The Physics laboratory nuclear spectroscopy system consists of a NaI(Tl) scintillation detector and the model 800A Nucleus multichannel pulse-height analyzer (MCA). When a high energy particle (alpha or beta particle, or gamma photon) passes through the NaI crystal, a current "pulse" is generated and the MCA converts the pulse into a "count" placed in one of the 256 channels of the analyzer. You should review the beginning of the Manual of Operation available in the lab to become familiar with the model 800A's front panel controls and general operation.
Initial SetUp for Experiments Numerous experiments which can be performed with the spectroscopy system are contained in the experiments manual, Spectrometry Experiments with Multichannel Analyzers. Most require calibration of the MCA. The first two experiments of the manual deal with detector resolution and energy calibration and should be performed before proceeding to other experiments. Copies will be distributed before the lab.
Essentially, the MCA performs an integration of the current pulse from the NaI detector and places a count in an appropriate channel corresponding to the size of pulse. The size of the pulse is proportional to the energy of the particle passing through the detector. The low and high end response of the MCA to particle energy is set in accordance with the range of particle energies expected in a particular experiment. This is done with the Zero, the Threshold, and the Amplifier Course and Fine Gain controls on the front panel of the MCA. Section 3.2, pages 16 through 19 of the operation manual, as well as Experiment #2 of the experiments manual, describe how to calibrate using the 32 keV and 662 keV gamma peaks from Cs-137.
Analysis:
A great deal of analysis can be done directly with the MCA front panel controls. It is possible to determine the counts in any region of the spectrum, the individual channel counts, and the FWHM and centroid of photopeaks. Peak stripping and other techniques are also described in both manuals. However, it is often useful to analyze the spectra using graphical analysis programs. The model 800A has an RS232 input/output port that can be used to transfer the spectroscopy data to a computer. On the lab's MacSE computers, you will find a folder called the MCAFolder. From this folder launch a Hypercard program called MCAProgramControl, which will guide you through the steps necessary to transfer the data stored in the MCA to the KaleidaGraph software program. A guide to the MCAProgramControl will be provided.