The conversion of the scintillation light to a usable electrical signal is traditionally accomplished using a photomultiplier tube, or PMT. This is essentially made of two parts: the photocathode and the electron multiplier tube.

The former is a thin layer of photosensitive material that converts a single photon into an electron with a probability, called quantum efficiency, that depends on the photon wavelength. The latter amplifies the weak primary current through a series of secondary emission electrodes, called dynodes. Once the energy of the primary electrons is transferred to the ones of the dynoes, a larger number of secondary electrons is emitted and accelerated to the next dynode. Typically tens of stages produce a gain of 107. The operational voltage of the PMTs used in ALPHA is between 1100 and 1200 V.