Development of the X-ray polarimeter using CMOS imager: polarization sensitivity of a $1.5~{\rm \mu m}$ pixel CMOS sensor

TL;DR

This study demonstrates that a CMOS sensor with 1.5 μm pixels shows improved X-ray polarization sensitivity over larger pixels, advancing the development of high-resolution X-ray polarimeters for 10-30 keV energies.

Contribution

The paper reports the first assessment of a 1.5 μm pixel CMOS sensor's modulation factor and efficiency for X-ray polarimetry, showing enhanced performance over previous sensors.

Findings

Measured modulation factor of 8.32% at 10 keV

Modulation factor of 16.10% at 22 keV

Quantum efficiency indicates a 2.67 μm detection layer

Abstract

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