  In low earth orbit, there are many cosmic rays composed primarily of high
energy protons. These cosmic rays cause surface and bulk radiation effects,
resulting in degradation of detector performance. Quantitative evaluation of
radiation hardness is essential in development of X-ray detectors for
astronomical satellites. We performed proton irradiation experiments on newly
developed X-ray detectors called XRPIX based on silicon-on-insulator technology
at HIMAC in National Institute of Radiological Sciences. We irradiated 6 MeV
protons with a total dose of 0.5 krad, equivalent to 6 years irradiation in
orbit. As a result, the gain increases by 0.2% and the energy resolution
degrades by 0.5%. Finally we irradiated protons up to 20 krad and found that
detector performance degraded significantly at 5 krad. With 5 krad irradiation
corresponding to 60 years in orbit, the gain increases by 0.7% and the energy
resolution worsens by 10%. By decomposing into noise components, we found that
the increase of the circuit noise is dominant in the degradation of the energy
resolution.
