Dates and Times: Thursday, April 30 – 9:00 a.m., 10:30 a.m., 2:30 p.m.
                               Friday, May 1 – 9:30 a.m., 2:30 p.m., 3:30 p.m.

Location: Dana, Room 127. I must be on hand to lead you through the exercise.

Report: Items in italics indicate material to be put on your report sheet.

Introduction:  In this exercise we will investigate and verify the random behavior of radioactive decay and determine the half-life of a radioactive isotope. You will use a computer-interfaced GM detector to monitor the decay rate of two different radioactive sources.

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Every time a decay product (such as an alpha or beta particle) flies out of the source and enters the detector through its window, a voltage signal is sent from the GM tube to an interface, and then on to the computer program which keeps a running tally of all such counts.

Initial Setup: Plug in the power cord for the GM detector and connect its phone jack to Channel 1 of the Science Workshop Interface box.  Log on to the computer.

• Open Data Studio. Click on Create Experiment.
• Click on the "add sensor or instrument" tab. Select "digital sensors" from the pull down menu. Click "Geiger Counter" and then click OK.
• Drag a Graph into digital channel 1.
• Under the "sample rate" tab, select seconds from the pull down menu. Then make the time box 5 seconds.

Carefully remove the plastic protective cap from the window of the LabNet Geiger-Müller detector. Clamp the GM detector vertically so its bottom edge is about 1 cm above the tabletop.

1. Carefully place the alpha source, paper side down, on the table and slide it directly underneath the GM detector. Lower the detector until it is just above the source, but so that you can slide a piece of paper between the source and detector.
2. Click on "Start" to begin data recording. A data point should appear every 5 seconds on the Graph. What characteristics about the counts do you notice?
3. After taking data for 60 seconds, click "Stop". Place a piece of paper between the source and the detector without moving the source. Click "Start" and record another 60 seconds of data. What do you notice about the new set of counts? Why has this happened?
4. Once you have completed the run, carefully slide out the alpha source and replace the source in the storage box. Close the box and put it aside.

1. Raise the GM detector so its bottom edge is about 4 cm above the tabletop.
2. Carefully place the long-half-life beta source, foil side down, on the table and slide it directly underneath the GM detector.
3. Before you start collecting data, choose "No Data" from the pop-up "Data" menu in the Graph window.
4. Click on "Start" to begin data recording. A data point should appear every 5 seconds on the Graph and in the histogram. Click "Stop" after 180 seconds.
5. Place a piece of paper between the source and the detector. Click "Start", record 180 seconds of data, and then click "Stop". What do you notice about the new set of counts? How many sheets of paper do you think will cut the count rate to basically zero? Now place one of the lead sheets over the source. Click "Start" and notice what happens. Now click "Stop".
6. On the Graph display, click on the Sigma icon button to show the "Stats" popup menu. From the pop-up menu that appears choose Mean. These values will appear in the label of the graph.  Record these numbers.
7. Once you have completed the run, carefully slide out the paper towel with the beta source and replace the source in the storage box. Close the box and put it aside.
8. Click on the Graph window and then choose "Print" from the "File" menu. Attach this graph to your exercise.

We are always subject to radiation from natural sources in the universe. Cosmic rays as well as radioactive atoms in water, soils, and even our bodies all contribute to the background count. The background count rate must be determined and subtracted from all determinations of count rate to yield the corrected count rate.

1. Remove all radioactive sources from the vicinity of the GM tube.
2. Before you start collecting data, choose "No Data" from the pop-up "Data" menu in the Graph window.
3. Click "Start". Again, the random nature of the activity can be noted. The counts per unit time will give the average background count rate and should be subtracted from any measured radioactive sample count rates in order to obtain the net rates due to the source alone.
4. After two minutes, the "Stop" button can be used to stop the collection of data and the GRAPH for this Run can be printed. Attach the graph to your exercise. Record a value for the background as the mean of counts per 5-second interval.

The plot you will be given shows the radioactive decay of Ba-137m. The decay curve shows the smooth exponential decay drawn through the randomness of the actual decay. Taking into account the mean background count rate you measured in Exercise 3, determine the half-life of this isotope. Use the table on your sheet to help you determine these times. Staple the plot to your final report.

1. How long will it take for Ba-137m to decay to 1/32^nd of the original counts/second?
2. How do your measurements of the half-life of Barium-137m compare to the accepted value of 2.6 minutes?