When a metal target in an x-ray tube is struck by a beam of electrons accelerated through a voltage V, two concurrent processes give rise to an x-ray emission spectrum.

First, the electrons lose kinetic energy in Coulomb interactions with nuclei in the target. This lost energy produces a continuous spectrum of photons called bremsstrahlung (braking radiation) covering a range of energies between 0 and $E_{max} = eV - \Phi$. This maximum energy is the kinetic energy of a single electron (eV) minus the work function, $\Phi$), or electron binding energy, of the metal. As the work function is typically on the order of a few eV and the kinetic energy due to the accelerating voltage is of the order of several keV, we can neglect $\Phi$. Therefore, this electron high energy cutoff can be related to an x-ray short wavelength cut-off by

Note that we can express $hc$ in units of electron-volt nanometers as $hc \approx 1240 \textrm{eV}\cdot\textrm{nm}$

Second, beam electrons knock atomic electrons in the target out of inner subshells, and giving the liberated electrons kinetic energy. Atoms with missing inner electrons are unstable. When electrons from outer shells of that same atom fall into the vacant inner shells, they radiate discrete energies, characteristic of the atomic species of the target. In this experiment we are particularly interested in K lines, which appear when electrons fall into vacancies in the $K_n = 1$) shell. The most prominent are the $K_\alpha$ line (from $n = 2$ to $n = 1$ transitions) and the $K_\beta$ line (from $n = 3$ to $n = 1$ transitions). See Fig. 1 for a schematic of $K$ and $L$ x-ray emission lines.

Figure 1: Atomic shell model showing the origins of $K$ (ending on $n = 1$) and L (ending on $n = 2$) emission lines

Thus, the x-ray spectrum produced is the superposition of the continuous (bremsstrahlung) and discrete (K, etc.) components as illustrated in Fig. 2.

Figure 2: X-ray spectrum produced when electrons are accelerated into a metal target. Note that a scale is not given on the x-axis; it will be quite different than the PHA.