Single channel analyzer has a lower and an upper level discriminators (LLD and ULD), and produces an output logic pulse when the input pulse lies between the differential of discriminator level or window. with this device all voltage pulses in a specific range can be selected and counted. If a full voltage or energy spectrum is desired, the SCA discriminators can be set to count a narrow range of voltage pulses and than gradually stepped up to higher voltages, The obtained count can than be plotted against the discriminator bias level to brain a gamma-ray spectrum.