X-rays are a form of electromagnetic radiation, and therefore their SI unit is the same as for all electromagnetic radiation: the joule (J).

However, it's important to note that:

* X-ray energy is often expressed in electron volts (eV), which is a more convenient unit for describing the energy of individual photons. 1 eV = 1.602 × 10⁻¹⁹ J.

* X-ray intensity is measured in various units depending on the context, such as:

* Gray (Gy): measures the absorbed dose of radiation.

* Sievert (Sv): measures the equivalent dose, taking into account the biological effects of different types of radiation.

* Becquerel (Bq): measures the activity of a radioactive source, which is the rate of decay.

So, while the SI unit of X-rays is the joule, other units are frequently used for specific purposes.