The reasoning
X‑rays are electromagnetic radiation of very short wavelength (~0.01–10 nm), massless and travelling at 3 × 10⁸ m/s. In the EM spectrum: radio < microwave < infrared < visible < ultraviolet < X‑ray < gamma. X‑rays and gamma rays overlap in energy; they differ by origin (X‑rays from electron interactions outside the nucleus, gamma from the nucleus).