Specifically, what makes x-rays easy to generate are these: https://en.wikipedia.org/wiki/Characteristic_X-ray In essence, smashing electrons into atoms allows you to ionize the inner shell of an atom and when an electron drops down from an outer shell, the excess energy is shed as high-energy photons. This constrains the energy range of X-ray tubes ("smash electron into metal") to wavelengths well below 13.5nm.

(These emission lines are also what is being used in x-ray spectroscopy to identify elements)

You can also generate broad spectrum bremsstrahlung radiation easily, this is widely used for medical X-rays.