Spectra. Escape peaks. Sum peaks.

Escape peaks arise when a strong element peak is recorded. Their formation occurs within the detector crystal. When an incident photon is passing through the detector volume and its characteristic energy is sufficiently high, it can produce a photoelectron from an inner shell of a crystal atom (Si). As a result, the excited atom can emit a fluorescence X-ray photon, mostly a K_α photon. It is most of the times reabsorbed thus contributing to the charge pulse. However, that photon can also escape from the crystal. In that case, it carries off the definite energy of the Si− K_α. The charge pulse appear as corresponding to a photon energy E_initial − E _Si− Kα and therefore show up in the spectrum as separate spurious peaks.

Sum peaks are due to the coincidence of two photons with different energies entering into the detector. Sum peaks are often found when a few large peaks at lower energy dominate the spectrum. It is important to note that the intensity of the sum peaks is count-rate dependent, they can be reduced and virtually eliminated by performing the measurement with lower primary beam intensity.