In pulse-height analysis (PHA) mode, incoming pulses are characterized based on their amplitude (peak voltage). The output spectrum is a histogram of these pulses, where the height of each channel corresponds to the number pulses counted within a narrow range of amplitudes. The resolution of the output spectrum depends on the number of channels of the MCA, which is on the order of a few thousand for typical instruments.
In alpha-, beta-, and gamma spectroscopy, PHA is used to measure the energy distribution of particles emitted in nuclear decay. Incoming particles are absorbed by a detector medium and excite voltage pulses whose amplitudes are proportional to their energy. After many pulses have been counted, the output spectrum shows the energy distribution of the radiation incident on the detector.