter sensitivity can be enhanced by using a sensor with smaller pixels.•Thicker detector and reduced thermal diffusion can improve polarimeter sensitivity.