DAPS is used to detect ionisation thresholds of core levels in atoms. This is done by bombarding the surface with electrons and modulating the energy of the primary beam. The electrons emitted from a surface can be detected with an RFA and measuring the current to ground from one of the hemispherical grids. If the primary beam voltage is such that a core peak is on the edge of being excited, then changes in the current emitted from the surface can be detected using standard modulation techniques.