29:30 Physics IV Experiment 1

Experiment 5 : The Multichannel Analyzer

Purpose:

To become familiar with the features of a multichannel analyzer, or MCA.
To use the MCA to observe several gamma-ray spectra.

Discussion:

In this experiment you will gain familiarity with the use of a multichannel analyzer to acquire pulse-height spectra. The idea is to learn the function of all the control knobs on the instrument and some of the capabilities and limitations of the device. Instruments of this general type are used in current research in many areas of physics to measure various kinds of pulse-height spectra. You will observe gamma-ray spectra from a NaI(Tl) scintillation counter. Review Experiment 4. The present experiment is the same except the amplifier, SCA and scaler are replaced by the MCA. As you perform the various operations described below, be aware of both of the purposes given above. Although familiarization with the MCA is the most obvious end result of this experiment, the introduction to the spectra of various common gamma-ray sources will be very valuable in your future work in this and other laboratories.

Procedure:

Connect the photomultiplier tube to the high voltage power supply and to the amplifier input of the MCA. Have the hookup checked and turn on the equipment. Set the high voltage at 1000 volts. Start with a Co-60 source. This source emits gamma- rays with energies of 1173 and 1332 keV. The following is specific to the Model 1024D made by The Nucleus. If other types of MCA are encountered, corresponding controls should be investigated.

Start collecting data by simply pushing the "Analyze" push button. The appearance of green dots at random places on the screen indicates that data is being accumulated. To observe a spectrum after accumulating it, just press the "Readout" button. Then to erase a spectrum press the "Erase" button.
Take several spectra with different settings of the fine gain and the course gain controls and observe the effect of these controls.
After taking one of the spectra, observe the effect of changing the "Counts FS", the "Vert. Expand" and "Horiz. Expand" controls. Observe the use of the cursor control with and without the "Horiz. Expand" turned on. Observe the LED readouts of the channel position and the counts.
Investigate the use of the livetime switch. Compare the timing indicated by the switch with a stop watch or clock. Do this both while observing a source which gives a moderate count rate and also while accumulating data such that there is a significant dead time. Does the dead time plus the setting of the livetime switch equal the clock time?
Investigate the use of the subtract feature. For example, collect a cobalt spectrum and then, without erasing, collect a spectrum of Cs-137 for some preset time. Then, with the cobalt source removed, put the add/subtract switch on the subtract position and subtract the cesium spectrum for the same preset time. How has the cobalt spectrum been modified by this procedure? What is the significance of the nearly full scale counts that appear at some points in the spectrum?
Investigate the feature that permits accumulation in a selected half or a selected quadrant of the full 1024 channels and which permits observation of two or more of the resulting spectra in an "overlap" mode. As an example of the use to which this feature might be put, take a spectrum of an isolated Cs-137 source in the 1/2 region for a preset time. Then take a spectrum of the same source with an aluminum block directly behind it to enhance the backscatter peak (recall experiment 4) in the 2/2 region. Now overlapping the two spectra should show clearly the enhanced backscattering.
Observe spectra of ThC", Na-22 and Ba-133 in addition to the Cs-137 and Co-60 spectra previously observed. In each case adjust the gain so the highest energy peaks lie between channels 800 and 900. Include in your report sketches of the spectra and add narrative comments to indicate the important features. Find out the energies of the peaks in the spectra and label each peak in your sketches with its energy.

Report:

The report should be written as if addressed to yourself, and should consist of a complete reference which you can use in your future encounters with MCA's. Just describe in your own words all of the effects you have observed and include additional sketches to illustrate.