NaI(Tl) is a scintillation crystal widely used in nuclear medicine;
The crystal is coupled to a photomultiplier tube to generate a voltage pulse representing the energy deposited in the crystal by the radiation;
A very sensitive amplifier is needed to measure such voltage pulses;
The voltages pulses range in amplitude depending on how the radiation interacts with the crystal, i.e. the pulses form a spectrum whose shape depends on the interaction mechanisms involved, e.g. for medium-energy gamma-rays used in in-vivo nuclear medicine: the Compton effect and the Photoelectric effect;
A Gamma-Ray Energy Spectrum for a medium-energy, monoenergetic gamma-ray emitter consists (simply) of a Compton Smear and a Photopeak;
Pulse Height Analysis is used to discriminate the amplitude of voltage pulses;
A pulse height analyser (PHA) consists of a lower level discriminator (which passes voltage pulses which are than its setting) and an upper level discriminator (which passes voltage pulses lower than its setting);
The result is a variable width window which can be placed anywhere along a spectrum, or used to scan a spectrum;
A single channel analyser (SCA) consists of a single PHA with a scaler and a ratemeter;
A multi-channel analyser (MCA) is a computer-controlled device which can acquire data from many windows simultaneously.
