  X-ray SOI pixel sensors, "XRPIX", are being developed for the next-generation
X-ray astronomical satellite, "FORCE". The XRPIX are fabricated with the SOI
technology, which makes it possible to integrate a high-resistivity Si sensor
and a low-resistivity Si CMOS circuit. The CMOS circuit in each pixel is
equipped with a trigger function, allowing us to read out outputs only from the
pixels with X-ray signals at the timing of X-ray detection. This function thus
realizes high throughput and high time resolution, which enables to employ
anti-coincidence technique for background rejection. A new series of XRPIX
named XRPIX6E developed with a pinned depleted diode (PDD) structure improves
spectral performance by suppressing the interference between the sensor and
circuit layers. When semiconductor X-ray sensors are used in space, their
spectral performance is generally degraded owing to the radiation damage caused
by high-energy protons. Therefore, before using an XRPIX in space, it is
necessary to evaluate the extent of degradation of its spectral performance by
radiation damage. Thus, we performed a proton irradiation experiment for
XRPIX6E for the first time at HIMAC in the NIRS. We irradiated XRPIX6E with
high-energy protons with a total dose of up to 40 krad, equivalent to 400 years
of irradiation in orbit. The 40-krad irradiation degraded the energy resolution
of XRPIX6E by 25 $\pm$ 3%, yielding an energy resolution of 260.1 $\pm$ 5.6 eV
at the full width half maximum for 5.9 keV X-rays. However, the value satisfies
the requirement for FORCE, 300 eV at 6 keV, even after the irradiation. It was
also found that the PDD XRPIX has enhanced radiation hardness compared to
previous XRPIX devices. In addition, we investigated the degradation of the
energy resolution; it was shown that the degradation would be due to increasing
energy-independent components, e.g., readout noise.
