A second basic type of detector relies on the interaction of a photon with an electron, resulting
in an electric current. In a photomultiplier tube, the emitted electrons are accelerated and used to
create an amplified current, which is measured. Photovoltaic cells may be employed as radiation
detectors. The photovoltaic effect results from the generation of a potential across a positive–
negative junction in a semiconductor when it is subject to a flux of photons. Electron-hole pairs are
formed if the incident photon has an energy level of sufficient magnitude. This process results
in the direct conversion of radiation into electrical energy, in high sensitivity, and in a fast response
time when used as a detector. In general, the photon detectors tend to be spectrally selective, so
that the relative sensitivity of the detector tends to change with the wavelength of the measured
radiation.