PURPOSE: Measure the half-life of Ba 137, approximately 156 seconds.

DESCRIPTION: The 660 keV gamma ray from a Cesium 137 source is actually from its beta decay daughter, Barium 137. The cesium decays into a metastable state of barium with a half-life of about 156 seconds. Using a chemical technique, easily done in class using the equipment in the photograph at the right above, the barium is separated out from the sample of radioactive powdered cesium. The 660 keV gamma rays from the decaying barium daughter are detected by a Geiger tube and a graph of counts versus time plotted for equal time intervals using a Macintosh computer with the Universal Lab Interface and the radioactivity and graph plotting software. The decay curve, taken in about 5 minutes, is shown below.

SUGGESTIONS: Think about this one before you do it and get instruction on operation of the system.

REFERENCES: (PIRA 7D10.20) See Demonstration Reference File for further information about the ULI and the nuclear physics involved.

EQUIPMENT: Macintosh IIci computer with Universal Laboratory Interface, radioactivity detector, and radioactivity and graph plotting software, with cesium/barium minigenerator.

SETUP TIME: 10 minutes.