In the X-ray spectrum, the units of mass are not directly associated with the spectrum itself. The X-ray spectrum refers to the range of X-ray wavelengths or energies, which are measures of electromagnetic radiation.
However, if you are referring to the interaction of X-rays with matter, mass can be relevant in terms of the units used to express the density or thickness of materials. The common units for mass in this context are grams (g) or kilograms (kg).
For example, in X-ray imaging or radiography, the density of a body part affects the attenuation or absorption of X-rays. The mass per unit volume (expressed in g/cm³ or kg/m³) determines the X-ray attenuation properties of the material. Different tissues or substances will have varying densities, and this influences how X-rays interact with and pass through them.
It's important to note that when discussing the X-ray spectrum itself, the units typically used are related to energy, such as electron volts (eV) or kilo-electron volts (keV), rather than mass.