Use of a charge-injection technique to improve performance of the Soft X-ray Imager aboard ASTRO-H

We are developing the Soft X-ray Imager (SXI), a charge-coupled device (CCD) camera system to be deployed onboard the ASTRO-H satellite. Using an engineering model system in which design specifications were the same as those of the flight model, we measured charge transfer inefficiency (CTI) and the effects of charge trailing. The CCD was irradiated with monochromatic X-rays produced by a radio isotope (⁵⁵Fe) and X-ray generator using alpha particles from ²⁴¹Am. We used four targets for the X-ray generator: (C₂F₄)_n, SiO₂, Ti, and Ge. Since CTI degrades energy resolution, we adopted the charge-injection technique to the SXI. With this technique, injected charges fill traps, and subsequent signal charges are transferred with less loss of charge. However, the charge-injection technique can cause positional variations in gain on the CCD chip. Thus, we constructed a method for correcting CTI. We also evaluated the charge trailing effect and tested a method for correcting its effects. After applying these corrections to charge injection, variations in gain improved from 0.5% to 0.1% over the CCD chip, and the energy resolution (FWHM) improved from ∼220eV to ∼180eV at 5.9 keV. - Highlights: • We measured CTI and charge trailing in the energy band from <1keV to ∼10keV. • We developed a method for correcting CTI and charge trailing effects. • Our data with high statistics required exponential functions as CTI model. • After applying the corrections, the remaining positional variations over the CCD satisfy requirements for flight operations