The Multichannel Analyzer

The multichannel analyzer (MCA) is an important laboratory instrument which can measure distributions of input signals consisting of pulses. It operates in two different modes: pulse height analyzer (PHA) mode, and multichannel scaler (MCS) mode. In PHA mode, the input pulses are sorted into bins (channels) according to their amplitude, while in MCS mode they are sorted according to the time when they arrive. The MCA provides a visual display of the resulting distributions and usually can output the data to a printer or computer for further analysis. Control in older MCA’s was done by hard-wired logic, whereas all modern MCA’s are microprocessor-controlled and have fairly extensive local arithmetic operation capabilities. Most microcomputers can function as MCA’s with the addition of a specialized MCA board. In this experiment, the MCA will be used in PHA mode to investigate nuclear γ-ray spectra, and in MCS mode to study statistical distributions. These two modes are discussed in turn.