*2.3. FIA-XRD*

The fixed incident angle X-ray diffractometry (FIA-XRD) was carried out by RINT 2500 MDG, (Rigaku Co., Ltd., Tokyo, Japan), with a fixed incident angle of 15◦ from the sample surface using the Cr Kα line (λ = 0.22896 nm) from an X-ray source. Different from the conventional θ*-2*θ method, the penetration depth of the incoming X-ray is constant for FIA-XRD, irrespective of the scattering angle 2θ. Since the ion irradiation effects often depend on the sample depth, the FIA-XRD method could be advantageous. The incident angle of 15◦ corresponds to the X-ray penetration depth of 2.76 µm normal from the sample surface. Here, the penetration depth defines where the incident X-ray intensity decreases to 1/*e*, where *e* is the Napier's constant. The density of 4.56 g/cm<sup>3</sup> was used for the penetration calculation.
